HVAC · Practice-Exam-2 · Problem 26PDFSolution in PDF ↓
HVAC · Practice-Exam-2 · Problem 26
Problem & Solution
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Problem: A thousand BTU per pound of heat is added to 70 degree 14.7 PSIA air.
Key step: A thousand BTU per pound of heat is added to 70 degree 14.7 PSIA air.
Watch out: And we don't even have to interpolate because it's sort of halfway between two values you can check this out for yourself in the table.
Result: It's an additional thousand and we'll just say h2 equals h1 plus delta h, which is 126 plus a thousand.
✅ Answer: A
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Student questions asked in live office hours about this problem
OH 102: HVAC: Full Practice Exam #26
OH 119 · April 28, 2026
Q: Why can't you use Q = C_p·ΔT with C_p = 0.24 BTU/lb·°F to find the exit temperature of air after adding 1,000 BTU/lb at very high temperatures?
A: The constant specific-heat assumption for air breaks down at very high temperatures where C_p varies significantly — this problem is intentionally set in that regime. Instead, use the Air Table: locate the starting enthalpy H₁ at the inlet temperature, add 1,000 BTU/lb to get H₂, then look up the corresponding T₂. The enthalpy–temperature relationship in the air table is linear enough to interpolate, but C_p is not constant enough to be reliable.
Q: The lookup of specific enthalpy in the air table makes sense. However, why can't we use Q equals MC delta T for this problem?
A: Yeah, this one was about, I think it was about the heating of air. And that's not crazy proposal. I think that's reasonable, but couple modifications.