Mathematics used to calculate gas compressor efficiency

First calculate the theoretical discharge temperature:

T2C = T1 * ((P2 / P1) ^ ((k - 1) / k))


T1 = Suction Temperature

P1 = Suction Pressure

P2 = Discharge Pressure

k = Ratio of Specific Heats

Next use theoretical discharge temperature and the actual measured discharge temperature to calculate the efficiency

isentropic efficiency = (T2C - T1) / (T2 - T1)


T2C = Calculated Discharge Temperature

T2 = Measured Discharge Temperature

T1 = Suction Temperature


ISENTROPIC (ADIABATIC) – No heat added or removed

POLYTROPIC – Heat leaves and enters

Empirical field experience has demonstrated that compression in positive displacement compressors (reciprocating and screw) closely follows a reversible adiabatic process (constant entropy). In dynamic compressors (centrifugal and axial) a Polytropic process is a more accurate method.

Air compressors efficiency is almost always stated on an isentropic basis. The isentropic model has been used in the application primarily because it gives more reliable results in field tests.