HVAC · Thermodynamics · Problem 3PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 3
Problem & Solution
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Problem: what is the ratio of specific heats for 120 degree Fahrenheit air?
Approach: And that one's a bit hard to look up, but if you search for CP, you may have to search through a few areas, but you'll eventual...
Calc: And by the way, r for air, since we're going to need it, is 53.35 foot pounds force per pound mass degree ranking.
Calc: Okay, so for our situation here, we're interested in the ratio of the specific heats at a particular temperature, 1020, but we ...
Result: So then Cv equals 0.195 BTO per pound-mass rank and and now K, the ratio of specific heat, which is what we want, we come back to this original rel...
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Student questions asked in live office hours about this problem
OH 87: HVAC: Thermo Module #3
Q: Why can't we use the standard K = CP/CV = 1.4 for air to calculate CV, instead of using the relationship CP - CV = R?
A: Because K is the unknown we're solving for in this problem, assuming K = 1.4 would create circular logic—you'd end up proving K = 1.4 by assumption rather than calculation. Since the problem involves air at an unusual temperature where K differs from 1.4, we must use CP - CV = R to find the actual properties without assuming the answer.
OH 94: HVAC: Thermo Module #3
Q: How do you convert humidity ratio from pounds per pound to grains per pound on the psychrometric chart?
A: Multiply the pounds of water per pound of dry air (from the reference handbook psychrometric chart) by 7,000 grains per pound to get grains per pound. Grains is useful because it gives you an integer instead of a small decimal, and you can reverse it by dividing by 7,000 if needed.