The working principle of air filters

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

The main airborne particle removal techniques are mechanical filtration, adsorption, electrostatic dust removal, negative ion and plasma methods, and electrostatic electret filtration.

Mechanical filtration generally captures particles in 3 main ways: direct interception, inertial collision, and Brownian diffusion mechanism, which is effective in collecting fine particles but has a high wind resistance.

Adsorption, which uses the large surface area and porous structure of the material to capture particulate pollutants, is easily blocked and is more effective for gas pollutant removal;

Electrostatic dust removal is a dust collection method that uses a high voltage electrostatic field to ionize the gas so that the dust particles are electrically adsorbed onto the electrodes, its wind resistance is small but poor for larger particles and fibers to be trapped, and it can cause electrical discharges and is troublesome and time-consuming to clean, it is easy to produce ozone and form secondary pollution.

The negative ion and plasma methods of removing indoor particulate pollutants work on a similar principle, both by making the airborne particles electrically charged, agglomerating to form larger particles and settling, but the particles are not removed, but merely attached to nearby surfaces, which can easily lead to re-dusting.

Electrostatic electret filtration effectively blocks airborne particulate contaminants such as dust, lint, pollen, and bacteria, while the ultra-low impedance ensures stable operation and cooling of the air conditioner.

Conventional standard filter media is very effective at removing particles above 10 microns.
When the particle size is reduced to the 5-micron, 2-micron, or even submicron range, efficient mechanical filtration systems become more expensive, and air resistance increases significantly.
With electrostatic electret air filtration, high capture efficiency can be achieved with low energy consumption, while at the same time combining the advantages of electrostatic de-dusting with low air resistance, but without the need for an external voltage of tens of thousands of volts, so no ozone is generated, and because the composition is made of polypropylene, it is easy to dispose of.


Interception

Dust particles in the air, with the airflow for inertial motion or irregular Brownian motion or by the action of some field force and move, when the particle movement hit other objects, objects exist between the Van der Waals force (is molecules and molecules, molecular groups and molecular groups between the force) so that the particles stick to the surface of the fibers.
The dust entering the filter media has more chance to hit the media and will be stuck when it hits the media.
Smaller dust colliding with each other will bond to each other to form larger particles and settle, and the concentration of particles of dust in the air is relatively stable.
Fading of the interior and walls is due to this.

It is a mistake to treat fiber filters like sieves.

Inertia and diffusion

Particulate dust moves inertially in the air stream.
When it encounters a disorganized fiber arrangement, the air stream changes direction and the particles deviate from the direction due to inertia and hit the fiber and are bound.
The larger the particles the easier the impact and the better the effect.

Small particles of dust for irregular Brownian motion.
The smaller the particle, the more violent the irregular motion, the more chances to hit the obstacle, and the better the filtering effect will be.
Particles smaller than 0.1 microns in the air are mainly in Brownian motion, with small particles and a good filtering effect.
Particles larger than 0.3 microns are mainly inertial, the larger the particles, the higher the efficiency.
Diffusion and inertia are not obvious, the particles are the most difficult to filter out.
When measuring the performance of high-efficiency filters, it is often specified to measure the most difficult-to-measure dust efficiency values.


Electrostatic effects

For some reason, fibers and particles can become electrically charged, creating an electrostatic effect.
The filtration effect of electrostatically charged filter materials can be significantly improved.
The reasons for this: static electricity causes the dust to change its trajectory and hit obstacles, and static electricity causes the dust to stick more firmly to the media.

Materials that are electrostatically charged for long periods are also called "electret" materials.
When the material is electrostatically charged the resistance remains unchanged and the filtration effect is significantly improved.
Static electricity does not play a decisive role in the filtering effect, but only a secondary role.
 

Chemical filtration

Chemical filters are mainly selective in their adsorption of harmful gas molecules.

Activated carbon materials have a large number of invisible micropores and a large adsorption area.
In activated carbon, the size of a grain of rice, the area inside the micropores is more than ten square meters.

After contacting the activated carbon, the free molecules coalesce into liquid in the micro-pores due to the capillary principle and stay in the micro-pores, some of which are integrated with the material.
The adsorption without an obvious chemical reaction is called physical adsorption.

Some of the activated carbon is treated and the particles that are adsorbed react with the material to produce solid substances or harmless gases, called chemisorption.

The adsorption capacity of the material decreases throughout its use and when it reaches a certain point, the filter is scrapped.
If the adsorption is only physical, heating or water vapor fumigation can be used to remove the harmful gases from the activated carbon and regenerate it.
 

Gravitational effect

When the particles pass through the fiber layer, under the effect of gravity, the displacement from the flow line occurs and settles on the fiber surface, this effect only exists when the particles are large (>0.5um), which is too small for the gravity of the particles, when it has not settled on the fiber with the airflow through the fiber layer.
Thus, for the filtration of particles smaller than 0.5um, gravitational settling can be completely ignored.