The correct method of calculating energy savings to justify adjustable-frequency drives on pumps

It is easy to make a bad business decision when using electrical energy savings as a justification to install adjustable-frequency drives (AFDs) on pumps. The simple hydraulic formulas and "rules of thumb" are easily misapplied and the errors will almost always economically favor the AFD installation. To use energy savings as a justification for an AFD installation it is necessary to accurately determine these savings over the life of the equipment. These savings are not dependent upon the AFD or motor characteristics but depend upon the characteristics of the process system. This paper is a tutorial in nature and will show why AFDs save electricity, give examples of the common errors that are made in performing the savings calculations, show how to do these calculations correctly, show how to mathematically model the process to assist in performing the analysis, and show how to perform the economic calculations to arrive at a rate of return and net present value on the AFD investment.