SELECTION OF FILTER MATERIAL FOR FILTER MATERIAL OF BAG FILTER

1. The principle of selection

The filter material of the dust bag filter is generally selected according to the nature of the dusty gas, the nature of the dust and the dust cleaning method of the dust collector. The following principles should be followed when selecting:

⑴ The performance of the filter material should meet the general conditions and special requirements of the production conditions and the dust removal process, such as the temperature of the gas and dust, the pH and whether there is an explosion hazard.

(2) Under the above premise, filter materials with long service life should be selected as much as possible, because long service life can not only save operating costs, but also meet the requirements of long-term gas emission standards.

(3) When selecting filter materials, sort and compare various filter materials, and should not use a so-called "good" filter material to adapt to various working conditions.

⑷ In terms of gas properties, dust properties and dust cleaning methods, the main influencing factors should be grasped to select filter materials, such as high-temperature gas, combustible dust, etc.

⑸ When selecting filter materials, various factors should be compared economically.

2. Select according to the nature of dust gas

⑴ Gas temperature

Dusty gas temperature is an important factor in the selection of filter materials. Dust-containing gas below 130°C is usually called normal temperature gas, and dust-containing gas above 130°C is called high-temperature gas, so the filter material can be divided into two categories, namely, normal temperature filter material below 130°C and filter material above 130°C high temperature filter material. Therefore, the appropriate filter material should be selected according to the flue gas temperature. Some people also refer to 130°C to 200°C as medium-temperature gas, but the filter material is mostly high-temperature type.

The temperature resistance of the filter material has two types: "continuous long-term use temperature" and "instantaneous short-term temperature": "continuous long-term use temperature" refers to the long-term temperature at which the filter material can be applied and continuously operated, and the filter material should be selected at this temperature; "Instant short-term temperature" refers to the high temperature where the filter material is not allowed to exceed 10 minutes per day. If the time is too long, the filter material will age or soften and deform.

⑵ Gas humidity

Dust-containing gas is divided into three states according to relative humidity: when the relative humidity is below 30%, it is dry gas; when the relative humidity is between 30% and 80%, it is normal; when the relative humidity is above 80%, it is high-humidity gas. For high-humidity gas, when it is in a high temperature state, especially when the dusty gas contains SO3, condensation will occur when the gas is cooled. This will not only foul and clog the surface of the filter bag, but also corrode the structural materials, so special attention is required. For wet gas, the following points should be paid attention to when selecting filter materials:

① Moisture gas makes the dust collected on the surface of the filter bag moist and bonded, especially for water-absorbing, deliquescent and moist dust, which will cause the bag to stick. Therefore, nylon and glass fiber should be selected with smooth surface, long fiber, The filter material is easy to clean, and it is advisable to impregnate the filter material with silicone oil and fluorocarbon resin, or use acrylic acid, polytetrafluoroethylene and other substances on the surface of the filter material for treatment.

② When high temperature and high humidity exist at the same time, the temperature resistance of the filter material will be affected, especially for materials with poor hydrolytic stability such as nylon, polyester, and imide, which should be avoided as much as possible.

③ For wet gas, round filter bags should be used in the design of dust removal filter bags, and flat filter bags and diamond-shaped filter bags with complex shapes and compact arrangements should not be used as much as possible.

④ The inlet temperature of the dusty gas in the dust collector should be higher than the dew point temperature of the gas.

(3) The chemical properties of the gas

Exhaust gases from various kilns and chemical industries often contain various chemical components such as acids, alkalis, oxidants, and organic solvents, and are often affected by various factors such as temperature and humidity. Therefore, the choice of filter material should be considered carefully.

Polyester fiber has good mechanical properties and acid and alkali resistance at room temperature, but it is very sensitive to water and air, and is prone to hydrolysis, which greatly reduces its strength. For this reason, the long-term operating temperature of polyester fiber in dry flue gas is less than 130°C, but in high-moisture flue gas, its long-term operating temperature can only drop to 60-80°C; Nomex fiber has good temperature resistance, Chemical resistance, but in high moisture flue gas, its temperature resistance will be reduced from 204°C to 150°C. Nomex fiber has better temperature resistance than polyester fiber, but it has poorer chemical resistance under high temperature conditions. Polyphenylene sulfide fiber has good performance of high temperature resistance and acid and alkali corrosion resistance, and is suitable for dust removal of coal-fired flue gas, but its anti-oxidation ability is poor: although polyimide can make up for its deficiency, its hydrolysis stability is not ideal . As the "King of Plastics", PTFE fiber has the best chemical resistance, but it is more expensive.

When selecting filter materials, the main factors must be grasped and comprehensively considered according to the chemical composition of the dusty gas.

3. Select according to the nature of the dust

⑴ Moisture and stickiness of dust

The wettability and wettability of dust are accomplished through the capillary action formed between dust particles, which is related to factors such as the atomic chain of dust, the surface state and the surface tension of the liquid, and can be characterized by the wetting angle; it is usually called hydrophilic if it is less than 60°C Sex, more than 90 ° C is hydrophobic. When the humidity of hygroscopic dust increases, the cohesive force and viscous force of the particles increase, and the fluidity and chargeability decrease accordingly. They adhere to the surface of the filter bag, and over time, the dust removal fails. Dust cake compacted.

Some dust contains CaO, CaCl, KCl, MgCl2, Na2CO3, etc. After absorbing moisture, further chemical reactions occur, and its properties and forms change, which is called deliquescence. The deliquescent dust sticks to the surface of the filter bag, which is taboo for the bag filter.

For wet and deliquescent dust, when selecting the filter material, attention should be paid to the smoothness, lint-free and water repellency of the filter material.

For dust with strong adhesion, needle-punched felt filter materials with surface singeing, calendering, and mirror surface treatment should be selected, and the technologies of impregnation, coating, and film coating should be fully utilized.

⑵ Combustibility and chargeability of dust

Certain dust in a certain concentration state will burn or explode when it encounters sparks in the air. The ignition source of dust combustion or explosion is usually caused by frictional sparks, static sparks, hot particles, etc., among which the chargeability is very harmful, so for flammable and easily chargeable dust, such as: coal powder, coke powder, alumina powder and magnesium powder, etc., it is advisable to choose flame-retardant filter materials and antistatic filter materials.

Antistatic filter material refers to mixing conductive fibers into the filter material fibers to make the filter material conductive in the warp or weft direction.

⑶ Flow and friction of dust

When the flow and friction of dust are strong, it will directly wear the filter bag and reduce the service life. The abrasiveness of the dust is directly proportional to the 2-3 power of the air velocity and the 1.5 power of the dust particle size. Therefore, the air velocity and its uniformity must be strictly controlled. Among the common dusts, aluminum powder, silicon powder, coke powder, sintered powder, sintered ore powder, etc. are all highly abrasive dusts. For abrasive dusts, filter materials with good wear resistance should be selected.

4. Choose according to the cleaning method of the dust collector

The dust cleaning method of the bag filter is another important factor in selecting the filter material structure. For the bag filter with different cleaning methods, due to the difference in the cleaning energy and the deformation characteristics of the filter bag, different structural types of filter materials should be selected.

⑴ Mechanical vibration bag filter

The filter material should be selected to be thin, smooth, and soft in texture, which is conducive to transmitting vibration waves and forming sufficient vibration force on the filter surface.

⑵ Compartment back-blowing bag type dust collector

Thin filter material with soft texture, easy deformation and stable size should be selected.

⑶ Vibration blowback type bag filter

A thin needle-punched felt with reinforced base fabric and stable dimensions should be selected.

⑷ Rotary reverse blowing type bag filter

Back-blowing bag type dust collectors should use relatively soft, stable structure, and good wear resistance filter materials, and medium-thick needle felt filter materials should be used first.

⑸ Pulse jet bag filter

The thick, wear-resistant and strong tensile filter material should be selected, such as chemical fiber needle felt or compressed felt filter material. The mass per unit area is: 500~650g/m2.