Compressed Air Potential Energy Calculator

Compressed air energy guide

1) Why model choice changes energy

For the same pressure and volume, the predicted work depends on the path of expansion. Isothermal assumes perfect heat transfer, adiabatic assumes none, and polytropic sits between. Many shop systems behave close to n = 1.1–1.3 during discharge, especially with metal tanks and moderate flow.

2) Typical storage pressures and tanks

Small compressors commonly fill receivers to 6–10 bar gauge (about 7–11 bar absolute at 1 atm ambient). Portable tanks often range from 10–50 L. Industrial receivers can be 0.5–5 m³ or larger. Enter gauge pressure for P₂ if your gauge reads “bar(g)” or “psi(g)”.

3) Energy scale example (quick check)

A 50 L receiver at 8 bar gauge has P₂ ≈ 9 bar absolute. Under isothermal expansion to 1 atm, the energy is on the order of 70–90 kJ, depending on exact ambient pressure. That is roughly 20–25 Wh, which helps validate whether outputs look reasonable.

4) Adiabatic behavior and γ

Dry air has γ ≈ 1.4. Adiabatic work is usually lower than isothermal for the same endpoints because temperature drops during expansion. If you see unexpectedly low energy, confirm you did not select adiabatic by accident, and verify your γ is greater than 1.0.

5) Polytropic exponent n as a practical knob

Polytropic modeling is useful when discharge is neither perfectly slow nor perfectly insulated. Using n = 1.2 often approximates real tanks. If n approaches 1.0, results approach isothermal. If n approaches γ, results approach adiabatic. Use n to match test measurements.

6) Why gauge vs absolute matters

Work formulas require absolute pressure. If you enter 8 bar as absolute when it is actually gauge, you understate P₂ by about 1 bar, which can reduce energy by 10–15% in common ranges. This calculator converts gauge P₂ to absolute using your entered ambient P₁.

7) Mass estimate and safety notes

If enabled, mass is estimated from m = P₂V₂/(R·T) with R = 287.05 J/kg·K. A 50 L tank at 9 bar absolute and 20°C holds about 0.53 kg of air. Use outputs for planning only; follow certified pressure-vessel rules and relief-valve sizing.

FAQs

1) What does this calculator output?

It estimates reversible expansion work from a compressed state (P₂, V₂) down to an ambient pressure P₁. Outputs include energy in J, kJ, MJ, and Wh, plus pressure ratio and the expanded volume V₁.

2) Should I enter gauge or absolute pressure?

Enter ambient P₁ as absolute. For P₂, choose gauge if your reading is relative to ambient (typical compressor gauges). The tool adds P₁ to convert gauge P₂ into absolute pressure for the calculations.

3) Which model should I use?

Use isothermal for slow discharge with strong heat exchange, adiabatic for very fast discharge with little heat transfer, and polytropic when reality is in between. If unsure, try polytropic with n ≈ 1.2 as a starting point.

4) Why is adiabatic energy lower than isothermal?

In adiabatic expansion, the gas cools while doing work, so pressure falls faster for a given volume increase. That reduces total work compared with isothermal expansion, where temperature is held constant by ideal heat transfer.

5) Can I use this for air tools runtime?

You can estimate stored energy, but runtime depends on tool flow, regulator losses, compressor cycling, and non-reversible effects. For runtime planning, combine this with measured SCFM consumption and an efficiency factor, then compare against tank capacity.

6) Why does my result change when I change P₁?

P₁ is the final pressure endpoint. Higher ambient pressure means the gas expands less and yields less work. It also changes gauge-to-absolute conversion for P₂ when you select gauge, so both effects can shift results.