The heat pump drying unit is mainly composed of a heat pump system (evaporator, compressor, condenser, throttle valve, etc.) and an air system (drying chamber, fan, electric heater, etc.) as shown in Fig. 2.1. Therefore, the heat pump drying cycle includes two cycles: (1) a vapor compression cycle of the refrigerant. (2) The circulation of dry air. Classification of heat pump dehumidifying dryers:
Refrigeration cycle: the high-temperature and high-pressure refrigerant gas from the compressor enters the condenser, transfers the heat to the air, and condenses it into a normal-temperature high-pressure liquid, which is throttled by the expansion valve and then enters the evaporator to absorb the humid air from the drying chamber. The heat is turned into a low-temperature low-pressure gas, which is compressed by the compressor, so that it reciprocates.
Air system: The humid air is cooled to below the dew point temperature at the evaporator, and the condensed water is precipitated. The moisture content drops, and then enters the condenser, absorbs the heat of the refrigerant and heats up, and the relative humidity is lowered, and then sent to the drying chamber.
(1) According to the air circulation system, it can be divided into closed circuit circulation system and open circuit circulation system.
Closed-circuit circulation system means that the drying medium is completely recycled in the dryer. The open-circuit circulation system means that the drying medium leaves the drying chamber and enters the heat pump. The evaporator and the heat pump are directly emptied after heat exchange.
(2) According to the refrigeration system, it can be divided into general type, cooling type and temperature regulation type.
The general type means that the air is cooled and dehumidified by the evaporator, heated by the condenser to lower the relative humidity, and the system outlet temperature cannot be adjusted, and can only be used for heating and drying.
The cooling type refers to the addition of an external water-cooled or air-cooled condenser to the general-type system. The condensation heat of the refrigerant is completely taken away by the added condenser. The temperature of the air after passing through the main condenser does not change. The system can be used to dry heat-sensitive organisms. Material, or for cryopreservation.
The temperature regulation type means that the heat of condensation of the refrigerant is carried away by the water-cooled or air-cooled condenser, and the remaining heat of condensation is used to heat the air passing through the evaporator, and the temperature of the system can be adjusted.