The drying process consumes a significant amount of heat energy. To conserve energy, materials with high moisture content, suspensions, or solutions containing solids are typically first mechanically dehydrated or heated to evaporate moisture before being dried in an air dryer to obtain dry solids.
The purpose of drying is to meet the needs of material use or further processing. For example, drying wood before making molds or woodenware prevents deformation, and drying ceramic blanks before firing prevents cracking. Additionally, dried materials are easier to transport and store; For instance, drying harvested grains to a certain moisture content prevents mold growth. Because natural drying cannot meet the needs of production development, various mechanized air dryers are increasingly widely used.
During the drying process, heat and mass (moisture) transfer must be completed simultaneously, ensuring that the partial pressure (concentration) of moisture vapor on the material surface is higher than that in the external space, and 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, causing the moisture on the material surface to vaporize and escape into the external space, thus creating a difference in moisture content between the material surface and its interior. Internal moisture diffuses and vaporizes towards the surface, continuously reducing the material's moisture content and gradually completing the overall drying process.
The drying rate of the material depends on the surface vaporization rate and the internal moisture diffusion rate. Typically, the drying rate in the early stages is controlled by the surface vaporization rate. Subsequently, as long as the external drying conditions remain unchanged, the drying rate and surface temperature of the material remain stable; this stage is called the constant-rate drying stage. When the material's moisture content decreases to a certain level, the diffusion rate of internal moisture to the surface decreases and becomes less than the surface vaporization rate. At this point, the drying rate is mainly determined by the internal diffusion rate and decreases continuously with further reduction in moisture content; This stage is called the falling-rate drying stage.