How this calculation works
A receiver stores usable air because its pressure can fall through an allowed band. Its geometric litre capacity is not the same as the number of reference-state litres released across that band. The pressure difference controls the usable fraction.
Usable reference litres = receiver litres × (high gauge pressure − low gauge pressure)/reference absolute pressure; minutes = usable litres/(demand − supply)A worked example
A 1000 L receiver moving from eight to six bar gauge releases 2000 reference litres when the reference pressure is one bar absolute and temperatures match. A 1000 L/min demand with 500 L/min concurrent supply removes 500 reference litres per minute, so the ideal band lasts four minutes. Raising concurrent supply to match demand removes the finite drawdown in this balance.
Good to know
Isothermal ideal gas and constant reference flows only; no adiabatic discharge, vessel strength, regulator dynamics or compressor control approval. Pressure limits must come from the actual rated installation. The calculator neither recommends pressure settings nor certifies a receiver for them.
Reference: Compressed Air Challenge — compressed air storage (opens in a new tab) ↗
What the model answers
A receiver stores usable air because its pressure can fall through an allowed band. Its geometric litre capacity is not the same as the number of reference-state litres released across that band. The pressure difference controls the usable fraction.
Concurrent compressor supply reduces net withdrawal. This distinguishes the calculation from assuming the vessel alone supplies the entire load, while still keeping supply and demand fixed during the modeled interval.
The Pump available NPSH calculator is another case where pressure-reference conventions matter. Its liquid-reservoir surface and vapor pressures must both be absolute; a gauge difference is not interchangeable with them.
Measure and enter consistently
High and low receiver pressures are gauge readings on the same reference datum. Their difference is unchanged by adding atmospheric pressure to both. The free-air reference pressure is absolute and is used in the storage conversion.
Receiver and reference air are assumed at equal temperature. Both supply and demand must be stated at that same reference condition; actual compressed-volume flow cannot be mixed with reference-state flow.
Interpret the result
When demand exceeds supply, stored air is consumed at their difference and a finite drawdown time is returned. Equal supply and demand produce no pressure decline in this ideal steady balance. Greater supply would increase pressure rather than drain the band.
The model is isothermal. Fast discharge can cool the vessel contents, and regulators, pressure-dependent equipment demand and compressor controls can change the real trajectory. The result does not describe those transients.
Work through the numbers
A 1000 L receiver moving from eight to six bar gauge releases 2000 reference litres when the reference pressure is one bar absolute and temperatures match.
A 1000 L/min demand with 500 L/min concurrent supply removes 500 reference litres per minute, so the ideal band lasts four minutes. Raising concurrent supply to match demand removes the finite drawdown in this balance.
Where this model stops
Isothermal ideal gas and constant reference flows only; no adiabatic discharge, vessel strength, regulator dynamics or compressor control approval.
Pressure limits must come from the actual rated installation. The calculator neither recommends pressure settings nor certifies a receiver for them.
A few questions, answered
Why is the receiver input gauge but reference pressure absolute?
The stored amount uses a pressure difference, for which a shared gauge offset cancels. The gas-state reference itself requires absolute pressure.
What if supply equals demand?
Net withdrawal is zero, so this ideal model has no pressure-band drawdown.