Good question.
Believe that poly E is used in compressors for a given gas composition, since this value remains (more or less) constant for this gas for a given flow and speed, even though pressure ratio across the multistage compressor may change for various operating cases. While isentropic E changes for a given flow and speed as pressure ratio changes.
Isentropic E is only used in gas expansion turbines. There is some explanation given for this in Perry Chem Engg Handbook, but I've yet to understand it.
