In recent years, air pollution control has attracted the attention of various industries, and the use of dust collectors is more common. This paper will introduce the selection of filter materials for bag filter, as follows: 18331752201
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In recent years, air pollution control has attracted the attention of various industries. The use of dust collectors is more common. This paper will introduce the selection of filter materials for bag filters:
In the production process of asphalt mixture mixer, the processes of drying, sieving, weighing and stirring 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 use a two-stage dust removal device consisting of a cyclone dust collector and a bag filter to collect dust pollutants as much as possible to reduce pollution to the natural environment and to comply with national environmental protection regulations. Claim. Taking the 4000 type asphalt mixture mixer as an example, the bag type dust collector has more than 600 heat-resistant (>200 °C) synthetic fiber bags in the box, with a working area of ​​1400 m2 and a maximum exhaust capacity of 65300 Nm3/h. The mesh is small, and dust of 0.3 μm or more can be collected in the filtration. At present, the dust removal efficiency of the bag type dust collector for the asphalt mixture mixer with advanced technical performance can reach 97% to 99%, and the dust-containing gas is purified to the extent of 15 mg/Nm3.
1 Principle of filter material selection
After the asphalt filter of the asphalt mixture mixer has been used for one stage, the filter material (bag) may be damaged and leaked due to various reasons, or “packing bagâ€, which must be replaced during maintenance. At this point, "what kind of filter material is used" causes people to think. Of course, the easiest way to deal with this problem is to follow the instructions and requirements of the asphalt mixing machine's instruction manual or maintenance manual. But the results are often unsatisfactory, or not applicable, or the cost is too great.
The filter material is usually made of natural fibers such as cotton or wool or synthetic fibers. Raw materials have different physical and chemical properties, suitable conditions for use or working environment (temperature, humidity, corrosion...). Therefore, the correct selection principle of the bag filter of the asphalt mixture mixer should be to fully understand the physical and chemical properties of the dusty gas discharged from the asphalt mixture mixer during the production process, and carefully compare the various fibers. Technical performance, make the right choice.
2 The correct choice of filter material
The correct selection of the filter material includes: selection according to the physical and chemical properties of the dust-containing gas; selection according to the properties of the dust; selection according to the cleaning method of the bag filter.
2.1 Select the filter material according to the physical and chemical properties of the dust-containing gas
The physical and chemical properties of dust-containing gases include temperature, humidity, corrosivity, flammability and explosiveness.
2.1.1 Temperature of dusty gas
The temperature of the dust-containing gas discharged from the asphalt mixture mixer is the primary factor for the correct selection of the filter material for the bag filter. According to the continuous use temperature, the filter material can be divided into three categories: normal temperature filter material (less than 130 ° C), medium temperature filter material (130 ~ 200 ° C) and high temperature filter material (greater than 200 ° C). For working conditions with large fluctuations in the temperature of the dusty gas, the safety factor should be chosen to be slightly larger, but the instantaneous peak temperature must not exceed the upper limit temperature of the filter. For the high-temperature dust-containing gas discharged from the asphalt mixture mixer, the high-temperature filter material can be directly used.
2.1.2 Humidity of dusty gas
The humidity of the dust-containing gas is another important factor in the correct selection of the filter material for the bag-type dust collector of the asphalt mixture mixer. The humidity of the dust-containing gas indicates how much water vapor is contained in the gas, and is usually expressed by the volume fraction of water vapor Xw or relative humidity ψ in the dust-containing gas. When Xw is more than 8%, or ψ is less than 80%, it is called a wet dust-containing gas. For wet dusty gases, the following points should be noted when selecting filter media:
1) The wet dust-containing gas wets and bonds the dust trapped on the surface of the filter bag, especially for water-absorbing, deliquescent dust, and even causes a paste bag. For this purpose, nylon, glass fiber and other filter materials with smooth surface and easy-to-clear fiber material should be used, and the filter material should be impregnated with silicone oil or fluorocarbon resin, or acrylic acid or polytetrafluoroethylene should be used on the surface of the filter material. Coating treatment.
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 the materials with poor hydrolysis stability such as polyamide, polyester, and imide. Therefore, when repairing the bag filter of the asphalt mixture mixer (replacement of the filter bag), the filter cloth material should be carefully selected, and the water content (≤ 3%) of the cold aggregate should be strictly controlled in the production process of the asphalt mixture mixer.
2.1.3 Corrosiveness of dusty gases
The chemical resistance of different fibers varies and is often affected by the influence of various factors such as temperature and humidity. In comparison, PTFE fibers known as "Plastic King" have the best chemical resistance, but are more expensive. Therefore, when selecting the filter material, it is necessary to grasp the main factors according to the chemical composition of the dust-containing gas, and to select a suitable material.
2.2 Select filter material according to the nature of dust
Dust traits include the chemical and physical properties of dust. The emphasis is now on the correct selection of materials, structures and post-treatments for the physical analysis of dust.
2.2.1 Dust shape and particle size distribution
As the dust removal object of the asphalt mixture mixer bag, it usually refers to dust of 0.1~10 μm, and its shape is divided into two types: regular shape and irregular shape. Usually, the high temperature combustion process is mostly regular dust, and most of the dust particles generated by the process are irregular. The regular-shaped dust has a smooth surface and a small specific surface area, and is not easily intercepted and agglomerated when passing through the filter cloth; on the contrary, the irregular shaped dust has different shapes, a rough surface, a large specific surface area, and is easily intercepted and aggregated when passing through the filter cloth. For the fine-grained dust after filtration of the asphalt mixture mixer through the cyclone dust collector (first-stage dust removal), the following points should be noted when selecting the filter material for the secondary dust collector bag filter:
1) Fiber should be selected as thin, short, curly, irregular section;
2) The structure is excellent in needle felt, if the fabric is used for twill weave or the surface is subjected to brushing;
3) Needle felt with density gradient when mixing thick and thin cotton wool layer, and new technology such as surface spraying, dipping or film coating to realize surface filtration is the development direction of ultrafine dust filter material;
4) Fine particle dust is difficult to capture and it is easy to form a dense dust layer after trapping, which is not conducive to cleaning. On the contrary, the coarse particle dust is easy to be collected, and the dust layer formed after the collection is loose, which is favorable for cleaning. In a sense, the dust of the combination of thickness and thickness is beneficial to both filtration and cleaning.
2.2.2 Adhesion and cohesion of dust
The dust has characteristics of adhering to each other or adhering to the surfaces of other objects, and when the suspended dusts come into contact with each other, they are adsorbed and aggregated with each other. The cohesive force of dust is related to various factors such as its type, shape, particle size distribution, water content and surface characteristics. The angle of repose can be used to characterize the cohesive force of dust. For example, an angle of repose of less than 30 ° is called low adhesion, and fluidity is good; when the angle of repose is greater than 45°, it is called high adhesion and poor fluidity. For the filter material for the bag filter of the asphalt mixture mixer, if the adhesion to the dust is too small, the ability to trap dust will be lost, and the adhesion is too large, resulting in dense dust condensation and difficulty in cleaning. Therefore, for the dust with strong adhesion, it is preferred to use a filament fabric filter material, or a needle-punched felt filter material which is surface-sintered, calendered, and mirror-finished. From the material of the filter material, nylon and glass fiber are superior to their varieties. For viscous dust, a raised fabric filter cannot be used as it may be sticky with dust and spread over the entire filter surface, making cleaning difficult.
2.2.3 Moisture absorption and deliquescence of dust
The ability of dust to absorb moisture in a gas is called hygroscopicity. If hydraulic power is the main component, it is called moisture infiltration. Hygroscopicity and wettability are achieved by the action of a capillary formed between dust particles. Hygroscopicity and wettability are related to factors such as the atomic chain of the dust, the surface state, and the surface tension of the liquid. The combination can be characterized by the wetting angle—hydrophilicity of less than 60° and hydrophobicity of greater than 90°. When the humidity of the hygroscopic dust increases, the cohesive force and adhesion of the powder increase, and the fluidity and chargeability decrease, which causes the dust to adhere to the surface of the filter bag. Over time, the dust is invalidated and the dust forms a plate.
In the dust discharged from the asphalt mixture mixer, such as CaO, CaCl2, KCL, MgCl2, etc., the chemical reaction will further occur after the moisture absorption, and its nature and morphology will change accordingly, which is deliquescence. They stick to the surface of the filter bag, which is the least desirable phenomenon for bag filters.
For hygroscopic and deliquescent dust, the precautions to be taken when selecting the filter material are the same as the "filter material selected according to the characteristics of the dust-containing gas".
2.2.4 Dust of the dust
When the filter material filters, intercepts, and agglomerates dust, the destructive effect of dust (especially irregular dust) on the filter material is called abrasion. It is related to the nature and shape of the dust, as well as the airflow velocity and dust concentration of the dust. For example: aluminum powder, silicon powder, carbon powder, sintered ore powder, etc. are hard and belong to high-abrasive dust; the surface of the particles is rough, the irregular edge of the dust is smoother than the surface, and the grinding of the spherical particle dust is many times larger. (About about 10); the dust with a particle size of about 90μm has the highest sharpness, and when the particle size of the dust is reduced to 5~10μm, the sharpness is very weak; the dust is grinded and the airflow speed is 2 ~3.5 times the square is proportional. Therefore, in order to reduce the grinding property of the dust on the filter material, the speed and uniformity of the exhaust gas flow of the asphalt mixture mixer must be strictly controlled. In addition, for the dust with high abrasion resistance, a filter material with good abrasion resistance should be used.
For the grinding of the dust, the following points can be used for reference when selecting the filter material:
1) Chemical fiber is superior to glass fiber, and expanded fiber is superior to general glass fiber;
2) Fine, short, crimped fibers are superior to coarse, long, straight fibers;
3) The felt material is superior to the fabric. In the felt material, the interlacing property between the fibers should be strengthened by needle punching. The satin fabric is optimal in the fabric, and the brushed surface of the fabric can improve the wear resistance.
4) Surface coating, calendering and other post-treatment can improve the wear resistance of the filter material. For the glass fiber filter material, silicone oil, graphite, and polytetrafluoroethylene resin can improve the wear resistance and folding resistance.
2.3 Select the filter material according to the cleaning method of the bag filter
The cleaning method of the bag filter is another important factor that must be considered when selecting the filter material correctly. That is, the bag type dust collector of different cleaning methods should adopt different structures due to the difference of the cleaning energy and the deformation characteristics of the filter bag. Variety of filter material.
2.3.1 Mechanical vibrating cleaning method
The bag filter of the mechanical vibration cleaning method uses mechanical means (including manual, electromagnetic vibration and pneumatic) to vibrate the filter bag, and the vibration frequency ranges from several times to several hundred times per second. In addition to small asphalt mixture mixers, such bag filters are mostly in the form of inner filter bags. The utility model is characterized in that the kinetic energy applied to the dust layer is less and the number of actions is more, and the filter material is required to be thin and smooth, and the texture is soft, which is favorable for transmitting vibration waves so as to form sufficient vibration force on all the filtering surfaces. Therefore, bag-type dust collectors for asphalt mixture mixers usually use satin or twill weaves made of chemical fiber staple fibers, with a thickness of 0.3-0.7 mm, a mass per unit area of ​​300-350 g/m2, and a recommended filtration speed of 0.6. ~1.2m / min (small asphalt mixture mixer can be appropriately increased to 1.5 ~ 2. 0m / min).
2.3.2 Room backwashing cleaning method
The baghouse type dust-removing bag type dust collector adopts a compartment structure and a valve to switch by chamber to form a reverse air flow, forcing the filter bag to contract or bulge to clear the ash. This kind of cleaning method is also a low-kinetic type cleaning, and the working pressure of the bag type dust collector is used as the cleaning power, and the back-blowing airflow power is additionally provided in special occasions. The bag type dust collector of the chamber back-flush type cleaning method requires a thin filter material with soft texture, easy deformation and dimensional stability. Since the bag type dust collector has internal filter and external filter, the filter material selection is slightly different. . Generally speaking, the inner filter type is usually a round bag with no frame, the bag diameter is 120~300mm, and the bag length to bag diameter ratio is 15~40. The satin (or twill) woven filter material is preferred, and the thickness is 1.0. ~1.5mm, the mass per unit area is 300~400g/m2; the outer filter type commonly used flat bag, diamond bag and honeycomb bag, with support frame, prefer to use thin needle felt filter material with good wear resistance and permeability. The unit area is 350~400g/m2.
2.3.3 Vibration backflushing and cleaning method
The bag type dust collector with vibration back-flushing and cleaning method refers to a bag type dust collector which has both the vibration and the reverse air flow double-cleaning action, and the vibration causes the dust cake to loosen and the reverse air flow to dissociate the dust. The two methods work together to improve the cleaning effect, especially for the filtration of fine particle viscous dust. The filter material for such bag type dust removal is selected to be substantially the same as the bag type dust collector of the chamber backflush type.
2.3.4 Nozzle back-blowing cleaning method
The bag type dust-removing bag type dust collector uses a high-pressure fan or a blower as the back-blowing cleaning power, and sequentially sprays the filter bag through the moving nozzle to form a strong reverse airflow, so that the filter bag is sharply deformed and cleaned. It is a medium energy cleaning type. According to the nozzle form and its movement track, it can be divided into three types: rotary back-blow type, reciprocating back-blow type and gas ring sliding back-blow type.
The bag-type dust remover of the rotary back-flushing type and the reciprocating back-flushing type is a compact outer bag with a frame. Such bag type dust collectors require filter materials with relatively soft structure, stable structure and good wear resistance. It is preferred to use medium thickness needle felt filter media with a mass per unit area of ​​350~500g/m2. China has more choices of tubular satin woven filter materials.
The bag type dust-removing bag type dust collector adopts an inner filter bag, and the nozzle is a ring-shaped slit. The sleeve is moved up and down on the outside of the bag, and the filter is required to be thick, wear-resistant, rigid, and lint-free. For the material, wool compression felt is preferred, and synthetic fiber needle felt is also available, and the mass per unit area is 600~800g/m2.
2.3.5 Pulse blowing type cleaning method
The bag-type dust-removing bag type dust collector is powered by compressed air, and uses a pulse blowing mechanism to release the compressed air stream instantaneously, inducing several times of secondary air to be injected into the filter bag at high speed, so that the filter bag is rapidly expanded. * Shock vibration and reverse gas and clear ash, which is a type of high kinetic energy cleaning. An outer filter bag or a flat bag with a frame is usually used. Such bag type dust collectors are required to use thick and wear-resistant and strong tensile filter materials. The chemical fiber needle felt or compressed felt filter material is preferred, and the mass per unit area is 500~650g/m2.
Types of filter materials commonly used in bag filters 
Bag type and application parameters 
Various industries 
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