Do you know how the air filter works?

The prototype of the air filter is a respiratory protection device that people use to protect their breathing. According to records, in Rome in the first century, when the mercury was purified, it was protected with a mask made of coarse hemp. For a long time after that, air filtration equipment has also made progress, but it is mainly used as respiratory protection. Appliances are used in some hazardous industries, such as the production of hazardous chemicals. In 1827, Bu Peng discovered the movement law of tiny particles, which made people have a further understanding of the mechanism of air filtration. The rapid development of air filtration equipment was closely related to the development of military industry and electronics industry. In the 1950s, the United States conducted in-depth research on the production process of glass fiber filter paper, which improved and developed air filtration equipment. In the 1960s, HEPA filters were introduced; in the 1970s, HEPA filters using fine glass fiber filter paper as a filter medium had a filtration efficiency of up to 99.9998 for D.3 micron particles. Since the 1980s, with the advent of new test methods, improved evaluation of use, and increased requirements for filtration performance, HE-PA filters have been found to have serious problems, resulting in higher performance ULPA filters. At present, countries are still working hard to study, and it is estimated that more advanced air filtration equipment will appear soon.

The filtration principle of the equipment separates the liquid water and liquid oil droplets in the compressed air, and filters out dust and solid impurities in the air, but cannot remove the gaseous water and oil.

The airborne particulate removal technology mainly includes mechanical filtration, adsorption, electrostatic dust removal, negative ion and plasma methods, and electrostatic electret filtration. Mechanical filtration generally captures particles mainly through the following three methods: direct interception, inertial collision, and Brownian diffusion mechanism. The collection effect on fine particles is good but the wind resistance is large. In order to obtain high purification efficiency, the filter element needs to be dense and periodically replaced.

Adsorption is the use of large surface area and porous structure of materials to capture particulate contaminants, which is easy to block and is more effective for gas pollutant removal. Electrostatic dust removal is the use of high-voltage electrostatic field to ionize the gas to make the dust particles charged to the electrode. Dust method, although its wind resistance is small, but the effect of trapping large particles and fibers is poor, it will cause discharge, and the cleaning is troublesome and time-consuming, and ozone is easily generated to form secondary pollution.

Negative ion and plasma methods work to remove indoor particulate contaminants. They are caused by charging particles in the air and coalescing to form larger particles to settle, but the particles are not actually removed, but are attached to nearby surfaces. It is easy to cause dust again.

Electrostatic electret filtration effectively blocks particulate pollutants in the air, such as dust, chips, pollen, bacteria, etc., while ultra-low impedance ensures stable operation and cooling effect of air conditioners.

Conventional standard filter media are very effective at removing particulate matter above 10 microns. When the particle size of the particles is divided into the range of 5 microns, 2 microns or even sub-micron, the efficient mechanical filtration system becomes more expensive and the wind resistance is significantly increased. Filtered by electrostatic electret air filter material, it can achieve high capture efficiency with low energy consumption, and has the advantages of electrostatic dust removal and low wind resistance. However, it does not need to connect tens of thousands of volts, so ozone will not be generated. Its composition is made of polypropylene, which is easy to dispose of.

Intercept

The dust particles in the air move with inertial motion or random Brownian motion of the airflow or by some field force. When the particle motion hits other objects, the van der Waals force exists between the objects (molecular and molecular, molecular group and The force between the molecular groups causes the particles to stick to the surface of the fiber. The dust entering the filter media has a greater chance of hitting the media, and it will stick if it hits the media. The smaller dust collides with each other to form larger particles and settle, and the particle concentration of the dust in the air is relatively stable. The fading of the interior and walls is for this reason.

It is wrong to treat the fiber filter like a sieve.

Inertia and diffusion

The particulate dust moves in inertia in the airflow. When encountering the fibers arranged in disorder, the airflow changes direction, and the particles are bound by the inertia deviating from the direction and hitting the fibers. The larger the particle, the easier it is to impact, and the better the effect.

Small particle dust is used for random Brownian motion. The smaller the particles, the more intense the irregular movement, the more chances of hitting the obstacle and the better the filtering effect. Particles smaller than 0.1 micron in the air are mainly used for Brownian motion, and the particles are small and the filtering effect is good. Particles larger than 0.3 microns are mainly used for inertial motion, and the larger the particles, the higher the efficiency. It is not obvious that the diffusion and inertia are the most difficult to filter out. When measuring the performance of high-efficiency filters, it is often prescribed to measure the dust efficiency value that is the most difficult to measure.

Electrostatic interaction

For some reason, fibers and particles may be charged, creating an electrostatic effect. The filtering effect of the electrostatically charged filter material can be significantly improved. Cause: Static electricity causes the dust to change its trajectory and hit an obstacle. Static electricity makes the dust stick more firmly on the medium.

Materials that can carry static electricity for a long time are also called "electret" materials. The resistance of the material after static electricity is unchanged, and the filtration effect is obviously improved. Static electricity does not play a decisive role in the filtration effect, but only plays an auxiliary role.

Chemical filtration

Chemical filters primarily selectively adsorb harmful gas molecules.

There are a large number of invisible micropores in the activated carbon material, which have a large adsorption area. In the activated carbon of rice size, the area inside the micropores is more than ten square meters.

After the free molecules are in contact with the activated carbon, they condense into a liquid in the micropores and remain in the micropores due to the capillary principle, and some are integrated with the material. Adsorption without a distinct chemical reaction is called physical adsorption.

Some treat activated carbon, and the adsorbed particles react with the material to form a solid substance or a harmless gas called chemical adsorption.

The adsorption capacity of the activated carbon during the use of the material is continuously weakened, and when it is weakened to a certain extent, the filter will be scrapped. If it is only physical adsorption, the activated carbon may be regenerated by heating or steaming to remove harmful gases from the activated carbon.

Gravity effect

When the particles pass through the fiber layer, under the action of gravity, the displacement of the flow line is released and settles on the surface of the fiber. This effect exists only when the particles are large (>0.5um), which is too small for the gravity of the particles. It also passes through the fibrous layer with the gas stream when it has not settled onto the fiber. Thus, for filtration of particles having a particle size of less than 0.5 um, gravity settling is completely negligible.

The above is all the content that is brought to you today. Through the introduction of the above small series, do you know the working principle of the air filter ? I also hope that this article can help everyone.