The vacuum drying is to put the materials to be dried in an enclosed space, use vacuum equipment to reduce the air pressure in the enclosed space to an atmospheric pressure, and at the same time continuously heat the materials.

In this way, the water molecules in the material gradually diffuse to the surface of the material due to the effects of pressure difference and concentration difference. After obtaining sufficient kinetic energy on the surface of the material, the binding of the intermolecular attraction is gradually overcome, and it escapes into the vacuum chamber of low pressure and then passes The vacuum pump is discharged into the atmosphere.

Vacuum drying processes:

The first is the heat transfer process, the material absorbs heat through the heat source, heats up and vaporizes the internal moisture;

Secondly, in the process of liquid mass transfer inside the material, the water inside the material moves to the surface in the form of liquid, and then vaporizes on the surface;

Finally, during the gaseous mass transfer process of the moisture on the surface of the material, the water vapor vaporized on the surface of the material gradually escapes into the inside of the vacuum chamber and flows to the outside through the vacuum chamber.

To complete the above heat and mass transfer process, temperature, pressure and concentration are the key control factors;

Temperature: The temperature of the heat source should be significantly greater than the temperature of the material to meet the energy required for its own temperature increase and vaporization of moisture. The higher the temperature, the faster the drying.

Concentration: The so-called concentration refers to the concentration of moisture in the material. The internal concentration of the material is higher than the surface concentration of the material. Under the action of capillary force and concentration difference, it migrates to the surface and eventually reduces the concentration of moisture.

Pressure: The steam pressure on the surface of the material is higher than the partial pressure of steam in the drying oven, which is the core theory of vacuum drying.

Vacuum drying thermodynamics mainly studies the relationship between the vaporization temperature of water, the saturated vapor pressure and the degree of vacuum. The theoretical basis can be Clapron. Clausius formula expression

The vaporization temperature of water is positively correlated with saturated vapor pressure and negatively correlated with vacuum degree. The smaller the saturated vapor pressure, the lower the vaporization temperature of water. Similarly, the greater the vacuum degree, the lower the vaporization temperature of water. Therefore, it can be seen that water is easier to achieve low-temperature vaporization in a vacuum environment. When the temperature is maintained at a certain value, increasing the vacuum in the drying chamber can convert more moisture on the surface of the material into water vapor, which can also accelerate the drying effect.

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