HVAC · Psychrometrics · Problem 14PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 14
Problem & Solution
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Problem: Psychometric's problem 14.
Given: 14. In Auditorium has a sensible heat load of 300,000 BTUs per hour and a moisture load of 100 pounds per hour; 000 B...
Approach: So this is an interesting problem because we don't know whether the sensible heat in the space, the 300,000 BTUs per hour, or t...
Calc: The space is not to exceed a temperature of 74 degrees or a relative humidity of 60%.
Calc: It may be that we're bumping into the 74 degree limit in order to meet the sensible load.
Result: Six eight six to the bow right here try to get that parallel and then 74 is about here so what is the relative humidity at that point it's just shy...
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Student questions asked in live office hours about this problem
OH 54: HVAC: Psychrometrics #14
Q: We assume 74°F return air drives the CFM calculation for the auditorium, but couldn't any temperature less than 74°F also yield a valid solution—how do we know 74°F is the only valid answer?
A: The problem establishes a maximum return air temperature of 74°F and asks for the design CFM—using the maximum temperature gives the most conservative (largest) CFM, which ensures the system can handle the worst-case sensible load. Any temperature lower than 74°F would yield a smaller delta T and thus a higher CFM requirement, exceeding the answer. You're solving for the design condition at the stated limit, not for all possible temperatures.
OH 80: HVAC: Psychrometrics #14
Q: I found max CFM from the sensible load with a max delta T of 12°F, then used the total heat formula 4.5·CFM·ΔH to find H2—is this a valid approach even though it differs from the solution?
A: Finding maximum CFM from the sensible load constraint is the right first step and valid. Using 4.5·CFM·ΔH to find H2 is also valid in principle, but you need to be careful that you're using the correct leaving conditions and that the enthalpy approach is consistent with how the latent load was established. If the sensible heat ratio approach in the solution gives a different answer, compare the intermediate values to find where the two paths diverge.