TFS · Heat Transfer · Problem 14PDFSolution in PDF ↓
TFS · Heat Transfer · Problem 14
Problem & Solution
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Topic: Heat exchanger / heat transfer — Water enters a closed feed water heater at 100 degrees and 200 psi a.
Approach: So let's start with state 1.
Key values: 200 psi
Reference: table to look up that enthalpy
✅ Answer: D
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Student questions asked in live office hours about this problem
OH 115
Q: Why can't we use ΔH = CP·ΔT to find the enthalpy change between superheated steam and subcooled water, even though we used it for the initial water enthalpy?
A: Because you're trying to bridge two different phases (liquid and vapor), and the formula only works for sensible heating within a single phase. When you cross a phase boundary, you have latent heat involved—a horizontal line on the T-H diagram where temperature stays constant while enthalpy changes dramatically—which CP·ΔT can't capture.