SDN Gas Treatment Industrial Baghouse Dust Collector Dust Collection Equipment

SDN Gas treatment
Baghouse Dust Collector

The working principle of bag type dust removal is the process of using filter cloth processed with cotton, wool, or artificial fibers to capture dust particles.

Bag filters have the following advantages:

① The dust removal efficiency is high, especially for fine powder, with a high replenishment efficiency of over 99%;

② Strong adaptability, it can handle different types of particulate pollutants (including high specific resistance dust that is difficult to handle by electrostatic precipitators). According to the processing capacity, it can be designed as a small bag filter or a large bag room;

③ When the operation flexibility is high and the dust concentration of the inlet gas changes significantly, it has little impact on the dust removal efficiency. In addition, the dust removal efficiency also has a certain degree of stability in relation to the variation of airflow velocity;

④ Simple structure, flexible use, easy to recycle dry materials, and no need for sludge treatment.

A typical bag filter is shown in the following figure:

From the figure, it can be seen that there are many filter bags suspended inside the box. When the dusty airflow passes through the filter bags, the dust is captured by the filter bags, and the purified gas is discharged from the outlet. At fixed intervals (adjustable according to working conditions), the air backflow system is automatically activated through an electromagnetic pulse valve, and the dust captured on the bag wall is blown and shaken by high-speed airflow into the lower ash hopper.

The process of bag type dust removal is divided into two stages: first, the dusty gas passes through the clean filter cloth. At this time, the fibers of the filter bag mainly play a role in dust collection. At this time, the gaps between the fibers on the clean filter cloth are large, so the dust removal efficiency is not high; Afterwards, as the amount of captured dust increases, a portion of the dust is embedded in the gaps between the fibers of the filter cloth, reducing the gap spacing and covering the surface of the filter cloth, forming a layer of dust. In this stage, the filtration of dusty gases mainly relies on the dust layer. At this point, the role of the dust layer is much greater than that of the filter cloth, greatly improving the efficiency of the bag filter. That is to say, bag filter can be said to use dust particles to block the escape of dust particles, achieving the purpose of collecting and filtering dust. However, it does not mean that the thicker the dust layer, the better. When the thickness of the dust layer reaches a certain thickness, the resistance loss of the gas is too large, resulting in excessive power consumption of the dust collector and even causing certain harm. Therefore, after the dust layer accumulates to a certain thickness, various cleaning methods need to be used to discharge these dust from the dust collector.

The above only provides a brief introduction to the dust removal process of bag filters, while the following categories introduce the dust removal mechanism of bag filters.

Firstly, it is the screening effect. When the particle size of dust is larger than the pores of the filter cloth or the inter particle pores deposited on the filter cloth, the dust is intercepted. Due to the fact that the pores of the new filter cloth are much larger than the dust particle size, the retention effect is very small. When a large amount of dust deposits on the surface of the filter cloth, the retention effect significantly increases.

Next is the inertial collision effect. When the dusty airflow approaches the filter cloth fibers, the airflow will bypass the fibers, and the dust particles will continue to move forward in a straight line due to inertia. When they collide with the fibers, they will be captured and collected. All large dust particles within the critical line of the dust trajectory can reach the surface of the fibers and be captured. This inertial collision effect increases with the increase of dust particle size and flow velocity.

Once again, there are electrostatic and diffusion effects. Dust particles smaller than 1 µ m are mainly removed through diffusion and electrostatic interactions at low airflow speeds. Dust particles smaller than 1 µ m detach from the streamline under the impact of gas molecules and undergo Brownian motion like gas molecules. During the motion, they come into contact with fibers and separate from the airflow. This phenomenon is called diffusion. It increases with the decrease of airflow velocity and the decrease of fiber and dust diameter. Generally, both dust and filter cloth may carry charges. When the charges carried by the two are opposite, dust is easily adsorbed on the filter cloth; On the contrary, if both have the same charge, the dust will be repelled. Therefore, if there is an external electric field, the electrostatic effect can be strengthened, thereby improving the dust removal efficiency.

The last action belongs to the action of gravity. When the slowly moving dusty airflow enters the dust collector, dust particles with high particle size and density may naturally settle down due to gravity.

The four dust removal mechanisms do not work simultaneously. The importance of various dust removal mechanisms varies depending on the actual conditions such as dust properties, bag filter structure characteristics, and motion conditions.

Main structure

The bag filter mainly consists of three parts, including filter material, dust removal method, and dust removal control system.

1. Filter material

The development of bag filter materials mainly revolves around three aspects: material, fabric structure, and surface treatment technology. Before the 1950s, filter media were mainly made of natural fibers such as cotton and wool, which had defects such as low operating temperature, high moisture absorption, and lack of acid resistance. In the 1970s, with the development of the petroleum industry, synthetic fibers emerged, and filter materials developed towards high operating temperatures, low moisture absorption, and corrosion resistance. Filter materials tended to use synthetic fibers and inorganic fibers to replace natural fibers, and excellent performance filter materials such as glass fiber, aromatic polyamide, and polytetrafluoroethylene (PTFE) emerged successively. With the expansion of the application scope of bag filters and the increasingly strict requirements for environmental emission indicators, the development of filter media only in terms of material and fabric structure can no longer meet the requirements. It is necessary to develop post-treatment technology to improve the physical and chemical properties and cleaning function of filter media, and increase filtration efficiency. The post-treatment techniques for filter materials include "pre coating treatment", "hot melt extrusion treatment", "surface coating treatment", etc. Pre coating treatment involves dissolving the prepared powder into the sewn filter bag material using a special process, and then fixing it with adhesive to achieve efficient dust collection before the filter bag is used, overcoming the disadvantage of low dust removal efficiency in the early stage of the new filter material; After hot melt rolling and surface treatment on the dust filtering surface of the needle punched felt, the surface is smooth and the surface pores become smaller, which is conducive to dust removal, improving dust collection rate, and reducing resistance; Surface coating treatment, which involves coating the filter material surface with PTFE film through adhesive bonding or hot pressing. The thin film filter material formed by surface coating treatment adopts membrane separation technology to achieve true surface filtration. Its filtration efficiency is high, resistance is greatly reduced, and energy is saved. It is increasingly widely used in areas with increasingly high environmental requirements.

In the 1980s, China successfully developed high temperature resistant aramid sulfone fiber and aramid needle punched felt, which can withstand high temperatures of 210 ℃ and are applied in high-temperature flue gas treatment in industries such as steel, non-ferrous metals, and carbon black. The development and production of synthetic fiber needle punched felt products that are anti-static, heat-resistant, corrosion-resistant, oil resistant, and waterproof have basically met the dust removal needs.

A considerable portion of domestic bag filters use traditional needle punched felt filter materials, which belong to deep filtration, that is, relying on the micro dust particle layer intercepted on the filter material for separation. Generally, there are problems such as high filtration resistance and high backwash frequency, which make the service life of filter bags relatively short and operation relatively difficult. In recent years, high-temperature flue gas has been generated in response to China's national conditions and metallurgical industry. The requirements for dust removal filter bags have led to the development of a series of high-temperature resistant needle punched filter materials, which are composed of two or more high-temperature resistant fibers mixed and layered composite. After different surface chemical treatments and vibration resistance technologies, they have the characteristics of easy dust removal, water resistance, oil resistance, anti-static, etc. They are widely used in departments such as steel, non-ferrous metallurgy, chemical industry, carbon black, building materials, and power.

2. Dust cleaning method

The cleaning effect is an important indicator of the performance of dust collectors, and the cleaning methods are mainly divided into three categories: mechanical vibration, reverse airflow cleaning, and pulse cleaning. In recent years, there have been some new methods of cleaning dust, such as using elastic vibration mechanism to clean dust and using sound waves to assist in cleaning dust, and so on.

Mechanical vibration cleaning is the process of using a mechanical vibration mechanism to drag a filter bag for horizontal vibration, or using an eccentric wheel device to vibrate the filter bag frame or regularly lift the filter bag frame for cleaning, or using a specialized mechanism to twist the filter bag at a certain angle to deform and clean the dust.

Reverse airflow cleaning is the use of airflow opposite to the filtered airflow to deform the filter bag and cause the dust layer to fall off. This includes pulse injection, rotary blowback, and gas ring blowback using compressed air, clean gas, or other gases. Its cleaning effect is twofold: on the one hand, the reverse cleaning airflow directly impacts the dust block, and on the other hand, due to the change in airflow direction, the filter bag undergoes expansion and contraction vibration, causing the dust block to fall off. There are two forms of reverse airflow cleaning: reverse blowing cleaning and reverse suction cleaning. When dealing with large flow of dust and gas, reverse suction air is often used for dust removal. Air ring reverse blowing ash cleaning is a hollow circular ring placed on the outer side of an inner filtering cylindrical filter bag, close to the surface of the filter bag. The circular ring can move up and down and is connected to high-pressure gas or high-pressure fan through a hose. The air ring reverse blowing has a strong ability to clean dust and is suitable for filter bags made of felt. The rotary backwash bag filter adopts a downward inlet external filter type, and the clean air chamber is equipped with a rotatable cantilever tube. The backwash airflow is sent from the center tube to the rotary cantilever, and is blown vertically downwards into the filter bag through the spraying hole, causing the filter bag to swell and shake off the dust, achieving the purpose of cleaning.

The bag filter for pulse jet cleaning relies on compressed gas to spray out a high-speed airflow (called primary gas) through the small hole of the blowing pipe through the pulse valve. The primary gas induces several times the surrounding gas (called secondary gas) into the filter bag through the inducer, forming a gas wave that causes the filter material to rapidly expand and vibrate, thereby achieving cleaning.

According to different injection pressures, it can be divided into two types: high-pressure pulse and low-pressure pulse. The high-pressure pulse provides a gas source injection pressure of 0.4~0.7MPa. Due to the high injection pressure, high-pressure pulse spraying affects the service life of the bag, and there are many gas source configuration equipment, complex spraying systems, large one-time investment, and many vulnerable parts. Therefore, high-pressure pulse has a trend towards low-pressure pulse development. Low pressure pulse is an addition of a storage box on the basis of high pressure pulse, which outputs a low pressure gas source for gas supply. The spray valve is fixed at the bottom of the air storage box. When the valve is closed, it presses the valve port. When it is opened, the low-pressure airflow from the air storage box enters the cloth bag, playing a role in cleaning the dust. This cleaning method has the characteristics of strong cleaning ability and good cleaning effect, and is currently widely used in many industries.

3. Ash cleaning control system

The oldest dust removal method was controlled by humans, followed by the use of a timer. With the development of electronic technology, pulse control systems are applied to the dust removal control of bag filters, with existing open-loop control and closed-loop control modes. Using a pulse control instrument, the electromagnetic valve is directly controlled using its output signal in an open-loop control mode: the cleaning is controlled based on time or the pressure loss of the dust collector. Closed loop control can be used to reduce the amount of gas used for cleaning, or the energy consumption of the reverse blower, extending the service life of the pulse valve and filter bag. With the widespread application of computers, the ash cleaning control system adopts computer fixed pressure difference or timed closed-loop control, which has functions such as detection, control, and alarm.

Main applicable industries:

All industries that generate industrial dust and exhaust gases, such as PCB factories, construction waste treatment plants, semiconductor factories, food factories, automotive parts processing plants, and so on.