HVAC · Heat-Transfer · Problem 3PDFSolution in PDF ↓
HVAC · Heat-Transfer · Problem 3
Problem & Solution
Video Synthesis
Problem: 10GPM of water is heated from 60 degrees to 125 degrees in a feedwater heater by steam.
Given: 10GPM of water is heated from 60 degrees to 125 degrees in a feedwater heater by steam; 5 psi g, steam enters at 350 ...
Approach: The steam side of the feedwater heater operates at 5 psi g, steam enters at 350 degrees, and leaves as a saturated mixture with...
Calc: So we've got water being heated from 60 to 125.
Calc: 60 degrees and then leaving at 125.
Result: So h2 equals hf plus the quality at state 2 times hfg.
Office Hours
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Student questions asked in live office hours about this problem
OH 15: HVAC: Heat Transfer #3
Q: Why is H1 superheated and H2 a saturated mixture in this steam heat exchanger problem?
A: The problem statement says steam enters at a given temperature and pressure and leaves as a saturated mixture—so H1 is superheated by specification (the inlet conditions place it above the saturation curve) and H2 is a saturated mixture by specification. Reading the problem statement carefully is how you know the phase at each state; you don't infer it from the process. Use the steam tables to look up enthalpy at each defined state.
OH 82: HVAC: Heat Transfer #3
Q: How do we know Q = ṁ·Δh when dealing with steam—is this formula in the reference handbook?
A: Q = ṁ·Δh is a foundational thermodynamics relationship that should be part of your working knowledge—don't look it up in the handbook during the exam. It states that the rate of heat transfer equals mass flow rate times the change in specific enthalpy, which applies any time fluid is flowing and exchanging heat without doing shaft work. Internalize this one; it's too fundamental to hunt for in a reference.