Comparison of 4 Different Flow Control Methods Of Pumps
Based upon the system head conditions, pumps may be able to pump at higher rates than needed when operating at 100% motor speed. This flow rate can be controlled by one of two ways, throttling the pump with a valve if the pump is a constant speed pump, or changing the motor speed with a variable speed drive. The former is only energy efficient if the higher flow operating point of the pump without throttling is to the right of the best efficiency point on the pump performance curve, and the throttling results in reducing the flow to a point closer to the best efficiency point on the pump curve. Otherwise, throttling the pump can result in using more energy than at the higher flow, as well as wasting energy because you end up using more energy than is needed. When the demand on the system fluctuates significantly, the pumping rate can be controlled automatically by varying the speed of the motor with a variable frequency drive (VFD), such that the pump output matches only what is needed to meet demands or the intended pumping conditions. The pump’s flow rate then increases or decreases based upon the affinity laws and the controlled speed of the motor. This way lower pumping rates can be achieved, which may result in lower efficiency than those at full motor speed; however, the energy consumption is still lower because the energy requirements to pump lower flows at lower heads are lower.