In the initial stage of vacuum pumping, due to the relatively high residual pressure in the vacuum system, the high-temperature water vapor coming out of the reduction tank is prone to condense on the pipe walls at lower temperatures, and the released latent heat is transferred to the ambient atmosphere through the pipe walls. In the middle and later stages of vacuum pumping, since the residual pressure in the vacuum system is relatively low, the water vapor is superheated steam. Obviously, at this time, the water vapor will not condense in the vacuum pipeline, or the water condensed on the pipe walls will evaporate again. When the water vapor passes through the filter medium of the vacuum filter, due to the sudden increase in local vapor pressure and the adsorption effect of the filter medium, the water vapor easily condenses into water droplets and adheres to the filter medium. Especially when the filter layer is blocked or the cross-sectional area is too small, the phenomenon of water vapor condensing into water is particularly severe. Due to the action of the vacuum pump, the condensed water will evaporate again while taking away a large amount of heat. The water vapor evaporated from the reduction tank is very similar to the vapor coming out of the compressor of a refrigeration system; the pipeline between the reduction tank and the filter is very similar to the condenser of a refrigeration system; and the filter medium in the filter is very similar to the throttle valve and evaporator of a refrigeration system. Usually, the residual pressure of the vacuum system is 5-10 Pa. Under such vacuum conditions, even ice will vaporize and take away heat. As long as the filter layer contains sufficient condensed water, icing is very likely to occur. If the filter medium is a thermally conductive material, it will accelerate the icing process.
In summary, the main reasons for icing are that the pumped gas contains a large amount of moisture, the filter is blocked or the cross-sectional area is too small, and high vacuum. However, high vacuum is necessary for production, so solutions can only be considered from the aspects of moisture content in the gas and the filter
Preventive Measures
After understanding the mechanism and causes of icing, the following active and effective measures can be taken to prevent icing:
1. Shorten the storage time of materials and adopt sealed storage to prevent materials from absorbing excessive moisture. A large amount of moisture will not only affect the activity of materials and reduce the output rate of magnesium, but also emulsify the vacuum pump oil, thereby affecting the normal operation of the vacuum pump. It is also one of the main sources of icing.
2. Clean the filter medium in a timely manner. After a period of use, the filter medium will be blocked due to dust adhesion. The filter medium should be cleaned regularly to ensure its smoothness.
3. Ensure that the filter has sufficient cross-sectional area. Some magnesium production manufacturers have filters with too small cross-sectional areas, which not only increases the workload of cleaning the filter medium, but also easily causes blockage in a short time, intensifying the condensation of water vapor in the filter layer.
4. Remove the cooling water jacket of the filter. The filters of magnesium production manufacturers have cooling water jackets, which will intensify the condensation of water vapor in the filter layer. If the filter medium is made of non-thermally conductive material, the icing process can be slowed down.
5. Use a water ring pump as the initial pumping pump. A large amount of dust and water vapor in the early stage of reduction can be directly discharged by the water ring pump. This not only reduces the workload of cleaning the filter medium, but also is not easy to cause blockage in a short time, reducing the condensation of water vapor in the filter layer, and also reducing the pollution of vacuum pump oil.
6. Prevent water from the water ring pump from flowing back into the tank. When the operation is improper or due to poor valve quality, it is easy for water in the water ring pump to enter the vacuum pipeline
