Contribution of internal heat exchanger to transcritical R-744 cycle performance

This paper presents results of 178 experiments evaluating the effects of internal heat exchange (IHX) on the performance of a well-instrumented prototype transcritical mobile air-conditioning system using R-744 (carbon dioxide) as the refrigerant. The effect on cycle efficiency is substantial, up to 25%, because of the relatively high irreversibility at the expansion device in the standard transcritical cycle. Three coaxial internal heat exchangers of various lengths, having identical cross sections, were used in the experiments in both parallel and counterflow configurations. These data were used to develop a simulation model that predicted accurately the IHX performance. Finally the model was used to develop an optimal design for a COP-maximizing IHX, which reduces material requirements by 50% while increasing effectiveness by 10%. The cycle optimization was guided by the trade-off between heat exchanger effectiveness and suction pressure drop.