Bman
Well-known member
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?
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?
Not sure what I was thinking there....
