Simulation of floating condensing temperature control for air-cooled chillers

This study aims to develop a thermodynamic model for an air-cooled screw chiller to investigate how the condensing temperature should be controlled to improve its coefficient of performance (COP). The model considers real process phenomena, including the capacity control of compressors and the overall heat transfer coefficients of an evaporator and a condenser at part load. A new configuration of variable speed condenser fans was proposed. 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 operating condition. The model was validated using the operating data and specifications of an existing chiller. The simulation results showed that the chiller COP drops considerably, especially at part load operation with moderate temperatures when using the conventional head pressure control with a fixed and high set point of 45 C for the condensing temperature. In order to maximize the chiller COP, the set point should be adjusted in response to changes of the chiller load and outdoor temperature. Such an adjustment could result in a 1.7-84.8% increase in the chiller COP. These findings are useful in developing highly efficient air-cooled chillers.