Experiment on performance of the non-azeotropic refrigerant closed-cycle heat pump drying bypass system

Drying is an ever-increasing demand in both industrial and agricultural fields. However, the traditional drying of heat pumps cannot fully meet the actual requirements of large-scale production, due to the low energy efficiency and serious environmental impacts. It is very necessary to develop an efficient and environment-friendly drying system. In this study, a drying bypass system of a closed-cycle heat pump was proposed for high efficiency using non-azeotropic mixed refrigerants. The heat recovery was optimized to enhance the product quality and the economic performance of the system, particularly with less energy consumption. White radish was used as the experimental material, and the optimization objectives included the heat transfer coefficient of the heat pump drying system, drying rate, dehumidification energy consumption ratio, final wet basis moisture content of the material, and exergy efficiency. Based on these indicators, kinetic and thermodynamic analysis models were developed, and experiments were conducted to investigate the effects of three factors—drying temperature, air bypass ratio, and non-azeotropic mixture ratio on the system's drying performance and energy efficiency. Experimental results show that the drying performance of the system was firstly improved and then declined, as the air bypass ratio increased. Once the air bypass ratio reached 60%, the best performance was achieved to optimize the heat transfer coefficient of the evaporator and the material drying rate. When the average temperature in the drying chamber increased to 55 ℃, the final wet basis moisture content of the white radish was further reduced to 23.4% within the same. In addition, the non-azeotropic mixed refrigerant R32/R600 was introduced to further improve energy efficiency and drying performance. There was a more homogeneous composition of the mixed refrigerant, as the xR32 increased. While the temperature glide decreased gradually. The more uniform thermal driving force was found inside the evaporator, thus improving the heat exchange efficiency and heat transfer coefficient. Furthermore, the maximum coefficient of performance (COP) of the system was achieved at the optimal R32 mass fraction of 0.70. The final content of wet basis moisture in the material decreased to the lowest level, indicating significant energy efficiency. Thermodynamic analysis revealed that the heat transfer losses in the evaporator and condenser accounted for about 50% of the total, where the condenser contributed the most (approximately 32%). The mixed refrigerants significantly improved the heat efficiency of the system. The heat efficiency reached the highest value of 40.37% when the R32 mass fraction was 0.70. There were the 67.5% and 27.0% increase, respectively, compared with the R32 mass fraction of 0.45 and 1.0. Experimental and thermodynamic analysis showed that the drying bypass system of a closed-loop heat pump with the non-azeotropic mixed refrigerant significantly improved the energy efficiency for the better performance of drying in the promising application. This finding can provide important theoretical and technical support to optimize the heat pump drying system.