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Engineering ToolBox > Calculating Expansion Tanks
An expansion tank is required in a heating, cooling or air condition system to avoid an unacceptable increase of the system pressure during heat-up.
Expansion tanks are in general designed as
The net expansion of water can be expressed as
Vnet = (v1 / v0) - 1 (1)
Vnet = necessary expansion volume of water (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)
Required volume of open expansion tanks can be expressed as
Vet = k Vw [(v1 / v0) - 1] (1)
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)
An open expansion tank has the disadvantage of allowing air to enter the system via absorption in the water. In general it must be located in the top of the building where it also may be exposed to freezing.
Closed compression tanks can be designed as
The required volume of closed expansion tanks can be expressed as
Vet = k Vw [ ( v1 / v0 ) - 1 ] / [ ( pa / p0 ) - ( pa / p1 ) ] (2)
pa = atmospheric pressure - 14.7 (psia)
p0 = system initial pressure - cold pressure (psia)
p1 = system operating pressure - hot pressure (psia)
The required tank volume of diaphragm expansion tank can be calculated as
Vet = k Vw [ ( v1 / v0 ) - 1 ] / [ 1 - ( p0 / p1 ) ] (3)
The minimum volume of an open expansion tank for a system with 1000 gallons of water heated from 68oF to 176oF can be calculated as
Vet = 2 (safety factor) 1000 (Gallons) [(0.01651 (ft3/lb) / 0.01605 (ft3/lb) ) - 1]
= 57 (gallons)