Refrigeration systems with minimum charge of refrigerant Bjo ¨rn Palm * Royal Institute of Technology, Department of Energy Technology, S 100 44 Stockholm, Sweden Received 20 January 2006; accepted 27 July 2006 Available online 12 October 2006 Abstract Concern for the environmental effects of HFC-refrigerants as well as the use of flammable refrigerants has resulted in a need of decreasing the refrigerant charge in refrigeration and heat pump systems. This paper discusses the possibility of such reductions, both at the systems- and the component level. It is shown that a move towards indirect systems, using secondary refrigerants, on both the cold and the hot side of the system may result in considerable reduction of charge. However, this reduction may come at the cost of slightly reduced system performance, which in itself is detrimental from an environmental point of view. At the component level, it may be shown that the main contents of refrigerant is usually contained in the heat exchangers. By selecting compact designs the charge may be reduced to extremely low levels. Specifically, mini-channel heat exchangers can be used for reaching low charge. With proper selection of heat exchangers, the system performance should not be influenced by the reduction of charge. For indirect systems, the amount of refrigerant solved in the compressor oil may be comparable to the amount in the (compact) heat exchangers. A possible solu- tion to reduce this amount is to use compressors with less oil. With components selected for minimum charge, the system design may be different than what is usual. Instead of a high pressure receiver and a thermostatic expansion valve, a capillary tube may be used in com- bination with a minimal low pressure receiver, similar to the system design used in household refrigerators. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Refrigerant charge; Charge reduction; Refrigerant inventory; Mini-channel heat exchanger 1. Introduction Since the realization of the CFC-refrigerants’ negative effect on the ozone layer in the end of the 1980s the refrig- eration industry has been facing the challenge of adapting the system solutions to new refrigerants. The global phase- out of CFCs within a ten year period, through the Mon- treal protocol, is probably the first successful example of an international agreement to overcome a global threat to the environment. During the last five to ten years the focus has shifted towards the threat of global warming and this has raised the demand for strict control of the emissions of HFCs, which do not contain chlorine and thus have no influence on the ozone layer, but which in most cases are strong greenhouse gases. Parts of the scientific community as well as environmen- talists have suggested that natural refrigerants, i.e. com- pounds which are naturally occurring in the environment, should be used as substitutes for the man-made HFC com- pounds now being used. In some countries, e.g. Denmark and Austria, legislative measures have already been taken towards a phase out of HFCs. Within the EC, the use of fluids with a global warming potential above 150 (including all commonly used HFCs) is being banned in mobile air conditioning according to a gradual phase out program. On the global scale, regulations on HFCs and other green- house gases are discussed within the Kyoto protocol and later amendments, however, without any agreements yet. The natural refrigerants discussed as substitutes for HFCs are hydrocarbons, ammonia, carbon dioxide and water. In this group, the hydrocarbons are most closely related to the HFCs and can be used in today’s H(C)FC systems without any major changes in the design. For example, a system designed for R22 may be run with 1359-4311/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.07.017 * Tel.: +46 8 7907453; fax: +46 8 204161. E-mail address: bpalm@energy.kth.se www.elsevier.com/locate/apthermeng Applied Thermal Engineering 27 (2007) 1693–1701