Because there are different kinds of compressors? Adiabatic exponents work well for positive displacement machines, but not so well for dynamic machines. Polytropic exponents are pretty much meaningless for PD machines.
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In Unisim/Hysys it is required to input one of the efficiency. (Adiabatic or Polytropic) Then it calculates the other efficiency irrespective of compressor type (Centrifugal/Reciprocating/Screw).
