Key factors to consider when selecting filter media (bags) for bag dust removal equipment

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Currently, whether it is bag filter equipment manufacturers or end-users, when selecting filter media, they all request filter media manufacturers to provide specific technical parameters for the media. Among the required technical parameters, emphasis is often placed on: 1. The weight of the filter media (mass per unit area). Since the filter media are textile products, there is inevitably a deviation between the measured values and the nominal values. According to the **standards, the mass per unit area of needle-punched felt filter media may allow a deviation of +/-5%. But some users do this in order to prevent filter material suppliers from cutting corners (are there other reasons as well? ) Manufacturers of filter bags are often asked to meet requirements such as the weight of the filter media having to be greater than or equal to 550 g/m2. If the filter media manufacturer agrees to meet these requirements, it means that the weight of the filter media during production must be controlled at 580 g/m2; in other words, the nominal value for the filter media should be 580 grams, and not 550 grams. 2. Thickness or density of the filter media ； The national standard also allows a deviation of +/- 10% for the filter media thickness. 3. Air permeability of the filter media ； Note: The density and air permeability of the filter media are related to each other. High-quality acupuncture equipment and techniques are a prerequisite for ensuring the production of high-quality filter media. If the filter media is not dense enough, its air permeability may be high, but its filtration efficiency and ash-removal performance will deteriorate. 4. Strength and elongation rate of the filter media. 5. Temperature resistance of the filter media (temperature allowed for continuous operation and maximum instantaneous operating temperature it can withstand). As a filter media manufacturer, it is the responsibility to determine, based on the expected service life of the filter bags requested by the users (such as 30,000 operating hours or 45 months), as well as the heat and chemical resistance (oxidation and hydrolysis resistance) of the fiber raw materials, the maximum allowable continuous operating temperature and the maximum instantaneous operating temperature for the filter media to be used. If the actual flue gas temperature often exceeds the maximum continuous operating temperature allowed by the filter media, then the user must consider taking measures in the system to improve the heat exchange efficiency of system components such as air preheaters, thereby reducing the flue gas temperature ； Or reduce the required service life of the filter bags (for example, waive the requirement of 30,000 operating hours). It is also reasonable to specify a maximum instantaneous operating temperature for the filter media, as the system can indeed experience transient over-temperature conditions (over-temperature refers to situations where the temperature exceeds the maximum continuous operating temperature). Two points must also be noted here: first, the maximum allowable instantaneous operating temperature of the filter media must not be exceeded ； Secondly, overheating is instantaneous, and there are time-related regulations; generally, it is specified how many minutes of overheating are allowed each time, and the total amount of overheating time per year cannot exceed a certain number of hours. 6. Thermal shrinkage rate of the filter media. Since most filter media are made of synthetic fibers, they undergo thermal shrinkage under certain temperature conditions; therefore, heat setting treatment is necessary during the production of filter media. The heat setting process is crucial and is often kept as a technical secret by filter media manufacturers. Both domestically and internationally, there have been cases where thermal contraction of the filter media caused the filter bags to protrude above the top plate (a situation in which the top of the bag cage was not secured), or led to the bottom of the dust removal filters being punctured. However, filter bags, especially those 8–9 meters long, experience a downward pulling force on them after being hung on the frame due to their own weight plus the weight of the dust accumulated on them; therefore, the thermal shrinkage and elongation rates of the filter material are also factors that need to be taken into account. Excessive heat setting, or overly strict requirements for a low heat shrinkage rate, can also cause the filter bag to stretch during use, thereby reducing its actual service life. In fact, the functional factors or properties considered when selecting filter media are not limited to the quality and technical requirements such as the durability (or expected service life) of these filters mentioned above. As a material for filtering dust particles, the functionality (performance) of the filter media must meet the following requirements: (1) Technical specifications regarding the filtration efficiency of the filter media ； (2) Technical specifications for the de-ashing performance of the filter media. When selecting filter media, users often require that dust emission levels be kept below 30–50 mg/Nm3, and that the operating resistance of the dust collector be less than 1000–1200 Pa. However, there are no specific criteria or indicators available to determine whether the chosen filter media can meet these requirements. In fact, as good manufacturers of filter media, they pay close attention to the filtration efficiency and ash-removal performance of these specific filter media during their research and development efforts. The reason why there are various surface-treated filter media available on the market today is the result of filter media manufacturers’ continuous focus on the filtration efficiency and ash-removal performance of such media. For the many users of bag dust collectors for flue gas purification in coal-fired boilers in domestic power plants, the most pressing issue at present is the service life of the filter bags. There are mainly two factors that affect the service life of dust removal bags: one is the thermal and chemical properties of the flue gas ； The second is the mechanical wear of the filter bags during filtration and cleaning processes. If the thermal and chemical properties of the flue gas have a significant impact on the choice of synthetic fiber filter media, such as oxidation of PPS filters, acid dew formation, and operation at extremely high temperatures, the filter bags may fail very quickly. At this point, even if the functional properties of the filter media itself (filtration efficiency and desulfurization performance) are of high quality, its true performance value cannot be realized. This is determined by the PPS fiber itself or its inherent resistance to heat and chemical corrosion. Faced with harsh flue gas, the fate of every filter media manufacturer is the same. However, when the filter media operates under acceptable flue gas conditions, its performance quality will determine the actual service life of the filter bags. The functional (performance) quality of filter media mainly refers to filtration efficiency and desulfurization performance. Good filtration efficiency of the filter media means that dust can be effectively trapped on the surface of the media, with little penetration into its interior. This not only reduces the wear on the fibers of the filter media caused by dust particles during their movement, but also allows for the maximum removal of dust from the surface of the media ； It boasts excellent de-ashing performance; the operating resistance caused by the accumulation of dust layers on the filter media is low, and the cycle before it rises back to the set de-ashing pressure after a single cleaning operation is **extended** ； The longer the dust cleaning cycle, the fewer times the filter bags need to be cleaned, which in turn reduces the frequency of mechanical damage to the filter bags and extends their service life. It should be specifically noted here that at present, many bag filter users in China do not opt for \"constant-pressure\" dust cleaning, but rather use \"fixed-time\" dust cleaning more frequently. The drawback of timed dust cleaning is that it cleans the filter bags when the operating pressure difference across them may not be high yet, which increases the likelihood and frequency of mechanical damage to the filter bags.