Hot Water Expansion Tanks  Sizing
Required hot water expansion volume in open, closed and diaphragm tanks.
Expansion tanks are required in heating, cooling or air condition system to avoid unacceptable increases of system pressures when water expands during heatup.
Explosive Power of Superheated Water
 1 lb (0.45 kg) of nitroglycerin > 2 000 000 ft lb _{ f } (2 700 000 J)
 1 lb (0.45 kg) of water flashed into steam > 750 000 ft lb _{ f } (100 000 J)
Expansion tanks are in general designed as
 open tanks
 closed compression tanks
 diaphragm tanks
Net expansion volume of water when heated can be expressed as
V _{ net } = (v _{ 1 } / v _{ 0 } )  1 (1)
V _{ net } = expansion volume of water ( ft ^{ 3 } , m ^{ 3 } )
v _{ 0 } = specific volume of water at initial (cold) temperature (ft ^{ 3 } /lb, m ^{ 3 } /kg)
v _{ 1 } = specific volume of water at operating (hot) temperature (ft ^{ 3 } /lb, m ^{ 3 } /kg)
Open Expansion Tanks
Required volume of an open expansion tank can be expressed as
V _{ et } = k V _{ w } [(v _{ 1 } / v _{ 0 } )  1] (2)
V _{ et } = required expansion tank volume (gallon, liter)
k = safety factor (approximately 2 is common)
V _{ w } = water volume in the system (gallon, liter)
v _{ 0 } = specific volume of water at initial (cold) temperature (ft ^{ 3 } /lb, m ^{ 3 } /kg)
v _{ 1 } = specific volume of water at operating (hot) temperature (ft ^{ 3 } /lb, m ^{ 3 } /kg)
Note that in an open expansion tank fresh air is constantly absorbed in the water and tends to corrode the system. Open expansion tanks must also be located above the highest heating element, in general on the top of buildings, where they may be exposed to freezing.
Closed Compression Expansion Tanks
Closed compression tanks can be designed as
 adjustable expansion tanks  air is evacuated or injected by automatic valves to the tanks to control system pressures when temperature and expansion of water rises or falls
 pumppressured cushion tanks  water is evacuated or injected to the systems to compensate for temperature rise or fall
 compression tanks with closed gas volumes  the tanks contains specific volumes of gas which is compressed when temperatures and system volumes increase
Required volume in a closed expansion tank
V _{ et } = k V _{ w } [( v _{ 1 } / v _{ 0 } )  1] / [( p _{ a } / p _{ 0 } )  ( p _{ a } / p _{ 1 } )] (3)
where
p _{ a } = atmospheric pressure  14.7 (psia)
p _{ 0 } = system initial pressure  cold pressure (psia)
p _{ 1 } = system operating pressure  hot pressure (psia)
 initial temperature 50 ^{ o } F
 initial pressure 10 psig
 maximum operating pressure 30 psig
Diaphragm Expansion Tanks
Required volume in a diaphragm expansion tank
V _{ et } = k V _{ w } [( v _{ 1 } / v _{ 0 } )  1] / [1  ( p _{ 0 } / p _{ 1 } )] (4)
 initial temperature 50 ^{ o } F
 initial pressure 10 psig
 maximum operating pressure 30 psig
 safety factor aprox. 2
 acceptance factor aprox. 0.5
Example  Volume in Open Expansion Tank
A system with 1000 gallons of water is heated from 68 ^{ o } F to 176 ^{ o } F .
Minimum expansion volume in an open expansion tank with a safety factor 2 can be calculated as
V _{ et } = 2 (1000 gallons) [((0.01651 ft ^{ 3 } /lb) / (0.01605 ft ^{ 3 } /lb))  1]
= 57 (gallons)
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