The dust filter bag is called the heart of the dust collector. The selection of the filter bag is very important. It directly affects the dust removal effect of the dust collector. The fabric and design of the dust filter bag should pursue high efficiency filtration, easy dust stripping and durability. The choice of filter bag should be selected from the following aspects: gas temperature, humidity and chemical properties; particle size; dust concentration; filtration wind speed;
Filter bag selection principles and precautions
Bag filter is generally based on the nature of dust-laden gas, the nature of dust, and the dust-removal method of the dust collector. When selecting a filter bag, the following principles should be followed:
1 The performance of the filter bag should meet the general conditions and special requirements of the production conditions and dust removal process.
2 Under the above premise, filter bags with long service life should be selected as far as possible, because long service life can not only save operating costs, but also meet the long-term emission requirements of gases.
3 When selecting the filter bag, it is necessary to comprehensively compare the sorting of various filter materials. It should not use a so-called “good” filter bag to adapt to various working conditions.
4 In the nature of gas, dust properties, and cleaning methods, filter bags should be selected based on the main influencing factors, such as high-temperature gas, corrosive gas, and flammable dust.
According to the nature of dusty gas selection
1 gas temperature. Dust gas temperature is an important factor in the selection of filter bags. Usually, the dust-containing gas less than 130°C is called a normal-temperature gas, and the dust-laden gas larger than 130°C is called a high-temperature gas. Therefore, the filter bags can be divided into two categories: the room temperature filter bags and the high-temperature filter bags. Therefore, a suitable filter bag should be selected according to the temperature of the flue gas. Some people called 130-170 °C medium temperature gas, but the filter bag multi-select high temperature type.
The temperature resistance of the filter bag is “continuous long-term use temperature” and “short-term instantaneous temperature”. “Continuous long-term use temperature” refers to the long-term continuous temperature that the filter bag can be applied to. The filter bag should be used at this temperature. “Instantaneous short-term temperature” refers to the maximum temperature at which the filter bag is not allowed to exceed 10 minutes per day. If the time is too long, the filter bag will be softened and deformed.
2 gas humidity. Dust-laden gases are classified into three types according to relative humidity: When the relative humidity is 30% or less, it is a dry gas, and it is prone to generate static electricity. Like high-humidity gases, dust tends to adhere and it is difficult to remove dust. The relative humidity of 30% to 80% is normal, and the dust collector works best. Relative humidity of more than 80% is high humidity gas. When the high humidity gas is in a high temperature state, especially when the dust-containing gas contains SO3, the condensation of gas will cause condensation.
This will not only foul the surface of the filter bag, but also will corrode the structural materials and cause serious damage to the dust removal device, filter bag, etc. Therefore, special attention is required. When selecting the filter bag for the moisture-containing gas, the following points should be noted:
1) Moisture-containing gas wets and binds the dust collected on the surface of the filter bag, especially for water absorption, deliquescent, and wet dust. For this purpose, filter bags with smooth surface, long fibers, and easy dust removal, such as nylon and glass fibers, should be used, and silicone oil and fluorocarbon resin should be used to impregnate the bags, or acrylic and PTFE should be used on the surface of the filter bags. And other materials for coating treatment. Plastic burning board and film materials have excellent moisture resistance and easy cleaning properties. However, it should be a waterproof and oil-removing dust bag as the first choice for high humidity gas.
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2) When the high temperature and high humidity exist at the same time will affect the temperature resistance of the filter bag, especially for cotton, polyester, polyimide and other materials with poor hydrolytic stability, should be avoided as much as possible.
3) For the design of the dust-containing filter bag, the circular filter bag should be used for the moisture-containing gas. Try not to use the flat filter bag and the diamond-shaped filter bag (except the plastic burning plate) with complex shapes and very compact arrangement.
4) The dust inlet gas temperature of the dust collector should be higher than the gas dew point temperature of 30°C to avoid sticking bags.
5) The chemical nature of the gas. In a variety of kiln exhaust gas and chemical waste gas, often contain a variety of chemical components such as acids, alkalis, oxidants, organic solvents, etc., and are often affected by a variety of factors such as temperature, humidity and other factors. Careful consideration should be given to the use of dust filter bags to avoid unnecessary losses.
Polyester fiber has good temperature resistance and chemical resistance at room temperature, but its chemical resistance is poor under high temperature conditions. Polyphenylene sulfide fiber has good properties of high temperature resistance and acid and alkali corrosion resistance. It is suitable for dusting coal-fired flue gas, but its antioxidant capacity is poor. Although polyimide fiber can make up for its deficiency, its hydrolytic stability is not ideal. Polytetrafluoroethylene fiber as “plastic king” has the best chemical resistance, but it is more expensive. When selecting the filter material, it must be based on the chemical composition of the dust gas, seize the composition, seize the main factors, and make a comprehensive consideration.
Choose according to the nature of dust
1 The wettability and adhesion of dust. Dust wettability and wettability are achieved through the capillary action formed between the dust particles, and are related to the atomic chain of the dust, the state, and the surface tension of the liquid, and can be characterized by wet corners. Usually less than 60 degrees is called hydrophilic, and greater than 90 degrees is called hydrophobic. After the hygroscopic dust increases, the cohesive force and the cohesive force of the particles increase, the fluidity and the chargeability decrease, and it adheres to the surface of the filter bag. As time passes, the cleaning dust fails and the dust cake compacts.
Some dusts such as CaO, CaCl2, KCL, MgCL2, NaCO3, etc., undergone further chemical reaction after moisture absorption, and their properties and morphology have changed, and they are called deliquescence. After deliquescence, dust sticks to the surface of the filter bag, which is the most taboo of the bag filter.
For wet, deliquescent dust, when selecting the filter material should pay attention to the smoothness of the filter material, not fluff and give water, of which the film filter bag plastic burning board is the best.
Many wet dusts have strong adhesion and are inextricably linked to the glue. For the bag filter, if the adhesive force is too small, it will lose the ability to collect dust, and the excessive adhesion will cause dust aggregation and dust removal difficulties.
For strong adhesion dust, filament filter bags, or singeing, calendering, and mirror treatment needled felt filter bags should also be used. For impregnation, coating, and coating techniques, full use should be made. From the dust bag material, nylon and glass fiber are superior to other varieties.
2 Dust flammability and chargeability. Some dust may burn or explode when exposed to sparks in air at certain concentrations. The flammability of dust is related to various factors such as particle size, composition, concentration, heat of combustion, and burning rate. The smaller the particle size, the larger the surface area and the easier it is to ignite. An important condition for dust explosion is closed air. In this space, the lower limit of its explosion concentration is generally tens to hundreds of grams per cubic meter. The higher the combustion heat and burning speed of dust, the greater the explosive power.
Dust combustion or explosive fire source is usually caused by friction sparks, static sparks, hot particles and other materials, of which the most harmful charge. This is because chemical fiber filtration is because the chemical fiber filter is usually easy to charge. If the dust is charged at the same time, it is easy to produce sparks. Therefore, for flammable and easily charged dust such as coal powder, coke powder, alumina powder and magnesium Powder, etc., should be selected flame-retardant filter and anti-static dust bag.
It is generally believed that the fibers with an oxygen index greater than 30 are mixed with conductive fibers so that the filter material has conductive properties in the warp direction or in the direction of the whiskers, so that the resistance is less than 1*109 ohms. Commonly used conductive fibers are stainless steel fibers and modified (carburized) chemical fibers. Compared with the former, the former has stable and reliable conductivity, and the latter easily decays after a certain period of time. The amount of conductive fiber mixed is about 2% to 5% of the basic fiber.
3 dust flow and friction. When the flow and friction of the dust are strong, the filter bag will be directly worn and the service life will be reduced. Surface coarse, diamond-shaped irregular particles than the smooth surface of the spherical particles 10 times greater wear. Dust particles with a particle size of about 90 microns have the highest abrasiveness, while when the particle size decreases to 5 to 10 microns, the abrasive properties are very weak. The 2-30000 power of abrasion and air flow rate is proportional to the 1.5th power of the particle show. Therefore, the air flow velocity and its uniformity must be strictly controlled. Among common dusts, aluminum powder, silicon powder, coke powder, carbon powder, sinter powder, etc. are high-abrasive dust. For wear dust, filter bags with good abrasion resistance should be used.
Dust filter bag wear parts and forms varied, according to experience, filter bag wear are in the lower part, because the filter bag upper filter speed is low, the gas dust concentration is small. In order to prevent the lower part of the filter bag from being worn out, the design should limit the rate of air flow rising in the lower part of the bag room.
For dust with strong abrasion, three points should be taken into consideration when selecting filter bags:
1Chemical fiber is superior to glass fiber. Extruded glass fiber is superior to general glass. Fine, short, and crimped fibers are superior to coarse, long, and smooth fibers.
2Felt should be used in the felt to strengthen the intertwining of the fibers. The satin fabric is the best in the fabric. The drawing of the fabric surface is also a measure to improve the wear resistance, but the felt, satin fabric and fleece Filter bag will increase the resistance value.
3 For the ordinary filter material surface coating, calendering and other post-treatment can also improve the wear resistance. For glass fiber filter bags, silicone oil, graphite, polytetrafluoroethylene resin treatment can improve wear and tear resistance. However, when the film-coated filter is used in a condition with strong abrasion, the film will be worn away prematurely and the film-covering effect will be lost.