A steam turbine at steady state is operated at part load by throttling the steam

Question

A steam turbine at steady state is operated at part load by throttling the steam to a lower pressure before it enters the turbine. Before throttling, the pressure and temperature are, respectively, 200 lbf/in.2 and 600 degrees F. After throttling, the pressure is 120 lbf/in . At the turbine exit, the steam is at 1 lbf/in. and a quality of 90%. Heat transfer with the surroundings and all kinetic and potential energy effects can be ignored. Determine. (a) the temperature at the turbine inlet, in degrees F. (b) the power developed by the turbine, in Btu per lb of steam flowing.

Explanation / Answer

State 1 (before throttling) : P1=200psi T1=600F -> h1=1321.8 Btu/lb
State 2 (after throttling) : P2=120psi h2=h1=1321.8 Btu/lb
use P2 and h2 to find T2 from steam table -> T2 = 589F -> temp at turbine inlet
State 3 (fter turbine): P3=1psi x=0.9
h3=hf+x(hg-hf) = 69.715+0.9(1105.356-69.715) = 1001.8 Btu/lb
wturbine = h2-h3 = 1321.8-1001.8 = 320 Btu/lb

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