Factors affecting the drying rate are: heat transfer rate, external diffusion rate, and internal diffusion rate.
1. Heat transfer rate
Speeding up the heat transfer rate can increase the drying rate:
① Increase the temperature of the drying medium, such as increasing the temperature of the hot gas in the drying kiln, increasing the hot stove, etc., but the surface temperature of the green body should not be raised too fast to avoid cracking;
②Increase the heat transfer area: such as changing one-sided drying to double-sided drying, layering the blanks or reducing the number of layers of the blanks, increasing the contact surface with the hot gas;
③ Improve convective heat transfer coefficient.
2. Outward diffusion rate
When the drying is in the constant drying stage, the external diffusion resistance becomes the main contradiction affecting the entire drying rate. Therefore, reducing the external diffusion resistance and increasing the external diffusion rate have the greatest impact on shortening the entire drying cycle. Diffusion resistance mainly occurs in the boundary layer, so it should be done:
④Increase the flow rate of the medium, reduce the thickness of the boundary layer, etc., and increase the convective heat transfer coefficient. It can also improve the convective mass transfer coefficient, which is beneficial to improve the drying speed;
⑤Reduce the water vapor concentration of the medium, increase the mass transfer area, and increase the drying speed.
3. Inner Diffusion Rate
The internal diffusion rate of moisture is a combination of wet diffusion and thermal diffusion. Wet diffusion is the moisture movement caused by the humidity gradient in the material, and thermal diffusion is the moisture movement caused by the temperature gradient in physics. To increase the internal diffusion rate should be done:
⑥ Make the direction of thermal diffusion and wet diffusion consistent, that is, try to make the center temperature of the material higher than the surface temperature, such as far-infrared heating and microwave heating;
⑦When the directions of heat diffusion and moisture diffusion are consistent, the heat transfer will be enhanced and the temperature gradient in the material will be increased. Diffusion is increased, so it dries faster;
⑧Thinning the thickness of the green body, changing from one-sided drying to double-sided drying;
⑨Reduce the total pressure of the medium, which is beneficial to increase the wet diffusion coefficient, thereby increasing the wet diffusion rate;
⑩ Other factors such as the nature and shape of the green body.