Some Surprises in Estimating the Efficiency of Variable-Speed Pumps with the Pump Affinity Laws

Abstract

The pump affinity laws are widely used in adjusting pump performance curves from one pump or speed to another. In most cases, they work fairly well and can be reliably used in modeling studies. This paper describes an experiment undertaken to determine how well these laws apply to a variable speed pump, in which the speed is controlled by a variable-frequency drive, the most common technology used today. The pump speed was varied from full speed to roughly 30 percent of full speed. Over that range, the affinity laws worked quite well for adjusting the pump head characteristic curves. However, the pump efficiency curves fell significantly lower than those predicted by the affinity laws. The explanation for this is most likely the loss of efficiency in the variable-frequency drive itself. Since energy costs are the largest component of pump life-cycle costs, the implications for pump selection are significant. With variable-speed drives costing almost as much as the pumps themselves, it becomes essential for life-cycle cost estimates, whether done manually or with models, to accurately account for the loss of efficiency.