Bag Filter

What is the bag filter working principle? | Updated for 2022

What is the bag filter working principle?

What is the principle of baghouse filter? When the dust-containing gas passes through the filter media, the dust is stopped on its surface and the clean air is discharged through the gaps of the filter media, which is the basic principle of air filtration technology for cloth bag dust collectors.

At present, the main types of air filtration are fiber filtration, membrane filtration (lamination or film) and dust layer filtration, all three of which can achieve the purpose of separating solid particles from aerosols, but their separation mechanisms are different.

The structure of cloth bag dust collector is mainly composed of: upper, middle and lower parts, dust cleaning system and dust discharge mechanism. The performance of the bag filter is good or bad is in addition to the correct choice of bag material, the dust cleaning system plays an important decisive role for the bag filter.

The technical characteristics of the bag filter are that the dust removal efficiency is up to 99%, the number of dust concentration in the gas at the outlet of the dust collector is within 10 mg/m2, there is this high resolution for fine dust of submicron particle size, the processing range is wide, it is used for dust removal from the flue gas of industrial furnaces, reducing the emission of air pollution, it is not sensitive to the characteristics of dust,

Bag filter is not affected by dust and resistance, it uses glass fiber, P84 and other high temperature resistant filter media and PTFE when it can be carried out above 200℃, simple structure, easy maintenance, and lower cost than electric dust collector at the same dust removal efficiency.

 

Bag filter is a combination of fiber filtration, or membrane filtration and dust layer filtration, and its dust removal mechanism is the result of the combined effect of sieving, inertial collision, hooking, diffusion, gravitational sedimentation and static electricity.

(1) Sieving effect: When the particle diameter of dust is larger than the space between the filter fibers or the pores between the dust on the filter media, the dust is blocked, called the sieving effect. For the commonly used fabric filter media, this utility is very small, because the gap between the fibers is often larger than the diameter of the dust particles. Only when a large amount of dust is deposited on the fabric, the sieving effect is fully displayed.

(2) collision effect: when the dusty airflow close to the filter fiber, airflow around the fiber, but more than 1μm larger particles due to inertia, deviating from the airflow streamline, still maintain the original direction, hit the fiber, dust collection down, called the collision effect.

(3) Hooking effect: When the dusty airflow is close to the filter fiber, the fine dust is still retained in the streamline, when the streamline is closer. If the radius of the dust particles is greater than the distance from the center of the dust to the edge of the fiber, the dust is captured, known as the hooking effect.

(4) Diffusion effect: when the dust particles are extremely small (0.2μm or less), the collision of gas molecules deviate from the streamline to do irregular motion (also known as thermal motion or Brownian motion), which increases the dust and fiber contact opportunities, so that the dust is captured. The smaller the dust particles, the more intense the motion, and thus the more opportunities for contact with fibers.

Collision, hooking and diffusion effects are increased as the diameter of the fiber decreases, and decreased as the porosity of the filter media increases, so the finer the filter media used, the denser the fiber, the higher the dust removal efficiency of the filter media.

(5) gravity sedimentation: large particles, the relative density of dust, under the action of gravity and settle down, which is the same mechanism of movement as 5 in the settling chamber dust.

(6) electrostatic effect: If the dust and the charge of the filter material is opposite, the dust is easy to adsorb on the filter material, thereby improving the efficiency of dust removal, but the adsorbed dust is difficult to be stripped down. Conversely, if the charge of both is the same, the dust is repelled by the filter media, the efficiency will be reduced, but the dust is easy to be stripped from the surface of the dust filter bag.

Differences in dust removal mechanism of different filter media

At present, the filter media used in the bag filter can be summarized into three main categories.

(1) textile filter media (including lint-free plain cloth and velvet).

(2) Felt or needle-punched felt filter media.

(3) Film (laminated) filter media.

The filtration mechanism of dust is different for different filter media.

The pores of textile filter media exist between warp and weft yarns (generally 300-700μm thread diameter, 100-200μm gap) and between fibers, and the latter accounts for 30%-50% of all pores. At the beginning of dust filtration, most of the airflow passes through the small holes between warp and weft yarns, and only a small portion of the dust passes through the gaps between the fibers (almost not for high twist yarns).

Coarse particles of dust are embedded in the small holes between the fibers, and the airflow continues to pass through the gaps between the fibers, at which time the filter media becomes an effective filter material for both coarse and fine dust particles, and forms a layer called the “initial adhesion layer ” or “second filter layer” of the dust layer, so the surface of the dust layer appears to force the filtering effect of dust collection process.

In addition, because the diameter of the dust in the airflow is usually smaller than that of the fibers, the effects of collision, hooking and diffusion increase significantly, and the efficiency of dust removal increases.

 

Felt or needle felt filter media, due to its own composition of thick porous filter bed, can give full play to the above effects, so the “second filter layer” of the filter role is not important. Focus on thermal power plant technology alliance!

Film filter media, its surface has a synthetic, internal grid structure, 50μm thick, per square centimeter contains 1.4 billion microporous special film, apparently its filtration effect is mainly sieve filter effect (also known as surface filtration).

Reasonable dust cleaning cycle

In actual operation, the bag filter requires periodic dust cleaning of the filter media. With the increasing amount of captured dust, the filter media shows a forced filtration effect on coarse and fine dust particles in the capture process, and as the initial dust adhesion layer thickens, its filtration efficiency increases, and the resistance of the bag filter gradually increases, while the air volume through the filter bag gradually decreases and the energy consumption of the system increases.

At this time, the filter bag needs to be cleaned. Both timely and evenly remove the accumulated ash on the filter bag, but also to avoid excessive ash cleaning, so that it can retain the “initial adhesion layer” to ensure stable work and high efficiency, which is more important for the larger pores or easy to clear the ash filter media. When the dust collector works 60min to clear the ash once, the bag resistance and ventilation can be immediately restored to the original value.

With the shortening of the dust cleaning cycle, the air volume and pressure of the bag filter will be more stable; however, if the dust cleaning is too frequent, it will print up the secondary dust and shorten the service life of the filter bag. Therefore, it is crucial to choose a reasonable dust cleaning cycle for a specific baghouse under specific operating conditions.

Hengke is a China trusted manufacture and supplier for dust filter bag in various countries. Hengke has designed and produced all types, sizes, and configurations of dust filter bag for more than 20 years. Being in the business for two decades, the company offers a wide range of products including dust filter bag, needle punched felts, dust collector bag, filter fabric, filter bag cage, Pulse valve etc. Till now, our products have been successfully exported to over 35 countries.