Mathematics used to calculate gas compressor efficiency
First calculate the theoretical discharge temperature:
T2C = T1 * ((P2 / P1) ^ ((k - 1) / k))
Where:
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)
Where:
T2C = Calculated Discharge Temperature
T2 = Measured Discharge Temperature
T1 = Suction Temperature
ISENTROPIC & POLYTROPIC EFFICIENCY
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 rotor.zone application primarily because it gives more reliable results in field tests.