The drying process needs to consume a lot of heat energy, in order to save energy, some materials with high wet content, suspension or solution containing solid matter are generally evaporated by mechanical dehydration or heating, and then dried in the dryer to obtain dry solids.
In the drying process, it is necessary to complete the transfer of heat and mass (wet separation) at the same time, to ensure that the wet vapor partial pressure (concentration) on the surface of the material is higher than the wet vapor partial pressure 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 vaporizes and escapes to the external space, so that the difference in moisture content between the surface and the inside of the material occurs. The internal moisture division 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 rate of vaporization of the surface and the rate of diffusion of the internal moisture. In general, 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, and this stage is called the constant speed drying stage; When the moisture content of the material is reduced to a certain extent, and the diffusion rate of the 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 continues to decrease with the decrease of the moisture content, this stage is called the deceleration drying stage.