How Bag-Type Dust Filters Work

Core Filtration Mechanisms

The filter media in a bag-type dust filter, such as nonwoven fabric, glass fiber, or synthetic fiber, has a porous structure. Particle capture relies mainly on the following four mechanisms, with different mechanisms dominating at different particle sizes.

Interception (for particles at least 1 μm)

As air flows through the media, larger particles—such as dust and fibers above 5 μm—are too large to pass through its pores and are stopped directly on the surface, much as a sieve captures coarse particles. This is the principal mechanism used by coarse G1–G4 bag filters.

Inertial Impaction (for particles from 0.5 to 5 μm)

Medium-size particles such as smoke particles and pollen have relatively high mass. When the airflow bends around the media fibers, their inertia prevents them from following the change in direction. They strike the fiber surface and are retained. Medium-efficiency F5–F9 bag filters rely mainly on this mechanism to capture particles in this size range.

Diffusion (for particles no larger than 0.5 μm)

Very small particles, including fine dust from 0.1 to 0.5 μm, undergo Brownian motion—the random movement caused by molecular thermal motion. They therefore collide randomly with the media fibers and are captured. The improved efficiency of higher grades of medium-efficiency filters, such as F8 and F9, against these particles results mainly from stronger diffusion effects. The finer and denser the fibers, the more pronounced the effect.

Electrostatic Capture (for particles no larger than 0.1 μm; available in some filters)

The media in some bag filters receives an electrostatic treatment, as in electret nonwoven fabric. Charges on its surface attract ultrafine particles through electrostatic forces, including smoke particles below 0.1 μm, further improving filtration efficiency for submicron particles.

How the Bag Structure Improves Performance

Compared with a panel filter, the bag design—which forms the media into multiple pleated bags—does not change the basic filtration mechanisms. Instead, it improves overall performance by increasing the effective filtration area.

Lower air velocity: At the same airflow, the unfolded media area of a bag filter is three to five times that of a panel filter. Velocity through the media is therefore lower, generally no more than 1.5 m/s. Particles have more time to contact the fibers, increasing the probability of interception and adsorption.

Lower localized resistance: Slower airflow reduces the pressure loss across the media and avoids the increased penetration that can occur when high-velocity air forces particles through the pores.

Higher dust-holding capacity: The three-dimensional space within the bags can accommodate more particulate matter. This prevents the media surface from becoming blocked quickly and extends filter life. With equivalent media, a bag filter can hold two to three times as much dust as a panel filter.

Airflow Path and Filtration Process

When dust-laden air enters a bag filter, it follows this path:

• Air flows from outside the filter into the bags, or from inside to outside depending on installation direction. Once expanded, each bag forms a hollow chamber that distributes air evenly across the media and prevents localized overloading.

• The four mechanisms described above capture particles in the media. Coarse particles of at least 5 μm deposit mainly on the media surface, while fine particles no larger than 1 μm are more often captured within the fiber matrix.

• As operating time increases, dust accumulates on the surface and within the media to form a dust cake. This layer itself becomes an additional filtration medium and further increases efficiency for fine particles, although it also raises airflow resistance.

Summary

The operating principle of a bag-type dust filter combines physical capture with structural optimization. Its porous media uses multiple mechanisms to capture particles of different sizes, while the bag design expands the filtration area to provide high dust-holding capacity, low resistance, and long service life. Its central objective is to maintain filtration efficiency while minimizing reductions in ventilation airflow and increases in energy use. Bag filters are therefore widely used in applications that handle large volumes of air with high dust concentrations, including industrial workshops and central HVAC systems.