A Brief Discussion on Bag Filters

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A Brief Discussion on Bag Filters Author: Wang Qingyun, Wang Wen, Zibo Highway Administration Bureau, Shandong Province Abstract: After providing a brief introduction to the functions, structure, and working principle of bag filters used in asphalt mixture mixers, this article focuses on the selection of filter fabrics, as well as the fault symptoms, cause analysis, and troubleshooting methods for bag filters. Keywords: asphalt mixture mixer, bag dust collection, filter cloth, faults. During the production process of an asphalt mixture mixer, processes such as drying, screening, weighing, and mixing generate large amounts of dust as well as harmful exhaust gases from fuel combustion, all of which contribute to environmental pollution. Modern asphalt mixture mixers all use a two-stage dust removal system consisting of cyclone dust collectors and bag filters, in order to capture dust pollutants as much as possible and reduce pollution of the natural environment, thus meeting the requirements of **environmental protection regulations**. Bag filters come in various structural types, including pulse-jet type, long-bag low-pressure pulse type, chambered ejector pulse type, filter-cylinder pulse-jet type, annular gap jet pulse type, flat-bag pulse-jet type, forward-jet pulse type, air-ring reverse blow type, rotary reverse blow type, atmospheric reverse blow type, and air-box pulse type. After decades of production experience, modern asphalt mixture mixers mostly use pulse (compressed air) jet bag or atmospheric back-pressure bag dust collectors. 1 Structure of bag filter: The bag filter (see Figure 1) consists of a housing, baffle plates, bag frames, throat pipes, pipe seat plates, injection pipes, pulse valves, differential pressure gauges, screw conveyors, and a controller. The chamber is filled with hundreds of heat-resistant (>200°C) synthetic fiber bags; it has a working area of 1400 m2, and a maximum exhaust volume of 65,300 Nm3/h (for a 4000-type asphalt mixture mixer). Its mesh structure is very fine, enabling it to capture dust particles larger than 0.3 μm during the filtration process. The dust removal efficiency of bag filters used in modern asphalt mixture mixers with advanced technical performance can reach 97%–99%, enabling the purification of dusty gases to a level of 15 mg/Nm3. Figure 1 Schematic diagram of the bag filter structure 1-Pulse valve ； 2-Blowing tube ； 3- Purify the air ； 4-Base plate ； 5-Laryngeal tube ； 6-Filter bag ； 7-Bag Skeleton ； 8- Baffle plates ； 9- Screw conveyor ； 10-Differential pressure gauge ； 11- During operation of the controller bag filter, the dust-laden gas after the first stage of dust removal enters the filter chamber. Under the action of the baffle plates, this dust-laden gas is dispersed and flows in such a way that it enters the inside of each filter bag from the outside. Through mechanisms such as screening, interception, impact, diffusion, and electrostatic attraction, the fine dust adheres to the gaps in the filter fabric, thereby separating the dust from the flue gas. The clean gas is then discharged into the atmosphere through a fan and exhaust pipes via the chimney. As dust accumulates on the filter bags, a layer of dust of a certain thickness is formed, which gradually reduces the air permeability of the filter fabric and hinders the proper operation of the dust collector. Therefore, during operation, bag filters must have the dust accumulated on their filter bags removed regularly and promptly. Currently, pulsed compressed air is commonly used for this purpose – a controller operates the pulse valves to inject small amounts of high-pressure compressed air in a timed, intermittent manner opposite to the direction of the flue gas above the filter bags, causing the filter bags to vibrate and shake, thereby knocking the dust off them into the lower part of the chamber. From there, the dust is transported by a screw conveyor to the limestone storage bin or the inlet of the hot aggregate elevator. To ensure the filtering function of the filter bags, a certain pressure difference must be maintained between the dust-containing gas and the purified gas after filtration. A differential pressure gauge is installed on the bag filter; if the pressure difference inside the chamber is too high, it indicates that too much dust has accumulated on the filter bags, resulting in high filtering resistance ； Conversely, it indicates that the filter bag is damaged or leaking and should be replaced. 2 Filter bag structure The filter bag (see Figure 2) consists of three parts: the bag mouth, the bag body, and the bag bottom. To achieve dust filtration, meet the corresponding dust removal requirements, ensure easy installation and reliable sealing, facilitate dust removal while minimizing damage, it is necessary to choose appropriate filter media, a well-designed structure with precise stitching, and proper assembly with related components (see Figure 3). For example: the external filtration type synthetic fiber filter bags should fit loosely with the frame ； The external filtration glass fiber filter bags should fit closely with the vertically arranged rebar frame at small intervals ； For internal filtration filter bags, the filter bag tension should be considered and set. Figure 2 Shape of filter bag a) External filtration type ； b) Internal filtration type. Figure 3 shows the modular bag dust collection system. For pulse-jet and box-type pulse bag dust collectors, the structural designs for the top and bottom of the bag filters include: 1) Spring ring top. A spring ring of a certain width is sewn at the opening of the bag, allowing it to be easily fitted over the opening in the ceiling grid, thereby significantly improving the sealing effect. 2) Flange top. An annular filter cloth is sewn at the opening of the bag to provide sealing between the ceiling plate and the slide frame. 3) Ring top. There is a metal ring at the mouth of the bag, sewn onto the flange edge. 4) Sleeve top. A separate filter cloth is sewn at the opening of the bag; when it is inserted into the cage frame, this filter cloth is folded into the frame, which helps to prevent dust leakage caused by folds in the bag itself. 5) Hem top. It can effectively prevent the opening of woven filter bags from coming loose during sewing. The structural types of the bag openings and bag bottoms for back-pulse and rapping filter bags include bag bottoms with rope holes, sling square-ring bag bottoms, adjustable sling bag bottoms, elastic ring bag bottoms, soft-core clamped bag openings, and spring-clamped bag openings with double-sided grooved seals. 3 Selection of filter media: Filter media are usually made from natural fibers such as cotton and wool, or synthetic fibers. Since raw materials have different physical and chemical properties, it is essential to fully understand the physical and chemical characteristics of the dust-containing gas when selecting filter media. By carefully comparing the properties of various fibers, an ideal dust removal effect can be achieved. The temperature of the dusty gas emitted by the asphalt mixture mixer is the primary factor in selecting the appropriate filter media for a bag filter. Based on the temperature at which they are used continuously, filter media are classified into three categories: normal-temperature filter media (≤130°C), medium-temperature filter media (130–200°C), and high-temperature filter media (≥200°C). In operating conditions with large fluctuations in gas temperature, it is advisable to choose a slightly higher safety factor, but the instantaneous temperature must not exceed the upper limit of the operating temperature for the filter media. For the high-temperature flue gas emitted by asphalt mixture mixers, high-temperature filter media should be used directly. The humidity of dusty gases is another important factor in selecting the appropriate filter media for bag filters in asphalt mixture mixers. The humidity of dust-containing gases indicates the amount of water vapor present in those gases, and it is usually expressed as the volume percentage Xw of water vapor in the dust-containing gases or as the relative humidity φ. When Xw>8% or φ>80%, it is referred to as a wet dusty gas. For wet dusty gases, the following points should be considered when selecting filter media (including designing dust removal devices): 1) Wet dusty gases cause the dust trapped on the surface of the filter bags to become wet and sticky; this is especially true for dusts that are hygroscopic or deliquescent, and it can even lead to the clogging of the filter bags. To this end, filter media with a smooth surface such as nylon or glass fiber, whose fibrous structure makes it easy to remove dust, should be used. Additionally, the filter media can be impregnated with silicone oil or fluorocarbon resins, or their surfaces can be coated with acrylics or polytetrafluoroethylene. 2) When the flue gas emitted by asphalt mixture mixers is characterized by both high temperature and high humidity, it affects the heat resistance of the filter media, especially for materials with poor hydrolytic stability such as polyamides, polyesters, and imides. Therefore, when maintaining the bag filter of the asphalt mixture mixer (replacing the filter bags), care should be taken in selecting the filter fabric material, and the moisture content of the cold aggregates must be strictly controlled during the production process of the asphalt mixture mixer (≤3%). 3) For wet dusty gases, the filter bags should be cylindrical in shape; flat filter bags with complex shapes and a very tight arrangement should be avoided as much as possible. Furthermore, the filter media should also be selected based on the physical properties of the dust (shape and particle size distribution, adhesiveness and cohesiveness, hygroscopicity and deliquescence, abrasiveness, etc.) as well as the dust removal method used in the dust collector (mechanical vibration type, chamber reverse blowing type, combined vibration and reverse blowing type, nozzle reverse blowing type, pulse jet type, etc.); details are omitted here due to space constraints. 4 Common faults of bag filters: The common fault symptoms, cause analyses, and troubleshooting methods for bag filters used in asphalt mixture mixers are shown in the table below. Common faults in bag dust removal: Fault symptoms, cause analysis, and solutions. Sudden increase in dust emission concentration: The filter bags are damaged and leaking air; replace the damaged bags. Rapid increase in negative pressure during dust discharge, with values that are too high (≥200 KPa): Excessive moisture content in the cold material, with excess water vapor sticking to the filter bags and sticking to the dust in the form of sheets or clumps. Poor performance of the dust removal motor: Low quality diesel, or improper adjustment of the burner’s air valves and fuel supply; reduce the moisture content in the cold material or the amount of material fed in, and increase the heating temperature. Check the motor’s speed or repair it. Increase the air flow and reduce the fuel supply to ensure complete combustion of the diesel. Difficulty in discharging dust: The shaft is stuck to the bearings (bushings), preventing the dust discharge mechanism from rotating; replace the bearings (bushings). (End)