Heat and mass transfer mechanism of drum drying

There are three parts of the heat transfer mechanism of drum drying: convective heat transfer of heating medium in drum, such as steam condensation heat transfer; heat conduction of layer, drum wall and material film; convective heat transfer outside material film.

Heat and mass transfer mechanism of drum drying

Drying is a transfer process of heat and mass transfer at the same time, and the two processes are closely related. In the drying operation of drum, the contact between the material film and the outer wall of the drum is quite good after the wet material is coated on the outer wall of the drum. Therefore, it can be considered that there is no additional thermal resistance between the material film and the outer wall of the drum.

There are three parts of the heat transfer mechanism of drum drying: convective heat transfer of heating medium in drum, such as steam condensation heat transfer; heat conduction of layer, drum wall and material film; convective heat transfer outside material film. Strictly speaking, there is a laminar bottom layer formed by the evaporation of material moisture at the junction of the film and the outer space. The heat transfer is heat conduction through the bottom layer of laminar flow, and then the heat transfer area dominated by convection heat transfer can be reached. Because the thickness of the laminar bottom layer is difficult to measure, the heat resistance of the laminar bottom layer can be calculated by using the method of heat transfer coefficient in heat transfer, and the thermal resistance of this laminar bottom layer can be incorporated into the convective heat transfer coefficient. The convective heat transfer coefficient includes the heat conduction of laminar bottom layer outside the drum and convective heat transfer in outer space.

In the drying process, the heat is transferred while the mass is transferred. The results show that the material film gets heat, and the temperature rises; the moisture molecules get heat energy, and the molecular movement speed is accelerated. When the kinetic energy path of the wet fraction molecules is enough to overcome the gravity between the moisture molecules and the solid material molecules, the wet partial pressure exceeds the partial pressure of the material in the environment, and the moisture molecules diffuse to the environment, and then cause 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 is consistent with the direction of heat transfer, and the higher the heat transfer rate, the greater the mass transfer rate. When the vaporization rate of external moisture separation is the control factor, the drying process shows a constant speed stage, and when the internal migration of moisture molecules in the material layer is the control factor, the drying process shows a slow down stage.

The speed of heat and mass transfer in drum drying is related to the physical properties, concentration, specific heat capacity, surface tension, film thickness, rotation speed and so on. The influence of these factors on drum drying is determined by experiment.