Technical Description

When variable frequency drives are employed to regulate pump speed in variable flow distribution systems, pump laws dictate that the pump can supply 50% of design flow at 50% speed and require only 12.5% of the full flow power, but only if the pressure at which the fluid is supplied is permitted to fall to 25% of the full flow pressure. This part load energy reduction potential is seldom attained because it is common practice to regulate the pump speed by means of a fixed pressure setpoint.

There are actually two costly energy penalties for such variable flow pumping systems. Loss of pump efficiency at part loads is one. The second is the additional pump head required due to the practice of employing valves one or more sizes smaller than the size of the piping serving each load. This is done in an effort to maintain a linear relationship between valve travel and capacity. However, the combination of reduced part load pump efficiency and a higher head pressures increase pump power requirements in chilled and heating water distribution systems at all load conditions (see Figure 1).

Many rules in use today for sizing valves and designing distribution systems date back to before variable speed drives were available. While these rules did achieve successful systems in the days of simple pneumatic controls, they are now outdated by the availability of variable frequency drives and networked digital controls.