About this calculator
This calculator estimates compressor power using the isothermal compression work formula (P = p₁ × V × ln(pressure ratio)), then divides by an overall efficiency factor to approximate real shaft or motor input power. Isothermal power is the standard theoretical baseline used in compressor engineering textbooks to compare compressor efficiency.
Formula
P (kW) = (p₁ × 100 × Q × ln(p₂ ÷ p₁)) ÷ (60 × η)
- p₁Intake absolute pressure — bar(a)
- p₂Discharge absolute pressure — bar(a)
- QFree air delivery — m³/min
- ηOverall efficiency — 0–1
- PShaft/input power — kW
Worked Example
Given: A screw compressor delivers 3 m³/min FAD, compressing from atmospheric (1.013 bar(a)) to 8 bar(a) discharge, at 70% overall efficiency.
Result: P = (1.013 × 100 × 3 × ln(8÷1.013)) ÷ (60 × 0.7) ≈ 15.0 kW.
How to Use
- Enter the intake absolute pressure — normally atmospheric, about 1.013 bar(a) at sea level.
- Enter the discharge absolute pressure (gauge pressure + ~1 bar).
- Enter the free air delivery (FAD) in m³/min.
- Enter an overall efficiency — 65–75% is a typical planning range for screw compressors covering thermodynamic, mechanical and motor losses combined.
- Click Calculate to get estimated power.
Important Assumptions
- Uses the isothermal (constant temperature) compression model, the standard theoretical reference used to rate compressor efficiency — real compression is closer to polytropic/adiabatic, which needs more energy; the efficiency factor is intended to bridge this gap.
- The single 'overall efficiency' figure is a simplification of several real losses (thermodynamic inefficiency, mechanical friction, motor losses) combined — for a specific machine, use the manufacturer's rated input power instead.
- Best used for budgetary estimation and comparison, not for selecting or guaranteeing a specific compressor's power draw.
Common Mistakes
- Using gauge pressure instead of absolute pressure for p₁ and p₂ — this is one of the most common compressor calculation errors.
- Assuming 100% efficiency, which significantly understates real power draw.
Frequently Asked Questions
Why use isothermal power instead of actual compression power?
Isothermal power is the standard theoretical minimum used across the industry to define and compare compressor efficiency; the efficiency factor then scales it up to a realistic estimate of actual power drawn.
Is this accurate enough to select a compressor?
Use it for budgetary sizing and comparison only — always confirm final compressor selection against the manufacturer's rated input power and performance curve for your exact operating conditions.
Why does discharge pressure use absolute, not gauge?
The compression ratio (p₂/p₁) that determines compression work is a ratio of absolute pressures; using gauge pressure gives an incorrect ratio, especially at lower discharge pressures.
These calculators are provided for informational and estimation purposes. Results depend on the assumptions and input values used. Always verify critical engineering calculations against applicable standards, manufacturer specifications, site measurements, and qualified professional advice.