HVAC · Practice-Exam-2 · Problem 22

Problem & Solution

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Problem: 5 pounds of 80 degree air is compressed at constant temperature from 15 PSIA to 90 PSI.
Approach: So in the old days of the murm, you had to look through 2 1.5 pages of different thermodynamic properties to figure out what was the best equation to ...
Key step: 5 pounds of 80 degree air is compressed at constant temperature from 15 PSIA to 90 PSI.
Watch out: That's 53.35 if you've memorized it but if not that's okay.
Result: And using those two phrases will get to this equation here says that the specific work equals R times T times the natural log of P1 over P2.
✅ Answer: D

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Student questions asked in live office hours about this problem

OH 56: HVAC: Full Practice Exam #22

Q: This problem, what was this one about?

A: Well, we can answer it kind of generally anyway. It's okay to use the equation Q equals MCP delta T, even though the problem is about air, the mechanical reference handbook says the equation is used to figure out heat transfer to or from water. Q equals MCP delta T.

OH 57: HVAC: Full Practice Exam #22

Q: The context of this one was it was a one and a half horsepower motor, which was driving a fan and heating a room with a volume of 7,500 cubic feet.

A: Yeah, let me just kind of break down those two options so we can put them side by side. Sure answer is yes, but I want to kind of explain why. So what you're proposing is to use the sensible heat rule of thumb, which says that Q dot equals 1.1 or 1.08, whatever you like.

OH 102: HVAC: Full Practice Exam #22

Q: There's an equation given in the handbook for ISO thermal compression that includes the molecular weight of gas M. Why is that not included in the solution and when do you use the equation that includes the molecular weight?

A: So this is a classic thermal properties equation right here for finding the work for an ISO thermal process, the compression process. So we have the pressure ratio, the mass flow it is in there. The temperature, they've included the efficiency, you may or may not see that if it's not included in the problem statement, maybe it's a 100%.