Problem: A 40GPM cold water booster pump is located 15 feet below the top of a storage tank held at atmospheric pressure from which water is supplied.
Given: 40GPM cold water booster pump is located 15 feet below the top of a storage tank held at atmospheric pressure from wh...
Approach: The suction piping is made up of 50 feet equivalent length of 2 inch schedule 40 steel.
Key formula: formula for that which we'll use in this problem
Calc: We have a tank of water and in that tank we have 60 degree water.
Calc: So, when it says it's never warmer than 60 degrees, we can just assume it's 60 degrees and that's the worst case scenario.
Result: So the best one is D.
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Student questions asked in live office hours about this problem
OH 21: OH21-2-FLUIDS-25
Q: Is it okay to use the 'design version' of the NPSHA formula from the reference handbook for this problem, which differs slightly from the one in the solution?
A: Yes—the design version leads to the same answer for this problem, so it's a valid approach. Both versions are ultimately the same thing expressed slightly differently. For exam purposes, the written solution (not the video) is the most current and accurate reference.
OH 25: HVAC Fluids 25
Q: I've only found part of the NPSHA equation used in the solution inside the reference handbook—is the solution equation correct?
A: Please refer to the written solution rather than the solution video for this one—the written solution was updated to align with the reference handbook and is accurate. The video may reference an older formula from a different source, and I wouldn't re-record the whole video just for that change. The written solution has exactly the right setup.
OH 42: HVAC: Fluids-25
Q: The equation used in the solution looks different from the one in the reference manual—is the solution correct, and does it account for HF?
A: The video used an older version of the NPSHA formula; the written solution has been updated to match the reference handbook and is what you should follow. Both versions reconcile to the same answer for this particular problem. If in doubt, always use the version of the formula in the reference handbook since that's what will be available on the exam.
OH 63: HVAC: Fluids Module #25
Q: The solution video includes a V²/2g term in the NPSHA formula that the reference handbook doesn't show—what is its importance, and when do you use the existing conditions equation versus the standard NPSHA equation?
A: There are two NPSHA equations in the reference handbook: the design (theoretical) version and the existing conditions version. The design version is used when you haven't built the system yet and you're computing what the suction head should be; the existing conditions version is used when the system already exists and you're checking whether cavitation will occur. For this problem, the existing conditions formula is the right one.
OH 71: HVAC: Fluids Module #25
Q: When solving an NPSHA question, should I automatically think about conditions occurring before (upstream of) the pump?
A: Yes—NPSHA is Net Positive Suction Head Available, meaning it's entirely about the suction side (upstream) of the pump. You need a certain column of fluid sitting on the pump's suction inlet at all times to prevent cavitation. Physical elevation, inlet pressure, and vapor pressure all factor into what's 'available' before the pump.
OH 76: HVAC: Fluids Module #25
Q: How did you get the NPSHA formula in the video when the reference handbook doesn't have all the same terms?
A: The video formula came from an older reference, which is an error—the reference handbook version is what you should use on the exam. Going term by term, both formulas reconcile to the same physical meaning, just expressed slightly differently. Use the reference handbook formula as your guide and verify each term represents a physical quantity you understand.
OH 104: HVAC: Fluids Module #25
Q: Why do we use the NPSHA (existing conditions) equation instead of the standard NPSHA design equation for this problem?
A: The existing conditions equation is used when the system is already built and you're checking whether cavitation will occur under actual operating conditions. The design version is used when you're designing a new system and specifying the required suction head. This problem presents a built system and asks whether conditions are safe—so the existing conditions formula applies.
OH 111: HVAC: Fluids Module #25
Q: Can you clarify the use cases between the standard NPSHA equation and the existing conditions equation in the reference manual?
A: The design (theoretical) NPSHA equation is used when you're designing a system from scratch and need to establish what the suction head must be to avoid cavitation. The existing conditions equation is used when the system is already in place and you're verifying whether the available suction head is sufficient given the actual operating conditions. As a shortcut: if the problem says 'existing system' or gives you real operating data, reach for the existing conditions version.