1. Qualitative high resistance The bag filter is a kind […]
1. Qualitative high resistance
The bag filter is a kind of high-efficiency dust removal and purification equipment that uses filter materials to filter dust-containing gas. It has the characteristics of high dust removal efficiency, simple structure, strong adaptability, and stable operation. It is widely used in metallurgy, thermal power generation, cement manufacturing and other industries. However, in the production process, the running resistance of the equipment is often high, which results in the reduction of dust removal efficiency and shortened filter bag life. When the running resistance of a bag filter exceeds 1500Pa, we can qualitatively say that the running resistance of this filter is on the high side. When the operating resistance of the dust collector exceeds 2000Pa, it will have a greater impact on the ventilation of the entire system.
2. Reasons for high resistance
The running resistance of the dust collector is caused by the body, the filter bag and the dust adsorbed on the filter bag. Structural design differences, filter material material, filter wind speed, gas dust concentration, dust particle size, gas humidity and other factors have a greater impact on the operation resistance of the dust collector.
2.1 The influence of structure design on the running resistance of dust collector
The structural design includes the cross section of the inlet and outlet air ducts, the air equalization device of the large bag filter, the cross-sectional dimensions of each part through which the air flows, and the design of air lock, inspection door sealing, and equipment insulation. Unreasonable structural design will make the equipment running resistance higher. Reasonable structural design has a body resistance of about 400Pa. The author recommends that the wind speed of the inlet and outlet ducts should be less than 15m/s, and the best wind speed is about 10m/s. The wind speed of the inlet and outlet air branch pipes of each bag chamber should be less than 10m/s, which is beneficial to uniform air and reduces resistance.
2.2 Dust concentration
When the dust concentration is too high, the collision probability between particles increases, causing the particles to stick together, which will also affect the running resistance of the dust collector. At the same time, the increase in concentration means that the thickness of the dust attached to the unit area of the filter bag per unit time will increase, and the pressure loss will increase accordingly. In this case, the frequency of cleaning dust is often increased to make the equipment work normally, but this will increase the expansion frequency of the filter bag and increase the friction between the filter bag and the bag cage, which will reduce the life of the filter bag and the filter material China Polypropylene Filter Cloths Supplierswill fail. The failure of the filter material will in turn lead to an increase in the pressure loss of the filter bag and an increase in the resistance of the equipment.
In addition, the positive pressure gas blown back during dust removal is also one of the reasons for the high operating resistance of the bag filter. Frequent injection of positive pressure gas into the bag chamber is bound to increase the operating resistance of the equipment. Therefore, when the dust concentration is too high, a block air equalization device should be added in the air inlet duct and the ash hopper to perform pre-dust collection treatment to reduce the load of the filter bag of the dust collector and reduce the resistance of the equipment.
2.3 Dust size
The main influence of dust particle size on bag filter is manifested in pressure loss and equipment wear. The greatest impact on pressure loss is fine dust, which can block the filter gap, reduce the air permeability of the filter material, and increase the resistance. It is generally believed that needle-like crystal particles and flake-like particles are easy to block the filter cloth, reducing the efficiency of dust removal and increasing the running resistance. For dusty gas with finer particles, membrane filter bags should be used, such as cement kiln tails and slag grinding systems.
The gas often contains a small amount of water vapor. When the relative humidity is between 30% and 80%, it is a normal state, and when the relative humidity exceeds this range, it is a high humidity state. When the air in the dust collector is in a high humidity state, under the action of the external cold air, condensation occurs, causing the dust to stick and block the filter bag, which reduces the dust catching ability of the filter material, and it is difficult to clean the dust. The increase. For condensation, generally try to choose a filter material with a smooth surface, such as a membrane filter material. In addition, heat preservation of equipment and increase the population temperature of dusty gases can reduce the negative effects of cold air and prevent condensation from occurring. Such as cement kiln tail, slag grinding system. In addition to the external insulation of the bag filter, the equipment inspection door cannot be ignored.
2.5 Air permeability of filter material
In general, the pressure loss of clean filter cloth is relatively small, and the air permeability is relatively high. In work, its small pressure loss means that its pores are relatively large, and the dust is easy to penetrate. Under normal circumstances, the dust collection performance is relatively low. In addition, the pressure loss of the dust layer on the surface of the filter bag is one of the factors affecting the pressure loss of the filter bag, which is related to whether the pores of the filter cloth are blocked. Therefore, the filter cloth is required to have the characteristics of not easy to block the pores, high dust collection efficiency, low pressure loss, good dust removal performance, and long life.
2.6 Filter bag filter wind speed
The filtration wind speed is an important technical parameter of the bag filter. When the cleaning method, filter material material, and soot characteristics are different, the designed filter wind speed should also be different, so the filter resistance produced is also different.
For dust in the cement industry, when collecting cement, due to the low humidity and relatively low viscosity of the dust, pulse bag filter is often used, and the filtering wind speed used is generally 0.8~1.0m/min; for drying dust removal or cement rotation When removing dust from the kiln exhaust, due to the relatively high moisture content of the dust, the dust humidity is relatively large: when the glass fiber bag anti-air bag type dust collector is used, the filtering wind speed is generally controlled between 0.3 and 0.5m/min, when the long bag pulse is used In the case of bag filter, the filtering wind speed is generally controlled between 0.7 and 0.9m/min; for the preparation of coal powder in the cement and metallurgical industries, due to the characteristics of large dust concentration, small particles, flammability and explosion, the filtering wind speed is generally controlled Between 0.6 and 0.8m/min; for slag ultrafine grinding, lime kiln lime and other dust collection, due to the small dust particles, high concentration, and certain viscosity, pulse bag filter for slag powder is usually used. The filtration wind speed is between 0.7～0.9m/min.
2.7 Ash cleaning efficiency
When the dust removal efficiency of the dust collector is not high, that is, it is incomplete, the adhesion of excessive dust on the surface of the filter bag will directly lead to the increase of running resistance. Therefore, improving the dust removal efficiency of the dust collector is also one of the guarantee measures for reducing the resistance of the equipment.
In addition to the condensation caused by humidity that we mentioned before, which will reduce the cleaning efficiency, unreasonable spraying is a common cause of low cleaning efficiency. The most obvious performance in this aspect is the long bag pulse spray dust collector. Due to the point-to-point vertical injection of high-pressure gas, during cleaning, the high-speed airflow injected into the filter bag may generate a vertical pressure from the outer surface of the filter bag to the inside of the bag near the mouth of the bag, so that the filter bag will not expand in this area. Can not achieve the purpose of cleaning dust.
When the filter bag is too long, the efficiency of cleaning the dust near the bottom of the filter bag may be low due to insufficient blowing strength or blowing air volume. The reasonable combination of the distance between the nozzle and the flower plate, the size of the nozzle, the length of the filter bag, and the model of the pulse valve can reduce the occurrence of low dust removal efficiency and high operating resistance caused by the injection system.
The impact of system positive pressure and equipment system air leakage pulse pipeline system uses high-pressure gas, which is in a positive pressure state. In order to achieve the effect of cleaning, the compressed air flow injected by the pulse valve must reach a certain amount, which is generally More than 2 times the internal volume of the cleaned filter bag.
In the working state, the equipment cabinet is under negative pressure. When too much positive pressure airflow is injected inward, the front and rear resistance of the equipment will inevitably increase. This effect is equivalent to equipment leakage. Therefore, reasonable selection of injection pressure and injection time can not only extend the service life of the filter bag, but also have a positive effect on reducing the operating resistance of the equipment. This situation is especially manifested when the number of equipment chambers is relatively small. . In actual production operations, some manufacturers blindly increase the cleaning pressure or extend the spraying time in order to blindly pursue the cleaning effect, which is very undesirable.
If the cleaning pressure is too high, it will accelerate the damage of the filter bag, and also affect the running resistance; while prolonging the spraying time, in addition to spraying more positive pressure gas and increasing the equipment running resistance, it will hardly cause any dust cleaning. Positive impact.
The air leakage of the equipment system is to allow the positive pressure gas from the outside to enter the negative pressure environment inside the dust collector, resulting in high equipment operation resistance, which will directly affect the output. This kind of air leakage is caused by static pressure difference. The structural feature of manhole doors, inspection doors, valves, etc. is that the cover plate plus the sealing soft material is supplemented by mechanical compression, and the mechanical force often only acts on a part of the cover plate. If it is too large, it is easy to cause local deformation of the cover plate and form a larger gap; too small is not enough to fill the gap with soft materials. In addition to the sealing force, the improper selection of sealing materials and the aging of the sealing materials are important reasons for system air leakage in actual production.
2.9 Reliability of accessories
Whether the parts that are matched with the dust collector are reasonable and whether they work normally are also possible reasons for the high running resistance of the bag filter. The supporting parts include a combination of electrical control devices, pneumatic components, and corresponding valve actions. For a bag filter, whether a reasonable cleaning sequence, correct type of pulse valve, effective pulse width and blowing pressure are selected, it also has a direct or indirect effect on ensuring the normal operation of the equipment and reducing operating resistance. In addition, whether the pulse valve, the cylinder and the solenoid valve are matched or not, and whether the air circuit components are working normally are directly related to whether the dust cleaning system works normally.
When the ash cleaning system is not working normally, the ash cleaning work cannot be carried out effectively, which will directly lead to high running resistance. In addition to whether the electrical components are working properly, for the offline cleaning bag filter, when the valve plate of the poppet valve falls off, causing the valve plate to cover the air outlet and the corresponding chamber is always offline, the actual number of working units of the equipment Less than the number of design units, that is, part of the chamber is in a non-working state, which will make the equipment running resistance always high. This situation often occurs in actual production.
For a bag filter, whether the design is reasonable and whether the operation is normal or not is mainly reflected in the service life of the filter bag and the resistance of the equipment. In some cases, the damage of the filter bag is related to the high resistance of the equipment. Scientific and reasonable structure design, parameter determination, and selection of accessories are important links for the successful operation of the bag filter. Reasonable, correct and effective management and maintenance of the equipment during operation are the guarantee for the successful operation of the bag filter.