The selection of industrial baghouse filter bags is of utmost importance. An unsuitable filter bag not only affects the dust removal effect of the baghouse machine, but also reduces the service life of the dust collector. The selection of dust filter bags should be comprehensively considered based on seven factors: the particle size of the filtered material, the filtration velocity, the concentration of dust, the dust cleaning method, the temperature, humidity, and chemical properties of the flue gas.
Dust filter bags are made of various high-precision, wear-resistant, high-temperature-resistant, and corrosion-resistant filter materials, which are made from high-performance fibers and high-strength base fabrics. These filter materials are produced through multiple processes such as needle punching, calendering, singeing, high-temperature setting, emulsion impregnation, and film coating, according to actual working conditions. Their ultimate goal is to achieve extreme and efficient dust removal and filtration, as well as longer service life. Generally speaking, the materials of dust filter bags include polyester, acrylic, fluoromethane, PTFE, PPS, and various composite materials. So, how to correctly choose from such a wide range of dust filter bag materials?
Selection Based on Gas Properties
1) Gas temperature
The temperature of dust-containing gas is an important factor to consider when selecting filter media. People usually refer to dust containing-gases below 130 ℃ as normal temperature gases, and those above 130 ℃ as high-temperature gases. Therefore, filter media are also divided into two categories, namely, normal-temperature filter media below 130 ℃ and high-temperature filter media above 130 ℃. Therefore, appropriate filter media should be selected based on the flue gas temperature.
There are two types of temperature resistance of filter media: “continuous long-term use temperature” and “instantaneous short-term temperature”. “Continuous long-term use temperature” refers to the long-term temperature at which filter media can be applied and continuously operated, and filter media should be selected based on this temperature; “Instantaneous short-term temperature” refers to the maximum temperature at which the filter media is not allowed to exceed 10 minutes per day. If the time interval is too long, the filter media will age or soften and deform.
If the temperature is lower than 130 ℃, conventional filter bag materials can be selected, such as polyester. If the temperature is higher than 130 ℃, high-temperature resistant dust removal filter bags need to be selected, such as Flumex and PTFE material dust removal filter bags.
2) Gas Humidity
Dusty gases can be classified into three categories based on the relative humidity: dry gas when the relative humidity is below 30%, normal gas when the relative humidity is between 30% and 80%, and high-humidity gas when the relative humidity is above 80%. For high-humidity gases, especially those containing SO in a high-temperature state, the cooling of the gas can cause condensation. This not only causes fouling and blockage on the surface of filter bags but also corrodes structural materials, so we should pay extra attention on this matter.
When selecting filter materials for moist gas, attention should be paid to the following points:
①Moist gas can wet and bond the dust captured on the surface of the filter bag, causing bag sticking. Therefore, smooth and long-fiber filter materials such as nylon and glass fiber should be selected, which are easy to clean and can be treated with silicone oil, fluorocarbon resin, or coated with substances such as acrylic acid and polytetrafluoroethylene on the surface of the filter material. Plastic burning boards and film-covered materials have excellent moisture resistance and easy-to-clean properties, and should be the preferred options for high-humidity gas.
②If the gas has both high temperature and high humidity characteristics, it may affect the temperature resistance of the filter material. Especially for materials with poor hydrolysis stability such as nylon, polyester, and polyamide, they should be avoided as much as possible.
Polyester fiber has good mechanical properties and acid and alkali resistance at normal temperature, but it is very sensitive to water and air, and is prone to hydrolysis, which significantly reduces its own strength. The long-term operating temperature of polyester fibers in dry flue gas can reach up to 130°C, but in high-humidity flue gas, the long-term operating temperature can only be reduced to 60-80°C. Nomex fibers have good temperature and chemical resistance, but in high-humidity flue gas, their temperature resistance will be reduced from 204°C to 150°C.
3) Chemical Property
In choosing filter materials for various furnace flue gases and chemical waste gases, it is important to consider the multiple chemical components, such as acids, alkalis, oxidizers, and organic solvents, which are often influenced by factors such as temperature and humidity. Therefore, the selection of filter materials must comprehensively consider the main factors based on the chemical composition of the dust-containing gas.
Nomex fibers have better temperature resistance than polyester fibers, but their chemical resistance is slightly weaker under high temperature conditions. Phenolic fibers have excellent properties in high temperature and acid-alkali corrosion resistance, and are suitable for coal-fired flue gas dust removal. However, they have poor antioxidant ability. This weakness can be compensated by using polyimide fibers, but their hydrolytic stability is not ideal. Polytetrafluoroethylene fibers have the best chemical resistance, but they are more expensive.
Selection based on Dust Properties
(1) Hygroscopicity of Dust
In case of hygroscopic dust, the cohesion and adhesion of particles increase with humidity, while the flowability and electrification decrease, leading to adherence to the filter bag surface. Over time, this leads to ineffective cleaning and dust cake formation. Some dust, such as CaO, CaCI, KCl, MgCl, NaaCO undergo chemical reactions after absorbing moisture, resulting in changes in their properties and morphology, known as deliquescence. Deliquescent dust sticks to the filter bag surface, which is most undesirable for bag-type dust collectors.
For wet and deliquescent dust, the selection of filter material should consider the smoothness, non-fuzzing and hydrophobicity of the filter material, with coated filter material being the best choice.
Many highly wet and adhesive dust particles require filter material made of long filament non-fuzzing fabrics or needle-punched felt that has been surface singed, calendered, and mirror-polished. Impregnation, coating, and film-forming techniques should be fully utilized. In terms of material, nylon and fiberglass are superior to other varieties.
(2) Combustibility and Electrification of Dust
Some dust can combust or explode in air at specific concentrations when exposed to sparks. The combustibility of dust is related to its particle size, composition, concentration, heat of combustion, and burning speed. Smaller particle size and larger surface area increase the likelihood of ignition.
Dust combustion or explosion sources are usually caused by friction sparks, static sparks, or incandescent particles, with electrification being the most harmful. This is because synthetic fiber filter materials are usually prone to electrification, and if dust particles are also electrified, sparks are highly likely to occur. Therefore, for combustible and easily electrified dust such as coal powder, coke powder, aluminum oxide powder, and magnesium powder, flame-retardant filter materials and conductive filter materials should be selected.
Fiber woven filter materials with an oxygen index greater than 30, such as PVC, PPS, P84, and PTEF, are generally considered safe. For fibers with an oxygen index less than 30, such as polypropylene, nylon, polyester, and polyamide, flame retardants can be used for impregnation treatment.
Antistatic filter materials refer to filter materials that mix conductive fibers into the filter fibers, making the filter material conductive in the longitudinal or transverse direction. Common conductive fibers include stainless steel fibers and modified (carbon-permeated) chemical fibers. The former has stable and reliable conductivity, while the latter’s conductivity deteriorates over time. The mixing ratio of conductive fibers is about 2% to 5% of the base fibers.
(3) Flowability and Friction of Dust
When dust exhibits high flowability and friction, it can directly wear out filter bags, reducing their service life. Particles with rough and irregular diamond-shaped surfaces have a 10-fold higher abrasiveness compared to smooth and spherical particles. The abrasive nature of dust particles is highest when their size is around 90 microns, while abrasiveness becomes very weak when particle size decreases to 5-10 microns. The abrasive nature is proportional to the 2-3 power of airflow velocity and 1.5 power of particle size. Therefore, airflow velocity and its uniformity must be strictly controlled.
Common dust such as aluminum powder, silicon powder, coke powder, carbon powder, and sintered ore powder belong to highly abrasive dust. For abrasive dust, it is advisable to use filter materials with good wear resistance.
When selecting a filter material for highly abrasive dust, there are three points to consider:
1. Chemical fibers are preferable to glass fibers, and expanded glass fibers are better than regular glass fibers. Fine, short, and curly fibers are superior to coarse, long, and smooth fibers.
2. In felt materials, it is best to use a needle-punching method to strengthen the interlacing of fibers, and satin weave fabrics are the most optimal for woven materials. Brushing the surface of woven materials is also a measure to improve abrasion resistance. However, felt materials, twill weave fabrics, and napped filter materials will increase the resistance value.
3. For common filter materials, surface coating and compression treatment can also improve abrasion resistance. For glass fiber filter materials, treatment with silicone oil, graphite, or polytetrafluoroethylene resin can improve wear and flex resistance. However, when coated filter materials are used in highly abrasive conditions, the coating film may wear out prematurely, losing its coating function.
Dust filter bags are widely used in industries such as steel, thermal power, chemical, building materials, waste incineration, and glass. They not only comprehensively improve the world’s air pollution prevention and control capabilities, but also achieve ultra-clean emissions to meet the major strategic needs of environmental protection. Therefore, careful consideration must be given to the selection of dust filter bags in various industries in order to achieve the desired results of their use.