What factors affect the filtration efficiency of air filters?

　　The filtration efficiency of an air filter is not a fixed value; rather, it is influenced by a combination of factors, including the material and structure of the filter element, the characteristics of the impurities, and operating conditions. These factors interact with one another and ultimately determine whether the filter can effectively trap impurities. The following is a detailed analysis of these specific influencing factors:

　　1. Core parameters of the filter element (decisive factors)

　　(1) Filter element material and microstructure

　　Fiber Type and Diameter: The finer the fibers, the greater the number of fibers per unit volume, the smaller the resulting micropore gaps, and the stronger the ability to capture fine particles (for example, nanofiber filter paper has a filtration efficiency 30%–50% higher than that of conventional paper fibers). Synthetic fibers (such as polyester and polypropylene), due to their lower surface smoothness, exhibit superior particle adsorption performance compared to paper fibers. In contrast, metal mesh fibers, with their larger diameters, are suitable only for filtering coarse particles.

　　Material porosity: If the porosity—the ratio of pore volume to total volume in a filter element—is too high, particles can easily pass through; if it’s too low, airflow resistance will increase. High-quality filters strike a balance between porosity and filtration accuracy; for example, the porosity of paper air filters used in engines is typically controlled within the range of 70% to 80%.

　　(2) Filter element structural design

　　Filtration area: The filtration area of a pleated filter element is significantly larger than that of a flat-type filter element. At the same air flow velocity, the probability of particles coming into contact with fibers is higher, thus improving filtration efficiency. For example, the pleated filter element used in automotive air filters has a filtration efficiency more than 20% higher than that of a flat filter element of the same size.

　　Filter element thickness: The thicker the deep-layer filtration filter element, the more times particles collide with and are intercepted by the filter material inside, resulting in higher filtration efficiency (for example, a thick nonwoven fabric filter element used in industrial applications can achieve a filtration efficiency of up to 99% for 5-μm particles, whereas a thin paper filter element achieves only 85%).

　　Surface treatment process: Synthetic fiber filter elements with electrostatic polarization (such as electrospun filter meshes) can capture tiny particles through electrostatic attraction, improving filtration efficiency by 15% to 25% compared to conventional filter elements. Filter elements treated with a hydrophobic coating can prevent clogging of the filter paper in humid environments, thereby indirectly maintaining filtration efficiency.

　　2. Impurity characteristics (objective influencing factors)

　　(1) Impurity particle size and shape

　　The larger the particle diameter, the easier it is for the filter element to trap particles (for example, the filtration efficiency for particles larger than 10 μm can reach 99.9%, whereas the filtration efficiency for ultrafine particles as small as 0.3 μm may be only 70%);

　　Irregularly shaped particles (such as needle-like metal debris) tend to get lodged in the gaps between filter fiber strands, making them more difficult to penetrate than spherical particles and thus achieving relatively higher filtration efficiency.

　　(2) Impurity Concentration and Composition

　　When the concentration of impurities in the air is too high, a “dust layer” tends to form rapidly on the surface of the filter element. Initially, this dust layer can enhance efficiency due to its secondary filtration effect. However, prolonged clogging will cause the airflow velocity to increase, ultimately allowing fine particles to penetrate the filter element—thus rendering the “bridging effect” ineffective.

　　Oily particles (such as cooking fumes and oil mist) tend to adhere to the filter media fibers, clogging the micropores and reducing the electrostatic adsorption capacity (if the filter uses an electrostatic mechanism), thereby lowering the filtration efficiency. In contrast, dry dust has a relatively minor impact on the filter element.

　　3. Operating conditions (dynamic influencing factors)

　　(1) Airflow Velocity and Pressure

　　The faster the airflow velocity, the shorter the residence time of particles in the filter element, reducing the probability of particle-fiber collisions. As a result, fine particles are more likely to penetrate the filter element along with the airflow (for example, when the airflow velocity increases from 2 m/s to 5 m/s, the filtration efficiency for 0.5-μm particles can drop by as much as 30%).

　　When the pressure difference between the filter’s inlet and outlet is too large, airflow will leak through damaged areas of the filter element or sealing gaps, directly causing a sharp drop in filtration efficiency.

　　(2) Temperature and Humidity

　　High-temperature environments—such as those found in engine intake systems—can cause the resin binder in paper filter elements to soften, leading to a loosening of the fiber structure and a decline in filtration efficiency (when the temperature exceeds 120°C, the efficiency of paper filter elements may drop by 10% to 15%).

　　In high-humidity environments, paper filter elements are prone to moisture absorption, mold growth, micro-pore blockage, and a decline in fiber strength. This not only reduces filtration efficiency but may also lead to filter element damage. In contrast, synthetic fiber filter elements exhibit better moisture resistance, and humidity has relatively little impact on their efficiency.

　　(3) Usage duration and maintenance status

　　The new filter element has a lower filtration efficiency. As usage time increases, the layer of dust that builds up on the filter surface begins to act as an "auxiliary filter," gradually improving the overall efficiency. However, if the filter element becomes clogged and is not replaced or cleaned in a timely manner, the airflow velocity will be forced to increase, causing the efficiency to drop rapidly.

　　After being washed multiple times, the structure of washable filter elements (such as metal mesh or synthetic fiber filters) may become damaged, and their micropores may enlarge, leading to a gradual decline in filtration efficiency (for example, after three washes, the filtration efficiency might drop from 95% to 80%).

　　4. Installation and Sealing Performance

　　Poor sealing between the filter and the installation housing—such as aging seals or improper installation alignment—can allow unfiltered air to enter the equipment through gaps, directly reducing overall filtration efficiency. Even if the filter element itself achieves an efficiency of 99%, when the leakage through sealing gaps reaches 5%, the overall efficiency will drop below 95%.

　　5. Adaptability of the Filtration Principle

　　Different filtration principles have different impacts on efficiency:

　　Surface-filtering cartridges (such as metal mesh) are highly efficient at capturing coarse particles but less effective at capturing fine particles.

　　Composite filter elements that combine deep-layer filtration with electrostatic adsorption (such as HEPA filters) are far more effective at capturing ultrafine particles than filter elements based on a single principle.

　　Centrifugal filters are highly efficient only in separating large particles (≥20 μm); for fine particles, they must be used in conjunction with filter cartridges; otherwise, their efficiency will be extremely low.