Per API 617 & ASME PTC-10
| Gas | k @ 60°F | MW | Compressor Type | η Range |
|---|---|---|---|---|
| Natural Gas | 1.27 | 18.9 | Centrifugal | 0.75–0.82 (poly) |
| Methane | 1.31 | 16.04 | Reciprocating | 0.80–0.88 (isen) |
| Ethane | 1.19 | 30.07 | Screw | 0.70–0.80 (isen) |
| Propane | 1.13 | 44.10 | Axial | 0.82–0.88 (poly) |
| CO₂ | 1.29 | 44.01 | Use Z < 0.9 near critical | |
Detailed coverage of adiabatic, polytropic, and isothermal compression with efficiency factors per API 617.
Compressor horsepower is calculated using either the isentropic (adiabatic) or polytropic method per GPSA 13th Edition, API 617, and ASME PTC-10. Key inputs are flow rate, suction and discharge pressures, gas properties (k-value, molecular weight), compressibility factor Z, and efficiency.
Isentropic (adiabatic) efficiency is used for reciprocating compressors per GPSA methodology. Polytropic efficiency is used for centrifugal compressors per API 617. For the same machine, polytropic efficiency is numerically higher than isentropic efficiency.
The compression ratio should be kept below 4 per stage for centrifugal compressors and below 6 per stage for reciprocating compressors to limit discharge temperature. If the overall ratio exceeds these limits, multi-stage compression with intercooling is required.
Natural gas has a typical specific heat ratio (k = Cp/Cv) of 1.27 at 60°F with a molecular weight of approximately 18.9. The k-value decreases with increasing temperature and varies with gas composition per GPSA Figure 13-4.