Classification of air filters

The term “filter” generally refers to an air filter for particulate matter. Filters that remove gaseous pollutants from the air are called chemical filters. There are many types of filters used in cleanrooms, and their names vary depending on their classification methods, such as filtration efficiency, intended use, filter material, and structural form. According to GB/T 14295-2008 “Air Filters,” they can be divided into four categories: coarse-efficiency, medium-efficiency, high-efficiency, and sub-high-efficiency air filters (Table 3-2). GB/T 13554-2008 “High-efficiency Air Filters” categorizes high-efficiency filters into four types based on filtration efficiency: high-efficiency A, high-efficiency B, high-efficiency C, and high-efficiency D (Table 3-2).

1. Classification by filtration efficiency

(1) Primary air filter. The coarse air filter element used for primary filtration of clean air conditioners generally adopts plate type, folding type, wedge bag type and automatic winding type. The filter material is mostly made of metal mesh, foam plastic, non-woven fabric, DV chemical composite felt and other materials that are easy to clean and replace. Coarse air filters are mainly used for fresh air filtration. The filter objects are generally sedimentation particles larger than 5μm and various foreign matter. Therefore, the efficiency of coarse filters is based on filtering 5μm. It requires large dust holding capacity, low resistance, low price and simple structure. Oil-immersed filters are not suitable for use as coarse air filters.

(2) Medium-efficiency air filters. The medium-efficiency air filters used for primary filtration in clean air filters generally have filter elements in the form of insert plates, wedge bags, plates, and pleated types. The filter media mostly uses medium- and fine-pore foam plastics or other fiber filter media, such as glass fiber mat (resin-treated), non-woven fabrics, composite non-woven fabrics, and filament non-woven fabrics. Because the coarse-efficiency air filter in front of it has retained large-size particles, it can also serve as the final filter for general air conditioning systems and a pre-filter for high-efficiency air filters in purification air conditioning systems. Therefore, it is mainly used to retain suspended particles of 1 to 10 μm. Its efficiency is based on filtering 1 μm. It is mainly used to filter fresh air and return air to extend the life of the high-efficiency air filter.

(3) High-efficiency air filters. These can be used as the final filter in general purification systems or as an intermediate filter to enhance the purification efficiency of air conditioning systems and better protect HEPA filters. They primarily intercept suspended particles sized 1-5μm, and their efficiency is based on filtering particles down to 1μm.

(4) Sub-high-efficiency air filters. Sub-high-efficiency air filters used in cleanroom air conditioners typically use glass fiber or cotton short-fiber filter paper as filter elements. Electrostatic filters can also function as sub-high-efficiency air filters. Sub-high-efficiency air filters can serve as final filters in cleanrooms to achieve a certain air cleanliness level, or as pre-filters for HEPA filters to further enhance and ensure the cleanliness of supply air. They can also serve as the final stage of fresh air filtration in air conditioning systems to improve fresh air quality. Like HEPA filters, they primarily intercept particles sized 1μm or smaller, and their efficiency is based on filtering particles down to 0.5μm. Sub-high-efficiency air filters are primarily used to filter fresh air and as the final filter in a tertiary filtration stage. They must be used under the protection of coarse and medium-efficiency air filters.

(5) High-efficiency air filters. The high-efficiency air filters used in cleanroom air conditioners come in three types: glass fiber filter paper, asbestos fiber filter paper, and synthetic fiber. They are primarily designed to filter dust particles smaller than 1μm. They must be used under the protection of coarse and medium-efficiency air filters and often serve as the final filter in a three-stage filtration process. They are the primary final filter in a cleanroom, ensuring air cleanliness levels are achieved. Their efficiency is typically based on filtering particles as small as 0.3μm. Further subdivided, ultra-high-efficiency air filters, designed to achieve air cleanliness levels between 0.1 and 0.3μm and with an efficiency of filtering particles as small as 0.12μm, are considered ultra-high-efficiency air filters.

2. Classification by purpose of use

(1) Filters for fresh air treatment. Used for the treatment of fresh air in clean air conditioning systems, that is, outdoor fresh air, usually use coarse efficiency, medium efficiency, high efficiency, sub-high efficiency, and sometimes high efficiency air filters to treat fresh air. If product production requires the removal of chemical pollutants, chemical filters are also required.

(2) Filters for indoor air supply. Usually used for terminal filtration in clean air conditioning systems, usually use sub-high efficiency, high efficiency, ultra-high efficiency or ULPA+chemical filters or HEPA+chemical filters.

(3) Filters for exhaust. In order to prevent pollutants (including various harmful substances, such as harmful gases, microorganisms – viruses, bacteria or allergens, etc.) generated during the production process of clean room products from polluting the atmosphere, reliable exhaust filters are often installed on the exhaust pipes of the clean room. The exhaust gas can only be discharged into the atmosphere after being filtered and treated to meet the specified exhaust standards. Generally, sub-high efficiency, high efficiency or high efficiency + chemical filters are used.

(4) Filters for equipment in clean rooms. These filters are used within cleanrooms to achieve the required air cleanliness level through internal circulation. They typically employ high-efficiency, ultra-high-efficiency, or HEPA+chemical filters, or ULPA+chemical filters.

(5) Filters for use within manufacturing equipment. These filters are integrated with product manufacturing equipment and typically employ HEPA, ULPA, or HEPA+chemical filters, or ULPA+chemical filters. These filters are closely associated with manufacturing equipment, and the requirements for these equipment vary widely, so they are generally non-standard filters.

(6) Air filters for high-pressure piping. These filters are typically used for gas transport processes with pressures greater than 0.1 MPa. These filters differ significantly from the aforementioned filters in terms of filter material and structure.

(3) Classification by filter material

(1) Filter paper filter. This is the most widely used filter in clean technology. Currently, filter paper is commonly made of glass fiber, synthetic fiber, ultrafine glass fiber and plant cellulose. Using different filter paper materials, it can be made into a 0.3μm-level ordinary filter or sub-high efficiency filter, or a 0.1μm-level ultra-high efficiency filter.

(2) Fiber layer filter. Fiber layer filter is a filter layer made of various fibers. The fibers used are natural fibers (a natural form of fiber such as wool, cotton fiber, etc.), chemical fibers (fibers made by chemical methods to change the properties of the raw materials) and artificial fibers (i.e. physical fibers, fibers separated from the raw materials by physical methods, and the properties of the raw materials are not changed). Fiber layer filters are low-filling filters with small resistance drop and are usually used as medium-efficiency filters.

(3) Foam material filter. Foam material filter is a filter made of foam material. The filtration performance of this type of filter is closely related to its porosity. At present, it is difficult to control the porosity of domestic foam plastics. The porosity of foam materials produced by different manufacturers varies greatly, and the performance of the filters made from them is unstable, so they are rarely used.

In addition, there are many other classification methods. For example, according to the structure of the filter, for example, filter paper filters can be divided into pleated and tubular shapes. Pleated filter paper filters can be classified according to whether there is a partition plate, with a partition plate, oblique partition plate, or without a partition plate. Currently, the most commonly used types are without a partition plate and with a partition plate. According to the filter particle size of 0.3μm and 0.1μm, the outer frame material can be divided into wood, plastic, aluminum alloy, ordinary steel, and stainless steel. According to the appearance, it can be divided into flat plate, V-shaped plate, etc.