The method of heating and drying materials in an enviro […]
The method of heating and drying materials in an environment below atmospheric pressure is called vacuum drying.
Theoretically, the water placed in a well-sealed vacuum tank will vaporize in the form of surface evaporation in a very short time when the vacuum is started. Since the water vapor pressure is equal to the total pressure in the tank, the entire vacuum process is always at the water temperature. It vaporizes in the form of boiling above the boiling point. Therefore, with the decrease of the ratio Pa/Ps of the pressure in the vacuum tank to the saturated vapor pressure corresponding to the surface temperature of the dried wood, the evaporation rate of the moisture on the surface of the wood is accelerated. The above is the biggest difference between the evaporation of water under negative pressure and the evaporation of water under normal pressure, which determines that the factors that control the evaporation of water on the surface of the wood are the absolute pressure Pa in the tank and the temperature of the wood.
In fact, there is a certain degree of air leakage in the vacuum tank, so a small amount of dry air and water vapor mixture will remain in the tank, and the heat transferred from the higher temperature part in the form of heat radiation and heat conduction will be received, and the temperature will be increased. This will reduce the relative humidity inside the tank and cause the evaporation of moisture on the wood surface.
As mentioned above, vacuum drying is different from drying under normal pressure. In addition to the evaporation of water on the surface of the wood, the main phenomenon is the boiling and vaporization of water in the wood. Therefore, the boiling point also becomes the evaporation surface. To achieve drying of wood, its internal moisture must move towards the evaporative surface. Moisture movement is different when free water is present and when the wood moisture content is below the fiber saturation point.
Wood vacuum drying equipment is mainly composed of drying tank (drying cylinder), vacuum pump, heating system, condensation system and control system.
The vacuum drying method mainly includes two kinds of drying: convection heating intermittent vacuum and hot plate heating continuous vacuum drying. Compared with:
(1) Equipment structure and investment: The continuous vacuum dryer has multi-layer heating plates, and the structure is more complicated.
(2) Equipment operation: When loading and unloading wood in the continuous vacuum dryer, it is necessary to hoist layers of heating plates, and connect each layer of heating plates to the main pipe through many hoses, so the loading and unloading of wood is more troublesome and time-consuming than intermittent.