Effect of Operating Temperature and Humidity in Heat Pump Drying on Energy Consumption and Drying Characteristics of Apple Slices

In the current work, a novel heat pump drying system with precise control of temperature and humidity of drying medium was developed and the impacts of drying temperature and humidity on the drying characteristics of apple slices and energy consumption of drying system were investigated. Experimental results indicated that the temperature and relative humidity (RH) of drying medium have a significant impact on drying efficiency and operating performance. During the first hour of the drying process, the heat pump drying of apple slices exhibited the highest drying rate throughout the entire process at a temperature of 40~50 °C and a relative humidity of 30~60%. And then the apple slices drying was in a falling-rate drying stage. When the relative humidity of the drying medium exceeded 50%, the final moisture content of the material increased significantly and exceeded 20% (dry basis, d.b.). Increased air medium temperature and humidity enhance the dehumidification rate of the evaporator. When the drying temperature was maintained at 40–60 °C, the condensation rate at 60% RH was 3.5–10 times that at 30% RH. The increased dehumidification rate significantly promoted the energy efficiency. The specific moisture extraction rate (SMER) was 2.53 kg/(kW·h) at 60 °C and 60% RH, which is 3.4 times that at 30% RH. It was appropriate to adopt high-temperature and high-humidity conditions in the early drying stage to improve drying energy efficiency. Meanwhile, the relative humidity should be reduced to promote moisture removal from the material in the late drying stage. The obtained results provided theoretical methods for the energy-saving control of heat pump drying for fruits.