85°F ECWT 1% A 100 90 80 Chiller Load % 70 60 50 40 30 20 10 0 12% of Operation 55°F 60°F 65°F 70°F 75°F 80°F Entering Condenser Water Temperature 85°F 45% of Operation 42% of Operation 1% of Operation Load Lift 42% B 45% C 12% D 75°F ECWT 65°F ECWT Figure 4: IPLV/NPLV operating buckets. Figure 3: Two-chiller efficiency comparison: custom analysis versus generic index ratings. plant operates 57% of the time at 65°F (18.3°C) entering condenser water temperature, and only 1% of the time above 75°F (23.9°C) entering condenser water temperature. As stated by the AHRI IPLV definition (Figure 1), all operating points above 75% load and warmer than 75°F (23.9°C) water are assumed to occur only 1% of the time. Said another way, 99% of the time, the chiller will be unloaded to less than 75% load and operate with 75°F (23.9°C) entering condenser water or colder. Fact 3: IPLV does not take variable-speed drives into consideration. Unfortunately, the values and methodologies originally intended to rate chillers did not fully consider the application of variable-speed drives, at least on centrifugal designs. Centrifugal compressors are dynamic compressors making them more dependent on lift reduction in lieu of load reduction. www.info.hotims.com/25211-32 ASHRAE Journal December 2009