Calculator Input Form
Use absolute pressures in bar(a). Choose a gas preset for quick values or select custom gas for manual inputs.
Formula Used
This calculator estimates compressor power from thermodynamic specific work and mass flow. It then adjusts the result using compressor efficiency and motor efficiency.
1) Pressure Ratio
2) Isothermal Specific Work
3) Isentropic Specific Work
4) Polytropic Specific Work
5) Power Relations
Where Z is compressibility factor, R is specific gas constant, T1 is inlet temperature, k is heat capacity ratio, n is polytropic index, and ṁ is mass flow rate.
How to Use This Calculator
- Select a gas preset or choose custom gas.
- Pick the compression method: isothermal, polytropic, or isentropic.
- Enter suction and discharge pressures in absolute bar.
- Enter mass flow rate and inlet temperature.
- Review gas properties, efficiencies, and number of stages.
- Add operating hours and electricity rate for annual cost estimates.
- Click the calculation button to view results above the form.
- Use the CSV or PDF buttons to export the result set.
Example Data Table
| Case | Gas | Method | Suction Pressure | Discharge Pressure | Flow | Inlet Temp | Stages | Approx. Motor Power |
|---|---|---|---|---|---|---|---|---|
| Plant Air Line | Air | Isentropic | 1.013 bar(a) | 7.0 bar(a) | 1.25 kg/s | 300 K | 2 | ~302 kW |
| Booster Service | Natural Gas | Polytropic | 2.0 bar(a) | 12.0 bar(a) | 0.85 kg/s | 315 K | 3 | ~274 kW |
| Low-Ratio Duty | CO₂ | Isothermal | 1.5 bar(a) | 3.2 bar(a) | 0.60 kg/s | 295 K | 1 | ~46 kW |
Frequently Asked Questions
1) What power does this calculator report?
It reports theoretical gas power, shaft power, and motor input power. Theoretical power comes from thermodynamic work, shaft power adjusts for compressor losses, and motor power adds motor losses. That separation helps with equipment sizing and energy planning.
2) Why must I use absolute pressure?
Compression equations use absolute pressure ratios. Gauge pressure can give a wrong ratio, especially near atmospheric conditions. Convert gauge readings to absolute values before entering them for a physically correct power estimate.
3) What is the difference between isothermal, isentropic, and polytropic models?
Isothermal assumes constant gas temperature and gives the lowest ideal work. Isentropic assumes reversible adiabatic compression. Polytropic sits between them for many practical machines, especially when heat transfer and real compression behavior matter.
4) How do stages affect the result?
More stages reduce the per-stage pressure ratio. With ideal intercooling, this usually lowers the total work for isentropic and polytropic compression. The calculator assumes equal stage ratios and ideal cooling back to inlet temperature between stages.
5) What efficiency should I enter?
Enter compressor efficiency for the compression section and motor efficiency for the driver. Use vendor data when available. If not, reasonable preliminary values often fall around 70–88% for compressors and 90–97% for motors.
6) What does compressibility factor Z do?
Z adjusts ideal-gas behavior toward real-gas behavior. A value of 1.0 means ideal behavior. For high pressures or gases with stronger real-gas effects, using a realistic Z improves the power estimate.
7) Can I use this for custom gases?
Yes. Choose custom gas and enter the specific gas constant and heat capacity ratio manually. That lets you estimate power for specialty gases, blends, or early design studies where only basic properties are available.
8) Is this enough for final equipment selection?
It is excellent for screening, comparison, and budgeting. Final equipment selection should also include vendor performance curves, suction conditions, gas composition, mechanical limits, cooler performance, and site-specific operating requirements.
Notes for Practical Use
This tool is intended for engineering estimation and comparison. Real compressor packages may include losses from coolers, drives, seals, controls, and pressure drops not modeled here.