Calculating Expansion Tanks

Calculate expansion volume in open, closed and diaphragm expansion tanks

An expansion tank is necessary 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 open, closed or diaphragm tanks.

Open Expansion Tanks

The necessary volume of an open expansion tank can be expressed as

V_et = 2 V_w [(v₁ / v₀) - 1] (1)

V_et = necessary expansion tank volume (Gallons)

V_w = water volume in the system (Gallons)

v₀ = specific volume of water at initial (cold) temperature (ft³/lb)

v₁ = specific volume of water at operating (hot) temperature (ft³/lb)

Closed Expansion Tanks

The necessary volume of a closed expansion tank can be expressed as

V_et = 2 V_w [ ( v₁ / v₀ ) - 1 ] / [ ( p_a / p₀ ) - ( p_a / p₁ ) ] (2)

p_a = atmospheric pressure - 14.7 (psia)

p₀ = system initial pressure - cold pressure (psia)

p₁ = system operating pressure - hot pressure (psia)

Diaphragm Expansion Tanks

The necessary volume of a diaphragm expansion tank can be expressed as

V_et = 2 V_w [ ( v₁ / v₀ ) - 1 ] / [ 1 - ( p₀ / p₁ ) ] (3)

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

V_et = 2 1000 (Gallons) [(0.01651 (ft³/lb) / 0.01605 (ft³/lb) ) - 1]

= 57 (gallons)

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