For example, the drying of the wood before the production of the wood mold and the wood can prevent the deformation of the product, and the drying of the ceramic material before the calcination can prevent the product from cracking. In addition, the dried material is also convenient for transportation and storage, such as drying the harvested food to a certain moisture content to prevent mildew. Since natural drying is far from meeting the needs of production development, various mechanized dryers are becoming more and more widely used.

The amount of water vapor in the compressed air is determined by the temperature of the compressed air: reducing the temperature of the compressed air reduces the amount of water vapor in the compressed air while maintaining the constant pressure of the compressed air, and the excess water vapor condenses. Into a liquid. The freeze dryer uses this principle to use refrigeration technology to dry compressed air. Therefore, the dryer has a refrigeration system. The refrigeration system of the freeze dryer is a compression type refrigeration system consisting of four basic components: a refrigeration compressor, a condenser, an evaporator, and an expansion valve. They are connected in series by pipes to form a closed system in which the refrigerant circulates continuously in the system, undergoes state changes and exchanges heat with compressed air and cooling medium. Compressed air dryers also have adsorption dryers and dissolution dryers.

The refrigeration compressor draws the low-pressure (low-temperature) refrigerant in the evaporator into the cylinder of the compressor, and the refrigerant vapor is compressed, and the pressure and temperature are simultaneously increased; the high-pressure high-temperature refrigerant vapor is pressed to the condenser, and in the condenser, The higher temperature refrigerant vapor exchanges heat with the cooler water or air, and the heat of the refrigerant is taken away by water or air to condense, and the refrigerant vapor becomes a liquid. This part of the liquid is then sent to the expansion valve, which is throttled into a low-temperature and low-pressure liquid through the expansion valve and enters the evaporator; in the evaporator, the low-temperature, low-pressure refrigerant liquid absorbs the heat of the compressed air and vaporizes (commonly called "evaporation") The compressed air is cooled to condense a large amount of liquid water; the refrigerant vapor in the evaporator is sucked away by the compressor, so that the refrigerant undergoes four processes of compression, condensation, throttling, and evaporation in the system, thereby Completed a loop.

In the refrigeration system of the freeze dryer, the evaporator is a device for conveying a cold amount, in which the refrigerant absorbs the heat of the compressed air to achieve the purpose of dehydration and drying. The compressor is the heart and acts to draw, compress, and deliver refrigerant vapor. The condenser is a device that emits heat, and the heat absorbed in the evaporator is transferred to a cooling medium (such as water or air) along with the heat converted from the input power of the compressor. The expansion valve/throttle valve acts as a throttle to reduce the pressure of the refrigerant, simultaneously controls and regulates the amount of refrigerant liquid flowing into the evaporator, and divides the system into two parts, a high pressure side and a low pressure side.