1. the principle of selection and precautions
Bag filter is generally based on the nature of the dust gas, the nature of the dust, and the dust removal method of the dust collector. When selecting the filter, the following principles should be followed:
(1) The filter material properties should meet the general conditions and special requirements of the production conditions and dust removal process.
(2) Under the above premise, filter media with a long service life should be selected as much as possible. This is because the long service life can not only save operating costs, but also meet the requirements for long-term gas discharge.
(3) Choose a comprehensive comparison of filter media sorting, you should not use a so-called “good” filter media to adapt to a variety of working conditions.
(4) In the nature of gas, dust properties and cleaning methods, the main influencing factors should be selected to select the filter materials, such as high-temperature gas, flammable dust and so on.
2.according to the nature of dust gas selection
(1) Gas temperature. Dust gas temperature is an important factor in the selection of filter media. Usually, the dust-containing gas less than 130°C is called normal-temperature gas, and the dust-laden gas larger than 130°C is called high-temperature gas. Therefore, the filter media can be divided into two categories: normal-temperature filter media and high-temperature filter media, so it should be based on flue gas. Use a suitable filter for the temperature. Some people called 130-170 °C medium temperature gas, but the filter more than the high temperature type. The temperature resistance of the filter material is “continuous long-term use temperature” and “instantaneous short-term temperature”. “Continuous long-term use temperature” refers to the long-term continuous temperature that the filter material can be applied to, and the filter material should be used at this temperature. “Instantaneous short-term temperature” means the maximum temperature at which the filter material is not allowed to exceed 10 minutes per day. If the time is too long, the filter material will soften and deform.
(2) Air humidity. Dust-laden gas is classified into three types according to relative humidity: when the relative humidity is below 30%, it is a dry gas, relative humidity is 30%-80% is a general state, and relative humidity is more than 80%, it is a high-humidity gas. For high-humidity gases, when they are in a high temperature state, especially when the dust-containing gas contains SO3, the condensation of the gas will cause condensation. This will not only foul the surface of the filter bag, but also will corrode the structural materials. Therefore, special attention must be paid to the following points when selecting the filter media for the moisture-containing gas:
a. Moisture-containing gas wets and binds dust trapped on the surface of the filter bag, especially for water-absorbing, deliquescent, and wet dust. For this purpose, nylon, glass fiber and other smooth, long-fiber, and easy-to-clean filter media should be used, and silicone oil and fluorocarbon resin should be used for impregnating the filter media, or acrylic and polytetrafluoroethylene should be used on the surface of the filter media. Ethylene and other substances are coated. Plastic burning board and film materials have excellent moisture resistance and easy cleaning properties. However, as a high-humidity gas, it should be a water-repellent and oil-repellent filter material.
b. When the high temperature and high humidity are present at the same time, the temperature resistance of the filter medium will be affected, especially for the materials with poor hydrolytic stability such as nylon, polyester, and imide, which should be avoided as much as possible.
c. For the design of the dust-containing filter bag for the moisture-containing gas, a circular filter bag should be used. Try not to use the flat filter bag and the diamond-shaped filter bag (excluding the plastic burning plate) with complex shapes and arrangement.
d. Dust collector dust gas inlet temperature should be higher than the gas dew point temperature of 30 °C or more to avoid paste bags.
(3) The chemical nature of the gas. In a variety of furnaces as 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 selection of filter media for this purpose.
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 dust removal from 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.
3. choose according to the nature of dust
(1) The wettability and adhesion of dust. The wettability and wettability of dust 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 the wet angle autumn. 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 will fail and the dust cake will harden.
Some dusts such as CaO, CaCl2, KCL, MgCL2, NaCO3, etc., undergone further chemical reaction after moisture absorption, and their nature 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 and deliquescent dust, the filter should be smooth, non-raised, and free from water when choosing the filter media. Among them, the plastic filter plate is the best.
Many wet dusts have strong adhesion, and in fact, they are inextricably linked with the adhesion. For bag filter, if the adhesive force is too small, it will lose the ability to capture dust, and excessive adhesion will cause dust aggregation and dust removal.
For strong adhesion dust should also use filament not fluff fabric filter material, or surface singeing, calendering, mirror treatment of needle felt filter material, for impregnation, coating, coating technology should be fully utilized. From the material of filter media, 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 choose flame-retardant filter and conductive filter material.
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 time. The amount of conductive fiber blended is about 2% to 5% of the base fiber.
(3) Dust flow and friction. When dust flow and friction are strong, it will directly wear the filter bag and reduce its service life. 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 is reduced to 5-10 microns, the wearability is very weak. The 2-30000 power of the 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, and sintered ore powder are high-abrasive dust. For wearable dust, filter media with good wear resistance should be used.
Dust filter media wear parts and forms varied, according to experience, the filter bag wear is in the lower part, this is 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 rise in the lower part of the bag room.
For the dust with strong wear, you should pay attention to 3 points:
a. Chemical fiber is superior to glass fiber. Expanded glass fiber is better than general glass. Fine, short, and crimped fibers are superior to coarse, long, and smooth fibers.
b. Felt should be used to strengthen the intertwining of the fibers with acupuncture. The satin fabric in the fabric is optimal, and the brushing on the fabric surface is also a measure to improve the wear resistance, but the felt, satin fabric and Filament filter will increase the resistance value.
c. For the ordinary filter surface coating, calendering and other post-treatment can also improve the wear resistance. For glass fiber filter, silicone oil, graphite, polytetrafluoroethylene resin treatment can improve wear and folding 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.