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Expansion tanks are required in heating, cooling or air condition systems to avoid unacceptable increase of system pressures during heat-up.




Expansion tanks are in general designed as

open tanks
closed compression tanks
diaphragm tanks
Net expansion of water can be expressed as

Vnet = (v1 / v0) - 1        (1)

Vnet = necessary expansion volume of water

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)

Open Expansion Tanks

   

Required volume of an open expansion tank can be expressed as

Vet = k Vw [(v1 / v0) - 1]     (2)

Vet = required expansion tank volume (gallon, liter)

k = safety factor (approximately 2 is common)

Vw = water volume in the system (gallon, 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)

With an open expansion tank fresh air absorbed to the water tend to corrode the system. Open expansion tanks must be located above the highest heating element, in general on the top of buildings, where it may be exposed to freezing.



Closed Compression Expansion Tanks
Closed compression tanks can be designed as

adjustable expansion tank - air is evacuated or injected by an automatic valve to the tank to control the system pressure when the temperature and expansion of the water rise or falls
pump-pressured cushion tank - water is evacuated or injected to compensate temperature rice or fall
compression tank with a closed gas volume - the tank contains a specific volume of gas which is compressed when the temperature and system volume increase.



The required volume of closed expansion tanks can be expressed as

Vet = k Vw [ ( v1 / v0 ) - 1 ] / [ ( pa / p0 ) - ( pa / p1 ) ]        (3)

where

pa = atmospheric pressure - 14.7 (psia)

p0 = system initial pressure - cold pressure (psia)

p1 = system operating pressure - hot pressure (psia)



initial temperature 50oF
initial pressure 10 psig
maximum operating pressure 30 psig
Diaphragm Expansion Tanks
The required tank volume of diaphragm expansion tank can be calculated as

Vet = k Vw [ ( v1 / v0 ) - 1 ] / [ 1 - ( p0 / p1 ) ]        (4)




initial temperature 50 oF
initial pressure 10 psig
maximum operating pressure 30 psig
safety factor aprox. 2
acceptance factor aprox. 0.5
Example - Volume Open Expansion Tank
The minimum volume of an open expansion tank for a system with 1000 gallons of water heated from 68 oF to 176 oF can be calculated as

Vet = 2 (safety factor) (1000 Gallons) [( (0.01651 ft3/lb) / (0.01605 ft3/lb) ) - 1]

    = 57 (gallons)