The Heat and Mass Transfer Mechanism of Drum Dryer

 Drying is a transfer process in which heat and mass transfer are carried out at the same time, and these two processes are closely related. In the drum drying operation, after the wet material forms a film on the outer wall of the drum, the contact between the material film and the outer wall of the drum is quite good. This assumption will not produce large errors in the calculation of heat and mass transfer for the drying of liquid materials, so it can be considered that there is no additional thermal resistance between the material film and the outer wall of the drum.

The reason for the heat transfer of the drum drying equipment consists of three parts: the convective heat transfer of the heating medium in the drum, such as steam condensation heat transfer; the heat conduction of the scale layer, the drum wall and the material film. The convective heat transfer outside the material film.

Strictly speaking, there is a laminar bottom layer formed by the vaporization of material moisture at the junction of the material film and the external space. The heat transfer is through this laminar flow through the way of heat conduction, before reaching the heat transfer area dominated by convective heat transfer. Because the thickness of this laminar flow bottom layer is difficult to measure, the calculation of its thermal resistance can be calculated by the convective heat transfer coefficient processing method in heat transfer, and the thermal resistance of this laminar flow bottom layer can be combined into the convective heat transfer coefficient. The convective heat transfer coefficient includes the heat transfer of the laminar bottom layer outside the drum and the convective heat transfer in the outer space.

During the drying process of the drum dryer, the heat transfer colleagues conduct mass transfer. The material film gets heat, the temperature rises; the moisture molecules get energy, and the molecular movement speeds up. When the kinetic energy of the moisture molecule is large enough to overcome the gravitational force between the moisture molecules and the gravitational force between the moisture molecules and the solid molecules, the moisture partial pressure exceeds the partial pressure of the substance in the environment, and the moisture molecules tend to The environment diffuses, and consequently causes the moisture molecules in the material film to migrate from the inside to the outside in the material layer. Therefore, the direction of mass transfer in the material film is consistent with the direction of heat transfer, and the greater the heat transfer rate, the greater the mass transfer rate. When the surface moisture vaporization speed is the controlling factor, the drying process is shown as a constant speed stage; when the internal migration of moisture molecules is a controlling factor, the drying process is shown as a slowing stage.

The heat transfer rate and mass transfer rate of the drum drying sheet are related to many factors such as the physical properties, concentration, specific heat capacity, surface tension, film thickness, heating medium temperature, and drum rotation speed of the material liquid. Experiments are often used in engineering to determine the influence of these factors on

.