4 - Efficiency Definitions for Compressors

Summary

The systematic definition of efficiency introduces isentropic, polytropic and isothermal efficiencies. The isentropic efficiency compares the actual work transfer to that which would take place in an ideal isentropic adiabatic process (with no losses and no heat transfer). Unfortunately, this does not represent the real thermodynamic process of a compressor very well. For example, a two-stage turbocharger using two stages each with a pressure ratio of 2 and an isentropic efficiency of 80%, has a pressure ratio of 4, but an isentropic efficiency of 78.1%. The polytropic efficiency overcomes this issue, and the two-stage compressor has the same polytropic efficiency as its individual stages. The kinetic energy present at the inlet and outlet of a stage can be identified by the difference between total and static states. The value of the kinetic energy in these planes is taken into account by comparing the total-to-total or total-to-static efficiencies. Care is needed as a radial compressor impeller may have a total-to-total polytropic impeller efficiency of over 90% but a static-to-static isentropic efficiency of well below 60%.