Problem: Fluids 19, a makeup pump that cycles on and off intermittently as required has a service factor of 1.25 and provides a water horsepower of 15 horse...
Given: 15 horsepower; 15 horsepower motor, being enough to produce a BHP of 17; 15 horsepower times the service factor of 1
Approach: What is the smallest motor that could be used?
Calc: We have the pump efficiency 84% and the motor efficiency is 93.
Calc: And we know that the pump provides a water horsepower of 15, so we know WHP.
Result: So the efficiency of the pump hurts because it means you need more power to produce 15 horsepower of hydraulic horsepower, but there's inefficiency.
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Student questions asked in live office hours about this problem
OH 34: HVAC: Fluids-19
Q: In the solution, the service factor of 1.25 is checked directly against water horsepower, not brake horsepower—shouldn't motor selection be based on BHP?
A: The service factor means you can run the motor 25% harder than its rated value because the pump operates intermittently. So you take the motor's nameplate rating and multiply by 1.25 to get an effective BHP capacity, then check that against the required BHP—you're making the motor rating larger, not the required BHP. The smallest motor whose effective capacity (rating × 1.25) exceeds the required BHP is the correct selection.
OH 46: HVAC: Fluids-19
Q: The problem says the pump has a service factor of 1.25—does the service factor apply to the pump or to the motor?
A: The service factor effectively applies to the motor selection, not the pump's hydraulic performance—we find the required BHP from the pump's water horsepower and efficiency, then use the service factor to see how much output we can get from a smaller motor. A better way to think about it: how much BHP can we get out of a motor rated below our requirement, given that the service factor allows it to run 25% harder? The smallest motor whose rated capacity times 1.25 exceeds the required BHP is the right pick.
OH 103: HVAC: Fluids Module #19
Q: Options C and D, when multiplied by the service factor, also give values greater than 17.9 HP—why isn't the answer C or D?
A: The service factor is like overclocking a processor: the manufacturer allows it when the pump runs intermittently, so you can select a smaller motor and run it harder than its rating for brief periods. The question asks for the smallest motor that still works, so once option B (15 HP × 1.25 = 18.75 HP > 17.9 HP) satisfies the requirement, you don't go larger. Options C and D also work, but selecting a larger motor than necessary is wasteful and not the engineer's job.