What are the basic knowledge and principles of air filtration?
Many people do not know what the principle of air filtration is. Why are the dust substances in the air filtered and purified by filter cotton, filter and other equipment? Let me explain the basic knowledge of air filtration.
Air filtration is a purification process in which the dust in the air is subject to a certain force, and the dispersed dust particles are removed from the gas by using a relatively porous body. A certain force mentioned above refers to inertial force, van der Waals force and electrostatic force. Large particles make inertial motion in the air flow, and the air flow detours when encountering obstacles. The particles deviate from the air flow direction due to inertia and hit the obstacles. Due to the large diameter and strong inertial force, the more likely they are to hit the obstacles, so the dust cannot pass through the Air Filter Raw Material, so the filtering effect is good. Small particles move irregularly. Although they have a certain direction, they mainly move in diffusion. Due to the fine fiber of the filter material, the van der Waals force between the two micromolecules makes them stick together, so the dust cannot pass through the filter material. At this time, the filtering effect is good. When we bring the filter material and keep the static electricity, the static electricity can keep the dust away, so that the dust cannot pass through the filter material. At this time, the filtering effect is good.
During air filtration, due to inertial collision, interception, diffusion, static electricity and other effects, the dust particles suspended in the gas will settle on the porous materials, which are fibrous, porous or the combination of these structures, collectively referred to as filter materials. The Air Filter Raw Material not only effectively intercepts dust particles, but also does not cause excessive resistance to the air flow. The air filter cotton and air filter paper meet this requirement. The jumbled fibers form countless barriers to particles, and the wide space between fibers allows air flow to pass smoothly.
The ratio of the amount of dust collected by the filter to the amount of dust in the unfiltered air is “filtration efficiency”. Less than 0.1 μ The particles of m (micrometer) mainly conduct diffusion movement. The smaller the particles, the higher the efficiency; More than 0.5 μ The particles of m (micrometer) mainly make inertial motion. The larger the particles are, the higher the efficiency is. At 0.1 μ M and 0.5 μ M, there is a lowest point of efficiency.
The moving air stream hits the filter material, and the fiber makes the air stream bypass, generating small resistance at the same time. The resistance of the filter is the sum of the resistance of numerous fibers. The filter resistance increases with the increase of air flow. By increasing the area of air filter material, the relative wind speed passing through the filter material can be reduced to reduce the filter resistance. The filter will form resistance to the air flow, and the filter will deposit dust. The resistance will increase. When the resistance increases to a specified value, the filter will be scrapped. The resistance of new filter is called “initial resistance”; The resistance value corresponding to filter scrapping is called “final resistance”.
The captured dust has additional resistance to the airflow, so the resistance of the filter in use gradually increases. The captured dust forms new obstacles, so the filtering efficiency is slightly improved. The captured dust is mostly concentrated on the windward side of the filter material. The larger the filter material area is, the more dust can be contained, and the longer the filter life is. The more dust accumulated on the filter material, the greater the resistance. When the resistance is too large to be allowed by the design, the life of the filter will end. Sometimes, too much resistance will make the dust caught on the filter fly away. When this danger occurs, the filter should also be scrapped.
According to the above principle, in the process of manufacturing air filters, it is necessary to select excellent air filter materials to meet people’s needs for keeping fresh space after air filtration. This is the filtering principle that we often mention.