This paper presents how to tune up the set point of the condensing temperature for improving the coefficient of performance (COP) of air cooled chillers under various operating conditions. The thermodynamic model of an air cooled screw chiller was developed using the simulation program TRNSYS and validated using the operating data and specifications of the chiller. A new configuration of variable speed condenser fans was proposed for the chiller. An algorithm was introduced to make use of a set point of condensing temperature to determine the number and speed of condenser fans staged at any given cooling capacity. A model was used to investigate how the COP changes with various set points of condensing temperature. It was found that the optimum set point for maximum COP is a function of outdoor temperature and chiller load. Depending on the operating conditions, the COP could increase by 12.1-110.4% when the variable speed condenser fans and the optimum set point of condensing temperature were applied to the existing air cooled screw chillers. This provides important insights into the development of more energy efficient air cooled chillers. (c) 2005 Elsevier Ltd. All rights reserved.