6-Water System Pressurization [Compatibility Mode]

7/30/2019 6-Water System Pressurization [Compatibility Mode]

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WATER SYSTEM PRESSURIZATION

Dr. Ahmed Farouk Elsafty Associate Prof. – Mechanical Engineering Dept.

College of Engineering & Technology

Arab Academy for Science, Technology and Maritime Transport

AASTMT

E-mail: [email protected]



•

• Hydronic Accessories

• z ng xpans on an s

Dr. Ahmed Farouk Elsafty



Open Hydronic System, Cooling Tower Side




Closed Hydronic system




An expansion tank is required in a heating, cooling or air conditioning system to

avoid an unacceptable increase of the system pressure during heat-up.




•

expressed as

net 1 0 -

– Vnet = necessary expansion volume of water ,

– v0 = specific volume of water at initial (cold)

,– v1 = specific volume of water at operating


,



Net Ex ansion of Wate




• For an open water system, the maximumopera ng gauge pressure s e pressureat a specific point in the system where the

pumps, to overcome the pressure drops, ,

fittings, and pipes plus the static head dueto the vertical distance between the

highest water level and that point, is at amaximum.




• In a closed chilled or hot water system, a variation in the

may raise the water pressure above the maximumallowable pressure.

closed water system are as follows:

– To limit the pressure of the water system to below its allowable

– To maintain a pressure higher than the minimum water pressurerequired to vent air

–suction head (NPSH) at the pump suction to prevent cavitation

– To provide a point of known pressure in the system




• The expansion chamber (also called an expansion or

compress on an serves o a erma unc on an a

hydraulic function.• In its thermal function the tank provides a space into

w c e non-compress e qu can expan or romwhich it can contract as the liquid undergoes volumetric

changes with changes in temperature.• ,

tank provides an interface point between the system fluidand a compressible gas.

,interface; thus, a system designed to function as aclosed system can have only one expansion chamber.




• Ex ansion tanks ressure relief valves

pressure-reducing valves for makeup water, andcorresponding controls are used to achieve.

• There are three types of expansion tanks for

– Open

– Closed

– Diaphragm.




Expansion tanks are of three basiccon guratons:

volume of compressed air and water, withan air water interface (sometimes called aplain steel tank);

2. An o en tank (i.e., a tank o en to theatmosphere);

3. A diaphragm tank, in which a flexible

membrane is inserted between the airand the water.




• An expansion tank is a device that allows for the

expans on an con rac on o wa er con a ne n a c ose

water system when the water temperature changesbetween two predetermined limits.

• no er unc on o an expans on an s o prov e apoint of known pressure in a water system.

• An open expansion tank is vented to the atmosphere.

point of the water system. Makeup water is suppliedthrough a float valve, and an internal overflow drain is

.• A float valve is a globe or ball valve connected with a

float ball to regulate the makeup water flow according to




• An open expansion tank is often connected to the suction

side of the water um to revent the water ressure inthe system from dropping below the atmosphericpressure.

• The pressure of the liquid level in the open tank is equal tothe atmospheric pressure, which thus provides a referencepoint of known pressure to determine the water pressure

at any point in the water system.• e mn mum tan vo ume s ou e at east percent o

the volume of water in the systemVs, ft3 (m3).• An open expansion tank is simple, more stable in terms of

sys em pressure c arac ers cs, an ow n cos . sinstalled indoors, it often needs a high ceiling. If it isinstalled outdoors, water must be prevented from freezing


, ,the outdoor temperature is below 32°F (0°C).



• Because the water surface in the tank is

exposed to the atmosphere, oxygen is moreeasily absorbed into the water, which makes thetank less resistant to corrosion than a dia hra mtank. Because of these disadvantages, an open

expansion tank has only limited applications.

• An open expansion tank has the disadvantageof allowing air to enter the system via absorption

n t e water. n genera t must e ocate n t etop of the building where it also may be exposedto freezin .




• A closed expansion tank is an airtight tank filled with air orother gases.

• en e empera ure o e wa er ncreases, e wa er

volume expands. Excess water then enters the tank. Theair in the tank is compressed, which raises the system

. ,volume contracts, resulting in a reduction of the systempressure.

•prevent corrosion and prevent air noise, a diaphragm, or abladder, is often installed in the closed expansion tank tose arate the filled air and the water ermanentl . Such an

expansion tank is called a diaphragm, or bladderexpansion tank. Thus, a closed expansion tank is either aplain closed expansion tank, which does not have a


,



• For a water system with only one air-filled space, the

water system is a point of fixed pressure. At this point,water pressure remains constant whether or not the

depends on only the volume of water in the system. The

pressure at this point can be determined according to the, .

• The pressure in a closed expansion tank during the initialfilling process or at the minimum operating pressure is

called the fill pressure pfil, [psia]. The fill pressure is oftenused as the reference pressure to determine thepressure characteristics of a water system.




.




Diaphragm expansion tank in a chilled water system




Diaphragm expansion tank




The required tank volume of diaphragm expansion

• Vet = k Vw [ ( v1 / v0 ) - 1 ] / [ 1 - ( p0 / p1 )

– Vet = required expansion tank volume (gallon, liter)

– k = safety factor (approximately 2 is common)

– V = water volume in the s stem allon, liter

– v0 = specific volume of water at initial (cold) temperature (ft3/lb, m3/kg)

– v1 = specific volume of water at operating (hot) temperature (ft3/lb, m3/kg)

– = s stem initial ressure - cold ressure sia initial ressure 10 si

– p1 = system operating pressure - hot pressure (psia)




• The location of the pump in a water

system that uses a diaphragm expansiontank should be arranged so that thepressure a any po n n e wa er sys emis greater than the atmospheric pressure.

• n suc an arrangemen , a r oes no eainto the system, and the required net

maintained at the suction inlet of the water




Diaphragm expansion tank in a hot water system

• A water pump locationcommonly used for hotwater systems with

diaphragm expansiontanks is just after theexpansion tank andthe boiler.

• In this arran ementthe pressure at thepump suction is thesum of the water

pressure and the fillpressure.




Tank Pressure Related to “ S stem” Pressure

•

pressure point in the hydronic system.•

electrical system

•air in the tank plus or minus any fluidpressure due to the elevation difference

between the tank liquid surface and thepipe.




Tank Pressure Related to “ S stem” Pressure

=




• In a closed re-circulated water s stem, air and

nitrogen are present in the following forms:– dissolved in water,– ,– pockets of air or gas.

• The behaviour of air or gas dissolved in liquids isgoverne an escr e y enrys equa on.• Henry’s equation states that the amount of gas

dissolved in a li uid at constant tem erature is

directly proportional to the absolute pressure of that gas acting on the liquid,




Penalties due to Presence of Air and Gas

• The presence of air and gas in a water system causesthe following penalties for a closed water system with aplain closed expansion tank:

– Presence of air in the terminal and heat exchanger, whichreduces the heat-transfer surface

– Corrosion due to the oxygen reacting with the pipes

– Water logging in plain closed expansion tanks

– Unstable system pressure– Poor pump performance due to gas bubbles

– Noise problems

– There are two sources of air and gas in a water system. One is

the ai -water interface in a plain closed expansion tank or in anopen expansion tank, and the other is the dissolved air in a citywater supply.




• Sizing the tank is the primary thermal consideration in incorporating

.

• However, prior to sizing the tank, the control or elimination of airmust be considered.• The amount of air that will be absorbed and can be held in solution

with the water is expressed by Henry’s equation (Pompei 1981):




• ’

Versus Temperaturefor Air and Water

(Coad 1980a)




•

VersusTemperature

and Pressure

for Air/Water o u ons

(Coad 1980a)




• Manual or Automatic Air Vent




Air EliminationSmaller Boiler System Using Dip tube and Tanks Connection




Com ression Tank Pi in with Air Se arator




Dia hra m Tank Pi in with air Se arator




Important Accessories for system operation:

ompress on an ppng w a r epara o

• Relief valve: protects for

• Pressure Reducing Valve(PRV) is set to provideminimum system pressure

maintenance

• A manual three-way valve

permits draining excesstank

• A glass sight gauge: indicatewater level.

•

blow down valve to draincollected sediment to sewer.




Expansion tank Location, Primary-Secondary system

• In primary-secondary or compound pumping systems, the tank should onlybe at one location, preferably on the suction side of the source pump.




Effect of Ex ansion Tank Location with Res ect to Pum Pressure




Effect of Expansion tank location with respect to Pump Discharge pressure




Sizin for closed tanks with air/water interface




Sizin For o en tanks with air/water interface


Si f Di h E i



Size of Diaphragm Expansion

Tank•

of air is too small, the system pressure will easilyexceed the maximum allowable pressure andcause water to discharge from the pressurerelief valve, thus wasting water. If the closedan s oo arge, w en e wa er empera uredrops, the system pressure may decrease to a

cause trouble in the air vent. Therefore, accuratesizin of a closed ex ansion tank is essential.




• ,

minimum volume of the water tank, Vt, gal,

formula, recommended by ASHRAE

,Equipment:




Size of Dia hra m Ex ansion Tank




• ,

temperature T2 is the highest anticipated

water system shuts down during summer.

is often the ambient temperature at fill

° °, .




•

system that will be operated at a designtem erature ran e of 80 to 104°C.

• The minimum pressure at the tank is 170

kPa and the maximum ressure is 270kPa.

• (Atmospheric pressure is 101 kPa.) The

volume of water is 10 m3. The piping issteel.




.

tank with an air/water interface.




.

lieu of the plain steel tank, what tank size




cm




cm


Flow Chart for Sizing Expansion Tank



Flow Chart for Sizing Expansion Tank

Phase 1:Review Piping Drawing

pec y ccessresfor Air/WaterControl

Phase 2:Determine OperationConditions

Calculate TankSize Phase 4:

Determine Max/MinPressures

Selection of Expansion

Tank Type (Closed orDiaphragm)


6-Water System Pressurization [Compatibility Mode]

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