TFS · Thermodynamics · Problem 17PDFSolution in PDF ↓
TFS · Thermodynamics · Problem 17
Problem & Solution
Video Synthesis
- Problem: A 60% efficient steam generating turbine system generates five megawatts using a 1100 PSIA steam at 850 degrees.
- Key step: The exhaust pressure is 200 PSIA.
- Watch out: We know that's not the case we're going to apply this 60% efficiency down the line but we want to find the enthalpy as though that entropy was equal.
- Result: But you'll notice that in this case it's actually still super heated steam and you can tell that by checking the entropy 1.578 and looking at this pre...
- ✅ Answer: A
Office Hours
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Student questions asked in live office hours about this problem
OH 113
Q: Why is my answer off when I assumed isentropic conditions (S1=S2) to find the temperature at that point, then used that as the isentropic exit temperature in the turbine efficiency equation?
A: You're double-counting the isentropic assumption—when you use the turbine efficiency equation, it already accounts for the isentropic process internally, so assuming S1=S2 as a separate step creates an inconsistency. You should use the actual entropy change between inlet and outlet states, not force an isentropic intermediate state.