The flash dryer is widely used in the dispersion and drying of granular, powdery, paste-like, and filter cake-like false agglomerates in the chemical, pharmaceutical, building materials, food, feed, and other industries. The flash dryer has the advantages of the small occupied area, low energy consumption, high thermal efficiency, and continuous mass production. What are the stages of material drying in flash dryers?
1. The speed-up drying stage of the material in the flash dryer. In the heat transfer medium, the particles are at a higher temperature and the relative humidity is less than 100%. The surface is heated to the wet bulb temperature of the drying medium in a short time, and the water evaporation rate increases rapidly. After a certain period of time, the heat absorbed by the particles is equal to the heat consumed by the evaporated water, reaching a balance. This stage is very short, the amount of water discharged is not large, and then enters the constant velocity stage.
2. The material in the flash dryer is at the same speed drying stage. At some stage, the water evaporated from the surface of the particles is constantly replenished from the inside to the surface of the particles, and the surface is always in a wet state. At this time, the drying rate remains unchanged and the particle surface temperature remains unchanged. The evaporation rate is related to the difference in water vapor concentration and temperature on the surface of the particles and the surrounding medium. The greater the difference, the faster the drying speed. In addition, the drying speed is also related to the air speed on the surface of the particles. There is always a layer of air film on the surface of the particles that is not easy to flow. The reduction of the air film is beneficial to the evaporation of moisture and heat exchange. Therefore, increasing the velocity of the airflow on the surface of the particles and thinning the air film can increase the drying rate. Drying lasts for a certain period of time, and the diffusion rate of water inside the particles starts to be lower than the surface evaporation rate. The moisture in the particles cannot completely wet the surface to keep the surface from evaporating, and then the next drying stage begins.
3. The material in the flash dryer is cooled and dried. When the amount of evaporation reaches a certain level, the moisture inside the particles is not enough to completely wet the surface, the wet surface gradually decreases, and the drying rate gradually decreases. At this stage, the evaporation and heat consumption decrease significantly, and the surface temperature of the particles is higher than the humidity of the medium. The ball temperature gradually rises, and the temperature difference with the carrier gradually decreases until it is close to or the same.
4. Material balance stage in flash dryer. At this time, the moisture absorption and evaporation on the particle surface reach equilibrium, and the drying rate is zero. The moisture in the particles is the final moisture in the dry, usually not lower than the equilibrium moisture during storage. The material particles are small after the rotary flash dryer is dried, and the residence time in the dryer is extremely short, usually 1-3s. Therefore, the surface temperature of the granules is the wet bulb temperature of the drying medium during the constant-speed drying stage. Using a rotary flash dryer, the particle size of the material is uniform, which is beneficial to ensure product quality.
Zhengzhou Dingli is a professional manufacturer of Flash Dryers, if you want to know more information, please feel free to contact us.
