Understanding IPLV/NPLV

By: John Fabian

The Integrated Part Load Value (IPLV) is a performance characteristic developed by the Air-Conditioning, Heating and Refrigeration Institute (AHRI).  It is most commonly used to describe the performance of a chiller capable of capacity modulation.  Unlike an EER (Energy Efficiency Ratio) or COP (coefficient of performance), which describes the efficiency at full load conditions, the IPLV is derived from the equipment efficiency while operating at various capacities.  Since a chiller does not always run at 100% capacity, the EER or COP is not an ideal representation of the typical equipment performance.  The IPLV is a very important value to consider since it can affect energy usage and operating costs throughout the lifetime of the equipment.  Energy codes such as ASHRAE Standard 90.1 specifies minimum values for the equipment.

The IPLV is calculated using the efficiency of the equipment while operating at capacities of 100%, 75%, 50%, and 25%.  For the purpose of chiller equipment, the operational conditions are shown in Table 3 of AHRI Standard 550/590-2003.  A water cooled chiller, for example, is required to run at a 44ºF evaporator LWT with a flow rate of 2.4 gpm/ton.  The condenser EWT will vary depending on the part load capacity utilizing a 3.0 gpm/ton flow rate.  If a chiller is designed to operate at different conditions than specified in Table 3, including lower water temperature or different flow rate, the efficiency is called a NPLV (non-standard part load value).  Both of these ratings can be calculated using the following equation:

IPLV (or NPLV) = 0.01A+0.42B+0.45C+0.12D

Where:

A = COP or EER @ 100% Load

B = COP or EER @ 75% Load

C = COP or EER @ 50% Load

D = COP or EER @ 25% Load

The derivation of this equation is extensive and includes various assumptions which create a lengthy discussion.  For more information refer to Appendix D, AHRI 550/590-2003.