TFS · Heat Transfer · Problem 26PDFSolution in PDF ↓
TFS · Heat Transfer · Problem 26
Problem & Solution
Video Synthesis
Topic: Psychrometrics — During summer operation, a 70% effective sensible heat recovery device pre-cools 8000 CFM of outside air at 95 degrees and 65% relative humidity.
Method: 10000 CFM of exhaust air at 75 degrees and 50% relative humidity is available for use.
Key values: 8000 CFM, 10000 CFM
Reference: reference handbook
✅ Answer: C
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Student questions asked in live office hours about this problem
OH 114
Q: Where does the effectiveness equation come from, and why can you drop the additional variables that appear in the reference handbook equation for sensible heat recovery?
A: For sensible heat recovery, effectiveness is based on temperature differences (ΔT), not enthalpy. The handbook's additional variables drop out because the specific heat capacity is essentially the same for both air streams regardless of moisture, and the mass flow rates are typically equal—so the numerator and denominator cancel, leaving only the temperature differences. When flow rates differ, you simply use the smaller flow rate as your constraint.