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TRG-TRC012-EN 39
period five
Application Considerations
notes
The standard rating conditions used for ARI certification represent typical
design temperatures and flow rates for which water-cooled and air-cooled
systems are designed. They are not suggestions for good design practice for a
given system, they are simply a common rating point. Trends toward improved
humidity control and energy efficiency have changed some of the actual
conditions selected for specific applications.
ARI’s part-load efficiency rating system establishes a single, blended estimate
of stand-alone chiller performance. The Integrated Part Load Value (IPLV)
calculator predicts chiller efficiency at the ARI standard rating conditions using
weighted averages representing a broad range of geographic locations,
building types, and operating-hour scenarios, both with and without an airside
economizer. While the weighted averages place greater emphasis on the part-
load operation of an average, single-chiller installation, they will not—by
definition—represent any particular installation.
Additionally, ARI notes that more than 80 percent of all chillers are installed in
multiple-chiller plants. Chillers in these plants exhibit different unloading
characteristics than the IPLV weighted formula indicates. Appendix D of the
Standard explains this further:
...The [IPLV] equation was derived to provide a representation of the average
part-load efficiency for a single chiller only. However, it is best to use a
comprehensive analysis that reflects actual weather data, building load
characteristics, operational hours, economizer capabilities, and energy drawn
by auxiliaries such as pumps and cooling towers, when calculating the chiller
and system efficiency. This becomes increasingly important with multiple
chiller systems because individual chillers operating within multiple chiller
systems are more heavily loaded than single chillers within single chiller
systems.
Remember that the ARI rating is a standardized representation. Many chillers
do not run at standard rating conditions and few are applied in single-chiller
installations. Performing a comprehensive energy analysis is the best method
of comparing the system operating cost difference between 2 chillers.
equipment certification standards
ARI Standard 550/590
▲ Standard rating conditions
◆ Common system conditions for published ratings
▲ Integrated Part Load Value (IPLV)
◆ Part-load efficiency rating
◆ Based on an “average” single-chiller installation
◆ Standard operating conditions
Figure 55