David W. Spitzer

A centrifugal pump is designed to flow 100 lpm of water at a pressure of 10 bar at full speed. Using the Affinity Laws, what is the approximate flow and pressure generated when a variable speed drive operates the pump motor at 70 percent speed? Approximately how much less energy is required at the lower speed?

A.    70 lpm and 7 bar using 70 percent less energy
B.    70 lpm and 7 bar using 30 percent less energy
C.    70 lpm and 5 bar using 70 percent less energy
D.    70 lpm and 5 bar using 30 percent less energy
E.    70 lpm and 3 bar using 70 percent less energy
F.    70 lpm and 3 bar using 30 percent less energy
G.    None of the above

Commentary
The Affinity Laws for centrifugal loads state that:
1.  Flow is proportional to pump speed.
2.  Pressure is proportional to the square of pump speed.
3. Power is proportional to the cube of pump speed.

Therefore, the calculated flow, pressure and power consumption at 70 percent speed using the Affinity Laws is approximately:
1. 70 lpm (0.7 * 100 lpm)
2.  5 bar (0.7 * 0.7 * 10)
3.  34 percent (0.7 * 0.7 * 0.7 * 100 percent)

Energy consumption of 34 percent represents a 66 percent energy savings, so the closest answer is Answer C.