With the decreasing cost of VFDs, a VFD equipped chiller is a smart choice for nearly any situation. But, once you’ve made that choice, figuring out exactly what the savings are and how to best sequence the plant can be a more difficult exercise. The two most common energy efficiency ratings given to chillers are full load kW/ton, and Integrated Part Load Value (IPLV). This is also measured in kW/ton. IPLV weights the chiller efficiency at different loads and condenser water temperatures to attempt to better approximate actual usage over a typical year. Full load is only given a 1% weighting, 75% load is given a 42% weighting, 50% load, a 45% weighting, and 25% load a 12% weighting.
The other factor that goes into the IPLV rating is relief. Relief is the added efficiency gained through a cooler condenser temperature. This is the reason that IPLV ratings generally have a lower kW/ton than full load ratings. AHRI sets the standards for rating a chiller, as well as other heating and cooling equipment. They allow for the ratings to include relief. The relief schedule is 85 deg F condenser water temperature at full load, 75 deg F at 75% load, and 65 deg F at 50% and 25% loads.
This relief that’s built into the chiller efficiency rating can make it difficult to determine how to sequence your chillers most efficiently. Looking at the standard power curves, it may look more efficient to run two chillers at 50% load than one at 100%. But, remember that it’s an apples and oranges comparison because the 50% efficiency ratings include relief. The only sure way to analyze true energy impacts and determine for certain the best sequencing is to gather a number of efficiency curves for each chiller at a variety of fixed condenser water temperatures. That way, you can manually select a given condenser water temperature and compare part load and full load ratings of each chiller.
The bottom line is that if IPLV ratings are great to run some back of the envelope numbers, but please recognize the limitations of these ratings in trying to nail down the energy savings or determine a chiller sequencing strategy.