Question 24 from the 2016 Chemistry and Engineering case study, afternoon session
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24. A centrifugal pump is used to transfer water at a rate of 25 m3/h at 20°C from an open container. The suction pipe of the centrifugal pump is 10 m long, with an inner diameter of 68 mm. Assuming that the fluid flow in the suction pipe has reached the region of constant resistance, with a friction coefficient for straight pipes of 0.03 and a total friction coefficient of 2, and given that the local atmospheric pressure is 90.15 kPa, what is the allowable installation height (in meters) for this pump? (At 20°C, the saturated vapor pressure of water is 2.33×10-3 Pa; at this flow rate, the required net positive suction head for the pump is 2.2 m, with a safety margin of 0.6 m.) (A) 4.95 (B) 5.45 (C) 5.95 (D) 6.55. There is some doubt here: using the formula H = (P – p)/((ρg) – (λ*l/d + ξ)) * U²/(2g) – 2.8, the answer is 4.95 m. But why isn’t the dynamic head loss taken into account? Textbooks on chemical engineering principles clearly state that the dynamic head loss at the pump inlet should be considered. If that loss were taken into account, the calculated value would be 4.78 m
I think since the problem already provides the inlet and outlet resistance coefficients, there’s no need to consider the dynamic head again; in other words, the loss due to dynamic head is already taken into account
The title mentions a total resistance coefficient of 2.
Is the resistance coefficient of a straight pipe the friction coefficient λ of the straight pipe? The saturated vapor pressure should be 2.33×10^3 Pa, which is 2.33 kPa, right?
Another issue is that the frictional losses associated with import and export are local losses; they have nothing to do with the dynamic head, right? Seeking advice~
Another issue is that the frictional losses associated with import and export are local losses; they have nothing to do with the dynamic head, right? Seeking advice~
The resistance coefficient of a straight pipe shouldn’t be the friction coefficient, right? According to my calculations, as shown in the picture, the answer is 5.95. Choose C
According to your calculation, it’s not 5.95 either; it’s 5.77. I think 0.03 should still be represented by λ, otherwise it’s useless even for determining the pipe length!