Simultaneous control of drying temperature and superheat for a closed-loop heat pump dryer

Closed-loop heat pump drying system has its particular characteristics, and in this paper a simultaneous control strategy is proposed to improve the precision of superheat and drying temperature. The stability of drying temperature guarantees the quality of drying material. And on the premise, superheat should be accurate and stable to improve energy usage. Two fuzzy controllers employing different membership functions and control rules are used to achieve more stable drying temperature. The superheat controller is mainly composed of a PID controller and a conversion emergency controller which accelerates the response of electronic expansion valve. The experimental results show that the temperatures at the outlet of indoor and outdoor condensers give smaller fluctuations. The variations of superheat are investigated when the temperature of drying chamber changes from ambient temperature to 25/30 °C, from 25 to 30 °C and from 30 to 25 °C, respectively. All the experimental data confirm that the new controller is applicable to decrease the nonlinearities of superheat and drying temperature.