Refrigerant Circuit Optimization of Dual-mode Single-Row Microchannel Heat Exchangers used for R410A Heat Pumps

Microchannel heat exchangers (MCHX) are being increasingly applied in heat pumps because of their compactness, significant charge reduction, lower refrigerant pressure drop and lower air-side fan power consumption compared to traditional round tube-plate fin (RTPF) heat exchangers. Using a microchannel condenser as well as evaporator in a heat pump system also offers significant potential for cost reduction. Very few studies on pass optimization of microchannel condensers and evaporators have appeared in the literature, and even fewer exist on the circuit optimization of dual-mode MCHX used in a heat pump. The influence of pass arrangement on the thermal-hydraulic performance of microchannel condensers and evaporators has been explored in this article. A total of 1982 configurations for 18 tube x 1.124 m, 36 tube x 0.562 m, and 54 tube x 0.375 m were simulated under conditions typically encountered by the outdoor unit of a R410A refrigerant-to-air heat pump. Two-, three-, and four-pass circuits with contracting, expanding, and equal pass designs were simulated using CoilDesigner. All designs had identical face area to allow a fair assessment of their performance. For optimal condenser performance, 36 or 18 tube configurations are preferred to 54 tube designs. The 36 tube31%/30%/25%/14%, 18 tube-56%/33%/11%, and 36 tube-25%/25%/25%/25% condenser coils have the best heat duty. Contracting or equal pass arrangements are superior to expanding pass arrangements for condensers. Unlike condensers, 54 or 36 tube configurations yield the best evaporator heat duty. Again, quite contrary to condensers, expanding pass arrangements are clearly favored for optimal evaporator heat duty, and the best expanding pass arrangements significantly outperform the best contracting or equal pass arrangements. The 54 tube, 2%/4%/24%/70% and 2%/6%/93% evaporator coils are the best performing ones. Again, unlike in condensers, a strongly disproportionate distribution of tubes among the passes is favored for evaporators, with very few tubes recommended in the first pass or two, and many more tubes in the last pass. Additionally, evaporator performance is found to be much more sensitive to pass arrangement than condenser configurations. Hence, evaporator pass arrangements need more careful consideration than condenser pass arrangements. Thus, quite contrary pass designs favor condenser and evaporator performance. This fact implies that to design dualmode MCHX, as in a heat pump, some compromises will be necessary. Future work will address the performance of dual-mode MCHX at the component and system levels. The influence of air and refrigerant maldistribution on optimal MCHX pass arrangement will also be investigated.