VFDs Controlling Chiller Pump Motors

By Jonathan Susser | January 15, 2021

This article was written by Charlie Martin, one of our senior energy engineers.

Winter is a perfect time to conduct maintenance on your cooling system and the heart of that system, chillers. It’s also a perfect time to make adjustments to the various components to ensure optimal system performance. Start by reviewing the system trend logs of the chiller, the chilled water and condenser water pumps, and the cooling tower.

If you have variable frequency drives (VFDs) on the chilled water or condenser water pump motors, review their performance. If your chillers operate at part load for much of the cooling season, the VFDs should be providing significant energy savings compared to constant-speed pump motor operation. If the information is not revealing the energy savings you expected, the issue may be with your system’s water flow control valves.

During a recent energy assessment, we reviewed a new chiller installation. The chiller plant consisted of two high-efficiency, 1,300-ton centrifugal chillers with VFDs for capacity control. The chillers also had variable water flow with VFDs on the 125-HP chilled water pump motors and the 60-HP condenser pump motors.

Our inspection found that the triple-duty valves in the chilled water valves were set at 70 percent open and the condenser water valves were set at 50 percent open (see Figure 1). In this arrangement, the valves are increasing resistance to water flow and reducing the respective maximum flows. This is a common practice for adjusting the valves to achieve desired flow for constant-speed pumps; however, having these valves set to limit flow is reducing the associated energy savings of the pump motor VFDs. With VFDs installed, these valves are not generally needed for flow adjustment and should be fully opened.

Figure 1. Condenser Water Valve

Centrifugal pumps are variable torque machines that follow what are known as the “Ideal Pump Laws” or “Affinity Laws.” In this equipment, a small change in speed can produce a large change in input power. For example, reducing pump speed by 20 percent reduces input power by more than 50 percent.

Through our inspection, we noticed an opportunity to reduce pump motor energy by opening the triple-duty valves in all the chilled water and condenser water circuits. Once the valves are opened to 100 percent, the VFD on the pump motor can be readjusted to achieve motor speed and obtain needed maximum flow. Our analysis revealed that opening the respective valves to 100 percent and utilizing the VFDs to fully modulate water flow would save an additional 129,000 kWh of electricity annually.