A Brief Discussion on the Correct Selection of Filter Materials for Bag Filters

1. Principles for selecting filter materials

After a period of use, the filter material (cloth bags) of the bag-type dust collector of the asphalt mixture mixer may be damaged and leak due to various reasons, or "clog the bags", and must be replaced during maintenance. At this point, the question of "What kind of filter material to choose" has aroused people's thoughts. Of course, the simplest way to handle this problem is to follow the regulations and requirements stipulated in the user manual or maintenance manual of the asphalt mixture mixer. However, the results are often unsatisfactory, or they are not applicable, or the cost paid is too high.

Filter materials are usually made from natural fibers such as cotton and wool or synthetic fibers. Raw materials each have different physical and chemical properties, and the suitable usage conditions or working environments (temperature, humidity, corrosion, etc.) . Therefore, the principle for the correct selection of filter materials for bag-type dust collectors of asphalt mixture mixers should be to fully understand the physical and chemical properties of the dust-laden gas discharged during the production process of asphalt mixture mixers, carefully compare the technical performance of various fibers, and make the correct choice.

2. The correct method for selecting filter materials

The correct methods for selecting filter materials include: choosing based on the physical and chemical properties of the dusty gas; Select based on the properties of the dust; The selection should be based on the dust cleaning method of the bag filter, etc.

2.1 Select filter materials based on the physical and chemical properties of the dusty gas

The physical and chemical properties of dusty gas include temperature, humidity, corrosiveness, flammability and explosiveness, etc.

2.1.1 Temperature of the dusty gas

The temperature of the dusty gas discharged from the asphalt mixture mixer is the primary factor for the correct selection of filter materials in bag-type dust collectors. According to the continuous operating temperature, filter materials can be classified into three types: normal-temperature filter materials (<130℃), medium-temperature filter materials (130-200℃), and high-temperature filter materials (>200℃). For working conditions where the temperature of the dusty gas fluctuates greatly, it is advisable to choose a slightly higher safety factor, but the instantaneous peak temperature must not exceed the upper limit temperature of the filter material. For the high-temperature dusty gas discharged from the asphalt mixture mixer, high-temperature filter materials can be directly selected.

2.1.2 Humidity of the dusty gas

The humidity of the dusty gas is another crucial factor for the correct selection of filter materials in the bag-type dust collector of the asphalt mixture mixer. The humidity of dusty gas indicates the amount of water vapor contained in the gas, and is usually expressed as the volume percentage of water vapor in the dusty gas Xw or the relative humidity ψ. When Xw>8% or ψ>80%, it is called wet dusty gas. When choosing filter materials for wet and dusty gases, the following points should be noted

Wet dusty gas causes the dust captured on the surface of the filter bags to wet and stick together, especially for hygroscopic and deliquescent dust, which may even lead to bag clogging. For this purpose, filter materials such as nylon and glass fiber with smooth surfaces and easy-to-clean fiber materials should be selected. It is also advisable to impregnate the filter materials with silicone oil or fluorocarbon resin, or apply coatings such as acrylic or polytetrafluoroethylene on the surface of the filter materials.

2) When the flue gas discharged from the asphalt mixture mixer simultaneously contains high temperature and high humidity, it will affect the heat resistance of the filter material, especially for materials with poor hydrolytic stability such as polyamide, polyester, and imide. Therefore, when maintaining (replacing filter bags) the bag-type dust collector of the asphalt mixture mixer, the filter cloth material should be carefully selected, and the moisture content of the cold aggregate (≤3%) should be strictly controlled during the production process of the asphalt mixture mixer.

2.1.3 Corrosiveness of dusty gas

The chemical resistance of different fibers varies and is often cross-influenced by multiple factors such as temperature and humidity. In comparison, polytetrafluoroethylene (PTFE) fiber, known as the "King of Plastics", has the best chemical resistance, but it is relatively expensive. Therefore, when choosing filter materials, it is necessary to take into account the chemical composition of the dust-laden gas, grasp the main factors, and select the most suitable materials.

2.2 Select filter materials based on the properties of the dust

The properties of dust include its chemical and physical properties. This article focuses on analyzing the correct selection of filter materials in terms of material, structure, and post-treatment based on the physical properties of dust.

2.2.1 Shape and particle size distribution of dust

As the dust removal object of the bag-type dust collector for asphalt mixture mixers, it usually refers to dust particles ranging from 0.1 to 10 μm, and their shapes are divided into regular and irregular types. Generally, the products of high-temperature combustion processes are mostly regular-shaped dust, while the dust particles produced by most technological processes are mostly irregular in shape. Regular-shaped dust has a smooth surface and a small specific surface area, making it less likely to be intercepted or coagulated when passing through the filter cloth. On the contrary, irregularly shaped dust varies in shape, has a rough surface and a large specific surface area, and is prone to being intercepted and aggregated when passing through the filter cloth. For the fine particle dust filtered by the cyclone dust collector (primary dust removal) of the asphalt mixture mixer, the following points should be noted when selecting the filter material for the bag filter of the secondary dust removal:

1) Fibers should be selected to be finer, shorter, curled, and have irregular cross-sectional shapes.

2) Needled felt is preferred for the structure. If fabric is used, it is advisable to be twill weave or have a brushed surface.

3) When using coarse and fine mixed cotton layers, needled felt with density gradients should be adopted, and new technologies such as surface spraying, impregnation or film coating should be used to achieve surface filtration. This is the development direction for the selection of filter materials for ultrafine dust.

4) Fine particulate dust is difficult to capture and, once captured, tends to form a relatively dense dust layer, which is not conducive to dust cleaning. On the contrary, coarse dust particles are easy to capture, and the dust layer formed after capture is relatively loose, which is conducive to dust cleaning. In a sense, a combination of coarse and fine dust is beneficial for both filtration and cleaning.

2.2.2 Adhesion and coagulation of dust

Dust has the characteristic of adhering to each other or to the surface of other objects. When suspended dust comes into contact with each other, they adsorb and coalesce together. The cohesion of dust is related to multiple factors such as its type, shape, particle size distribution, moisture content and surface characteristics. In summary, the Angle of repose can be used to characterize the cohesion of dust. For instance, those with an Angle of repose less than 30 ° are considered to have low adhesion and good fluidity. Those with an Angle of repose greater than 45° are called having high adhesion and poor fluidity. In addition, the degree of stickiness (adhesion and agglomeration) between dust and solid surfaces is also related to the roughness and cleanliness of the solid surfaces.

For the filter material used in the bag-type dust collector of the asphalt mixture mixer, if the adhesion to the dust is too weak, it will lose the ability to capture the dust, while if the adhesion is too strong, it will cause the dust to coagulate tightly and make cleaning difficult. Therefore, for dust with strong adhesion, long-fiber fabric filter materials or needle-punched felt filter materials that have undergone surface singling, calendering and mirror finishing treatments are recommended. In terms of the material of the filter material, nylon and glass fiber are superior to their varieties. For sticky dust, pilling fabric filter materials should not be selected as they may adhere to sticky dust and spread it throughout the entire filter surface, making dust cleaning extremely difficult.

2.2.3 Hygroscopicity and deliquescence of dust

The ability of dust to absorb moisture from gas is called hygroscopicity. If water force is the main component, it is called the wettability of moisture on dust. Hygroscopicity and wettability are accomplished through the action of capillaries formed between dust particles. Hygroscopicity and wettability are related to factors such as the atomic chains of dust, surface conditions, and the surface tension of the liquid. Collectively, they can be characterized by the wetting Angle - those less than 60° are hydrophilic, and those greater than 90° are hydrophobic. When the humidity of hygroscopic dust increases, the cohesion and adhesion of the powder particles increase accordingly, while the fluidity and electrification decrease, causing the dust to adhere to the surface of the filter bag. Over time, the dust cleaning fails and the dust forms plates.

The dust discharged from the asphalt mixture mixer, such as CaO, CaCl2, KCL, MgCl2, etc., will undergo further chemical reactions after absorbing moisture, and their properties and forms will change accordingly. This is called deliquescence. They stick to the surface of the filter bags, which is the most undesirable phenomenon for bag-type dust collectors.

For hygroscopic and deliquescent dust, the matters needing attention when selecting filter materials are the same as those for "selecting filter materials based on the characteristics of the dusty gas".

2.2.4 Abrasive properties of dust

When filter materials filter, intercept and coagulate dust, the destructive effect of dust (especially irregularly shaped fine dust) on the filter materials is called abrasiveness. It is related to factors such as the nature and form of the dust, as well as the velocity of the airflow carrying the dust and the concentration of the dust. For instance, materials such as aluminium powder, silicon powder, carbon powder, and sintered ore powder are hard and belong to highly abrasive dusts. Dust particles with rough surfaces and irregular sharp edges are many times more abrasive than those with smooth surfaces and spherical particles (about 10 bits or so). Dust with a particle size of about 90μm has the greatest abrasiveness, while when the particle size of the dust is reduced to 5-10μm, the abrasiveness is very weak. The abrasiveness of dust is directly proportional to the 2nd to 3.5th power of the airflow velocity carrying it. Therefore, in order to reduce the abrasiveness of dust on the filter material, it is necessary to strictly control the speed and uniformity of the exhaust gas flow of the asphalt mixture mixer. In addition, for dust with high abrasiveness, filter materials with good wear resistance should be selected.

For the abrasiveness of dust, the following points can be referred to when selecting filter materials:

1) Chemical fibers are superior to glass fibers, and expanded fibers are superior to common glass fibers.

2) Fine, short and curly fibers are superior to thick, long and straight fibers.

3) Felt materials are superior to fabrics. In felt materials, needling should be used to enhance the interweaving between fibers. Among fabrics, satin fabrics are the best. The brushing on the fabric surface can improve its wear resistance.

4) Post-treatment such as surface coating and calendering can enhance the wear resistance of filter materials. For glass fiber filter materials, treatments with silicone oil, graphite, and polytetrafluoroethylene resin can improve their wear resistance and folding endurance.

2.3 Select filter materials based on the cleaning method of the bag filter

The cleaning method of bag-type dust collectors is another crucial factor that must be considered in the correct selection of filter materials. That is to say, bag-type dust collectors with different cleaning methods should choose filter materials of different structures and types due to the differences in cleaning energy and the deformation characteristics of filter bags.

2.3.1 Mechanical vibration type ash cleaning method

The bag-type dust collector with mechanical vibration cleaning method uses mechanical devices (including manual, electromagnetic vibration and pneumatic) to make the filter bags vibrate, with the vibration frequency ranging from several times to hundreds of times per second. Except for small asphalt mixture mixers, most of these bag-type dust collectors adopt the form of inner filter round bags. Its characteristic is that the kinetic energy applied to the dust layer is relatively small but the number of actions is relatively large. It requires the filter material to be thin and smooth, with a soft texture, which is conducive to the transmission of vibration waves, so as to form sufficient impact force on the entire filter surface. Therefore, bag-type dust collectors for asphalt mixture mixers typically use satin or twill fabrics made of short chemical fibers, with a thickness of 0.3 to 0.7mm and a unit area mass of 300 to 350g/m ². It is also recommended to select a filtration speed of 0.6 to 1.2m/min(for small asphalt mixture mixers, it can be appropriately increased to 1.5 to 2.0 m/min).

2.3.2 Compartmentalized reverse-blowing ash cleaning method

The bag-type dust collector with the compartmental-reverse-blowing cleaning method adopts a compartmental-structure, with valves switching chamber by chamber to form a reverse airflow, which forces the filter bags to contract or expand for cleaning. This type of ash cleaning method also belongs to low-kinetic energy ash cleaning. It uses the working pressure of the bag filter as the ash cleaning power, and only in special circumstances is a reverse blowing air flow power provided. The bag-type dust collector with compartmental-reverse-blowing cleaning square type requires the selection of thin filter materials that are soft in texture, easy to deform and stable in size. As this bag-type dust collector is divided into internal filter and external filter, the selection of filter materials is slightly different. Generally speaking, for internal filtration, circular bags are commonly used, without frames, with bag diameters ranging from 120 to 300mm, and the ratio of bag length to bag diameter is 15 to 40. Satin (or twill) woven filter materials are preferred, with a thickness of 1.0 to 1.5mm and a unit area mass of 300 to 400g/m ². For external filtration, flat bags, rhombic bags and honeycomb bags are commonly used, with support frames. Thin needled felt filter materials with good wear resistance and air permeability are preferred, with a unit area of 350 to 400g/m ².

2.3.3 Vibration and reverse blowing combined ash cleaning method

The bag-type dust collector with the combined vibration and reverse blowing cleaning method refers to the bag-type dust collector that has the dual cleaning functions of vibration and reverse air flow. Its vibration loosens the dust cake and the reverse air flow separates the powder dust. The two methods work in combination to enhance the dust cleaning effect, especially suitable for the filtration of fine and sticky dust particles. The selection of filter materials for this type of bag-type dust collector is generally the same as that for bag-type dust collectors with compartmentalized reverse blowing cleaning methods.

2.3.4 Nozzle reverse blowing ash cleaning method

The bag filter with nozzle reverse blowing cleaning method uses a high-pressure fan or blower as the reverse blowing cleaning power. By moving the nozzles to blow the filter bags in sequence, a strong reverse airflow is formed, causing the filter bags to deform sharply and clean the dust. It belongs to the medium-energy cleaning type. According to the form of the nozzle and its movement trajectory, it can be classified into three types: rotary back-blowing type, reciprocating back-blowing type and air ring sliding back-blowing type.

The bag-type dust collectors with rotary back-blowing and reciprocating back-blowing cleaning methods adopt the form of outer filter flat bags with frames, featuring a compact structure. This type of bag-type dust collector requires the selection of relatively soft, structurally stable and wear-resistant filter materials. Medium-thickness needled felt filter materials are preferred, with a unit area mass of 350 to 500g/m ². In China, cylindrical satin woven filter materials are widely used.

The bag-type dust collector with the air ring sliding reverse blowing cleaning method adopts inner filter round bags, and the nozzles are in the shape of annular seams, which are placed outside the round bags and move up and down for blowing. It is required to select thick, wear-resistant, rigid and non-fuzzing filter materials. Wool compressed felt is preferred, and synthetic fiber needled felt can also be selected. The unit area mass is 600 to 800g/m ².

2.3.5 Pulse jet cleaning method

The bag-type dust collector with pulse jet cleaning method uses compressed air as the power source. It utilizes the pulse jet mechanism to release the compressed air flow in an instant, inducing several times the secondary air to be injected into the filter bags at high speed, causing the filter bags to expand rapidly. The dust is cleaned by impact vibration and reverse air flow, which belongs to the high kinetic energy cleaning type. Usually, outer filter round bags or flat bags with frames are adopted. This type of bag-type dust collector requires the selection of thick, wear-resistant and highly tensile filter materials. It is preferred to use chemical fiber needled felt or compressed felt filter materials, with a unit area mass of 500 to 650g/ m ².