HVAC · Practice-Exam-2 · Problem 26PDFSolution in PDF ↓
HVAC · Practice-Exam-2 · Problem 26
Problem & Solution
Video Synthesis
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
Office Hours
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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.
OH 124 · July 6, 2026
Q: Can this air-table problem be simplified with delta h = cp delta T?
A: Not safely here, because the temperature change is thousands of degrees and the specific heat of air will vary too much over that range. Dan recommended using the air table for this kind of problem, even if only to get enough exposure to know where it is and how it works.
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.