The principle is to fold and intercept dust particles in the air, move with the inertia motion or irregular Brownian motion of the air flow or under the action of a certain field force. When the particle motion hits other objects, the van der Waals force (the force between molecules and molecules, molecules and molecular clusters) between objects makes the particles stick to the surface of the fiber. Dust entering the filter medium has more opportunities to hit the medium, and if it hits the medium, it will be stuck. Smaller dust collides with each other to form larger particles and settle, and the concentration of dust particles in air is relatively stable. It is for this reason that the interior and walls fade. It is wrong to treat the fiber filter like a screen. The folded inertia and diffusion particles move in the air flow. When the fibers are arranged disorderly, the air flow changes the direction, and the particles are bonded because the inertia deviates from the direction and hits the fiber. The larger the particle, the easier it is to impact, and the better the effect. Small particles of dust do irregular Brownian motion. The smaller the particles, the more irregular motion, the more chance of hitting obstacles, the better the filtration effect. The particles less than 0.1 microns in the air mainly do Brownian motion, the particles are small, and the filtration effect is good. Particles larger than 0.3 microns mainly move in inertia, and the greater the particle efficiency, the higher the efficiency. Diffusion and inertia are not obvious, making it most difficult to filter out particles. When measuring the performance of high efficiency filters, people often stipulate the most difficult to measure the dust efficiency value. Folding electrostatic action for some reason, fibers and particles may be charged, resulting in electrostatic effect. The filtration effect of filter material with static electricity can be obviously improved. Reason: static electricity changes the trajectory of dust and hits obstacles, and static electricity makes dust stick better on the medium. Materials that can carry static electricity for a long time are also called electret materials. After the material has static electricity, the resistance will not change, and the filtration effect will be improved obviously. Static electricity does not play a decisive role in the filtration effect, but only plays an auxiliary role. Folding chemical filtration chemical filter mainly adsorbs harmful gas molecules selectively. There are a large number of invisible micropores and large adsorption area in activated carbon materials. In the activated carbon of rice grain size, the micropore area is more than ten square meters. After contact with activated carbon, free molecules condense into liquid in micropores because of capillary principle, some of them are integrated with materials. Adsorption without obvious chemical reactions is called physical adsorption. Some of the activated carbon are treated and the adsorbed particles react with the material to form solid matter or harmless gas, which is called chemical adsorption. In the process of using activated carbon, the adsorption capacity of the material is weakened, and when it weakens to a certain extent, the filter will be scrapped. If it is only physical adsorption, heating or steam fumigation can separate the harmful gas from activated carbon and regenerate activated carbon. When the folded gravity effect particles pass through the fiber layer, under the action of gravity, the displacement from the airflow streamline occurs and settles on the surface of the fiber. This effect only exists when the particles are large (> 0.5um). This is that the gravity effect of the particles is too small, and it passes through the fiber layer with the air flow when it has not yet settled on the fiber. Therefore, the heavy force settlement can be ignored for the filtration of particles with particle size smaller than 0.5um.