2-Water Systems [Compatibility Mode]

7/30/2019 2-Water Systems [Compatibility Mode]

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WATER SYSTEMS

Dr. Ahmed Farouk Elsaft

Associate Prof. – Mechanical Engineering Dept.College of Engineering & Technology

Arab Academy for Science, Technology and MaritimeTransport – AASTMT

E-mail: [email protected]



The conventional chilled-water system

following primary components:• ater c ers

• Load terminals(chilled-water cooling coils in comfort-cooling applications)

• Cooling towers in water-cooleds stems

• Chilled- and condenser-waterpumps

• e - an con ense -wa erdistribution systems that include

piping, an expansion tank, control

valves, check valves, strainers,and so forth.

Dr. Ahmed F. ELsafty



• Water systems that are part of an air

conditionin s stem and that link thecentral plant, chiller / boiler, air-handling

,classified into the following categories




. .

• In a chilled water system, water is first cooled in the water

chiller—the evaporator of a reciprocating, screw, or centrifugalrefri eration s stem located in a centralized lant—to atemperature of 40 to 50°F (4.4 to 10.0°C). It is then pumped tothe water cooling coils in AHUs and terminals in which air iscooled and dehumidified. After flowing through the coils, the

° .

18.3°C) and then returns to the chiller

hydronic air conditioning systems. When the operatingtemperature is below 38°F (3.3°C), inhibited glycols, such aseth lene l col or ro lene l col, ma be added to water tocreate an aqueous solution with a lower freezing point.




Freezing point of aqueous

solutions

5045403530252015105% ethylenel col

by volume

–33.3 –26.7 –20.6 –16.1 –12.8 –8.9 –6.7 –3.9 –2.2 –1.1Freezing point

°C

1.0401.0371.0321.0281.0241.0201.0171.0121.0061.004Specific gravityd15.6°

• Table obtained from Lange's Handbook of Chemistry , 10th ed.

Specific gravity is referenced to water at 15.6°C.







• Ethylene glycol and propylene glycol normally include an

.checked each year using a suitable refractometer to

determine glycol concentration.• Certain precautions regarding the use of inhibited

ethylene glycol solutions should be taken to extend theirservice life and to reserve e ui ment:

1. Before injecting the glycol solution, thoroughly clean and flushthe system.2. Use waters that are soft and low in chloride and sul hate ions to

prepare the solution whenever possible.3. Limit the maximum operating temperature to 120°C in a closed

hydronic system. In a heat exchanger, limit glycol filmempera ures o o s eam pressures a or essto prevent deterioration of the solution.

4. Check the concentration of inhibitor periodically, following


.



2. Evaporative-Cooled Water System.

• In arid and dry areas, evaporative cooled water

is often roduced b an eva orative cooler tocool the air.

3. Hot Water Systems.

• ese sys ems use o wa er a empera ures

between 450 and 150°F (232 and 66°C) forspace an process ea ng purposes.




4. Dual-Temperature Water System.

• n a ua - empera ure wa er sys em, c e wa er or owater is supplied to the coils in AHUs and terminals and

the following two distribution systems:– Use supply and return main and branch pipes separately.

– se e common suppy an re urn ma ns, ranc ppe, an cofor hot and chilled water supply and return.

– The changeover from chilled water to hot water and vice versa ina building or a system depends mainly on the spacerequirements and the temperature of outdoor air. Hot water is

often produced by a boiler; sometimes it comes from a heatrecovery sys em.




5. Condenser Water System.

• n a con enser wa er or coo ng wa er sys em, e a enheat of condensation is removed from the refrigerant in

• This condenser water either is from the cooling tower oris surface water taken from a lake, river, sea, or well.

• For an absorption refrigeration system, heat is alsoremoved from the solution by cooling water in the

.depends mainly on the local climate.




• Flow Generation (Forced Flow or Gravity

Flow)• Temperature (Low, Medium and High)

,

• Piping Arrangement

• Pumping Arrangement




Hydronic systems may be divided into several

general piping arrangement categories:

• Single or One-pipe• wo pe s eam, rec e urn or

Reverse Return)

• Three Pipe• Four Pi e

• Series Loop




Water systems also can be classified

accordin to their o eratin characteristicsinto the following categories:

– .

– Open System.

– Once-Through System.




• In a closed system, chilled or hot water flowing through the coils,, , ,

recirculating loop.

• In a closed system, water is not exposed to the atmosphere durings owng process. e purpose o rec rcua on s o save wa er an

energy.




• In an open system, the water is exposed to the atmosphere, For example, chilled

washer, and condenser water is exposed to atmosphere air in the cooling tower.Recirculation of water is used to save water and energy.

• Open systems need more water treatments than closed systems because dust and.

quantity of makeup water is required in open systems to compensate for evaporation,drift carryover, or blow-down operation.




- .• In a once-through system, water flows through the heat exchanger

ony once an oes no rec rcua e.• Lake, river, well, or seawater used as condenser

• cooling water represents a once-through system. Although the

temperature after absorbing the heat of condensation, it can still beused for other purposes, such as flushing water in a plumbings stemafter the necessar water treatments to conserve water. In

many locations, the law requires that well water be pumped backinto the ground.




Volume Flow and Temperature

Difference




governed mainly by pipe erosion, noise

and water hammer.

• Generally, the pressure drop for waterpipes inside buildings is in a range of 1ft/100 ft to 4 ft/100 ft 100 to 400 Pa/m

with a mean of 2.5 ft/100 ft (250 Pa/m)used most often.

• Because of a lower increase ininstallation cost for smaller-diameter

pipes, it may be best to use a pressure.Pa/m) when the pipe diameter is 2 in.or less.







'

Head loss to friction (m)=hf

Friction factor (dimensionless)=f

Length=L

Flow velocity (m/s)=u

Gravitational constant (9.81 m/s2)=g

Pipe diameter (m)=D




Friction factor (Relates to the SI Moody chart)=f Absolute pipe roughness (m)=ks

Pipe bore (m)=D

Reynolds number (dimensionless)=Re




' '




Reynolds number=Re

1000 kg/m3=Density of water= ρ

0.71 m/s=Velocity of water=u

0.15 m=Pi e diamete=D

1.138 x 10-3 kg/m s (from steamtables)

=Dynamic viscosity of water (at15°C)

=μ




Friction chart for water in steel

pipes (Schedule 40).

ow a e

Pipe Diam

Water velocit


Head loss



• ,

(ft/100 ft), water velocity v w

(ft/s), and water pipe

diameter D (in.).• Given any two of these parameters, the other two can be

determined. For instance, for a steel water pipe that hasa water volume flow of 1000 m if the ressure dro is

2 ft 100ft, the diameter is 8 in. and the correspondingvelocity is about 8 ft/s.

more than 4 ft/ s (1.2 m/s) for water pipes 2 in. (50 mm)or less in diameter in order to prevent an excessive Hf .

Pa/m) for water pipes of greater than 2-in. (50-mm)diameter.




Friction chart for water in steel


ow a e

Pipe Diam

Water velocit


Head loss



Friction chart for water in plastic





Friction chart for water in copper

tubing (types K, L, and M).




Exam le• Determine the pipe size for a circuit requiring 1.25 L/s flow.• Solution:

. ,400 Pa/m), and select 40 mm.

• Velocity is 1 m/s and pressure loss is 300 Pa/m.




' '




Relation Between Friction Factor and Reynolds

Number (Moody 1944)




• Pipes, valves, fittings, etc. will be more

ex ensive than necessar .• Higher installation costs will be incurred,

, , .

• For steam pipes a greater volume of condensate will be formed due to the

Wet steam is delivered to the point of use.




• A lower pressure may only be available at

the oint of use. This ma hinderequipment performance due to only lower

.

• There is a risk of water starvation.

• There is a greater risk of errosion,

inherent increase in water velocity.


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2-Water Systems [Compatibility Mode]