CENTRIFUGAL PLANT EFFICIENCY AUDIT
Chiller Part-Load
(IPLV) Index
Audit staging efficiency markers against standard part-load benchmarks to isolate compressor performance drift.
Understanding Chiller IPLV and Compressor Part-Load Staging
Centrifugal chillers rarely operate at 100% capacity for their entire service life. In reality, they spend the vast majority of their operational hours at 50% to 75% load. The Integrated Part Load Value (IPLV) is a weighted metric developed by ASHRAE to account for this. It acknowledges that a chiller’s efficiency changes as the cooling demand changes. A system might be extremely efficient at full capacity but struggle with “surge” or “stall” conditions when loads drop, causing efficiency to crater. Understanding your chiller’s IPLV allows engineers to predict seasonal energy consumption rather than just peak-load performance.
Frequently Asked Questions
Q: Why does a chiller’s kW/Ton rating often improve when the load drops to 75% but worsen again at 25%?
A: At 75% load, the compressor lift requirements are lower because the heat exchanger surfaces have more “excess capacity,” leading to more efficient heat transfer. However, as the load drops toward 25%, the system approaches the “surge line”—a point where the pressure differential across the compressor becomes too high for the volume of gas being moved. Systems often activate energy-hungry anti-surge hot-gas bypass valves at these low loads, which kills overall efficiency.
A: At 75% load, the compressor lift requirements are lower because the heat exchanger surfaces have more “excess capacity,” leading to more efficient heat transfer. However, as the load drops toward 25%, the system approaches the “surge line”—a point where the pressure differential across the compressor becomes too high for the volume of gas being moved. Systems often activate energy-hungry anti-surge hot-gas bypass valves at these low loads, which kills overall efficiency.