# Six Minute Solutions T&F Question #8



## Bman (Mar 25, 2010)

Having a little trouble with this concept.... The problem reads:

The feedwater piping system of a boiler is designed for an operating pressure of 2200 psi and a temperature of 280oF at the outlet. The friction pressure drop in the piping system under the design conditions is 33 psi. The design flow rate is 1.4 x 10^6 lbm/hr. The velocity pressure differentials and the static pressure drops are negligible; the lowest mass flow rate is 740,000 lbm/hr. Assuming a laminar flow under all operating conditions, the lowest pressure drop in the feedwater piping is most nearly:

A.	9.2 lbf/in2

B.	17 lbf/in2

C.	33 lbf/in2

D.	120 lbf/in2

Looking at the friction loss equation (hf = (fLv^2)/(2Dg)), I would assume that as v decreases, hf will increase (since the numerator is smaller) which would indicate the pressure drop due to friction will increase as the flow rate decreases. The answer is A though, which means the pressure drop due to friction decreases as the flow rate decreases.

The author derives the equation delta P(lowest) = delta P(design) x (m^2 lowest / m^2 design)

The m = mass flow rate.

Am I missing something? Shouldn’t the pressure drop be inversely related to the velocity?


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## Bman (Mar 25, 2010)

Ok, nevermind, I'm an idiot  Not sure what I was thinking there....


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