A centrifugal pump produces head by accelerating fluid to a velocity approaching the tip velocity of the rotating impeller. The liquid leaving the pump is at a high velocity and is forced into the discharge pipe from the pump, where it comes into contact with relatively slow-moving fluid in the pipe. As the fluid decelerates, the velocity head is converted into pressure head. For a given pump the following relationships can predict performance:

The speed of rotation of the impeller can be adjusted, even when a constant speed motor is used, by the use of a pulley system as shown in Figure P21.19.

figure p21.19

The pump curve given in Figure 21. 4 was obtained when the impeller was directly coupled to a constant-speed motor ( \(3500 \mathrm{rpm}\) ). Develop performance curves for this pump, with a 7-indiameter impeller, assuming that the pump is driven via a pulley system in which the constant- speed motor ( \(3500 \mathrm{rpm}\) ) has a 6-in-diameter pulley and the motor impeller is connected to a series of pulleys with diameters of 4 in, 5 in, and 6 in. Construct performance diagrams for each pulley combination.

figure 21.4

Transcribed Image Text:

h2/h = [(N2/N)(D2/D)] and / = [(N/N)(D2/D)]