Problem: During design review, an engineer notices a 1.1-1-4-inch nominal hot water pipe intended to carry 6GPM of 130 degree water takes a sub-optimal rout...
Given: 6GPM of 130 degree water takes a sub-optimal route along the perimeter of the building, containing 250 linear feet of...
Approach: In case 1, we have a head loss HF1, let's say, which is going to have some major losses FLV squared over 2DG and some minor los...
Calc: In case 1, we have a head loss HF1, let's say, which is going to have some major losses FLV squared over 2DG and some minor los...
Calc: FL1 over D plus K times V squared over 2G.
Result: So using epsilon over D and the Reynolds number we can go to the moody diagram and look up the friction factor 0.002 all the way down to 2.7 times ...
Office Hours
4
Student questions asked in live office hours about this problem
OH 46: HVAC: Fluids-28
Q: I compared HF using the steel pipe friction tables versus the Darcy equation and got significantly different results—why is there such a large discrepancy?
A: The deviation is simply the inherent difference between the two methods—the steel pipe friction tables and the Darcy equation are both valid, but they don't always reconcile exactly. In this problem, the discrepancy is large enough to push you to a different answer choice, which is unfortunate but not uncommon for problems that compare percentages or ratios where small differences get amplified. The takeaway is to be aware of which method you're using and to be consistent throughout a given calculation.
OH 73: HVAC: Fluids Module #28
Q: I solved head loss for fittings and piping separately and got 55% reduction, but the solution combined them algebraically and got 44%—did I do something wrong?
A: You didn't go wrong—both approaches are valid in principle, and the simplification in the solution just cancels common terms to arrive more elegantly at 44%. The difference in your result (55%) suggests a setup error somewhere in how you defined the 'reduced' case rather than a conceptual error. Check whether your denominator (original total loss) and numerator (reduced total loss) are consistent with what the problem actually changed.
OH 80: HVAC: Fluids #28
Q: I got 22% (answer A) instead of 44% (answer C)—I got the same friction factor, but my percent reduction formula was slightly different. Can you check my work?
A: This problem compares two scenarios: original piping with elbows at full length versus redesigned piping at 30% shorter length with elbows removed. The percent reduction formula must correctly capture both changes—reduction in pipe length and elimination of elbow equivalent lengths. A common error is defining the numerator or denominator of the percent reduction incorrectly, which can cut the result in half (hence getting 22% instead of 44%).
OH 96: HVAC: Fluids Module #28
Q: If the engineer only straightened the run without reducing the total pipe length by 30%, would you solve it by adding the elbows' equivalent length from the table to the original 250 feet?
A: Yes—if the elbows are removed but the total pipe length doesn't change, you'd use the elbow equivalent length from the fitting tables, add it to the original pipe length, and compare that total to the case without elbows. The equivalent length method is perfectly appropriate here since you're only dealing with elbows and the handbook provides those values. The problem as written includes both the length reduction and the elbow removal, so both effects compound.