Question 1

Why are compressor and turbine efficiency defined differently?

Accepted Answer

So that both come out below 1. A real compressor needs more work than the ideal, so its efficiency is ideal ÷ actual. A real turbine delivers less work than the ideal, so its efficiency is actual ÷ ideal. Applying the wrong definition gives a number above unity — or, worse, a plausible number below it that is simply wrong. This catches people out regularly when moving between compressor and turbine calculations in the same analysis.

Question 2

What is the difference between isentropic and polytropic efficiency?

Accepted Answer

Isentropic efficiency compares the whole machine against a single ideal process end to end. Polytropic (or small-stage) efficiency applies the same comparison to each infinitesimal stage and integrates. For a compressor the polytropic figure is always slightly higher, because of the reheat effect — each stage works on gas already warmed by the last. Vendors quote both, so check which one you are comparing before choosing between machines.

Question 3

Why does an inefficient compressor run hotter?

Accepted Answer

Because the lost work does not disappear — it becomes heat in the gas. Every joule of inefficiency raises the discharge temperature, which is why discharge temperature is such a good diagnostic. An 8:1 stage at 80% isentropic efficiency discharges around 350 °C where the ideal would be 260 °C, and that difference is precisely the loss.

Isentropic Efficiency