The sizing of compression tanks for closed loop hydronic systems can be calculated by using one of these two formulas. The high and low pressures are going to be determined by two factors. The pressures at the tank connection are selected as follows; The low pressure at the tank connection, is selected to hold a positive pressure at the highest point in the system usually about 10 psig, when the system is cold(ie;50°F)in the case of a heating system. The high pressure is usually set by the maximum pressure allowable at the safety relief valve(s) without opening them, at operating temerature. In the case of a chilled water system the reverse will be used. The high pressure will be when the system is off(ie;90°F), and the low pressure will be when the system is at operating temperature. The formula used will depend on the type of tank you are selecting.
For a closed tank with an air/water interface use;
For a diaphram tank use;
Vt=volume of tank,gal. Vs=volume of water in system,gal. Δt=(t2- t1),°F t1=lower temperature°F t2=higher temperature°F Pa=atmospheric pressure psia P1=pressure at lower temperature, psia P2=pressure at higher temperature, psia v1=specific volume of water at lower temperature ft3/lb v2=specific volume of water at higher temperature ft3/lb σ=linear coefficent of thermal expansion,in/in* °F =6.5 x 10-6in/in*°F Copper =9.5 x 10-6in/in*°F Steel
These are the methods used in the ASHRAE Systems Handbook