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This column compares the performance of a typical constant-speed, two-stage centrifugal chiller with a comparable variable-speed centrifugal chiller at similar price points. In an attempt to do a comparative performance benefit based on similar chiller price, the base efficiency of the constant-speed chiller was improved by 10% by taking the additional dollars spent on a variable-speed drive and applying it to improve the design performance for the constant-speed unit (generally in the form of more effective heat exchangers). In addition, a 3% efficiency penalty was used for the variable-speed chiller to account for inverter losses. The overall performance for the constant-speed chiller is 13% better than that of the variable-speed chiller at design conditions.

Citation: ASHRAE Journal, vol. 56, no. 12, Dec 2014