In the filtration mechanism of the filter material fiber, diffusion, gravity, inertia collision, static electricity and other functions all exist on the dust layer, but the main role is screening.
When the bag filter starts to operate, there is no dust on the new filter bag. After several minutes of operation, a very thin dust film is formed on the surface of the filter bag. Because the filter bag is made of fiber, before the dust layer is formed, the dust layer will gradually form the bridging phenomenon of dust between fibers under the effect of diffusion and other effects. The fiber diameter of the filter bag is generally 20-100 microns. The diameter of needled felt fiber is generally 10-20 microns. The distance between the fibers is generally 10-30 μ m, and the phenomenon of bridging is easy to appear. The dust layer of 0.3-0.5mm after the completion of bridge construction is often called dust film or primary dust layer. The dust accumulated again above the primary dust layer is called the secondary dust layer.
The dedusting efficiency of plain fabric filter cloth is 85% ~ 90%, which is relatively low. However, when the dust on the surface of the filter cloth adheres and accumulates, more than 99.5% of the dust removal efficiency can be obtained. Therefore, it is necessary to remove 0.3-0.5mm thick dust layer on the surface of filter cloth after cleaning the dust, and only remove the second dust layer. This problem is not only a technical problem but also a test of disposal experience for design and manufacturing plants. So we can say that the history of bag filter is the history of gradual improvement. Based on the influence of dust layer on the efficiency of dust removal, the efficiency of dust removal in the part where the dust layer peels off drops sharply; at the same time, the pressure loss is reduced, and the flue gas flows through this part intensively. Therefore, after a few seconds, a dust layer is formed on the surface of the filter cloth, and the dust removal efficiency rises again, that is, a certain amount of dust can be discharged in each cleaning cycle. On the other hand, if the filtering wind speed is properly designed, there will be a certain pressure loss (usually 1000-1500pa) on the filter layer on the surface of the filter cloth, that is, when the filter layer reaches a certain thickness in the required time, the time is inversely proportional to the filtering wind speed.
In addition, the needled felt of non-woven type is used as the filter cloth, generally 1.5-2.5mm thick. This layer is equivalent to the primary dust layer mentioned above, which exists in the inner layer of the filter cloth.
Smoke and dust permeate through the surface of the filter cloth, and at the same time, some methods are used to remove dust. After the above two functions are balanced, the inner layer (felt type) of the filter cloth forms a layer with a thickness of 0.5-0.7mm, which is formed by the intersection of dust and filter cloth fiber. This is the inner filter layer, which is equivalent to the primary filter layer of the plain fabric. However, the dust is accumulated on the surface of the filter cloth again and becomes the secondary dust layer.
In this way, the inner filter layer is interwoven with the fiber, and its property is greatly different from that of the secondary dust layer. Therefore, if only the secondary dust layer is removed, the inner filter layer is easy to be completely retained, so the dust removal efficiency will not decrease after the dust is cleared.
The adhesion of the dust on the filter cloth is very strong. When the filtering wind speed is 0.28m/s, the adhesion of the dust particles with a diameter of 10 microns on the filter material can reach 1000 times of the self weight of the particles, and the adhesion of the dust particles with a diameter of 5 microns on the filter material can reach 4200 times of the self weight of the particles. Therefore, the dust layer will continue to exist after the dust removal of the filter bag. The existence of the dust layer greatly strengthens the screening effect in the filtration process and improves the filtration efficiency. Compared with the gap of the filter material fiber, the screen hole formed by the dust layer is much smaller, and its screening effect is obvious.
The formation of dust layer is related to the filtration speed. The formation of dust layer is faster when the filtration speed is high, and slower when the filtration speed is low. If only considering the filtering effect of dust layer, low filtering speed is not favorable. When the dust layer continues to thicken, it must be removed in time with the method of dust removal, otherwise the resistance will be too high, or the dust layer will fall automatically, resulting in the "air leakage" phenomenon between the dust and reducing the effect of dust collection.
