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    How to Select PTFE Filter Bag
     Oct 15, 2020|View:372

    The PTFE filter bag is the key part of the bag filter, and the temperature of dust gas plays a key role in the selection of the PTFE filter bag. Because the temperature resistance value of PTFE filter bag is limited, for example, the temperature resistance value of glass fiber is 280 ℃, sometimes cooling measures must be taken to reduce the temperature of dust bearing gas below the temperature resistance value of PTFE filter bag. The main cooling methods are as follows:

    1. Use a humidification tower or flue to spray water directly to cool down. For bag type dust collector with glass fiber filter material, the temperature can be reduced to below 260 ℃. However, if the temperature drops much more, the flue gas flow will be much smaller, which is conducive to reducing the filtering wind speed and prolonging the service life of the PTFE filter bag.

    2. Use the cooler to cool down. In order to improve the efficiency of the cooler, an axial flow fan can be used to blow the cooler to speed up the cold and heat exchange. When the air volume is too large and the temperature is very high, this method will consume a lot of electricity and is not economical.

    3. Mixing cold air to reduce the temperature of dust containing gas to below the temperature resistance value of filter bag by mixing cold air. This method is simple with less investment. Due to the increase in air volume, larger specifications should be considered in the selection of bag filters.

    PTFE Filter Bag

    The humidity of gas has a great influence on the normal use of bag filters and the service life of the PTFE filter bag. The humidity of gas indicates how much water vapor is contained in the gas. Generally, there are two ways to express it: A. humidity, the mass of water vapor contained in unit mass or unit volume of gas. B. Relative humidity, the mass ratio of water vapor in unit volume gas to that in saturated state at the same temperature and pressure. When the relative humidity is between 30% and 80%, it is in a general state, higher than 80% is high humidity, and less than 30% is an abnormal dry state. Under the condition of high humidity, water film may be formed on the surface of dust particles to increase the adhesion, which is conducive to dust capture, but it is difficult to remove dust. The dust-containing gas is easy to generate static electricity when it is in an abnormal dry state, and it is not easy to leak electricity due to its high insulation. It is easy to form high voltage electrification, which is easy to adhere to and difficult to remove dust. Especially for high temperature and high humidity gas, the phenomenon of condensation and leakage may occur due to the cooling effect of external gas, which makes the captured dust scale on the surface of the PTFE filter bag, that is, the paste bag, which seriously affects the filtering function and service life of the PTFE filter bag, and also corrodes the internal materials of the dust collector. Therefore, it is necessary to control the humidity of the gas to ensure the normal use of the dust collector and prolong the service life of the PTFE filter bag.

    At present, the filter materials used in the bag filter mainly include weaving filter material, nonwoven filter material, and composite filter material. Woven filter material is made of twisted warp and weft yarn or single thread. Through the gap between the warp and weft lines, the dust-containing gas is filtered and separated, and the porosity of the filter material is generally 30% ~ 40%. The weaving filter material has high strength and strong wear resistance, but the resistance of the filter material itself is large. The surface filtration can be realized after covering the membrane, which can improve the purification efficiency and reduce the working resistance of the PTFE filter bag. Nonwovens filter media are directly screened without the usual spinning process and fixed together by mechanical, chemical, or other methods. Composite filter material is to give full play to the advantages of materials, using two or more kinds of materials with their own characteristics, processed into filter materials.

     

    For the weaving filter material, in the initial stage, the dust particles bridge into the hole of the cloth to form a dust layer, and then the dust collection efficiency is increased. Because the pore of the weaving filter material is three-dimensional, the dust particles can not only adhere to the fiber to form a dust layer but also enter the interior of the filter cloth. During this period, the dust collection efficiency is 50% ~ 80%. After the formation of the dust layer, the dust layer plays the role of dust collection, and the dust collection efficiency is above 99%. The structure of the dust layer changes with time. After half a year or a year, even if it is the same amount of dust layer, the resistance loss will increase due to the blockage of the hole. When the resistance loss increases to a large extent, it is necessary to replace the PTFE filter bag. The microporous membrane covered on the surface of the membrane filter material replaces the dust layer, and because of the small pore size of the microporous membrane, it is difficult for dust particles to enter the filter material and block the internal pores, so the resistance of long-term use is lower than that of the uncoated filter material.

     

    For non-woven filter materials, most of the filter materials are needle punched felt, which can be divided into two types: base cloth and non-base cloth. Increasing the base cloth can improve strength. The pre-woven base cloth is placed between the upper and lower fiber webs, reinforced by pre needling and main needling, and then after necessary post-treatment, it becomes the needled felt filter material. The fibers in the needle felt filter material are arranged in a three-dimensional staggered manner, which is conducive to the formation of the dust layer. There is no direct gap, and the dust catching rate is higher than that of ordinary woven filter materials. Needle felt filter material has no or only a small amount of base cloth, so the porosity is generally in the range of 70% ~ 80%, with good air permeability and low resistance.

     

    For the composite filter material, because it is made of two or more kinds of materials with different characteristics, this kind of filter material basically maintains the good properties of various materials. For example, the composite filter material made by covering the surface of the needle felt filter material or woven filter material with a microporous membrane can realize surface filtration and improve the capture rate. Moreover, because the dust only adheres to the surface of the PTFE filter bag, it is easy to peel off. The resistance of the membrane-covered filter material is increased compared with that of the non-coated filter material. However, after the dust collector is operated, it has good peeling property, easy dust removal, and the dust particles are not easy to block the hole of the filter material, so the pressure loss is low and the growth is slow in the normal service period, and the service life is longer than that of the non coated filter material. Due to the dust particles blocking the holes of the filter material, the resistance of the PTFE filter bag increases with the increase of working time. The resistance of the PTFE filter bag in the later service life is very large, which is uneconomical from the point of view of power consumption and treatment effect. At this time, a new PTFE filter bag should be replaced.

     

    At present, glass fiber filter material is mostly used to treat high-temperature flue gas. Glass fiber filter material is drawn from glass liquid. It is an inorganic non-metallic material with high tensile strength, low relative elongation, good high-temperature resistance, and chemical stability. At 280 ℃, the shrinkage rate of glass fiber filter material is 0, and it can operate stably at 260 ℃. It is an ideal filter material for treating high temperature and high humidity flue gas. At present, the more commonly used glass fiber filter materials are glass fiber open width filter material, glass fiber bulky yarn filter material, glass fiber needle felt filter material. Glass fiber open width filter material is a kind of fabric made of warp and weft yarns, which is the leading product in the domestic market. Glass fiber bulky yarn filter material is woven by adding glass fiber bulky yarn into the weft of traditional filter material. Due to its bulky yarn and strong covering capacity, compared with glass fiber open width filter material, under the same capacity, the filtration wind speed can be increased by 1 / 3, and the system running resistance can be reduced by 1 / 4, It can collect particles with a particle size of about 1 um, and the dust collection rate is more than 99.5%. At present, the amount of particles in foreign countries accounts for about 70% of the total amount of glass fiber filter materials. Glass fiber needle felt filter material has a three-dimensional microporous structure, high porosity, and low resistance to gas filtration. It is a high-temperature filter material with high air cloth and high dust collection rate. It can be used in a high-speed pulse bag filter. According to the pressure of the bag filter system and the requirements of the dust removal effect, different filter materials are selected. In general, glass fiber open width filter material or glass fiber bulky yarn filter material is selected for reverse blow bag filter, and glass fiber needle felt filter material is selected for pulse bag filter. According to the filtering wind speed, different thickness of filter media is selected. The high wind speed requires the thickness of the filter material. Too high filtering wind speed has a negative impact on the service life of the filter material, dust collection efficiency, dust removal effect, and system operation resistance.


    The selection of filter materials should consider the dust concentration of flue gas, the fineness of dust, the temperature, moisture content, emission requirements, and the strength of filter materials. At present, high-temperature resistant filter materials mainly include ordinary glass fiber filter material, membrane-covered glass fiber filter material, PPS filter material, P84 filter material, PTFE fiber filter material, and high-temperature composite material filter material. Using ordinary glass fiber filter material, the filtration wind speed is low, the same air volume, dust collector volume is much larger, steel is also used more, dust collector cost is high, the service life of PTFE filter bag is generally about 1 year. The price of the PTFE filter bag is high when membrane filter material and P84 filter material are selected. The price of a PTFE filter bag often exceeds the cost of a dust collector body, and the service life is relatively long, generally 2-3 years. In order not to affect the normal production of the equipment and discharge up to the standard of environmental protection as far as possible, the PTFE filter bags of domestic high-temperature bag filters mostly use coated filter material and P84 filter material.

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