HVAC · Heat-Transfer · Problem 5PDFSolution in PDF ↓
HVAC · Heat-Transfer · Problem 5
Problem & Solution
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Problem: Heat transfer 5 in uninsulated 8 inch chilled water pipe carries 55 degree chilled water through a factory maintained to 80 degrees.
Given: 5 in uninsulated 8 inch chilled water pipe carries 55 degree chilled water through a factory maintained to 80 degrees...
Approach: The length of the run is 75 feet.
Calc: So it's 55 degree water.
Calc: And the length of the run is 75 feet and the diameter of the pipe is 8 inches.
Result: 3560 BTU per hour closest to answer choice A.
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Student questions asked in live office hours about this problem
OH 76: HVAC: Heat Transfer #5
Q: The reference handbook defines T1 as the body surface and T2 as the surroundings—based on that, T1 should be 55°F (512°R) and T2 should be 80°F (540°R), so why did the solution switch them?
A: The subscripts in radiation formulas are somewhat arbitrary—what matters is that you subtract the colder temperature from the warmer one to get a positive result, since heat always flows from hot to cold. If you get a negative result, that's a red flag that your temperatures are assigned backwards. As long as the subtraction yields a positive number, the direction of heat transfer is correctly represented.