A dryer is a specialized drying equipment for materials, effectively evaporating the moisture from them. The drying process can be divided into four stages: preheating, constant-speed drying, decreasing-speed drying, and equilibrium drying until the material’s moisture content reaches a certain level. Once all four stages have been completed, the drying process is finished.

1. Preheating Stage: Initially, the material being dried has very low moisture content. When the material comes into contact with hot air, the hot air first raises the temperature of the material and its moisture content, bringing them to the temperature at which moisture vaporizes. This stage is called the preheating stage. The characteristic of this stage is that the material temperature rises from its critical point to a certain value in a very short time.

2. Constant-rate stage: As heat continues to transfer, the moisture content and surface moisture of the material continue to rise. However, surface moisture evaporates and diffuses into the air due to vapor pressure, reducing the surface moisture content. After the surface moisture evaporates, internal moisture continues to flow towards the surface, keeping the surface moist, and the drying rate remains constant. The characteristic of this stage is that moisture evaporates from the material, and the rate of diffusion from the inner layer of the material to its surface is very rapid, keeping the material surface moist and maintaining a constant, high drying rate.

3. Decreasing-rate stage: Because the rate of moisture outflow from the material’s interior is lower than the rate of surface vaporization, a portion of the material surface will remain dry. Therefore, the amount of vaporized water decreases, meaning the drying rate gradually decreases. This is called the decreasing-rate stage of drying. The characteristics of this stage are the same as the second stage: moisture vaporizes from the material surface, the moist surface gradually decreases, and the drying rate decreases uniformly.

4. When the material in the dryer reaches the equilibrium or critical moisture content stage: As the drying rate decreases, the central temperature of the material surface will continuously rise, and heat will be transferred inward, causing the evaporation surface to move inward. The moisture will turn into steam and then diffuse and flow to the surface. In this way, the moisture content in the material becomes less and less, while the resistance encountered by the moisture flow becomes greater and greater, and the drying rate drops rapidly. Finally, the moisture content in the material reaches equilibrium with the temperature of the air; that is, the moisture content of the material has reached the equilibrium moisture content under certain conditions. At this point, the drying rate is zero, and the drying process is completely finished.