The origin history and development trend of desiccant air dryer

Since ancient times, humans have been accustomed to using natural heat sources and natural ventilation to dry materials, which is completely restricted by natural conditions and low production capacity. With the development of production, they are gradually replaced by artificially controllable heat sources and air dehumidifiers.
Modern air dryers began to use fixed-bed air dryers with intermittent operation. In the middle of the 19th century, the use of tunnel air dryers marked the desiccant air dryer from intermittent operation to continuous operation. The rotary cylinder air dryer better realizes the agitation of the granular materials, and the drying capacity and strength are improved. Some industries have developed continuous operation air dryers that meet the requirements of the industry, such as drum air dryers in the textile and paper industries.
At the beginning of the 20th century, the production of dairy products began to use spray air dryers, which provided powerful tools for large-scale drying of liquid materials. Since the 1940s, with the development of fluidization technology, high-strength, high-productivity fluidized beds and airflow air dryers have appeared one after another. The freezing sublimation, radiation and dielectric air dryers provide new means to meet special requirements. In the 1960s, far infrared and microwave air dryers were developed.
The drying process requires a large amount of heat energy. In order to save energy, certain materials with high moisture content, suspensions or solutions containing solid substances are generally dehydrated or heated to evaporate mechanically, and then dried in an air dryer to obtain dry solids.
The purpose of drying is for material use or further processing. For example, drying of wood before making wooden molds and woodware can prevent product deformation, and drying of ceramic blanks before calcination can prevent finished products from cracking. In addition, the dried materials are also easy to transport and store, such as drying the harvested grain to a certain moisture content to prevent mildew. Since natural drying is far from meeting the needs of production development, various mechanized air dryers are more and more widely used.
During the drying process, it is necessary to complete the transfer of heat and quality (moisture) at the same time to ensure that the partial pressure (concentration) of moisture vapor on the surface of the material is higher than that in the external space, and ensure that the temperature of the heat source is higher than the temperature of the material.
The heat is transferred from the high-temperature heat source to the wet material in various ways, so that the moisture on the surface of the material is vaporized and dissipated to the outside space, so that the difference in moisture content appears on the surface and inside of the material. The internal moisture 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 drying rate of the material depends on the surface vaporization rate and the diffusion rate of 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 remain unchanged, the drying rate and surface temperature of the material remain stable. This stage is called the constant-rate drying stage; when the moisture content of the material decreases to a certain When the diffusion rate of 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 continuously with the decrease of moisture content. This stage is called the reduced-rate drying stage.
Air dryers can be classified according to different characteristics such as operating process, operating pressure, heating method, wet material movement mode or structure. According to the operation process, air dryers are divided into two types: batch type (batch operation) and continuous type;
According to the operating pressure, air dryers are divided into two types: atmospheric air dryers and vacuum air dryers. Operating under vacuum can reduce the partial pressure of moisture and steam in the space and accelerate the drying process, and can reduce the boiling point of moisture and the drying temperature of materials , The steam is not easy to leak, so the vacuum air dryer is suitable for drying heat sensitive, easily oxidized, explosive and toxic materials and occasions where moisture vapor needs to be recovered;
According to the heating method, air dryers are divided into convection, conduction, radiation, and dielectric types. Convection air dryers, also known as direct air dryers, use hot drying media to directly contact wet materials, transfer heat by convection, and take away the generated steam; conductive air dryers are also known as indirect air dryers It uses conduction to transfer heat from the heat source to the wet material through the metal partition. The generated moisture vapor can be removed by vacuum suction, a small amount of purge gas or condensation on the surface of a separate low-temperature condenser. This type of air dryer does not use drying medium, has high thermal efficiency, and the product is not contaminated, but the drying capacity is limited by the heat transfer area of ??the metal wall, and the structure is also complex. It is often operated under vacuum; the radiant air dryer uses various This kind of radiator emits electromagnetic waves in a certain wavelength range, which is selectively absorbed by the surface of the wet material and then converted into heat for drying; the dielectric air dryer uses the action of a high-frequency electric field to cause the thermal effect of the wet material to dry.
The future desiccant air dryer will be based on the in-depth study of drying mechanism and material drying characteristics, and the development and improvement of air dryers under the optimal operating conditions for different materials; in addition, large-scale, high-strength, high-economic, and improved The adaptability of raw materials and product quality are the basic trends in the desiccant air dryer; at the same time, further research and development of new efficient air dryers that meet special requirements, such as refrigerated dehumidifiers, rotary dehumidifiers and air freeze dryers.
The desiccant air dryer should also pay attention to energy conservation and comprehensive utilization of energy, such as adopting various combined heating methods, transplanting heat pump and heat pipe technology, developing solar air dryers, etc.; also developing automatic control technology of air dryers to ensure optimal The realization of operating conditions; in addition, as human beings pay more attention to environmental protection, improving the environmental protection measures of air dryers to reduce the leakage of dust and exhaust gas will also be a direction that needs in-depth research.