Compressor power calculator
Ideal-gas isentropic compression power from suction/discharge pressure, suction temperature and flow — then isentropic and motor efficiency stacked on top to get shaft and electrical power, plus displacement sizing from volumetric efficiency.
How to use this calculator
- Pick the refrigerant/gas — it fills in k and molar mass below, editable.
- Enter suction and discharge pressure, and suction temperature.
- Enter the flow, either as mass flow or volumetric flow at suction conditions.
- Enter isentropic and motor efficiency.
- Read the ideal, shaft and electrical power, plus displacement if volumetric efficiency was given.
What this calculates — and its one big limitation
This calculates ideal-gas isentropic compression work, the standard textbook thermodynamic reference point for compressor sizing, treating the refrigerant vapour as an ideal gas:
R = R₀ / M specific gas constant
cp = k·R / (k−1) specific heat at constant pressure
w_s = cp·T₁·[ (P₂/P₁)^((k−1)/k) − 1 ] specific isentropic work
T₂s = T₁·(P₂/P₁)^((k−1)/k) isentropic discharge temperature
ρ₁ = P₁ / (R·T₁) suction density (ideal gas)
This is not a real refrigerant-cycle simulation. Real refrigerant vapour near saturation deviates from ideal-gas behaviour, sometimes significantly — an actual cycle calculation needs real enthalpy data from that refrigerant's own equation of state, which this site has no verified source for (the same limitation noted on the Refrigerant Pipe Pressure Drop and COP / EER calculators). Ideal-gas isentropic work is nonetheless a genuinely standard first-pass estimate used throughout compressor engineering, not a shortcut invented for this tool — treat the result as a solid order-of-magnitude reference point, not a substitute for a manufacturer's selection software.
Where k and molar mass come from
Picking a refrigerant auto-fills k (the isentropic exponent, cp/cv) and molar mass with rough typical values for that refrigerant's vapour near ordinary operating conditions — the same confidence tier as the density/viscosity typical values on the Refrigerant Pipe Pressure Drop Calculator: a reasonable starting point, not a verified per-condition figure. Molar mass is the more solid of the two (it's a fixed molecular property); k varies somewhat with temperature and pressure even for the same refrigerant. Edit either if you have more specific data.
Efficiency stack: isentropic → shaft → electrical
Isentropic power is a lower bound — a real compressor always needs more shaft power to do the same compression, captured by isentropic efficiency (typically roughly 60-80% for reciprocating and scroll compressors, higher for well-matched screw compressors, lower near very high pressure ratios). Shaft power becomes electrical power via the motor's own efficiency (typically 90-96% for a reasonably modern motor). Both are editable inputs, not looked-up values — they depend on the specific compressor and motor.
Displacement sizing
If you enter a volumetric efficiency, the required compressor displacement is the suction-side volumetric flow divided by that efficiency — the swept volume flow rate a reciprocating or scroll compressor would need to deliver the required mass flow, accounting for re-expansion and leakage losses that mean actual delivered flow is always less than geometric swept volume would suggest.