HVAC · Systems-and-Components · Problem 22PDFSolution in PDF ↓
HVAC · Systems-and-Components · Problem 22
Problem & Solution
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Student questions asked in live office hours about this problem
OH 19: HVAC: Systems-22
Q: I multiplied Q_steam by 0.9 (i.e., 1 – 0.1 for the 10% bypass) and got a different answer than the solution — was my approach incorrect?
A: Take a look at the solution video more carefully — the bypass factor applies to the air that bypasses the coil, not to the steam side. The steam coil heats 100% of the air flowing through it; the bypass factor describes the fraction of air that skips the coil entirely and rejoins at the outlet.
OH 42: HVAC: Systems And Components Module #22
Q: My understanding is that bypass factor is the fraction of air that skips the coil — so shouldn't only 90% of the total airflow pass through the coil heating calculation, not the full 1,000 CFM?
A: The air that touches the coil — the 90% — exits at the fully conditioned (leaving) coil temperature; the 10% bypasses and stays at the entering temperature. The total leaving condition is a mixture of those two streams. You still use the full 1,000 CFM for the final energy balance to the space, but the coil itself only conditions the 90% fraction — re-watch the solution video with that framing and it should click.
OH 96: HVAC: Systems And Components Module #22
Q: How do you know to use hfg when calculating Q_steam? I'm unsure when to use hfg versus another enthalpy value when multiplying by mass.
A: When steam enters a coil and condenses back to liquid, the energy it gives up is the latent heat of vaporization — that's hfg by definition. hfg is the enthalpy difference between saturated vapor (hg) and saturated liquid (hf) at the same pressure, which is exactly the heat released when steam fully condenses. Any time you have a phase-change process — condensation or vaporization — look for hfg.