by Lev Nelik, P.E., APICS

In Pump Challenge #3 (Pumping Prescriptions, September 2009), the specified parameters for energy estimation were 4,000 gpm and 400 ft. Using the Energy Estimator tool, you should receive the following data when comparing two speeds (3,600 rpm and 1,800 rpm):


Solution to Pump Challenge #3 Figure 1

Solution to Pump Challenge #3 Figure 2

The 3,600 rpm pump is smaller (11.30 in impeller versus 20.72 in) and more efficient (85.2 percent versus 84.6 percent). If this pump operates non-stop all year, it will burn for $2,613 less at a typical cost of energy of $0.10 per kWh.

The reason for the efficiency difference is in the pump's Specific Speed (Ns), as shown in the tabulations above. In our case, the faster running pump is more efficient, but that may not always be the case. It depends on Specific Speed.

In the other set of conditions specified in Pump Challenge #3 (2,000 gpm and 200 ft), will a faster running pump be more efficient?

Solution to Pump Challenge #3 Figure 3

Solution to Pump Challenge #3 Figure 4

he answer is no-the faster running pump is actually less efficient. The trick is an optimal value of the Ns. Just like the more familiar shape of the efficiency curve versus flow (for a given pump), this program gives a useful estimate for comparing pumps. Entering several speeds-starting with rather low values and increasing to high-reveals that Specific Speed will change. Efficiency will grow, reach the maximum value and then drop past that Ns.

This information is useful in selecting optimal motor speed and evaluating the effect of a VFD on efficiency.

Click here for Pump Challenge #4: Energy Costs