HVAC · Practice-Exam-2 · Problem 57PDFSolution in PDF ↓
HVAC · Practice-Exam-2 · Problem 57
Problem & Solution
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Topic: Pump system — A pump running at 1,750 rpm delivers 250 gpm and generates 150 feet of head.
Method: What is the head generated by the pump after the impeller is trimmed by 25% assuming the speed remains the same.
Key values: 150 feet of head, 4 feet now it, 250 gpm and generates 150 feet of head
Reference: search the reference handbook for pump affinity laws, you can jump
✅ Answer: A
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Student questions asked in live office hours about this problem
OH 119 · April 28, 2026
Q: In a pump affinity law problem with constant speed, why does head still change — and why doesn't N₂/N₁ = 1 keep it constant?
A: Pump affinity laws have two independent columns: speed change and diameter change. Here the speed is held constant, but the impeller is being trimmed (diameter reduced), which causes a linear reduction in flow, quadratic reduction in head, and cubic reduction in power. The problem is about diameter change, not speed change — recognizing which variable is changing is the first step before applying any affinity law equation.