Filtering Overview

Filter Material

Effectively traps dust particles without creating excessive resistance to airflow. The randomly interwoven fibers form numerous barriers against particles, while the ample spaces between the fibers allow for smooth airflow.

Efficiency

The ratio of the amount of dust captured by the filter to the amount of dust in the unfiltered air is called "filtration efficiency." Particles smaller than 0.1 mm (micrometers) primarily undergo diffusion; the smaller the particle, the higher the efficiency. Particles larger than 0.5 mm primarily undergo inertial motion; the larger the particle, the higher the efficiency.

Resistance

The fibers cause airflow to bend around them, generating minute resistance. The sum of the resistance of countless fibers is the filter's resistance.

Filter resistance increases with increasing airflow. Increasing the surface area of the filter material can reduce the relative air velocity passing through the filter media, thus reducing filter resistance.

Dynamic Performance

The captured dust creates additional resistance to airflow, causing the filter's resistance to gradually increase during use. The captured dust forms new obstacles, thus slightly improving the filtration efficiency.

Most of the captured dust accumulates on the windward side of the filter media. The larger the filter media area, the more dust it can hold, and the longer the filter's lifespan.

Lifespan

The more dust accumulates on the filter media, the greater the resistance. When the resistance becomes unacceptable in the design, the filter's lifespan ends. Sometimes, excessive resistance can cause the captured dust to scatter, resulting in secondary pollution, and the filter should also be discarded.

Static Electricity

If the filter media or dust particles are statically charged, the filtration effect can be significantly improved. This is because static electricity causes the dust to change its trajectory and collide with obstacles, with electrostatic force participating in the adhesion process.

Filtration Efficiency

Among the factors determining filtration efficiency, the meaning of the "amount" of dust varies, resulting in different calculated and measured filter efficiency values. In practical applications, there are total dust weight and the number of dust particles; sometimes it's the amount of dust with a specific particle size, sometimes it's the amount of all dust; there are also methods that indirectly reflect concentration, such as light transmittance (colorimetry) and fluorescence (fluorescence method); there are instantaneous quantities for a certain state, and there are weighted average quantities of efficiency values throughout the entire dust generation process.

Using different methods to test the same filter will yield different efficiency values. Without the testing method, filtration efficiency is meaningless.

Filter Resistance

Filters create resistance to airflow. Dust accumulates in the filter, increasing resistance. When the resistance increases to a certain specified value, the filter is rendered unusable.

The resistance of a new filter is called "initial resistance"; the resistance value at which the filter is rendered unusable is called "final resistance."

Final Resistance

The choice of final resistance directly affects the filter's lifespan, the range of system airflow variations, and system energy consumption.

In most cases, the final resistance is 2 to 4 times the initial resistance.

Recommended Final Resistance Values

The dirtier the filter, the faster the resistance increases. An excessively high final resistance value does not necessarily mean a significantly longer filter lifespan, but it will drastically reduce the airflow of the air conditioning system. Therefore, it is unnecessary to set the final resistance value too high.

Low-efficiency filters often use coarse fiber filter media with a diameter ≥10mm. Due to the large gaps between the fibers, excessive resistance may blow away the accumulated dust on the filter. In this case, the resistance no longer increases, but the filtration efficiency drops to zero. Therefore, the final resistance value of filters below G4 should be strictly limited.

Each filter section should be equipped with a resistance monitoring device. The final resistance must be determined by instruments, not solely by the operator's feeling.

Dust Holding Capacity

Dust holding capacity is the weight of a specific test dust that the filter can hold under specific test conditions. The term "specific" here refers to:

a. Standard test wind tunnel and related testing and measurement equipment;

b. Standard "road dust" much larger than actual atmospheric dust particles;

c. Test methods and calculation methods agreed upon by the client and the testing provider, or as specified in the standard;

d. Conditions for terminating the test agreed upon by the client and the testing provider.

Dust holding capacity is not directly related to the actual weight of dust held by the filter; isolated dust holding capacity data is meaningless to the user.

Inhalable Particulate Matter

Large dust particles in the air are blocked by the nasal cavity, while small dust particles may enter the trachea and lungs with the airflow. These dust particles are engulfed and digested by macrophages in the trachea and lungs. Bacteria and viruses that macrophages cannot completely eliminate are eliminated by white blood cells.

The nasal hairs, secretions, and mucous membranes of the nose can filter out most dust particles larger than 10 mm. Only particles smaller than 10 mm will enter the trachea and lungs with the airflow. Therefore, "inhalable particulate matter" is defined as "particulate matter in the air ≤10mm".

The total amount of dust in the air is called "total suspended particulate matter". Removing particles larger than 10mm leaves "inhalable particulate matter", technically designated TM10. The "inhalable particulate matter" we often hear about is this TM10. If particles larger than 5mm are removed, the remaining "inhalable particulate matter" is TM5.

Inhalable Particulate Matter and Health Effects

Chemical Filters

Chemical filters remove gaseous pollutants from the air. In ventilation and air conditioning systems, activated carbon is used as the primary filter material. Typical applications of chemical filters include: chip factories, nuclear industry, airports, environmental protection, museums, and some household appliances also use chemical filter materials.

Chemical Filtration Principle

Chemical filters selectively adsorb harmful gas molecules, rather than mechanically removing impurities like ordinary filters.

Activated carbon materials contain numerous micropores invisible to the naked eye, most of which have a pore size between 5 Å and 500 Å. The total internal surface area of the micropores per unit material can reach 700–2300 m²/g. This means that the internal surface area of the micropores in a single rice-grain-sized activated carbon particle is equivalent to the size of a large living room wall.

Adsorption without a significant chemical reaction is called physical adsorption, which primarily relies on van der Waals forces. When free molecules in the air with high boiling points (at room temperature or higher) come into contact with activated carbon, some condense into liquid within the micropores and remain there due to capillary action, while others fill the micropores, becoming integrated with the material. The main components of the atmosphere, such as nitrogen, oxygen, carbon dioxide, hydrogen, and argon, have very low boiling points, and activated carbon cannot adsorb them. Ordinary activated carbon is hydrophobic, so its adsorption capacity for water vapor is also limited. In addition, activated carbon can adsorb and kill certain airborne microorganisms.

Chemical adsorption refers to the adsorption process where the material reacts chemically with harmful gases through chemical treatment. Activated carbon uses van der Waals forces to capture gas molecules, and the chemical components on the material react with the pollutants to produce solid components or harmless gases. The main method of chemical treatment is to uniformly incorporate specific reagents into the activated carbon; therefore, chemically treated activated carbon is also called "impregnated carbon."

During use, the adsorption capacity will continuously weaken. When it weakens to a certain level, the filter becomes unusable. If only physical adsorption is used, heating or steam fumigation can remove harmful gases from activated carbon, thus regenerating it.

Activated Carbon Materials

Activated carbon materials are classified into granular activated carbon, fibrous activated carbon, and powdered activated carbon.

Fiber activated carbon is made from carbon-containing organic fibers. It has a small pore size (<50 Å), large adsorption capacity, fast adsorption, and fast regeneration. Commonly used fiber base materials include phenolic resin, plant fibers, polyacrylonitrile, and asphalt.

Adsorption Performance

Adsorption Capacity: The maximum amount of pollutants that a unit of activated carbon can adsorb is called the adsorption capacity. Different materials have different adsorption capacities; the same material will have different adsorption capacities for different gases; changes in temperature and background concentration will also affect the adsorption capacity.

Retention Time: The time air stays in the activated carbon layer is called the retention time. The longer the retention time, the more complete the adsorption. To maintain sufficient retention time, the carbon layer should be thick enough, and the filtration velocity should be as low as possible.

Service Life New activated carbon has high adsorption efficiency, but this efficiency gradually decreases during use. When the concentration of harmful gases downstream of the filter approaches the permissible limit, the filter becomes unusable. The time it takes to be used before it becomes unusable is its service life, also known as the effective protection time.

Selectivity. Generally speaking, gases that are easily adsorbed through physical adsorption include: gases with large molecular weights, gases with high boiling points, and volatile organic gases. If activated carbon is chemically impregnated, it can also remove gases that are normally difficult to handle, or highlight its adsorption capacity for certain types of gases.

Selection of Activated Carbon Filters

The main factors affecting the adsorption effect and service life of activated carbon filters are: the type and concentration of pollutants, the residence time of the airflow in the filter material, and the temperature and humidity of the air.

In actual selection, the filter type and activated carbon type should be determined based on the type and concentration of pollutants and the air volume to be processed.

Both upstream and downstream of the activated carbon filter should have good dust removal filters with an efficiency specification of not less than F7. The upstream filter prevents dust from clogging the activated carbon material; the downstream filter blocks dust generated by the activated carbon itself.