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Low charge system behaviour: interaction of heat exchanger volumes and charge.

Author(s) : MADER G., LARSEN L. F. S., FÖSEL G. P. F.

Summary

Currently, a shift in the heat exchanger technology in residential air conditioning is ongoing; the traditional fin and tube type is replaced by microchannel heat exchangers. For this application the low internal volume of the microchannel heat exchangers allows for a considerable reduction of the refrigerant charge. Reducing the charge and changing the volume ratio between evaporator and condenser emphasizes the effect of charge migration on the system performance. Especially when introducing a variable speed compressor, large variations in subcooling over the range of the compressor capacity have been observed. This behaviour is hitherto unknown from systems with fin and tube condensers and is expected to affect the system performance negatively. The objective of this paper is to analyze the interaction of heat exchanger volume ratio and system charge, using a static system model of a basic refrigerant cycle, where the heat exchangers are divided into zones depending on the state of the refrigerant-liquid, two-phase or gas. The model basically shows that the system behaviour is governed by the heat exchanger in which the bulk of refrigerant is present. For very small relative internal volumes of the evaporator, the condenser characteristics are mainly influencing the variations in the subcooling. Enlarging the relative evaporator volume the location of the charge is increasingly determined by the two phase zone in the evaporator. Charge then is either pushed to or pulled from the condenser, thereby influencing the subcooling. The numerical results are compared to experimental data and the model shows to predict the right trends.

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Details

  • Original title: Low charge system behaviour: interaction of heat exchanger volumes and charge.
  • Record ID : 2010-1862
  • Languages: English
  • Source: RCR 2010. 2nd IIR Workshop on Refrigerant Charge Reduction in Refrigerating Systems: Stockholm, Sweden, June 16-17, 2010.
  • Publication date: 2010/06/16

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