HVAC · Psychrometrics · Problem 15PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 15
Problem & Solution
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Problem: Cypher metrics problem 15, a thousand CFM of air 76 degrees and 60% relative humidity is removed from a room.
Given: 300 CFM is cooled to 54 degrees dry bulb and 52 degrees wet bulb; 700 CFM is being bypassed because 300 is being cool...
Approach: 300 CFM is cooled to 54 degrees dry bulb and 52 degrees wet bulb.
Calc: We know that it's 76 degrees dry bulb and 60% relative humidity in the room.
Calc: But 700 CFM is being bypassed because 300 is being cooled by a coil and the remaining bypasses.
Result: And that results in a mixed humidity ratio of 0.0104 pounds of water per pound of drier.
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Student questions asked in live office hours about this problem
OH 121 · May 25, 2026
Q: Can wet bulb temperature be used as a linear interpolation axis when solving air-mixing problems on the psychrometric chart, the same way dry bulb temperature can?
A: No — wet bulb temperature lies on the saturation curve, which is nonlinear, so it cannot be treated as a linear axis for interpolation. For mixing problems, only the horizontal axis (dry bulb temperature) and vertical axis (humidity ratio) are linear and valid for proportional calculations. Using wet bulb as a linear interpolant will yield incorrect results, as it did here when the student arrived at 65% RH instead of the correct answer.