Medium-efficiency bag filters are the most widely used structural form of medium-efficiency filters in ventilation and air-conditioning systems. Thanks to the large face area and low resistance provided by the bag design, they serve as the second line of defense in the filtration system, protecting downstream high-efficiency filters and system equipment.
Filtration Efficiency Grades (F5–F9)
Medium-efficiency filters are designated by F, and primarily provide pre-filtration for particles above 0.5 μm. The efficiency of each grade (colorimetric method) is as follows:
| Grade | Filtration Efficiency (@0.5 μm) | Remarks |
| F5 | 45% | Basic medium efficiency |
| F6 | 65% | Common specification |
| F7 | 85% | Common specification |
| F8 | 90% | Common specification |
| F9 | 95% | High-end medium efficiency |
It should be noted that some references specify the capture particle size of medium-efficiency filters as 1–5 μm, with efficiencies ranging from 60–95%. The definition given here—”above 0.5 μm”—is stricter and is consistent with the test basis of the F-series colorimetric efficiency. For actual selection, the design specifications of the specific project and the product test reports should prevail.
Structural Features and Types
Medium-efficiency bag filters typically consist of a metal frame (aluminum alloy, galvanized steel, or powder-coated cold-rolled steel) and bag-shaped filter media. The number of bags can be configured from 3 to 8 depending on airflow requirements. The filter media are usually made of non-woven fabric, synthetic fiber, or glass fiber, and some products contain electrostatic fibers to enhance the capture of submicron particles.
The key advantages of the bag structure are:
- Large filtration area: The unfolded effective filtration area of the bags is much larger than that of a plate filter of the same dimensions, resulting in higher dust-holding capacity;
- Uniform airflow distribution: Unique hot-melt or ultrasonic welding processes ensure that airflow is evenly distributed and fully fills each bag, preventing bag-to-bag compression or leakage;
- Low resistance: The design initial resistance typically does not exceed 80 Pa, helping to reduce system fan energy consumption.
In terms of structural form, medium-efficiency filters also include plate type (FB), header-frame type (FS), and combination type (FV) in addition to the bag type. Bag filters perform better in dust-holding capacity and low pressure loss, while plate filters are more compact and suitable for space-constrained applications.
Application Areas
Medium-efficiency bag filters are the most common type of air filter product in industrial and civil ventilation systems. Main applications include:
1. Intermediate filtration in air-conditioning and ventilation systems
As the second filtration stage in central air-conditioning and centralized air supply systems, they follow the pre-filter and further remove medium-sized particulate matter from the air. In locations with low air purification requirements, air treated by medium-efficiency filters can be supplied directly to the room.
2. Purification air conditioning in pharmaceutical, electronics, food, and other industries
These industries have explicit requirements for air cleanliness. Medium-efficiency bag filters serve as the intermediate filtration stage in purification air-handling units, providing protection for terminal high-efficiency filters. If pre-filtration is missing or ineffective at the front end, the bag filters will be directly exposed to high-dust environments and their service life will be significantly shortened. Therefore, proper systems should ensure that pre-filters are in normal operation.
3. Front-end protection for high-efficiency filters
Medium-efficiency bag filters intercept most medium-sized particles, thereby reducing the load on downstream HEPA/ULPA filters and extending their replacement cycles and service life. This “protecting the downstream stage” function is one of the core design rationales for the medium-efficiency stage.
4. Combined use of bag and plate types
In some air-conditioning systems, the medium-efficiency stage adopts a combination of bag and plate configurations, balancing high dust-holding capacity with compact structure to suit different unit space layouts.
Selection and Maintenance Guidelines
- Not suitable for standalone use: Medium-efficiency filters should be combined with pre-filters to prevent the bag media from being directly exposed to high concentrations of dust and rapidly clogging.
- Replacement cycle: Generally recommended to replace every 3–6 months, or when the resistance reaches twice the initial resistance (approximately 400 Pa). The actual cycle should be adjusted according to on-site dust concentration and operating time; in some clean environments, it can be extended to annual replacement.
- Installation orientation: Bag filters should be installed vertically, ensuring that the length direction of the bag opening is perpendicular to the ground, to avoid bag folding or airflow short-circuiting.