Figure 1: Schematic diagram of the heat pump drying system. 1—Compressor set. 2—Water-cooled tube condenser. 3—Air-cooled fin condenser. 4—Fluid reservoir. 5—Drying filter. 6—The first evaporator. 7— The second evaporator. 8—The third evaporator. 9—Air-discharging valve. 10—Fluid-feeding valve. 11—Thermal expansion valve. 12—System door. 13—Additional electric heating device. 14—Air flow valve. 15—Adjustable valve. 16—Tunnel. 17—Temperature–humidity probe.