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The development history of air dryers

The air dryer is a mechanical equipment that vaporizes and escapes the moisture in the material (generally refers to moisture or other volatile liquid components) by heating to obtain a solid material with 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. 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 development of air dryers 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. The development of far-infrared and microwave air dryers began in the 1960s.

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, the drying of wood before making wooden molds and woodware can prevent product deformation, and the drying of ceramic blanks before calcination can prevent the finished product 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 on the surface of the material is higher than that in the external space, and to ensure that the temperature of the heat source is higher than the temperature of the material.
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 to the surface and vaporizes, 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 for 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 level 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.