Definition - What does Polytropic Efficiency mean?
Polytropic Efficiency is a process whereby compression is divided into numerous small steps with the steps contain similar isentropic efficiency. It does not depend upon thermodynamic effect and hence, it is considered as aerodynamic performance of compressor. It can be observed that the isentropic efficiency of small stages is greater than the efficiency of the finite compression process. It is a limited value of isentropic efficiency because pressure ratio reaches 1.0 and the Polytropic Efficiency value is higher than the corresponding adiabatic efficiency.
Petropedia explains Polytropic Efficiency
Polytropic Efficiency relates to real and ideal processes. It arises in context to the reversible compressor that compresses the gas from similar initial state to final state as irreversible adiabatic compressor but also obeys the relation that polytropic index is constant. Efficiency for turboexpander can be calculated depending on the isentropic expansion instead of polytropic expansion even when the efficiency is not hundred percent. It is done because losses are largely introduced at discharge of a machine in the form of disk friction and seal leakage that heats the gas.