TFS · Practice Exam 1 · Problem 70PDFSolution in PDF ↓
TFS · Practice Exam 1 · Problem 70
Problem & Solution
Video Synthesis
- Problem: Steam enters a 75% efficient turbine at a pressure of 800 PSIA and a temperature of 700 degrees and exits at atmospheric pressure.
- Key values: 75 %, 800 PSIA, 100 %, 14.7 PSIA
- Reference: look up, steam table
- Result: So if this is 0.75 if this is less than 100% we expect that the actual change in enthalpy the specific work coming out of the turbine is less than wha...
- ✅ Answer: A
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Student questions asked in live office hours about this problem
OH 117 · March 30, 2026
Q: For a turbine efficiency problem involving steam, can you substitute temperatures in place of enthalpies in the isentropic efficiency formula (η = (H1 - H2_actual) / (H1 - H2_ideal)) to find the actual outlet temperature, then look up H2 in the superheated steam tables?
A: Replacing enthalpies with temperatures in the efficiency formula is only valid if the working fluid is an ideal gas with constant specific heats — a condition not given for a steam problem. Steam does not behave as an ideal gas, so enthalpies must be used directly rather than substituting Cp·T terms. The correct approach is to find H2_ideal from the steam tables using the isentropic condition (S1 = S2), apply the efficiency to get H2_actual, and compute the work from that enthalpy difference.