Power-efficiency curve, centrifugal pumps

A set of curves for h, rj, and P as a function of Q are shown in Figure 8.25 from which it is seen that when die pump is operating near optimum conditions, its efficiency remains reasonably constant over a wide range of flowrates. A more general indication of the variation of efficiency with specific speed is shown in Figure 8.26 for different types of centrifugal pumps. The power developed by a pump is proportional to Qghp ... 

Fig. 11.—Head-capacity, power and efficiency curves for centrifugal pumps.

Power consmnption the operating point of centrifugal pump should be chosen at the highest efficiency possible. Generally, the vendors/manufacturers offer a range of models, which cover the requirements of capacity, head, etc. However, the final selection should be done while keeping in mind the most likely (normal) condition in which the pump will be ran. In case of positive displacement pumps the performance curves should be studied to select proper speed for the capacity and required head... 

Where the driven machine is a centrifugal type i.e. pump or compressor, the shaft power may be taken as that which occurs at the end of curve operating point. This rule-of-thumb point is defined as being 125% of the power required at the maximum operating efficiency point on the designed curve of pressure (head) versus fluid flow rate, at the rated shaft speed. 

Figure 18.11 shows the performance curve for a centrifugal conpressor. It is immediately observed that the y-axis is the ratio of the outlet pressure to inlet pressure. This is in contrast to punp curves, which have the difference between these two values. Curves for two different rotation speeds are shown. As with punp curves, curves for power and efficiency are often included but are not shown here. Unlike most pumps, the speed is often varied continuously to control the flowrate. This is because the higher power required in a conpressor makes it economical to avoid throttling the outlet as in a centrifugal punp. 

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