The bag filter is a dry dust filter device that uses the filtering action of the dust filter bag to filter the dust containing gas. It is suitable for capturing fine, dry, non-fibrous dust. The dust filtration process of a bag filter is complex. Generally speaking, the settling of dust particles on the dust filter bag, i.e. separation and filtration, is not the result of only one settling and filtration mechanism, but the result of the combined action of multiple settling and separation and filtration mechanisms.
Principle of filtration and dust removal
According to the different mechanical characteristics of the movement of different particle sizes of dust in the fluid, the filtration and dust removal mechanism involves the following aspects.
1. Screening and filtering effect
Filter media mesh is generally 5 to 50μm, when the dust particle size is larger than the mesh or pore diameter or dust deposited in the interparticle space between the filter media, the dust is blocked to stay. For the new fabric filter media, due to the gap between the fibers that the pore diameter is much larger than the dust particle size, so the screening effect is very small, but when the surface of the filter media deposited a large amount of dust to form a dust layer, the screening effect is significantly enhanced.
2, inertia collision effect
General particle size of the larger dust mainly relies on inertial collision effect trapping. When the dust-containing airflow is close to the fiber of the filter media, the airflow will bypass the fiber, where the larger particles (greater than 1μm ) due to inertia, deviating from the airflow streamline, continue to follow the original direction of motion, hit the fiber and arrested. All the large dust particles within the critical line of the dust trajectory can reach the fiber surface and be arrested. This inertial collision effect is enhanced with the increase of dust particle size and airflow velocity. Therefore, to increase the airflow velocity through the filter media, the inertial collision effect can be improved.
3、Intercepting effect
When the dusty airflow is close to the filter fiber, the finer dust particles with the airflow around the flow, if the radius of the dust particles is greater than the distance from the center of the dust particles to the edge of the fiber, the dust particles are intercepted because of contact with the fiber.
4、Diffusion effect
For less than 1μm of dust particles, especially less than 0.2μm submicron particles, the impact of gas molecules from the streamline, like gas molecules for Brownian motion, if in the process of movement and fiber contact, can be separated from the airflow. This effect is known as diffusion, which is enhanced with the reduction of flow velocity, fiber and dust diameter reduction.
5、Electrostatic effect
Many fiber woven filter media, when the airflow through, due to friction will produce electrostatic phenomenon, while the dust in the process of transport will also be due to friction and other reasons and charged, so that a potential difference between the filter material and dust particles, when the dust with the airflow tends to the filter material, due to the Coulomb force prompted the dust and filter fiber collision and enhance the adsorption of the filter material on the dust and arrested to improve the capture efficiency.
6、Gravity sedimentation
When the slow-moving dusty airflow enters the dust collector, the dust particles with large particle size and density may settle down naturally due to gravity.
The various dust removal mechanisms of a baghouse are not effective at the same time, but one or several in combination. During operation, the effect of each mechanism on the filtration performance of different dust filter bags varies with the change of filter media voids, air flow rate, dust particle size and other reasons.
The performance of filter dust collector
1、Filtration speed.
v=Q/60×A
Where v - filtration speed (apparent filtration gas velocity), m/min
Q-air volume of filter dust collector, m3/hour
A-Filtering area of filter media, ㎡
It is generally believed that the true velocity vp of gas passing through the filter layer is
vp=v/εp
where vp - the real velocity of filtration through the filter layer, m/min
εp - the average void ratio of the dust layer, generally 0.8 to 0.95.
In actual operation, the filtration speed is determined by the type of filter media, the size of dust particle size, physical and chemical properties and its dust cleaning method. The large filtration speed will increase the pressure difference between the two sides of the filter media and squeeze the fine dust that has been attached to the filter media, so that the filtration efficiency will be reduced to the specified emission value or wear the single fiber of the filter media, especially the glass fiber filter media to accelerate the damage. If the filtration speed is small, the volume of the dust collector will be increased, thus increasing the investment.
2、Dust removal efficiency
That is, the ratio of different particles of dust being filtered down.
3. Filtration resistance.
Pt = Pf + Pc + Pm.
Pf = K1*Vf ---- is the pressure drop when the flue gas passes through the clean filter bag.
Pc = K2*Ci*Vf2*t ---- is the pressure drop of the flue gas through the dust layer or filter cake.
Pm ---- represents the mechanical resistance loss of the dust collector equipment
K2 is the resistance coefficient of the dust layer or cake and is related to the nature and structure of the dust. For example, in an electric bag filter, after the dust has passed through the electrostatic field, the nature of the dust changes and the charged dust repels each other, making the structure of the dust layer formed on the filter bag more sparse and the operating resistance will be smaller than that of a pure bag filter.
Ci is the dust concentration at the dust collector inlet. As the dust concentration increases, the operating resistance will definitely rise. So, a baghouse in a dry desulfurization system in a power plant will have a higher operating resistance than one without dry or semi-dry desulfurization; likewise, an electric baghouse, due to the retained electrostatic field effect, reduces the concentration of dust entering the filter bag, which can make the dust collector operating resistance much lower.
Vf is still the filtration speed here, 2 is the squared relationship (sorry, writing using a blog, I can't place 2 in the squared position); t is the time for which filtration takes place. The significance of this is to illustrate that: the change in operating resistance or pressure drop rising through the dust layer increases with operating time, in a squared exponential increment to the designed filtration speed. In addition, it is more emphatic: among these three resistance factors, the only factor that can change the operating resistance of the dust collector with the effect of dust cleaning is the dust layer factor.
4、Cleaning method
Ash cleaning methods mainly include vibration and reverse airflow joint, pulse spraying, reverse blowing air.
Anhui Yuanchen Environmental Technology Co., Ltd (hereinafter referred to as "Yuanchen Technology") is a high-tech enterprise integrating R&D, production and sales of dust removal bags and denitrification catalysts. Over the past sixteen years, Yuanchen Technology has been focusing on environmental protection field, and now has 4 international PCTs, 33 authorized invention patents, 77 invention patents in process, among which many products such as "dust removal and denitrification integrated technology products" have been awarded the Scientific Progress Award of Anhui Province. Our dust removal bags (mainly PPS, PTFE, P84 and composite series filter materials) and SCR denitrification catalysts have been widely used in cement, steel, glass kilns, waste incineration power generation, biomass power generation, non-ferrous metal smelting and other industries. In the future, Yuanchen Technology will be guided by "becoming the guardian of global ecological environment", always rooted in environmental protection, and insist on the great cause of guarding the blue sky and white clouds. Leveraging on the national ecological civilization construction pattern, we will continue to deepen technology, optimize management, strengthen brand, refine industry and solidify advantages, and create synergistic value for industry through comprehensive and integrated governance and services.
This article was first published on http://www.shychb.com/, if you need to reproduce, please indicate the source.
