air conditionning systems.pdf

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Thermal exchange guide

Air conditioning systems


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Themal exchange guide

Air conditioning systems

Chapter 1 : Definitions

Chapter 2 : All-air systems

Chapter 3 : All-water systems

Chapter 4 : Air-water systems

Chapter 5 : Direct-expansion systems

Chapter 6 : Computer room systems

Chapter 7 : Energy storage


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Page 1/1

Thermal exchange guideVersion 1.0

Preamble

Having been informed of the type of problem to be solved and after analysis of the consequences,the designer will try to outline the type of installation which will guarantee the best results.

In order to carry out the air handling installation, the designer has at his disposal some basic

elements to be used alone or in association with others.

However the choice or implementation of these various elements will take into account the

following factors :

Nature of the enquiry :

room cooling,

comfort air conditioning,

precision air conditioning

. Nature and use of spaces to be treated :

new or existing,

number and respective locations

Presence or absence of equipments which could be used :

Heating generator,

Cooling generator,

Distribution networks

Summer-winter thermal reports concerning spaces to be treated.

This appraisal determines the choice of a system.


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Air conditioning systems Page 1/37 Definitions

Air conditioning systemsVersion 1.0

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Contents

Introduction............................................................................................................... 3

Space cooling ........................................................................................................... 4

Definition..................................................................................................................... 4

Aim ............................................................................................................................. 4

Parameters taken into account ................................................................................... 4

Fields of application.................................................................................................... 4

Comment .................................................................................................................... 4

Comfort air conditioning.......................................................................................... 5

Definition..................................................................................................................... 5Aim ............................................................................................................................. 5



Air conditioning ........................................................................................................ 6

Definition..................................................................................................................... 6

Aim ............................................................................................................................. 6



Systems..................................................................................................................... 8Definition..................................................................................................................... 8

Potential system components..................................................................................... 9

System identification................................................................................................. 10

Method 1: Local systems .......................................................................................... 11

Method 1: Central systems ....................................................................................... 12

Heating and refrigeration equipment .................................................................... 13

Definition................................................................................................................... 13

Heating equipment.................................................................................................... 13

Refrigeration equipment............................................................................................ 16

Fresh-air conditioning............................................................................................ 21

Definition................................................................................................................... 21

Heat recovery units................................................................................................... 26


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Introduction

This chapter defines the various terms used in the industry to address the field of

air handling: space cooling,

comfort air conditioning,

air conditioning.

It also defines what is meant by a system and its components, and identifies the

criteria for categorising systems.

The types of heating and refrigeration equipment are also redefined.

Last but not least, it discusses fresh-air conditioning and examines the energy

savings afforded by various aspects of heat recovery.


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Space cooling

Definition Cooling is the simplified conditioning of ambient air in order to lower the

temperature by a few degrees.

Aim Provide a feeling of coolness in the summer.

Parameters takeninto account

Only temperature is taken into account. The system must maintain it two or

three degrees lower than the outdoor air (for calculations at least).

Note:

Relative humidity is not taken into account. Occupants may therefore

occasionally experience a feeling of discomfort.

Fields ofapplication

They remain rather varied in cases where cost outweighs technical aspects.

Comment This conditioning method must be used with precaution. It is not suitable forsites where significant amounts of moisture are released.


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Comfort air conditioning

Definition The term comfort air conditioning encompasses all the conditioning processes

applied to ambient air to obtain an indoor environment that is comfortableinterms of temperature and relative humidity.

Aim Obtain, all year round, conditions that are favourable to occupants, their healthand well-being or even improve their conditions.

Comfort can mean two things:

In homes, comfort targets the well-being of occupants,

In businesses, it targets:

improved worker productivity,

increased customer satisfaction.


Air conditioning takes into account the following parameters:

temperature,

relative humidity,

air cleanliness (impurities, odours),

noise level,

quality of diffusion (air motion, velocity).

Fields ofapplication

Comfort air conditioning applies to:

single-family and multi-family housing,

businesses (shops, offices, public buildings).


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Air conditioning

Definition Air conditioning encompasses all the processes applied to the air in a space to

obtain specific temperature and humidity levelsfor: an activity,

a process,

product storage conditions.

Aim Obtain specific, constantand reliable conditionsall year round for the activityor process being carried out in the space.

Example:

Abattoir cutting room:

temperature of +12C, dew point temperature: +4C to prevent moisture forming on carcasses

taken out of coolers at +4/+5C,

Operating theatre:

highly efficient filtration (HEPA filter), no return air,

temperatures of +20C to +22C (or even 18C),

relative humidity greater than 50% (static electricity).


The parameters vary by activity:

air cleanliness (impurities, odours),

temperature,

relative or absolute humidity,

drying (or dehumidification) capacity.

In most cases, more or less stringent requirements must be met to maintain these

parameters:

result accuracy:

temperature C C (e.g. 25C 1C),

humidity % % (50% RH 10%),

result stability:

over time,

in the space,

controlled atmosphere:

cleanrooms.


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Air conditioning (continued)

Fields of

application

Air conditioning is used in:

industries: microelectronics,

avionics,

optics,

clockmaking,

micromechanics,

automotive,

paints,

biology:

food processing: beverages, dairy products, meat products, pharmaceuticals and cosmetics,

biotechnology: research laboratories,

hospitals: operating theatres, sterile rooms,

high-tech industries:

fine chemicals,

space (miniaturisation),

packaging,

glassware,

plastics.


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Systems

Definition Whether used for comfort or process applications, the air conditioning system

is the backbone or flow chart connecting all the basic components used tosolve the problem at hand.

The components listed in detail on the following page are divided into four main

categories:

energy production,

air conditioning,

coolant supply (air, water, refrigerant),

air diffusion.

waterchiller

recycling

fan

chilled watercoil

filter

filter

energy recoverydevice

heating coilhot watergenerator

CIATCOOLERI T OOLER

humidifier

fan

In the example opposite:

Cooling and heating energy is generated by awater chiller and a water boiler,

Air, distributed by a system of ducts, is used asthe coolant,

The air is conditioned in an air handling unit(filter, recovery unit, cooling coil, heating coil,

humidifier, forced-draught fan),

The air flows out of ceiling registers,

And is drawn out of the space by the exhaust fan.


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Systems (continued)

Potential system

components

The potential components of a system are the basic elements:

hot water production: independent boiler,

exchanger (from a distribution system),

heat pump,

heat transfer cooling unit (heat recovery condenser),

chilled water production:

independent chiller,

exchanger (from a distribution system),

direct-expansion refrigeration:

packaged, split,

multisplit,

air conditioning:

air handling unit,

terminal units,

ductwork:

distribution,

return,

supply of fresh air,

removal of stale air,

air diffusion equipment:

supply registers,

return grilles.


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Systems (continued)

System

identification

Depending on the criteria used, there are two methods of categorising air

conditioningsystems (see note): Method 1:

The cooling of one or more spaces is the main criterion. It does not take into

account the type of system:

individual air conditioning, or local system,

shared air conditioning, or central system,

Method 2:

The main coolant is the main criterion.

If Then

the system is an all-AIR system air is the coolant

the system is an all-WATER system water is the coolant

the system is an AIR-WATER system both water and air are used

the system is a DIRECT EXPANSION system the refrigerant is the coolant

Note:

These two categorisation methods are the most common.


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Systems (continued)

Method 1:

Local systems

Definition Each space is served by self-contained equipment containing:

a refrigeration and/or heating unit,

air conditioning and diffusion components.

Generally, the equipment is either packagedor split, low or medium capacity,

and direct expansion.

INDOOR UNIT

HEAT EXCHANGER

REFRIGERANT/INDOOR AIR

EXPANSION VALVE

REFRIGERANT LINE

OUTDOOR UNIT

COMPRESSOR

HEAT EXCHANGER

REFRIGERANT/OUTDOOR AIR OR WATER

Use Local systems are generally used in existing single-family homes and smallbusinesses.


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Systems (continued)

Method 1: Central

systems

Definition Each space is supplied with airconditioned in a central unit containing:

an air handling unit (AHU),

heating equipment (+),

refrigerationequipment (-),

the necessary ductwork

Exhaust air

Fresh air

Hotwater

Chilledwater

Supply

Return

A H U

DuctworkAir handling unit

Water boiler

Water chiller

Use As old buildings were not designed with air conditioning in mind, this type ofsystem is found more often in new or recent buildings. A buildings design takes

into account the entire system, i.e. mechanical room and duct runs.


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Heating and refrigeration equipment

Definition The following equipment is needed in order for air to be conditioned:

water (coolant, humidifier),

electricity (to power the motors),

heating equipment,

refrigeration equipment.

This section will discuss local heating and refrigeration equipment.

Heatingequipment

The term encompasses a number of items of equipment:

boiler,

heat exchanger, heat pump,

transfer unit.

Boiler A self-contained vessel used to heat water. Electricity, gas or fuel oil may beused as a heating source. The temperature of the hot water can vary from 50 to

80C.

SUPPLY

BOILER

RETURN

SUPPLY

RETURN


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Heating and refrigeration equipment (continued)

Heating

equipment(continued)

Heat exchanger Actual production is centralised (district heating or the boiler room in a factoryor for a group of buildings),

A heat exchanger is used locally to raise the hot fluid to the desired temperature.

The operating temperature can vary from 50 to 80C.

EXCHANGER

SUPPLY

RETURN

DISTRICT HEATING


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Heating


Heat pump Heat pumps draw heat from a cold source and transmit it to a heat source.

The cold source may be:

Water: ground water, river, lake, sea, process cooling water,

Air: outside air, exhaust air (buildings, processes).

The temperature of the hot water (heat source) is between 40 and 55C.

SUPPLYRETURN

WATER - WATER

DISCHARGE

HEAT PUMP

SUPPLYRETURN

AIR - WATER

air

Transfer unit Transfer units are found on cooling systems that operate all year long (processcooling).

The refrigeration circuit may be equipped with an additional water-cooled

condenser known as a heat recovery condenser.

No heat is produced unless refrigeration is also produced.The temperature of

the hot water is between 40 and 50C.

SUPPLYRETURN

WATER-cooled heatrecovery condenser AIR-cooledcondenser

Evaporator


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Refrigeration

equipment

The term covers a number of items of equipment:

water chiller,

direct-expansion refrigeration unit,

heat exchanger,

cooling tower,

drycooler.

Water chiller A water chiller is a machine that cools water, which is used as a refrigerant.

Temperature of the chilled water:

5 to 12C depending on the operating conditions,

mean inlet/outlet temperature difference: 5C,

Freezing temperatures:

An antifreeze such as glycol or similar is added to the water.


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Refrigeration


Direct-expansion

refrigerationunit

The evaporator is removed from the central system and placed inside the air

handling unit, where it cools the air that will be supplied to each space.

Direct expansion coil

Thermostaticexpansion valve

Air flow

Water-cooled condenser(or air-cooled)

Motor compressor

Heat exchanger Actual production is centralised (district cooling or factory system).

More and more large cities are turning to chilled water district cooling systems.

In industry, heat exchangers are used in applications requiring water at

temperatures different from that in the distribution system.

EXCHANGER

SUPPLY

RETURN

DISTRICT HEATING


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Refrigeration


Cooling tower A cooling tower is a specific type of cooling device. It uses humidified ambientair to produce chilled water. The air is cooled to its wet bulb temperature.

Example: Air at a temperature of 30C with 40% RH is cooled to its wet bulb

temperature of +20C. It serves as a refrigerant.

There are two types of cooling tower:

Closed-circuittowers, in which heat is exchanged between air and water by asprayed heat exchange coil. The air and the water in the condenser circuit

do not enter into contact.


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Refrigeration


Cooling tower(continued)

Open-circuit towers, in which air and water come into direct contact. Dropsof water run along a distribution panover which air flows. The water in the

condenser circuit and the air are in direct contact with each other.

The diagram below shows a water chiller operating on an open-circuit cooling

tower.

Fan

Spray nozzle

Air inlet

Drain pan

Wet deck

Gutter

Droplet separator

Cleaning tap

Makeupwater

Bypassvalve

pump

Treated makeup water inlet

Overflow

Screen filter

Drain

Water-cooled condenser


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Refrigeration


Drycooler A drycooler is a type of cooling device that uses dry air as a refrigerant.

Example: When the air temperature is 30C with 40% RH, the temperature taken

into account is +30C (not +20C, as in the case of the cooling tower). Heat is

transferred from water to air by a dry heat exchanger.

As with a closed-circuit cooling tower, the water in the condenser circuit of a

drycooler never enters into contact with the air.

This eliminates the risk of spreading Legionellosis.


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Fresh-air conditioning

Definition Before discussing the subject of fresh-air handling, one should understand what

is meant by the following terms: pollution,

removal of stale air,

supply of fresh air,

fresh-air handling,

heat recovery units.

Pollution People, their health and activities, as well as processes are all sources ofpollution:

dust,

germs,

water vapour,

carbon dioxide, etc.

The concentration of contaminants that can endanger health and adversely affect

processes rises quickly in enclosed rooms.

Removal ofstale air

The concentration of contaminants must be reduced to a safe level. This is

accomplished by extracting all or part of the stale air.

Supply of freshair

The extracted air is replaced with air from the outside. This air is often referred

to as fresh air. This outside air is assumed to be of better quality than the

extracted air. This is not always the case however. This is why fresh air must be

conditioned before it is introduced into a space.


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Fresh-air conditioning (continued)

Definition

(continued)

Fresh-airconditioning

Fresh air is conditioned in a number of ways:

It is filtered to ensure the right level of cleanliness,

It is heated or cooled to ensure the right temperature,

It is either humidified or dehumidified to ensure the correct level of humidity.

These processes are energy consuming.

Fresh air

+32C

Stale air

discharge

Forced-draught fan

-10C

+20Cor

+25C

ExtractionfanFilters

Preheating or precooling coil

or both if need be

to

Winter : ambient +20Cair supply +30C

Summer : ambient +25Cair supply +15C

ENERGY-

CONSUMING

SYSTEM

Fresh air can be conditioned:

For specific environmental conditions. In this case it is not used to heat orcool the space,

For specific air flow conditions in a space. In this case a portion is usedfor heating and cooling. As the volume of fresh air is small, it is simply

an addition.


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Definition

(continued)

Heat recoveryunits

The phenomenon is most flagrant in winter. Fresh air drawn in from outdoors is

at low temperature (e.g. -10C) and stale air discharged outdoors is at high

temperature (+20C at comfort levels).

A heat recovery unit is a device used to transfer heat without any direct contact

between both types of air. More specifically, a portion of the heat in the exhaust

air is transferred to the supply air. This exchange of heat results in lower energy

consumption.

Exhaustair

Freshair

Outdoors

-10C/90%32C/40%

Air-conditionedspace

20C/50% (winter)25C/50% (summer)

Energy recoverysystem

A H U


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Air purification:

the No concept

The No concept is based on the combination of an adsorbent such as

activated carbon and photocatalysis.

Polluted air

mineralization byphotocatalysis

Contaminant retention Purified air

NEO operation principle

Contaminant removal

ACTIVATED

carbon

ULTRAVIOLETradiation

desorptionabsorption


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Air purification:

the No concept(continued)

The activated carbon [Navarri et al., 2001] absorbs large quantities of

contaminants along its surface (it has a specific surface area ofapprox. 1000 m/g). These contaminants are trapped by low-intensity

electrostatic forces, called van der Waals bonds, with interaction energies of

5 to 40 kJ/mol. The main drawback of this kind of filter is the saturation

point of activated carbon. Known as the breakthrough point, this threshold

is very difficult to predict in cases where concentrations and flow rates vary.

Once this point is reached, the filter can no longer achieve the desired

concentration efficiency. Titanium dioxide (TiO2) photocatalysis in the gas

phase [Nguyen, 2001] is a heterogeneous catalysis process in which the

solid catalyst is activated only by ultraviolet radiation. Under certain

conditions, the heterogeneous photocatalysis process is capable of

mineralising pollutants completely. It is split into five phases:

transfer of gaseous reagents to the photocatalytic surface,

adsorption of the gaseous reagents on the photocatalytic surface,

photochemical reaction between the adsorbed gaseous reagents and thephotocatalytic surface; mineralisation of organic compounds,

desorption of gaseous photocatalytic reaction products,

diffusion of the gaseous products off the photocatalytic surface.

The main drawback of photocatalysis used on its own is the low adsorption

capacity of the catalyst (titanium dioxide) which prevents it driving down

high pollution levels. Pollutants are thus only partially mineralised when

concentrations and/or flow rates are high. Combining a filter containing anadsorbent such as activated carbon with a photocatalysis system eliminates

the drawbacks inherent to each process and also significantly cuts down on

maintenance.


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Heat recovery

units

The term heat recovery unit, or HRU, covers various types of equipment:

dual-coil recovery units,

plate heat exchangers,

heat pipes,

heat recovery wheel.

Dual-coilrecovery units

As is implied by its name, a dual-coil recovery unit consists of two standard

finned coils connected by a circuit through which refrigerant (antifreeze if

necessary) is circulated by a circulator pump.

One coil is placed in the exhaust air circuit and the other in the supply air circuit.

The air and water circuits are arranged for counter-current circulation.

Safety valve

HEAT

RECOVERY

COIL

HEAT

RECOVERY

COIL

Circulator

+20C

50%

-10C90% Drain

Circuit

filling

Preheate

d

freshair

Exhausta

ir

Expansionvessel

Freshair

Cooled

exhaust

air

Drain valve


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Heat recovery

units (continued)

Dual-coilrecovery units

(continued)

This system allows for much flexibility, and the coils can be integrated inside the

air handling unit.

Water

Exhaustair

Freshair

Outdoors

AHU+/-

+/-

Return air

Supplyair

Air-conditionedspace

In more complex cases where an air extraction system cannot be installed in the

same space as a fresh-air conditioning system, both can be connected

hydraulically.

Forced-draughtfan

Extractionfan

Conditionedsupply air

Pump

Glycol/water heat transfer circuit

Airextrait

Freshair

Exhaustair

Water


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Heat recovery

units (continued)

Heat pipe A heat pipe is a metal tube usually with fins on its outside and containing a fluidin the form of two balanced phases:

a liquid phase,

a gas phase.

It transfers heat through a cycle of evaporation and condensation.

1. The fluid evaporates in the hot end (evaporator).

2. The vapour thus formed condenses on the cold end (condenser).

3. The fluid from the cold end returns to the hot end:

By gravity, in which case the condenser section must be above theevaporator section,

Via a capillary (or wick) structure lining the inside wall of the tube. Theheat pipe can also operate horizontally in this case; a slight angle of 7-8

will promote movement.

HORIZONTAL HEAT PIPE

Liquid

Vapor

Liquid

Evaporation section Condensation section

Capillary

structure

Extracted warmair enters

Warmed

air exits

Cooled airexits

Cool freshair enters(cold source)

Partition

Heat transfer

(hot source)

Heat

recovery


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Heat recovery

units (continued)

Heat pipe(continued)

Condensed

liquid

Heat transfer

(hot source)

Heat

recovery

Vapour

Capillary

structure

Extracted

warm air

enters

Cooled air

exits

Warmed

air exits

Cool fresh

air enters(cold source)

The pipes are arranged in arrays split into two by a sealed partition separating the

two streams of air.

The evaporation and condensation temperatures are highly similar and the

operation is virtually isothermal. The transfer occurs only through the latent heat

of the change in state.

gravity causes liquidto flow back down

condensation

vaporisation

vapour

Warmedair exists

Liquid

Extracted warmedair enters

C

ondensation

section

Evaporation

section

Heat

recovery

Cooledair exits

Cool freshair enters

(cold source)

Heat transfer

(hot source)

VERTICAL HEAT PIPE

Vapor


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Heat recovery

units (continued)

Heat pipe(continued)

partitionheat pipe

coolai

r

warm

air

warmedair

cooled

air

The figure below illustrates a constant-flow model of a ClimaCIAT dual-flow

AHU with heat pipes:

Exhaust air

Fresh air-10C

Exhaust air+20C

Air supply

The figure below illustrates a ClimaCIAT dual-flow AHU with variable-flow

heat pipes and a mixing box:

Freshair

-10C

Exhaustair

+20CExhaust

air

Airsupply


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Heat recovery

units (continued)

Plate heatexchanger

In this type of heat exchanger, horizontal and vertical streams of air flow

between thin plates stacked in parallel:

The exhaust air transfers its heat to the air flowing between the plates.

There are two types of flow arrangement:

cross-flow,

counter-flow.


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Heat recovery

units (continued)Plate heat

exchanger (continued)

Cross-flowexchanger

Counter-flowexchanger

Exhaustair

Fresh air

Exhaustair

Cooledexhaust

air

Inside

Outside

Fresh air

Preheatedfresh air

Platethickness

Gap

This figure illustrates a ClimaCIAT dual-flow AHU with a plate heat exchanger

but no mixing box:

constant flow of exhaust air,

constant flow of fresh air.

Fresh air-10C

Exhaustair

+20C

Exhaust air

Air supply


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Heat recovery

units (continued)

Plate heatexchanger

(continued)

The figure below illustrates a ClimaCIAT dual-flow AHU with a plate heat

exchanger and a mixing box. The flow of air over the plate heat exchanger can

be adjusted.

exhaust air+20C

exhaust air

fresh air-10C

supply air

Example of an air handling process: fresh air in with recovery of heat from

exhaust air by a plate exchanger.

Dual-flow AHU

exhaustair

AIR-TO-AIRheat exchanger

frost protection dampers freshair

supply

air

exhaust

air

R


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Heat recovery

units (continued)

Heat recoverywheel

A heat recovery wheel is a low-speed wheel (10-20 rpm) with many small

channels through which air passes. A little less than half the front surface is

connected to the exhaust air circuit. A little less than half the surface is

connected to the supply air circuit. A small surface is used as the purge sector.

As the wheel rotates, the section heated by the warm air gives up its heat to the

stream of cool air flowing through it.

The two air streams thus flow through the channels alternately.

motor

fresh

air

exh

aust

air

purge sector

drive belt


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Heat recovery

units (continued)

Heat recoverywheel

(continued)

Apurge sectoris usually built over the wheel to minimise carryover from the

exhaust air. To facilitate this, the pressure of the exhaust air must be lower than

that of the supply air.

direction of

rotation

purge sector

P4

P1

P1>P4 exhaust air

fresh air

duct exhaust air

exhaust air

supply air fresh air

seal

purge sector

purge stream

whell

direction ofrotation


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Heat recovery

units (continued)

Heat recoverywheel

(continued)

The figure below illustrates a ClimaCIAT dual-flow AHU with a rotary heat

exchanger without amixing boxbypass:

constant flow of exhaust air,

constant flow of fresh air.

fresh air-10C

exhaustair

exhaust air+20C

supplyair

The figure below illustrates a ClimaCIAT dual-flow AHU with a rotary heat

exchanger, mixing box and adjustable air flow:

fresh air-10C

supply air

exhaust air+20C

exhaust air


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Heat recovery

(continued)

Importantinformation

Choosing the right fresh-air conditioning method is part of the design process.

It depends on the type of air handling equipment that will be used:

When the air in a large number of spaces is to be handled by standardterminal units (e.g. fan coil units), fresh air should be handled completely by

one central unit,

When the air in a space is handled by an AHU, a single or double mixing boxmay be added to help to condition fresh air.


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Air conditioning systems Page 1/29 All-air systems


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Contents

Contents .................................................................................................................... 2

Basic concept ........................................................................................................... 3

Definition..................................................................................................................... 3

Air handling unit .......................................................................................................... 5

Possible solutions....................................................................................................... 6

Local air handling unit.............................................................................................. 7

Concept ...................................................................................................................... 7

Operation.................................................................................................................... 8

Mollier chart ................................................................................................................ 9

Single-zone air handling unit................................................................................. 10Concept .................................................................................................................... 10

Operation.................................................................................................................. 11

Mollier chart .............................................................................................................. 13

Field of application.................................................................................................... 14

Dual-duct AHU ........................................................................................................ 15

Concept .................................................................................................................... 15

Operation.................................................................................................................. 16

Mollier chart .............................................................................................................. 17


Multizone unit.......................................................................................................... 18

Concept .................................................................................................................... 18

Operation.................................................................................................................. 19

Mollier chart .............................................................................................................. 21


Air handling unit with variable-volume diffusion boxes for each space ........... 22

Concept .................................................................................................................... 22

Operation.................................................................................................................. 23


Impulsair ................................................................................................................... 25


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Basic concept

Definition Air is preconditioned by an air handling unit then supplied to a space via a duct.

All the heatneeded to cool a space is carried by air:

Air conditioning is generally centralised,

The mechanical room contains:

refrigeration equipment,

heating equipment,

an air handling unit in which air, including fresh air, is conditioned,

The ductwork contains:

return ducts running from each space,

stale air exhaust ducts,

fresh air supply ducts, supply ducts running to each space.


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Basic concept (continued)

Definition

(continued)

The drawing below illustrates the basic design of an all-air system.

exhaust air

fresh air

hotwater

chilledwater

supplyair

retrurnair

A.H.U.

ductworkair handling unit

water boiler

water chiller


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Air handling unit The flow of the air handled by the unit is determined by:

thermal loads,

the acceptable t between the supply and return air (see comfort airconditioning and air handling).

For obvious reasons of economy, the supply of fresh air is limited (unless

required otherwise for safety) to minimum healthy levels set out by regulations.

The type of handling needed is defined by the influencer, who then buildsthe air

handling unit to fit the needs of the space.

For example, the AHU below contains the following equipment:

1 return air fan,

2 exhaust air/recirculated air mixing

dampers,

3 prefilter,

4 cooling coil and heating coil,

5 humidifier,

6 forced-draught fan,

7 f inal filter.

exhaust air

return air

fresh air

1

2

3

supply air

45

6

7

In some cases (such as cleanrooms; see our document on filtration) the final filter

may be placed at the entrance to a space instead of inside the AHU to allow

for potential pollution from ductwork.


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Air handling unit

(continued)

The drawing below shows an air handling system with a heat recovery unit

installed on the fresh air/exhaust air circuits.

heat recovery unit on exhaust air circuit

exhaust

air

return air from

served spaces

supply airfresh

air

128

43 5 6 7

mixes exhaust air and

recirculated air

Possible solutions The solutions depend on the type of space and changes in loads, as well as

investment possibilities and the requirements for each space.

The most common solutions include:

individual space equipment: a separate AHU and ducts for each space,

multispace equipment: a unit with ducts for a series of spaces,

a single unit connected to two ducts, one for hot air and the other for cool air.A mixing box ensures the adequate mix of air for each space,

a multizone AHU with separate ducts for each space,

basic equipment (air handling unit) with, for each space, a variable-volume

air diffusion box.


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Local air handling unit

Concept Each space is supplied with conditioned air by its own specific unit:

constant flow of air,

fresh air (suction) is usually conditioned in the AHU which also extracts staleair,

the components (filter, cooling and heating coils, humidifier) are determinedbased on the requirements of each space.

Heating and refrigeration equipment is with the unit in the mechanical room or

outside (cooling system with air-cooled condenser).

exhaustair

Example of a unit with electric heating coil and chilled-water

cooling coil with temperature control only

freshair

volume = constantT = f (space requirement)

R


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Local air handling unit (continued)

Operation The unit adjusts conditions based on the space loads. It:

regulates the temperature (summer and winter),

regulates the humidity (winter),

and, in certain cases, dehumidifies the air (summer).

In the drawing below, regulation occurs with the following sequences:

Sensors Effect on

Temperature cooling coil (chilled water) (1)

heating coil (hot water) (2)

Relative humidity cooling coil (chilled water) (1)

humidifier (3)

Occupancy fresh air damper

Outdoor temperature free cooling (energy savings)

room

supply air

A

exhaustair

extraction air

T : Temperature sensorH : Humidiy sensorO : Occupancy meter

O

T

T

S

EFM C

H

freshair

(1) and (2) 2-way or 3-way modulating or on/off valve(3) 2-way on/off valve

(1) (2) (3)

R


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Local air handling unit (continued)

Mollier chart Changes in air temperature and humidity are plotted in the following manner on

what is called aMollier chart.

O

M

t

wq'

HM

space linesegment

summer operation

O : outdoor airI : indoor environmentM : mixing

S : supply airC : supply from cooling coil

H : supply from heating coil Ssummer line segment

Hwinter line segment

winter operation

S

I

I

SS

W

C

100%

O

H

Field ofapplication

This type of system is generally used for:

large-volume spaces,

low-tolerance environmental controls,

spaces with heavy load conditions that vary,

meeting rooms,

theatres and concert halls.


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Single-zone air handling unit

Concept Air is supplied to several spaces by a single air handling unit:

the air flow for each individual space is constant and calculated based on itsmaximum heat load,

fresh air (suction/extraction) is generally conditioned in the AHU beforebeing supplied to all the spaces,

the components (filter, cooling and heating coils, humidifier, etc.) aredetermined based on the needs of each space,

heating and refrigeration equipment is generally located in the mechanicalroom,

detection devices (temperature and humidity sensors) are arranged in acontrol room.

exhaust air

fresh air

hot

water

chille

dwater

supp

lyair

retur

nair

A.H.U.

ductworkair handlingunit

water boiler

water chiller

This type of system is also called a simplified all-air system.


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Single-zone air handling unit (continued)

Operation This system is used for spaces where the following loads are identical:

distribution of sensible heat and latent heat, changes and variations in the same direction and proportions,

The properties of the air supplied to each space are the same,

Fresh air is distributed based on the total load, not on the density of occupantsin the space.

The control devices and temperature and humidity sensors may be installed:

In a control room (e.g.

A1)

The other spaces (A2, A3etc.) are governed by the A1

law.

In the main return airsection

All the spaces (A1, A2, A3etc.) are governed by thelaw of averages.

Space conditions cannot be adjusted to personal preferences,

As a matter of fact, spaces rarely experience the same changes in heat.Deviations occur in environment parameters on account of the fact that the air

supply conditions are the same for all spaces. The system would not be an

adequate choice for precision air conditioning.


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Operation

(continued)

The figure below shows an installed single-zone air handling unit.

SA3

A2

A1

S


S

(1)(2)(3)

T

T H

exhaustair

fresh

air

R


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Mollier chart Changes in the air supplied to the spaces are plotted on the Mollier chart.

S

2

A3

A1=A

A2

3

1

t

wq'

This diagram does not show the various handling/conditioning processes the air

undergoes in the unit. It shows changes in the air in each space based onsensible and latent heat.

1, 2, and 3 are the line segments. A is the space setpoint. When the same

amount of air S is distributed to all the spaces, it is clear that the space

conditions A cannot be controlled when their loads are different,

the space line segments 1, 2, 3 are rarely completely identical,

all pass by the same supply air point.

The indoor environment A1, A2, A3, should have the same value A, but as the

loads are not really identical, the value of A is controlled only for the space

where the sensors are installed (i.e. space A1for which A1will equal A).


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Mollier chart

(continued)

To sum up:

The simplified all-air system is adequate only if the thermal loads in each zoneare identical and vary in the same direction and proportions,

The balance and stability of the air flows are deceptive for branched orlengthy systems.

fresh air

zone 1 zone 2 zone 3 zone 4

return air

duct

supply air

duct

no means ofregulation

supply air conditions are

the same for every zone

Field ofapplication

Because it is economic, the system may be used for spaces with identical thermal

loads and where conditions do not need to be adjusted to personal preferences.


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Dual-duct AHU

Concept This system offers the following features:

A singleair handling unit that is simplified for air preconditioning. It is madeup of:

a mixing box,

a preheating coil for winter,

The supply air duct leaving the AHU is split in two:

warm air duct with preheating coil and humidifier,

cool air duct with cooling coil,

Three ducts are routed to and from spaces:

cool air supply duct,

warm air supply duct,

extraction duct.

The unit is generally operated at high speed (V >6 m/s) for reasons of space.

Each space contains a motorised mixing box controlled by the space sensor.

The supply air is delivered at a constant rate but the proportions of cool and

warm air vary.

C

F

S

spacesensor


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Dual-duct AHU (continued)

Operation Contrary to what might be thought, the system is not particularly energy

consuming: Most often (in spring and autumn, amongst other periods) only one of the

ducts (heating or cooling) is supplied with air that has been conditioned

simply by mixing the outdoor air with the recirculated air,

When both ducts are needed, the hot water is delivered by the condenser in thewater chiller unit (the condenser then becomes a heat transfer unit).

Fresh air is introduced when the outdoor temperature drops below room

temperature and cooling is needed.

A3

S3

A2

A1

T

T

exhaustair

freshair

S2

T

S1

T

(1) and (2) 2-way modulating or on/off valve

(3) 2-way on/off valve

(2) (2)(1)(3) R


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Dual-duct AHU (continued)

Mollier chart Changes in air temperature and humidity are plotted on the Mollier chart.

O

M

H

C

t

w

q'

C

M

I

I

SS

W

C

E

S

O : outdoor airI : indoor air

M : mixingH : warm air stream properties

S : supply airC : cool air stream properties

Ssummer line segment Wwinter line segment

Field of

application

This system is advantageous for handling the air in spaces with heavy load

conditions that vary.

It is often used with laminar-flow ceilings:

business spaces,

exhibition halls,

upscale spaces: large glazed surfaces subject to varying loads.


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Multizone unit


The conditions of the supply air are adjusted by mixing the cool and warm air,

Both air types are mixed inside the unit and occupants in each space (or zone)choose the level they want,

Each space has its own separate supply air duct:

constant air flow,

variable supply air temperature,

The return air ducts may be shared.

The AHU has a different design.

recirculatedair

filter fan

thermostat

heating

coil

cooling coil

freshair

warm air

cool air

zone 3 (H)zone 2 (H)

zone 1 (H)

zone 3 (C)zone 2 (C)

zone 1 (C)

R

(1) and (2) 2-way or 3-way modulating or on/off valve

(1)

(2)outdoor sensor

R

R

outdoor sensor

The heating and cooling coils are installed in parallel (not in series) on the aircircuit. This creates a stream of warm air and a stream of cool air,

The fan is installed upline and blows air on the coils,

Two rows of dampers downline of the coils (one on the warm stream, theother on the cool stream) are used to mix warm air and cool air,

Vertical barrier walls are used to divide the free areas (i.e. volumes of air) andcreate zones inside the unit. The air flows at work are a function of the thermal

balances. A unit is typically split into no more than four or five zones.


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Multizone unit (continued)

Operation The figure below shows a multizone unit.

recirculated air

return air A

freshair

limit swich for : 3 rooms 3 zones 3 air supply ducts the return air duct may be shared

A3A2A1

T1

T2

T3

S1 S2 S3

T

exhaustair

M3

M2

M1

M3M2

M1

C

F

The sets of dampers are used to mix air in all types of proportions possible for each zone :Warmed air (H) mixed with conditioned air (M) by the mixing box in winterCooled air (C) mixed with conditioned air (M) by the mixing box in summer

M

(2)

(1)

(1) and (2) 2-way or 3-way modulating or on/off valve

R


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Operation

(continued)

the coils ensure constant cool-air and warm-air temperatures,

servomotors controlled by the space thermostat adjust the dampers foradequate air mixing:

a detection unit measures the outdoor air temperature and controls themixing box servomotor (free cooling),

limit switches (servomotors zone) also participate in regulating air flows.Closing all the zones during heating will cause the heating coil supply to

close.

Note:

as stated previously, the energy-efficient operation described below isfrequently observed:

only one coil (cooling C or heating H) is supplied and the air delivered bythe mixing box (M = mix of fresh air and recirculated air) is divided

between the supplied coil and the unsupplied one.

this gives a new mix:

C + M or H + M depending on the need,

when adjusted correctly, this type of system is relatively energy efficient,

by using refrigeration equipment such as a heat transfer unit, heat from thecondenser can be used in cases where both cooling and heating are necessary.


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Mollier chart Changes in the air supplied to a given space are plotted on the Mollier chart.

O

O

MH

M

C

t

w

q'

CC

S

W

S1

I1

S1

I1

O : outdoor airI : indoor airM : mixingH : supply from heating coilS : supply airC : supply from cooling coil

Ssummer line segment Hwinter line segment

Field ofapplication

The systems advantage lies in its quick responsiveness and the ability to adjust

spaces to individual needs:

Groups of spaces with load conditions that vary quickly,

Series of spaces, each with occupancy loads that are never the same or varyfrom zone to zone (this makes it possible to lower the total installed capacity).

It is therefore well-suited to spaces such as:

company canteens, dining halls, homes (day zone, night zone).


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Air handling unit with variable-volume diffusion boxes for eachspace


variations in thermal loads are handled by adjusting the flow of conditionedairsupplied to each space,

a single air handling unit for all spaces,

the AHU may be equipped with a flow control system (suction deflectors orspeed control),

it may be constant volume; if so, each space is equipped with a bypass.

variable-volumebox

variable-speedforced-draught fan

variable-speedreturn air fan

air handlingunit

return air

exhaust air fresh air

air outlet

return airgrille

System with variable volumes of air per zoneUnit equipped with a flow controller


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Air handling with variable-volume diffusion boxes for eachspace (continued)

Operation How the system operates:

air is regulated in the air handling unit to ensure a constant supply airtemperature. The AHU operates in either heating or cooling mode; differing

needs cannot be met,

the flow of supply air to each space is modulated by the space sensor based onthe heat load.

In the example below:

Air is delivered to Type A spaces by the variable air volume system; the returnair flows along passageways (halls),

Type B spaces are equipped with their own individual units. Stale air is drawnout by a separate extraction unit.

AB

T

T

exhaust air

return air

discharge

outside outsidehall

distribution ofconditioned air bypass

fresh air

T


(1)(2)(3)

window

fan coil unit with fresh air inlet

Constant-volume unit with

space bypasses

H

R


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Mollier chart Changes in the air in the AHU are plotted as follows on the Mollier chart.

O :outdoor airI : indoor airM : mixingS : supply air

Ssummer linesegment

O

M

t

w

q'

S S I

Air is conditioned in the conventional manner inside the unit. The regulation

system ensures that the temperature and humidity of the supply air are at

particular levels. However, the amount of air delivered to each space is adjusted

as needed.

Issues andprecautions foruse

Variations in flow in a given space should not disrupt the balance of the wholeor the flows delivered to the other spaces.

A system is needed to control and maintain the pressure in the distributioncircuits.

High variations and significant drops in flow rates may cause the geometry ofthe air stream introduced and its range to create uncomfortable conditions for

occupants.

As regards air change, a number of problems may arise:

The fresh air is conditioned entirely inside the unit.

Each space receives the required amount of fresh air under full load

conditions.

Losses (e.g. insulation) may cause heat load variations in the spacewhereas the occupant density (i.e. the need for fresh air) stays even. The

modulation of the flow of supply air in turn modulates the amount of fresh

air introduced.

Adjustingflow and pressure may be tricky; an adjusting device may need tobe installed to maintain the pressure inside the ducts at a constant level.

Installing ceiling registers specially designed to keep air streams horizontal isrecommended.


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Field ofapplication

This system, quite rate in France, may be used in:

office buildings,

hotels.

Impulsair The impulsair is a variable-air-volume adjustment system used in the 1970s.

The Coandaeffect

The operation of the impulsair is based on a well-known aerodynamic

phenomenon known as the Coanda effect, or the extremely odd tendency of air

to adhere to the surface along which it is flowing.

Henri Coanda discovered this phenomenon in 1928 when he noticed that hot

gases from his experimental jet-propelled aircraft hugged the sides of the

fuselage, damaging the tail unit.

On closer look at this phenomenon, it can be seen that a fluid will stay attached

to a surface, even a convex one, as is moves along it.

Coanda effect

stream of fluid


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Impulsair(continued)

Concept A stream of air directed towards a Y-duct can be deflected to either branch if twolateral openings A and B are placed just before the Y-duct.

Opening A and closing B will direct all the air to the right branch. Conversely,

closing A and opening B will direct all the air to the left branch.

A B A B

Variable-volume impulsair systems have the following main features:

constant supply air velocity resulting in even space temperature,

air flows are adjusted without the means of moving components such asdampers or motors (zero breakdowns or servicing),

constant volume in the air handling units, air is supplied to spaces intermittently. The supply time, not the volume

supplied, is modulated.


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Concept(continued)

If Then

Space cooling under full load is

required.

The control damper is maintained in a position set by the

thermostat while the main damper directs the entire volume of air

to the space.

The temperature in the space

decreases, as does the need for cool

air.

The thermostat controls the impulsair. The air from the unit is

directed to either the space or the recirculation duct. At the start,

the supply time is longer than the recycling time.

Cooling requirements continue to

decrease.

The impulsair responds by gradually shortening the supply time

and extends the recycling time.

suspended ceiling

diffuser

damper

damper

damper

return air

supply air

return air grille

ceiling

T

impulsair

Operation Note:

Although it is theoretically conceivable that the entire volume of air be

recycled and that no more air be directed to the space, this possibility should be

excluded in a well-designed system. Firstly, lighting and people are a basic

cooling load. By carefully selecting the supply air temperature, however, a high

rate of air can be obtained even if the heat gain is minimal.

What occurs between 100% supply air and 100% recycling is depicted in the

figure on the following page. The surfaces at the top show the air flows

delivered to the space. The surfaces at the bottom show the flow rates of

recirculated air.


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Operation(continued)

100% constant-volume air stream

time in seconds

spaces100

100

returnpercentageofveloc

ity

spaces100

250

100

return

p

ercentageofflow

2 HZ (max.frequency)

2 54 565248 504642 444036 3834323026 282420 221814 16128 10640

AIR VELOCITY

AIR FLOW


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The figure below shows an installed system with impulsair units.

return air grille

impulsair

diffuser

diffuser

forced-draught fan

return air fan

airhan

dlingunit


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Air conditioning systems Page 1/33 All-water systems


CCChhhaaapppttteeerrr333:::AAAllllll---wwwaaattteeerrrsssyyysssttteeemmmsss


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Contents

Contents .................................................................................................................... 2

Basic concept ........................................................................................................... 3

Definition of an all-water system ................................................................................................ 3

Possible solutions........................................................................................................................ 4

Fan coil units............................................................................................................. 5

Basic concept ............................................................................................................................... 5

Two-pipe fan coil unit .................................................................................................................. 7

Four-pipe fan coil unit.................................................................................................................. 9

Two-pipe, two-wire fan coil unit................................................................................................ 11

Air-change fan coil unit.............................................................................................................. 13Non self-contained air-handling terminal units.................................................... 15

Basic concept ............................................................................................................................. 15

Equipment type .......................................................................................................................... 15

Fan coil units .............................................................................................................................. 15

Mini air handling units ............................................................................................................... 20

Individual water-loop heat pumps......................................................................... 29

Basic concept ............................................................................................................................. 29

Advantages................................................................................................................................. 29

Heating and refrigeration equipment........................................................................................ 30

Heat pumps................................................................................................................................. 32

Field of application..................................................................................................................... 33


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Basic concept

Definition of an

all-water system

An all-water system offers the following features:

The space is equipped with a terminal unitsupplied with hot and/or coldwater by pipes (water loop).

The pipes transfer energy between the central plant(refrigeration andheating equipment) and the terminals.

All the energy needed to cool a space is carried by water.

Note: With this type of system, fresh air may be conditioned inside the terminal

unit or introduced directly in the space without being conditioned (see fan coil

unit, air change).


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Possible solutions The solutions generally depend on the type of space, as well as investment

possibilities and the requirements for each space.The most common solutions include:

A fan coil unit that can also be used in a number of versions (two-pipe,four-pipe, two-pipe/two-wire).

Cassette or mini non self-contained air-handling terminal units.

Individual water-loop heat pumps.

Each of these solutions will be discussed in detail in this chapter.


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Fan coil units

Basic concept A fan coil unit is a terminal unit with:

a heat exchange coil,

a fan motor,

a filter,

a condensate drain pan.

Fan coil units can be in cased or uncased configurations, vertical or horizontal,

and withor withoutfresh air inlet.

Ceiling-mounted

horizontaluncased fan

coil unit

Fresh air inlet with mixing dampers fitted in a suspended ceiling.


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Fan coil units (continued)

Basic concept

(continued)

Vertical casedfan coil unit

with fresh airinlet

Fresh air inlet with mixing dampers attached to a partition wall.

Horizontalcased fan coil

unit

An horizontal cased fan coil unit is also called a ceiling unit.


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Two-pipe fan coil

unit

Operation The unit has just one coil supplied by two pipes (supply and return).

The coil operates as follows:

The circuit is supplied with warm water for winter heating,

And with chilled water for summer cooling.

The term changeoverrefers to the changing from heating to cooling and vice

versa.

The output power is adjusted by: Turning the fan on and off on ultra-simple systems (for economical reasons).

Opening or closing an electric on/off valve on the water circuit.

Modulating valve that varies progressively the rate of water supplied to thecoil.


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Two-pipe fan coil

unit (continued)


Two-pipe fan coil units

coil :air/water

exchanger

fan

filter

central plant :hot water / cold water

Principle

Energyproduction

Heating and cooling energy is produced in a central plant consisting of:

water heating equipment (e.g. furnace or exchanger),

water chilling equipment (water chiller).

An air-to-water or water-to-water heat pump can also be used to produce hot

or chilled water depending on the season.

Field ofapplication

This system is used for divided spaces, such as offices and hotels, where needs

fluctuate but are of the same type.

It does not satisfy simultaneous heating and cooling needs; heating is provided

in the winter, and cooling in the summer.


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Four-pipe fan coil

unit

Operation The unit consists of two coils, one connected to a hot water loop, the other to acold water loop.

Both water loops are necessary. The flow of water is usually regulated by valves

(on/off or modulating) on each water circuit.

Energyproduction

Energy may be provided by a:

Water boiler and a water chiller.

Heat pump providing hot and cold water simultaneously. Transfer unit that recovers heat from the condenser during cooling.


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Four-pipe fan coil

unit (continued)

Field ofapplication

This system is used for offices, hotels and other spaces with significant heating

and cooling needs that fluctuate and are not of the same type.

It delivers heating and cooling at the same time.

or

Four-pipe fan coil units

heatingcoil

filter

coolingcoil

fan

Principle


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Fan coil units

Two-pipe, two-wirefan coil unit

Operation The unit has two coils: a two-pipe water coil and a two-wire electric heater.

Under conventional operation, the water coil is connected to the chilled water

circuit to provide cooling. There is no hot water circuit; the electric heater

serves as a heating battery.


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Fan coil units

Two-pipe,two-wire fan coilunit (continued)

Energyproduction

A water chiller is located in the mechanical room.

A reversible water chiller could be installed in order to have chilled water in the

summer and hot water in the winter. In this case the unit serves as a

low-temperature hot water coil and back-up electric heater in the winter, when

no cooling is needed.

or

Two-pipe, two-wire fan coil units

electricheater

filter

cooling orheating

coil

fan

Principle

Field ofapplication

This system is advantageous for heavily insulated modern buildings, such as

offices and hotels, where heating needs are reduced.


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Air-change fan coil

unit

Concept Although there are fan coil units with fresh air inlets, in most cases the air inspaces is changed by independent, central equipment:

single-flow ventilation,

dual-flow ventilation.

Single-flowventilation

Single-flow ventilation generally consists of using a roof ventilator (T) to expel a

portion of the air from a building with passageways and spaces containing

specific pollutants. Fresh air is introduced via inlets along the exterior walls.

Although inexpensive, this type of ventilation has the drawback of being unable

to adequately control air change in each separate room. Nothing guarantees that

fresh air will actually enter through the inlets and in the expected proportions. As

a result, some spaces could be under-ventilated while draughts could be created

in others.

Furthermore, the inlets along the outer walls also allow outside noise to enter

indoor spaces. This technique is therefore to be avoided in noisy environments.

It should also be avoidedin polluted environments.


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Air-change fan

coil unit(continued)

Dual-flowventilation

Dual-flow ventilation makes it possible to avoid these risks:

The desired amount of fresh air is filtered (and warmed in the winter) by an airhandling unit before being introduced into each space. This conditioned fresh

air is carried through ducts:

Directly to served spaces.

Or to the return air section of terminal units . This solution makes itpossible to limit the risks of occupants being bothered by air draughts. It

also allows fresh air to be warmed at lower temperatures (energy savings in

spring and autumn). On the other hand, the fans in the terminal units mustbe left on occupancy periods.

Stale indoor air is expelled outside by an exhaust fan.

Fresh air conditioning and air extraction may be accomplished inside a single

dual-flow air handling unit. A heat recovery unit, or HRU, can also be installed

to recover heat from exhaust air.

Comment: A chilled water coil can be added to cool incoming fresh air slightly

in the summer. The aim here is not to chill the indoor air, but rather to avoid

supplying fresh air at too high of a temperature in the summer (this is not a true

chilled dual-ventilation system, which is much more elaborate).


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Non self-contained air-handling terminal units

Basic concept These units have the same components as conventional fan coil units (heat

exchange coil, fan motor, filter, condensate drain pan), but are designedspecifically for installation in ceilings, suspended ceilings and raised floors.

Equipment type There are two types of such terminal units: fan coil units and mini air handlingunits.

Fan coil unitsare designed for installation in suspended ceilings. They bothsupply air to and recirculate the air in spaces directly. They have no available

pressure and cannot be connected to ductwork.

Mini air handling unitscan be installed in false ceilings or raised floors(uncased models) or directly in spaces (cased ceiling or vertical models).

They can be connected to a mini-duct system (available static pressure).

Fan coil units There are two models of cassette type fan coil unit:

Melody fan coil units, designed specifically for installation in the centre of aspace, far from walls.

Coadis fan coil units, designed for easy installation near walls.


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Non self-contained air-handling terminal units (continued)

Fan coil units

(continued)

Melody type fancoil units

The composite drawing below shows the components of a cassette type fan coil

unit:

6 5 111

7 3 2 14

A Melody cassette type fan coil unit discharges air la

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