Optimal design of vapor-bypassed heat exchanger for performance improvement of air source heat pump system

Abstract

The vapor-bypassed technique can bypass part of the vapor inside the evaporator to the evaporator outlet, which can effectively increase the heat transfer rate and reduce the pressure drop. In addition, when the vapor-bypassed technique applied to the outdoor heat exchanger of the air source heat pumps (ASHPs), the affect on both the evaporator and condenser performance should be comprehensively considered. Therefore, this paper proposes the vapor-bypassed heat exchanger for the outdoor heat exchanger of a multi-split ASHP system, aiming to enhance both cooling and heating performance. Firstly, simulation models for the vapor-bypassed evaporator and condenser based on the distributed parameter method was developed to determine the circuitry configuration for optimal comprehensive performance. According to the simulation results, the optimal vapor-bypassed circuitry under four APF conditions is identified as 3-2-9 (RP1-R1-RP2). When the heat transfer capacity of the evaporator simulation model is adjusted from 5.5 to 13.5 kW, the pressure drop of the evaporator using the optimized vapor-bypassed circuitry can be reduced by 23.7–98.8 kPa. Meanwhile, under various cooling conditions, using the optimized circuitry can increase the condenser heat transfer capacity by 42–309 W. Subsequently the experimental results indicated that the annual performance factor (APF) of the vapor-bypassed (VB) system increased by 4.0 %, and the annual energy consumption (AEC) of the VB system decreased by 3.8 %. Furthermore, the number of operating indoor units directly influences VB system performance. Experimental results demonstrated that the system's performance enhancement potential increases progressively as fewer indoor units remain active. Finally, the evaluation of Life Cycle Climate Performance (LCCP) showed that the vapor-bypassed technique could reduce carbon emissions by 2.9 % over the life-cycle of the multi-split ASHP. The contribution of this paper can provide new thoughts for optimizing refrigerant circuitry in ASHPs using vapor-bypassed technique.