HVAC · Systems-and-Components · Problem 20PDFSolution in PDF ↓
HVAC · Systems-and-Components · Problem 20
Problem & Solution
Video Synthesis
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Office Hours
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Student questions asked in live office hours about this problem
OH 44: HVAC: Systems And Components Module #20
Q: Could you find the dew point temperature (76.7°F) for state 1 and the enthalpy for state 3 directly from the chart without using the heat exchanger efficiency?
A: Yes — if you start from around the 4-minute mark in the solution video, that's exactly what we do there. Take another look and it should answer your question; if there's something specific that still doesn't land, let me know and I'm happy to go deeper.
OH 48: HVAC: Systems and Components-20
Q: I found enthalpy H3 using the outdoor air enthalpy H1 and return air enthalpy H2 with the heat exchanger efficiency and got 39.9, rounding to answer C — is this approach valid?
A: Yes, that's equally valid — this is a good example of two different but correct approaches landing in the same place. The slight deviation between our values is just chart reading precision, not a methodological error.
OH 79: HVAC: Systems And Components Module #20
Q: Where does the dew point of 76.7°F for state 1 come from? I'm getting 82.9°F using a dry bulb of 90°F and 80% relative humidity.
A: The state is defined by 90°F dry bulb and 80°F wet bulb — not 80% relative humidity. If you re-run the psychrometric calculation with 80°F wet bulb, you'll get the 76.7°F dew point used in the solution. Those two 80s look identical but represent completely different states.
OH 96: HVAC: Systems And Components Module #20
Q: Can you use enthalpy values from the outdoor and return air conditions to find the leaving enthalpy, or must you always subtract dry-bulb temperatures?
A: For an enthalpy wheel (energy recovery device), efficiency equals the ratio of actual to maximum change in temperature, humidity ratio, or enthalpy — so all three can be used interchangeably. But this problem is an air-to-air heat exchanger, which is only a sensible heat recovery device — only temperature ratios apply, not enthalpy. That distinction between sensible-only vs. total energy recovery is the key to setting up the efficiency equation correctly.