Can I assume that any steam turbine is isentropic?

[Remusco]

Homework Statement

Steam enters an adiabatic turbine at 1000 psia and 900F and leaves at 2 psia. The turbine efficiency is 80%. The steady-state specific work (btu/lbm) developed by the turbine is most nearly:

Relevant Equations

efficiency=W_t/W_t,i

As I'm studying for my PE and taking practice exams, I came across several problems where the solution assumes that a steam turbine is isentropic. I am now all confused and questioning my knowledge up to this point. It says in the problem statement that the turbine is "80% efficient". If that is the case than how can the turbine be isentropic? Doesn't isentropic always mean the efficiency will be equal to 1? Am I incorrect or is this practice exam incorrect in this assumption?

"solution:"

 

[BvU]

Hi,

For turbines and compressors, 'efficiency' is understood to be 'isentropic efficiency' (unless otherwise stated). In your exercise the turbine is clearly not isentropic.

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