The operational efficiency, stability, and economy of an air filtration system depend primarily on the scientific selection and hierarchical matching of air filters with different efficiency levels. In air purification engineering, the hierarchical configuration of filters must follow the core principle of “graded filtration and step-by-step protection”. By reasonably matching the efficiency specifications of each level of filters, the established air cleanliness requirements can be achieved, while maximizing the overall service life of the system and controlling operational costs.
I. Principles for Hierarchical Matching and Efficiency Selection of Air Filters
In an air filtration system, the functional positioning of each level of filter is clearly differentiated: the final stage filter (terminal filter) directly determines the cleanliness level of the air output by the system and is the core executive unit of the filtration system; all upstream filters play a protective role and are collectively referred to as “pre-filters”. Their core function is to intercept large-particle dust in the air, reduce the dust load of the terminal filter, thereby extending the service life of the terminal filter and reducing the replacement frequency.
The matching of efficiency specifications of each level of filters is the key to selection, and two types of unreasonable matching should be avoided: first, the efficiency gap between two adjacent levels of filters is too large, resulting in the pre-filter being unable to effectively intercept dust, and the terminal filter directly bearing the impact of high-concentration, large-particle dust, which quickly accumulates dust and clogs, significantly shortening its service life; second, the efficiency gap between two adjacent levels is too small, leading to excessive interception load on the pre-filter and insufficient load on the terminal filter, resulting in waste of filter materials and increased system operational costs.
Combined with industry engineering practice, the current mainstream efficiency specification grading matching adopts the “G～F～H～U” series grading system, which can quickly and scientifically determine the efficiency level of each level of filter. Within the G2～H12 efficiency range, a filter is usually set every 2～4 grades to form a progressive filtration gradient. This not only ensures the full play of the protective role of the pre-filter but also enables the terminal filter to stably meet the designed cleanliness requirements, achieving a balance between filtration efficiency and economy.
II. Classification and Definition of Air Filters (Classified by Filtration Efficiency)
According to the national standard GB/T 14295-2019 “Air Filters”, combined with filtration efficiency and interception particle size, air filters can be divided into three categories: primary efficiency, medium efficiency, and high efficiency. Their core definitions and technical indicators are as follows:
(1) Primary Efficiency Air Filter
As the primary pre-treatment unit of the filtration system, it mainly intercepts suspended particles with a particle size ≥5.0μm in the air (such as dust, lint, large-particle dust, etc.). Its core technical indicator is: the atmospheric dust counting efficiency for particles with a particle size equal to or greater than 5.0μm is ≥20% and ＜80%. The commonly used efficiency levels are G2～G4, which are widely used in air intake pre-treatment of air conditioning systems, equipment air intake protection and other scenarios.
(2) Medium Efficiency Air Filter
Located downstream of the primary efficiency filter and upstream of the high efficiency filter, it undertakes the secondary filtration task. It mainly intercepts suspended particles with a particle size ≥1.0μm in the air, further purifies the air, and provides key protection for the high efficiency filter. Its core technical indicator is: the atmospheric dust counting efficiency for particles with a particle size equal to or greater than 1.0μm is ≥20% and ＜70%. The commonly used efficiency levels are F5～F9, which are suitable for air purification in places with medium cleanliness requirements (such as ordinary offices, general wards of hospitals, food processing workshops, etc.).
(3) High Efficiency Air Filter
As the core terminal unit of the filtration system, it mainly intercepts fine suspended particles with a particle size ≥0.3μm in the air (such as bacteria, viruses, fine dust, etc.) and directly determines the air cleanliness level. Its core technical indicator is: the atmospheric dust counting efficiency for particles with a particle size equal to or greater than 0.3μm is ≥99.99%. The commonly used efficiency levels are H13～U17, which are widely used in high cleanliness places (such as operating rooms, laboratories, electronic clean workshops, biosafety laboratories, etc.).
III. Definition of Core Technical Parameters of Air Filters
Filtration efficiency, resistance, service life, and dust holding capacity are the four core technical parameters measuring the performance of air filters, which directly determine the selection, operational effect, and economy of filters. Their standardized definitions are as follows:
(1) Filtration Efficiency
Filtration efficiency refers to the ratio of the amount of dust captured by the air filter to the amount of dust in the unfiltered air under stable operating conditions, and is the core indicator evaluating the filtering capacity of the filter. It should be particularly noted that for the same filter, the measured efficiency values vary significantly when different test methods are adopted (such as atmospheric dust counting method, artificial dust weighing method, DOP method, etc.). Therefore, the expression of filtration efficiency must clearly specify the corresponding test method, and efficiency data divorced from the test method has no reference significance.
(2) Resistance
Resistance refers to the obstructive effect of the air filter on the air flow under rated air volume, with the unit of Pa. The resistance of the filter changes gradually with service time: the resistance of a new filter under rated air volume is called “initial resistance”, which is a key parameter to consider when selecting the filter and directly affects the energy consumption of the air conditioning system; as the dust accumulation of the filter increases, the resistance gradually rises. When the resistance reaches a specified limit (i.e., “final resistance”), the filter is blocked, resulting in insufficient ventilation and decreased filtration efficiency, and needs to be replaced in a timely manner. The final resistance is usually clearly specified by the manufacturer according to the filter type and application scenario.
(3) Service Life
The service life of an air filter refers to the cumulative service time from the start of operation to the time when the resistance rises to the specified final resistance. Its length directly affects the operational cost and maintenance frequency of the system. The service life mainly depends on five factors: first, the initial resistance of the new filter (the lower the initial resistance, the longer the service life usually); second, the performance of the filter material (filter materials with high bulkiness and reasonable pore structure can accommodate more dust and have a longer service life); third, the effective filtration area (the larger the filtration area, the more sufficient the dust interception space, and the longer the service life); fourth, the air dust concentration (the higher the air dust concentration in the environment, the faster the filter accumulates dust, and the shorter the service life); fifth, the operating air volume (exceeding the rated air volume will accelerate dust penetration and shorten the service life).
(4) Dust Holding Capacity
Dust holding capacity refers to the cumulative amount of artificial test dust captured by the air filter when the resistance rises to the specified final resistance under specified test conditions, with the unit of g. Dust holding capacity is an important indicator evaluating the dust holding capacity of the filter. The larger the dust holding capacity, the stronger the anti-clogging ability of the filter, and the relatively longer the service life. Its value is usually determined through laboratory simulation tests, and it is an important reference basis for filter selection and maintenance cycle formulation.
IV. Conclusion
The reasonable selection and matching of air filters and the scientific understanding of core parameters are the basis for ensuring the stable, efficient, and economic operation of the air purification system. In engineering practice, it is necessary to combine factors such as designed cleanliness requirements, environmental dust concentration, and system air volume, follow the principle of “graded filtration and step-by-step protection”, reasonably match the efficiency levels of primary, medium, and high efficiency filters, and at the same time combine the core parameters of the filter such as resistance, service life, and dust holding capacity to achieve the optimal balance between filtration efficiency and operational cost, and provide a clean air environment that meets the requirements for various places.Core Technical Parameters