OPTIMIZATION OF PASS ARRANGEMENTS FOR MICROCHANNEL HEAT PUMP CONDENSERS USING ISOBUTANE

Microchannel heat exchangers (MCHX) have seen increased application in heat pumps because they offer greater compactness, significant charge reduction, lower refrigerant pressure drop and lower airside fan power consumption compared to traditional round tube-plate fin heat exchangers. Employing MCHX condenser (MCC) as well as evaporator (MCE) in a heat pump system using natural refrigerants offers significant potential for cost reduction as well as environmentally benign operation. Typically, for a given tube and fin surface geometry and operating conditions, the system designer has the freedom to choose the correct circuit arrangement to optimize the heat exchanger capacity. Very few studies on the circuit optimization of isobutane (R600a) MCHX have appeared in the open literature. In this work, we present the results of a simulation study that explores the circuit optimization of a MCC used in a R600a heat pump. Two-, three-, and four-pass circuit arrangements in expanding, contracting, and equal configurations were explored. The number of MCC tubes and the tube length were simultaneously varied so as to maintain a constant coil face area at a typical condenser operating condition in a residential heat pump. 36 tube x 0.562 m condenser coils are shown to provide the best heat duty among all possible contracting, expanding, and equal pass arrangements. The best contracting pass arrangement, 13-13-6-4, has about 27% higher heat duty compared to the worst pass arrangement, 28-26. The results are also explained based on the variations in refrigerant mass flow rate and enthalpy change among the various pass arrangements. The importance of designing coils with the optimal pass arrangement to maximize the heat duty is emphasized.