34974988 District Heating Danfoss

8 STEPS - CONTROL OF HEATING SYSTEMS


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1 8 STEPS - CONTROL OF HEATING SYSTEMS



Preface 1

Chapter 1 Definitions 3District heating 3Pressure 5Level pressure 5Steam pressure 5

Chapter 2 District heating systems used inWestern Europe 7

Production 71. Environmental requirements 72. Fuel 83. Exhaust emission control 94. Water quality 105. Flow and return temperatures 126. Expansion systems 137. Open expansion system 148. Closed expansion system 14

Distribution 151. Pre-insulated pipes 152. Construction, material 153. Heat losses 164. Linear expansion due to variations

in temperature 165. Design 166. Flow 177. Pumps 188. Pressure control 18

Consumption 191. Heat exchanger 192. Connection design 203. Electronic temperature controls 214. Self-acting controls 215. Control valves 226. Differential pressure control 23

7. Flow limitation 248. Energy metering 25

Chapter 3 Secondary systems used in Europe 27Preface 27Comfort 28Heat requirement 29

Transmission requirements 29Ventilation 30Wind influence 30Incidental heat gain from heat sources other than the heating system 31Domestic hot water 31

Production 321. Control 322. Control valves 333. Temperature controller 334. Periodic set back of the flow temperature 345. Expansion systems 346. Closed system 347. Open systems 348. High-rise buildings 35

Distribution 361. Definitions 362. Pipe material 373. Piping 374. Compensation for the linear

expansion due to temperature variations 385. Insulation 386. Flow 387. Pumps 398. Pump control 39

Consumption 401. Radiator and convector systems 402. Pressure distribution 42

Contents



3. Differential pressure controls 434. Control of the room temperature 445. Correct flow temperature 446. Floor heating systems 457. Control 468. Ventilation 46

Chapter 4 Evaluation of systems and products 47District heating 47Central boiler plant 48Fuel 49Combustion 50Exhaust emission control 51Temperatures 52Static pressure 52Expansion system 53Distribution - Consumption 54

1. Accumulator 542. Temperature 553. Static pressure 554. Pre-insulated pipes 555. Flow 56

Control valves 56Differential pressure control 58Flow limitation 59

6. Heat exchangers 607. Pump 618. Metering 63

Heating systems 651. One-pipe systems 66

Existing one-pipe systems 67Two or three–way valves 69

2. Two-pipe systems 70Vertical or horizontal systems 71Gravity 72

3. Thermostatic or manual valve 73

4. Weather compensation 75Setting of the right flow temperature 76Periodic set back of the flow temperature 77

5. Flow 78Differential pressure control 78Flow limitation 80

6. Static pressure 82The circulation pump in the flowor in the return pipe 82

7. Pump 84Principles for pressure control 85

8. Metering 88

Chapter 5 Instructions for designing districtheating systems. 89

Environment 901. Durability 902. Production 903. Fuel 914. Combustion 915. Flue gas purification 916. Handling of ashes 927. Handling of coal 928. Water quality 93

Local district heating system 941. Effect ranges 942. Existing boilers 943. New boilers 954. Accumulator 965. Expansion systems 976. Circulation pumps 98

Dynamic pressure 98Flow 98

7. Pre-insulated pipes 99Material 99Linear expansion due to variations in temperature 99Sizing of pipes 100

8. Heat exchangers 100

Operating conditions 1011. Temperature levels 1012. Return temperatures 101



3. Temperature drop in the distribution network 1024. Static pressure 1025. Available differential pressure 1036. Water quality 1047. Pressure testing 1058. Operating times 105

Local control and supervision 1061. The control of boilers 1062. Control of the accumulator 1073. Control of the outgoing temperature

in the district heating network 1074. Flow limitation 1085. Differential pressure control 1096. Pressure control of pumps 1107. Heat metering 1118. Central control and supervision 112

Chapter 6 Instructions for designing heating systems 113

Comfort 1141. Room temperature 1142. Temperature on the surfaces of the room 1143. Down draught 1154. Ventilation 1155. Wind influences 1166. Distribution of the heat 1167. Domestic hot water 1168. Hot water circulation 117

Conditions 1181. Heat requirement 1182. Calculation of the transmission losses 1183. Ventilation 1194. Incidental heat gain 1195. The wind influence on the heat requirements 1206. Heat requirement per room 1207. Control of the actual heat requirement 1208. Domestic hot water 120

Heating systems 1211. Heat exchangers 1212.Expansion system 1223. Circulation pump 122

4. Horizontal distribution pipe 1235. Risers 1246. High-rise buildings 1257. Radiator circuit, two-pipes horizontal 1268. Radiators – convectors 128

Operating conditions 1301. Temperature levels 1302. Return temperature 1303. Temperature drops in the pipe system 1314. Static pressure 1315. Expansion vessels 1316. Available differential pressure 1317. Water quality 1328. Heat losses in the sub-station 132

Control 1331. Control and supervision 1332. Control of flow and return temperature 1343. Control of the room temperature 1344. Pressure control of pumps 1355. Control of the available differential pressure 1366. Flow metering per apartment 1367. Control of domestic hot water 1378. Control of domestic water in an apartment 137

Chapter 7 How to select size of products and components 139

Thermostatic valves 139Choice of valve size 139Existing one-pipe systems 139Two-pipe systems 139Flow 139Valve size 140Pre-setting 141Choice of control unit 141

Control valves 142Primary systems 142

Available differential pressure 142Valve size 143



Secondary systems 144Available differential pressure 144Two-way valve 144Valve sizes 145

Differential pressure controls 146Primary systems 146

Available differential pressure 146Valve size 147Setting value 148

Secondary systems 148Available differential pressure 148Valve size 148

Differential pressure control of risers 150Setting value 151

Flow limitation 152Primary systems 152Secondary systems 153

Control equipment 155Radiator systems 155Hot water heating 156

Pipes and heat exchangers 157Pipes for heating 157Pipes for domestic water 158Heat exchangers 158

Heat meters 159The primary network 159The secondary network 159

Pressure control of pumps 160The primary network 160The secondary network 160

Chapter 8 Technical data, Formulas and charts 161



Heating a home has always been and still is a basic human requirement.This requirement. This requirement enables us to live and work in loca-tions with low temperaure. In the beginning the solutions were simple.An open fire on the floor of a tent or a simple hut, made it possible tosurvive in a hostile environment. As civilisation developed there wasmigration from the countryside to the towns and cities and into biggerand bigger houses, creating a requirement for more elborate heatingsystems. This requirement stimulated technical development, but alsocreated a problem, namely the use of a finite resource (fossil fuels) withthe resulting pollutions from the burned fuels.

The purpose of a good heating system is to create the best environmentpossible. The construction of the building with a well designed heatingsystem, associated with good automatic controls, minimises the heatingrequirements and emissions radically.

Preface.


8 STEPS - CONTROL OF HEATING SYSTEMS2


8 STEPS - CONTROL OF HEATING SYSTEMS 3

CHAPTER 1 • DEFINITIONS

District heatingDistrict heating is a system which provides a number of buildings withheat from a central boiler plant through pre-insulated pipes.(Pre-insulated pipes are in fact a modern kind of heat culvert or districtheating duct, but since these systems nowadays are pre-manufactured,they will from here on be referred to as pre-insulated pipes.) The smallest systems cover 200-300 houses or a block.

The connection to the secondary heating system can be direct or indi-rect, i.e. with or without a heat exchanger. Domestic hot water is alsoproduced with the help of district heating. As a result, the heating plantsare also in operation during non-heating seasons.

There is a difference between heating plants, pure heat producers andcombined heat and power plants. The main purpose of the last-namedis to produce electricity through a steam turbine. The connected buil-dings are used to cool down the condensate to such a low temperature aspossible in order to increase the capacity of the steam turbine.

The efficiency for coal-fired power plants is low, 30-40 %. By combiningthe power production with the heat delivery, the efficiency has increasedright up to 90 %, which corresponds to the efficiency of well-kept dis-trict heating plants.

A district heating plant, (the primary circuit), can be divided into threeparts:• Production (central boiler plant)• Distribution (pre-insulated pipes)• Consumption (sub-station)

Definitions.

Central boiler plant Distribution ConsumptionFig. 1:2

Combined heating and power plant.Fig. 1:1



In the production plant, the water temperature is increased to the re-quired level. Distribution implies heat transfer to the consumers with assmall a loss as possible. Consumption implies heat transfer from thewater of the primary side to the water of the secondary side, and a largetemperature drop in the primary water. It may also imply directlyconnected systems, detached houses for instance, with a differentialpressure control as protection against too high differential pressures.

District heating systems with a large production plant, an efficient dis-tribution network and a sub-station with heat exchanger and automaticcontrols, can be made very effective in respect of consumption as well aspollution.

The choice of material and operating conditions such as static pressure,temperature and water quality are important factors concerning the ope-ration of the system, its maintenance and its durability.

The heating system in a building, (the secondary circuit), can bedivided into three parts:• Production (heat transfer through the heat exchanger)• Distribution (the main piping system of the building, including

the circulation pump)• Consumption (radiators, convectors, or floor heating for the rooms)

In the production plant, the secondary water temperature is increasedto the required level.

Distribution implies heat transfer to the consumers with the smallestlosses possible and small temperature drop.

Consumption implies heat transfer from the water to the rooms andlarge temperature drop in the water.


Direct connectionFig. 1:3

Indirect connectionFig. 1:4

Production Distribution ConsumptionFig. 1:5



PressureIn district heating systems and heating systems, you make a distinctionbetween static and dynamic pressure. In an open system, the static pres-sure is equal to the weight of the water column. The word static repre-sents something stationary. The dynamic pressure appears when thewater begins to circulate and a circulating resistance is formed . The worddynamic means that something is in motion.

The static pressure has two functions in a distric heating system. It hasto ensure that all parts of the system are filled with water (level pressure)and that the water does not begin to boil (steam pressure).

Level pressureAll the parts of a system are filled with water if the static pressure, cal-culated in meter water gauge, is equal to the level of the system, at itsmeter. 10m WG = 1 bar = 100 kPa, providing the circulation pump is notin operation. If the circulation pump is placed in the flow line, which isusually the case with the district heating systems of today, the pump willprovide a higher total pressure (static + dynamic pressure) in the flowline, when in operation.

Correspondingly, the total pressure is lower in the return line, and lowestat the return connection to the pump. By placing the pump in the flow,you will have an additional guarantee that there is water in all parts of thesystem.

If the pump is placed in the return line, the case will be the opposite, andthe static pressure must be increased by 60-70 % of the pressure increaseacross the pump in order to get all parts filled with water.

Steam pressureThe boiling point of the water is depending on the current pressure.A low pressure decreases the boiling point and a high pressure increasesit. At sea level the water boils at 100°C in an open vessel, and already at120°C, an over-pressure (the pressure shown on the pressure gauge) ofapproximately 1 bar, 100 kPa, is required to avoid boiling. An over-pres-sure of 2 bar, 200 kPa, corresponds to approximately 130°C.

In order to avoid boiling, the over-pressure required must be available ineach unit of the system.


H

M

Height in meter is equal to static pressure.Fig. 1:6

Stat

ic p

ress

ure

Definition of pressure in district heating systems.Fig. 1:7

Dynamic pressureDifferential pressure

Tota

l pre

ssur

e

Stat

ic p

ress

ure

Steam generatingpressure

System or level pressureis determined by difference in altitudebetween heating plantand highest situatedsub-station




6



CHAPTER 2 • DISTRICT HEATING SYSTEMS USED IN WESTERN EUROPE

7

ProductionThe production takes place in a plant in which the energy of the fuel inquestion is converted into heat through combustion and then transferredto the water of the distribution network.

1. Environmental requirementsThe environmental requirements on fuel are made more and morestringent. The contents of environmentally hazardous substances in coaland oil have diminished considerably during the past ten years. There arealso requirements on the volume of dust discharges of the ashes aftergood combustion. In cases where the requirements made on the fuelcannot be fulfilled, a penalty tax is imposed, and/or a plant reducing theenvironmental influence to the established level is requested.

The pollutants, set free by the combustion, are spread with the windscovering very large areas. It is not sufficient only to limit the dischargeslocally, but the same requirements are necessary all over Europe. Certainvalues have been established and a tightening-up of the requirements willbe carried out, as people in many countries find the values too high.

Sulphur causes acidification of the ground which kills both plants andanimals. Nitrogen also causes acidification and have negative effects onthe ozone layer. Both these substances travel great distances and measu-res must be taken right at the source.

Opposite, see tabel, are allowed discharges according to IEA Coal Researchair pollutant emission standards for coal-fired plants database, 1991.

District heating systemsused in Western Europe.

Allowed discharges according to IEA CoalResearch air pollutant emission standards forcoal-fired plants database, 1991.

Particles mg/m3 SO2 mg/m3 NOx mg/m3

EC 50 – 100 400 – 2.000 650 – 1.300

Minimum 40 160 - 270 80 - 540

The values relate to new plants. The first valueis for big plants and the second value for smallones.

Central boiler plant Distribution Consumption

Smoke gets in your eyes wherevere you are.Fig. 2:1




Hydrocarbons derived from motor-driven vehicles and industrial proces-ses contribut to the fact that ozone is formed close to the ground and thefact that the ozone layer is demolished.

Greenhouse gases, carbon dioxide, nitrous oxide and methane are allcontributing to the so-called greenhouse effect. Carbon dioxide isformed by different sorts of combustion, in central heating plants, in carengines etc.

Heavy alloys, which influence the germ plasm, are stored all the time, andgradually they end up at the top of the food chain, i.e. in predators andin human beings.

2. FuelOil and coal are the fuels most frequently used. Natural gas is more andmore used as well as biofuel (renewable energy such as forest waste andstraw).

Coal is refined through washing so that the content of pollutants andashes will be less than before. The sulphur content is under 0.8 %. Byspraying with surface chemicals or with water only, the dust amount fromtransport and handling has been reduced. Pulverized coal is a processingoperation that increases the efficiency of handling and combustion. Effi-cient purification of the exhaust gases is required, bearing in mind solidparticles, sulphur and nitrogen gas.

Because of the large volumes in connection with district heating, thetransport must be carried out by ship, unless of coal mine is located nearthe district heating plant.

Oil for large district heating systems, so called heavy oil, contains amaximum of 0.8 % sulphur and can be very efficiently burnt with presenttechniques, but to reduce the discharges to the accepted level, purifica-tion of the exhaust gases is required.

The oil is tranported by ship and lorry or by train.

Gas can be purified from possible pollutants before combustion, butnitrogen remains even after the combustion.

When dealing with large quantities in liquid form, transport is undertakenby special tankers or through gas pipe-lines.

Biofuel is mostly used in minor plants, up to 10.000 apartments,700.000 m2. Biofuel is not considered to have negative effects on theenvironment, as the carbon dioxide, released by the combustion, is usedwhen the corresponding amount of biofuel is building up.

Oil, gas and coal is transported by ships.Fig. 2:2

Lorries are used for shorter transportation of oil and gas.Fig. 2:3

Pipe-lines are often used for transportation of gas.Fig.2:4




The resistant ashes are to be brought back to the specific site from wherethe fuel has been collected. Purification of the gas fumes is required.

When using biofuel, it is essential from an economic as well as environ-mental point of view, that the combustion plant is located close to thearea from where the fuel is collected. The biofuel is transported by lorry.

Waste heat or surplus heat from an industrial process, e.g. cooling waterwith a high temperature can be used in the district heating network.Classic examples of such processes are the manufacturing of glass and therefining of oil.

3. Exhaust emission controlIn earlier years chimneys were built higher when the dust quantities werea nuisance, but experience has shown that this method only shifted theproblem further away from the chimney. Nowadays the exhaust gases are,as a rule, mostly purified as for as sulphur, nitrogen oxide and particulates.

Particles are separated with the help of cyclones, mechanical filters orelectro-filters.

Sulphur is separated by adding lime, with plaster as the end product.There are several methods and they are developing all the time. Theseparation degree is as high as 95%.

Nitrogen oxide is separated by injecting ammonia. A separation level of90% can be reached.

9

Principle for purifying the exhaust gases.Fig. 2:5

Dilution air

Ammonia

Catalyticreactor

Air pre-heater

Boiler Primary air Electro-filter Fabric filterSOx reactor



4. Water qualityThe water quality is of great importance and effects the whole system’srequirements for maintenance and durability.

When installing boilers, complete with equipment, welding and laying ofpre-insulated pipes, and also when installing heat exchangers in the sub-station, a lot of strange impurities end up in the district heating system.They can be anything from welding sparks and iron oxides to sand andgravel. If these impurities remain in the system during operation, theywill damage valves, pumps and other components, and also some blockparts and form layers reducing the heat transfer. To prevent this, all partsof the system must be carefully flushed before filling it with water, andstrainers installed upstreams of sensitive equipment, such as regulatingvalves and flow meters.

Leakage threatens the operation safety, and that is why all welded jointsare X-ray tested.

The temperature, and pressures in the systems are so high that pipes andcomponents are classified as pressure vessels. After the pressure test ofthe plant has been made, it still remains to protect it against corrosion.

Corrosion may occur on the inside or on the outside. External corrosioncan be avoided by securing a dry environment. To prevent internal cor-rosion, a water quality that does not cause corrosion is required.

Oxygen causes corrosion and ordinary water contains oxygen. Water,with a temperature of 10 ºC, may contain 11,25 mg oxygen per kg at apressure of 0,1 Mpa (1 bar).

Once the water has been heated to 100 ºC, it cannot contain any oxygen.Each mg oxygen supplied to a district heating system uses about threetimes as much iron. Consequently, the water is pretreated by, for instance,heating it to about 100 ºC before using it in the system.

Water contains other “pollutants“ which may cause problems in heatingsystems, for example lime, sludge, chloride and sulphate.

When calcareous water is heated in the boiler or in the heat exchanger,calcium carbonate (CaCO3) or limestone is formed on the heat transfer-ring surfaces. A layer of 1 mm thickness increases the heat consumptionby 10%, a layer of 2 mm thickness increases the heat consumption by18% and a layer of 10 mm increases the heat consumption by 50%.The problem with limestone is solved by using a wet filter, whichexchanges the lime and the magnesium salts in the water for sodiumsalt.


Standard values applied in Europe for thewater quality in district heating and largeheating systems, are stated below:

Circulating water Water for re-filling

Conductivity <10 µS/cm <10 µS/cm

PH-value 9,5 – 10 9,5 – 10

Hardness 0,1 tHº <0,5

Appearance clear and clear and mud free mud free

O2 0,0 mg/l 0,02 mgl




There may be sludge or mud in the water used for re-filling, but mud canalso be formed in a chemical reaction between the water and the compo-nents being part of the system. The result could be calcium carbonate,iron and copper oxides, copper sulphides (providing the water pipes aremade of copper) and calcium phosphate. The sludge sinks and ends upin places where the water speed is low, for example at the bottom of radi-ators. Pitting (corrosion), which may rapidly lead to leakage, especially inradiators of sheet metal, is easily formed under these situations.

A mechanical filter is used to remove mud from the water.

Large contents of chloride and sulphate in the water result in high con-ductivity, which may lead to corrosion. These salts are removed throughreverse osmosis.

The water that is used for re-filling, after the first filling, is treated in thesame way before re-filling. There is no leakage in modern pre-insulatedpiping systems. The re-filling of water is to compensate for the water thathas been let out as a result of coupling up of new parts of pre-insulatedpipes or sub-stations. Various chemicals are added to the systems in orderto reduce the risk of corrosion, and checks are made regularily in order toensure the quality of the water.

11

Feed waterReturn line

Ion reduction

Dosageof chemicals

ParticlefilterWater treatment.

Fig 2:6

Heat exchanger

Heating

Thermaldeaeration



5. Flow and return temperaturesThe flow temperatures in a district heating system vary a great deal, fromunder 100 ºC up to 160-170 ºC. The flow temperatures have one thingin common, a large temperature drop, and that also applies to pure heatproduction. A large temperature drop leads to a reduced flow whichmeans smaller pipe dimensions and smaller pumps. The operating costsare lower for the smaller pumps, and the losses from the smaller pipes arealso less.

Heating plants are often built with the boilers, including all the requiredequipment, as a system which transfers heat to the distribution networkthrough a heat exchanger and an accumulator. This is also the only solu-tion regarding combined power and heating plants, as the boilers are pro-ducing steam for the steam turbines.

The purpose of the accumulator is to store heat in order to level off thepeaks of the consumption, which also generates more permanent condi-tions and higher efficiency for the combustion plant.

Consequently, there are usually three temperature levels in a districtheating system with connected sub-heating systems. At each heatexchanger the temperature drops a few degrees.

Temperatures below 100 ºC are working at a normal air pressure, whiletemperatures above 100 ºC require overpressure to avoid boiling and for-mation of steam. At temperatures above 100 ºC, the systems are classi-fied as pressure vessels, which put greater demands upon material as wellas the quality of the workmanship.


Different temperatures in different units in district heating.

Fig 2:7




13

As the district heating systems are also responsible for the production ofdomestic hot water, they have to be in operation throughout the year. Acommon way to deal with this is to have the flow temperature at a con-stant level during the summer months, 60-70 ºC, which is enough forproducing hot water. When the local heating system requires a highertemperature, in order to keep desired room temperature, the primaryflow temperature is raised up to the maximum value, according to theoutdoor temperature.

The outgoing temperature on domestic water is to be kept as low as pos-sible, preferably below 65 ºC. Higher temperatures cause scalding or skinburns.

The legionella bacteria, a malicious bacteria that may cause LegionairésDisease, sets a lower limit to the temperature on the domestic hot water,55-60 ºC.

Larger systems of domestic water are equipped with circulation so thathot water is available without any uneccesary delay. In these systems,with the help of an automatic control, there is the facilit to run highertemperatures at regular intervals through the system in order to preventthe germ growth.

Primary return temperatures of 60 ºC or lower, are desirable whether it isa matter of pure heat production or combined power and heat produc-tion. In the first case there is an exhaust gas condenser; economizer,which requires low return temperatures to perform well, and in thesecond case the condensate has to be cooled down to improve the powerproduction. A large temperature drop also reduces the amount of watercirculating in the system, and it also reduces the operation costs for thecirculation pump.

6. Expansion systemsThe purpose of the expansion system is to manage the volume change ofthe system water at varying temperatures and to sustain the static pres-sure level of the system.

Expansion systems can be designed in two ways:

• open or

• closed

Open systems are in direct contact with the environment, while closedsystems are not. Open expansion system.

Fig 2:9

Air vent

Overflow pipe

Expansion tank

Expansion pipe

Expa

nsio

nvo

lum

e

60

70

80

90

100

110

120

-20 -10 0 10 20

Primary flow temperature when producingdomestic hot water.Fig 2:8

Outdoor temperature ºC

Flow temperature ºC




In precious years, most of the systems were open, but gradually there hasbeen a change-over to select closed systems. The closed systems can bemore easily adapted to changes in the district heating network. Largedifferences in the elevation within the networks have made it moredifficult to work with open systems, as they require sufficient head ofwater above the production unit.

7. Open expansion systemNormally an open expansion system consists of a tank of the necessaryvolume with the tank placed higher up than all the other parts of thesystem.

There are also other cases, where the tank is positioned in the boilerhouse and a pump fills up or taps off the system as required. The staticpressure is sustained because a pipe has been installed to the necessarylevel.

Open expansion tanks are mostly situated in cold spaces and have to beprotected against freezing, which is done by insulation or by supplyingheat. A circulation pipe is installed from the boiler up to the expansiontank, and thus the required amount of heat is supplied.

8. Closed expansion systemClosed expansion vessels consist of a tank, in which the required pressureis sustained by air or by nitrogen. Nitrogen is preferable as it eliminatescorrosion. A compressor maintains the pressure at the right level.

In smaller systems a diaphragm may be used, dividing the expansion tankinto two parts. The heating system is connected to one side of thediaphragm, and on the other side nitrogen is supplied with a suitableoverpressure. When the system is filled, the gas will be compressed andwhile heating, it will be even more compressed. When the water volumechanges, due to temperature fluctuating, the gas is adapting its volume.

Saftey valves, which opens and lets out exessive pressure if there is any,are required for closed expansion system. The safety valves are regularlytested in order to guarantee this function.

Expansion tank

By-pass forcirculation

Boiler or heatexchanger

Expansion volume

Requiredwater level

Reversiblepump

Open expansion system.Fig 2:10

Closed expansion system.Fig 2:11

Pressurised gas

Diaphragm

System water

Expansion pipe

Closed expansion system.Fig 2:12

Expansion tank

Pressure gauge

Safety valves

Boiler or heatexchanger

Gas



DistributionThe distribution part consists of circulation pumps and preinsulatedpipes.

1. Preinsulated pipes.A preinsulated pipe consists of water-bearing pipes, insulation and a con-struction preventing the ground water from getting in contact with insu-lation and pipes.

2. Construction, material.The water-bearing pipe is, as a rule, made of steel. For smaller dimen-sions, used when connecting to small units, detached houses and so on,copper pipes or pipes made of heat resistant plastic are also used, forexample in direct connected systems with lower temperatures.

The greatest risk, as far as the preinsulated pipes are concerned, is exter-nal corrotion since there is treated system water in the pipes.

In earlier years the whole heat culvert was built on site. A concrete struc-ture, open upwards, was built in a well drained excavation. The steelpipes, insulated after pressure test, were installed in the structure andthen a concrete cover was placed on top. Manholes were placed at regularintervals. The big problem with this type of heat culvert is making theconcrete structure leakproof.

The heat culverts of today (preinsulated pipes) are manufactured in afactory with water-bearing pipes of steel, insulation of expanded polyu-rethane and waterproof pipes of polyethylene. The insulation is foamedbetween the steelpipe and the polyethylene pipe.

The steel pipes are jointed through welding, and the polyethylene pipesare equipped with divided muffs of plastic-coated plating, fastened withbolts. The muffs are filled with polyurethane foam. Branchings are madein the same way and there is no need for manholes.

15



Heat culvert produced on site.Fig 2:13

Steelpipes, insulation of expanded polyuret-hane and waterproof pipes of polyethylene.Fig 2:14


3. Heat losses.The heat losses from a heat culvert can be considerable if the pipes arenot well insulated. The pre-insulated pipes with polyurethane foam asinsulation show small losses particularly where there are several insula-tion thicknesses.

A pre-insulated pipe with a nominal diameter of 100 mm (DN), with aninsulation of 35 mm and a water temperature of 100 ºC emits 28,4 W/munder given circumstances. The same pipe with a thicker insulation of 45mm, emits 23,8 W/m under the same circumstances. The correspondingvalues for a pipe with the DN of 400 mm and an insulation thickness of45 and 65 mm respectively is 62,3 and 49 W/m respectively. The samepipe without insulation emits 168 and 203 W/m respectively.

The heat losses are as much as 30% in old heat culvert systems. In pre-insulated pipes the losses are reduced to less than 3%.

4. Linear expansion due to variations in temperature.The pre-insulated pipes are installed at a temperature way below thenormal operation temperature. The pre-insulated pipes are thereforeinclined to expand when they are in operation, 0,12 mm/m pipe and10 ºC temperature rise from the installation temperature. The pre-insu-lated pipes are working as one unit, i.e. the forces caused by the expan-sion of the steel pipes are transferred through the insulation to the exter-nal plastic pipe. The plastic pipe, in turn, is held in position by the fric-tion against the sand with which it is covered. A linear expansion doesnot occur, but the wall of the steel pipe picks up the expansion by gettinga bit thicker.

Installation and re-filling can be done in several way with regard to theexpansive forces, but the final result remains the same:

• no measures taken for expansion pick-up, pre-heating to half of the temperature difference, thereafter re-filling

• no measures taken for expansion pick-up, thereafter re-filling

5. Design.To design the pre-insulated pipes means an optimization of the pipecosts and the operation cost for the circulation pump. A low water rategives large pipe dimensions and a low pressure increase across the pump,a high water rate has the opposite effect.

There should be turbulent flow.



100 mm

3545

28,4 W/m

100 °C

400 mm100 °C45

62,3 W/m

65

49 W/m

23,8 W/m

Heat losses from preinsulated pipes.Fig 2:15

Preinsulated pipes with no measuretaken for expansion.Fig 2:16



6. Flow.The adjustment of the heat supply, applied with two-way valves, resultsin a varying flow in the pre-insulated piping, which in turn results in avarying flow resistance. The resistance varies by the square of the flowchange. If the flow is halved, Q = 0,5, the resistance is reduced to aquarter, 0,52 = 0,25.

17


0,1

0,20,3

0,50,71,0

23

57

10m3/h

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 kPa

l/s

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 mvp

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 Bar

∆p

2

1

3

Fig 2:17

Reducing the flow 5 m3/h, 1 , to 2,5 m3 /h will reduce the resistance from 60 kPa to 15 kPa, 2 . 0,52 x 60 = 15 kPa.A reduction to 25%, 3 , gives the new resistance 0,252 x 60 = 3,75 kPa


7. Pumps.Centrifugal pumps are used in the district heating systems. They are runby electric motors and the sealing around the shaft into the pumphousing is a mechanical sealing, which prevents leakage.

8. Pressure control.The heat supplier signs a contract to supply a certain amount of heat. Tobe able to fulfil this contract, a lowest available pressure of 100-150 kPais required at each sub-station.

The available pressure at the sub-station situated farthest away is keptconstant with a pressure control, which controls the rotation speed of thepump via a pump control, a frequency converter.

The available pressure is, in spite of the pump control, different at fullflow, depending on where the sub-station is connected in the system. Thecloser to the production unit the higher available pressure. At minimumflow the differences in available pressure are small between the first andthe last connected station. The control valves must be sized for this lowpressure, and therefore, they are too large at full flow in the system, whichmay cause problems with a poor control, a high return temperature anda pendulum effect throughout the whole system.



100

200

300

150 0

50

100

Pump for district heating.Fig 2:18

Pressure control, with the sensor at the end of the system, guarantees a minimum available pressure in thesystem. There will still be big differences in available pressure at different flow.Fig 2:19

∆psystem

∆ppump

∆pmin

Flow

%

Min ∆p = 150 kPa



Consumption.The consumption part consists of heat exchangers for heat and domesticwater, with relevant control equipment and heat meters.

1. Heat exchangers.There are two kinds of heat exchangers:

• coil units

• plate heat exchangers

Coil units consist of flat or profiled copper pipes, wound to a compactunit and is surrounded by a jacket through which the primary mediumflows. The secondary medium is connected to the copper pipes.

The plate heat exchanger consists of profiled plates, which are placedagainst each other so that a space is formed, in which the water is able toflow. Every second space contains primary water and every second onecontains secondary water.

The heat exchangers are externally insulated.

The pollutants in the primary and secondary water are deposited inlayers in the heat exchangers, due to the rather large temperature diffe-rences on the surfaces. Even a very thin layer reduces the heat transferconsiderably. Pure water and a high water rate neutralizes the deposit.

19


The coil unit in a jacket and coil heat exchanger.Fig 2:20

Plate heat exchanger.Fig 2:21


Plate heat exchanger.Fig 2:22


2. Connection designThere are many different ways for connecting the various systems tobuildings. In principle there are three types:

• direct connection

• one heat exchanger and with a secondary division to the various systems

• a separate heat exchanger for each part of the system

From a safety point of view, direct connection is used only when the flowtemperature to the radiators is well below 100 ºC.

One heat exchanger for all the systems in the building provides greatflexibility and excellent possibilities for low return temperatures. Shuntgroups with circulation pumps are then installed for radiator-, floorheating- and ventilating circuits. The domestic water is heated in a sepa-rate heat exchanger

When using a separate heat exchanger for each system part, the exchang-ers can be connected in parallel or the domestic hot water can be heatedin two stages. At first the domestic water is heated by the return waterfrom the radiator circuit, and if that is not sufficient, a re-heating takesplace by supplying the re-heater with primary system water.



Direct connection.Fig 2:23

Secondary circuit

Prim

ary

circ

uit

Indirect parallel connection.Fig 2:25

Indirect semi-parallel connection.Fig 2:26

One heat exchanger with two separated circuits.Fig 2:24



3. Electronic temperature controls.In heating systems, the secondary flow temperature is controlled accor-ding to the outdoor temperature via an electronic control station com-plete with a sensor a weather compensator. As a rule the control valve isplaced on the primary side. The temperature on outgoing domestic hotwater is controlled in the same way. The weather compensator has aspecial control function for this purpose.

The control stations and other electronic temperature controls are oftenconnected to a computer so that monitoring and adjustments may bemade from a central location.

4. Self-acting controls.Self-acting controls have a sensor filled with a substance which changesits volume as the temperature changes. The volume change is transmit-ted through a capillary tube to an adjusting device placed on a controlvalve. The adjusting device contains a bellows, and when the bellowschanges in volume - expands or contracts - this motion is transferred tothe cone in the valve. Self-acting controls can only keep the set tempe-rature constant, and they are therefore not suitable for the control of thevariable flow temperature to a radiator system. They are, however, wellsuited to keep the flow temperature of the domestic hot water or the ven-tilating air at a constant level.

21


Self-acting control.Fig 2:27

Self-acting regulator controlling domestic hot water temperature.Fig 2:28

Self-acting regulator controlling air temperature in a ventilation unit.Fig 2:29

Shut-off valve Control unit

Pump

Sensor

Primarypump Shunt pipe

∆p-valve

∆p-control



5. Control valvesThe valve capacity is stated as a kvs –value, fully open valve.

The kv –value states the actual flow, Q, in m3/h at a pressure drop across

the valve, ∆pv, at 1 bar (100 kPa).

Two-way valves are always used in district heating system to preventmore water than necessary from circulating. This means that the flowand the available pressure will vary considerably under varying operatingconditions. The variations become more significant the closer the sub-station is to the circulation pump, even if the pump is pressure control-led.

The valve must be sized for the lowest available pressure existing, 100-150 kPa, minus the resistance across the heat exchanger. If there is toogreat a difference between the lowest and the highest available pressure,the valve could start to hunt. The valve is too big when the available pres-sure is higher than the one for which it has been sized.


3

0,1

0,20,3

0,50,71,0

23

57

10

1 2

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 400 600 1000 kPa0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 15 20 30 40 60 100 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 3 4 5 6 10 Bar

47

1 2

150 200

,4

1,01,6

4,0

2,5

,63

6,3

Flow chart for sizing control valves.Fig 2:31

Cut away of a two way valve.Fig 2:30

m3 /h Lowest available ∆p in sub-station Valves kvs - value l/s

∆pv




6. Differential pressure controlA differential pressure controller senses the differential pressure betweentwo points in a piping system and can, via two impulse tubes, keep a con-stant differential pressure by activating a diaphragm and a cone in thevalve housing.

If a differential pressure valve is placed in the flow direction after thecontrol valve, with one impulse tube connected before and one after thecontrol valve, the differential pressure across the control valve will beconstant, independent of the volume of the flow. Variations in the avai-lable pressure, that may occur, will not influence the control valve, evenif they are substantial.

A differential pressure controller can serve several control valves, but onlyone of the valves can then reach optimum conditions.

23

0

0

1

1

2

2

3

3

4

4

∆p control and controlled circuits.Fig 2:32

Impulsetube

Built-in impulsetube

Controlledcircuit

Avai

labl

edi

ffere

ntia

lpr

essu

re

Nec

essa

ry∆p ∆p

con

trolle

dci

rcui

t

A differential pressure control can reduce theavailable pressure to an acceptable level or equalize big variations in available pressure.Fig 2:34

Available differentialpressure.

Differential pressureacross controlled circuit.

Fig 2:33 Controlled circuits



7. Flow limitationWhen a house owner buys heat, he is also contracting for a maximumeffect. The heat supplier too wants to make sure that the client cannotconsume more. This limitation of the flow is important to the supplier,bearing in mind that he has to be able to deliver to all his clients at thesame time.

A constant differential pressure across a fixed resistance causes a limitedflow. This can be obtained in several ways. A constant differential pres-sure is obtained by a differential pressure control valve, and a fixed resis-tance, which could be a throttle orifice, an adjustment valve or a fullyopen control valve. A differential pressure control valve with a built-insetting device is also a solution.

If the resistance is fixed - pressure adjusting orifice or fully open controlvalve – the limitation is done by adjusting the differential pressure. Whenthe resistance as well as the adjustment valve and the differential pressurecan be adjusted, the limitation can be done with the help of both theadjustment valve and the differential pressure control. At a fixed diffe-rential pressure, (a combined differential pressure controller and anadjustment valve), the limitation must be done with the adjustmentvalve.


0

20

40

60 80

100

120

140

Fig 2:38 Q Q

Q

Flow limitation to a sub-station

Fig 2:39

Flow limitation issimple when you havea constant ∆p

Flow limiter.Fig 2:35

ΣQ

Differential pressure control and fully open control valve.Fig 2:37

Flow limiter and differential pressure control.Fig 2:36

∆p control

Flow control

Max flow

Resistance

Cons

tant

∆p



8. Energy metering.The energy supplied to a building is measured by metering the flow andby registering the temperature difference across the heat exchanger.

The flow meters can be mechanical or electronic, working with ultra-sound. Flow and temperature drop readings are accumulated in a comp-uterized unit where the consumption can be read straight away or byusing a small computer. The information can also be transmitted througha cable or a modem to a central unit.

Tests have to be made on how to read the consumption in smaller units,in each apartment of a larger building for instance, but this is difficultbecause heat is transferred between the apartments. (An apartment,located in the centre of the building, with the heat completely turned off,only recieves about 2 ºC lower room temperature than the surroundingapartments.)

In order to keep down the costs for the metering equipment, flow metersare used for the distribution of the total consumption between the diffe-rent apartments, provided that all the apartments have access to water,holding the same temperature.

25


Ultrasonic flowmeter.Fig 2:40





CHAPTER 3 • SECONDARY SYSTEMS USED IN EUROPE


PrefaceSecondary systems are the parts of the heating systems with a lowerpressure and temperature level, installed in buildings. A lower pressureand a lower temperature can be obtained with a shunt connection and adifferential pressure control, (direct connected systems). The most com-monly used system is, however, the connection through a heat exchanger,completely separating the two systems from each other, (indirect con-nected systems).

The secondary systems consist of three parts:

• production, boiler or heat exchanger

• distribution

• consumption

When speaking of district heating, the production unit is in fact only atransformation from one temperature- and pressure level to another, butregarding function, it is a production unit.

Secondary systems usedin Europe.

Production Distribution Consumption




ComfortThe purpose of the heating system is to create environmental conditionsin the building, comfortable for people to live in.

Generally an air temperature of 20-23 ºC is considered acceptable, butthere are also other factors influencing the comfort:

• the temperature of surrounding surfaces

• air movements, convection

• activity level

• clothing

The heat transfers which we can influence, towards and from a person ina room, are from radiation, convection and/or conduction. A minor sharecomes from breathing.

Heat transfer by radiation has the biggest influence. We are receivingheat from surfaces with a higher temperature than our skin, and we areemitting heat to surfaces with a lower temperature. The greater diffe-rence the larger the heat transfer.

Air with a lower temperature that flows over a surface removes heat fromthe surface. The higher velocity of the air-flow the more heat is removed.The greater the temperature difference the larger the heat flow.

Heat conduction requires direct contact, for instance when you are sittingon a cold chair, but it is normally short-lived as the chair is quicklywarmed up by your body heat.

The result of the factors mentioned above and the temperature of theroom air at a given point in a room, can be calculated. It is thus possibleto determine in advance if a heating system will provide an acceptablecomfort in a given room. Surface temperatures close to 20 ºC on all sur-faces in a room and air-flow velocities lower than 20 cm/s provides verygood comfort.

Our activity level is also of great importance for how we are experiencingcomfort. The temperature can be kept several degrees lower in a sportscentre than in a living room.

We adapt to present conditions with our clothing.

0,5

0,4

0,3

0,2

0,1

018 19 20 21 22 23 24 25 26 27 28

0510

15

20

30

40%

Heat radiates to surfaces with lower temperatures.Fig. 3:1

Percent of unsatisfied persons as result of air tempe-rature and air velocity.Fig. 3:2

Different people react differently at the same tem-perature depending on age, activity, clothing etc.Fig. 3:3

18

40

15 17

17

20

Air velocity m/s




Heat requirements.The heat requirements in a building consist of:

• transmission requirements

• ventilation

• domestic water

Transmission requirements.When designing a house, we can influence the transmission require-ments, heat loss through walls, floors, roofs, windows and doors, basedon differences between the outdoor and the indoor temperature.

In northern Europe, with long and cold winters, the standard requires20-30 cm of high-quality insulation in the external walls and sealed tripleglazing in the window units. Transmission requirements of 20 W/m2

floor area are normal.

The calculations made to determine the transmisison requirements arebased on data containing large safety margins. The real requirementstherefore are far below the theoretically calculated ones. This is veryobvious when you look at the flow temperature required and the tempe-rature difference obtained when the heating plant is taken into operation.During the first year, the heat requirements will be about 30% more dueto the drying out of the dampness of the building. Here therefore, partof the surplus will be needed.

Standard insulation thicknesses in northern Europe.Fig. 3:5

There are big differences between calculated and required heat.Fig. 3:4

Outdoor temperature-200C - -300C

Indoor temperature+200C

200 mm mineral wool

400 mm mineral wool

Actual value

Calculated value

Drying period

Required heat




Ventilation.The purpose of ventilation is to remove pollutants (water vapour, odour,dust etc.). The air removed from a building must be replaced by coldoutdoor air, heated to room temperature. Ventilation also requires heatand the colder the outdoor temperature, the more heat is required. Inorder to lower the heat consumption, the buildings are constructed astightly sealed as possible in cold areas, and the ventilation is carried outso that the lowest air change is maintained, 0,5 change per hour. Thewarm air which is exhausted from a building contains much heat. Diffe-rent devices are used to recover this heat, for example heat pumps andheat exchangers. It has turned out that a too few air change and tootightly sealed houses are causing problems with damp, condensation andmould.

Wind influence.The wind has a great influence on the air changes and thus the heat con-sumption, in very tightly sealed houses. In many parts of Europe thewind is blowing more and stronger in the temperature range around 0 ºCthan at other temperatures when heat is required. Even moderate windvelocities of 10 m/s can double or treble the air changes, depending onhow tightly sealed the house is built. As regards the heating system, theflow temperature must be raised considerably in order to keep the roomtemperature at the desired level.

+20 °C

+ +18-18

+2

Wind has a big influence on the air change in housesFig. 3:8

Systems for recovering heatFig. 3:7

Ventilation systemFig. 3:6

VVX

-12 oC +22 oC

+8 oC+5 oC

+22 oC

+8 oC

5 +oC

-12 oC




Incidental heat gain from heat sources other than the heating system.The irregular incidental heat gain from, people, the sun, cooking andelectrical appliances is so great that ,it will cause over temperatures if nomeasures are taken. This is to much so that it is definitely profitable toequip, for example radiators with thermostatic valves in order to adjustthe heat supply to present requirements. Furthermore the comfort level willincrease owing to the more even temperature from the thermostaticvalves.

Domestic hot water.It was evident early on that it was not enough to just supply heat to theradiators. When in addition hot water could be offered to each apart-ment, the leakage was reduced and corrosion damage ceased in theheating system.

The consumption of domestic hot water forms a rather substantial partof the total heat requirements in a building, and that part becomes moresubstantial the better the house is insulated. After the discovery of thelegionella bacteria and legionairés Desease, the control of domestic hotwater temperature has become important. Stationary hot potable watershould hold a temperature of at least 60 ºC.

The pipes for domestic hot water are made of copper or of heat resistantplastic, for example PEX. The domestic water system in large buildingsis equipped with a circulation pipe and a circulation pump so that domes-tic hot water always is available at all taps, without long delay s in deli-very.

There are lot of heat soarces in an apartment.Fig. 3:9

New buildings Old buildingskWh/m2 year kWh/m2 year

Heating and ventilation 40 - 80 100 - 200*Hot water 20 - 30 20 - 30Common electricity 5 - 10 5Electricity in dwelling 20 - 40 20 - 40

Energy consumption in dwellings.* The lower values are for single houses and the higher for

multi-story buildings.




Production.The production unit is the part of the system in which energy is trans-formed into heat (separate houses), or in which heat is transferred to thesystem (buildings connected to district heating)

There exist a lot of heat sources, for example:

• oil

• gas

• coal

• biodynamic fuels, wood, straw etc.

• solar heat

• heat pumps

• district heating

The three first-mentioned are the prevailing sources, while biodynamicfuels and heat pumps are continually increasing. Solar heat is marginal.From now on we are going to deal only with systems connectedto district heating, in which the four first mentioned heat sources areprevailing.

1. ControlThe control is to guarantee that the required heat volume is available inthe building and that the return temperature does not become too high.





2. Control valvesOnly two-way valves are used on the primary side, and this generallyapplies to the secondary side as well. Three-way valves may be used ifthey are connected in a way that the flow towards the exchanger varies.

Control valves are sized according to the current flow and to the avail-able pressure, independent of the pipe dimension in question.

3. Temperature controllerThe flow temperature to the radiators is controlled by a temperature con-troller according to the outdoor temperature. There is also a control pos-sibility in the return temperature of the domestic hot water in most of theweather compensators.

Should the domestic hot water be produced in a secondary connectedwater heater, the control of the temperature coming from the main heatexchanger will be made at the secondary connected water heater, at leastwhen domestic hot water is produced.

Self-acting controls.There are also self-acting controls for the control of the domestic watertemperature.

115 °C 115 85

606585

115 °C 85

6065

Use two-way valves in district heating systems. In secondary systems use three-wayvalves only when there is no pump in the circuit from the heat exchanger.Fig 3:10

Weather compensator controlling flow and returntemperature according to outdoor temperature anddomestic hot water temperature and return tempe-rature.Fig 3:11




4.1 Periodic set back of room temperatureSetting back the room temperature during the night is to a great extentapplied in order to reduce the heat consumption. The actual energy saving depends on several factors, e.g. the lighter the building is con-structed (poor insulation) and the longer the set back period, the larger the savings will be. 4.2 Periodic set back of flow temperatureIn a system with thermostatic valves, a reduced flow temperature meansthat the valve authority gets smaller; the thermostatic valves open com-pletely as the TRV's tries to maintain the set temperature. Furthermore the hydraulic balance disappears. To prevent this, the flow to each heater (radiator) is preset so that a fair hydraulic balance is maintained, even during these circumstances.

5. Expansion systemsSecondary systems, directly connected to a district heating network, donot need to be equipped with their own expansion system, if there is onein the network.

Other secondary systems are equipped with expansion systems. The con-ditions are the same as for the primary circuit.

6. Closed systemsClosed systems are for practical reasons the most commonly used.The pump is mounted in the flow pipe, and the static pressure has tocorrespond to the height of the pipe system.

7. Open systemsOpen systems are less and less used even in smaller systems. The reasonfor this is problems with corrosion at the connection to the expansiontank, and to some extent the risk of freezing.

τb25

τb50

τb100

1,5

1

,5

0

0 1 2 3 4 5 6 7 8 9 10 11

Closed expansion systemFig. 3:13

Night set back does not pay.Time constant, τb,100 = good apartment building.Fig. 3:12

∆t room

Hours




8. High-rise buildingsThe heating system is divided vertically in high-rise buildings in order toprevent the static pressure from becoming higher than the maximumworking pressure of any of the components, usually of the radiators. Notethat this is working pressure, not test pressure.

In order to avoid the exposure of heat exchangers, expansion tanks,pumps, control valves etc. to high static pressures, a sub-station is placedon ground level for, let us say, the first 15 floors.

The sub-station for the floors 16-30 is placed on the 16th floor.

In high-rise buildings the heating system willbe separated into high and low systemsdepending on the actual work load for theused components.Fig. 3:14




DistributionThe distribution unit consists of circulation pumps, horizontal distribu-tion pipes and risers.

1. DefinitionsHorizontal distribution pipes distribute the water from the sub-stationto other buildings and/or risers. Distribution pipes can be pre-insulatedpipes or steel pipes lying in a passage in the cellar of the building.

The risers are vertical distribution pipes, distributing the water to theradiators on each floor.

A radiator circuit consists of pipes distributing the water from the riserto each radiator. The radiator circuit can be made for one or two-pipesystems.Horizontal distribution pipe from ceiling in passage

and branchings with valves.Fig. 3:15


Shut-off and differential pressurevalves.

Drain valve

Fig. 3:15

Centrallyplacedriser

Visiblyplacedriser

Radiatorcircuit

Horizontal dis-tribution pipes

Cut away of a building Cut away of a building with duct




2. Pipe materialStandard steel pipes are used for larger pipe dimensions, joined togetherthrough welding.

The connection to bigger valves and devices are made with flanges.

Smaller pipes are of threaded steel pipes with its dimensions adapted tostandardized pipe threads.

Soft pipes delivered in coils of steel, copper or heat resisting plastic witha diffusion barrier, are used for the connection between riser and radia-tors. The joint is made with compression fittings of various types.

3. PipingThe distribution pipes can be laid as pre-insulated pipes, in the groundor under a building, or be hung from the ceiling in the cellar of thebuilding.

The risers are placed centrally, in shafts in the building, or at an outerwall, exposed or in shafts.

Soft pipes are laid insulated on the load-bearing system of joists and arecovered with a layer of concrete.

Insulated pipes which will be embedded in concrete.Fig. 3:18

Soft pipes of steel, copper or plastic are, of smallerdimensions, used in heating systems.Fig. 3:17




4. Compensation for the linear expansion due to temperaturevariationsThe linear expansion for steel pipes is about 0,12 mm/m pipe at 10 ºCtemperature change. Measures should be taken when it is a question oflong exposed piping of steel or copper pipes.

The linear expansion is absorbed in special compensators, for examplebellows which can expand or contract. An easier way would be to makean expansion loop on the pipeline or to move the pipeline sideways toobtain an expansion loop. It is important that the pipes can movetowards the expansion unit, and that the branches are not blocked.

The pipes must be fastened so that they will not touch walls or otherparts of the building, otherwise, any pipe movement may causedisturbing noises.

5. InsulationAll the pipes including those within a building are insulated to make theheat losses to the consumers radiators as small as possible. The radiatorsare to emit heat and the emitted heat volume is controlled by the ther-mostatic valves.

6. FlowThe flow in the distribution unit is going to vary in systems with ther-mostatic valves, in spite of the fact that the flow temperature is adjustedto the outdoor temperature. The reason for this is that the conditionsvary from room to room in the building and the flow temperature mustbe adjusted to be able to hold the room temperature in the room whichdoesn´t receive any incidental heat. Less heat is required in rooms withincidental heat gain from various sources. In those cases the radiatorthermostats reduce the heat transfer, i.e. the flow.

Incidental heat gain comes from people, the sun, cooking and electricalappliances and it is very unevenly spread throughout the building.Furthermore, the thermal mass in the building has to be considered.

Insulated pipe.Fig. 3:20

Expansion of pipes can easily be picked up byintelligent mounting of the pipes.Fig. 3:19

Expansion loop

Fixing point Fixing point

Fixing point




7. PumpsThe pumps on the secondary side are of two kinds:

• pumps with a dry motor

• pumps with a wet motor

In a pump with a dry motor, the motor and pump housing situated somedistance from each other. The shaft connecting the motor and impelleris visible, and there is a sealing joint where the shaft enters the pumphousing. The sealings, mechanical flat sealings, are nowadays very safeand tight, requiring practically no maintenance.

In a pump with a wet motor, the pump housing and motor are builttogether as one unit. The rotor of the motor is located in the systemwater, and a thin wall made of non metallic material separates the statorfrom the system water.

8. Pump controlThe varying flow in the secondary system makes it beneficial to controlthe pump according to its pressure and varying flows.

The control can be made in accordance to several principles of function:

• constant pressure at the pump

• constant pressure at the last valve at the end of the system

• proportional pressure

• pressure control parallel to the pipe resistance

Constant pressure means that the pressure is not increasing when theflow is decreasing.

Proportional pressure means that the pressure decreases at decreasingflow along a straight line which, at the flow 0, is equal to half of thepressure at calculated flow.

Pressure control parallel to the pipe resistance means that the pumppressure follows in accordance with the graph for the pipe resistance atdecreasing flow, but only down to half of the calculated pressure.

The differential pressure controls can be integrated in to the wet pumps,and it is the pressure increase across the pump that is controlled.Frequency converters and separate pressure sensors can be used for allsizes of pumps.

100

0 100%0

50

50

Q

Pumps with dry motors.Fig. 3:21

Pump with wet motorFig. 3:22

The resistance varies by the square and the effectfor the pump by the cubic of the flow changeFig. 3:23

∆pn=∆Q2x∆p0

Pn=∆Q30xP0

% ∆p, P




ConsumptionThe units emitting heat to the rooms are the heat consumers. They maybe called heaters from the aspect of the rooms.

The most commonly used type of heaters are the radiators of pressed andwelded steel. There are also radiators of cast iron, but they are seldomused, and finally there are convectors and convection radiators indifferent models.

Floor heating based on heat resistant plastic pipes has been used to agreat extent during the past twenty years.

1. Radiator and convector systemsThe connection of heaters can be made according to two principles:

• one-pipe systems

• two-pipe systems

The one-pipe systems can for instance comprise one apartment. Theheaters are equipped with special valves in which the distribution of theflow between heater and heating coil takes place. According to therequirement the flow to the heater is controlled with a thermostaticvalve. The flow in the circuit is always constant and the circuit must bethoroughly insulated to prevent heat from being supplied to the roomwhen there is no need. Soft copper pipes are the most commonly usedpipe material, but soft thin-walled steel pipes and pipes of heat resistingplastic with a diffusion barrier are also being used. As a rule the pipes arefixed directly onto the insulation and embedded in concrete. The

60°C 52 °C 47°C

6040

52 47


One-pipe system with temperature drop.Fig. 3:25

Unit with thermostatic valve for connecting radiatorto a one-pipe system.Fig. 3:24




requirements for controlled reduced heat consumption have resulted in areduced use of one-pipe systems, from about 15% five years ago to about12% today.

Two-pipe systems offer greater flexibility and more options regardingpiping layout and efficient control of the room temperature. The heatersare equipped with special valves for the connection to flow and returnwith a thermostatic valve in the flow. From a centrally placed riser, thepipes can be laid parallel with T-branchings to each radiator or as aTichelmann-coil (very seldom used in Europe), in order to provide thesame available pressure for each heater.

The piping can also be made with a separate flow and return pipe to eachheater. The riser can be laid exposed on a wall with visible connectingpipes to a heater on each side of the riser, but this solution can causedisturbing noises between the floors.

6040

6040

Two-pipe system with temperature drop.Fig. 3:26

Tichelmann-coil.Fig. 3:28

Separate flow and return pipe to each radiator.Fig. 3:29

Risers on the outside walls.Fig. 3:30

Unit with thermostatic valve for connecting radiatorto a two-pipe system.Fig.3:27




2. Pressure distributionTwo-pipe systems cause different available pressures in various units ofthe system. The risers and the distribution pipes to which horizontalone-pipe systems are connected are also two-pipe systems and each one-pipe circuit has its own available pressure.

One-pipe systems have a constant flow, and a distribution of pressure andflow can therefore be made with manually adjustable valves, and ahydraulic balance can be obtained.

In two-pipe systems, with a control of the temperature in each room, theflow will vary and thereby also the available pressure, which in turnmeans that a pre-set adjustment will only function at full flow. At adecreasing flow the resistance reduces by the square of the flow changeacross the adjustment, and the exceeding differential pressure must behandled by the thermostatic valve or by the floor heating valve. Imbalanceand disturbing noises may arise. Thermostatic valves should not have ahigher differential pressure than 25 kPa.

Pressure distribution in a two-pipe radiator circuit.Fig. 3:31

Excessive pressure

Required pressure

Pressure distribution in a two-pipe system.Fig. 3:32

Required ∆p for horizontal pipes

Required ∆p for risers

Excessive ∆p for risers

Excessive ∆p for the radiators

∆p for pipes

∆p for risersAvailable ∆p




3. Differential pressure controlsIn order to accommedate varying flows and pressures in a heatingsystem, there are automatic adjusting valves differential pressure controls.Via impulse tubes they sense the pressure in the flow and return of theriser, and possible pressure changes are transferred via a diaphragm toa cone in the valve housing, and thus the differential pressure remainsconstant.

Thermostatic valves connected to a riser with differential pressure controlwill be exposed to virtually insignificant changes of the differential pressure,and above all they will never be exposed to a higher differential pressurethan the one set on the differential pressure control.

Thermostatic valves are generally used in most installations.Fig. 3:34

Differential pressure control.Fig. 3:33

Differential pressure control on every riser.Fig. 3:35




4. Control of the room temperatureIn order to reduce the heat consumption, but nevertheless offer comfort,there are requirements or recommendations in most countries to usethermostatic valves on radiators and convectors as well as the correspondingcontrol equipment for floor heating.

Thermostatic valves have to be mounted on each heater to give a goodresult. They are to have a heat authority larger than 1,0, which meansthat they are to have at least that heat amount available at the valve whichis required to keep the temperature set on the thermostat. The thermo-static valves must also be able to sense the present room temperature.

5. Correct flow temperatureIt is important that the room control has the right pre-conditions inorder to work with a control of the temperature in each room:

• available pressure should be equal to or higher than required

• available heat amount should be equal to or larger than required

The mounting of differential pressure controls at the bottom of each riserand a control, if necessary an adjustment, of the available pressure at theriser located farthest away can manage the first item.

The available heat volume is adjusted by the flow temperature. If the flowtemperature is raised, more heat is emitted from the heater, the roomtemperature increases a little, the thermostat reduces the flow and therewill be a larger temperature difference and a larger amount of heat isavailable. The room control should have heat authority.

A higher flow temperature reduces the p-band for a thermostatic valve.The p-band is the temperature increase by the sensor, required to makethe thermostatic valve pass from a nominal position to a closed valve.Thermostatic valves are tested at 2ºC p-band, but in practice the p-bandis less than 1ºC and the thermostatic valve therefore reacts efficientlyeven to small temperature changes in the room.

At a too low flow temperature, the heater does not emit sufficientamount of heat. In these cases the thermostat opens the valve completelyand the whole system gets unbalanced, unless a rough pre setting of theflow has been made.

75 80 85 90

0

1

2

3

4

22

21

20

19

18

75 80 85 90

0

1

2

3

4

22

21

20

19

18

75 80 85 90

0

1

2

3

4

22

21

20

19

18

The thermostat is working within the recommended area.Fig. 3:36

The flow temperature is important for the functioning of the thermostat Fig. 3:37

Good heat authority gives a small p-band and gooduse of the incidential heat gain.Fig. 3:38

Good heat authority No heat authority

Recommended Closed valve

p-ba

nd 0 C

troom0C

tflow0C

p-ba

nd 0 C

troom0C

tflow0C


Correctflow tem-perature

p-ba

nd 0 C

troom0C

tflow0C





6. Floor heating systemsFloor heating provides a very high comfort level. The whole floor area iswarm and all the surrounding surfaces will obtain radiant heat whichincreases their temperature.

Modern floor heating systems are based upon light pipes of plastic,which can be manufactured and handled in substantial lengths. The mostcommon pipe material is cross linked polyethylene, PEX, with an exter-nal diffusional barrier, which on the whole eliminates any penetration ofoxygen through the pipe wall.

The coils emanate from centrally placed distributors. They can be laidaccording to three different methods:

• single laying, which is the easiest way of laying

• double laying

• helical laying

The coils are cast into concrete, and there must always be an insulationunder the coils in order to reduce the heat emission downwards. Eachroom should have its own coil to make it possible to control the heatsupply to the room.

Floor heating emits, at a room temperature of 20ºC, about 11 W/m2

floor area and per ºC temperature difference between the floor surfaceand the room air. The temperature of the floor surface should not exceed27ºC if you are going to stay on the floor for a long time. The requiredflow temperature is low, often not more than 40ºC, and the temperaturedrop across the coils is calculated to be between 5 and 10ºC.

PEX-pipes for floor heating.Fig. 3:39

Different kinds of laying.Fig. 3:40

HD PEXGlueOxygen barrage

Single Helical layingDouble

Floor heating in different floor constructions.Fig. 3:41

Concrete

Insulation

Concrete

Concrete

Insulation

Pipes Pipes




7. ControlThe control of the room temperature is made with an electric thermostat,opening and closing a control valve via a thermo-hydraulic motor. Theelectric thermostat contains an electric resistance, which is activatedwhen the thermostat opens the control valve. The resistance emits heatin the thermostat, which after a while, believes that the room tempera-ture has increased and closes the control valve. This type of on-offcontrol has proved to be very efficient in the use with floor heating. Thetype of circuit layout chosen is of little or no significance.

8. VentilationIn the colder parts of Europe, mechanical exhaust air systems (a fanexhausting air out of the building), is the most common in dwellings. Inthe southern parts, natural ventilation is applied. Offices and industrialbuildings have other requirements, and in these buildings both supplyand exhaust air are mechanized. The supply air volumes in these systemsare also considerably larger and require a pre-heating of the supply air toobtain an acceptable comfort.The supply air is treated in special units before being distributed to thedifferent rooms through a ducted system. Special inlet terminal devicesare used to diffuse the supply air into the rooms without creating draughtor noise.The supply air devices consist of a filter unit for cleaning the air. There-after the air is heated to a little below the room air temperature and thenit passes the exhaust fan of the unit. Beside these functions, the devicescan be used to cool or humidify the air.The control of the temperature of the supply air is made by a shuntcircuit, containing a control valve and a circulation pump. The controlvalve supplies the required heat and a control station with a sensor in thesupply air duct ensures that the correct temperature is obtained. Thecontrol can also be made by self-acting controls.Air has a low heat capacity. You can change its temperature rapidly, andthat is why the control must be stable. Oscillations in the control systemsare devastating. The distance between battery and shunt circuit should bethe shortest possible. A change of the temperature in the air supply ductmust result in a changed temperature of the radiator as quickly as possible.For the same reasons, differential pressure controls are mounted to keepa constant pressure across the control valves.The flow in the battery circuit should be constant, which can be accom-plished by adjustment of a valve or with a pressure controlled circulationpump.

Room temperature will be controlled for every room.Fig. 3:42

Room thermostat

TransformerActuator

Shunt for ventilation unit with ∆p-control.Fig 3:43

Shunt for ventilation unit with ∆p-control and self-acting control valvesFig. 3:44

Principle for supply air unit.Fig. 3:45

Constantflow

Varia

ble

flow

co

nsta

nt te

mpe

ratu

re

Ventilation unit

Control valve

∆p-control.

Ventilation unit

Constantflow

Varia

ble

flow

co

nsta

nt te

mpe

ratu

re

∆p-control.

Control valve

Sensor

Air damper

Filter

Heater

Heat exchanger

Fan

Man

ifold






CHAPTER 4 • EVALUATION OF SYSTEMS AND PRODUCTS

47

Evaluation of systemsand products.The evaluation is based on experiences from systems used in Europe andthe systems currently used in China.The results are described in thechapters ”Design instructions” as proposals for ready systems. However,a rapid development continues within all fields and new evaluationsought to be made at regular, not too long, intervals

District heatingDistrict heating means that the combustion, the central boiler plant,including all the required transports, are located at one site, serving alarge area. This location should be chosen so that the disturbances of theresidents is kept at a minimum, as regards noise, pollutants and trans-ports. District heating systems can be designed for direct or indirect con-nection. Direct connection is cheaper in the construction process of thesystem, but in the long run, the whole system becomes more sensitive. Aleakage in an installation can even empty the pre-insulated pipes and thecentral boiler plant of water. The static pressure for the central boilerplant also prevails in the radiators of the apartments.Indirect connectionmeans that the installations of the building form a system completelyseparated from the pre-insulated piping network by a heat exchanger. Inthe same way, the pre-insulated piping network is separated from theboiler by a heat exchanger. Each part of the system can therefore work atits own temperature and its own static pressure.

Recommendation: District heating with indirect connection should beused.

Direct connection.Fig. 4:2

Indirect connection.Fig. 4:3

Combined heating and power plant Local central boiler plant Sub-stationFig. 4:1



Central boiler plantEfficient operation of a central boiler plant requires automatic controland supervision. On the whole, the cost of the automatic controls are thesame regardless of the size of the central boiler plant. Automatic controldoes not become profitable until the produced effect exceeds 50 MW.The efficiency of new boilers of this size is about 88 – 90%. Theproduction of electricity by steam turbines becomes profitable if it iscombined with district heating in so-called combined heating and powerplants. The boiler effect in the combined heating and power plant shouldbe at least 200 MW. About 40% of the production is electricity and 60%heat. A combined heating and power plant should be in operation all yearround. During the winter months, the combined heating and powerplant delivers heat to the local district heating networks and uses thereturn water for cooling the condensate from the steam turbine. Coolingtowers are used for condensing of the steam in cases when the districtheating network is not sufficient for cooling. During the summer, theheat can also be used to run cooling cycles. The efficiency of combinedheating and power plant is about 90 – 92%.

Recommendation: Local central boiler plants should be larger than 50MW, and they should eventually be connected to a combined heatingand power plant of at least 200 MW. Local central boiler plants consi-dered to have a long remaining life, should be equipped with flue gascooling in order to improve their efficiency.


48

Local central boiler plants connected to a combined heating and power plant.Fig. 4:4



Fuel.With modern techniques, it does not matter what sort of fuel you areusing since the exhaust gases can always be purified. But a fuel containingless pollutants also emits less pollutants and therefore requires lesspurification of the flue gases. Coal is a domestic fuel and will thereforebe used for the a foreseeable future. The local heating plants should beusing as pure coal as possible even before renovation or rebuilding. Afterrebuilding of boilers with a fluidized bed, coal of the best quality shouldstill be used. Coal of a lower quality can be used in the combined districtheating and power plants, which have been provided with large scalepurification equipment. The quality of the coal should be improved asmuch as possible before delivery. A reduction of the ash content can bemade by washing the coal. This in turn has a great influence on combus-tion, efficiency and discharges.

Recommendations: All the coal for local heating plants should be of ahigh quality with low content of sulphur and ash.


49

Open coal mine.Fig. 4:5



Combustion.Presently, the most efficient method for combustion of coal is the fluidizedbed. Combustion can occur at atmospheric pressure or at overpressure.Coal, ground to pieces 6 mm or smaller, is mixed with water or air andthen sprayed into the fire, where a glowing and whirling mass is formed,emitting heat to the tubes of the boiler. The temperature in the fire iskept at a constant and relatively low level, about 850 – 870ºC bycontrolling the supplied fuel amount and the percolation through thetubes. The low combustion temperature results in a decrease of thedischarges of SOx to about 400 mg/nm3, about 75% purification. Thedischarges of NOx are less than 500 mg/nm3.

Recommendations: Small central boiler plants, up to about 40 MW,should be removed, and the pre-insulated pipes should be connected to alarger local district heating network. The boilers in local heating plantsrequiring a thorough renovation, should instead be replaced by modernboilers with a fluidized bed or gas boilers. New plants are only built withthese modern boilers. Smallest size 50 MW.


50

AtmosphericFluidizedBedCombustion

System for fluidized bed combustion.Fig. 4:6

AirCoaland lime Air

Coaland lime

PressurisedFluidizedBedCombustion

CirculatingFluidizedBedCombustion



Exhaust emission control.The combined heating and power plants which are in operation all yearround, should be provided with equipment for thorough purification ofthe exhaust gases, above all SOx and NOx and particles, but also heavymetals. Equipment for sulphur purification normally removes more than90 % and when it comes to nitrogen oxides, the discharges are lower than200 mg/nm3. The local heating plants must concentrate on better coalqualities in order to reduce the discharges and also on bag filters tocollect the particles. Discharges lower than 5 mg/nm3 are common.When the local heating plants are connected to a combined heating andpower plant, the operation time will be considerably reduced and hope-fully to under 20%. Under these circumstances the total dischargesduring one year may be accepted at the present. The local heating plantsare thereafter equipped with boilers with a fluidized bed and only usedwhen the production capacity of the combined heating and power plantis not sufficient.

Recommendations: All boilers in local heating plants should, as soon aspossible, be equipped with filters to remove the particles from the fluegases, and exhaust gas coolers to increase the efficiency as well as toreduce the discharges of SOx.


51

A simple but effective purification of the exhaustgases can be done by using bag filters.Fig. 4:7

Boiler

Bag filter




130 °C

15 m

0 500

100

200 300

400

kPa

Static pressure for boiler.Fig.4:9

Boiler

Temperatures.The flow temperature of the water, through which heat exchangers trans-fer heat to the local pre-insulated piping network, should be 130ºC, andthe return temperature about 70ºC. These temperatures are chosen sothat existing systems can be operated under these circumstances.

Recommendations: The flow temperature of the boiler circuit should be130ºC and the return temperature 70ºC.

Static pressure.The constant pressure of the boiler circuit is determined by the presentsteam pressure and the highest point of the boiler circuit. The steam pres-sure must be available also at the highest point in the system. At 130ºC,at maximum boiler temperature, the steam pressure is 200 kPa (2 bar)and to that must be added the height of the system converted into kPa.

Recommendations: The static pressure should not be higher than thatwhich is technically justified.

130 0C


Expansion systems.An open expansion system requires that the tank be placed with its loweredge 20 meters over the highest point of the boiler circuit. Such aplacement is difficult to accomplish without having to take expensivemeasures. In any case, there will be difficulties in accomplishing serviceand maintenance. A closed expansion system can be placed at any levelwithin the central boiler plant. The only disadvantage is the requiredsupervision and control of the safety valves, and that there is qualifiedpersonnel in the central boiler plant, capable of handling the safetyvalves.

Recommendations: Closed expansion systems should be used wheretechnically qualified personnel are available for supervision andmaintenance.



15 m

20 m

kPa

130 °C

A closed expansion system is most preferable.Fig. 4:10

Boiler


Distribution-Consumption.

1. Accumulator.The main purpose of the accumulator is to even out differences betweenthe heat delivered from the boilers and the consumption in the buildings.The heat requirement in a building can vary rapidly when, for instance,the sun shines on a whole wall face, or lights are turned on in the wholebuilding at nightfall. When the local systems are connected, the accu-mulator can be used to manage a short period with a larger heat require-ment, without having to start up another boiler. When the combinedheating and power plant is in operation, the accumulator may allow theplant to manage the variations during a twenty-four hour period withoutthe assistance of other boilers. An accumulator is a large tank of waterand it must be made for the working pressure of the system. Byincreasing the volume of the accumulator with the expansion volumerequired by the system plus 20% for the gas, the accumulator alsofunctions as a closed expansion tank.

Recommendations: An accumulator should be part of every localdistrict heating network, which eventually should be connected to acombined heating and power plant or to other local district heatingnetworks. The accumulator is charged via heat exchangers from the localboiler and from the combined heating and power plant. The accumulatoris also used as an expansion system.



Expansion volume in the accumulator.Fig. 4:11

Safety valve

Primaryside

Secondaryside

Expansion volume

Accumulator

Distribution Consumption


2. Temperature.The flow and return temperatures in the local district heating networkshould be at 120ºC and 65ºC respectively. The temperatures are basedupon current values for existing systems. The flow temperature can beadjusted according to the outdoor temperature, down to about 70ºCwhen producing domestic hot water, otherwise down to 30 – 40ºC,which leads to reduced losses from the pre-insulated pipes.

Recommendations: Flow temperature of 120ºC, return temperature of65ºC. The flow temperature should be adjusted according to the outdoortemperature, but all the sub-stations must have access to at least therequired heat amount.

3. Static pressure.The temperature of 120ºC requires a steam pressure of 100 kPa (1 bar)in the highest located part of the system. The static pressure makes thelevel difference, converted into kPa, from the pressure gauge to thehighest point plus the steam pressure 100 kPa. The same problem applieswith the placing of an open expansion tank as for the local boiler. Theaccumulator functions well as a closed expansion tank.

Recommendations: The static pressure should not be higher than thatwhich is technically justified. A closed expansion tank should be used.

4. Pre-insulated pipes.For systems with working temperatures over 100ºC, there are today onlypre-insulated pipes available, consisting of steel pipe, polyurethane foamand a mantle of HD polyethylene.The systems are highly developed andthere are pipes in all required dimensions. Laying and mounting is safeand relatively straightforward. The heat losses in the pre-insulated pipingnetwork should be as small as possible.

Recommendations: Pre-insulated pipes should be used. Check all thewelding with X-rays, they are pressure vessels. All the systems should bepressure tested with a pressure of 1,3 times the maximum working pres-sure. A leakage alarm should be installed.



120-70 °C

Temperatures for the primary system.Fig. 4:12

Safety valve Expansion volume

Accumulator

An elevated sub-station influences the static pressure.Fig. 4:13

Pre-insulated pipe.Fig. 4:14



5. Flow.The type of flow in the pre-insulated piping network, varying orconstant, is determined by the way the joint is made where the heatexchangers are connected. A well functioning district heating systemimplies low return temperatures, which can only be obtained with avarying flow. A two-way valve, increasing or decreasing the flow throughthe heat exchanger according to needs, provides a low return temperatureand varying flow.

Control valves.There are two and three-way seat valves. The seat valves have a coneworking towards a seat. The cone is shaped differently depending on thefield of application. We usually speak of the characteristics of the cone,which describes the ratio between the lift height of the cone and the flowchange which is the result thereof. In order to obtain a satisfactory func-tioning in a radiator system, it is a good thing if a certain change of thelift height of the cone in the primary control valve results in the corres-ponding change of the heat emission from the radiators. For this purposea cone with a logarithmic characteristic is required. Other characteristicsare linear ones, for instance in thermostatic valves, and also exponentialones.

Valve authority.The valve authority or the pressure authority of the valve states the valvesshare of the resistance in the circuit where it is placed, 30% for three-wayvalves and 50% or more for two-way valves. These values are only appli-cable to the sizing circuit. With regard to other valves the available dif-ferential pressure has to be calculated, and the valve should preferably usethe whole pressure available to the valve.


56

0

50

100

0 50 100

s %

1 2 3

Linear 1 , quadratical 2 , and logarithmic 3 ,characteristics for valves. The lift range for the cone,s, shown in %.Fig. 4:16

Two and three-way valves.Fig. 4:15


Two-way valves.A two-way valve has one inlet and one outlet, and the cone and the seatare placed in between, making it possible to control the flow through thevalve.

Connection.The design of the connection determines its function.

The simplest connection design is when a pump is feeding water to thevalve which increases or decreases the flow as required. When the waterhas passed the consumer unit, a heat exchanger for example, it returns tothe pump. The flow in the circuit will vary. Two circuits are obtained if ashunt is placed after the control valve, between flow and return, and afterthat a circulation pump. The circuit before the shunt will give a varyingflow, when the control valve is adjusting the flow as required, and thecircuit after the shunt will have a constant flow with varying temperatures.Whether the control valve is placed in the flow or in the return pipe is ofno significance as far as control is concerned, but if the shunt is placedhigh up in the system, the best situation is to have the valve in the returnpipe, which will reduce the risk of air entering the consumer units. Ashunt for a ventilation device should be placed as close to the radiator aspossible to avoid temperature oscillations. A two-way valve may be usedto provide a constant flow in the supply circuit, but in that case a shuntis required before the control valve, in which the resistance is as large asthe resistance through the control valve in nominal position. (Sincethree-way valves already have an automatic shunt in the control valve,they would be the natural choice).



More or less flow in the primary circuit controls thetemperature in the secondary circuit.Fig. 4:17

Shunt for control of the temperature in secondarycircuit.Fig. 4:18

The above shunts with three or two-way valves with no pump in the main circuitgive the same result. A pre-setting valve in the by-pass is required when using two-way valve. The resistance in the by-pass should be equal to that of the two-wayvalve.Fig. 4:19



Differential pressure control.In systems with varying flows, large variations arise in the availabledifferential pressure, which means that the control valves, sized for thelowest available differential pressure, are forced to work with a manytimes larger pressure. At these high pressures the valves become too largeand this could easily result in oscillations which, except for unnecessarywear, causes higher return temperatures and affects the other valves inthe system. The differential pressure controls keep a constant pressure atvarying flows.

Construction.A differential pressure control consists of:• valve body• control unit

The valve body contains a cone and a seat.The control unit consists of a diaphragm, a setting unit with a springpack and a connection for impulse tubes on each side of the diaphragm,and also the impulse tubes. An impulse tube can be built into the valvebody.

Fucntion.The differential pressure control can be mounted before or after the partof the system over which it is to control, the controlled circuit. Oneimpulse tube is connected before the controlled circuit and on the posi-tive side of the diaphragm. The other one is connected after the control-led circuit and on the negative side of the diaphragm. Differential pres-sure controls with a built-in impulse tube are made to be mounted eitherbefore or after the controlled circuit.


58

0

12

3

4

5

6

7

8

9∆p

0 5 10

3

2

1

The figure above illustrates the pump head atvarious flows. The area 2 represents the necessarypump head for the circuit apart from the valve.Area 3 represents the differential pressure the valvehas to handle.Fig. 4:20

Differential pressure control.Fig. 4:21

Setting handle

Connection for the+ impulse tube

Diaphragm house

Valve

This combination provides the controlvalve with the same available pressurewhen the flow fluctuates.Fig. 4:22

Impulse tube


Flow limitation.In large systems, there may be requirements for limiting the flow to theconnected units, so that none of them can take any flow away from theothers.

Principle.The principle is: The flow is limited by keeping a constant differentialpressure over a resistance.

Solutions.The differential pressure is kept constant with a differential pressurecontrol. The resistance can be a throttle orifice, a fully open control valveor an adjusting valve. There are also complete flow controllers, in whicha differential pressure valve and an adjusting valve are built together asone unit.

Recommendations: The flow in the pre-insulated piping networkshould be varying. Two-way valves should be used for controlling theheat supply to the heat exchangers. Differential pressure controls shouldbe mounted at the control valves and they should also be used for themaximum limitation of the flow, along with the control valve.



Limitation of flow requires constant ∆p and some kind of resistance.Fig. 4:23

Flow control can be arranged with a flow controllerwith built in resistance or with a ∆p controller andthe fully open control valve as resistance.Fig. 4:24

Resistance

Cons

tant

∆p

Resistance

Constant ∆pMax flow

Flow control

∆p control



6. Heat exchangers.Modern district heating systems, with requirements of low returntemperatures, work well together with heat exchangers with a smallwater content.

There are in principle two kinds of heat exchangers:• plate heat exchangers• coil units

Both types provide a comparatively small resistance in spite of a highwater rate. High water rate is good, because it leaves less depositions inthe exchanger.

Recommendations: Plate heat exchangers or coil units should be used.


60

Heat exchangers.Fig. 4:25

Coil heat exchangerPlate heat exchanger



7. Pump.Circulation pumps used in district heating systems give a larger pressureincrease at lower flows. At the same time, the requirement for pressure isless as the resistance decreases by the square of the flow change. The highdifferential pressure causes problems at the control valves in the form ofnoise, poor control and hunting, but it also involves unnecessary electricconsumption for operation of the pumps. While the resistance alters bythe square of the flow change, the electric consumption alters by the cubeof the flow change. Consequently here is money to be saved.


61

100

200

300

∆p

150 0

50

100

Pressure control, with the sensor at the end of the system, guarantees a minimum available pressure in the system.There will still be big differences in available pressure at different flows.Fig. 4:26

∆ppump

∆psystem∆pmin

Flo

w %

Min ∆p = 150kPa



Principles for pressure control.There are several principles for controlling the differential pressure pro-vided by a pump:• constant differential pressure at the last consumer• constant differential pressure at the pump• proportional differential pressure• parallel to the resistance in the pipe system

A constant differential pressure at the last consumer guarantees that allthe sub-stations have the required pressure, which gives a lower differen-tial pressure by decreasing consumption, and at the flow of almost zero,the low differential pressure is predominant throughout the wholesystem. The available differential pressure for valves is determined at aminimum flow. Valves close to the pump will at a maximum flow, have aconsiderably higher differential pressure than that they are sized for.

Principles for the control of electric motors.There is one type of control for the electric motors in question:• a frequency converter

A frequency converter converts the alternating current into directcurrent and then into alternating current for the moment requiredfrequency. Frequency converters are used together with standardinduction motors and are available in sizes from 1,1 – 200 kW shafteffect. The efficiency is high, about 96%, and installation and use aresimple.

Recommendations: Pumps on the primary side should be equippedwith frequency converters for pressure control. The pumps should beplaced in the flow pipe to ensure the water level in sub-stations locatedhigh up. The lowest available pressure at the last consumer should bekept constant. The control valves should be sized for this lowest pressure.The problems with large variations remain, though somewhat smaller,considering the available differential pressure at the control valves.Differential pressure control is always a requirement for reliable, safefunction.


62

Even with a pressure controlled pump the availablepressure will fluctuate so a differential pressurecontrol is required.Fig. 4:28

Frequency converter.Fig. 4:27



8. Metering.Heat meters are used in district heating networks to distribute the costsaccording to consumption, which is an efficient way of lowering the heatconsumption.

Metering can be made centrally for the whole building, and then thecosts are distributed according to apartment area.

Principles.Heat meters for district heating consist of:

• flow meter

• temperature sensor

• counter


63

Heat meter.Fig. 4:30

Heat meters register consumption and heat losses from the pipe network.Fig. 4:29

Heat meter

Heat meter

Heat meter

Temperature sensor

Flow meter

Counter


There are several kinds of flow meters:• impeller indicator

• ultrasonic meters

The impeller indicators are the oldest ones, and they consist of an impel-ler set in motion by the water flow. At small flows, the impeller indica-tors have great margins for error, and they are sensitive to larger flowsthan those which they have been sized for. The impeller, its shaft and itsbearings are exposed to hard wear by impurities in the water. Regularservices are therefore required. Servicing every second year is standard fora district heating network.

Ultrasonic meters have no moving parts, and they work with a soundsignal, transmitted to a receiver from where it is transmitted back. Thedifference in frequency between the two signals is a measure of the flowrate, which multiplied by the pipe area gives the flow.The ultrasonic metersare insensible to impurities and have a large accuracy of metering withinthe whole metering range, which is considerably larger than the one forthe corresponding impeller indicator.

Temperature sensors should be placed in the flow and in the return tometer the temperature drop across the plant.

In the counter, in this case a computer, the temperature drop is multi-plied by the flow, and the result is the consumed heat amount. Theconsumption can be read from the meter, via a modem or a cable, laid inconnection with the pre-insulated pipes.

Recommendations: Metering of the outgoing heat amount from thecentral boiler plant, as well as of the heat deliveries to the variousbuildings, is to be made in order to check the efficiency of the productionand distribution. This metering makes it possible to study the effect ofdifferent measures taken, but it also gives a signal if something is wrong.

For this purpose, ultrasonic meters are presently the only alternative.



00135

Mechanical flow meter, principle.Fig. 4:31

Ultrasound flow meter, principle.Fig. 4:32

Transmitter, receiver

Reflector Reflector

Counter

Housing

Turbine wheel


Heating systems.Heating systems is the comprehensive term for all the installations forthe heating in a building i.e. the production, the distribution and theconsumption unit.

One or two-pipe systems.The great difference between one and two-pipe systems is the flowtemperature to the radiators connected to the circuits respectively, andconsequently the resulting return temperature. In the one-pipe systems,the flow temperature becomes lower for each connected radiator, and inthe two-pipe systems, the flow temperature is the same for all theradiators, irrespective of the heat losses from the pipes between them.The temperature drop across a one-pipe coil, 20 - 25ºC, is the same asthe one across each radiator in a two-pipe system, but at an incidentalheat gain, the thermostatic valves are closing, and the return temperaturethen decreases in a two-pipe system, while it increases in one-pipesystems. The flow is constant in one-pipe systems and varying in two-pipe systems. The differential pressure controlling of the pump in two-pipe systems may cut the operation costs for the pump between 70 to80%. You cannot make that kind of saving in a one-pipe system.



The flow is the same to all radiators in a one-pipe circuit. In two-pipe systems the flow will be determined from required heat andtemperature drop across the radiatorFig. 4:33

Q=100%

Q=P/∆t

∆t 250C

Q=n%

Q=100-n%




1. One-pipe systems.The gradually lower flow temperature in one-pipe systems is compensa-ted by the increase of the radiator surface. The surface increases the lowerthe flow temperature becomes. If the flow temperature decreases belowthe required value, i.e the available heat amount is too small, it becomesimpossible to compensate by an increased flow.

The heat emission from the pipes in a one-pipe circuit cannot be con-trolled, and the emission can be substantial especially from uninsulatedpipes. If one or several thermostatic valves have closed the flow to respec-tive radiators, the flow in the circuit continues with a higher temperatureand the heat emission from the pipes increases. The gravity forces,especially in high-rise buildings, increase the circulation in the circuitconsiderably. The flow in the one-pipe systems is constant and has to beadjusted for each circuit.


66

0

100

200

300

400

0 20 40 60 80 100 120

80/89 65/76

50/60

40

32

25

20

10

15

1

25

25

ca 3

m ca 1,5 m

ca 0,6 m

There are roughly 6 meters of uninsulated pipes in each room

Room temperature: 20 0CFlow temperature: 90 0C

Vertical pipe DN 25Heat losses : 105 W/m x 0,8 = 84 x 3 = 252 W

Horizontal pipe DN 25Heat losses : 105 x 3 = 315 W

Sum: 567 W

Fig. 4:34Heat losses from uninsulated horizontal pipe.For vertical pipe reduce by 20 %One pipe above another reduce by 12 %Three pipes above each other reduce by 20 %Fig. 4:35

Temperature above room temperature 0C

W/m pipe Pipe size



Existing one-pipe systems.Existing one-pipe systems usually have problems with the distribution ofheat between various one-pipe circuits and between the separate rooms.

The distribution between one-pipe circuts.The distribution between the one-pipe circuits can be adjusted withadjusting valves, providing the available differential pressure is constant.In high-rise buildings, the gravity forces will cause a varying availablepressure, depending on the current flow temperature, and then an auto-matic flow control is required on each one-pipe circuit in order to distri-bute the flow properly.

Heat emission from radiators.The heat supply to a radiator is controlled by the flow temperature, thetemperature drop and the flow amount.

The heat emission from a radiator is controlled by the difference in tem-perature between the radiator surface and the air temperature of theroom.

If we increase the flow through a radiator from zero, at a constant flowtemperature, the heat emission will increase considerably up to a certaintemperature drop across the radiator. A further increase of the flow willabove this level give a very small increase of the heat emission.


67

∆t °C 40 30 25 20 16 12 10 8 6 5

4

0 503010 20 40 60 70 80 900

,5

1

13

100

54 m

1,5 m

3 m

95 °C

70 °C

Heat emission from radiator with 900C flow temperature at 30% to radiator.Small temperature drop means that the flow must be reduced with some 70-80% before there will be a significant change in heat emission from theradiator.Fig. 4:37

Heat emissionΦΦ0

Vertical one-pipe system.Fig. 4:36

∆ t °C 40 30 25 20 16

12

108

654

0

,5

1

0 50 100

13

Heat emission from radiator with 900C flow temperature at 10% to radiator. Small changesin flow will have a larger influence on the heatemission from the radiator.Fig. 4:38

Tflow 90 0C

Heat emissionΦΦ0

tflow 90 0C

Q% Q%



The reason for this is that when the surface temperature of the radiatoris almost the same across the whole surface, the heat emission cannotbecome any larger. It is the difference in temperature between the surfaceof the radiator and the room air which determines the heat emission. Theheat emission will not change if the difference in temperature isn’taltered.

A small flow through a radiator provides a large temperature drop. Alarge flow results in the opposite. It is first at a temperature drop of 15-20ºC that a flow change really affects the heat emission.

The temperature drop has to be relatively large, more than 15ºC if youwant to be able to control the heat emission from the radiators in a one-pipe system.

Flow distribution to the radiatorsA flow distribution to the radiators requires a by-pass pipe throughwhere the remaining flow can pass.

It is the difference in resistance between the radiators that determines theflow distribution. A large flow through a radiator requires a largeresistance in the by-pass. In cases where the radiators and the by-passpipes are serially connected in a one-pipe system, the resistance in all theradiators and by-pass pipes are added to the resistance in the one-pipecircuit. The available differential pressure is the same for both radiatorcircuit as well as by-pass, so the difference in resistance across the circuitsrespectively is determined by the ratio between the two different flows.This means that the resistance in the by-pass pipes should, at a 30% flowthrough the radiator circuit, be 0,3/0,7=0,45 of the resistance in theradiator circuit.

At a 10% flow through the radiator, the resistance in the by-pass pipesshould be 0,11 of the resistance in the radiator circuit.

How does the temperature drop across the radiator affect thetemperature drop across the circuit?The temperature drop across the radiator does not affect the temperaturedrop across the circuit. The emitted heat amount however, affects thetemperature drop across the circuit.

If a thermostatic valve should reduce the flow through the radiator, toreduce the heat emission, the result would be a larger temperature dropacross the radiator. But the water temperature in the circuit after theradiator will have a somewhat higher temperature because less heat hasbeen taken from the water.


68

A by-pass is required when using thermostatictwo-way valves.Fig

30

5070100

200300

5007001000l/h

0,1 ,2 ,3 ,4 ,5 ,7 1,0 2 3 kPa0,01 ,02 ,03 ,07 ,1 ,2 ,3 mWG

,001 ,002 ,004,006 0,01 ,02 ,03 Bar

,01

,02,03

,05,07,1

,2,3l/s

,05

1

2

25 20

15

A comparison of the resistance in the by-pass andthe radiator circuit at 30 and 10% flow respectivelythrough the radiator circuit shows that the 10%flow is preferable.The resistance through a thermostatic valve and aradiator is only slightly larger than the resistancethrough the valve only. A flow chart for thermostaticvalves can therefor be used to illustrate thedifference in increased resistance at 30 and 10%respectively through the radiator circuit. The resis-tance in the radiator circuit is at 30% three timesas large as the square of the change in flow, 32= 9.Example:1 Q = 300, ∆p = 1,7 kPa. Ten equally large radia-

tors in a one-pipe circuit requires 10 x 1,7 = 17 kPain Higher ∆p.2 Q = 100, ∆p = 0,19 Kpa ( 1,7/9 = 0,19 ). Ten

equally large radiators in a one-pipe circuit onlyrequires 10 x 0,19 = 1,9 kPa in higher ∆p.

RTD-G 15, 20 and 25


Two or three-way valves.The heat emission from a radiator in a one-pipe circuit can be controlled byinfluencing the flow through the radiator. The largest heat requirement,at a design outdoor temperature, is adjusted with the flow temperatureand a full flow in the one-pipe circuit and the calculated distribution toeach radiator. The flow temperature is then adapted according to thepresent outdoor temperature. A control valve on the radiator can onlydecrease the heat supply from the level in question.

The two-way valve in a one-pipe system is to have a low resistance witha size equal to the pipe having a resistance which provides a desireddistribution to the thermostatic valve and radiator in question.There arespecially made inserts which are pressed down into the by-pass providinga suitable distribution for the valve sizes respectively. The heat emissionfrom the radiators is then determined by the flow temperature and thereis no reason for changing the distribution.

Two-way valves are cheap, easy to install and do not require any specialsettings to function.

A three-way valve in these systems requires an adjustment of the distri-bution to the radiators, and there must be the same distribution to all theradiators of the circuit in question. When the required adjustments havebeen made, the functioning is the same for the three-way valve as for thetwo-way valve with its by-pass pipe. Three-way valves are more expen-sive, the adjustments are difficult to make in a proper way, and there isalways the possibility of changing these adjustments afterwards.

Recommendations: Install two-way high capacity thermostatic valveson all the radiators, with the same dimension as the one of the circuit.Install a by-pass pipe of the same size and equip the by-pass with a by-pass insert which provides the required resistance corresponding to thecontrol valve in question. Equip all one-pipe circuits with flow limiters.



Thermostatic three and two-way valves.Fig. 4:41


2. Two-pipe systems.For two-pipe systems, nominal size is applicable as well as the same tem-perature drop for all the radiators. The thermostatic valves are chosenaccording to the current flow and the flow temperature determines howlarge the p-band will be. The resistance across a two-pipe valve and aradiator is normally so large, 5 kPa, that the gravity forces are insignifi-cant.

An increased flow temperature in a two-pipe system means that the ther-mostatic valves will decrease the flow through the radiators, and thetemperature drop becomes larger throughout the whole system. At thesame time, the p-band of the valves decreases, which makes the thermo-static valves more efficient. They are, in other words, saving more heat.

The thermostatic valves are maintaining the hydraulic balance in thetwo-pipe systems as long as they have good heat authority. The availableheat amount should be sufficient to keep at least the set temperature. Ifthe flow temperature is decreasing during a twenty-four hour period ormore, so that the heat authority becomes less than 1,0, the room tempe-rature will decrease after a while and the thermostatic valves open com-pletely. An adjustment of the flow to each radiator is under these cir-cumstances required to maintain the hydraulic balance.

Two-pipe systems are superior to one-pipe systems. Some advantages are:

• the same nominal radiator size for all the radiators• a better use of the incidental heat gain• the p-band is set by the flow temperature• the return temperature is set by the flow temperature• a lower return temperature at incidental heat gain• pre-setting of the flow to each radiator• easier to adjust at changed operation conditions• considerably lower operation costs for a pressure controlled circulationpump



75 80 85 90

0

1

2

3

4

22

21

20

19

18

75 80 85 90

0

1

2

3

4

22

21

20

19

18

A higher tflow 86 instead of 820C, gives a reductionof the p-band from 1,5 to 0,40C. That means thethermostat will use more of the incidental heat gain,it will be more effective.Fig. 4:42

Night set back of the tflow to any point under thetemperature for good heat authority, takes thethermostat out of order.Fig. 4:43

troomoC

tflow0C

Closed valve

p-ba

nd o C

troomoC

tflowoC

Closed valve

p-ba

nd o C

No

heat

aut

horit

y

Goo

d he

at a

utho

rity


Vertical or horizontal systems.Vertical radiator systems imply that the riser is laid at an outer wall, andthat one, maximum two radiators per floor are connected to the riser.There are two important disadvantages with this system. For one thing,there are many risers conducting noise between the apartments.Secondly, when using one-pipe systems there are problems in how tolimit the number of radiators per one-pipe circuit. One as well as two-pipe systems can be used. There are also difficulties insulating the risersplaced visually in the rooms.

Horizontal radiator systems imply that several apartments on the samefloor share a riser, how many depending on the planning. The riser can,in this case, be laid centrally in the house and be insulated so that allfloors obtain the same flow temperature. The piping to the radiators isinstalled horizontally on a wall or embedded in the floor and can beinstalled separately for each apartment as well as for multiples.

When using two-pipe systems, there is the possibility of metering theflow to the radiators in each apartment and also keeping the availabledifferential pressure constant on each floor. The disadvantage is thelaying of the pipes to the radiators. Horizontally laid pipes on a wall bythe floor or by the ceiling are neither pretty to look at nor hygienic, andnear the floor cause problems if doors are to be passed. The casting ofpipes into floors requires that the floor construction is made in two steps,one bearing construction, upon which the pipes are laid and one screedlaid after having pressure tested the pipes.

Embedded pipes ought to be insulated and require such conditions thatthey do not need to be exchanged until the building has served its time.One- as well as two-pipe systems can be used.

Centrally placed risers and horizontal laying to the radiators areadvantageous, above all when constructing a new building, but this canalso be made in existing buildings. Some advantages are:

• a smaller number of risers• no noise transfer between the apartments• the possibility of flow metering per apartment• differential pressure control for each floor• small radiator circuits reducing the requirement of adjusting



Noice is transferred adjoining apartments throughraiser.Fig. 4:44

Pipes embedded in floor.Fig. 4:45



Gravity.The forces arising due to the differences in water density at various tem-peratures, gravity forces, become large in high-rise buildings and at hightemperatures. There are also great variations due to the current flow tem-perature, if the flow temperature is controlled according to the outdoortemperature.

In an 18-storey building, the gravity forces are 8,3 kPa at 95ºC flowtemperature and at 25ºC temperature drop. At a heat requirement of50%, the temperature drop is 12,5ºC and the flow temperature 55ºC,which gives gravity forces of 3,1 kPa (approximate values).

The pressure conditions in the systems are affected equally, whether it isa question of one or two-pipe systems, or vertical or horizontal ones.

Regarding the one-pipe systems with thermostatic valves, the flow willincrease in the one-pipe circuits. The thermostatic valves close a little topreserve the set room temperature, but the flow in the circuit increasesand the return temperature becomes higher. The solution to this problemis to install a flow limiter on each one-pipe circuit. Then the flow willremain the same, independent of the variation of the gravity forces inflow temperature and temperature drop. Note that a stationary adjust-ment does not work because of the varying available differential pressure.

Two-pipe systems with thermostatic valves on all the radiators will alsoadapt themselves to the new pressure conditions so that the heat supplyis preserved. The size of the flow will be the same, as well as the returntemperature. You may have a problem with noise disturbance, if the totalavailable differential pressure at the thermostatic valves becomes toohigh, more than 25 kPa. Thermostatic valves for two-pipe systems canmanage a differential pressure of 80 kPa, as far as controlling isconcerned. The same conditions are guaranteed, independent of the sizeof the gravity forces, if differential pressure controls, with a stationaryvalue of 10 kPa, are installed at the bottom of the risers up to the 6thfloor, or for the apartments of each floor.

Recommendations: Centrally placed risers, differential pressure controland horizontal two-pipe radiator circuits provide the best conditions toobtain a well functioning system with good possibilities of metering andreducing the heat consumption, as well as cutting operational costs for apressure controlled circulation pump. This solution can also manage largegravity forces as well as other variations of the differential pressure.


72

54 m

1,5 m

3 m

95 °C

70 °C

One-pipe systemFig. 4:46

Circuits resistance:ca 0,1 kPa/m, incl.single resistance.54 × 2+ 18 × 3 = 162 m162 × 0,1 = 16,2 kPa

Gravity forces:density kg/m3

95oC = 962,270oC = 977,8∆ρ = 15,6 kg/m3

∆p = 54 × 15,6 x 9,81= 8.264 Pa = 8,3 kPa.Total ∆p at 95 oC= 16,2 + 8,3 = 24,5 kPa

Flow at 95oC:The resistance varies by thesquare of the flow change.∆Q2 × ∆p1 = ∆p2;∆Q2 × 16,2 = 24,5;

∆Q = ;

∆Q = 1,23;The flow will increase with23%

24,516,2

Two-pipe systemFig. 4:47

Flow limiter

Circuit resistance:2 × 54=111 m111 × 0,1 = 11,1 kPa

Thermostatic valve inclradiators = 5 kPaTotal ∆p = 16,1 kPa

Gravity forces: 8,3 kPaTotal ∆p at 95oC = 16,1 +8,3 = 24,4 kPa

The increase in flow willbe very small due to thethermostatic valves.

∆p control

∆p control on each riserwill secure the same ∆peven when the gravityforces are large.



3. Thermostatic or manual valve.Radiator valves are intended to be used when controlling the heatemission from radiators. There are in principle two types:

• manually controlled

• thermostatically controlled

Manually controlled valves are adjusted by hand when someone finds ittoo hot or too cold. The flow temperature must be adapted to theoutdoor temperature at the building in question with great accuracy.Incidental heat gains from heat sources other than the heating systemcause over-temperatures and over-consumption.

Manual valves have very steep characteristics, which make it difficult toadjust to intermediate values. They are either closed or fully open.

Thermostatically controlled valves, thermostatic valves.

The thermostatic valve holds the set temperature, i.e. it detects the roomtemperature in question and adjusts the heat supply to the radiatoraccording to the current requirement. With the correct setting of thesystem, (the flow temperature and the constant differential pressure) thethermostatic valve uses incidental gains from other heat sources andovertemperatures are avoided.

A thermostatic valve consists of two parts:

• valve body

• control unit (a thermostat built-in to a construction mounted on thevalve body)


73

100500

100

50

0

100500

100

50

0

Heat characteristics for radiator with manual valve.Fig. 4:48

Heat characteristics for radiator with thermostatic valve.Fig. 4:50

Thermostatic valveFig. 4:49

Heat emission %

Lift range %

Heat emission %

Lift range %



Valve body.There are several kinds of valve bodies, straight and elbow and alsodifferent sizes. The sealing around the spindle affecting the cone, isconstructed as one unit making it, easy to exchange during operation.

Control unit.There are several kinds of control units. The most common ones are:

• control unit with a built-in thermostat sensor

• control unit with a separate sensor, connected with a capillary tube

Principle for a thermostat.The principle for the thermostat is simple. A substance, liquid, wax orgas, is enclosed in a body, and when the substance changes its tempera-ture, it also changes its volume. The body, often a bellows, then expandsor contracts, and this change in form is transferred to the valve cone sothat the flow to the radiator increases or decreases. Experience has shownthat a gas/liquid filled bellows gives the best result and the best safety ofoperation.

Thermostatic valves are proportional controls, regulating the heat supplyin relation to the difference between the temperature set on thethermostat and the temperature detected by the thermostat. If thethermostat detects a much lower temperature than the one set on thethermostat, the valve opens more than if the difference is smaller.

The thermostatic valves should be set at the desired room temperature,and the flow temperature at the valve should be at least so high that theset room temperature can be obtained.

Recommendations: Thermostatic valves with the right valve size, theright control unit, with the possibility to set a maximum temperature andthe correct setting of the system (pressure, flow and flow temperature)provide an improved comfort and a reduced heat consumption. A wellconstructed system can save more than 20%, in one as well as two-pipesystems.


74

Control units with built in sensor, wax cartridge andgas/liqued filled bellowsFig. 4:52

Control unit with separate sensor.Fig. 4:51



4. Weather compensation.In a heating system with manually controlled valves, it is obvious that theflow temperature must be adjusted according to the outdoor tempera-ture, (the requirement) so that the approximate desired room tempera-ture is obtained.

Function.A weather compensator consists of:

• control unit

• control motor, control valve

• sensor for outdoor temperature

• sensor for flow temperature

• sensor for return temperature, optional

• timer, optional

The centrally placed control station adjusts the flow temperatureaccording to the outdoor temperature. A sensor placed outdoors on thenorth side of the building detects the temperature and sends this infor-mation to the control station. A curve can be set in the control station,which governs the desired flow temperature at different outdoor tempe-ratures. The control station compares the desired value with the real valuevia a sensor in the flow pipe. If the two values does not correspond, theposition of the cone is altered in the control valve via a control motor.

The controls can also check that the return temperature does not becometoo high, via a special sensor mounted in the return pipe.

Timers are used to decrease and to increase the flow temperature atcertain times.


75

Weather compensator.Fig. 4:53

It is difficult to reach a good result withoutautomatic control.Fig. 4:54


Why is weather compensation necessary?It is important that the flow temperature does not become too high forone-pipe systems, as the whole flow in a circuit always passes through thepipe circuit emitting heat to the rooms, even if the heat requirement iszero or very small. The lowest required flow temperature must first andforemost always be available, so that the desired room temperature canbe maintained.

The same thing applies to the two-pipe systems , i.e the lowest requiredflow temperature must always be available at all the radiators to maintainthe desired room temperature. A too high flow temperature causes eitherlosses from pipes passing through rooms which are not supposed to bewarm, or over-temperatures in rooms where the thermostatic valves haveclosed the supply to the radiators.

Setting of the right flow temperature.The flow temperature providing the worst located room with desiredroom temperature is the right one. The curve set in the control stationgives the required flow temperature at various outdoor temperatures, butthere are different ways of setting it. The curve can be parallel displacedupwards or downwards, and it can be made steeper or more flat accor-ding to requirements.

The setting of the curve in the control station can be made quitetheoretically, but it is better to set it at some degrees below zero and tobase the flow temperature upon the actual requirement.

Read the flow temperature, the temperature drop and the room tempe-rature at the worst located radiator. Has the desired room temperaturebeen obtained and is the temperature drop sufficiently large?



100

90

80

70

60

50

40

30

20-15 -10 -5 ±0 5 10 15 20

The flow temperature will be controlled by theweather compensator according to the outdoortemperature.Fig. 4:55

75 80 85 90

0

1

2

3

4

22

21

20

19

18

The flow temperature for one-pipe systems must bevery close to the minimum temperature for goodheat authority. For two-pipe systems the range offlow temperatures that gives good control is wider.Fig. 4:56

tflowoC

toutdooroC

troomoC

tflowoC

Closed valve

p-ba

nd o

C

No

heat

aut

horit

y

Goo

d he

at a

utho

rity

0ne-pipe system

Two-pipe system



Periodic setting-back of the flow temperature.A setting-back of the flow temperature during a shorter or a longerperiod of time is made to reduce the heat consumption. A condition formaking a saving is a decrease of the room temperature and that it doesn’ttake as much heat consumption when resetting the room temperatureafter a set-back period as it would have, had the system been run withoutthe set-back period.

Buildings accumulate much heat, heavy buildings more than light ones.The accumulation means that it takes a long time before the room tem-perature drops when the heat has been completely or partly turned off. Ifa decreased room temperature is obtained, it also means that the tempe-rature of the building body has dropped and that the same heat amountmust also be supplied before the room temperature comes up to normalagain.

A simple calculation shows that there is almost no saving to be made ina short temperature set back period over one night. We could for instanceassume a building with no accumulation, where the room temperaturecan be lowered from 20ºC to 16ºC and raised from 16ºC to 20ºCwithout any time consumption. If this set back is made for one night,eight hours in such a building, the mean temperature over twenty-fourhours will be:

(20×16+16×8)/24=18,7ºC; The temperature decrease during twenty-fourhours is 1,3ºC and each degree with a lower temperature is calculated togive a saving of 5%, 5×1,3=6,5%.

If we make the same calculation with a reasonably heavy building; adecrease of the room temperature with 0,4ºC takes four hours and there-heating takes just as long a time, we will receive the following values:the mean temperature during the eight hours will be about 0,2ºC lower,

20-0,2 =19,8ºC (20×16+19,8×8)/24=19,9ºC; The temperature decreaseduring twenty-four hours will be 0,1ºC and the saving 5×0,1=0,5%.

Recommendations: Weather compensation has a function in theheating systems with thermostatic valves. It is essential that the heatauthority is always kept over 1,0 for the worst located radiator.

A periodical set-back of the flow temperature gives no saving for onlyone night, but longer set-back periods may be profitable, several days forexample. Note that the re-heating period must start in good time, whena decrease of the room temperature has been obtained, and that a higherflow temperature than the outdoor temperature requires is requiredduring the re-heating period.


77

20

19

18

17

16

15242 4 60 8 10 12 14 16 18 20 22

Night set back with no time constant gives a smallreduction of heat comsumption:1,3°C × 5 %=6,5 %.Fig. 4:57

20

19

242 4 60 8 10 12 14 16 18 20 22

19,5

Night set back with a normal time constant gives inpractice no reduction of heat comsumption:0,1°C × 5 %=0,5 %.Fig. 4:57

troomoC

troomoC with night set back

Mean temperature

troomoC

Time

Time

troomoC with night set back

Mean temperature



5. Flow.Thermostatic valves in two-pipe systems give a varying flow, providedthat they have heat authority. It is true that the weather compensatoradjusts the temperature according to the requirements, but the inciden-tal heat gains from people, electricity, cooking and the sun are substan-tial. Besides, there is a certain decrease of the flow temperature betweenthe first and the last connected radiator, despite well insulated pipes. Asthe last radiator is supposed to have access to the required heat amount,this means that the first radiator has access to much more heat, which isthrottled by the thermostatic valve. The thermostatic valve keeps the settemperature, and this fact in addition to all the incidental heat gains givesvariations in the flow, in spite of the set flow temperature.

Differential pressure control.There are large variations in the available differential pressure in systemswith varying flows, which means that thermostatic valves sized accordingto the lowest available differential pressure, are forced to work with manytimes greater pressure. The valves are too large at these high pressures,and oscillations easily arise, a fact which, except for unnecessary wear,gives higher return temperatures and affects the other valves in thesystem. The differential pressure controls keep the pressure constant evenat varying flows.


78

∆t °C 40 30 25 20 16

1,00,90,80,70,60,50,40,30,20,10

1,11,2

0 1,0 2,0 Q

100

90

80

70

60

50

12

1095

8

6

45

Fig. 4:59

Available differential pressure in a high-rise building.Fig. 4:60

tflow for the last radiator in a circuit is 90 oC ∆t is 25oC.

tflow for the first radiator is 95oC which willgive 5% more heat.

The thermostatic valve will reduce the flow by9% to give the same room temperature and∆t will be 28oC.

∆p radiator circuit ∆p radiator

∆p 5 kPa

Available ∆p on 18th floor

∆p riser

∆p radiator ∆p radiator circuit

Heat

em

issio

n TflowoC

1

2

3

3 21

Avai

labl

e∆p

on

1st f

loor



Construction.There are specially designed differential pressure controls for heatingsystems. One type with a constant differential pressure of 10 kPa and onetype with an adjustable differential pressure of between 5 and 25 kPa.

A differential pressure control consists of:

• valve body

• control unit

The valve body contains a cone and a seat.

The control unit consists of a diaphragm, a setting unit with a springpack and a connection for an impulse tube. An impulse tube is built-into the valve body.

Function.The differential pressure control can be mounted in the flow or in thereturn of the riser or the branch, across which it is to control thedifferential pressure, the controlled circuit. Usually the mounting is madein the return pipe. An impulse tube is then connected between the flowpipe and the plus side of the diaphragm. The second impulse tube isbuilt-in to the valve body.


79

Connection for setting handle.Fig. 4:61

Differental pressure controls provide every floor with the same available pressureon every floorFig. 4:62

Reduced by the ∆p controller

∆p radiator

Available ∆p on the 1 floor

∆p radiator circuit

∆p 1

0 kP

a

∆p controls with fixed and adjustabledifferential pressure respectively.

Drain valve

Valve bodyShut offscrew

Connection forimpulse tube

Adjustment handle



Flow limitation.The thermostatic valves in two-pipe systems are responsible for the flowlimitation as long as they have heat authority. It is, should the availableheat amount become too small, sufficient to make a rough pre-setting tomanage the flow distribution, thanks to the constant differential pressurekept by the differential pressure control.

One-pipe systems have, as a rule, a constant flow and the current flowmust be set separately for each circuit which theoretically can be madewith a pre-set adjustment valve. As shown above, the gravity forces arelarge in high-rise buildings and they also vary with the flow temperatureand the temperature drop. A manually adjusted valve will therefore inthese cases not work, but an automatic flow limiter is required here.

Principle.The principle is: The flow is limited by keeping a constant differentialpressure over a resistance.

Flow limiters for heating systems consists of:• valve body• control and setting unit

The valve body contains a cone, a seat and a drain valve.

The control and setting unit consists of a diaphragm, a spring pack anda handle for the setting.


80

0,1

0,2

0,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 Bar

l/s

∆p

1

2

The flow through a fixed adjustment will vary when the differential pressurefluctuates.1 ∆p 16 kPa, Q 600 l/h 2 ∆p 25 kPa, Q 740 l/h

Fig. 4:63

Flow control valveFig. 4:64

Drain valve

Valve bodySettinghandle

Shut off screw

m3/h



Function.The flow limiter is mounted in the return pipe and the built-in dia-phragm keeps the differential pressure constant at 15 kPa across the coneand the seat. The setting of the flow is made by altering the resistanceover the cone and the seat. The valve also has a shut-off function.

Recommendations: The flow will vary in the two-pipe systems withthermostatic valves. Pre-set adjustments are therefore only functioningwhen the thermostatic valves have no heat authority. A rough pre-settingcan be made to manage the distribution at longer set-back periods of theflow temperature.

Differential pressure controls with a pre-set differential pressure of 10kPa should, in buildings of a maximum six floors, be mounted at thebottom of the risers and in the branches on each floor in taller buildings.The available differential pressure for the riser or for the radiator circuitson each floor will then always be the same, independent of the gravityforces.

Theoretically speaking, the flow in one-pipe systems is constant, but inhigh-rise buildings, the gravity forces will give a flow, varying with theflow temperature and the temperature drop. Each one-pipe circuit,vertical or horizontal, must therefore be equipped with an automatic flowlimiter.


81

0,1

0,2

0,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 kPa0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 Bar

12

2

2

2

2

Constant ∆p, 1 over an adjustable restriction, 2 creates a flow limitation.Fig. 4:65

∆p

m3/h l/s



6. Static pressure.At maximum temperatures below 100ºC, there is no requirement for asteam pressure. Only the height of the building/system determines thestatic pressure.

Expansion systems.Closed expansion systems with safety valves require regular supervisionand control.They are therefore not suitable since you cannot have qualified person-nel available in all the buildings all the time.

Open expansion systems do not require as much supervision andno service, providing they are made of the proper materials. All theexpansion systems must always be in open connection with the part ofthe system from where the heat is supplied.

The circulation pump in the flow or in the return pipe?A heating system with an open expanison tank is a communicating vesseland the location of the circulation pump, in the flow or in the return, isof great importance.

The open expansion tank has two functions. It is to:

• pick up the volume change of the system caused by temperature varia-tions• see to it that all parts are filled with water, whether the pump is in ope-ration or not

If the pump is placed in the return pipe, the available pressure, i.e thestatic and dynamic pressure put together (the pressure which can be readfrom the water guage) will increase at the connection of the expansionpipe to the system. The present total pressure for the expansion system,pE, can be calculated. Then, from the water guage after the pump, readpressure, p1, reduced by the resistance in the pipe, appliances if there areany, and the level difference up to the connection of the expansion pipegives pE. pE converted to meter is equal to the difference in level betweenpE and the highest point of the expansion pipe, i.e. at the bottom of theexpansion tank. Experience shows that the static pressure should be equalto the highest point in the system plus 65% of the pump head convertedinto meters. The bottom of the expansion tank should be located at thisheight.If the pump is placed in the flow, the available pressure will be lower atthe beginning of the expansion pipe, i.e. the water level in the expansionpipe sinks when the pump is in operation.


82

Static pressure.Fig. 4:66



Yet, the whole system should be filled with water even when the pump isnot in operation to avoid corrosion, and the bottom of the expansiontank should be placed about 0,5 – 1 m higher up than the highest pointof the system.

Recommendations: The pump should be placed in the flow. An openexpansion tank is placed in a warm area, with its bottom 0,5 – 1 m overthe highest point in the system.


83

Expansiontank

Boiler

Heating circuit

RadiatorMinimum levelfor filled system

The expansion system andthe heating circuit are com-municating vessels. Thewater level in the expan-sion system, or the pressurein closed systems, are equalto the level in the circuitwhen there are no circula-tion.

Communicatingvessels

Necessary static pressure depending on whether the circulation pump is in the return or the flow pipe.

ppump =p1 - p2p2p1

P lev

el

pEpE pE

pE = pp1 - pp1-pE - plevel

p1

ppump =p1 - p2

pE p2 p1 pE

pE

p2

pE = pp2 + pp2-pE + plevel

The pump in the flowpipe will give the oppo-site result to the pump inthe return pipe and thereis no risk of air enteringthe heating circuit.

What you read on the pressure guage is thetotal pressure, i.e. the static and the dynamicpressure. The pump in the return pipe will,when activated, raise the water level in theexpansion pipe and lower it in the heatingcircuit.

The total pressure where the expansion pipeis connected to the system, pE, is equal to thepressure after the pump, p1, minus the resis-tance in the pipes and the difference in levelbetween the pump and the connection of theexpansion pipe. pE converted to meters.

is equal to the difference in level betweenpE and the highest point of the expansionpipe, i.e. at the bottom of the expansiontank.Experience shows approx. 0,65× ppump.


7. Pump.

Pressure control of pumps.Circulation pumps used in heating systems, give a larger pressureincrease at lower flows. At the same time, the requirement for pressure isless as the resistance reduces by the square of the flow change. The highdifferential pressure causes problems at the thermostatic valves in theform of noise, worse control and oscillation, but it involves an unneces-sary electric consumption for operating the pumps. While the resistancealters by the square of the flow change, the electric consumption alters bythe cube of the flow change. Consequently money is to be saved here.

Principles for pressure control.There are several principles for controlling the differential pressureprovided by a pump:

· constant differential pressure at the pump· constant differential pressure at the last consumer· proportional differential pressure· parallel to the resistance in the pipe system

A constant differential pressure at the pump gives a higher availabledifferential pressure at a decreasing flow, and at a flow of almost zero thedifferential pressure will be the same throughout the whole system. Theavailable differential pressure for valves and branches is determined at afull flow. At a decreasing flow the differential pressure will increase moreand more the farther out in the system you go and valves and branchesreceive a higher available differential pressure.

A constant differential pressure at the last consumer gives a lower availabledifferential pressure at a decreasing consumption and at a flow of almostzero the low differential pressure prevails throughout the whole system.The available differential pressure for valves and branches is determinedat a maximum flow. At a maximum flow the valves and the branchesclose to the pump will have a considerably higher differential pressurethan which they are sized for.






1009080706050403020100

0 50 100 %Q

1009080706050403020100

1009080706050403020100

0 50 100 %Q

1009080706050403020100

Flow chart showing pump head and resistancein the heating circuit. The diagram to the rightshows available pressure for each riser at 50and 100% flow.∆pdim shows the lowest pressure available foreach ∆p control and risers.Fig. 4:70 ∆p

nec

=20

kPa

∆pnec =40 kPa

∆p kPa ∆p kPa

∆ppump

∆p horizontal pipe

∆p available for each riser at 50 and 100% flow.

Q 50

Q 100 ∆pdim

∆p kPa ∆p kPa

∆ppump



Q 50

Q 100 ∆pdim

∆pne

c∆p

nec

Fig. 4:72 Constant pressure control.

Fig. 4:71 Without pressure control.




1009080706050403020100

0 50 100 %Q

1009080706050403020100

1009080706050403020100

1009080706050403020100

0 50 100 %Q

Flow chart showing pump head and resistance inthe heating circuit. The diagram to the right showsavailable pressure for each riser at 50 and 100%flow.∆pdim shows the lowest pressure available for each∆p control and risers.Fig. 4:73

∆pne

c=2

0 kP

a

∆pnec =40 kPa

∆p kPa ∆p kPa

∆ppump



Q 50

Q 100

∆p kPa ∆p kPa

∆ppump



Q 50

Q 100

∆pdim

∆pne

c∆p

nec

Fig 4:75 Parallel pressure control.

∆pdim

Fig. 4:74 Proportional pressure control.


A proportional differential pressure means that the differential pressure,available after the pump at maximum flow will be reduced to half at aminimum flow. The available differential pressure for valves and branchesis determined at a minimum flow. At a maximum flow, the valves and thebranches close to the pump will have a considerably higher differentialpressure than that which they are sized for.

A differential pressure controlled parallel to the resistance in the pipesystem means that the pump curve will run parallel to the system curve,but only down to half of the differential pressure at a maximum flow. Theavailable differential pressure for valves and branches is determined at aminimum flow. At a maximum flow the valves and the branches close tothe pump will have a considerably higher differential pressure than thatwhich they are sized for.

Principles for the control of electric motors.There are different types of control for the electric motors in circulationpumps:

• A frequency converter is the most flexible solution.

A frequency converter is used together with standard induction motorsand is available in sizes from 1,1 – 200 kW shaft effect. The efficiency ishigh, about 95%. Installation and use are simple.

Recommendations: Pressure controlling of pumps should be used inlarger systems. A constant differential pressure at the last branch providesthe best possibility for the largest cut in operational cost for the pump.The pressure sensor is placed at the last branch and is set at the lowestrequired differential pressure. The resistance of a riser is equal to theresistances across a thermostatic valve with a radiator, the pipes in theriser and the resistance across a differential pressure valve, 8 kPa forDanfoss ASV-P or PV. The lowest required differential pressure at ashunt is equal to the resistances across the control valve, the differentialpressure valve and also in the pipes between the pressure sensor and theshunt, should there be any. A frequency convertor controls standardinduction motors. Considering available differential pressure at risers andbranches, the problems with large variations remain even thoughsomewhat smaller. Controlling the differential pressure is a requirementfor good and safe functioning.



Pressure controlled pump with a frequencyconverter.Fig. 4:76


8. Metering.Metering of the heat volume per apartment implies a more personalresponsibility for the heat costs but is not as accurate. Someone living inthe centre of the building may turn off the heat completely withoutreceiving much lower temperature than his neighbours, while someonehaving a gable apartment, highest up in the building, will get considerablyhigher heat costs for the same apartment area. A conversion factor can becalculated, based upon the theoretical heat requirement for apartmentswith a gable wall and/or roof surfaces, compared to the correspondingapartments without these surfaces.

Flow metering per apartment.In buildings with insulated, centrally placed risers, and a two-piperadiator circuit per apartment, the heat consumption can be metered foreach apartment with a flow meter, preferably an ultrasonic one, bearingservice and precision in mind. The flow meter is placed in an easilyaccessible position, in the stairwell. It can be equipped with a remotecontrol for metering. An adjustment of the consumption for gableapartments and apartments with a roof should be made.

Heat metering per radiator.Heat meters, installed on each radiator and providing a measure of theconsumption through evaporation, seem to be a simple solution, even forexisting buildings. For vertical one-pipe systems however, much heat issupplied from the pipes and can not be metered by using this method.The tenants also have the possibility meter manipulating of the metersand meter reading is also time-consuming.

Recommendations: A heat meter per apartment is the most efficientand safest way of metering the consumption. This method requires two-pipe systems, a separate connection for each apartment.

It is (not) advisable to meter the heat consumption of one-pipe systemswith an evaporation meter for each radiator, however there is no othermethod.

To be consistent the consumption of domestic hot water should also bemetered per apartment. A flow meter placed in the stairwell has provedto be the best solution.

To be consistent the consumption of domestic hot water should also bemetered per apartment. A flow meter placed in the stairwell is the bestsolution.



Flow meters are used to measure the heat amountfor each apartment.Fig. 4:78

Evaporational heat meter on radiator.Fig 4:79

A gable apartment on the top floor requires moreheat than an equally large apartment in the centreof the building.Fig. 4:77



Chapter 5 • Instructions for designing district heating systems

89

Instructions for designingdistrict heating systems.

The methods and systems chosen at new systems and restoration ofexisting systems should be subject to a long-term planning. At the sametime, we have to consider future development. The solutions listed beloware those which seem to be the most suitable for this purpose during thenext few years.

District heating is considered the long-term solution, but it has to bemade more efficient. Small production plants are hard to manage froman environmental- and an efficiency point of view. Smaller systemsshould be removed, the load should be connected to larger productionplants. When the local district heating networks have reached a suffici-ent number, they should be connected to a combined heating and powerplant. Distances of 14-15 km or more between district heating networkscreates no problem with the modern preinsulated pipes. The combinedheating and power plants produces electricity and heat all year round.The local heating plant are on “Stand-by”. They are started when thecombined heating and power plants cannot meet the total heat require-ment.

Large plants in operation day and night. They are required to achieve themost efficient combustion with the smallest possible discharges. Whenall the local district heating networks share the same operationconditions, it will, in the long run, be possible to connect them to a largecombined heating and power plant.

The primary system/ the district heating can be divided into:• production, central boiler plant• distribution, preinsulated pipes• consumption, sub-station

Production Distribution Consumption.Fig. 5:1




Environment.Most of what we are doing in our ordinary lives affects the environmentin one way or the other. Some things are clear to the naked eye: thesmoke and the soot when we are lighting a fire, for instance. Other thingscan be more difficult to detect: as how much more smoke and soot thatis formed if we do not efficiently utilize the heat we are producing. Or: aunit in a central boiler plant must be exchanged after five years insteadof 20 years, due to inefficient operation.

The consumption of coal has negative effects on the environment in thecentral boiler plant and its closest surroundings, but the area from wherethe coal is collected is also indirectly affected. The transport to and fromthe central boiler plant also has negative effects on the environmentthrough its consumption of energy. The most efficient way of reducingthe negative effects on the environment is to reduce the consumption ofcoal through a more effective use.

1. Durability.There are two reasons for exchanging components in a district heatingsystem:• the component is worn out, for example a bearing in a pump• a new product provides a better efficiency

Components with no moving parts do not wear out, and their technicallife is calculated to 50 years. Boilers of a good quality can last for about30 years with a proper maintainance.

2. Production.In the production plant the temperatures are high and the wear isextreme. An efficient operation process, a reduced consumption of fuel,a large reduction of discharges and an increased durability of thecomponents are measures that have to be considered. Small central boilerplants, up to 30 MW, should be replaced by connection to districtheating networks, with larger boilers combusting more efficiently withfluidised beds.

90

Efficient systems reduce the negativ effects on theenviroment.Fig. 5:2

Worn out components should be exchanged.Fig. 5:3




3. Fuel.The fuel in the district heating Main Power Stations will in generalduring a foreseeable future be coal and gas (only within certain areas).Irrespective of fuel type, impurities in the fuel has to be kept as low aspossible. International standards apply.

A decrease of the ash content in coal causes an immediate increase of theefficiency and reduces the discharges radically. This decrease can beaccomplished by washing the coal, and this should be made withoutdelay, even for existing boilers.

Crushing and washing of coal should not be made at the district heatingPower Station but rather in connection with the mining process.

By choosing coal containing small quantities of sulphur, the discharge ofsulphur decreases in the combustion process. Internationally there is onlycoal with less than 1% sulphur for sale.

The use of better coal in all the boilers results immediately in smallerdischarges and ought to be used as soon as possible. Coal of high qualityshould, in the long run, be used in the local heating plants, while coalwith a lower quality is to be used in the combined heating and powerplants where an efficient purification of the flue gases takes place.

4. Combustion.The combustion has to be efficient, as it reduces the impurities in the fluegases and utilizes the heat contents of the coal.

Combustion of coal, based upon pulverised coal and burned with afluidised bed, has proved to be the best combustion technique at present.The impurity content in the flue gases is already low without thepurification. This combustion technique should be used in new plant andwhen replacing old boilers, both local and in combined heating andpower plants. Combustion that is efficient and durable for a long time,requires automatic operation and sound operating conditions.

5. Flue gas purification.All the discharges coming from the flue gases should be reduced to thelowest possible level.

No distinction is made between small and large plants, regardingdischarge of particles. The best result in 1998 is, up till now, 40 mg/m3.The local heating plants, with effects of more than 40 MW,

91

Boiler with fluidised bed combustion.Fig. 5:4

Air

Boilers with fluidised bed combustion are very effec-tive even from the environmental point of view.

Coal and lime

Particles mg/m3 SOx mg/m3 NOx mg/m3

EC 50-100 400 - 2.000 650 - 1.300Mininmum 40 160 - 270 80 - 540

Allowed discharges according to IEA Coal Rese-arch air pollutant emission standards for coal-fired plants database, 1991.The values regard new plants. The first value is forlarge plants and the second value for small ones.




should be equipped with bag filters. Electric filters may be more efficientwhen it comes to combined heating and power plants.

The discharges of SOx and NOx should be reduced to internationallevels, in the CHP plant 1998 =25 mg/MJ, but it is not possible tointroduce such an efficient reduction of SOx and NOx into the localheating plants for economic reasons as they are only allowed to be usedat peak load. Boilers with a fluidised bed emits small quantities of SOx

and NOx. Local heating plants do not, for that reason, have to beequipped with further purification of the flue gases, as they are only inoperation for a short time, after having been connected to combinedheating and power plants.

The combined heating and power plants should be equipped with puri-fication of SOx and NOx.

6. Handling of ashes.The ash quantity is dependent on the quality of the coal. The washing ofcoal reduces the ash contents and the better washing the less ashes.

The handling of ashes is important regarding the environment andshould be carried in closed vehicles. The large volume of ash also involvesconsideration for it’s long term use.

The transport of ashes should be made in tight vehicles or containers sothat the surrounding environment is not affected.

7. Handling of coal.Coal which is stored or moved openly should be handled in a way thatthe wind cannot carry away dust. Spraying with water or chemicals aretested methods.

Unloading, tipping, crushing or grinding of coal should be made in sucha way that the surroundings are not disturbed by noise, or dust.

92

Transport of coal and ashes can effect the environ-ment in more than one way.Fig. 5:5



8. Water quality.The water in the primary system should be of such a quality that there isno risk of corrosion or coatings. All the water brought to the systemshould be within the following requirements:

Conductivity max 10 µS/cmpH-value 9-10Hardness 0,1 tHºAppearance clear and sediment freeO2 0,02 mg/litre

Leakage is not acceptable. The material and the construction of shaft-and spindle inlets should be made so there can be no leakage.

The water for refilling should be treated in the same way.

The systems should not be emptied of water, even though they are not inoperation.

Flushing of the systems.During the whole installation process of a production plant, all impuri-ties, such as scales, sand, gravel etc., should be removed from the system,and the connections should be flushed before the system is finally filled.

The requirements for flushing and water quality applies to the produc-tion- as well as to the distribution unit.

93


Water treatmentFig. 5:6

Feed water Return line

Ion reduction

Heating

Heat exchanger

Thermal deairiation

Dosage of chemicals

Particlefilter



Local district heating system.

1. Effect ranges.Local district heating systems should lie within the range of 40-60 MW.The effect refers to the actual heat requirement in the buildings.

The combined heating and power plant should deliver about 60% of thetotal connected heat requirement, at optimum distribution betweenelectricity (40%) and heat production (60%). Minimum output, electri-city and heat, is to be 200 MW.

The local heating plants should be connection to a combined heating andpower plant only be used at peak loads and at operational break downand maintainance the combined heating and power plant.

2. Existing boilers.Existing boilers, of 40-60 MW, in good condition that do not need to beexchanged within a reasonable time, should use coal with a low contentof ashes, the combustion should be made with a high efficiency. Flue gascoolers should be installed to raise the effictively and then thecondensate, SOx must be taken care of effectivly. The boilers should bein operation night and day and turned off only for cleaning.

94


Purification of the exhaust gasesFig. 5:7

A simple but effectivepurification of theexhaust gases can bedone with bag filtersand cooling the gaseswill raise the effeciency

Boiler

Flue gascooler

Bag filter



Flue gas purification with bag filter should be installed, and as mainte-nance work is required, other measures should be taken for a change-overto the supply from a combined heating and power plant.

In principle, the same procedure applies on both smaller and largerboilers, but the smaller ones should be removed as soon as possible, andtheir system is to be connected to larger plants with flue gas purification.

3. New boilers.When new boilers are installed, either in new or existing district heatingnetworks, the out put should be of about 40-60 MW, the combustionshould be made with a fluidised bed.

The coal quality has to be good, i.e. low contents of sulphur and ashes,and the combustion should be done continuously as long as there is anyneed of it.

Two or more local heating plants of this size can at an early stage beconnected to preinsulated pipes. It is better to have one heating plantwith a capacity of 100% in operation, than to have three with a capacityof 33% each.

The discharges of SOx and NOx stays at an acceptable level with thiscombustion technique, even without purification. When the localheating plants are later connected to the combined heating and powerplant, the operation times will be reduced to perhaps 15-20% per year,the SOx -and NOx levels are then acceptable. The discharges of particlesmust be limited. This is done by using bag filters.

Heat losses in the production units.There are many surfaces with high temperatures emitting a lot of heat inthe production units.

All warm surfaces should be well insulated in order to increase the effi-ciency of the plant.

A high room temperature which is a result of a bad insulation or none, isshortening the life of the devices required in an advanced plant of thistype, not to mention the electronic equipment. Furthermore, people haveto be able to work efficiently within the plant.

95


Connecting two or more district heating networkswill rise the efficiency and the reliability.Fig.5:8




4. Accumulator.The accumulator has two purposes:• to level off the differences between production and consumption• to be an expansion system for the distribution unit

The accumulator should have volume enough to manage a heat require-ment of 12 hours.

The accumulator is made of steel with the same pressure class as the restof the distribution network. It is anti-corrosively treated on the outsideas well as on the inside and is equipped with outside insulation, forexample extruded polyurethane, and a tight surface layer.

In order to be able to pick up the water volume change, the requiredexpansion volume plus 20 % is added to the volume of the system. Theexpansion volume is filled with nitrogen gas and the pressure is raised tothe required level.

Safety valves with the required capacity, opening at a maximum workingpressure, should be installed. They have to be easily accessible for serviceand testing.

Heat exchangers.Heat exchangers for transfer of heat from the local boiler as well as fromcombined heating and power plant are connected to the accumulator.The exchangers are installed outside the accumulator, a charging pump,transfers the heat into the accumulator. An additional pump or valvesystem is required, to allow the stored heat in the accumulator to be usedin the district heating on demand.

The local boilers are detached from the distribution network with a heatexchanger before installing the accumulator. The boiler circuit can be runwith the optimum conditions for the fuel consumption, temperatures andpressures.

An accumulator must be protected from corrosionboth on the outside as well as on the inside andalso be well insulated.Fig. 5:9

Waterproofcoating

Insulation

Rustproof steel




Safety valve.Fig. 5:10

Expansion volume in the accumulator.Fig. 5:11

Safety valve

Primaryside

Secondaryside

Expansion volume

Accumulator

5. Expansion systems.Closed expansion systems should be used. It is easier to adjust them topossible changes in systems or in operating conditions. It is also easy toincrease the static pressure, should, for example, a cavitation arise.

Closed expansion vessel with inert gas for the keeping pressure.Closed expansion vessels are exposed to the same pressure as the rest ofthe system and must therefore be constructed as pressure vessels.Closed systems must be equipped with safety valves, opening andletting excessive pressure out if boiling should occur. The opening pres-sure is equal to the maximum working pressure of the plant. The safetyvalves require a permanent control.

Expansion systems for the boiler circuit.A pressure vessel with a volume corresponding to the expansion of thesystem plus approximately 10%, as a margin, is installed in a suitablelocation in the production plant.

The expansion circuit of the boiler is connected at the bottom of thevessel. The pressure is maintained by assistance of a compressor or withnitrogen straight from gas bottles. A gas pillow lies above the watersurface at a constant pressure. Nitrogen is used because it preventscorrosion.

Expansion systems for the distribution unit.Before installing the accumulator, the same type of closed vessel is usedas for the boiler circuit.

The accumulator is sized for an extra volume (for the gas), which is 20%larger than the expansion volume required for the distribution unit. Thepressure maintenance is effected in the same way as for the closed expan-sion vessels.




6. Circulation pumps.When considering the required static pressure, the circulation pumpsshould be placed in the flow.

The pumps in the boiler circuit and the charging pumps for the accu-mulator should be sized to be able to manage the resistance in the currentcircuits, including heat exchanger and control valves, if there should beany.

Dynamic pressure.With regard to the local distribution systems, the lowest obtainabledifferential pressure of 150 kPa has to be available in all the sub-stations,and the stated maximum water rates should be strived for in the pre-insulated piping network.

The same conditions also apply to the central distribution network.

Flow.The flow is determined on the basis of heat requirements and tempera-ture drop. The theoretically calculated heat requirements are usuallyhigher than the real ones, and therefore an exact calculation of the flowis not necessary.

When the system has been commerioned, a measurement of the realvalues is important, in order to run the plant in the best way possible.

0

200300

1000

50

100

400500600

Dynamic pressureFig. 5:12

The minimum ∆p, 150 kPa,should always be availablein all sub-stations.

Flow and return pipeMin ∆p = 150 kPa

∆p

Flow

%

∆p pump

∆p system ∆p min

The specific heat amount of water is based on 1kg at 15oC. Calculations for heating systems arenormally made with 1 kg water equal to 1 litreand that is not physically correct because thevolume and the specific heat will change with thetemperature. This deviation is still small comparedto the differences between calculated and realrequirements.



7. Pre-insulated pipes.

Pre-insulated pipes with a high-quality insulation and a safe waterproofprotective cover should be used for all the heating distribution systems.They should be constructed and installed in such a way as they last aslong as possible.

Stated maximum water rates should be strived for which will give asmaller external diameter and therefore a smaller heat emitting surface.The standard insulating thickness should be used.

Material.Pre-insulated pipes consist of an internal steel pipe. On the outside thereis foam insulation and the waterproof external layer consists of a poly-ethylene pipe. The insulation is foam with the steel pipe as an internaland the polyethylene pipe as an external mould. The constructionfunction as one unit from an expansion point of view. There are preinsu-lated pipes in dimensions from the smallest to the largest, dout 27 – 1.220mm.

Linear expansion due to variations in temperature.The mounting of pre-insulated pipes is made at temperatures far belowthe normal operating temperature. The pipes therefore expand operating,0,12 mm/m pipe and a temperature raise of 10°C. The pre-insulatedpipes are functioning as one unit, i.e. the forces arising when the steelpipe is expanding are transferred to the external plastic pipe through theinsulation. The plastic pipe, in turn, is held in position by the frictionagainst the poured sand. A linear expansion does not occur, but the wallof the steel pipe picks up the expansion.

The mounting and the re-filling are done without any special measurestaken for an expansion pick-up. Once the pipes have been welded andthe joint has been tested, the caps for the external mantle are mountedand the cavity filled with foam. After that there is a re-filling of sandaround the joint. Open pipe ends should be covered to avoid sand andother impurities from entering the pipes. The system should be flushedbefore use.

99


Open pipe ends should be covered.Fig. 5:14

Pre-insulated pipe.Fig. 5:13


100



Sizing of pipes.A high water rate in pre-insulated pipes is important from severalaspects. It results in smaller pipes which have the advantage of beingcheaper and causes smaller heat losses. The temperature drop across acertain distance becomes twice as large at the same temperatures if theflow is halved. At the same time, the resistance in the circuit is reducedto a quarter, and the operational cost of the pump will only be an eighth.

8. Heat exchangers.

A sub-station is situated in each building, maybe several in long, high-rise buildings. It is cheaper to distribute heat in a primary distributionsystem than to construct up a secondary one.

Coil units or plate heat exchangers can be used for hot water as well asfor domestic water systems. Both these types contain very small waterquantities, and therefore an increased consumption requires that thewhole primary system reacts quickly.

The water flow rate in heat exchangers ought to be high, so that thedeposits do not remain in them.

The flow resistance across an exchanger is usually 20-50 kPa.



Operating conditions.

1. Temperature levels.The boilers in the combined heating and power plant with an electricalproduction will work with steam, but a temperature of 130ºC ought tobe chosen for the distribution out to the local district heating networks.The same temperature should also be applied to the boilers in the localdistrict heating systems.

Maximum temperature in the local distribution unit is 120ºC.

2. Return temperatures.Low return temperatures should be strived for, partly because the flue gascoolers, if any, require it, partly because a low return temperature meansa large temperature drop, i.e. the flow pumped around in the districtheating network is low.

The return temperature should be around 70ºC.

101


Temperatures in a local district heating system.Fig. 5:1

BoilerAccumulator

70 oC70 oC

130 oC 120 oC



3. Temperature drop in the distribution network.Proper functioning requires the same flow temperature at all the sub-stations. Good insulation and a relatively high water rate through thepipes are required to achieve this. Heat losses of up to 30% may occur ina distribution network with low consumer energy density. In energydense areas with pre-insulated pipes the losses are 3% or less.

The stated maximum water rates should be strived for, see graph inchapter 8.

4. Static pressure.The local distribution unit will be working with a static pressure, whichis the sum of the steam pressure (100 kPa at 120 ºC) and the differencein height between the pressure gauge in the production unit and thehighest located sub-station. The pump should be placed in the flow.

The static pressure of the local boilers depends on their maximumworking temperature. The steam pressure is 200 kPa at 130 ºC and thatpressure is to be added to the height of the boiler converted into kPa.

If the circulation pump is placed in the flow pipe, it is enough with anaddition of 10-20 kPa (as a safety margin) to the static pressure to get allthe parts of the system water filled at operation.

102


The static pressure is determined by the maximumwater temperature and the height of the highestpart of of the system. To avoid boiling, a pressurethat is higher than the steam pressure at thetemperature in question is required at the highestpoint of the system.Water temperature ºC Steam pressure

kPa/bar(gauge pressure)

110 47/0,5120 99/1130 193/2140 262/2,6160 518/5,2

Static pressure in a local district heating system.Fig. 5:16

12 m 36

m

Static pressure, boiler 130 oC.Steam pressure = 200 kPaLevel pressure = 120 kPaTotal = 320 kPa

14 m

Static pressure, district heating network 120 oC.Steam pressure = 100 kPaLevel pressure = 360 kPaTotal = 360 kPa



Under certain operating condition, the pressure in the control valvescould become so low that cavitation occurs. Cavitation means that steambubbles are formed (through boiling), and when these steam bubbles arepressed together, imploding, large forces arise damaging the valve coneand the valve seat. Cavitation ceases if the static pressure is raised.

All the components included must be officially approved for the currentworking pressure.

5. Available differential pressure.The available differential pressure in the primary distribution system willvary with the flow in the system. The pump keep the differential pressurein the last sub-station constant at 150 kPa, at all flows. The differentialpressure will vary for the rest of the connected sub-stations, from themaximum at 100% flow to approximately 150 kPa at a minimum flow.The control valves should be sized for the lowest possible available diffe-rential pressure, 150 kPa, minus the resistance in the heat exchanger inquestion.

103


100

0 100%0

50

50

Q

When water passes through a valve the speed willincrease over the seat and cone and then decrease.The increase in speed will use up some pressure.The result is ∆pvalve min.some of that pressurereturns when the speed decreases and the result is p2.If the ∆pvalve min becomes lower than the steampressure, cavitation occurs and the water will boiland bubbles of steam will form. When the speeddecreases, the pressure will rise and the bubblesimplode. This causes a loud noise and the largeforces could damage surfaces of the valve.

Fig. 5:17

Valve minSteam

∆p

p1p2

∆pvalve

% ∆p, P

∆pn=∆Q2xp0

Pn=∆Q3xP0

Q

The resistance changes by the square of the flow change and the effect for thepump by the cube.Fig. 5:18



6. Water quality.The water quality is very important as regards to the durability of all thecomponents included in the primary system. A plant can provide theproduction as well as the distribution units with water. The plant forwater treatment should be designed so that it can also manage the refil-ling of the secondary systems. Refilling pipes to the various systemsshould be equipped with meters to obtain control over the refilledvolumes. With regard to systems with mixed new and old constructions,a water change of 0,5 times per year is taken into account. The new pre-insulated pipes are only refilled when considering new systems andsub-stations and a possible filling of the secondary systems.

The following values apply of the water after purification.

Conductivity max 10µS/cmpH-value 9-10Hardness 0,1 tHºAppearance clear and sediment freeO2 0,02 mg/l

104


A flow meter register the amount of water feeded.Fig. 5:19

Feed water Flow meter


7. Pressure testing.Before a system or parts of it are commissioned it has to be pressuretested. The system in question is filled with treated water and all the airis purged. After that, the pressure is increased, with a pump, to at least1,3 times the maximum working pressure. The pressure should beconstant for at least 60 minutes, without dropping. Joints, connectionsand components should be visually checked during the pressure testing,to make sure that there is no leakage. The supervisor in charge shouldkeep records of the pressure tests. The records should contain informationon time, place, scope, current pressures at the beginning and the end ofthe test, and also leakages attended to. The records are then to be signedby the supervisor in charge.

8. Operating times.The local central boiler plants must be in operation until they areconnected to an accumulator, another district heating network or acombined heating and power plant. If domestic hot water is to beproduced as well, this applies all year round.

When several local central boiler plants have been connected, just asmany boilers are used as necessary to obtain the required effect. Afterbeing connected to a combined heating and power plant, they onlyrespond to the peak loads.

Combined heating and power plants producing electricity are to be inoperation all year round. During the non-heating seasons, the combinedheating and power plants should use the requirement of domestic hotwater in the buildings for cooling, as far as possible. If there is require-ment for air conditioning the heat can be used to run a cooling process.

105



Pressure testing of pre-insulated pipes.Fig. 5:20



Local control and supervision.All the information required to operate a local district heating systemefficiently can be gathered and processed only by computers. The concernsinformation about everything from the air temperature supplied to thecombustion chamber in the boiler to the temperature in an apartment.The gathering of information is important to improve the operatingprocess. The gathering comes out as statistics, and these statistics will atthe same time serve as a control function for the operation.

There is today, in 1998, computer software for this purpose which is welltried and able to co-operate with weather compensators, control motorsand other equipment. Temperatures and pressures can be adjusted fromthe centrally placed computer if and when there is a need for it.

1. The control of boilers.There are a lot of operations to be automated and supervised will regardto local boilers to making the plant effective and less pollutive.

On the whole, the supply of fuel should correspond to the requirementof heat. The introduction of an accumulator to which the boiler isconnected, has made the task easier. A shortage or an excess of fuelduring a short time is evened out by the accumulator.

The operating temperature as well as the return temperature of the boilermust be controlled the whole time. The filter and the flue gas tempera-ture are important from an environmental and an efficiency point of viewand must be checked regularly.

Flue gas fans and circulation pumps should be controlled according tothe current requirement.

106


A computer network control center can control thesystem and record and analyse large amounts ofinformation.Fig. 5:21



2. Control of the accumulator.The accumulator is a buffer between the boiler and the load. By

containing water with a high temperature in the accumulator duringperiods with a smaller consumption, the boiler can work with a moreeven load, which gives better combustion and a smaller amount ofimpurities. Heat is avilable continuously.

At low outdoor temperatures, the accumulator is completely charged,while it is only partly charged during spring and autumn.

The heat transfer from the heat exchanger to the accumulator is control-led by a variable-speed circulation pump.

The temperature of the water to the accumulator should correspond tothe current charging temperature, and it can vary with the outdoortemperature or the expected outdoor temperature. Weather forecasts,expected temperature and wind force are all parts of the decision recordsat the operation and the control of a district heating network with anaccumulator.

3. Control of the outgoing temperature in the district heatingnetwork.The maximum outgoing temperature is 120ºC and the return tempera-ture is 60ºC. The outgoing temperature should be adjusted according torequirement, i.e. the outdoor temperature, down to the temperaturerequired for the production of domestic hot water, 65-70ºC. The advan-tage of this is that the losses from the pre-insulated piping networkdecreases, and the flow down to this temperature is relatively constant.When the heating system requires lower temperatures, large variations inthe flow are obtained. The same temperature is required for the opera-tion of cooling processes during the summer as for the production ofdomestic hot water.

The outgoing temperature can be lowered further according to the outdoortemperature in systems where domestic hot water is not produced.

The outgoing temperature must never be so low that the required heatvolume is not available at each sub-station or that the return temperaturebecomes too high. The control valves should at all times have good heatauthority.

107

1009080706050403020

-15 -10 -5 ±0 5 10 15 20

120130

110


The flow temperature will be controlledby the weather compensator according tothe outdoor temperature. The dotted linerepresents the lowest temperature fordomestic hot water production.Fig. 5:22

tflow oC

toutdoor oC



4. Flow limitation.In cases where there is a rapidly increasing heat requirement, or when theproduction unit hasn´t got enough energy, the solution would be to limitthe flow to each heat exchanger. Flow limitation means that anexchanger does not receive a higher flow than it is set for. One exchangercannot steal heat from the others.

The most simple flow limitation consists of a control valve and adifferential pressure control. The differential pressure control keeps thedifferential pressure across the control valve constant. The currentdifferential pressure is the one required in order to assure a fully opencontrol valve to provide the maximum required flow.

With such equipment at each heat exchanger there will only be amaximum flow at each exchanger, even if the heat amount is not suffici-ent. When the flow temperature then increases, all the exchangers arereceiving the same heat, until sufficient heat volume is available and thecontrol valves begin to close up.

k 4,0vs

108


0,1

0,20,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 Bar

∆pvalve 55 kPa

Maximum flow 3 m3/h

Fig. 5:23 Fig. 5:24

Constant ∆p, across a fully open valve, creates limitation.

m3/h l/skvs 4,0



5. Differential pressure control.In systems with a varying flow, large variations arise in the available dif-ferential pressure for the control valves. A differential pressure controlshould be used if the difference between the calculated and the highestdifferential pressure is more than 50 % of the calculated one.

If a differential pressure control is installed in the flow direction after thecontrol valve, with one impulse tube connected before and one after thecontrol valve, the differential pressure across the control valve will beconstant. Possible variations in the available differential pressure, evenvery large ones, will not affect the control valve.

If a control valve appears to be too large, a reduction of the differentialpressure can adjust the control valve to the real requirement, with thehelp of the differential pressure control. This also applies in the oppositecase.

109


0,1

0,20,3

0,50,71,0

2

35710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 Bar∆p

316,4

4

1,6

1

2

2,5

Fig. 5:26Fig. 5:25

∆pvalve kPa

kvs ∆pvalve kPa Flow m3/h

1 6,4 10 2,02 4,0 25 2,03 2,5 61 2,0By changing ∆p, across the valve you can make

it correspond exactly to the requirement.

kvs -valuem3/h l/s



6. Pressure control of pumps.The pumps should be pressure or temperature controlled in all circuitswith a varying flow. Temperature control applies to the charging pumpsto the accumulator. Pressure control applies in all the systems where thecontrol valves are adjusting the flow according to requirement, the localpre-insulated piping network with sub-stations for example.

The variations in the differential pressure available to the control valveswill be so large, even with pressure controlled pumps, that a differentialpressure control is required to obtain the best operation. If the connec-tion is made according to the Tichelmann principle, the variations willbe the same.

0

100

0

100

0 150

0100

100

200

200

110


∆ppump

∆pmin

Flow %∆p

Flow pipe

Min ∆p = 150 kPa

Return pipe

With a Tichelmann laying of the distribution pipes the same ∆p is always available in all sub-stations.The differences in ∆p depending on various flows will however be the same as with conventional two-pipelaying.

Fig. 5:27



7. Heat metering.The only way of establishing what has been produced and what has beendelivered is by metering the heat volume. The metering also makes itpossible to estimate the efficiency of different units. The deliveries con-stitute a basis for invoicing.

The modern heat meters based upon ultrasound are very efficient andsafe in operation and they are available in all required sizes.

A heat meter is used to register the outgoing heat from the combinedheating and power plant. The obtained heat volume and the outgoingdelivery to the sub-stations is registered at each local production unit.The production in the local boiler should be registered as well. Finally,the heat volumen obtained in each sub-station should be registered.

The records made with values from these meters can reveal possibledefects in pre-insulated pipe construction or in the control of certainunits.

111


Heat metering is the basis for invoicing.Fig. 5:28

Powerplant Boilerplant



8. Central control and supervision.The question of control and supervision becomes even more importantin combined heating and power plants that also produce electricity.For one thing, there are more items to take into consideration when theproduction of electricity items is effected. Secondly, the local districtheating systems, in any case the operation of the accumulator, should becontrolled from the combined heating and power plant. It is essentialthat centrally you have the knowledge of how much cooling that can beobtained from the local district heating systems.

When a local boiler is to be connected to assist the existing ones, youhave to be able to control the operating process from a central point.

The local computers should be connected to the computer in the com-bined heating and power plant.

112


The operations and maintenance are assisted and made more efficient viacentral data control and supervision.Fig. 5:29

Boilerplant Sub-stations



CHAPTER 6 • INSTRUCTIONS FOR DESIGNING HEATING SYSTEMS.

113

Instructions for designingheating systems.

In order to obtain the required effect, a lower consumption, a smallerquantity of impurities and improved comfort, heat and domestic hotwater installations in new and existing buildings have to be installed in away that fits into the total pattern.

The solutions stated below are those which appear to be the mostsuitable for this purpose for the years to come.



Comfort.The purpose when building houses and to supply them with heating anddomestic hot water is to create better conditions for the residents.

Comfort here is a question of creating conditions so that an apartmentwill be comfortable to live in.

1. Room temperature.Metering the room temperature with a thermometer is not a very goodmeasure of comfort, but it is the simplest method of measuring we have.

Heating systems are usually designed for a room temperature of 18-20 ºC,and that is for most cases sufficient. Elderly or sick persons may need ahigher temperature to experience the same comfort as younger andhealthy ones.

The temperature difference vertically in a room should not be too greateither. It is not nice if the feet are cold, while it is too warm in the arearound the head.

2. Temperatures on the surfaces of the room.The heat transfers from warm to cold surfaces.A person sitting close to a cold window emits heat to the window, andafter a while he/she will experience unpleasedt conditions. All surfaceswith a lower temperature than the skin receives radiant heat from theperson. How much is depends on the difference in temperature.


114

Measured air temperature is not a measure ofcomfort.Fig. 6:1

16

4

6

8

10

12

14

-20 -15 -10 -5

U=3,0

U=2,0

U=1,5U=1,2

±0

Difference in temperature between room, 20°C, and window of different windowconstructions. Window temperatures below +12°C can cause radiant cooling.Window with sealed double glazing give a U = 3.0.Fig. 6:2

20oC

18 oC

16 oC

16oC

∆t K

Outdoor temperature oC



A room with many cold surfaces (a corner room with a roof ) provides alower level of comfort than a room with few cold surfaces (a room withonly one exterior wall), due to the radiant heat.

To increase the comfort, the temperature of the cold surfaces must beraised, which can be done in two ways; either by raising the roomtemperature or improving the insulation.

A better level of insulation with regard to windows means having sealeddouble glazed units.

Roofs should be better insulated, and a heat transmission coefficient of0,3 W/m2K, about 100 mm mineral wool, is a minimum requirement.Gable walls should be insulated in the same way as roofs.

3. DowndraughtDowndraught is a reverse convection. Air coming into contact with asurface that holds a lower temperature, cools down, becomes heavier anddescends.

Downdraught occurs mostly in the window areas, as the window has thelowest temperature in a room, but all the surfaces with a lower temperaturethan the room air causes downdraught. How much will depend on thedifference in temperature.

The cold air descends to the floor where it stays. Radiators below thewindows can remove the downdraught providing they cover the wholewidth of the window.

Heat emission through radiation to a cold surface, cold radiation, is oftenmistaken for down draught. The same measures apply on both downdraught as well as cold radiation. To counter balance this, raise the tem-perature of the cold surfaces!

4. Ventilation.Ventilation removes impurities, such as small particles, odour and mois-ture, from the rooms. Odour and moisture are secreted from the humanbody, but they are also produced by cooking. The introduction of showersin the apartments increases the moisture production greatly, and a briefand effective ventilation is required.

An easy way to ventilate is to simply open a window. If this is donebriefly with a fully open window or even with cross draught, it is bothefficient and cheap. If no extra measures are taken to insulate round thewindows, these leaks are sufficient during the winter months, togetherwith an efficient airing, to remove the odour and moisture.


115

The down draught from the window can beprevented by the heat from the radiator if thewindow bay and the windowledge are designedproperly.Fig. 6:3

A brief cross draught is cheap and efficient.Fig. 6:4



5. Wind influences.The air changes in the building increases when it is windy, and if onewants to preserve the room temperature at the set level, a higher flowtemperature to the radiators is required.

6. Distribution of the heat.In principle, the room temperature should be the same in all the roomsof a building. If roof and gable walls are not insulated, it should be pos-sible to keep a somewhat higher temperature in rooms with roof and/orgable walls.

7. Domestic hot water.Each apartment should have access to domestic hot water in the kitchenand in the bathroom. The bathroom should be equipped with a floordrain and a shower.

When a combined heating and power plant is in operation and is usingthe apartment, heating systems for cooling, the costs for a shower aresmall, but the value of hygiene and comfort is substantial.


116

The room temperature must be somewhat higher in rooms with more cold surfacesto keep the same comfort.Fig. 6:5


Keysmart

Note


8. Hot water circulation.When the water in a riser for domestic hot water stands without tappingfor a long time it will take room temperature. The first person wantinghot water will therefore run away a large amount of water before hotwater reaches the tap. If a small circulation pipe is laid parallel to the riserand connected with the riser at the top, a gravity circulation is obtainedso that hot water always is available.


117

A gravity circulating system makes water of the right temperature availablethroughout the whole system.Fig. 6:06



Conditions.

1. Heat requirement.The heat losses in a building consist of:

• transmission

• ventilation

• domestic water

2. Calculation of the transmission losses.The transmission losses are losses through walls, floors, ceilings/roofs,windows and doors, arising due to the outdoor and indoor temperaturedifferences.

The size of these losses should be calculated at the outdoor design tem-perature for the specific geographical area, for example –10ºC, and aroom temperature of 18-20ºC.

The calculated transmission loss will always be considerably higher thanthe real value.

When starting up the system the real losses are to serve as a basis for theadjustments made.


118

40 30 25 20 16

12

10

8

654

1,00,90,80,70,60,50,40,30,20,10

1,11,2

0 1,0 2,0 Q

100

9095

80

70

60

50

1

2

The calculated heat requirement is never equal to the actual requirement.Fig.6:7

Heat

emiss

ion

tflow oC

∆t oC

tflow treturn ∆t Q Required heat1. Calculated 95 70 25 1 1,02. Measured 75 66 9 2 0,74



3. Ventilation.It is rather easy in buildings with mechanical supply and exhaust air, tocalculate the heat requirement for the ventilating air. The size of the airflow and the specific heat content of the air are known as well as therequired temperature rise. These factors are multiplied and the heatrequirement for ventilation is determined.

A fan exhausting air from an apartment, a kitchen fan for instance, takesin air through leaks in the building, and that air is warmed up by theradiators in the rooms.

It is difficult to determine the size of the air flow at self draught or atairing through leaks. A lowest standard value is 0,5 air change per hour.However, the cold incoming air is to be warmed up by the radiators.

Large forces arise in high-rise buildings due to differences in tempera-ture between the air outside and the air inside the building, so called selfdraught forces. The stair-well in a high-rise building becomes aventilating duct, removing large amount of heat from the building,especially if the outer door on the ground floor is open. Keep the outerdoor closed and put another door a few meters inside the outer door, a socalled airlock !

4. Incidental heat gain.Incidental heat gain from other heat sources than the heating systemhave to be used to reduce the heat consumption. The incidental heat gainwill give over-temperatures if the heat supply from the heating system isnot reduced correspondingly. Thermostatic valves are well suited to usethe incidental heat gains with a preserved room temperature.

The amount of the incidental heat gain will largely depend on theactivity of the residents, and the amount of the incidental heat gain aspart of the heat requirement of a room becomes larger the better theroom is insulated.


119



5. The wind influence on the heat requirements.The air change increases in the buildings in windy conditions. Theharder the wind, the larger the air changes. An increase in volume of coldair supplied to the room has to be warmed up to room temperature.Otherwise the room temperature will decrease.

It is not usually windy at the same time as we have design outdoortemperatures. Thus the radiator size needs no compensation for the windinfluence, unless the experience/statistics from the area is/are showingsomething else. The flow temperature, however, must be raised in windyconditions. As an alternative, a slightly too high flow temperature is usedand the thermostatic valves will keep the room temperature at the rightlevel. Then the heat is there even in windy conditions.

6. Heat requirement per room.The total heat requirement per room is equal to the sum of the trans-mission and the ventilating requirement. The size of the radiators andthe required flow are determined according to this value, at maximumload.

7. Control of the actual heat requirement.The actual heat requirement for a building cannot be obtained until thebuilding is built and the system is in operation. The simplest way is tometer the current flow and the flow and return temperatures. Amultiplication of the temperature difference and the flow gives the heatamount.

8. Domestic hot water.The heat requirement for heating domestic water is rather easy tocalculate, the flow multiplied by the temperature raise, but the size of theaccumulated flow is difficult to determine.

The pipes for domestic hot water have to be made of copper or of heatresistant plastics.


120

Find ∆ppump; p2 - p1;Find the flow from the flow chart of the pump and∆t for the circuit = tflow - treturn

oC.Heat consumption = ∆t x flow;Fig. 6:8

∆ppumpp1 p2 tflow

treturn

Heat requirement for domestic hot water

Cold water: +8 oCHot water: 65 oCFlow: 1 l/s

P = 1 × 3.600 × 57 × 0.86 = 176.472 W;P = 176 kW;



Heating systems.The heating system should be constructed and operated in a way that thestated requirements can be reached with regard to environment, comfort,operating economy and a low return temperature.

Before a system or part of it is taken into operation it have to be pressuretested. The system in question is filled with treated water and all the airis let out. After that, the pressure is increased with a pump, up to at least1,3 times the maximum working pressure. The pressure should be keptconstant for at least 60 minutes, without dropping. Joints, connectionsand equipment should be checked visually during the pressure testing tomake sure that there is no leakage. The supervisor in charge should keeprecords, of the pressure tests. The records should contain information ontime, place, scope, current pressures at the beginning and the end of thetest, and also possible leakages attended to. The records are then to besigned by the supervisor in charge.

1. Heat exchangers.Each building ought to be equipped with its own sub-station. It is appro-priate in long buildings to have several sub-stations. The same applies tohigh-rise buildings, of more than 18 floors. These are however dividedvertically.

Sub-station.In the sub-station, the high temperatures in the primary system are con-verted to the level required by the system in the building. The systems arecompletely separated from each other, a fact which requires an expansionsystem and a circulation pump to make the secondary system work.

Circuit diagram.If there is only one heat exchanger in the sub-station, there are no pro-blems in connecting it, but a parallel connection of the exchangers isrecommended when it is a case of several exchangers. Then each systemwill have its own control equipment and expansion vessel, as well ascirculation pump.


121

Two parallel connected heat exchangers.Fig. 6:9



2. Expansion system.

Expansion system.An open expansion system, with the circulation pump installed in theflow pipe, is a simple and practical solution.

There has to be room around the vessel for inspection and repair work.

The connection of the expansion pipe to the heat exchanger must not beequipped with a shut-off device.

Corrosion protection of expansion vessels.The expansion vessel and the upper part of the expansion pipe should bemade of rust-proof material.

3. Circulation pump.Circulation pumps should be installed in the flow, which will guaranteethat there is water in all the radiators when the pump is in operation. Thepumps should be reliable and equipped with a tight sealing shaft thatrequires no maintenance. It is advisable to place a unit for sludge separa-tion after the pump, a filter for instance. The filter unit is constructedwith shut-off devices so that it can easily be emptied of sludge.

The flow is determined from the calculated heat requirements and thetemperature drop. The pressure increase over the pumps is obtained fromthe pipe calculation. There is no reason for making any increases in thesevalues. The pumps are already oversize with the increases made whencalculating the heat requirements for the building. A too high differentialpressure can cause flow noise in valves and radiators.


122

The design circuit is the pipes from the heat exchanger to the radiator locatedfarthest away. The resistance in this circuit is equal to the pump head.Fig. 6:11

Pump in the flow pipe and no shut-off valvebetween the heat exchanger and the expansion tank.Fig. 6:10

Expansion tank

Expansion pipe



4. Horizontal distribution pipe.

Definitions.The horizontal distribution pipes distributes the water from the sub-station to other buildings and/or risers.

Pipe material.Standard pipes joined together by welding are used for the larger units.The connection of valves and devices is made with flanges.

Smaller pipe installations are of threaded steel pipe and the sizes areadapted to standardized pipe threads.

Piping.The distribution pipes can be laid as pre-insulated pipes, in the ground,under a building or hung from the roof in the basement of the building,depending on how the building is constructed.

Compensation of the linear expansion due to variations in temperature.The linear expansion for steel pipes is 0,12 mm per meter of pipe and atemperature change of 10ºC. The temperature change 10-95ºC gives 85 ºC,i.e. 8,5×0,12 mm =1,02 mm/m. Measures must be taken with regard tolong pipework seetions.

The linear expansion is absorbed up by expansion loops on the pipeworkor by shifting the pipe course sideways to create an expansion loop. It isimportant that the pipes can move towards the device picking up theexpansion and that the branches are of such length, up to a passagethrough a wall or a vault, that they can pick up the expansion withoutfailing.

Insulation.The distribution pipes are insulated in such a way that the heat losses tothe consumers are as small as possible. When the piping is visible, theinsulation is provided with a protective surface layer.

It is important for the functioning of the system that the flow tempera-ture is the same for each connected riser.


123

Horizontal distribution pipes.Fig. 6:12

Length of expansion loops.Fig. 6:13



5. Risers.

Definition.Risers are the vertical pipes emanating from the horizontal distributionpipes up through the building.

Each riser is equipped with shut-off and draining valves and possibly adifferential pressure valve.

Pipe material.Standard pipes joined together through welding are used for the largerinstallations.

The connection of valves and devices are made with flanges.

Smaller pipe dimensions are made of threaded steel pipes and the sizesare adapted to standardized pipe threads.

Piping.The risers are placed in central shafts with branches on each floor. Thebranches are equipped with shut-off valves.

Compensation of the linear expansion due to variations in temperature.The linear expansion for steel pipes is 0,12 mm per meter pipe and atemperature change of 10ºC, i.e. approximately 1 mm/m in heatingsystems. Measures should be taken when the piping is 15 m long ormore.

The linear expansion is absorbed by expansion loops on the piping or byshifting the pipe course sideways to create an expansion loop. The pipesshould be fixed so that they can move towards the device absorbing theexpansion.

The branches on each floor should be of a sufficient length or have aflexible insulation to be able to absorb the expansion. They must not belocked.

Insulation.The risers are insulated in a way that the heat losses to the consumers areas small as possible. When the piping is visible, the insulation is providedwith a protective surface layer.

It is important for the functioning of the system that the flowtemperature is the same at the branches on each floor.


124

Riser with shut-off and draining valves.Fig. 6:14

The expansion of the riser must be taken intoconsideration when the branch is installed.Fig. 6:15

Riser

Floor

Pipe embeddedin concrete



6. High-rise buildings.The equipment included in a heating system, radiators, pumps, valvesetc., are designed to the highest working pressure, usually 600 kPa(6 bar). Each meter vertically corresponds to about 10 kPa. With anapartment height of three meters and an open expansion vessel placed atthe roof on the top floor, it is possible to accommodate 19 floors((600-30)/30=19 floors), but then there are no margins. A maximum of18 floors would be more realistic.

In buildings with more than 18 floors, the heat installation ought to bevertically divided. A building with 28 floors receives two heating systems,managing 14 floors each. There are two options for the upper floors. Theheat exchanger can either be placed in the sub-station on the groundfloor (A), or a separate sub-station is set up on the 15th floor (B). Thesub-station on the 15th floor might also serve the 14 first floors, but inthat case with a separate heat exchanger. (D)

If the sub-station of both the heating systems is placed on the groundfloor (A, C), the equipment installed for the highest located heatingsystem (C) have to manage the higher static pressure occurring, morethan 600 kPa.

If the sub-station is placed on a floor halfway up the building, it willprovide a correspondingly higher static pressure for the primary system,(steam pressure at 120ºC 100 kPa, height to the sub-station placed onthe 18th floor 300 kPa, plus the possible difference in level between thefloor in the production unit and the floor on the 1st floor of the connec-ted building, sum = at least 400 kPa). The material on the primary sideusually manages these pressures if the boilers are separated with heatexchangers.


125

28th floor

15th floor

14th floor

1st floor

Sub-station in high-rise buildings.A or D for the lower part of the buildingB or C for the upper part of the building

A C

B D



7. Radiator circuit, two-pipe horizontal.The horizontal two-pipe system emanates from a centrally placed riserwith branches on each floor. A differential pressure control, keeping thedifferential pressure constant at 10 kPa, is installed on the branch on eachfloor. Then, branches are made for one radiator circuit to each apart-ment. Each branch is equipped with shut-off valves and flow meters. Theradiator circuit is either laid as a two-pipe system with a parallel flow andreturn pipe, or as a Tichelmann-coil with the pipes insulated in thescreed. With regard to existing buildings, the pipes are laid uninsulatedon a wall.

Adjustment.The thermostatic valves in a two-pipe system provide a varying flow anda varying differential pressure. A pre-set adjustment will only function ata maximum flow, when the flow decreases, the resistance over the adjust-ment changes by the square of the flow change.


126

Insulated pipes for the radiator circuit embeddedinto the floor.Fig. 6:17

Available differential pressure in the riser without differential pressure control.Fig. 6:18

∆p radiator circuit ∆p radiator

Available ∆p on the 18th floor

Available ∆p on the 1st floor ∆p radiator circuit ∆p radiator

∆p riser

∆p 5 kPa



A simple and safe method for the balancing of two-pipe systems with avarying flow is based upon differential pressure controls at the bottom ofeach riser or for each radiator circuit/floor. The differential pressurecontrol keeps the differential pressure constant independent of thechanges in flow. Maximum differential pressure across the thermostaticvalves is 25 kPa to prevent excessive noise.

It is sufficient with an approximate adjustment based upon heatrequirement for each radiator. The adjustment will only take effect if andwhen the available heat volume is not enough to keep the temperatureset on the thermostatic valve, i.e. at a long decrease in the flow temperatureor at disturbances in the heat supply.


127

Available ∆p on the 18th floor

∆p radiator circuit

∆p radiator

∆p 1

0 kP

a

Differential pressure control gives the same available pressure on each floor.Fig.6:19



8. Radiators – convectors.There are three types:

• radiators

• convectors

• convection radiators, convector with a front plate giving radiated heat.

Radiators emit heat through radiation but not through convection, or airmovement, until higher temperatures are reached – above 40ºC surfacetemperature at 20ºC room temperature.

Convectors emit heat through convection.

Convection radiators emit a smaller part of heat through radiation.

Approximate distribution among radiation and convection for differentheaters.:

Radiation % Convection %

Section radiators 15 85

Panel radiators, single 32 68

Convectors – 100

Convection radiators 10 90

As systems they are pretty much equal but they should not be mixed inthe same system, and from now on they will all be treated as radiators.

Radiator size.The radiators are sized for a nominal heat requirement, and the flow willvary when the thermostatic valves adjust the heat supply to the currentrequirement. The best effect is reached if the connection with the flow ismade to the upper tap-in, and the return to the lower tap-in on the sameside of the radiator.

Mounting.In order to prevent downdraught, the warm air from the radiators mustbe able to rise and meet the cold glass in the windows. Window ledges,if any, should be constructed so there is a gap along the whole window.The gap should be at least 30 mm wide, as close as possible to thewindow.


128

Section radiator.

Fig. 6:20

Panelradiator

Convector



The covering of radiators reduces the heat emission by obstructing theheat emission from the radiator to the room. The most common type ofcovering, which has only a grille in front of part of the radiator, reducesthe heat emission disastrously. A better solution, if the radiator is to beconcealed, is to make the front tight but with a 10 cm high opening atthe floor. The width of the opening should be equal to the width of theradiator. An opening is made in the horizontal protection plate of thecover of the same length as the radiator. The depth of the opening shouldbe 15-20 cm and it should be as close as possible to the window.

This function will be even better if plates are placed closely, at both endsof the radiator so that a vertical passage is formed. The reason why manyresidents are using radiator screens is that the high surface temperaturecauses a strong radiant heat which is experienced as unpleasant, whenyou are close to the radiator. The flow temperature should therefore notbe higher than that which is necessary to manintain the desired roomtemperature.


129

Acceptable cabinetca -8 - 10%

Open lattice> -15%

Small and tightlattice.Not recommended.> -30%

Alternativeopenings

a

a

aa+40

Covering a radiator reduces the heat emission.Fig. 6:21



Operating conditions.

1. Temperature levels.A flow temperature of 95ºC and a temperature drop of 20-25ºC havebeen referred to as the calculated values. If the radiators are sized cor-rectly, a flow temperature of 95ºC is required for the last radiator. Thetemperature drop from the sub-station to the last radiator is about 5ºCin larger systems. Consequently the outgoing temperature from the sub-station should be 100ºC and that is not possible.

90ºC is the highest temperature at which a radiator system can be ope-rated under these circumstances, which means an outgoing temperatureof about 95ºC from the sub-station. The adjoining heat emission curvefor radiators is therefore made for a flow temperature of 90ºC.

A lower flow temperature or a smaller temperature drop requires largerradiators. The lower flow temperature should be used if it turns out thata lower flow temperature can be used without influencing the desiredroom temperature, (the heat authority of the thermostatic valves or a lowreturn temperature). A lower flow temperature provides improvedcomfort by reducing the difference in surface temperature betweendifferent surfaces in the rooms.

A two-pipe radiator system requires that the flow temperature to all theradiators is pretty much the same if the system is going to function well.By metering the temperature drop across a radiator and then reading theroom and the outdoor temperatures, it is possible to get an idea of howlarge the system in question is, compared to the actual requirement. Theflow temperature is set at a level providing good heat authority for thelast thermostatic valve of the design circuit. The calculated temperaturedrop across radiator or radiator circuit should be strived for.

2. Return temperature.The return temperature from the radiators should be at least as low as therequired primary return temperature. 70ºC is the calculated value, but alower temperature is preferable and should be strived for.

A two-pipe system is the only solution that can guarantee a low returntemperature at the right conditions.


130

∆t °C 40 30 25 20 16

5

1,00,90,80,70,60,50,40,30,20,10

1,11,2

0 1,0 2,0 Q

12

10

8

6

4

1 2 3

4

10090

80

70

60

50

1 first radiator in circuit, tflow 95oC, Q = 1,0

2 last radiator in circuit, tflow 90oC, Q = 1,0

If Q = 2,0 the temperature drop across radiator willbe 50% lower. The flow temperature can be reducedwhile the heat emission remain the same.

3 first radiator in circuit, tflow 87,5oC, Q = 1,0

4 last radiator in circuit, tflow 85oC, Q = 1,0



3. Temperature drops in the pipe system.Excessively large pipes result in low water rates and large heat losses.Large heat losses in pipes with a small area result in a large temperaturedrops, and it is important for a good functioning that the flow temperatureis the same to all the radiators.

Good insulation and the highest rates applies for a good functioning.

The stated maximum water rates should be strived for. See graph inchapter 8.

4. Static pressure.At temperatures below 100ºC, the static pressure is equal to the heightconverted into kPa from the pressure gauge in the sub-station to thehighest point of the system. The pumps should be installed in the flow.

The static pressure is to ensure that all the parts of the system are filledwith water, whether the circulation pump is in operation or not.

5. Expansion vessels.The expansion vessel should be placed on the roof on the top floor. Thebottom of the vessel should be at the level for the static pressure, 0,5-1meter above the highest point of the system. The space is warm, thusthere is no risk of freezing, and it can be equipped with a floor drain sothat a possible overflow does not cause any water damage.

6. Available differential pressure.The available differential pressure must not be so high that it causes dis-turbing noise. With regard to thermostatic valves, 25 kPa is appliedtoday, as a maximum for the highest quality thermostatic valves.Differential pressure controls with a set constant differential pressure of10 kPa guarantees quiet and well-controlled thermostatic valves.


131

The lower edge of the expansion tank should beplaced above the highest point of the system.Fig. 6:23

Expansion tank

0,5 - 1 mExpansion pipe

The differential pressure controls give the same available differential pressure toeach radiator circuit.Fig. 6:24



7. Water quality.The requirements of the water used for filling the primary system alsoapplies to the secondary system.

The system must never be emptied of water, not even during longerbreaks in operation. As regards possible repairs, only the parts of thesystem directly affected should be emptied.

Leakages should be attended to immediately.

Water for filling the system is taken from the primary system. Therefilling pipe should be equipped with a flow meter so you can registerthe quantity of the refill in order to control the losses.

8. Heat losses in the sub-station.There are many surfaces with high temperatures emitting a lot of heat inthe production units.

All warm surfaces should be well insulated in order to increase the effi-ciency of the plant.

A high room temperature, which is a result of a bad insulation or none,is shortening the life of the equipment required in a modern plant of thistype, not to mention the electronic controls. Furthermore, people have tobe able to work efficiently within the plant.

Ventilation assisted by of a thermostatically controlled fan reduces theover-temperatures which arise.


132

Good insulation increases the efficency and lower the temperature in thesub-station.Fig. 6:26

The secondary system can be filled up with treatedwater from the district heating system butcontrolled. The amount of water fed into systemis to be measured and leakages are not acceptable.

Fig. 6:25

Feedingpipe

Flowmeter



Control.It is in the apartments that the actual consumption occurs. Hot wateremits heat to a room and comfort is created with the right heat supply.

Comfort requires a control of the heat supply, that is, it must not be toowarm or too cold.

Comfortable environment and living conditions require efficient systemswith control over heat supply and heat emission.

1. Control and supervision.The regular supervision of pressures and temperatures in sub-stations isnecessary for an economic and environmentally sound operation of thelocal district heating system and in due course the combined heating andpower plants.

Information data on temperatures, water level or pressures in expansionvessels, the position of the cone in the control valve, current primary flowetc., is transferred to the computer in the local production plant, andalarms for excessive temperatures, a low water level etc. can be recorded.

A computerized control and supervision makes it possible to optimizethe operation and also increases the operating safety.

Control valves.Two-way valves should be used on the primary side, which means thatno more water than required is circulating in the system and that a largetemperature drop can be maintained.

Each heat exchanger should have its own control valve, which should besized according to the current flow and the lowest available differentialpressure. Avoid too large valves!

The valve capacity is stated with a kv-value. The flow through the valvein m3/h, Q, at a differential pressure across the valve, ∆pv, on 1 bar (100kPa). The kvs-value states the flow at a fully open valve.


133

Control and supervision will be efficient whencomputerized.Fig. 6:27


2. Control of flow and return temperature.The flow temperature should be adjusted according to the outdoortemperature with a weather compensator which can be connected to acomputerized control and supervision system.

The flow temperature should be set so that the worst located thermostaticvalve will have good heat authority. Measure the flow and returntemperature across the radiator! A too high return temperature is obviatedby gradually increasing the flow temperature.

3. Control of the room temperature.The room temperature is controlled by having thermostatic valves oneach radiator. Even the last thermostatic valve is to have good heatauthority. A rough adjustment of the required flow is made on eachthermostatic valve.

The thermostats can be limited to a maximum temperature of 18-22ºC.The temperatures should be higher where elderly or sick persons live.

A thermostatic valve with a built-in thermostat should in most cases beused. When the valve cannot sense the actual room temperature, it isreplaced by a separate capillary connected sensor, placed at a suitablepoint, in the room.



k 1 2 3 4 5 6 7 NRTD-N 15 0,04 0,08 0,12 0,20 0,27 0,36 0,45 0,60RTD-N 20 0,10 0,15 0,17 0,25 0,32 0,41 0,62 0,83

v

RTD-N 25 0,10 0,15 0,17 0,25 0,32 0,41 0,62 0,83

72 °C

40 30 25 20 16

5

1,00,90,80,70,60,50,40,30,20,10

1,11,2

0 1,0 2,0 Q

10012

10

8

6

4

123

90

80

70

60

50

Measure the flow temperature to and thetemperature drop across the last radiator in thedesign circuit.A small increase in flow temperature has a biginfluence on the temperature drop as well as onthe flow.

tflow

Heatemission ∆t oC

tflow oC

1 : tflow 72oC, ∆t 9oC, heat requirement 0,73.2 : ∆t 16oC, requires tflow 75oC.3 : ∆t 25oC, requires tflow 80oC.

Fig. 6:28

Pre-set values for thermostatic valves.Fig. 6:29



4. Pressure control of pumps.All the circulation pumps in systems with varying flow should beequipped with pressure control. A constant differential pressure at thelast branch/valve provides the largest saving. Using pressure controldoesn’t mean that differential pressure control valve should be excluded.


135

1009080706050403020100

0 50 100 %Q

Available differential pressure for a circuit close to the sub-station but with ∆pcontrol for the circuit. The pump has proportional pressure control.Fig. 6:30

1009080706050403020100

0 50 100 %Q

Available differential pressure for a circuit close tothe sub-station. The pump has proportional pressurecontrol.Fig. 6:31

7,5 4 kPa

8

8,5

9

9,5

55

55,5

49,5

50

49

∆p kPa ∆p

Radiator

Circuit

Flow and returnpipe to circuit

∆p kPa ∆p

Circuit

∆p - control

Flow and returnpipe to circuit

∆p kPa

∆p 10 kPa ∆p 50,5 kPa

∆p 10 kPa

∆p 10 kPa 50 m

Available differential pressure with the differential pres-sure control in the riser, up to six floors.Fig. 6:32

Available differential pressure with one differentialpressure control for each radiator circuit in high-raisebuildings.Fig. 6:33

∆p 4 kPa

≈ 17 m


5. Control of the available differential pressure.In buildings of maximum 6 floors, each riser is equipped with a differen-tial pressure control providing 10 kPa.

When the building has more than 6 floors, a differential pressure control,providing 10 kPa, should be installed on each floor.

6. Flow metering per apartment.

Heat.Each apartment is equipped with a flow meter for the distribution of theheating costs. The flow meter should be accessible for reading from thestair-well, and possibly connected with the control and supervisionsystem of the building.

With regard to gable apartments and apartments with a roof, acompensating factor is calculated on the basis of heat requirementcalculations made for a similar apartment in the centre of the building.

Domestic water system.Domestic hot water is produced in a heat exchanger of the percolationtype in the sub-station.

A distribution pipe is laid in the ground floor of the building, from whichrisers are drawn up centrally through the building. Each riser is equippedwith shut-off and draining valves.

The branches on each floor are equipped with shut-off valves.

Distribution pipes and risers should be made of a non-corrosive materialand well insulated.

A gravity pipe for the of hot water should be laid parallel to the tap waterpipe.

The circulation pipe should be laid uninsulated in the riser, and at theconnection with the horizontal circulation pipe be equipped with anadjustment valve.

Flow meters for domestic hot water should be installed in the stair-well,one for each apartment.



The heat comsumption, for apartments of the samesize, will vary depending on how many outer walland roof surfaces there are. A correction factor canbe calculated based on the heat requirement perapartment or per square meter.Fig. 6:34

Principles for domestic hot water in installation withcirculation pipe, shut-off and adjustment valves.Fig. 6:35

Requirement 5.600+9.400=15.000 W.Correction factor=5.600/15.000=0.37.



Requirement 5.600 W.Correction factor=0.


7. Control of domestic hot water.The outgoing temperature from the heat exchanger should be kept con-stant. A control valve on the primary side, regulated by an electroniccontrol, which is either built-in to the weather compensator or placedseparately, keeps the outgoing temperature constant. Self-acting controlscan also be used.

The maximum temperature at the tap is 65ºC and the minimum is 60ºC.

The return temperature from the heat exchanger for domestic watershould be below 60ºC, by a comfortable margin.

8. Control of domestic water in an apartment.The taps for personal hygiene, shower and wash-basin should bedesigned so that hot and cold water can be mixed to a suitable temperature.

Max flow in the shower, 0,2 l/s.

Max flow in the wash-basin, 0,1 l/s.

Max flow in the kitchen, 0,2 l/s



>60 °C

<65 °C

Control of flow temperature for domestic hot water.Maximum and minimum temperatures are impor-tant.FIG. 6:36

Shower with mixer.FIG. 6:37






CHAPTER 7 • HOW TO SELECT SIZE OF PRODUCTS AND COMPONENTS.

139

How to select size of products and components.Thermostatic valves.

Choice of valve size.

Existing one-pipe systems.All the radiators must be equipped with thermostatic valves to be able tocontrol the room temperature, use the incidental heat gain efficiently anddistribute the heat according to requirements. This requires a by-pass ateach radiator, and the resistance in the by-pass has to be larger than inthe main pipe so that a certain amount of water is let to in the radiator.

Good operation is obtained if the thermostatic valve has a low resistance,like valves intended for gravity circulation, and the by-pass is of the samedimension as the main pipe. The by-pass is equipped with a restrictioncreating the required resistance.

Two-pipe systems.The valve size is determined on the basis of the required flow and theavailable differential pressure. Maximum differential pressure is limitedto 25 kPa as far as noise is concerned. The available differential pressurefor each thermostatic valve is obtained from the pipe calculation.

Flow.The flow is calculated from the heat requirement in watts, W, and thetemperature drop across the radiator in Kelvin, K. The valve size can theneither be determined from a selection flow chart or be calculated.

Existing one-pipe system with thermostatic valveand by-pass.Distribution through radiator and by-pass.Fig. 7:1

By-pass insert



Valve size.The last valve in the design circuit, (which determines the pump headthroughout the entire system) ought to have a resistance of about 5 kPa.The other valves should be sized according to the differential pressureavailable for them, i.e. the penultimate valve in the design circuit has anavailable pressure equal to the resistance across the last valve plus theresistance in the pipes between the two valves.


140

Available ∆p for the risers in a two-pipe system.Fig. 7:2

1

6

7140 133 125∆p 80 kPa

∆p 72 kPa ∆p 9 kPa

∆p 5 kPa

3

5710

2030

5070100

,001

,002,003

,005,007,01

,02,03

1 2 3 4 5 7 10 20 30 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 Bar

,1

,05,07

500

300200

0,60 0,45

0,36 0,27

0,20 0,12

0,080,04

N

1

23

456771

Finding the pre-set values for the thermostatic valves in the above heating system.Fig. 7:3

l/h RTD-N 15

Pre-set value l/s

∆pva

lve

Radiator l/h ∆p kPa Pre-set

1 140 5 N

7 140 9 7

125 140 72* 3,5

140 140 80* 3,5

*too high ∆p, will create a problem with noise.

kvs -value

k 1 2 3 4 5 6 7 NRTD-N 15 0,04 0,08 0,12 0,20 0,27 0,36 0,45 0,60RTD-N 20 0,10 0,15 0,17 0,25 0,32 0,41 0,62 0,83

v

RTD-N 25 0,10 0,15 0,17 0,25 0,32 0,41 0,62 0,83


Pre-setting.Adjusting a valve implies a calculation of the difference between theavailable and the required pressure for the valve. The resistance across thevalve should then be increased, through adjustment, so that all theavailable pressure is utilized. The setting values providing the requiredresistance can be read from the selection flow charts.The values for eachvalve should be stated on the drawing so that the setting can be made inconnection with the installation.

Choice of control unit.There are many conditions influencing the function of the thermostaticvalve. The control unit has to sense the room temperature to be able tocontrol it.This is not possible if it is covered by a long curtain or acabinet.

Heat radiation from warmer surfaces, for example heating pipes, a warmfloor, electrical devices etc., deceives the sensor into believe that it iswarmer than it actually is in the room.

Downdraught and draught from open windows or doors deceives thesensor into believe that it is colder in the room than it actually is.

A control unit with a built-in sensor has difficulties in managing theseproblems. A control unit with a separate capillary tube connected sensortherefore should be chosen. The sensor can then be placed where itdetects the right room temperature.



The control unit has to sense the room temperature to be able to control it.Fig. 7:4


Control valves.

Primary systems.Two-way valves and consequently varying flow are recommended for theprimary systems.

Available differential pressure.A resistance of 100-120 kPa is recommended to be available in thedesign circuit for the control valve.

As regards other control valves in systems without a pressure controlledpump the available differential pressure is obtained from the pipecalculation.

When using pressure controlled pumps with the sensor located farthestaway in the system all the control valves should be sized for the lowestavailable differential pressure of the system. In other words, the differentialpressure set on the sensor, 150kPa, is recommended, minus the resistancein the heat exchanger in question, 20-50 kPa. Check the resistance in theheat exchanger with the supplier!

If the available differential pressure at a valve should increase by 50% ormore of the designed differential pressure a differential pressure controlis recommended for that particular valve. The designed differential pres-sure is shared between the control valve and the differential pressurecontrol.


0

200300

1000

50

100

400500600


If the pump is equipped with pressure control the valves must be calculated for thelowest available ∆p. In this case 150 kPa, 1,5 bar, minus the resistance in the heatexchanger.Fig. 7:5

∆ppump

∆psystem

∆pmin

Flow

%

∆pMin ∆p = 150 kPa


Valve size.Enter information of flow and available differential pressure into thevalve selection flow chart and then select the valve size! The dimensionof the pipe in which the control valve is to be installed has no influenceon the required valve size.



0,1

0,2

0,30,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

10 20 30 40 60 100 kPa

1 2 3 4 5 7 10 15 20 mWG

0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 Bar

10

57

150

1550

3020

1

2

3

,4,631,01,62,54,06,3

101625

40

Example, control valve:∆t = 50 oC.1 P = 1.500 kW; Q = 1.500 × 0,86 / 50 = 25,8 m3/h.

∆p available = 1,5 bar. ∆p heat exchanger = 0,3 bar.

2 ∆p = 1,5 bar - 0,3 = 1,2 bar.

Values from diagram:3 kvs = 25 m3/h, ∆pv = 1,1 bar

Sizing of the control valves in the adjoining district heating circuit.Fig. 7:6

m3/h Valve kvs - value l/s

∆pva

lve


Secondary systems.Two-way valves should also be used in the secondary systems, with amain pump supplying the water out to each mixing loop or shunt.

Available differential pressure.Two-way valve.A resistance of 10-15 kPa is recommended to be available for the controlvalve in the design circuit.

The available differential pressure for other control valves in systemswithout a pressure controlled pump is obtained from the pipe calculationand as much as possible of the differential pressure should be used.

With regard to the pressure controlled pump with the sensor at thepump, all the control valves should be sized for the lowest differentialpressure they will obtain. The designed differential pressure depends onwhich type of pressure control that is used:

• a constant differential pressure gives design values according to thepipe calculation

• a proportional control gives that design value which is 50% of themaximum differential pressure

• a pressure control parallel to the pipe resistance gives a design valuethat is 50% of the maximum differential pressure

• a constant ∆p at control valve located the farthest away gives designvalues for all the control valves equal to the lowest set differential pres-sure


1 2 3 4 5 6 7 8 9


0

2030

10

405060

Available ∆p with or without pump control at different flow.Fig.7:7

This combination provides the control valve with thesame available pressure when the flow fluctuates.Fig 7:8

Impulse tube

∆ppump

∆psystem

∆pvalve< 15 kPa

∆pvalve

∆p k

Pa

∆psystem ∆p 100% flow Design ∆pwithout pump control or with constant ∆p

With proportional or parallel pump control

∆p at 0% flowwith max ∆p

with proportional ∆p


If the available differential pressure at a valve should increase by 50% ormore of the designed differential pressure, a differential pressure controlis recommended for that particular valve. The designed differential pres-sure is shared between the control valve and the differential pressurecontrol.

Valve size.Enter information of flow and available differential pressure into thevalve selection flow chart and then select valve size! The dimensions ofthe pipe in which the control valve is to be installed has no influence onthe required valve size.


145

0,1

0,20,3

0,50,71,0

2

35710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 200 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 15 20 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 Bar

10

57

150

50

3020

100

200

2030

50

,4,631,0

1,62,54,06,3

1016254063

100145

68 7 2

139 5 4


Sizing valves from a diagram will not give the same mathematical accuracyas a calculation, but it is good enough when considering the inaccuracy of theunderlying calculations.

Fig 7:9

Sizing of the control valves in the above heating circuit.

m3/h Valve kvs - value l/s

∆pva

lve

Exampel.Q = 3 m3/h∆ppump = constant∆pavailable = from the calculation of the designcircuit, including valve 9.Here: from the above diagram + ∆p valve no 9.Excessive ∆p, ∆pexc. = ∆pavailable - ∆pvalve∆ppump = ∆psystem + ∆p valve 9.∆psystem = 60 kPa.Sizing of control valve 9.

See diagram: 3 m3/h, ∆p<15 kPakvs 10, ∆p valve 9 = 9 kPa.Selecting valve size from diagram:Valve ∆pavailable kvs ∆pvalve ∆pvalve

1 53+9=62 4,0 55 72 47+9=56 4,0 55 13 40+9=49 6,3 23 264 34+9=43 6,3 23 205 26+9=35 6,3 23 126 20+9=29 6,3 23 67 14+9=23 6,3 23 -8 7+9=16 10 9 7


Differential pressure controls.Only the differential pressure control can eliminate the pressure varia-tions being the result of a varying flow in the systems, and only the dif-ferential pressure control can provide the control valves with goodworking conditions.

The valve size is determined on the basis of the required flow and theavailable differential pressure. A differential pressure control keeping thepressure constant across a control valve has to be sized along with thecontrol valve.

Primary systems.Differential pressure controls are used in primary systems to keep the dif-ferential pressure constant across a sub-station or a valve in the sub-station.

Available differential pressure.The available differential pressure for the sub-station, 150 kPa, minus theresistance across the heat exchanger, 30 kPa, is the available differentialpressure for both the control valve and the differential pressure control,∆pv2 =150-30=120kPa.



Two parallel connected heat exchangers.Fig. 7:11

Controlled ∆p gives the best result.Fig.7:10

∆p

h


Valve size.

Divide ∆pv2 by two and choose a control valve from the valve selectionflow chart according to the ∆p and the flow in question. The remaining∆p, i.e.120 kPa minus ∆pv is the available differential pressure for thedifferential pressure control. Enter the differential pressure and the flowfor the differential pressure control into the selection flow chart andthen select size!


147

0,1

0,20,3

0,50,71,0

23

57

10

0,1

0,20,3

0,50,71,0

23

,03

,05,07

10 20 30 40 60 100 kPa

1 2 3 4 5 7 10 15 20 mWG

0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 Bar

10

57

150

1550

3020

,4,63

1,62,54,06,3

10162540

1

1,0

423


0

200300

1000

50

100

400500600

m3/h

Min ∆p = 150 kPa

∆ppump

Valve kvs - value l/s

∆pva

lve

∆psystem ∆pminFl

ow %

If the pump is equiped with pressure control, the valves must be calculated for thelowest available ∆p. In this case 150 kPa, 1,5 bar, minus resistance in the heatexchanger. All valves for which the available ∆p will exceed the design ∆p withmore than 50% require a ∆p control.Fig, 7:12

∆p

Example, control valve and differential pressure control:∆t = 50 oC

1 P = 1.500 kW; Q = 1.500 × 0,86 / 50 = 25.800 l/h.∆p available = 1,5 bar. ∆p heat exchanger = 0,3 bar.∆p = 1,5 - 0,3 = 1,2 bar.

2 ∆p available for ∆p valve = 1,2/2 = 0,6 bar;Values for ∆p - valve from diagram: kvs = 40 m3/h;

3 ∆pv = 0,41 bar

4 ∆p available for control valve = 1,2-0,41 = 0,79barkvs = 40 m3/h; ∆pv = 0,41 bar;

Pre-set value for the ∆p control = 0,41 bar;

Fig, 7:13


Setting value.A differential pressure control keeps the differential pressure constantacross a circuit. The setting value for the differential pressure control isequal to the resistance in that particular circuit.

Secondary systems.In the secondary systems differential pressure controls are used to keepthe differential pressure constant across a control valve or a part of thesystem, for example a riser or a two-pipe radiator circuit containingseveral thermostatic valves.

Available differential pressure.In secondary systems, the resistance in the design circuit, of which thedifferential pressure control is a part, is calculated. It is important whencalculating to check the requirements for the differential pressure controlin question. Some of these differential pressure controls require aminimum differential pressure to function properly.

The resistance across the differential pressure control in the designcircuit is obtained from the selection flow chart. Enter the flow inquestion into the selection flow chart then select valve size and read theresistance.

For the other circuits the available differential pressure is obtained fromthe pipe calculation.

Valve size.Differential pressure control across a control valve.

In the designed circuit first of all check if the differential pressure controlrequires a minimum differential pressure. Is this the case, select a size ofcontrol which requires at least this pressure. Even if the resistance acrossthe smallest valve is not large enough make sure that at least theminimum differential pressure is available. Select accordingly the size ofthe control valve.

The available pressure in the other circuits is divided by two. The controlvalve is selected first and the remaining differential pressure is used forselection of the differential pressure controller.





149

0

2030

10

405060

1 2 3 4 5 6 7 8 9

0,1

0,2

0,30,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 200 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 15 20 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 Bar

10

57

150

50

3020

100

200

2030

50

,4,631,0

1,62,54,06,3

1016254063

100145

9 87 2

1

4

536


Example

Q = 3m2/h∆ppump = constant∆pavailable = from the calculation of the designcircuit, including valve no 9.Here: from the above diagram ∆p valve 9.

Excessive ∆p ∆pexc. = ∆pavailable - ∆pvalve∆ppump = ∆psytem + ∆p control and differentialpressure valves no 9.∆psystem = 60 kpa.

Sizing of the control and differential pressurecontrol valves no 9.See flow chart: 9. 3m3/h ∆p control valve <15 kPa.

∆p valve no 9 kvs 10, ∆p = 9 kPa.The ∆p control valve will be the same sizeand ∆p ∆pvalves = 18 kPa.∆ppump = 60 + 9 + 9 = 78 kPa.Selecting valve size from flow chart:Divide the total available ∆p by 2.Find in flow chart the cutting point between flow,3 m3/h, and the ∆p available for the valve.Choose the first valve size which is big enough.Find ∆p across the chosen valve, that is the set pressurefor the differential pressure control.The two valves will have the same size.Valve ∆pavailable kvs ∆pvalve ∆pvalves

Total 1 valve

1 53+18=71 35 6,3 23 462 44+18=6 31 6,3 23 463 39+18=57 28 6,3 23 464 34+18=52 26 6,3 23 465 29+18=47 23 6,3 23 466 24+18=42 21 10 9 187 19+18=37 18 10 9 188 12+18=30 15 10 9 18

Calculation of valve no 9.

kv = ; kv = 7,8; => valve with kv 10,0;

∆p = ; ∆pv = 0,09 bar; => 9 kPa.

3√ 0,15

3( )10

∆ppump

∆p Pa ∆psystem ∆p 100%

∆pvalve< 15 kPa

Design ∆pWithout pump control or with constant ∆pWith proportional or parallel pump contol

∆p at 0%With max ∆pWith proportional ∆p

∆pvalves no

Available ∆p with or withoutpump control at different flowFig. 7:14

Choosing valves from a flow chart will not give the mathematical accuracyas a calculation, but it is good enough when considering the inaccuracy of theunderlying calculations.Fig. 7:15

Sizing of the control valves and differential pressure control valves in the aboveheating circuit.

m3/h Valve kvs-value m3/h l/s

∆pva

lves

2




Differential pressure control of risers.With regard to the design circuit, it is first of all a question of checkingif the differential pressure control requires a lowest differential pressure.If so, choose a size of control that requires at least this pressure, or makea reservation for the lowest required pressure for the control, even if theresistance across it is not very large.

Concerning the other circuits, the flow and the available differentialpressure are entered in the selection flow chart and a suitable valve sizeis chosen.

∆p 5 kPa

∆p 9 kPa

1

6

7∆p 80 kPa 140 133 125

∆p 72 kPa

∆p 9

∆p 9

3

5710

2030

5070100

,001

,002,003

,005,007,01

,02,03

1 2 3 4 5 7 10 20 30 kPa0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 Bar

,1

,05,07

500

300200

0,60 0,45

0,36 0,27

0,20 0,12

0,080,04

N

1

23

456771

125140

l/h Valve kvs - value

∆pva

lve

l/s

Calculation of the pre-set values for the valves in the above system with ∆p controlvalves in the riser.Fig. 7:17

Radiator l/h ∆p kPa Pre-set1 140 5 N

7 140 9 7

125 140 9 7

140 140 9 7

Available ∆p for the risers with ∆p-control valves.Fig. 7:16

Set values

k 1 2 3 4 5 6 7 NRTD-N 15 0,04 0,08 0,12 0,20 0,27 0,36 0,45 0,60RTD-N 20 0,10 0,15 0,17 0,25 0,32 0,41 0,62 0,83

v

RTD-N 25 0,10 0,15 0,17 0,25 0,32 0,41 0,62 0,83




Setting value.A differential pressure control keeps the differential pressure constantacross a circuit. The setting value for the differential pressure control isequal to the resistance in that particular circuit.

0,3

0,50,71,0

23

5710

kPa

mWG

7 10 20 30 40 60 100

,7 1 2 3 4 5 7 10

0,1

0,20,3

0,50,71,0

23

1,6

4,06,3

10

2,5

1

2 18-20

,06 ,1 ,1,5

1

,2 ,3 ,4

∆p 5 kPa

∆p 9 kPa

1

6

7

89

140 133 126

∆p kPa

∆p 9

∆p 9

12181920

85 81 1822

Q in each riser = 980 l/h

ASV-P, ASV-PV Min. available ∆p

m3/h Valve kvs-value l/sMax. ∆pvalve

∆pva

lve

Sizing of ∆p-valve in riser.Q in each riser = 980 l/h∆p riser = 9 kPa.Valve no 1 , se diagram.kvs = 4,0, ∆pvp = 6 kPa∆p-valve with fixed ∆p = 10 kPa andminimum available ∆p = 8 kPa gives 18 kPa.

Valves 2, 18, 19 and 20.∆p-valve Q l/h ∆pavail.-∆priser = ∆pvp avail kvs ∆pvp

2 980 22 10 12 4,0 6

18 980 81 10 71 1,6 37

19 980 85 10 75 1,6 37

20 980 89 10 79 1,6 37

Sizing of the ∆p-valves in the riserFig. 7:19

Available ∆p for the ∆p-control valves at each riser.Fig. 7:18




Flow limitation.Flow limitation is required in both primary and secondary systems.

Primary systems.In a primary system, it is the flow to a whole sub-station or to the appliedheat exchangers that should be limited.

The heat supply is controlled by a control valve and if the differentialpressure across this valve is kept constant with a differential pressurecontrol the sub-station contains the required components to limit theflow.

Calculate the differential pressure that is necessary across the fully openvalve to obtain required flow. Set the differential pressure control so thatit will provide the differential pressure and the maximun flow is limited.

Combined flow limiters consisting of a differential pressure control anda setting valve are available. The differential pressure control keeps aconstant differential pressure across the integrated pre-set valve. The sizeof the flow is determined by changing the resistance across the settingvalve. When large sizes are required a flow limitation is obtained as adifferential pressure control can keep a constant differential pressureacross a integrated pre-set valve. The valve size is determined in aselection flow chart on basis of the available differential pressure and theflow.

0,1

0,20,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 Bar

∆p

4

1 2

3

k 4,0vs

Example, limiting the flow in a primary circuit.Control valve kvs 4,0

Ex. no Q m3/h ∆pvalve. ∆pvp-set

1. 3 55 55

2. 4 100 100

3. 1 6,3 6,3

The ∆p necessary for a specific flow through a fullyopen control valve is equal to the setting ∆p for thedifferential pressure control.

Calculation1 ∆pv = ; ∆pv = 0,56 bar => 56 kPa;

2 ∆pv = ; ∆pv = 1 bar => 100 kPa;

3 ∆pv = ; ∆pv = 0,0625 bar => 6,3 kPa;

Fig. 7:20

kvs-valuem3/h

Limiting the flow in a sub - station equiped with ∆p control valveFig. 7:21

( )44

2

( )14

2

( )34

2

∆pvalve kPa

l/s




Secondary systems.In secondary systems the limitation of the flow could come intoquestion to a shunt coupling, a riser or a one-pipe circuit.

If there already is a control valve and a differential pressure control in ashunt coupling, use these for the flow limitation too! Calculate the resis-tance across a fully open control valve at the maximum required flow andset the differential pressure control on this differential pressure!

In other cases there are flow limiters keeping the differential pressureconstant across a built-in adjustment valve. They are often sizedaccording to the available differential pressure and the required flow.Setting value is read in the selection flow chart.

0,1

0,2

0,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 kPa0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 Bar

∆p

1

2

3

1,6

Limiting the flow for a control valve in a secondary circuit with ∆p control.Fig. 7:22

Example, limiting the flow in a primary circuit.Control valve kvs 1,6∆p-valve kvs 1,6

Ex. no Q m3/h ∆pvalve. ∆pvp-set ∆p-contr

1. 0,4 5,8 5,8 ASV-PV

2. 0,8 25 25 ASV-PV

3. 1,5 90 90 AVP

The ∆p necessary for a specific flow through a fullyopen control valve is equal to the setting ∆p for thedifferential pressure control.ASV-PV: setting range 5-25 kPa.AVP: setting range 5-50, 20-100 and 80-160 kPa.

Calculation1 ∆pv = ; ∆pv = 0,0625 bar => 6,3 kPa;

2 ∆pv = ; ∆pv = 0,25 bar => 25 kPa;

3 ∆pv = ; ∆pv = 0,88 bar => 88 kPa;

Fig. 7:23

( )0,81,6

2

( )1,51,6

2

( )0,41,6

2

kvs-valuem3/h l/s




∆p one-pipe circuit

∆p available

∆pv

Flow limitation in a one-pipe circuitFig. 7:24

Flow limitation in a one-pipe circuit∆p available > ∆p1-pipe circuit + ∆pv∆pv = 25 kPa

Example, ASV-QASV-Q Capacity l/h Setting value15 100-800 1-820 200-1400 2-1425 400-1600 4-1632 500-2500 5-3

Q = 1100 l/hChoose ASV-Q 20 (always choose the smallest possible valve)Setting value = 11




Control equipment.Different control equipment is required for different purposes. Thecontrol of the flow temperature to radiators requires one type of control,hot water heating requires another, and ventilation devices require a thirdtype. For the last two cases there is also a choice between electronic andself-acting control.

Radiator systems.The flow temperature in radiator systems is controlled according to theoutdoor temperature by a weather compensator.

The electronic central control can be equipped with timers with twenty-four hours or weekly functions. This is however only the case if the heatsupply is set back during a period of several days and nights and if thesystem is not connected to a computer.

A pump stop is an optional function which shuts off the circulationpump when the outdoor temperature is so high that the building requiresno heating.

The limitation of the return temperature is usually not required in thetwo-pipe systems with thermostatic valves.

A computerized supervision and control system is a labour-saving andefficient way of controlling large systems with many sub-stations.

Necessary control equipment for sub-stationFig.7:25

Weather compensator

Outdoor temperature sensor

Surface sensor

Reversible gear motor




Hot water heating.Water is heated in a heat exchanger or in an accumulator.

The heat supply for the two types of hot water heating can be controlledby a weather compensator with an extra function for this purpose or self-acting controls for the accumulating hot water tanks.

For heat exchangers up to 30 apartments there are self-acting controlswith flow compensation available.

Flow compensated thermostatic valve for control of domestic hot water tempera-ture.Fig. 7:26




Pipes and heat exchangers.

Pipes for heating.When designing pipe systems an economic water rate has to be maintained.Too low a rate will give large-size pipes, deposits in the pipes, larger heatlosses and temperature drops, but of course also a lower flow resistanceand thereby lower operating costs for the pump.

An optimization reflecting the costs for pre-insulated pipes gives waterrates of approximately 0,6 m/s for the internal diameter of 27 mm to 3,6m/s for the internal diameter of 1.220 mm.

The corresponding values for insulated standard pipes in the heatingsystem of a building will give about 0,3 m/s for pipes with an internaldiameter of 10 mm and 1,5 m/s for an internal diameter of 150 mm.




Pipes for domestic water.There are three types of pipe material to choose from for the domesticwater - galvanized steel, copper and plastic. All of them can as a rule beused for cold water, but copper and plastic are superior. For hot wateronly copper and special plastic pipes can be used.

Copper pipes are sensitive to high water rates and they are environmen-tally hazardous, (copper is transported together with the sewage down tothe purification plant and will there affect the purification process nega-tively).

Maximum rates in an easily exchangable pipe:

• cold water 2 m/s

• hot water 1,5 m/s

For plastic pipes there are no limits to the water rate, but pipes intendedfor domestic hot water must endure the temperature in question formany years – 50 years according to international standards, NKBProduct rules, 3, July 1986 and DIN 16892.

Heat exchangers.Modern heat exchangers, plate and coil units, contain small quantities ofwater and the flow channels are narrow. By making them short and bylaying a large number of them parallel, the flow resistance is kept at a lowlevel in spite of a relatively high water rate.

The high water rate is necessary to prevent deposits from settling on theheat transferring surfaces.

The resistance across the coil unit is in the range of 20-30 kPa and forthe plate heat exchanger the resistance is up to 50 kPa. The choice of sizeis made according to the instructions from the manufacturer. There aredomestic water selection flow charts, based on empirical values, givingthe total consumption for various number of apartments.

0

0,5

1,0

1,5

2,0

2,5

050100150200250300350400

1 10 50 100 150 200 250

Maximum required flow according to the SwedishBoard for District HeatingFig. 7:27

Domestic hot water, Q l/s Effect, P kW

Number of apartments




Heat meters.Heat meters register the delivery to each building/apartment, but theyalso indicate if anything goes wrong in the system. As there are largevariations in the flow, a flow meter must also be able to measure low flowswith great accuracy.

The primary network.Meters on the primary side register the heat consumption, i.e. flow andtemperature drops. The meters should be based on ultrasound, and theintegration unit should be able to communicate with a central computer.

The theoretical maximum flow determines the size of the flowmeters.The ultrasonic meter has an advantage of being able to measurethe lowest flows very well, independent of size.

Each heat exchanger for heating and for domestic hot water should beequipped with a heat meter.

The secondary network.On the secondary side, it is sufficient to measure the flow for eachapartment. Based on this, make a percentage calculated distributionbetween the apartments of the total heat supply to the building. Then usea flow meter, mechanical or ultrasonic to register the flow to eachapartment.

The variations in flow can be considerable, so it is important to carefullyregister the low flows here.

Flow meters based upon ultrasound are therefore the most suitablechoice, especially when considering the large numbers and the fact thatthe ultrasonic meters require practically no maintenance.

The choice of the flow meter sizes is made according to the theoreticalmaximum flow to each apartment.

If the distribution of the heating costs is to be consistent, the hot domes-tic water to each apartment ought to be registered too, which requiresthat the riser for hot domestic water be placed centrally, in the stair-well,and that separate pipes are laid from there to each apartment.

Flow meters register the flow to each apartmentFig. 7:29

Heat meters register consumption and heat lossesfrom pipe network.Fig. 7:28

AccumulatorHeat meter

Heat meter


Pressure control of pumps.The pressure control of pumps should be applied on the primary and thesecondary sides to reduce the consumption of electricity. The effect onthe available pressure will be marginal as the differential pressure controlis applied on control valves or parts of the systems.

The primary network.The required pressure and flow on the primary side is always so highthat it requires a pump with a separate motor. The motor is a standardinduction motor and a frequency converter is therefore the most suitablechoice for control.

Frequency converters are available in the same sizes as the ones beingstandard for the standard induction motors. There are therefore noproblems in selecting the size. Choose a frequency convertercorresponding to the size of the motor!

The secondary network.There are pumps with a wet motor and a built-in pressure controlavailable for the secondary side. These pumps should be used as far aspossible and when their capacity isn’t sufficient to meet the requirements,dry pumps and frequency converters should be chosen. The largest cut inthe operating costs for the pump is obtained when the differential pres-sure is kept constant at the last riser/valve.



% p, P∆100

0 100%0

50

50

Q

∆ ∆p = Qn2x p 0

P = Qn 0∆ 3

x P 0

Q

The resistance varies by the square of the flow change and the effect of the pumpby the cubicFig. 7:30



CHAPTER 8 • TECHNICAL DATA, FORMULAS AND CHARTS

161

TECHNICAL DATA, FORMULAS AND CHARTS

Diagram for local district heating plants and heating and power plant . . . . . . . . . . . . . . .162

Diagram for heating and domestic hot and cold water . . . . . . . . . . . . . . . . . . . . . . . . . . .163

Heat emission from radiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .164

Conversion chart for radiators in one-pipe systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .165

Reduction of heat emission from radiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .166

Heat losses from uninsulated pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167

Pressure drops in steel pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .168

Resistance in heating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .169

Sizes of steel pipes for heating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .169

Flow chart for thermostatic radiator valves in one-pipe system . . . . . . . . . . . . . . . . . . . . .170

Flow chart for thermostatic radiator valves in two pipe system . . . . . . . . . . . . . . . . . . . . .171

Flow chart for ∆p control valves for risers or circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .172

Flow chart for control valves in heating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .173

Flow chart for control valves in district heating systems . . . . . . . . . . . . . . . . . . . . . . . . . . .174

Flow chart for ∆p control valves in district heating systems . . . . . . . . . . . . . . . . . . . . . . . .175

Heat requirements for domestic hot water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177

Flow limiters for one-pipe circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .178

Calculation of one-pipe systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .180

Calculation of two-pipe systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .182

SI-units, Greek alphabet, Physical properties for water . . . . . . . . . . . . . . . . . . . . . . . . . . . .184




130 °C70 °C

130 °C

130 °C

70 °C

70 °C

130 °C

130 °C

70 °C

70 °C

130 °C

130 °C

70 °C

70 °C

Diagram for local district heating plants connected to a heating and power plant.

Heating andpower plant

Local heating plant

Flue gascooler

Safetyvalve

Exp. tank

Bo

iler

Safetyvalve

Heat exchanger

Accumulator

Heat meter

Flue gascooler

Safetyvalve

Exp. tank

Bo

iler

Safetyvalve

Heat exchanger

Accumulator

Heat meter

Flue gascooler

Safetyvalve

Exp. tankB

oile

r

Safetyvalve

Heat exchanger

Accumulator

Heat meter




163

<6 >6

120-70 °C

90 °C65 °C

Diagram for heating and domestic hot and cold water.

Expansion tank

∆p - control

Flow meter

Domestic hot water

Domestic cold water

Flowmeter> 6 storeys

Heat meterDomestic hot water 60

Domestic cold waterCirculation

Control valve< 6 storeys

Storeys


40 30 25 20 16

0

1,00,90,80,70,6

0,50,40,30,2

0,1

1,1

1,2

0 1,0 2,0 Q

90

60

70

80

50

0,5 1,5 2,5

1

2

4

5

6

3



Heat emission from radiators.

Two-pipe system with thermostaticvalves. Measured 1 : tflow 75 oC, ∆t 8 oCHeat requirement : 0,83, Q = 2,47tflow 80 oC : 2 ∆t 16 oC, Q = 1,23Every point along the horizontal line0,83 gives the same heat emission.

The influence of gravity forces on heat emission from a radiator in a two-pipe system For a correctly sized radiator 3 ( with manual radiator valve in a two-pipesystem ) the heat emission will increases only by 5% when the flowincreases by 23%, 4 , depending on gravity forces. The temperature dropacross the radiator however will decrease by 5 oC and that is significant,because it reduces the capacity of the whole system all the way down tothe heating and power plant.

Resuls ∆t for one- and two - pipe circuits, and required pump capacitywhen thermostatic valves utilize internal and external heat gains.

Two-pipe circuit One-pipe circuitPoint Heat Flow ∆t Circuit resi- Pump ca- Flow ∆t Pump ca-

gain % % oC stance % pacity % % oC pacity %3 0 100 25 100 100 100 25 1005 10 66 33 44 29 100 22,5 1006 20 47 39 22 10 100 20 100

n = 1,3 troom = 20 oC tflow = 90 oC ∆t = 25 oC

∆t oCH

eat

emis

sio

n

Q

12

10

8

6

5

4


165



0,8

0,9

1,0

1,1

1,2

1,3

1,4

1,5

1,6

1,7

1,8

1,9

2,0

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

90

85

80

75

70

60 65

2 1

Conversion chart for radiators in one-pipe circuits.

Conversion chart for panel and section radiators in one-pipe circuits.Enter the current tflow and temperature drop and find the conver-sion factor, Fc.Multiply the heat requirement by Fc and select size of the radiatoraccording to the new value.

Example.Calculated heat requirement: 1.230 W.tflow : 82 oC, ∆t: 15 oC, 1Fc = 1,16 2Converted heat requirement: 1.230 x 1,16 = 1.427 W.

Formula for calculating Fc:

49,33 x ln

t1 - t2[ ]t1 - tr n

t2 - tr( ) nPanel radiator 1,28Section radiator 1,29Convector 1,3 - 1,33

F =

tflow oCFc

∆t oC




a

The control unit has to sense the room temperature to be able to control it.

No enclosure0%

Shelf withopening0%

Shelf close tothe wall10 -2%

Open frontedrecess with ashelf12 -6%

Encased withgrille in front> -15%

Encased withsmall grille infront. Notrecommended.> -30%

Acceptablecabinet.≈ -8 - 10%

Reduction of heat emission from radiators fixed in some type of enclosure

Radiation from a radiator depending on the treatment of thesurface.

Material Surface treatment Radiation %

Steel, cast iron 100

Oil paint 100

Aluminium orcopper bronzes 75

Zinc white 101

Lead white 99

Enamelled White 101

Matt green 96

Aluminium 8

10 - 100 mm 30 - 100 mmAlternativeopenings a+40

> 10

0 m

mSimpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com

167

0

100

200

300

400

0

20 40 60 80 100 120

80/89 65/76

50/6

40

32

25

20

10

15



Heat losses from uninsulated horizontal pipe.

For vertical pipe reduce by 20%

One-pipe above another reduce by 12%

Three pipes above each other reduce by 20%

Temperature above room temperature oC

Heat emissionW/m pipe DN/0




5 7 10 20 30 mmWG/m

,05 ,07 0,1 0,2 0,3 0,4 0,5

kPa/m

25

15

20

32

40

80

50

100

125

150

,01,015,02

,03,04,05

,07

,1

,15,2

,3

,5,4

,7

1

1,52

345

7

10

4050

1520

30

1,0

2,0

10

100

1000

10

100

1,52

345

7

1520

304050

70

150200

150200

300400500

700

70

5040

15 40 50

0,15

65

3,0

m/s 0

,2

0,3 0,4

0,5

k = 0,00003 mDensity = 1.000 kg/m3

Pressure drop in steel pipes for heating installations.

m3/h DN mm l/s

l/h


0,1

0,20,30,5

1,0

23

,01 ,02 ,03 ,05 0,1 ,2 ,3 ,5 1

12

64

3

2 3 4 5 107

169



∆p for ζ values at differnt rates.

Symbol Units Coefficient of resistance, ζ

Branch tee 1

Through tee 1

Elbow, smooth 0,2

Bend 1

The values for the coefficient of resis-tance for tees, elbows and bends.

The pressure drop is calculated from: ∆p = ζ 0,5 ρ ν2 ,

Recommended portion of pipe losses for different systems or part ofsystems.Type of system Unit Friction %Heating Small buildings 50 - 60

Large buildings 60 - 70Sub-stations Primary and secondary side 20 - 30Distribution pipe net work Primary side 80 - 90

ζ valuem/s

∆p kPa

Sizes of steel pipes for heating systems. Working pressure 1,0 MPa (10 bar)Nominal diameter External diameter Wall thickness Internal diametermm inch mm mm mm8 1/4 13,50 2,25 910 3/8 17,00 2,25 12,515 1/2 21,25 2,75 15,7520 3/4 26,75 2,75 21,2525 1 33,50 3,25 27,0032 1 1/4 42,25 3,25 35,7540 1 1/2 48,00 3,50 41,0050 2 60,00 3,50 53,0065 2 1/2 75,50 3,75 68,0080 3 88,50 4,00 80,50100 4 114,00 4,00 106,00125 5 140,00 4,50 131,00150 6 165,00 4,50 156,00


30

5070100

200300

5007001000

0,1 ,2 ,3 ,4 ,5 ,7 1,0 2 3 kPa

0,01 ,02 ,03 ,07 ,1 ,2 ,3 mWG

,001 ,002 ,004,006 0,01 ,02 ,03 Bar

,01

,02,03

,05,07,1

,2,3

,05

4 5 7 10 20

,7 1 2

,04 ,06 0,1 ,2

,5

15

2025



Flow chart for RTD-G 15, 20 and 25

RTD - G 15, 20 and 25

l/h l/sValve size

∆pva

lve


171



3

5710

2030

5070100

,001

,002,003

,005,007,01

,02,03

1 2 3 4 5 7 10 20 30 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 Bar

,1

,05,07

500300200

N

1

23

4567

3

5710

2030

5070100

,001

,002,003

,005,007,01

,02,03

1 2 3 4 5 7 10 20 30 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 Bar

,1

,05,07

500

300200

N

1

23

456

7

Flow chart for thermostatic valves in two-pipe system

l/h l/sPre-set value

∆pva

lve

Pre-set value 1 2 3 4 5 6 7 N

kv values 0,04 0,08 0,12 0,20 0,27 0,36 0,45 0,60

Pre-set value 1 2 3 4 5 6 7 N

kv values 0,10 0,15 0,17 0,25 0,32 0,41 0,62 0,83

l/h l/sPre-set value

∆pva

lve

RTD - N 15

RTD - N 20 - 25




0,1

0,20,30,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 80 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 Bar

520

8

,8

1,62,54,06,310

1

Flow chart for ∆p control valves for riser or circuit in heating systems.

ASV-P, PV 15-40 and ASV-M 15-40

m3/h l/skvs-value

∆pva

lve

Working range: ASV-P 10 kPaASV-PV 5 - 25 kPa.

Minimum available ∆p for good functioning: 8 kPa.

ExampleQ: 300 l/h. ∆p riser: 7kPa. ∆p radiator including valve: 5 kPa.∆p-control kv 1,6. ∆pvp = 3,4 kPa, 1Necessary ∆p = 7+5+8 = 20 kPa.


173



0,1

0,2

0,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 200 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 15 20 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 Bar

10

57

150

50

3020

100

200

2030

50

,4,631,01,62,54,06,3

1016254063

100145

Flow chart for control valves in heating systems.

m3/h l/skvs-value

∆pva

lve

Formulas.∆p : bar. Q: m3/h. kv = ; ∆p = ; Q = kv √ ∆p ;

Q√∆p

Qkv( )2

∆p : kPa. Q: l/h. kv = 0,01 ; ∆p = 0,01 ; Q = 100x kv √ ∆p ;Q

√∆pQkv( )2

∆p : kPa. Q: l/s. kv = 36 ; ∆p = 36 ; Q = √ ∆p ;Q

√∆pQkv( )2 kv

36

Q

Q

Q




0,1

0,2

0,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 200 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 15 20 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 Bar

10

57

150

50

3020

100

200

2030

50

,4,631,01,62,54,06,3

1016254063

100145

Flow chart for valves in district heating systems.

m3/h l/skvs-value

∆pva

lve


175



0,1

0,20,30,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 80 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 Bar

520

8

,8

1,62,54,06,310

1

m3/h l/skvs-value

∆pva

lve

AVP 15 - 32

Flow chart for ∆p control valves in district heating systems.




0,1

0,2

0,3

0,50,71,0

23

5710

0,1

0,20,3

0,50,71,0

23

,03

,05,07

1 2 3 4 5 7 10 20 30 40 60 100 200 kPa

0,1 0,2 ,3 ,4 ,5 ,7 1 2 3 4 5 7 10 15 20 mWG

0,01 ,02 ,04 ,06 0,1 ,2 ,3 ,4 ,5 ,7 1,0 1,5 2 Bar

10

57

150

50

3020

100

200

2030

50

,631,01,62,54,06,3

101625

5080

125

20

Flow chart for ∆p control valves in district heating systems.

m3/h l/skvs-value

∆pva

lve

IVD-IVFS kvs 0,63 - 25,0 m3/h

AFP kvs 50 - 125 m3/h

∆p-regulator, working range: IVD 5 - 50 and 20 - 250 kPa.AFP 20 - 120 and 50 - 250 kPa

Maximum ∆p valve IVF kvs: 0,63 and 1,0 = 1.000 kPa2,5 = 630 kPa4,0 - 25 = 800 kPa

Maximum ∆p valve AFP: 1.200 kPa


177



0

0,5

1,0

1,5

2,0

2,5

050100150200250300350400

1 10 50 100 150 200 250

Heat requirement for hot water according to the Swedish Board of District Heating

Domestic hot water, Q L/s. Effect, P kW

Number of apartments.




0,07

0,1

0,15

0,2

0,3

0,4

0,5

0,6

0,70,8

1,00,9

20 30 40 50 60 70 80m /h

3 ∆p kPav

0,2 0,3 0,4 0,5 0,6 0,7 0,8

∆

p Barv

1

2

3

4

5

8

6,5

0,2

0,3

0,4

0,5

0,7

2,0

1,5

1,00,90,8

0,6

20 30 40 50 60 70 80m /h

3 ∆p kPav

0,2 0,3 0,4 0,5 0,6 0,7 0,8

∆

p Barv

10

1214

2

4

6

8

Flow limiter, ASV-Q 15, Flow limiter, ASV-Q 20

Set values

Set values

ASV-Q Capacity l/h Set value

15 100 - 800 1 - 820 200 - 1400 2 - 1425 400 - 1600 4 - 1632 500 - 2500 5 - 30


179



2,0

1,71,5

1,2

1,00,90,80,7

0,6

0,5

0,4

30 40 50 60 70 80m /h

3 ∆p kPav

0,3 0,4 0,5 0,6 0,7 0,8

∆

p Barv

10

121416

4

6

8

4,0

3,0

2,0

1,5

1,2

1,00,90,80,70,6

0,5

0,4

m /h3

30 40 50 60 70 80∆p kPav

0,3 0,4 0,5 0,6 0,7 0,8

∆

p Barv

5

15

10

20

25

30

Flow limiter, ASV-Q 25, Flow limiter, ASV-Q 32

Set values

Set values




Calculation of one-pipe system

6

31 m6 m6 m6 m6 m

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

6 m 6 m 6 m

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1200

3 x

12 =

36

m

1,5 m

0

2345

23456

789

1

1010 m


181



Calculation of one-pipe system




Calculation of two-pipe system

12001200 1200 120012001200 1200 1200

1200 12001200 12001200 12001200 1200

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

3 m 6 m 6 m 6 m

123456

7

8

9

10

11

12

13

14

15

16

17

3 m

3 m

31 m


183



Calculation of two-pipe system




SI-units.Effect, P. Pressure, p.W kcal/h Pa kPa bar mWG1 0,85985 1 0,001 0,00001 0,0001

1,163 1 1.000 1 0,01 0,1

100.000 100 1 10

10.000 10 0,1 1

Flow, Q (ϕ). Temperature, t (θ).l/s m3/h Kelvin K Celsius oC1 3,6 0 -273,15

0,278 1 273,15 ± 0

373,15 100

Greek alphabet.

Α α Β β Γ γ ∆ δ Ε ε Ζ ζ Η η Θ θ Ι τ alfa beta gamma delta epsilon seta eta theta iota

Κ κ Λ λ Μ µ Ν ν Ξ ξ Ο ο Π π Ρ ρ Σ σkappa lamda my ny xi omikron pi ro sigma

Τ τ Υ υ ϑ ϕ Χ χ Ψ ψ Ω ωtau ypsilon phi chi psi omega

Physical properties for water.

Temperature Pressure Density Isobaric heatυ oC p kPa ρ kg/m3 capacitivity

cp J/ (kg x K)

0 - 999,84 421810 - 999,70 419220 - 998,205 418230 - 995,65 417840 - 992,2 417850 - 998,14 418160 - 983,21 418470 - 977,78 419080 - 971,80 419690 - 965,33 4205100 1,3 958,35 4216110 43,26 951,0 -120 98,54 943,1 4245130 170,11 934,8 -140 261,36 926,1 4287150 375,97 916,9 -


185




Documents

34974988 District Heating Danfoss