Problem: Fluids 12, 200 GPM of 90 degree water flows through 500 feet of 4-inch nominal schedule 40 steel pipe with a discharge that is 30 feet higher than ...
Given: 200 GPM of 90 degree water flows through 500 feet of 4-inch nominal schedule 40 steel pipe with a discharge that is 3...
Approach: For the computer-based test using the reference handbook instead of the murm, we're going to be looking up k factors from a tab...
Calc: So that's the difference between P1 and P2.
Calc: Now we're going to be looking for the difference between P1 and P2 presumably P1 is greater than P2 because it's driving a flow...
Result: So to specify F the friction factor we need the Reynolds number and the relative roughness Reynolds number is vd over new and the velocity we're go...
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Student questions asked in live office hours about this problem
OH 58: HVAC: Fluids-12
Q: The final answer divides P1 minus P2 by specific weight, but the question asks for the pressure difference—why do we divide by specific weight?
A: The result in feet of head needs to be converted to PSI using the rule of thumb: divide by 2.31 (since 2.31 feet of water equals one PSI). You can also multiply by 62.4 lb/ft³ and divide by 144—it's the same operation. Either way, the specific weight step is required to convert head in feet to pressure in PSI.
OH 63: HVAC: Fluids Module #12
Q: I used the equivalent length tables instead of K-factors for minor losses and got 19.4 PSI instead of 14.6 PSI—are both methods valid?
A: Both methods are valid and you did it correctly. The equivalent length method was common in the open-book exam era, but now the reference handbook only includes equivalent lengths for elbows, making K-factors more broadly applicable. Know both methods, but lean on K-factors for their flexibility with mixed fitting types.
OH 77: HVAC: Fluids #12
Q: How do you use the two equivalent length charts on page 216, and why do we use K-factors instead?
A: The equivalent length tables require you to know pipe size and velocity, and they only apply to specific 90-degree elbows—for anything else you're back to K-factors. K-factors are preferred because you can sum all K values into one total and plug it into the minor loss formula KV²/2g. Know both methods, but K-factors are more flexible when dealing with mixed fitting types.
OH 103: HVAC: Fluids Module #12
Q: What's the difference between the steel pipe friction tables and the Darcy equation? I'm getting HF major of 21.5 feet using the tables and a final answer of 23.74 PSI.
A: The Darcy equation is more general and applies to any fluid; the steel pipe friction tables are specific to steel pipes but fast and convenient for the typical water-in-steel-pipe PE problem. If both methods are applied correctly, they should reconcile—a large relative error may look alarming, but if the absolute difference is small it's acceptable. Both are valid for steel pipes; if it's a different material, default to Darcy.