How to choose filter bag
Dust removal filter bag can be called the heart of the dust collector, the selection of filter bag is very important, it directly affects the dust removal effect of the dust collector, dust removal filter bag fabric and design should try to pursue efficient filtering, easy to dust stripping and durable effect. The selection of filter bag should be made from the following aspects: temperature, humidity and chemical properties of the gas; Particle size; Dust concentration; Filter wind speed; Comprehensive consideration of factors such as ash cleaning method.
1. Principles and precautions for filter bag selection
According to the nature of dust gas, the nature of dust and the different cleaning methods of the dust collector, the selection of filter bags should follow the following principles:
(1) filter bag performance should meet the production conditions and dust removal process of the general situation and special requirements.
(2) in the above premise, should choose the long service life of filter bag, this is because long service life can not only save operating costs, but also can meet the requirements of long-term gas emission.
(3) the selection of filter bag should be a variety of filter material sorting comprehensive comparison, should not use a so-called "good" filter bag to adapt to a variety of operating conditions.
(4) in the nature of gas, dust and dust cleaning way, should seize the main influencing factors to choose filter bag, such as high temperature gas, corrosive gas, flammable dust, etc.
2. Choose according to the properties of dusty gas
Gas temperature. Dust gas temperature is an important factor in the selection of filter bag. Generally, dust-containing gas less than 130℃ is called normal temperature gas, and dust-containing gas greater than 130℃ is called high temperature gas. Therefore, filter bags can be divided into two categories: normal temperature filter bag and high temperature filter bag. Therefore, appropriate filter bags should be selected according to the flue gas temperature. Some people put 130-170℃ call medium temperature gas, but filter bag more choose high temperature type.
The temperature tolerance of filter bag has two kinds: "continuous long-term service temperature" and "instantaneous short-term temperature". "Continuous long-term service temperature" refers to the long-term temperature at which the filter bag can be used for continuous operation, and the filter bag shall be selected at this temperature. "Instantaneous short-term temperature" refers to the maximum temperature of the filter bag that is not allowed to exceed 10min per day. If the temperature is too long, the filter bag will be softened and deformed.
(2) gas humidity. Dust-containing gas is divided into three states according to the relative humidity: when the relative humidity is less than 30%, it is a dry gas, easy to generate electrostatic, and like high-humidity gas, dust is easy to adhere and difficult to clear ash. The relative humidity is in the general condition of 30% ~ 80%, and the dust collector is in the best working condition. When the relative humidity of the gas is above 80%, it is considered as high humidity gas. When the high humidity gas is in the high temperature state, especially when the dust gas contains SO3, the phenomenon of condensation will occur in the gas cooling. This will not only cause fouling and blockage on the surface of filter bag, but also corrode structural materials and cause serious damage to dust removal device and filter bag. Therefore, special attention should be paid to the following points when selecting filter bag with wet gas:
1) the wet gas will moisten and bond the dust collected on the surface of the filter bag, especially the hygroscopicity, deliquescence and wetting dust, which will cause the pasting of the bag. For this reason, filter bags with smooth surface, long fiber and easy ash removal, such as polyamide and glass fiber, should be selected, and should be impregnated with silicone oil and fluorocarbon resin, or coated with acrylic acid, polytetrafluoroethylene and other materials on the surface of filter bags. Plastic burning board and film coated materials have excellent wet resistance and easy to clean ash. But as a high humidity gas preferred should be waterproof oil - proof dust - removal cloth bag.
2) when high temperature and high humidity exist simultaneously, the temperature resistance of filter bag will be affected, especially for materials with poor hydrolysis stability, such as cotton fiber, polyester, imide, etc., which should be avoided as far as possible.
3) circular filter bag should be used for dedusting filter bag with wet gas, and flat and diamond filter bag with complicated shape and compact arrangement should not be used as far as possible (except plastic burning board).
4) the inlet temperature of the dust containing gas of the dust collector should be above 30℃ above the dew-point temperature of the gas to avoid bagging.
5) chemical properties of gases. In various furnace exhaust gas and chemical waste gas, often contain acid, alkali, oxidant, organic solvent and other chemical components, and often by temperature, humidity and other factors of the cross influence. Therefore, the selection of dust filter bag should be considered, so as to avoid unnecessary losses.
Polyester fiber has good temperature and chemical resistance at room temperature, but at high temperature, chemical resistance is poor. Polyphenylene sulfide fiber has good properties of resisting high temperature and acid and alkali corrosion, and is suitable for dust removal of coal-burning flue gas. Teflon, the "king of plastics", has the best chemical resistance but is expensive. In the selection of filter material, must be based on the chemical composition of dust gas, grasp the composition, grasp the main factors, comprehensive consideration.
3. Choose according to dust properties
(1) dust wetting and adhesion. Wetting and wetting of dust are completed by capillary action between dust particles, which is related to the atomic chain, state of dust and surface tension of liquid and other factors, and can be characterized by wetting Angle. It is usually called hydrophilic when it is less than 60 degrees and hydrophobic when it is more than 90 degrees. When the moisture content of the hygroscopic dust increases, the cohesion and viscosity of particles increase, the fluidity and electrical charge decrease, and the particles adhere to the surface of the filter bag. As time goes by, the dust removal fails, and the dust cake is consolidated.
Some dust, such as CaO, CaCl2, KCL, MgCL2 and NaCO3, will undergo further chemical reaction after hygroscopic, and its properties and morphology will change, which is called deliquescent. After delisting, dust paste on the surface of filter bag, which is the most taboo bag filter.
For wetting, deliquing dust, in the selection of filter material should pay attention to the filter material smooth, no fleece-free and give water, which is the best plastic coated filter bag burning board.
A lot of dust with strong wetting force is stronger, actually have inseparable relation with adhesion. If the adhesive force of bag filter is too small, it will lose the ability to collect dust, and the excessive adhesive force will cause dust aggregation and dust removal difficulty.
For dust with strong adhesion, filament non-flapping fabric filter bag should also be selected, or needle felt filter bag with surface singeing, pressing and mirror treatment, and make full use of impregnation, coating and film covering technology. From the dust cloth bag material on the nylon, glass fiber is better than other varieties.
(2) the combustible dust and charge. Certain dusts, at certain concentrations, will ignite or explode in the air when caught in sparks. The flammability of dust is related to particle size, composition, concentration, combustion heat and combustion speed. The smaller the particle size, the larger the surface area and easier to ignite. An important condition of dust explosion is closed space, in this space its explosion concentration lower limit is every cubic meter generally dozens to several hundred grams, the higher the combustion heat and combustion speed of dust, the greater its explosion power.
The ignition source of dust combustion or explosion is usually caused by friction spark, electrostatic spark, hot particles and so on. This is because the chemical fiber filter this is because the chemical fiber filter is usually easy to charge, if the dust is charged at the same time is easy to spark, so for combustible and easily charged dust such as coal, coke, aluminum oxide powder and magnesium powder, it is appropriate to choose flame-retardant filter and anti-static dust cloth bag.
It is generally believed that conductive fibers with an oxygen index of more than 30 are mixed into the fibers to make the filter material have conductive properties in the longitudinal or lateral direction, and the resistance is less than 1*109 ohm. Common conductive fibers are stainless steel fibers and modified (carbonized) chemical fibers. Compared with the former, the conductivity of the latter is stable and reliable. The mixing amount of conductive fiber is about 2% ~ 5% of the basic fiber.
(3) dust flow and friction. When the flow and friction of dust are strong, it will directly wear away the filter bag and reduce the service life. Particles with rough surface and irregular diamond shape have 10 times more wear than spherical particles with smooth surface. Dust particles with particle size of about 90 microns have the largest wear property, while when the particle size decreases to 5 ~ 10 microns, the wear property is very weak. Wear and airflow speed of 2 ~ 30 million power and particle play 1.5 power is proportional to, therefore, the airflow speed and uniformity must be strictly controlled. Among the common dusts, aluminum powder, silicon powder, coke powder, carbon powder and sinter powder belong to high wear dust. Wear resistant filter bag should be used for wear dust.
The wear parts and forms of the filter bag are various. According to experience, the wear of the filter bag is at the lower part, because the upper part of the filter bag has a low filtration rate and a small concentration of gas dust. In order to prevent the wear of the lower part of the filter bag, the design should limit the speed of airflow rising at the lower part of the bag room.
For dust with strong abrasion, three points should be paid attention to when selecting filter bag:
(1) chemical fiber is better than glass fiber, expanded glass fiber is better than general glass, thin, short, curly fiber is better than thick, long, smooth fiber.
(2) needling should be used to strengthen the interlock between fibers in felt, satin fabric is the best among fabrics, the raising of fabric surface is also a measure to improve wear resistance, but felt, satin fabric and flapping filter bag will increase the resistance value.
(3) for the common filter surface coating, calendering and other post-treatment can also improve wear resistance. For glass fiber filter bag, silicon oil, graphite, ptfe resin treatment can improve wear resistance and folding. But mulch - coated filter material used for wear - out conditions, the film will wear out prematurely, lose the role of mulch.