Power Calculation Based on Pressure and Flow: Compressor Shaft Power Calculation Principle and API 619 Reference Method

In compressor engineering selection and energy efficiency evaluation, accurate estimation of shaft power is the key link. Power is not a direct measurement, but indirectly derived through medium pressure, volume flow, variable efficiency and gas thermodynamic parameters.

Basic rating formula

for ideal adiabatic (isentropic) process, compressor shaft power (kW) can be approximately expressed:

• P = (Q × Δh_isen)/N_isen(Based on isentropic enthalpy rise)
• or use common engineering simplification:P ≈ (Q × P_d× k) / [(k−1) × η_poly] × ln(P_d/P_s)

where:Qvolume flow under import conditions (m³/s),P_s, P_dfor suction/exhaust absolute pressure (Pa),kratio of specific heat,N_polyfor variable efficiency (usually 0.7-0.85, depending on the model and speed).

API 619 guiding significance

API RP 619 Rotary positive displacement compressor for petroleum, chemical and gas industries clearly requires that the manufacturer shall provide the shaft power under rated working conditions according to the measured performance curve or verified changeable model, and indicate the test conditions (such as suction temperature, relative humidity and gas composition). This standard does not provide a single conversion formula, but emphasizes that gas compressibility, leakage loss and mechanical efficiency correction must be considered.

Practical points of air compressor parameters

• nominal traffic (such as 6 m³/min “) usually refers to the free air flow (FAD), which needs to be converted to the compressor inlet state;
• the pressure unit must be unified as absolute pressure (for example, 0. 7 MPa (g) ≈ 0.8 MPa (a));
• the actual power is usually 10-higher than the theoretical value- 25%, should be included in the coupling loss, gearbox loss and motor efficiency.

Schematic of compressor power calculation relationship

Q, Pₛ
P_d

→ non-linear increase of power with pressure ratio and flow rate

Note:this calculation is only applicable to preliminary estimation. Engineering Design and contract acceptance must be based on GB/T 3853, ISO measured performance test results specified in 1217 or API 619. It is forbidden to use the fan formula sleeve for positive displacement compressor.