The principle of dust collection by bag filter mainly depends on five functions.
Working Principle of Bag Dust Collector
Bag filter is one of the dust collectors. It is mainly used in some large-scale production rooms, especially those factories with a lot of dust. In our daily life, we seldom see it, but it has an intuitive and important role in the quality of ambient air, and the filter material of the general bag filter is synthetic fibers, natural fibers or glass fibers woven cloth or felt.
The working principle of bag filter is: depending on woven or felt filter cloth as filter material, when dust-containing gas passes through the filter bag, the dust is blocked on the surface of the filter bag, while the dry air is discharged through the gap between the filter bag fibers, so as to achieve the purpose of separating dust-containing gas. Its working mechanism is that dust is trapped by screening, inertia, adhesion, diffusion and static electricity generated by filter cloth.
When dust-containing gas passes through the filter cloth, the gap between the filter cloth fibers or between the dust adsorbed on the surface of the filter cloth separates the dust larger than the diameter of the gap, which is called screening. For the new filter cloth, because of the large gap between the fibers, the effect is not obvious, and the dust removal efficiency is also low.
Only after a certain period of time, a certain thickness of dust layer was established on the surface of the filter bag, and the screening effect was significant. After dust removal, due to a certain amount of dust remaining on the surface and inside of the filter bag, it can still maintain a good dust removal efficiency.
For needle-punched felt or pile filter cloth, because the felt or pile filter cloth itself constitutes a thick porous filter layer, it can give full play to the screening function, and not rely entirely on the dust layer to maintain a higher dust removal efficiency.
2. Inertial action
When dust-containing gas passes through the filter cloth fiber, dust larger than 1 micron is trapped because inertia still keeps straight line motion and impacts on the fiber. The larger the diameter of dust particles, the greater the inertia effect. The higher the filtration gas velocity is, the greater the inertia effect is, but the gas velocity is too high, and the gas flow through the filter cloth also increases. The air flow will break through the weak part of the filter cloth, resulting in lower dust removal efficiency. The higher the gas velocity is, the more serious the penetration is.
When the dust particles are below 0.2 um, Brownian motion, such as thermal motion of gas molecules, occurs due to the extremely small size of the dust particles, which increases the contact opportunities indicated by the dust and filter cloth and makes the dust trapped. This diffusion effect is contrary to inertia effect, and increases with the decrease of filtration gas velocity and dust particle size. Take glass fiber as an example, the finer the fiber, the higher the dust removal efficiency (see table). But the pressure loss of fine fibers is larger than that of coarse fibers, and the thinner the fibers are, the worse the corrosion resistance is.
Dust removal technology of dust collector
When the dust-containing gas is close to the filter cloth, fine dust still moves with the airflow. If the radius of the dust is larger than the distance between the dust center and the edge of the filter cloth, the dust-dust will be trapped by the filter cloth adhesion. The smaller the void of the filter cloth is, the more remarkable the adhesion is.
5. Electrostatic interaction
Dust particles collide with each other to generate static electricity. If the filter cloth is an insulator, it will charge the filter cloth. When the charges of dust and filter cloth are opposite, the dust is adsorbed on the filter cloth, which improves the efficiency of dust removal and makes it difficult to clean up the dust.
On the contrary, if the charges are the same, the repulsion force will be produced, and the dust can not be adsorbed on the filter cloth, which will reduce the dust removal efficiency. Therefore, the electrostatic effect can improve or hinder the dust removal efficiency of the filter cloth. In order to ensure dust removal efficiency, filter cloth must be selected according to the charge property of dust. Generally, the electrostatic effect is only shown when the particle size of dust is less than 1 micron and the filtration gas velocity is very low. Under the condition of applied electric field, the electrostatic effect can be strengthened and the dust removal efficiency can be improved.