TFS · Thermodynamics · Problem 23PDFSolution in PDF ↓
TFS · Thermodynamics · Problem 23
Problem & Solution
Video Synthesis
- Problem: 200 degree water enters an open feed water heater operating at a hundred PSIA and is heated by a hundred PSIA 400 degree steam.
- Key step: 200 degree water enters an open feed water heater operating at a hundred PSIA and is heated by a hundred PSIA 400 degree steam.
- Watch out: It's not a known temperature it's a known pressure because everything here is at a hundred PSIA.
- Result: The right side here we can actually multiply together and just call this now 298.5 BTU per hour and now we have some terms with BTU per hour on both s...
- ✅ Answer: C
Office Hours
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Student questions asked in live office hours about this problem
OH 77
Q: Why can't I assume H₃ = H₁ + H₂ when the mass balance M₃ = M₁ + M₂ is true in an open feedwater heater problem?
A: You must distinguish between intrinsic (specific, per unit mass) and extrinsic (total) properties. The mass balance equation works with mass flow rates directly, but the energy balance requires you to account for mass: Ṁ₁h₁ + Ṁ₂h₂ = Ṁ₃h₃. You can't add specific enthalpies directly because they're not additive properties—you need to weight them by their respective mass flows first.