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Application of VFDs to Multiple Pump Sizes in HVAC Systems


Jiajun Liao, PhD, CEM, PE, LEED AP BD+C, Retro Commissioning Project Manager, contributed to the ASHRAE technical paper, “Application of VFDs to Multiple Pump Sizes in HVAC Systems.”

Variable-frequency drives (VFDs) have been widely applied to chilled-water pumps and heating hot-water pumps to meet variable loads in building heating, ventilation, and air conditioning (HVAC) systems. However, pumps with VFDs have a minimum speed near 30% of rated speed due to the motor overheating at lower speeds. Thus, they have to run at this minimum power and flow rate to cover a large percentage of hours when the system loads are below 30% of the designed condition. This results in a minimum pump power of ~10% of rated power when the pump power could be as low as 1% of rated power during these hours if the pump were capable of operating effectively at lower speeds. A new configuration called multiple pump sizes with VFD is proposed. A lower capacity pump with a VFD is added to cover the operating hours with lower loads and maintain a constant-loop differential pressure in the pumping system. It can significantly reduce pump energy consumption. A laboratory building and a hospital building both running for 24 hours per day, 7 days per week are analyzed as case studies, and results show 10.5% to 32.1% pumping power energy savings with the new configuration and 1.5 to 7.1 year simple payback.

The full paper can be purchased at the link below.

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