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

Abstract

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 formulae and "rules-of-thumb" are easily misapplied and the errors will generally 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 tutorial in nature and shows why AFDs save electricity, gives examples of the common errors that are made in performing the savings calculations, shows how to do these calculations correctly, shows how to mathematically model the process to assist in performing the analysis and shows how to perform the economic calculations to arrive at a rate-of-return on the AFD investment.