1. The Accelerated Drying Phase of Raw Materials in a Flash Dryer

Within the heat-transfer medium, particles are exposed to elevated temperatures and an air humidity of less than 100%. Within a very short timeframe, the particle surfaces are heated to the wet-bulb temperature of the drying medium. Consequently, the rate of moisture evaporation accelerates rapidly. After a certain period, the heat absorbed by the particles balances with the heat consumed by the evaporating moisture, thereby reaching a state of equilibrium. This phase is extremely brief, and the volume of moisture expelled is relatively small; the process then transitions into the constant-rate drying phase.

2. The Constant-Rate Drying Phase of Raw Materials in a Flash Dryer

During this phase, moisture evaporating from the particle surface is continuously replenished by moisture migrating from the interior of the particle to its surface, ensuring that the surface remains in a consistently wet state. At this juncture, both the drying rate and the surface temperature of the particles remain constant. The evaporation rate is directly correlated with the differences in water vapor concentration and temperature between the particle surface and the surrounding medium; the greater these differences, the faster the drying rate. Furthermore, the drying rate is influenced by the velocity of the airflow across the particle surface; a stagnant boundary layer of air—a "gas film"—invariably forms on the surface. Reducing the thickness of this gas film facilitates both moisture evaporation and heat exchange. Therefore, increasing the velocity of the airflow across the particle surface can effectively accelerate the drying process. As drying continues for a certain duration, the rate at which moisture diffuses from the particle's interior begins to fall below the rate of surface evaporation. Consequently, the internal moisture can no longer fully saturate the surface to sustain surface evaporation, marking the transition to the subsequent drying phase.

3. The Decelerated Drying Phase of Raw Materials in a Flash Dryer

Once the evaporation rate reaches a certain threshold, the internal moisture within the particles becomes insufficient to keep the entire surface saturated. The area of ​​the wet surface gradually diminishes, causing the drying rate to progressively decline. During this phase, both the volume of evaporation and the rate of heat consumption decrease significantly. The surface temperature of the particles rises above the wet-bulb temperature of the drying medium and continues to climb, while the temperature differential between the particles and the drying medium gradually narrows until the two temperatures are nearly or exactly equal.

4. The Equilibrium Phase of Raw Materials in a Flash Dryer

At this stage, the rate of moisture absorption by the particle surface reaches equilibrium with the rate of evaporation. Consequently, the drying rate drops to zero. The residual moisture remaining within the particles constitutes the "final moisture content"; typically, this level should not fall below the equilibrium moisture content required for safe storage. In a rotary flash dryer, the dried material particles are typically fine, and their residence time within the drying chamber is extremely brief—generally ranging from just 1 to 3 seconds. Consequently, during the constant-rate drying phase of the particles, their surface temperature corresponds to the wet-bulb temperature of the drying medium. The use of a rotary flash dryer ensures a uniform particle size distribution of the raw materials, thereby facilitating the maintenance of product quality.