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 evaporated by heating before being dried in a dryer to obtain dry solids.
During the drying process, the transfer of heat and mass (moisture) 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 towards the surface and vaporizes, 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 diffusion rate of internal moisture. Typically, the drying rate in the early stages of drying 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 moisture content of the material 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 continues to decrease with decreasing moisture content; this stage is called the falling-rate drying stage. Furthermore, technological advancements will reverse the export situation of domestic drying equipment. Currently, China's drying equipment exports have not yet reached a significant scale, accounting for less than 5% of the total output, and are mainly sold to Southeast Asia. However, it is predicted that with technological development, the proportion of China's exported drying equipment will increase from 5% to 10% of total production in the next few years, and the export market will expand from Southeast Asia to Europe and America. This is expected to improve the situation where the manufacturing of large-scale drying equipment in China lags significantly behind international standards.