the mechanism of filtration and dust removal

Update time : 2019-03-28 16:49:28

According to the different mechanical characteristics of the movement of dust with different particle sizes in fluids, the mechanism of filtration and dust removal involves the following aspects:
1.1.1 Screening
The mesh of filter material is usually 5-50 micron. When the particle size of dust is larger than mesh or pore diameter or when the dust deposits in the inter space between the particles, the dust will be blocked.
For the new fabric filter material, because the pore size between the fibers is much larger than the particle size of the dust, the screening effect is very small, but when a large amount of dust deposits on the surface of the filter material to form a dust layer, the screening effect is significantly enhanced.
1.1.2 Inertial Collision
Generally, the dust with larger particle size is mainly trapped by inertial collision. When the dust-laden airflow approaches the filter material, the airflow will bypass the fiber, and the larger particles (larger than 1 micron) will deviate from the airflow line due to inertia, continue to move along the original direction of motion and impact on the fiber and be trapped.
All the large dust particles in the critical line of dust trajectory can reach the surface of the fiber and be trapped.
The inertial collision effect increases with the increase of particle size and airflow velocity.
Therefore, the inertial collision can be improved by increasing the flow rate through the filter material.
1.1.3 Interception
When the dust-laden airflow is close to the filter material fiber, the finer dust particles flow with the airflow. If the radius of the dust particles is larger than the distance between the center of the dust particles and the edge of the fiber, the dust particles will be intercepted because of the contact with the fiber.
1.1.4 Diffusion
For dust particles less than 1 micron, especially sub-micron particles less than 0.2 micron, they break away from streamline under the impact of gas molecules and make Brownian motion like gas molecules. If they are in contact with fibers during the movement, they can be separated from the airflow. This effect is called diffusion, which increases with the decrease of flow rate and the diameter of fibers and dust.
1.1.5 Electrostatic Action
Many fiber-woven filter materials, when air flows through, will produce static electricity due to friction, while dust will be charged due to friction and other reasons in the transport process, which will form a potential difference between the filter material and dust particles. When dust tends to filter material with air flow, due to the Coulomb force, dust and filter material fibers collide and enhance the adsorption of dust by the filter material. Force is captured to improve the efficiency of capture.
1.1.6 GRAVITY SETTLEMENT
When the slowly moving dust-laden airflow enters the dust collector, the dust particles with large particle size and density may naturally settle due to gravity (see follow Table1-1).

Table1-1
Particle size range of various trapping mechanisms

Serious No.	Mechanism	particle size range	Effect of Wind Speed Increase on Mechanism Efficiency
1	Interception	>1μm	reduce
2	Inertial collision	>1μm	improve
3	diffusion	<0.01～0.5μm	reduce
4	Electrostatic Action	<0.01～5μm	reduce
5	Screening	> Micro pore size of filter layer	reduce

Generally speaking, various dust removal mechanisms are not effective at the same time, but one or several combined functions.
Moreover, with the change of the void, airflow velocity, dust particle size and other reasons, the effects of various mechanisms on the filtration performance of different filters are also different.
In fact, when the new filter material starts to filter dust, the efficiency of dust removal is very low. After using for a period of time, the coarse dust will form a layer of dust on the surface of the filter cloth.
The effects of various mechanisms on the filtration performance of different filters are also different due to the changes of the voids, airflow velocity, dust particle size and other reasons. In fact, when the new filter material starts to filter dust, the efficiency of dust removal is very low. After using for a period of time, the coarse dust will form a layer of dust on the surface of the filter cloth. Because of the dust filtering effect of the initial layer of dust and the dust layer gradually accumulated on it, the filtering efficiency of the filter material is continuously improved, but the resistance is also correspondingly enhanced.
When cleaning ash, the primary layer should not be destroyed, otherwise the efficiency will decrease. The structure of dust initial layer plays a very important role in the efficiency, resistance and ash removal effect of bag filter.

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