Performance Investigation of a Constant Temperature Heat Pump Dryer

Heat pump drying systems (HPDS) are systems composed of a heat pump (HP) that carries out a drying process which is energy efficient and environmentally friendly. However, in the HPDS, HP technology is used in the drying process, which makes it a very complex system coupling a hot air cycle and compressed refrigeration cycle. The control strategy of the drying process is important when developing heat pump dryers (HPDs) with high efficiency and reliability. An auxiliary condenser can be used to release excess heat from the system and maintain a constant temperature drying process, while equipping an evaporator bypass is a simple method for controlling the heat pump dehumidification rate. In this study, a novel enclosed drying system that incorporates a commercial 2.65 kW HP dehumidifier with an evaporator bypass was designed and constructed. The drying performance, such as the drying time and drying degree, and the HP performance, such as the coefficient of performance of an enclosed system with an air bypass, were experimentally investigated. According to the results of the investigation, increasing the circulating air bypass ratio can greatly reduce the drying time and improve the drying degree and the drying performance of the drying system. However, the air bypass reduces the performance of the HP, especially at a higher air bypass ratio, which should be taken into account in research and design of HPDS in the future. Besides, an attempt has been made to investigate the feasibility of using this novel HPD to preheat the drying room before drying process and refrigeration storage in drying room after drying process. Contribution of this research can diversify the function of the HPD.