Classification of Air Filters in China
General Filter Classification
In 1992 and 1993, China issued the national standards for air filters: “High-Efficiency Air Filters” (GB 13554-1992) and “Air Filters” (GB/T 14295-1993). The filters are categorized into five types: coarse filters, medium-efficiency filters, high-medium-efficiency filters, sub-high-efficiency filters, and high-efficiency filters. Among these, high-efficiency filters are further subdivided into four types. Coarse filters, medium-efficiency filters, and high-medium-efficiency filters are collectively referred to as general air filters.
Table 3-1: Classification of General Air Filters by Performance
| Category | Dust Removal Efficiency η at Rated Airflow (%) | Initial Resistance at Rated Airflow (Pa) |
|---|---|---|
| Coarse Air Filter | Particle size ≥ 5μm, 80 > η ≥ 20 | ≤ 50 |
| Medium-Efficiency Filter | Particle size ≥ 1μm, 70 > η ≥ 20 | ≤ 80 |
| High-Medium Efficiency Filter | Particle size ≥ 1μm, 99 > η ≥ 70 | ≤ 100 |
| Sub-High Efficiency Filter | Particle size ≥ 0.5μm, 99.9 > η ≥ 95 | ≤ 120 |
In general air filters, coarse filters and medium-efficiency filters are widely recognized, while high-medium-efficiency filters and sub-high-efficiency filters are newer classifications in China.
High-Medium Efficiency Filters (1) Bag-Type High-Medium Efficiency Filters
Made from non-woven fabric, the nominal size is 610mm×610mm (24in×24in), with an actual frame size of 592mm×592mm. In air handling units, if the filtering section is made up of 610mm×610mm units, sometimes to cover the entire filter area, the edges are equipped with bag-type filters of 305mm×610mm.
Performance Parameters of Bag-Type High-Medium Efficiency Filters
| Model | Size (mm) | Bag Length (mm) | Rated Airflow (m³/h) | η (%) (≥ 1μm) | Initial Resistance (Pa) |
|---|---|---|---|---|---|
| KG-D-K-2-Ⅲ | 592×592 | 533 | 2400 | ≥80 | ≤90 |
| KG-D-K-3-Ⅲ | 592×592 | 737 | 3400 | ≥80 | ≤90 |
| KG-D-K-4-Ⅲ | 592×592 | 915 | 4100 | ≥80 | ≤90 |
In combined air purification air-conditioning units, bag-type filters, though occupying more space, are widely used due to their high airflow, large dust holding capacity, low resistance, and long service life.
(2) Tube (Cylinder)-Type High-Medium Efficiency Filters
The main feature: space-saving. When there is no installation space, this filter can be directly installed in a straight duct to save space. However, in actual use, it has been found that the filter tubes may detach, affecting the filtering efficiency.
Tube-Type High-Medium Efficiency Filter
Sub-High Efficiency Filters
Sub-high-efficiency filters are divided into two types: folded paper type and tube type. The former has the same appearance and processing technology as the high-efficiency filter with partitions. The latter is the only type of sub-high-efficiency filter that does not require glue, thus preventing secondary pollution, making it ideal for use in medical buildings. When replacing, only the filter tubes need to be replaced (although this requires more work), which saves construction costs. It can be made into any shape and is suitable for use in air purification equipment and civilian purification units.
The purification system with three stages of sub-high-efficiency filtering can be equipped with fans that have a speed of ≤ 1450r/min and spring vibration dampers, resulting in low noise levels. The author has used sub-high-efficiency filters at the air supply end of a 10,000-class biological clean room. The detection results showed that, except for a longer self-purification time, other parameters such as cleanliness, sedimentary bacteria, and airborne bacteria met the required standards (static). Some technical personnel, however, hold a negative view and do not agree with the low-resistance advantages of sub-high-efficiency filters.
Engineers with practical experience understand that if the final resistance of a three-stage filter is considered to be twice the initial resistance, and under the same other conditions, the resistance of a sub-high-efficiency system will be 160-360Pa lower than that of a high-efficiency system. If the final resistance is greater than twice the initial resistance, this resistance difference will be even greater. When selecting fans, this difference could cause the fan speed to increase from 1450r/min to 2900r/min, resulting in a noticeable difference in noise levels.
Therefore, for biological clean rooms with cleanliness levels of ≤ 100,000, the use of low-resistance sub-high-efficiency filters at the air supply end is more beneficial than detrimental. The YCG-type low-resistance high-efficiency filter’s filter tubes are bonded using a hot-melt method, so the quality of processing is crucial. If the quality is poor, leakage may occur during use. Thus, it is important to select reliable products.
High-Efficiency Filter Classification
High-efficiency filters are classified by performance as shown in Table 3-3. The efficiency of Class A, B, and C high-efficiency filters is measured using the sodium flame method, while Class D high-efficiency filters are measured using the counting method.
Table 3-3: High-Efficiency Filter Classification by Performance
| Category | Efficiency Test Method |
|---|---|
| Class A | Sodium flame method (≥99.9%) |
| Class B | Sodium flame method (≥99.99%) |
| Class C | Sodium flame method (≥99.999%) |
| Class D | Counting method (≥99.999%) |
High-efficiency filters can also be classified by the materials of their separators, into those with separators and those without separators.
(1) High-Efficiency Filter with Separators
The filter paper is separated by corrugated separators to ensure the flow of air between the pleated filter papers. The filter’s edges are fixed to the outer frame with adhesives. The filter material is glass fiber paper, and the separator materials are aluminum foil and white cardboard. The outer frame materials include galvanized steel plate, aluminum plate, and plywood. The strength of aluminum plate frames is lower than that of galvanized steel plates, and when installed using compression methods, uneven force can cause deformation and affect the sealing effect. Wooden frames, such as plywood, should not be used in biological clean rooms. Common adhesives include polyurethane, and special cases may use silicone, PVC, etc. Previously, epoxy resin and polyamide were used.
(2) High-Efficiency Filter without Separators
The “separatorless” design does not mean there are no separators. Otherwise, airflow would be obstructed. The filter material is also glass fiber paper, and the separators are made from hot-melt adhesive lines, wires, or glass fiber strips.
Sodium Flame Method and Counting Scanning Method for Efficiency Testing
Sodium Flame Method for Efficiency Testing
Origin: United Kingdom (currently the standard method in China)
Test dust source: Monodisperse sodium chloride mist (average diameter 0.5μm)
Instrument: Photometer
Principle: The salt mist gas causes a hydrogen flame to turn blue, and the concentration is determined by the change in brightness.
Counting Scanning Method for Efficiency Testing
International mainstream method (common in Europe, similar in the U.S.)
Instrument: High-flow laser particle counter / Condensation nucleus counter
Features:
✓ Can scan the entire air outlet surface (detect local efficiency)
✓ Dust source types:
• Laskin nozzle multi-dispersed solution
• Specific solid dusts (customizable for users)
High-Efficiency Filter Sizes
484 Series
Classic size: 484mm×484mm×220mm (mainstream in China)
Installation modulus: 500mm×500mm (space for suspension rods must be reserved)
Model example: GB-01-G (G = glass fiber, 01 = 484mm frame size)
Rated airflow: 1000m³/h
Table: Main Size Series Comparison
| Series | Outer Frame Size (mm) | Thickness (mm) | Application Scenarios |
|---|---|---|---|
| 484 | 484×484 | 220 | Industrial clean rooms |
| 610 | 610×610 | 150 | International standard equipment |
| 320 | 320×320 | 220 | Special layout spaces |
The 650mm×650mm size, considering the position of the installation suspension rods, the filter cross-sectional size is designed as 630mm×630mm. The derived size for 630mm is 315mm, 945mm, 1260mm.
The early product codes for this filter were GS-03, GB-03, where “03” represents a 630mm frame length, with “G,” “S,” and “B” corresponding to the 484 series. This filter has a rated airflow of 1500m³/h.
320 Series
The 320mm×320mm×260mm size coexists with the 484mm and 630mm series. The early product codes for this series were GS-02 and GB-02, where “02” indicates a 320mm frame size. “G,” “S,” and “B” have the same meanings as before. The rated airflow for this filter is 500m³/h. Currently, the filter size has been adjusted to 320mm×320mm with reduced thickness, so the rated airflow is less than 500m³/h, and care should be taken when selecting.
610 Series
High-efficiency filter frames in developed countries have always been 610mm (24in) in width. In the 1980s, imported filters became popular in China. In the 1990s, the 610mm size became prevalent domestically. The derived sizes for 610mm are 203mm, 305mm, 762mm, 915mm, 1219mm, 1524mm, and 1829mm (8in, 12in, 30in, 36in, 48in, 60in, 72in).
In engineering design, designers can choose any of these high-efficiency filter series based on their preferences. However, mixing different filter series (except for the 320 series) in the same project should be avoided to prevent issues with construction, future maintenance, and spare parts procurement. The 320 series filters are generally used in smaller cleanrooms or in large cleanrooms with relatively wide layouts (low ceiling height).
