Problem: Fluids 10, a hotel with 400 units requires a maximum cold water supply of 5GPM per unit with a diversity factor of 60%.
Given: 5GPM per unit with a diversity factor of 60%; 000 feet long and made of 8 inch standard weight steel with a roughness...
Approach: The supply line is 5,000 feet long and made of 8 inch standard weight steel with a roughness coefficient of C equals 130.
Calc: The start of the supply line is 100 feet above the delivery end at the hotel.
Calc: And it says the start of the supply line is 100 feet above the delivery end at the hotel.
Result: So the best answer choice is A.
Office Hours
3
Student questions asked in live office hours about this problem
OH 15: Fluids 10
Q: I used the Darcy equation and roughness coefficient method on problem 10 and got results that differed by more than double—should they be closer, and is the Darcy equation the assumed method on the exam unless roughness is given?
A: Yes, they should reconcile, but consider absolute versus relative error—doubling a small head loss is still small in absolute terms, so discrepancies are more pronounced at lower loss ranges. Use the roughness tables for water in steel pipe (faster), but the Darcy equation is always a solid fallback; both should agree within reasonable engineering error.
OH 53: HVAC: Fluids #10
Q: Can we use the Hazen-Williams equation on the PE exam, and why did I get different results when I tried it?
A: Yes, you can use Hazen-Williams, but the exam tends to favor the Darcy equation and friction tables because they test fundamentals more directly. Your different results likely stem from assumptions about the C value—if you're not given it explicitly, you're stuck; all three methods should theoretically align within engineering tolerance, but minor differences are expected and acceptable.
OH 55: HVAC: Fluids #10
Q: In Bernoulli equation problems, why does pressure divided by gamma equal pressure? I keep getting confused about the units.
A: Strictly speaking, it doesn't—you need to distinguish between SI and US customary units. In US customary units, pressure = ρgh/gc (not just ρgh), and gamma is defined as ρg/gc, where g and gc have the same magnitude but different units, so they cancel numerically while their unit differences allow the algebra to work out correctly.