The Energy Savings Calculator (also in Figure 4) requires the efficiency of the pumps operating at synchronous speed, the motor efficiency and the cost per kW of power. It calculates the total BHP and cost per hour of operation for each control scheme. As shown, synchronous operation requires an additional 2.9 BHP, and the cost per hour is increased by 24 cents.
When the efficiencies of the two control techniques at 400 gpm are compared, individual control trumps synchronous control by about six percentage points, and at 500 gpm, it will still have a one point advantage. As flow increases to 600 gpm, the two efficiencies get closer, but individual control is still higher by about 0.3 percentage points. This trend continues until flow reaches 700 gpm and both control techniques operate at 78 percent.
For this particular pump, individual speed control is the best control choice. For others, synchronous control may work equally as well, or even better. It will depend upon the application and the efficiency range. When three or more pumps operate in parallel, individual speed control should still be compared. However, it will have less of an impact as more pumps are brought online. For example, when the second pump is brought online and operated synchronously, each pump will initially operate at 50 percent of its BEP flow. When a third pump is added, each will begin at 66.6 percent, and if a fourth pump is added, each will begin at 75 percent of BEP flow. These increased minimum flows will also result in an increase in average efficiency.
Now, you may wonder if a savings of $0.24 per hour is worth the trouble. I think that it is if the system operates just above one pump flow for extended periods, even this small savings will add up over several years of operation. Also, the savings can be much larger as application BHP increases. Finally, it costs nothing because both schemes require the same components. Think of it as one small step that when included with others, allows the best possible increase in overall efficiency.
The beta version of VFPPA is available for download from the “Pump Sizing & Selection Tools” section of www.pumped101.com. The final version will support the generation of a system curve that is composed of both static and friction head. It should be available early next year.
Pumps & Systems, October 2010