International Journal of Thermal Sciences 48 (2009) 825–836 www.elsevier.com/locate/ijts Modelling of improved energy performance of air-cooled chillers with mist pre-cooling F.W. Yu ∗ , K.T. Chan Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China Received 28 February 2008; received in revised form 16 April 2008; accepted 9 June 2008 Available online 19 August 2008 Abstract Air-cooled chillers are widely used to provide cooling energy and yet pragmatic and simple energy efficient measures for them are still lacking. This paper considers how their coefficient of performance (COP) can be improved by using mist to pre-cool ambient air entering the condensers. The benefit of this application rests on the decrease of compressor power resulting from the reduced condenser air temperature with insignificant consumption of water and pump power associated with the mist generation. Based on a simulation analysis of an air-cooled screw chiller operating under head pressure control, applying such mist pre-cooling enables the COP to increase by up to 7.7%. Precise control of mist generation rate and integration with floating condensing temperature control are the major challenges of using a mist system to maximize electricity savings. The results of this study will prompt low-energy operation of existing air-cooled chillers working for various climatic conditions.  2008 Elsevier Masson SAS. All rights reserved. Keywords: Air-cooled chiller; Coefficient of performance; Electricity consumption; Water mist 1. Introduction Air-cooled chillers are an inevitable choice for central cool- ing systems in buildings whenever fresh water is considered to be a precious resource which cannot be used freely for evapora- tive cooling towers in the heat rejection process. They may even be a preferable choice for most small to medium scale build- ings due to the ease of installation, the simplicity of operation and maintenance, and the lower installation and maintenance costs as compared to water-cooled chillers. Yet concern has been expressed about the considerable electricity consumed by these chillers when in use because they are recognized as “less- efficient” equipment compared with water-cooled chillers. According to many studies [1–8], the deficient performance of air-cooled chillers is mainly due to head pressure control (HPC) under which the condensing temperature floats around a high set point of 50 ◦ C based on a design outdoor temperature of 35 ◦ C, irrespective of different chiller loads and weather con- ditions. The condenser fan power under HPC can be kept low * Corresponding author. Tel.: +852 37460416; fax: +852 23647375. E-mail address: ccyufw@hkcc-polyu.edu.hk (F.W. Yu). but with considerable compressor power at such high condens- ing temperature. To overcome this, proper control of condens- ing temperature should be developed to optimize the trade-off between the compressor power and condenser fan power, given that the condensing temperature can vary widely in response to outdoor temperatures ranging between 10 to 35 ◦ C. Floating condensing temperature control (CTC) is proposed as an alter- native to HPC to lower the condensing temperature in response to changes of ambient and load conditions [6–8]. The condenser fans under CTC are staged as many as possible in most operat- ing conditions to enhance the heat rejection airflow required to decrease the condensing temperature. Simulation analyses on air-cooled screw or centrifugal chillers [7,8] confirmed that the coefficient of performance (COP) of the chillers could in- crease by up to 237.2% when variable speed condenser fans are used to complement CTC. A COP is defined as the chiller load in kW divided by the power input in kW. The variable speed fans are capable of modulating smoothly the heat rejection air- flow with reduced power while enhancing the controllability of condensing temperature. To implement CTC, the set point of condensing temperature should be adjusted in response to variations of outdoor temperature and chiller part load ratios. Although the idea of CTC is easily recognized and there should 1290-0729/$ – see front matter  2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2008.06.016