A dryer is a mechanical device that vaporizes moisture in a material (generally referred to as moisture or other volatile liquid component) by heating to obtain a solid material of a specified moisture content.

Since ancient times, humans have been accustomed to drying materials with natural heat sources and natural ventilation, which are completely restricted by natural conditions and have low production capacity. As production progressed, they were gradually replaced by artificially controllable heat sources and mechanical ventilation dehumidification.

Modern dryers have begun to use intermittently operated fixed bed dryers. In the mid-19th century, the use of tunnel dryers marked the development of dryers from intermittent operation to continuous operation. The rotary cylinder dryer better achieves the agitation of the granular material, and the drying ability and strength are improved. In some industries, continuous operation dryers have been developed to meet the requirements of the industry, such as drum dryers in the textile and paper industry.

In the early 20th century, the use of spray dryers in dairy production provided a powerful tool for large-scale drying of liquid materials. Beginning in the 1940s, with the development of fluidization technology, high-strength, high-productivity fluidized bed and airflow dryers appeared one after another. Frozen sublimation, radiation and dielectric dryers offer new means to meet specific requirements. Far infrared and microwave dryers were developed in the 1960s.

The drying process requires a large amount of heat energy. In order to save energy, some materials with high moisture content, suspensions or solutions containing solid substances are generally first mechanically dehydrated or heated to evaporate, and then dried in a desiccator to obtain a dry solid.

The purpose of drying is for the purpose of material use or further processing. 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.

In the drying process, heat and mass (moisture) transfer must be completed at the same time to ensure that the moisture partial pressure (concentration) of the surface of the material is higher than the moisture partial pressure in the external space to ensure that the heat source temperature is higher than the material temperature.

Heat is transferred from the high-temperature heat source to the wet material in various ways, so that the surface of the material is vaporized and vaporized to the external space, so that the difference in moisture content occurs on the surface and inside of the material. The internal moisture distribution diffuses and vaporizes to the surface, so that the moisture content of the material is continuously reduced, and the overall drying of the material is gradually completed.

The rate of drying of the material depends on the rate of surface vaporization and the rate of diffusion of the internal moisture. Generally, the drying rate in the early stage of drying is controlled by the surface vaporization rate; then, as long as the external conditions of drying are constant, the drying rate and surface temperature of the material remain stable. This stage is called the constant-speed drying stage; when the moisture content of the material is reduced to a certain To the extent that the rate of diffusion of the internal moisture to the surface decreases and is less than the surface vaporization rate, the drying rate is mainly determined by the internal diffusion rate and decreases with the decrease of the moisture content. This stage is called the slow-down drying stage.

The dryer can be classified according to different characteristics such as operation process, operating pressure, heating mode, wet material movement mode or structure. According to the operation process, the dryer is divided into two types: batch type (batch operation) and continuous type;

According to the operating pressure, the dryer is divided into two types: atmospheric pressure dryer and vacuum dryer. The operation under vacuum can reduce the moisture partial pressure of space and accelerate the drying process, and can reduce the boiling point of the wet water and the drying temperature of the material. Leakage, therefore, the vacuum dryer is suitable for drying heat sensitive, oxidizable, explosive and toxic materials and wet steam needs to be recycled;

According to the heating method, the dryer is divided into a convection type, a conduction type, a radiation type, a dielectric type and the like. The convection dryer, also known as the direct dryer, uses a hot drying medium to directly contact the wet material to transfer heat in a convective manner and to carry away the generated steam. The conductive dryer is also called an indirect dryer, which utilizes conduction. The heat is transferred from the heat source to the wet material through the metal partition wall, and the generated wet steam can be removed by vacuum suction, a small amount of purge gas or condensation on the surface of the separately provided low temperature condenser. This type of dryer does not use drying medium, has high thermal efficiency, and the product is not polluted, but the drying capacity is limited by the heat transfer area of ​​the metal wall, the structure is also complicated, and it is often operated under vacuum; the radiant dryer uses various radiations. The electromagnetic wave emitted by a certain wavelength range is selectively absorbed by the surface of the wet material and then converted into heat for drying; the dielectric dryer uses a high-frequency electric field to cause a thermal effect inside the wet material to be dried.

According to the movement mode of wet materials, the dryer can be divided into fixed bed type, agitated type, spray type and combined type; according to the structure, the dryer can be divided into a box dryer, a conveyor dryer, a drum dryer, and a stand. Dryers, mechanically agitated dryers, rotary dryers, fluidized bed dryers, airflow dryers, vibrating dryers, spray dryers, and combined dryers.

The future development of the dryer will be in-depth study of the drying mechanism and material drying characteristics, mastering the optimal operating conditions for different materials, developing and improving the dryer; in addition, large-scale, high-strength, high-economic, and improved raw materials Adaptability and product quality are the basic trends in dryer development; at the same time, further research and development of new and efficient dryers for special requirements, such as combined dryers, microwave dryers and far-infrared dryers.

The development of dryers should also pay attention to energy conservation and comprehensive utilization of energy, such as the use of various joint heating methods, transplantation of heat pumps and heat pipe technology, development of solar dryers, etc.; development of dryer automatic control technology to ensure optimal operating conditions In addition, as humans pay more attention to environmental protection, improving the environmental protection measures of the dryer to reduce the leakage of dust and exhaust gas will also be the direction that needs further research.