HVAC · Systems-and-Components · Problem 14PDFSolution in PDF ↓
HVAC · Systems-and-Components · Problem 14
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Student questions asked in live office hours about this problem
OH 42: HVAC: Systems And Components Module #14
Q: Why did you multiply the pressure drop by 30/100 — wasn't that already accounted for in the delta-T calculation? My cross-check with the refrigerant table gave about 4 psi.
A: The pressure drop from the table or equation comes out per 100 feet of pipe — multiplying by 30/100 simply scales it to the actual 30-foot line length. Your 4 psi cross-check aligns with my 4.25 psi/100 ft, so we're essentially getting the same result. The only difference is that the line is 30 feet, not 100, hence the final step of multiplying by 30/100 to get 1.3 psi total.
OH 105: HVAC: Systems And Components Module #14
Q: Why can't we use Q = MΔH for this problem the way we did in problem 12? I tried applying the mass flow rate to the Type L copper table but got confused.
A: Problem 12 is a classic refrigeration cycle analysis — you're finding the flow rate of refrigerant around the cycle using energy balance. Problem 14 is fundamentally different: it's asking you to size the suction line, which means finding the capacity of the pipe from the copper tube tables in the reference handbook. The dead giveaway in the problem statement is 'Type L copper line' — that's telling you to go find those suction-line capacity tables, not to set up an energy balance.