Editing Steam turbine (section)

====Isentropic efficiency====
To measure how well a turbine is performing we can look at its [[isentropic]] efficiency. This compares the actual performance of the turbine with the performance that would be achieved by an ideal, isentropic, turbine.{{sfn|Moran|Shapiro|Boettner|Bailey|2010|p=}} When calculating this efficiency, heat lost to the surroundings is assumed to be zero. Steam's starting pressure and temperature is the same for both the actual and the ideal turbines, but at turbine exit, steam's energy content ('specific enthalpy') for the actual turbine is greater than that for the ideal turbine because of irreversibility in the actual turbine. The specific enthalpy is evaluated at the same steam pressure for the actual and ideal turbines in order to give a good comparison between the two.

The isentropic efficiency is found by dividing the actual work by the ideal work.{{sfn|Moran|Shapiro|Boettner|Bailey|2010|p=}}
<math display="block">\eta_t = \frac{h_3 - h_4}{h_3 - h_{4s}} </math>

where
* {{mvar|h<sub>3</sub>}} is the specific enthalpy at state three
* {{mvar|h<sub>4</sub>}} is the specific enthalpy at state 4 for the actual turbine
* {{mvar|h<sub>4s</sub>}} is the specific enthalpy at state 4s for the isentropic turbine

(but note that the adjacent diagram does not show state 4s: it is vertically below state 3)