Rotate the drying process of the flash dryer
Drying machine in the heat exchange, mainly in the air and particles, cylinder wall and the two main heat exchange. The essence of the drying process is the diffusion of water. Is carried out by the proliferation and diffusion. The process of diffusion of particles on the surface of the particles in the form of steam from the surface to the surrounding medium. Internal diffusion is the process of moisture moving inside the particles.
Rotary flash dryer uses high-speed hot air tangential direction into the cylinder, the air flow in the barrel of the spiral movement, on the one hand to reduce the particles around the medium temperature, while increasing the media flow rate and temperature, which greatly improved the external diffusion speed. On the other hand, the high-temperature air flow at high speed impact in the lower part of the cylinder particles or small wet material, while the cylinder stirring the stirring effect of the particles, the particles are broken, the particle size decreases, the length of the internal capillary is also reduced, The effect of diffusion, reduce the resistance within the diffusion. Most of these particles in the lower part of the cylinder at high temperature, high mixing strength of the cycle was repeatedly broken. Elimination of material agglomeration, but also to promote the proliferation of effective measures to strengthen the grain moisture evaporation.
Particles and hot air flow: in the lower part of the cylinder both convection and co-flow, the coarse material is convection and repeated heat transfer. For the fine material is the same with the hot air flow in parallel, so the drying process can be completed instantly. Drying of the granules is actually drying with hot air with high temperature and low humidity. These particles are mainly adsorbed by water, filled in the gap between the particles, the use of high temperature and low humidity conditions, the entire particles of heat conduction is slow, resulting in local stress concentration and chapped, broken, dispersed, accelerated drying process.
Material into the dryer, the hot air first heat transfer to the particle surface, causing the surface of the evaporation of water, the material particles within the water continue to spread to the surface and spread to the outside world, and finally to the entire particle dry. If the material does not undergo chemical reaction during drying, the moisture content, temperature and drying rate of the material change with the drying time can be divided into the following stages:
1. Slow drying stage. The particles are placed in a heat medium with a relatively high temperature and a relative humidity of less than 100%. In a short period of time, the surface is heated to the wet bulb temperature of the drying medium, the evaporation rate of the water is increased rapidly, and the heat and evaporation of the particles after a certain time Moisture consumption to the same heat, to balance. This time is very short, the discharge of water is not large, then enter the constant speed stage.
2. Equivalent speed drying stage. In some stages, the moisture from the surface of the particles is continuously replenished from the inside to the surface of the particle, and the surface remains wet, and the drying rate remains constant and the particle surface temperature remains constant. The evaporation rate is related to the difference in water vapor concentration and temperature difference between the surface of the particles and the surrounding medium. The greater the difference, the greater the drying rate. In addition, the drying rate is also related to the air velocity on the surface of the particles. There is always a layer of air film which is not easy to flow on the surface of the particles. The reduction of air film Baidu is favorable for water evaporation and heat exchange. Thereby increasing the velocity of the particle surface air flow, so that the air film is thinned, can improve the drying rate. Drying continues for a certain period of time, the internal water diffusion rate of the particles begins to be less than the surface evaporation rate, and the moisture in the particles can not completely wet the surface to maintain the surface evaporation, and then enter the next drying stage.
3. Cooling drying stage. After the moisture evaporation reaches a certain amount, the moisture content inside the granule is not enough to wet the surface, the wet surface gradually decreases, the drying rate decreases gradually. The evaporation rate and heat consumption are greatly reduced in this stage, the surface temperature of the particles is higher than the wet bulb temperature Gradually increases, and the temperature difference between the carrier is reduced until close or the same.
4. Balance stage. At this point the particle surface moisture absorption and evaporation to achieve a balance, the drying rate of zero.
The moisture in the particles is dry and final moisture, usually not below the equilibrium moisture at the time of storage. Rotary flash dryer Since the material after drying is very small, the time spent in the dryer is very short, usually 1-3s. Thus, the dry temperature of the dry card is dry, and the temperature of the surface is the wet bulb temperature of the drying medium. The use of rotary flash dryer, the material size uniformity, is conducive to ensuring product quality.