The working principle of bag dust removal
The structure of a bag filter consists of: dust collector ash hopper, inlet and outlet air ducts, filter chamber (middle and lower boxes), cleaning room, filter bag and (bag cage bone), manual air inlet valve, pneumatic butterfly valve, pulse cleaning mechanism, etc.
Dust removal working principle: It is to introduce dust containing gas into the middle box through the air inlet, enter the bag from outside, block the dust on the surface outside the bag, purify the air into the bag, and then enter the upper box through the upper part of the bag before being discharged through the exhaust pipe.
One Dust removal mechanism
When dusty gas passes through the filter material, the dust is trapped on its surface, and clean air is discharged through the gaps of the filter material. Air filtration technology is the basic principle of bag filters. At present, the main methods used for air filtration are fiber filtration, membrane filtration (film or membrane), and dust layer filtration. All three methods can achieve the goal of separating solid particles from aerosols, but their separation mechanisms are different. A bag filter is a combination of fiber filtration, membrane filtration, and dust layer filtration. The dust removal mechanism is divided into:
1. Collision effect: When the dusty airflow approaches the filter fiber, the airflow bypasses the fiber, but larger particles larger than 1 μ m deviate from the airflow streamline due to inertia and still maintain their original direction, hitting the fiber and capturing the dust, which is called collision effect.
2. Screening effect: When the particle diameter of dust is larger than the gaps between filter fibers or the pores between dust particles on the filter material, the dust is retained, which is called screening effect. For commonly used fabric filter media, this effect is minimal because the gaps between fibers are often larger than the diameter of dust particles. Only when a large amount of dust deposits on the fabric, the filtering effect is fully displayed.
3. The hook attachment effect should be such that when the dusty airflow is close to the filter fiber, fine dust still remains in the streamline, and the streamline is relatively tight. If the radius of dust particles is greater than the distance from the center of the dust to the edge of the fiber, the dust is captured, which is called the hook effect.
4. Diffusion effect: When dust particles are extremely small (below 0.2 μ m), they deviate from the streamline and make irregular movements (also known as thermal motion or Brownian motion) under the collision of gas molecules, which increases the chance of contact between dust and fibers and captures dust. The smaller the dust particles, the more intense their movement, and thus the more opportunities they have to come into contact with fibers. The collision, hook attachment, and diffusion effects all increase with the decrease of fiber diameter and decrease with the increase of porosity of the filter material. Therefore, the finer and denser the fibers used in the filter material, the higher the dust removal efficiency of the filter material.
5. If the electrostatic charge of the dust is opposite to that of the filter material, the dust is easily adsorbed on the filter material, thereby improving efficiency, but the adsorbed dust is difficult to peel off. On the contrary, if the charges of the two are the same, the dust will be repelled by the filter material, resulting in a decrease in efficiency, but the dust is easily peeled off from the surface of the filter bag.
6. Dust with large particles and high relative density settles under the action of gravity, which is similar to the movement mechanism of dust in the settling chamber.
II Differences in dust removal mechanisms of different filter media
The filter materials used in bag filters can be classified into three categories:
1. Textile filter media (including lint free plain cloth and flannel cloth); The pores of textile filter media exist between warp and weft yarns (generally with a thread diameter of 300-700 μ m and a gap of 100-200 μ m) and between fibers, with the latter accounting for 30% to 50% of all pores. When starting to filter dust, most of the airflow passes through the small holes between the warp and weft yarns, and only a small part of the dust passes through the gaps between the fibers (almost not through 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 this point, the filter material becomes the filtering material for both coarse and fine dust particles, and a dust layer called the "initial adhesion layer" or "filtering layer" is formed. Therefore, the surface of the dust layer undergoes a process of capturing dust through forced screening effect. In addition, due to the fact that the diameter of dust in the airflow is usually smaller than that of fibers, the effects of collision, hooking, and diffusion are significantly increased, resulting in improved dust removal efficiency.
2. Felt or needle punched felt filter material; Felt or needle punched felt filter material, as it constitutes a thick porous filter bed, can fully exert the above-mentioned effects, so the filtering effect of this "filter layer" is not significant.
3. Thin film (laminated) filter material. Coated filter material, with a layer of artificially synthesized, grid like structure on its surface, a 50 μ m thick film containing 1.4 billion micropores per square centimeter, obviously its filtering effect is mainly sieve filtration (also known as surface filtration)
