The isentropic efficiency has one significant drawback in that there is no way to separate the fluid dynamic losses from the total (fluid dynamic + thermodynamic) losses. This means that devices having different pressure ratios will have different isentropic efficiencies even though they may both be of similar fluid dynamic quality. An example would be two compressors of different pressure ratios. The higher-pressure ratio compressor will have a lower isentropic efficiency because of thermodynamic losses. This property can make it difficult to comparatively evaluate different compressor designs. A similar argument applies to turbine design.

To work around this drawback, the assumed "ideal" path followed by the process does not have to be isentropic. Instead, one can evaluate polytropic efficiency by following a path along a line of constant efficiency. Aungier [31] discusses how a constant efficiency path on a T-s diagram can be approximated by the following equation:

(25–48)

This equation can be rearranged to give the following two relationships:

(25–49)

The entropy change along this path is evaluated by integrating the last expression:

(25–50)

Rearranging this expression gives the value of the constant A along the given path:

(25–51)

To evaluate the polytropic efficiency, evaluate the polytropic enthalpy change along the alternative path defined by the path equation. Starting with the second law:

(25–52)

The term is the polytropic work, or enthalpy change, and is what must be solved for. First, the term is substituted using Equation 25–49 to give:

(25–53)

Integrating this equation along the polytropic path gives:

(25–54)

The polytropic enthalpy can be obtained from the polytropic enthalpy change simply by omitting the subtraction of . Also, substituting for A gives the final form of the polytropic total enthalpy :

(25–55)

Polytropic static enthalpy is evaluated by replacing total quantities at point 2 with static quantities:

(25–56)

Using and instead of and in formulas Equation 25–40-Equation 25–42 and Equation 25–45-Equation 25–47, the corresponding value of the polytropic efficiency can be evaluated.