Problem: 50GPM of cold water 60 degrees enters a 100-foot tall building via 3-inch schedule 40-steel supply piping.
Given: 50GPM of cold water 60 degrees enters a 100-foot tall building via 3-inch schedule 40-steel supply piping; 85 PSIG; 5...
Approach: Street pressure prior to the service entrance to the building is 85 PSIG.
Calc: There's this 3-inch pipe, 3-inch nominal and there's some pressure in the street.
Calc: We'll call that P1 equals 85 PSIG and then branching off of that supply piping in the street.
Result: I'll keep the left side the way it is P2 over gamma equals 85 psi times 2.31 feet per psi and you have to remember that this is gauge not absolute ...
Office Hours
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Student questions asked in live office hours about this problem
OH 31: HVAC_FLUIDS-9
Q: Can you use the head loss per 100 feet from a schedule 40 steel pipe table multiplied by equivalent length instead of the Darcy equation?
A: Yes, both methods are acceptable and equally reliable for steel pipes. Know how to do both, but the steel pipe friction tables are faster and my go-to first approach. Be aware the two methods may give different results (sometimes 50-100% relative error), but focus on absolute error—the difference is often negligible in practical terms, and answer choices usually won't be close enough to matter.
OH 39: HVAC: Fluids-9
Q: When should we use the standard weight steel pipe table versus schedule 40, and if using that table, why shouldn't we also use the head loss values from it? The student got a slightly lower answer than 38.9 and would have gotten it wrong if 38 was an option.
A: You can absolutely use the head loss values from the table—it's often faster. Know both methods for studying, but on the exam use whichever is faster. The slight deviations you're seeing are normal; I intentionally don't put answer choices too close together because engineering problems often have inherent uncertainty, and you need to develop judgment to handle that variability rather than expect false precision.
OH 45: HVAC: Fluids-9/11/12
Q: When using steel pipe tables versus the Darcy equation to find head loss, I'm getting different results (e.g., 6.2 vs 3.2 feet, 2 vs 1 foot, 19.4 vs 15.2). How precise should these two methods agree, and will the PE exam have close answer choices where this discrepancy matters?
A: Some variability between methods is normal, but if answers are close enough to create ambiguity, the problem is flawed—unlikely on the PE exam. Focus on absolute error, not just percentage difference: a 100% difference in head loss might only be 2-3% error in the final answer if head loss is a small contributor to total pressure. Context matters—know what the question is actually asking.
OH 68: HVAC: Fluids Module #9
Q: For Bernoulli equation problems: what units should pressure be in (the problems give PSI G), and how do you know if elevation change is positive or negative—is it positive when state 2 is higher?
A: For pressure units, you can use PSI or feet depending on which form of Bernoulli you're using—just be consistent throughout (either all PSI G or all PSI A, not mixed). For elevation, it's entirely your choice where you set the datum; what matters is consistency with your chosen reference point, and ultimately aligning with the answer format.
OH 68: HVAC: Fluids Module #9
Q: When the PE handbook gives a range for the relative roughness factor (absolute roughness), how do we determine which value to use for finding friction factor on the Moody diagram?
A: Just use the average of the range every time. That's the standard approach for Fluids problems and should work fine for the exam.
OH 79: HVAC: Fluids Module #9
Q: I used the 2.31 feet per PSI rule of thumb and got 42 PSI for top floor pressure, but the answer is 40 PSI—why didn't my estimation get closer?
A: You didn't account for head losses using the Darcy-Weisbach equation (HF = fLV²/2DG), which adds about 2-3 feet of losses and brings your answer down to approximately 39-40 PSI. Check the solution video for the detailed calculation.
OH 96: HVAC: Fluids Module #9
Q: Does NPSHA (Net Positive Suction Head Available) apply only to pumps, or should I use Bernoulli's equation if there's no pump in the problem?
A: NPSHA and NPSHR are pump-specific concepts used to prevent cavitation—they don't apply without a pump. You likely pattern-matched on the word 'available' in the Fluids 9 question, but slow down and match the tool to what the problem is actually asking; in that case, a simple pressure equation was the answer, not NPSHA.
OH 111: HVAC: Fluids Module #9
Q: Why can we use kinematic viscosity from atmospheric pressure water tables when the problem involves water at 85 psi gauge, which is much higher than atmospheric pressure?
A: Liquids are essentially incompressible, so pressure has negligible effect on viscosity—less than 1% change even at that pressure. However, temperature is what significantly affects kinematic viscosity, which is why the property tables have different rows for different temperatures; treat viscosity as a function of temperature only, not pressure.