Three stages of drying work of the dryer

Under the dryer's drying conditions-that is, the temperature, humidity, flow rate, and contact pattern with the material remain constant during the drying process-we observed changes in the dryer's moisture content, temperature, and drying rate. When a large amount of hot air at a constant temperature, humidity, and flow rate is used to dry a small amount of wet material, it can essentially approach constant drying conditions.

The following is a dryer manufacturer's explanation of the three stages of dryer drying:

1. Preheating Stage

When the material comes into contact with the hot air, it transfers heat to the material surface. At this point, the amount of heat transferred per unit time to the material surface is far greater than the amount of heat required to evaporate surface water, causing the surface temperature to rise rapidly. When the hot air reaches its wet-bulb temperature, the amount of heat transferred per unit time to the material surface equals the amount of heat required to evaporate the surface water, achieving equilibrium. During the preheating stage, the moisture content of the material decreases, the surface temperature increases, and the dryer's drying rate increases. The preheating stage is short, and little water is removed.

2. Constant-Rate Drying Stage

During the drying process, the material's drying speed remains constant. This is known as the constant-rate drying stage of a dryer. During the constant-rate drying stage of a constant-rate dryer, due to the high moisture content of the material, internal moisture readily diffuses to the surface. This migration of moisture to the surface can accommodate the evaporation rate of the material's surface moisture, resulting in the material's surface being wetted by unbound water, while maintaining a constant heat transfer rate and evaporation rate. The drying rate during this stage of the dryer is independent of the material's particle size and initial moisture content, but rather depends on the dryer's drying temperature, air humidity, and the speed and direction of the air flow. This is characterized by constant surface moisture retention, a stable temperature increase, and the removal of unbound water from the material.

3. Decreasing Drying Stage

When the material's moisture content drops to the critical moisture content, the drying rate decreases as the moisture content decreases. The decreasing drying rate of the material dryer indicates that the rate of moisture migration from the material's interior to the surface is slower than the evaporation rate of the surface moisture. Consequently, the material, especially the protruding portions, dries out per unit time area. The amount of water evaporation decreases. Consequently, the heat required for surface moisture evaporation decreases, the material's surface temperature gradually increases, and the drying rate decreases. As the drying process of the dryer proceeds, the moisture in the material gradually decreases, but the rate of decrease becomes smaller; the drying speed of the dryer gradually decreases and then decreases.