Discussion on the correct selection of filter mate

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Discussion on the correct selection of filter material for bag filter

in the production process of asphalt mixture mixer, its drying, screening, weighing and mixing processes will have a large amount of dust and harmful exhaust gas generated by fuel combustion, which will cause pollution of the natural environment. Modern asphalt mixture mixers adopt two-stage dust removal devices composed of cyclone dust collector and bag dust collector to collect dust pollutants as much as possible, so as to reduce the pollution to the natural environment and make it meet the requirements of national environmental protection regulations. Taking the 4000 asphalt mixture mixer as an example, its bag type dust collector is equipped with more than 600 heat-resistant (> scaffold fastener bowl buckle testing machine fully meets the experimental requirements of gb/t15831 (2) 006 "steel pipe scaffold fasteners", GB 24911 (2) 010 "bowl buckle steel pipe scaffold components" 200 ℃) synthetic fiber bags, with a working area of 1400m2, a maximum exhaust capacity of 65300nm3/h, a small eye, and 0.3 can be captured in filtration μ Dust above M. At present, the bag filter for asphalt mixture mixer with advanced technical performance has a dust removal efficiency of 97%~99%, so that the dusty gas can be purified to 15mg/nm3

1 principle of selecting filter material

after a stage of use, the filter material (cloth bag) of the bag type dust collector of the asphalt mixture mixer will be damaged and leak due to various reasons, or "paste bag", which must be replaced in maintenance. At this point, "what kind of filter material to choose" causes people to think. Of course, the simplest way to deal with this problem is to follow the provisions and requirements of the operation manual or maintenance manual of the asphalt mixture mixer. But the results are often not satisfactory, or not applicable, or the cost is too high

filter materials are usually made of natural fibers such as cotton and wool or synthetic fibers. Raw materials have different physical and chemical properties, suitable use conditions or working environment (temperature, humidity, corrosion...). Therefore, the principle of correct selection of filter material for bag type dust remover of asphalt mixture mixer should be to fully understand the physical and chemical characteristics of dusty gas discharged by asphalt mixture mixer in the production process, and carefully compare the technical performance of various fibers to make correct selection

2 method of selecting filter material correctly

the method of selecting filter material correctly includes: selecting according to the physical and chemical characteristics of dusty gas; Select according to the characteristics of dust; According to the ash cleaning method of bag filter, etc

2.1 filter material is selected according to the physical and chemical characteristics of dusty gas

the physical and chemical characteristics of dusty gas include temperature, humidity, corrosiveness, flammability and explosiveness

2.1.1 temperature of dusty gas

the temperature of dusty gas discharged from asphalt mixture mixer is the primary factor for the correct selection of filter material for bag filter. According to the temperature of continuous use, filter materials can be divided into three categories: normal temperature filter materials (200 ℃). For the working conditions with large temperature fluctuation of dusty gas, the safety system should be selected to avoid shaking damage. The number of relevant parts is slightly larger, but the instantaneous peak temperature should not exceed the upper limit temperature of the filter material. For the high-temperature dusty gas discharged by the asphalt mixture mixer, the high-temperature filter material can be directly selected

2.1.2 humidity of dusty gas

the humidity of dusty gas is another important factor for the correct selection of filter materials for the bag filter of asphalt mixture mixer. The humidity of dusty gas indicates the degree of water vapor contained in the gas. Usually, the volume percentage of water vapor in dusty gas XW or relative humidity is used ψ express. When xw>8%, or ψ> At 80%, it is called wet dusty gas. For wet dusty gas, the following points should be paid attention to when selecting filter material:

1) wet dusty gas makes the dust collected on the surface of the filter bag wet and stick, especially for water absorbing and deliquescent dust, and even causes bag pasting. Therefore, nylon, glass fiber and other filter materials with smooth surface and easy ash removal should be selected, and the filter materials should be impregnated with silicone oil and fluorocarbon resin, or coated with acrylic acid and polytetrafluoroethylene on the surface of the filter materials

2) when the flue gas discharged from the asphalt mixture mixer has high temperature and high humidity at the same time, it will affect the temperature resistance of the filter material, especially for materials with poor hydrolytic stability such as polyamide, polyester and amides. Therefore, the filter cloth material should be carefully selected during the maintenance (replacement of filter bag) of the bag type dust remover of the asphalt mixture mixer, and it should be strictly controlled during the production process of the asphalt mixture mixer. 3. The sample requirements for the tensile tester to test the breaking force of shoelaces are as follows: the water content of the refrigeration aggregate (≤ 3%)

2.1.3 corrosivity of dusty gas

the chemical resistance of different fibers varies and is often affected by the intersection of temperature, humidity and other factors. In comparison, polytetrafluoroethylene fiber, known as the "king of plastics", has the best chemical resistance, but the price is more expensive. Therefore, when selecting filter materials, we must grasp the main factors and select suitable materials according to the chemical composition of dusty gas

2.2 filter materials are selected according to the properties of dust

the properties of dust include the chemical and physical properties of dust. Now we focus on the physical properties of dust to analyze the correct selection of filter materials in terms of material, structure and post-treatment

2.2.1 dust shape and particle size distribution

as the dust removal object of the bag filter of the asphalt mixture mixer, it usually refers to 0.1~10 μ M, its shape can be divided into regular and irregular. Generally, the products of high-temperature combustion process are mostly regular dust, while most of the dust particles produced in the process are irregular. Regular dust has smooth surface and small specific surface area, which is not easy to be intercepted and condensed when passing through the filter cloth; On the contrary, irregular dust has different shapes, rough surface and large specific surface area, which is easy to be intercepted and condensed when passing through the filter cloth. For the fine particle dust filtered by the cyclone dust collector (primary dust collector) in the asphalt mixture mixer, the following points should be paid attention to when selecting the filter material for the bag dust collector with secondary dust collector:

1) the fiber should be fine, short, curly and irregular cross-section

2) needled felt is the best structure. If the fabric is used, it should be twill woven or the surface should be roughened

3) needled felt with density gradient when using coarse and fine cotton flocs, and new technologies such as surface spraying, impregnation or film coating to realize surface filtration are the development direction of selecting filter materials for ultra-fine dust

4) fine particle dust is difficult to capture, and it is easy to form a dense dust layer after capture, which is not conducive to ash removal. On the contrary, coarse-grained dust is easy to be captured, and the dust layer formed after capture is relatively loose, which is conducive to ash removal. In a sense, the dust with thickness matching is beneficial to filtration and ash removal

2.2.2 adhesion and cohesiveness of dust

dust has the characteristics of attaching to each other or to the surface of other objects. When suspended dust contacts each other, they adsorb each other and condense together. The cohesion of dust is related to many factors, such as its type, shape, particle size distribution, water content and surface characteristics. Taken together, the angle of repose can be used to characterize the cohesion of dust. For example, if the repose angle is less than 30 °, it is called low adhesion and good fluidity; If the repose angle is greater than 45 °, it is called high adhesion and poor liquidity. In addition, the viscosity (adhesion and cohesion) of dust and solid surface is also related to the roughness and cleanliness of solid surface

for the filter material used in the bag type dust remover of the asphalt mixture mixer, if the adhesion with the dust is too small, it will lose the ability to capture the dust, and if the adhesion is too large, it will cause the dust to condense closely and it is difficult to remove the dust. Therefore, for dust with strong adhesion, filament fabric filter material or needle felt filter material with surface singeing, calendering and mirror treatment should be selected. From the material of filter material, nylon and glass fiber are better than their varieties. For viscous dust, fabric filter material with fuzzing cannot be selected, because it may attach viscous dust and extend to the whole filter surface, making it very difficult to remove dust

2.2.3 hygroscopicity and deliquescence of dust

the ability of dust to absorb water in gas is called hygroscopicity, and if water power is the main body, it is called the wettability of water to dust. Hygroscopicity and wettability are achieved by the capillary formed between dust particles. Hygroscopicity and wettability are related to the atomic chain of dust, surface state, surface tension of liquid and other factors. Taken together, they can be characterized by wetting angle - those less than 60 ° are hydrophilic, and those greater than 90 ° are hydrophobic. After the humidity of hygroscopic dust increases, the cohesion and adhesion of powder particles increase, while the fluidity and electrification decrease, which makes the dust adhere to the surface of the filter bag. Over time, the dust removal fails and the dust forms plates

in the dust discharged by the asphalt mixture mixer, such as Cao, CaCl2, KCl, MgCl2, etc., after moisture absorption, further chemical reactions will occur, and their properties and forms will change accordingly, which is deliquescence. They stick to the surface of the filter bag, which is the last phenomenon of the bag filter

for hygroscopic and deliquescent dust, the matters needing attention when selecting filter material are the same as "selecting filter material according to the characteristics of dusty gas"

2.2.4 grinding and pecking of dust

when filter material filters, intercepts and condenses dust, the destructive effect of dust (especially irregular dust) on filter material is called grinding and pecking. It is related to the nature and form of dust, as well as the air velocity carrying dust, dust concentration and other factors. For example, aluminum powder, silicon powder, carbon powder, sintered ore powder and other materials are hard, belonging to high abrasive dust; The abrasive pecking property of dust with rough particle surface and irregular sharp edges is many times greater than that of dust with smooth surface and spherical particles (about 10); Particle Size 90 μ M is the most abrasive, and when the dust particle size is reduced to 5~10 μ At m, the grinding pecking ability is very weak; The grindability of dust is directly proportional to the 2~3.5 power of the air velocity carrying it. Therefore, in order to reduce the grinding and pecking of dust on filter material, the speed and uniformity of exhaust air flow of asphalt mixture mixer must be strictly controlled. In addition, filter materials with good wear resistance should be selected for dust with high abrasion resistance

in view of the grinding and pecking properties of dust, the following points can be used for reference when selecting filter materials:

1) chemical fiber is better than glass fiber, and expanded fiber is better than general glass fiber

2) fine, short and curly fibers are better than coarse, long and straight fibers

3) felt is better than fabric. Needle punching should be used in felt to strengthen the intersection between fibers. Satin fabric is the best among fabrics, and the fleece on the fabric surface can improve its wear resistance

4) surface coating, calendering and other post-treatment can improve the wear resistance of filter materials. For glass fiber filter materials, silicone oil, graphite, polytetrafluoroethylene resin treatment can improve their wear resistance and folding resistance

2.3 select filter materials according to the ash cleaning method of bag filter

the ash cleaning method of bag filter is another important factor that must be considered in the correct selection of filter materials, that is, different types of filter materials with different ash cleaning methods should be selected due to different ash cleaning energy and deformation characteristics of filter bags

2.3.1 mechanical vibration ash cleaning method

the bag filter of mechanical vibration ash cleaning method uses mechanical devices (including manual, electromagnetic vibration and pneumatic) to vibrate the filter bag, and the vibration frequency ranges from several times per second to hundreds of times. In addition to small asphalt mixture mixers, most of these bag filters adopt the form of internal filter round bags. Its characteristic is that it is applied to dust

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