How to choose filter bag for Asphalt Mixing Station

Asphalt mixing machine in the production process,the drying, screening,weighing and mixing processes will be a lot of dust and fuel combustion of harmful exhaust emissions,they will cause the natural environment pollution. Modern mixers are used cyclone dust collector and baghouse two-stage dust removal device,as much as possible to collect dust pollutants to reduce the pollution of the natural environment,so that it meets the requirements of national environmental laws and regulations.

Taking 4000 asphalt mixing machine as an example,the baghouse box contains more than 600 heat-resistant (> 200 ℃) synthetic fiber bags, the working area reaches 1400 m2,the maximum displacement reaches 65300 Nm3/h,its mesh is very small,in the filter can capture more than 0.xn--3m-99b dust. At present,the dust removal efficiency of the bag-type dust remover with advanced technical performance of asphalt mixture mixing machine can reach 97%-99%,and the dust-containing gas can be purified to a degree of 15 mg/Nm3.

1.The principle of selection of filter cloth

Filter cloth is usually made of cotton,wool and other natural fibers or synthetic fibers as raw materials. Raw materials have different physical and chemical properties,suitable conditions of use or working environment (temperature,humidity,corrosion).

Therefore,the principle of proper selection of the filter material of the bag filter should be to fully understand the physical and chemical characteristics of the dust-laden gas discharged from the mixer during the production process,carefully compare the technical performance of the various fibers to be the correct choice.

2.The correct choice of filter cloth method

Proper selection of filter cloth methods include: According to the physical and chemical characteristics of dust-containing gas selection; According to the characteristics of dust selection; According to the selection of bag filter cleaning methods and so on.

2.1 According to the physical and chemical characteristics of dusty gas filter cloth

Physical and chemical properties of dust-laden gases include temperature,humidity,corrosivity,flammability and explosiveness.

2.1.1 Temperature of dusty gas

The temperature of the dust-laden gas discharged by the mixer is the most important factor for the correct selection of the filter bag filter. According to the continuous use of temperature,the filter cloth can be divided into three categories: normal temperature filter cloth (less than 130 ℃),medium temperature filter cloth (130 ~ 200 ℃) and high temperature filter cloth (greater than 200 ℃). For dusty gas temperature fluctuations in the working conditions should choose a little larger safety factor,but the instantaneous peak temperature shall not exceed the upper limit of the filter cloth temperature. For mixing high-temperature dust discharge mixer,you can directly use high-temperature filter cloth.

2.1.2 Dusty gas humidity

The humidity of dusty gas is another important factor for the proper choice of filter material for the bag filter of the mixer. The humidity of the dusty gas indicates how much water is contained in the gas,usually expressed as the volume percentage of water vapor in the dusty gas,Xw,or relative humidityψ. When Xw is greater than 8%,orψ less than 80%,it is called wet dusty gas. For wet dusty gas,the following points should be noted when choosing filter cloth:

1) The wet dusty air makes the dust trapped on the surface of the filter bag moisten and bond,especially for water absorption, deliquescence dust and even paste bag. This should be used nylon,glass fiber surface smooth,fiber material easy to clean the filter cloth,and should be used on the filter cloth silicone oil,fluorocarbon resin impregnated,or in the filter cloth surface using acrylic,polytetrafluoroethylene for coating treatment.

2) When the fume discharged from the mixer is mixed with high temperature and high humidity,it will affect the temperature resistance of the filter cloth,especially the materials with poor hydrolytic stability such as polyamide,polyester and imide. Therefore, the filter bag material should be carefully selected for the maintenance of the bag filter (bag replacement) and the water content (≤3%) of the cold aggregates should be strictly controlled during the production of the mixer.

2.1.3 Corrosive dusty gas

Different fibers have different chemical resistance and are often influenced by the intersection of multiple factors such as temperature and humidity. In contrast,polytetrafluoroethylene fibers known as "plastic kings" have the best chemical resistance but are more expensive. Therefore,the selection of filter cloth,the gas must be based on the chemical composition of dust,seize the main factors,choose the appropriate selection of appropriate materials.

2.2 According to the characteristics of dust selection filter cloth

The traits of dust,including the chemical and physical properties of dust,now focus on the physical analysis of the dust from the material,structure and post-processing aspects of the correct selection.

2.2.1 Dust shape and particle size distribution

Usually,most of the products produced by high-temperature combustion process are regular dust,while most of the dust produced by the process is irregular. Regular-shaped dust smooth and smooth,small surface area,not easy to be intercepted and agglomerated when passing through the filter cloth; On the contrary,the irregularly shaped dusts have different shapes,rough surfaces and large specific surface area,which are easily intercepted and aggregated when passing through the filter cloth. Bag filter should pay attention to the following points:

1) Fiber should be used thinner,shorter,curly,irregular section;

2) The structure of acupuncture felt is better,if the fabric should be twill weave or surface brushed processing;

3) Needle-felt with density gradient should be adopted when using the mixed thickness of batt,and the new technology such as surface spraying,dipping or laminating should be adopted to realize the surface filtration,which is the development direction of the filter cloth with ultra-fine dust;

4) Fine particles of dust more difficult to capture and capture after the formation of more dense layer of dust is not conducive to cleaning. On the contrary,coarse particles of dust easy to capture, capture dust formed after the more loose layer is conducive to cleaning. In a sense,the dust with the thickness of both filtration and cleaning are beneficial.

2.2.2 Dust adhesion and cohesion

Dusts have the characteristics of being attached to each other or attached to the surface of other objects,and adsorbed and condensed together when the suspended dust contacts each other. Dust cohesion and its type,shape,particle size distribution,water content and surface characteristics of a variety of factors,can be used in combination to characterize the dust angle of cohesion. For example, the angle of repose of less than 30°is called low adhesion,good fluidity; angle of repose of more than 45°is called high adhesion,poor fluidity. In addition,the adhesion of the dust to the solid surface (adhesion and cohesion) is also related to the solid surface roughness and cleanliness.

2.2.3 Dust hygroscopicity and deliquescence

Dust on the absorption of water in the gas is called hygroscopicity,if the main body of water is called moisture on the infiltration of dust. Hygroscopicity,wettability is accomplished through the action of capillaries formed between dust particles. Moisture absorption,wettability and the atomic chain of dust,the surface state and the surface tension of the liquid and other factors,can be used together to characterize the wetting angle - less than 60°for the hydrophilic,more than 90°for the hydrophobic. After the humidity of the hygroscopic dust increases,the cohesion and adhesion of the particles will increase,while the fluidity and chargeability will decrease,which will cause the dust to adhere on the surface of the filter bag. Over time,the cleaning will fail,and the dust will form a plate.

Mixture mixer dust emissions,such as CaO,CaCl2,KCL,MgCl2 and other moisture will further chemical reaction,the nature and morphology will change,this is deliquescence. They paste the filter bag surface,which is the most undesirable phenomenon bag filter.

For hygroscopicity,deliquescent dust,the selection of filter cloth should pay attention to the matter and "according to the characteristics of dusty gas filter cloth selection" the same.

2.2.4 Grinding dust peeking

Filter cloth in the filter,block,aggregate dust,the dust (especially irregular shaped dust) on the destructive filter cloth known as the pecking sex. It is related to the nature and shape of the dust,the speed of the airborne dust,the dust concentration and other factors. For example:aluminum powder,silicon powder,carbon powder,sinter powder and other hard material,belonging to the high pecking sex dust; Particle surface roughness,irregular sharp edge of the dust than the surface smooth,spherical pearl dust particles to many times (about 10 bits); The pebble grinding of about 90μm in particle size is the largest,while the pecking property is very weak when the particle size of the particles is reduced to 5 ~ 10μm; The pebability of dust and the air velocity ~ 3.5 times the square is proportional. Therefore,in order to reduce the grinding resistance of the dust to the filter cloth,the speed and the uniformity of the exhaust gas flow of the mixer mixer must be strictly controlled. In addition,for the pebbled big dust to wear good wear-resistant filter cloth.

Pearly grinding for the dust,the following points for the selection of filter cloth Reference:

1) Chemical fiber is better than glass fiber,expanded fiber is better than ordinary glass fiber;

2) Fine,short,curly fiber is better than coarse,long,straight fiber;

3) The felt is better than the fabric,the felt material should be acupuncture way to strengthen the intercrossing between fibers,fabric satin fabric optimal,brushed fabric surface can improve its wear resistance.

4) The surface coating,calendering and other post-treatment can improve the wear resistance of the cloth,for glass fiber cloth,silicone oil,graphite,PTFE resin can improve the wear resistance and folding endurance.

2.3 According to the bag filter cleaning method selection filter cloth

The cleaning method of the bag filter is another important factor that must be considered when selecting the filter cloth correctly. That is,the bag filter with different cleaning methods should adopt different structures due to the different cleaning properties and the deformation characteristics of the bag filter, Variety of filter cloth.

2.3.1 Mechanical vibration cleaning method

Mechanical Vibration The dust-cleaning baghouse uses mechanical means (manual,electromagnetic and pneumatic) to vibrate the filter bag,with vibration frequencies ranging from several to several hundred times per second. In addition to this type of bag filter mixers outside the small,mostly used within the filter round bag form. It is characterized by less kinetic energy applied to the dust layer and a greater number of actions,requiring the filter cloth thin and smooth,soft texture is conducive to the transmission of vibration waves in order to form enough in all the filter surface vibration force. Therefore,the compound mixer bag filter usually made of staple fiber made of satin or twill fabric,a thickness of 0.3 ~ 0.7mm,mass per unit area of 300 ~ 350 g/m2,and recommended the use of filtration rate of 0.6 ~ 1.2 m/min (small mixer can be properly raised to 1.5 ~ 2.0 m/min).

2.3.2 Denitrification cleaning method

Chamber cleaning type bag filter cleaning method is the use of sub-room structure,the valve switch from room to room,the formation of reverse air flow,forcing the bag shrinkage or bulging and cleaning. This cleaning method also belongs to the low kinetic energy cleaning,with the help of the working pressure of the bag filter as the cleaning power,repacking the airflow force in special occasions. Chamber bled cleaning dust bag filter requires the use of soft texture,easy deformation and stable size of the thin filter cloth,because the bag filter with the filter outside the filter points,so the choice of filter cloth slightly different. In general,the commonly used internal circular filter bag,frameless,bag diameter of 120 ~ 300 mm,bag length and bag diameter ratio of 15 to 40,preferably satin (or twill) woven filter cloth,the thickness of 1.0 to 1.5 mm,the unit area of the quality of 300 ~ 400 g/m2; Common external flat filter bag,diamond bag and honeycomb bag,with a support frame,preferably wear-resistant,breathable thin needle- 350 ~ 400 g/m2.

2.3.3 Vibration backflushing and clean with the way

Vibration backflushing and cleaning with the bag-type dust bag means both vibration and counter-flow double cleaning dust bag filter,the dust loosening of the vibration,the anti-airflow to dust off. Two ways to cooperate with each other,so that the cleaning effect can be improved,especially for fine particles of viscous dust filter. Such bags of dust filter cloth selection,in principle, with the sub-room cleaning bag cleaning dust the same way.

2.3.4 Nozzle blow-cleaning method

Nozzle blow-back cleaning bag filter is the use of high-pressure blower or blower as a cleaning power blow through the moving nozzle in turn on the filter bag,the formation of a strong reverse flow,the bag drastically deformed and clean,it's a medium energy cleaning type. By the nozzle and its trajectory can be divided into rotary blowback,reciprocating blowback and gas ring sliding blowback and other three.

Rotary Blowback and Reciprocating Blowback bag filter with a frame with the outer filter flat bag form,compact structure. Such bag filter requires the use of relatively soft,stable structure,good wear resistance of the filter,preferably medium thickness needle felt filter,the unit mass of 350 ~ 500 g/m2. China more use of tubular satin woven filter cloth.

Air ring sliding anti-blowing cleaning bag filter with filter bag,the nozzle is ring-shaped,set in the round bag outside the move up and down the injection,requiring the choice of thick,wear-resistant, rigid,lint-free filter cloth,the preferred choice of wool compression felt,but also the choice of synthetic needle felt,the unit area of quality is 600 ~ 800 g/m2.

2.3.5 Pulse jet cleaning method

Pulsed jet cleaning ash bag filter is driven by compressed air,the use of pulsed blowing mechanism to release compressed air in an instant,inducing several times the secondary air into the bag at a high speed,the rapid expansion of the filter bag,according to shock vibration and reverse gas and cleaning,it's a high kinetic energy cleaning type. Usually with a frame with a round filter bag or flat bag. Such bag filter requires the choice of thick wear-resistant,tensile strength of the filter cloth,the preferred choice of chemical fiber needle felt or compressed filter cloth,the unit mass of 500 ~ 650 g/m2.