The filtering principle of the air filter

22/07/2020

    Air filters are widely used in commercial electrical appliances, automobiles, rail transit, aerospace, aviation, electronics, pharmaceuticals, biological engineering, and other fields to provide cleaner air. So how does the air filter the air?
    The following will introduce the filtering principles of the following air filters. The principles are divided into four types: interception, inertia and diffusion, electrostatic interaction, and chemical filtration.
    Ⅰ. Interception
    Dust particles in the air move with the airflow in inertial motion or irregular Brownian motion or under the action of a certain field force. When the motion of the particle hits other objects, the van der Waals forces existing between the objects (which are molecules and molecules, molecular clusters, and The force between the molecular clusters) makes the particles stick to the fiber surface. The dust entering the filter medium has more chances to hit the medium, and it will be stuck when it hits the medium. Smaller dust collides with each other and will bond to each other to form larger particles and settle, and the particle concentration of dust in the air is relatively stable. Fading indoors and walls are, for this reason, it is wrong to treat the fiber filter like a sieve.
    Ⅱ. Inertia and diffusion
    Particle dust moves inertially in the airflow. When it encounters disorderly arranged fibers, the airflow changes direction, and the particles deviate from the direction due to inertia and hit the fibers to be bonded. The larger the particle, the easier it is to hit, and the better the effect.
    Small particles of dust make irregular Brownian motion. The smaller the particles, the more violent the irregular movement, the more chances of hitting obstacles, and the better the filtering effect. The particles smaller than 0.1 microns in the air mainly make Brownian motion, the particles are small and the filtering effect is good. Particles larger than 0.3 microns are mainly used for inertial motion, the larger the particle, the higher the efficiency. Diffusion and inertia are not obvious and particles are the most difficult to filter out. When measuring the performance of high-efficiency filters, people often specify the dust efficiency value that is the most difficult to measure.
    Ⅲ. Electrostatic effect
    For some reason, fibers and particles may be charged, causing electrostatic effects. The filtering effect of electrostatically charged filter materials can be significantly improved.
    Reason: Static electricity makes the dust change its trajectory and hit obstacles, and static electricity makes the dust stick to the medium more firmly. Materials that can hold static electricity for a long time are also called "electret" materials. After the material is charged with static electricity, the resistance will not change, and the filtering effect will be significantly improved. Static electricity does not play a decisive role in the filtering effect, but only plays a supporting role.
    Ⅳ. Chemical filtration
    The chemical filter mainly selectively adsorbs harmful gas molecules. The activated carbon material has a large number of invisible micropores and a large adsorption area. Inactivated carbon the size of a rice grain, the area of ​​the micropores is more than ten square meters. After the free molecules contact the activated carbon, they condense into liquid in the micropores and stay in the micropores due to the capillary principle, and some of them are integrated with the material.
    The adsorption without obvious chemical reaction is called physical adsorption. Some processes the activated carbon, the adsorbed particles react with the material to produce solid matter or harmless gas, which is called chemical adsorption.
    The adsorption capacity of the activated carbon material is constantly weakened during use, and when it weakens to a certain degree, the filter will be scrapped. If it is only physical adsorption, heating or steam fumigation can remove harmful gases from activated carbon and regenerate activated carbon.