Enter system volume, temperatures and pressures. The page computes how much the liquid grows when heated, the fill pressure and the maximum allowed by the relief valve, and the volume of a diaphragm or plain tank.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Same expansion and fill pressure; only the maximum pressure allowed at the tank changes.
Liquid volume increase net of pipe expansion, as % of system volume.
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In a closed loop, water grows in volume as it heats and, since it does not compress, pressure would rise until the relief valve opens. The tank stores that extra volume against a cushion of compressed air or gas and keeps pressure between fill and maximum. It applies to heating, chilled water and glycol loops.
First the net expansion: Va = Vs × [(v2/v1 − 1) − 3α·Δt], which in the example gives 22.1 gal for an 800 gal system from 50 to 180 °F. Then divide by the diaphragm tank acceptance factor, 1 − P1/P2 in absolute pressure: with 25.7 psig fill and 45 psig maximum, 32%. The tank needs 68 gal, and the next standard size is 86 gal.
In a diaphragm tank, precharge should equal the fill pressure at the tank location: the static height of water above it (0.433 psi per foot) plus the minimum pressure wanted at the high point, usually 4 to 7 psi. In the example, 50 ft of height gives 25.7 psig. Set the precharge with the tank isolated and empty of water, before filling the system.
Add equipment contents from the data sheets (boiler, chiller, exchangers, buffer tanks, coils) to the pipe volume: about 0.66 gal per foot of 4" pipe and 1.50 gal/ft of 6". On an existing system, a meter on the fill line gives a measured value. When in doubt, round up.
In a plain tank the air starts at atmospheric pressure, and much of the volume only serves to be compressed up to fill pressure. In the example the same duty needs 188 gal plain versus 68 gal with a diaphragm. Plain tanks also lose air into the water over time and need the cushion recharged.
The maximum allowed pressure: in the example, 14.5 psi more cuts the tank from 68 gal to 49 gal. It is limited by the relief valve and the weakest equipment, so only raise the setting if everything is rated for it. Placing the tank higher in the system also lowers fill pressure and size.
Leaving the factory precharge different from the fill pressure, which robs the tank of capacity; underestimating maximum temperature; and forgetting that glycol expands differently from water. The tank does not replace the relief valve, which is still required, and larger tanks are pressure vessels subject to ASME and state boiler and pressure vessel rules.
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