Isothermal Efficiency=Actual measured input power / Isothermal Power
Isothermal power (kW) = P1 x Q1 x loger/36.7
Where
P1 = Absolute intake pressure kg/ cm2;
Q1 = Free air delivered m3/hr; and
r = Pressure ratio P2/P1.
The calculation of isothermal power does not include power needed to overcome friction and generally gives an efficiency that is lower than adiabatic efficiency.