When unsaturated air with higher temperatures comes into contact with wet materials, there is a transfer of heat and mass between gas and solids. According to the characteristics of different periods in the drying process, the drying process is divided into two stages.
The first stage is the constant speed drying stage. At the beginning of the process, due to the high moisture content of the whole material, the moisture inside it can quickly reach the surface of the material. Therefore, the drying rate is controlled by the vaporization rate of moisture on the surface of the material, so this stage is also called the surface gasification control stage. At this stage, all the heat transferred by the drying medium to the material is used for the vaporization of water, the temperature of the surface of the material is kept constant (equal to the temperature of the wet bulb of hot air), and the partial pressure of water vapor at the surface of the material is also kept constant, so the drying rate is constant.
The second stage is the deceleration drying stage, when the material is dried to reach the critical wet content, it will enter the deceleration stage. At this time, the moisture contained in the material is less, and the rate of moisture transfer from the inside of the material to the surface is lower than the vaporization rate of the moisture on the surface of the material, and the drying rate is controlled by the transfer rate of moisture inside the material, so this stage is also called the internal migration control stage. The moisture content of water gradually decreases, and the migration rate of moisture inside the material also decreases gradually, so the drying rate decreases continuously. The main influencing factors of the drying rate and critical water content of the constant velocity section are: the type and nature of solid materialsThe thickness or particle size of the solid material layer;Temperature, humidity and velocity of the air;The relative movement of air and solid materials.
Drying experiments. Purpose of drying experiment.
1. Master the experimental measurement methods of drying curve and drying rate curve, and deepen the understanding of drying operation process and mechanism
2. Understand the use of dry and wet bulb thermometers;
3. Understand and analyze the factors that affect the drying rate.