Calculator Inputs
Enter absolute pressures. Convert any gauge pressures before using this page.
Example Data Table
| Gas | P1 (bar abs) | P2 (bar abs) | T1 (°C) | Mass Flow (kg/s) | Model | Actual Power (kW) | Actual Outlet Temp (°C) |
|---|---|---|---|---|---|---|---|
| Air | 1.0133 | 7.0000 | 25.0000 | 1.2000 | Polytropic | 305.0034 | 229.3711 |
This example assumes Z = 1.000, k = 1.400, n = 1.300, efficiency = 82%, and two stages with intercooling back to 25 °C.
Formula Used
Pressure ratio: r = P₂ / P₁
Inlet density: ρ₁ = P₁ / (Z × R × T₁)
Inlet volumetric flow: Q₁ = ṁ / ρ₁
Isothermal specific work: w = Z × R × T₁ × ln(r)
Polytropic specific work: w = [n / (n - 1)] × Z × R × T₁ × [r^((n - 1)/n) - 1]
Isentropic specific work: w = [k / (k - 1)] × Z × R × T₁ × [r^((k - 1)/k) - 1]
Actual compression work: wactual = wideal / η
Power: Power = ṁ × wactual
Ideal outlet temperature, polytropic: T₂ = T₁ × r^((n - 1)/n)
Ideal outlet temperature, isentropic: T₂ = T₁ × r^((k - 1)/k)
Approximate actual outlet temperature for compression: T₂,actual = T₁ + (T₂,ideal - T₁) / η
Temperatures use kelvin inside the calculations. Pressures use pascals internally after converting from bar absolute.
How to Use This Calculator
- Choose a gas preset or select custom values.
- Enter inlet and outlet absolute pressures in bar.
- Enter inlet temperature and mass flow rate.
- Set compressibility, heat capacity ratio, and gas constant.
- Enter the polytropic exponent and compressor efficiency.
- Set the number of stages and intercooler outlet temperature.
- Select the reporting model you want highlighted.
- Press the calculate button to show results above the form.
- Review the comparison table and Plotly graph.
- Download the summary as CSV or PDF.
FAQs
1. Should I enter gauge pressure or absolute pressure?
Enter absolute pressure. If your instrument shows gauge pressure, add local atmospheric pressure before using the calculator. Compression ratios require absolute values for correct thermodynamic work and temperature results.
2. What is the difference between isothermal, polytropic, and isentropic results?
Isothermal assumes constant temperature, giving the lowest ideal work. Isentropic assumes reversible adiabatic compression. Polytropic sits between them and often matches practical compressor behavior better when heat transfer and losses exist.
3. Why do staged results often need less power?
Intercooling lowers gas temperature between stages, reducing specific volume and compression work. That is why multi-stage arrangements commonly improve efficiency, especially for large overall pressure ratios.
4. What does the compressibility factor Z change?
Z adjusts the ideal-gas relation for real-gas behavior. When Z differs from 1, density and compression work shift accordingly. For moderate pressures, many gases remain close to ideal.
5. How should I choose the polytropic exponent n?
Use measured compressor data when possible. Otherwise, pick a value between 1 and k. Lower values indicate more heat removal during compression, while values near k behave closer to adiabatic compression.
6. Why is actual outlet temperature higher than ideal temperature?
Real compressors are not perfectly efficient. Extra work becomes additional heating, so the actual discharge temperature rises above the ideal prediction for the same pressure ratio.
7. Can I use this tool for blower or vacuum calculations?
Yes, if the process still behaves like gas compression and you enter consistent absolute pressures. For extreme vacuum systems or very high pressures, more advanced real-gas models may be necessary.
8. Does this calculator replace detailed compressor design software?
No. It is excellent for estimates, comparisons, and quick screening. Detailed mechanical sizing, surge limits, clearance losses, and manufacturer performance curves still require dedicated engineering analysis.