HVAC · Psychrometrics · Problem 10PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 10
Problem & Solution
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Problem: Psychometric problem 10.
Given: 0112 pounds of water such that the total mass in the box is the sum 1
Approach: How much above 0.24 maybe not very much maybe not enough to make a difference so it might not get from B to C it may still roun...
Result: How much above 0.24 maybe not very much maybe not enough to make a difference so it might not get from B to C it may still round off to be we'll fi...
Note: Well, moist air is a mixture of dry air and water vapor.
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Student questions asked in live office hours about this problem
OH 80: HVAC: Psychrometrics #10
Q: Why is using the formula CP = 0.24 + 0.45 × W (where W is humidity ratio) to find specific heat capacity of moist air giving me answer D instead of the correct answer C?
A: Your approach is actually brilliant and conceptually sound—the formula correctly adds the water vapor contribution to the dry air CP. The small difference between C and D comes from a precision issue: the formula doesn't account for the fact that as water vapor increases, the mass fraction of dry air decreases (only ~98.9% in this problem), whereas the mixture calculation does. This level of precision rarely matters on the PE exam, so your method will get you very close to the right answer.