HVAC · Psychrometrics · Problem 7PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 7
Problem & Solution
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Problem: Psychometrics Problem 7.
Given: 000 CFM of outside air at 40 degrees and 60% relative humidity is preheated with a 50KW electric coil, then humidifie...
Approach: And then from there, we're going to humidify, so we're going to lay and heating up to some state 3.
Calc: We're starting at some state 1, which is on the colder side.
Calc: And we're heating with electric coil, so we're heating sensibly to some temperature at a state 2, which we don't know exactly w...
Result: And that's closest to answer choice D.
Office Hours
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Student questions asked in live office hours about this problem
OH 21: OH21-3-PSYCHROMETRICS-7
Q: Student got 50 kW for sensible heat from state 1 to state 2, but obtained 28 Btu/lbm for enthalpy at state 2, which is throwing off their answer by 10-15%.
A: You misread the problem—state 2 should be 50% relative humidity, not whatever you used. That changes the enthalpy to 26.2 instead of 28 Btu/lbm, which will fix your answer. Double-check your enthalpy lookup based on the correct RH value.
OH 80: HVAC: Psychrometrics #7
Q: I determined the answer was 300,000 BTU/hr (choice D) just by knowing the sensible load is 170,000 BTU/hr so the total must be higher—was this coincidence, or can some problems be solved without calculations using the given information?
A: Engineering judgment to eliminate choices is valuable, but don't rely on it too heavily during studying—work through the full solution path so you're flexible if answer choices are rearranged. Use study time to pull the work toward you and build competence; on exam day you can combine both methods for extra certainty.