Leelananda, Rajapaksha Kankanamge; (2003) Performance evaluation of reversible heat pumps when using refrigerant mixtures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes research work conducted at University College London. The investigation focused on the implications of using refrigerant mixtures in vapour compression reversible heat pumps (RHPs) in relation to the influences of the special mixture attributes; temperature glide and composition shift, on the design and the performances. The analysis was based on a purpose-built simulation program that incorporates thermal-physical property routines from a refrigerant(s) database. The result presents the performance of a water-to-water RHP with three selected reversing methods, emphasizing on the glide match/mismatch situation in the reversed mode. The influences of a liquid receiver on the circulating composition and the performances were also looked into. Further, the possibility of using the two-accumulator concept to obtain capacity control of the RHP was investigated. The investigation found that the composition shift and the temperature glide influence the component sizing and the performance in both forward/reversed modes. System design needs to consider the effects of composition shift and pressure drops on the performance and glide match. Different reversing methods yielded distinct performance trends in the reversed mode. These trends were found to be dependent on the refrigerant(s) and the heat exchanger volumes. Redirecting the heat transfer fluid between the two heat exchangers to implement the change of mode appeared to be the best method for mixture applications. The flow temperatures of the heat transfer fluid in mixture systems were generally better in both forward and reversed modes when a counter flow configuration is available, and the use of a higher glide mixture improves the temperatures. Both modes showed positive composition shifts, and the running compositions are different to each other in the two modes; a liquid receiver enhanced the shift. Further it was found that the two-accumulator concept could be applied to reversible systems to obtain capacity change in the mode where excess charge occurs.

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