This paper considers how to apply optimum condensing temperature control and variable chilled water flow to increase the coefficient of performance (COP) of air cooled centrifugal chillers. A thermodynamic model for the chillers was developed and validated using a wide range of operating data and specifications. The model considers real process phenomena, including capacity control by the inlet guide varies of the compressor and an algorithm to determine the number and speed of condenser fails staged based oil a set point of condensing temperature. Based oil the validated model, it was found that optimizing the control of condensing temperature and varying the evaporator's chilled water flow rate enable the COP to increase by 0.8-191.7%,, depending oil the load and ambient conditions. A cooling load profile of an office building in a subtropical climate was considered to assess the potential electricity savings resulting from the increased chiller COP and optimum staging of chillers and pumps. There is 16.3-21.0% reduction in the annual electricity consumption of the building's chiller plant. The results of this paper provide useful information on how to implement a low energy chiller plant. (C) 2007 Elsevier Ltd. All rights reserved.