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 drying external 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 moisture content of the material decreases to a certain level, 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 varies with the moisture content The reduction of the drying rate is continuously reduced. This stage is called the deceleration drying stage.