1 The principle of selection of filter cloth
The filter cloth is usually made of natural fibers such as cotton and wool or synthetic fibers. Raw materials have different physical and chemical properties, suitable conditions of use or working environment (temperature, humidity, corrosion, …). Therefore, the principle for the correct selection of filter materials for compound mixer baghouses should be to fully understand the physical and chemical characteristics of the dust-laden gas emitted during the production process of the blender mixer, and to carefully compare the technical performance of various fibers. And choose it correctly.
2 The correct choice of filter cloth
The correct selection of filter cloths includes: selection according to the physical and chemical characteristics of dust-laden gas; selection according to the characteristics of dust; selection according to the dust removal method of bag filter and so on.
2.1 According to the physical and chemical characteristics of the dust gas selection filter cloth
The physical and chemical characteristics of dust-laden gas include temperature, humidity, corrosion, flammability and explosiveness.
2.1.1 Temperature of dust gas
The temperature of the dust-laden gas discharged from the blender is the primary factor for the correct selection of filter cloths for the bag filter. According to the continuous use temperature, the filter cloth can be divided into three categories: normal temperature filter cloth (less than 130 °C), medium temperature filter cloth (130 ~ 200 °C) and high temperature filter cloth (more than 200 °C). For work conditions where the temperature of the dust-laden gas fluctuates greatly, the safety factor should be selected to be slightly larger, but the instantaneous peak temperature must not exceed the upper limit temperature of the filter cloth. For high-temperature dust-laden gas discharged from a blender, a high-temperature filter cloth can be directly used.
2.1.2 Humidity of Dust-laden Gases
The humidity of the dust-laden gas is another important factor in the correct choice of filter for the mix-mixer bag filter. The humidity of dust-containing gas indicates the degree of water vapor in the gas, which is usually expressed as the volume fraction of water vapor Xw or relative humidity ψ in the dust-laden gas. When Xw is greater than 8%, or ψ is less than 80%, it is referred to as wet dust gas. For wet dust gas, the following points should be noted when selecting the filter cloth:
1) Wet dust-containing gas wets and binds dust trapped on the surface of the filter bag, especially for water-absorbing, deliquescent dust, and even causes paste bags. For this purpose, filter cloths with smooth surfaces such as nylon and glass fibers, and easy-to-clean ash for fiber materials should be used, and the filter cloths should be impregnated with silicone oil or fluorocarbon resin, or acrylic cloth, PTFE or the like should be used on the surface of the filter cloths. Coating treatment.
2) When the flue gas discharged from the mix mixer has both high temperature and high humidity, it will affect the temperature resistance of the filter cloth, especially for materials with poor hydrolytic stability such as polyamides, polyesters, and imides. Therefore, the filter cloth material should be carefully selected during the maintenance of bag filter (replacement filter bag) of the mix mixer, and the moisture content (≤ 3%) of the cold aggregate should be strictly controlled during the production process of the mix mixer.
2.1.3 Corrosiveness of Dust-laden Gases
The chemical resistance of different fibers varies and is often influenced by various factors such as temperature and humidity. In contrast, polytetrafluoroethylene fibers, known as “plastic kings,” have the best chemical resistance but are more expensive. Therefore, when selecting the filter cloth, the main factors must be grasped according to the chemical composition of the dust-containing gas and the appropriate material must be selected.
2.2 According to the characteristics of dust filter cloth
The properties of dust, including the chemical and physical properties of dust, are now focused on the proper selection of materials, structures, and post-processing of filter cloths from the physical analysis of dust.
2.2.1 Dust Shape and Size Distribution
. Generally, the high-temperature combustion process produces mostly regular dust, and most of the dust particles produced during the process are mostly irregular. The regular-shaped dust has a smooth, smooth surface and a small specific surface area. It is not easily intercepted and agglomerated when it is filtered. Conversely, irregular-shaped dusts have different shapes, rough surfaces and large specific surface areas, and are easily intercepted and agglomerated when they are filtered. The following points should be taken into consideration when using filter cloths for bag filter:
1) The fiber should be fine, short, curled, irregular cross-section;
2) The structure is excellent with needle felt, if the fabric is suitable for twill weave or surface treatment;
3) Needle felt with density gradient should be used when mixing thick and thin cotton layers, and new techniques such as surface spraying, dipping, or laminating should be adopted to realize surface filtration as the development direction of ultrafine dust filter cloth;
4) Fine-grained dust is more difficult to capture and tends to form a denser dust layer after trapping, which is not conducive to cleaning. On the contrary, coarse-grained dust is easily trapped and the dust layer formed after trapping is relatively loose, which is beneficial to cleaning. In a sense, the thick and thin dust is beneficial to filtration and cleaning.
2.2.2 Adhesion and Cohesion of Dust
Dust has the property of adhering to each other or adhering to the surface of other objects, and when suspended dusts contact each other, they agglomerate together. The cohesion of dust is related to many factors such as its type, shape, particle size distribution, water content, and surface characteristics. Together, the angle of repose can be used to characterize the cohesion of dust. For example, an angle of repose less than 30° is referred to as low adhesion and good fluidity; a repose angle greater than 45° is referred to as high adhesion and poor fluidity. In addition, the viscosity (adhesion and cohesion) of the dust and solid surface is related to the roughness and cleanliness of the solid surface.
2.2.3 Hygroscopicity and deliquescence of dust
The ability of dust to absorb moisture in gas is called hygroscopicity. If it is based on water, it is called the infiltration of water by dust. Hygroscopicity and wettability are achieved by the action of capillaries formed between dust particles. Hygroscopicity and wettability are related to the atomic chain of the dust, the surface state, and the surface tension of the liquid. In combination, they can be characterized by wet corners—hydrophilicity of less than 60° and hydrophobicity of greater than 90°. When the humidity of the hygroscopic dust increases, the cohesive force and adhesion force of the powder increase, and the fluidity and chargeability decrease, which causes the dust to adhere to the surface of the filter bag. As time passes, the cleaning dust will fail and the dust will become a plate.
In the dust discharged from the mixer, for example, CaO, CaCl2, KCL, MgCl2, etc. will undergo further chemical reaction after moisture absorption, and their nature and morphology will change accordingly. This is deliquescence. They stick to the surface of the filter bag, which is the most undesirable phenomenon for the bag filter.
For moisture-absorbing and deliquescent dust, the items to be noted when selecting filter cloths are the same as “selecting filter cloths according to the characteristics of dust-laden gas”.
2.2.4 Abrasiveness of dust
When the filter cloth is filtering, intercepting, and agglomerating dust, the destructive property of the dust (especially irregular dust) on the filter cloth is called grinding property. It is related to the nature and form of the dust, the speed of the airstream carrying the dust, the concentration of the dust, and other factors. For example: aluminum powder, silicon powder, carbon powder, sinter powder and other materials are hard, it is a high-abrasive dust; particle surface rough, irregular sharp dust than the smooth surface, spherical particle dust grinding more than a lot of times (about 10 or so); the dust particle size of about 90μm is the highest, while the dust particle size is reduced to 5~10μm, the grinding property is very weak; the grinding property of the dust and the airflow velocity It is proportional to the power of ~3.5. Therefore, in order to reduce the abrasiveness of the dust on the filter cloth, the speed and the uniform velocity of the exhaust gas flow of the compound material mixer must be strictly controlled. In addition, a filter cloth with good abrasion resistance should be used for dust with high abrasiveness.
In view of the abrasiveness of dust, the following points can be used when selecting a filter cloth:
1) Chemical fiber is superior to glass fiber, and extruded fiber is better than general glass fiber;
2) Fine, short, and crimped fibers are superior to coarse, long, and straight fibers;
3) The felt material is better than the fabric. The needle-punching method is recommended to strengthen the interfabricating property between the fibers. The satin fabric is the best in the fabric. The brushing on the surface of the fabric can improve the wear resistance.
4) Surface coating, calendering, and other post-treatments can improve the abrasion resistance of the filter cloth. For glass fiber filter cloths, the treatment of silicone oil, graphite, and polytetrafluoroethylene resin can improve its wear resistance and folding endurance.
2.3 Select filter cloth according to the cleaning method of bag filter
The cleaning method of the bag filter is another important factor that must be taken into consideration when selecting the filter cloth correctly. That is, baghouse dust collectors with different cleaning methods should use different structures because of the different cleaning energy and deformation characteristics of the filter bag. Variety of filter cloths.
2.3.1 Mechanical vibration cleaning method
Baghouse dust collectors using mechanical vibration cleaning methods use mechanical devices (including manual, electromagnetic vibration, and pneumatic) to cause the filter bags to vibrate at frequencies ranging from several to several hundred times per second. This type of bag filter is mostly used in the form of an inner filter round bag in addition to the small mix mixer. It is characterized by less kinetic energy applied to the dust layer and a greater number of functions. The filter cloth is required to be thin and smooth, and the texture is soft, which is favorable for the transmission of vibration waves so that sufficient vibration force can be formed on all the filter surfaces. Therefore, the blender bag type dust collector usually uses satin or twill weave made of chemical fiber staple fiber, the thickness is 0.3~0.7mm, the mass per unit area is 300~350g/m2, and the recommended filter speed is 0.6~ 1.2m/min (small-scale mixing machine can be properly increased to 1.5 ~ 2. 0m/min).
2.3.2 Chamber-backflushing method
Chamber bag-type dust collectors with chamber-backflushing method adopt a sub-chamber structure and the valves switch from room to room to form a reverse air flow, forcing the filter bags to contract or bulge and clear the dust. This kind of cleaning method also belongs to the low kinetic energy type of cleaning, and the working pressure of the bag filter is used as the cleaning power. In special situations, the regenerative air flow power is not provided. The baghouse dust collector with chamber backflushing method requires the use of a thin filter cloth with soft texture, easy deformation, and dimensional stability. Since the bag filter is divided into inner filter and outer filter, the choice of filter cloth is slightly different. . In general, the inner filter type commonly used round bags, no frame, bag diameter of 120 ~ 300mm, bag length and bag diameter ratio of 15 ~ 40, preferred Satin (or twill) woven filter cloth, thick 1. 0 ~1.5mm, mass per unit area is 300~400g/m2; external filter type commonly used flat bags, diamond bags and honeycomb bags, with support frame, preferred wear thin, high permeability breathable thin needle felt filter cloth, The unit area is 350~400g/m2.
2.3.3 Vibration Blowback and Dust Removal
The baghouse dust collector with vibratory back-flushing and dust-removing method refers to a baghouse dust collector that has both the function of vibration and countercurrent air cleaning. The vibration of the baghouse removes the dust cake and reverses the flow of air to separate the dust. The two methods cooperate with each other to improve the cleaning effect and are particularly suitable for the filtration of fine-grained viscous dust. The choice of filter cloths for bag dust collection is generally the same as that of baghouse dust collectors with chamber backflushing.
2.3.4 Nozzle Blowback Cleaning
The bag-type dust collector of the nozzle-reverse-blowing cleaning method utilizes a high-pressure blower or a blower as a back blowing cleaning power, and injects the filter bag in sequence by moving the nozzle to form a strong reverse air flow, so that the filter bag is rapidly deformed and dust-removed. A medium energy cleaning type. According to the shape of the nozzle and its movement trajectory, it can be divided into three types: rotary backflushing, reciprocating backflushing, and air ring sliding backflushing.
The bag-type dust collector of the rotary blow-back type and the reciprocating back-flushing type dust-removing method adopts the form of an outer flat bag with a frame and has a compact structure. This type of bag filter requires a relatively soft, stable structure, and good abrasion resistance. The medium-thickness needle felt filter is preferred. The mass per unit area is 350-500 g/m2. In China, a cylindrical satin woven filter cloth is used more.
The baghouse dust collector with air ring sliding back flushing method adopts the inner filter round bag, the nozzle is a ring-shaped slot, and the outer sleeve moves upwards and downwards around the round bag, requiring thick, wear-resistant, rigid, and lint-free filter. The cloth is preferably made of wool compressed felt, and synthetic fiber needle felt can also be used. The mass per unit area is 600~800g/m2.
2.3.5 Pulse Jet Cleaning Method
The pulse jet cleaning type bag filter is powered by compressed air. The pulse jet mechanism is used to instantaneously release the compressed air flow, which induces multiple times of secondary air to enter the filter bag at high speed, causing the filter bag to expand rapidly. * Impact vibration and reverse gas and dust removal, is a type of high kinetic energy cleaning. An outer filter round or flat bag with a frame is usually used. Bag type dust collectors of this type require the use of thick wear-resistant and strong-resistance filter cloths, preferably using chemical fiber needle felt or compressed felt filter cloths, and the mass per unit area is 500 to 650 g/m2.