TFS · Heat Transfer · Problem 17PDFSolution in PDF ↓
TFS · Heat Transfer · Problem 17
Problem & Solution
Video Synthesis
Topic: Heat exchanger / heat transfer — A run-around loop used for energy recovery pre-heats 20,000 pounds per hour of 20 degree outside air.
Method: 150,000 BTU per hour of heat is extracted from the same amount of exhaust air which is initially at 80 degrees.
Key values: 150,000 BTU per hour of heat is extracted from the same amount of exhaust air which is initially at 80 degrees, 20,000 pounds per hour of 20 degree outside air, 20,000 pounds per hour and that
✅ Answer: C
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Student questions asked in live office hours about this problem
OH 101
Q: How is the tolerance of ±15 achieved in this run-around coil problem? Did I miss something or is this a standard value I should know?
A: The ±15 isn't a tolerance—it's derived from the energy balance on the coils. Using the sensible heat rule (500 GPM × ΔT for water), we calculate the heat extracted from the leaving air, which tells us the water's ΔT across each coil is 30°. The temperature range of the loop is then the difference between the two extremes (35°F to 65°F), which gives us the 30° delta and eliminates wrong answers.