Airflow drying, also known as instantaneous drying, is the process of bringing the heating medium (both a heat carrier and a moisture carrier, such as air) into direct contact with the solid material to be dried.
The material is suspended in the gas stream, and the heating medium transfers heat to the material in a convective heat transfer mode to vaporize part of the moisture in the material, thereby obtaining a solid product having a certain moisture content. During the process of air drying the material, the movement of the material particles in the air flow is divided into an accelerated motion phase and a constant velocity motion phase. In the accelerated motion phase, the sum of the drag and buoyancy of the particles is greater than the gravity, with an upward acceleration, so the relative velocity of the particles and the airflow is a variable; as the velocity of the particles increases, the drag gradually decreases until 3 The vector sum of the forces is zero, and the particles enter the isokinetic motion phase, where the relative velocity between the gas flow and the particles is constant. The relative motion of the particles and the gas flow has a great influence on the heat transfer rate between the particles and the gas flow. In the initial drying stage, the ascending velocity of the particles just enters the drying pipe is zero, and meets the hot gas flow with higher velocity to obtain an upward direction. Speed, at this time, the convective heat transfer coefficient between the two phases is very large, the material particles are continuously accelerated and enter the accelerated motion drying stage, and the heat obtained by the solid particles in the acceleration phase accounts for more than half of the heat obtained in the entire drying stage. In the later stage of drying, when the rising speed of the solid material approaches or even reaches the gas flow rate, the convective heat transfer coefficient is greatly reduced, and the drying efficiency is lowered. In the drying process, the relative velocity of the gas-solid two phases is continuously changed, the turbulence intensity at the boundary layer around the particles is increased, the contact area of ​​the gas-solid two phases is increased as much as possible, and the contact time of the two phases is increased, which is an effective measure for improving the drying efficiency.
The material is suspended in the gas stream, and the heating medium transfers heat to the material in a convective heat transfer mode to vaporize part of the moisture in the material, thereby obtaining a solid product having a certain moisture content. During the process of air drying the material, the movement of the material particles in the air flow is divided into an accelerated motion phase and a constant velocity motion phase. In the accelerated motion phase, the sum of the drag and buoyancy of the particles is greater than the gravity, with an upward acceleration, so the relative velocity of the particles and the airflow is a variable; as the velocity of the particles increases, the drag gradually decreases until 3 The vector sum of the forces is zero, and the particles enter the isokinetic motion phase, where the relative velocity between the gas flow and the particles is constant. The relative motion of the particles and the gas flow has a great influence on the heat transfer rate between the particles and the gas flow. In the initial drying stage, the ascending velocity of the particles just enters the drying pipe is zero, and meets the hot gas flow with higher velocity to obtain an upward direction. Speed, at this time, the convective heat transfer coefficient between the two phases is very large, the material particles are continuously accelerated and enter the accelerated motion drying stage, and the heat obtained by the solid particles in the acceleration phase accounts for more than half of the heat obtained in the entire drying stage. In the later stage of drying, when the rising speed of the solid material approaches or even reaches the gas flow rate, the convective heat transfer coefficient is greatly reduced, and the drying efficiency is lowered. In the drying process, the relative velocity of the gas-solid two phases is continuously changed, the turbulence intensity at the boundary layer around the particles is increased, the contact area of ​​the gas-solid two phases is increased as much as possible, and the contact time of the two phases is increased, which is an effective measure for improving the drying efficiency.
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