Bag filter is one of the dust collectors. It is mainly used in some large delivery rooms, especially in factories with a lot of dust. We seldom see it in our daily life, but it plays an intuitive and important role in the quality of ambient air. The filter material of general bag filter is cloth or felt made of synthetic fiber, natural fiber or glass fiber.

Working mechanism

The working principle of the bag filter is: the woven or felt woven (pressed) filter cloth is used as the filter material. When the dusty gas passes through the filter bag, the dust is blocked on the surface of the filter bag, and the dry air is discharged through the gap between the fibers of the filter bag, so as to achieve the purpose of separating the dusty gas and dust. Its working mechanism is that the dust is trapped by screening, inertia, adhesion, diffusion and static electricity when passing through the filter cloth.

1. Screening function

When the dusty gas passes through the filter cloth, the gap between the fibers of the filter cloth or the gap between the dust adsorbed on the surface of the filter cloth separates the dust larger than the diameter of the gap, which is called screening. For the new filter cloth, due to the large gap between the fibers, this effect is not obvious, and the dust removal efficiency is also low.

Only after a certain time of use, a certain thickness of dust layer is established on the surface of the filter bag, the screening effect is more significant. After ash removal, a certain amount of dust remains on the surface and inside of the filter bag, so it can still maintain a good dust removal efficiency.

For needle felt or pile filter cloth, because the felt or pile filter cloth itself forms a thick porous filter layer, it can give full play to the screening function, and does not completely rely on the dust layer to maintain a high dust removal efficiency.

2. Inertial action

When the dusty gas passes through the filter cloth fiber, it is greater than 1 μ M dust still keeps linear motion due to inertia, and is trapped when it strikes the fiber. The larger the diameter of dust particles, the greater the inertia effect. The higher the filtration gas velocity is, the greater the inertia effect is. However, if the gas velocity is too high, the air volume passing through the filter cloth will also increase, and the air flow will break through the weak part of the filter cloth, resulting in the reduction of dust removal efficiency. The higher the gas velocity, the more serious the perforation phenomenon.

3. Diffusion

When the dust particles are 0.2 μ Below m, the Brownian motion such as the thermal motion of gas molecules is generated due to the extremely small dust, which increases the contact opportunity between the dust and the filter cloth and makes the dust trapped. This diffusion effect is opposite to the inertia effect. It increases with the decrease of filtration gas velocity and dust particle size. Taking glass fiber as an example, the finer the fiber, the higher the dust removal efficiency (see table). However, the pressure loss of fine fiber diameter is greater than that of coarse fiber, and the finer the fiber diameter, the worse the corrosion resistance.

4. Adhesion

When the dusty gas approaches the filter cloth, the fine dust still moves with the air flow. If the radius of the dust is greater than the distance from the dust center to the edge of the filter cloth, the powder dust will be adhered by the filter cloth and arrested. The smaller the gap of the filter cloth, the more significant the adhesion.

5. Electrostatic action

The impact between dust particles will produce static electricity. If the filter cloth is an insulator, it will charge the filter cloth. When the charge of the dust and the filter cloth is opposite, the dust will be adsorbed on the filter cloth, so as to improve the dust removal efficiency and make it difficult to clean the dust.

On the contrary, if the charges carried by the two are the same, repulsive force will be generated, and the dust cannot be adsorbed on the filter cloth, which will reduce the dust removal efficiency. Therefore, electrostatic effect can improve or hinder the dust removal efficiency of filter cloth. In order to ensure the dust removal efficiency, the filter cloth must be selected according to the charge property of the dust. Generally, the electrostatic effect is only when the dust particle size is less than 1 μ M and when the filtration gas velocity is very low. Under the condition of external electric field, the electrostatic effect can be strengthened and the dust removal efficiency can be improved.

Structure of bag filter

Bag filter is mainly composed of box, filter bag (including frame), ash cleaning device, ash hopper and ash removal device. When the dusty flue gas enters the box and passes through the filter bag, the dust is blocked on the outside of the filter bag, and the purified flue gas is discharged through the inside of the filter bag.

Characteristics of bag dust removal

1. High dust removal efficiency, especially for fine dust, generally up to more than 99%.

2. It has strong adaptability and can capture dust with different particle sizes. For example, for dust with high specific resistance, bag filter is superior to electrostatic precipitator.

3. The use is flexible, and the treatment air volume can range from hundreds of cubic meters to hundreds of thousands of cubic meters per hour.

4. The structure is simple, and the simple bag type dust collector with direct bagging can be adopted according to local conditions, or the pulse ash cleaning bag type dust collector with higher efficiency can be adopted.

5. Stable operation, easy to recover dry materials, no sludge treatment, corrosion and other problems, and simple maintenance.

Classification of bag filter

The large body structure of the bag filter has various forms, which can be classified according to the section shape of the filter bag, the direction of the dusty air flow passing through the filter bag, the layout of the air inlet, the internal gas pressure of the dust collector and the ash cleaning method. The important parameters of bag filter, such as dust removal efficiency, pressure loss, filtration speed and filter bag life, are related to the ash cleaning method. The common product structure of bag filter is mainly classified according to the ash cleaning method.

The important parameters such as dust removal efficiency, pressure loss, filtration speed and filter bag life of bag filter are related to the ash cleaning method. Therefore, bag filter is mainly classified according to the ash cleaning method, which is divided into three categories: mechanical vibration, pulse injection and reverse blowing.

According to the ash cleaning method:

Mechanical vibration dust cleaning bag type dust collector

The bag filter uses mechanical transmission to vibrate the filter bag, so that the dust layer deposited on the filter bag falls into the ash hopper.

It is a way for the filter bag to vibrate along the vertical direction. It can not only use the method of regularly lifting the hanging frame of the filter bag, but also use the eccentric wheel to vibrate the frame.

Advantages: the bag type dust collector using the vertical vibration of eccentric wheel has the characteristics of simple structure, good dust removal effect and low energy consumption. It is suitable for dust removal with small dust concentration and intermittent dust sources.

Bag filter unit has irreplaceable advantages in small dust removal equipment. Compared with electrostatic precipitator, it has the advantages of simple structure, low cost and small volume; Compared with the cyclone combined unit, it has high efficiency and wide application range; Compared with the air filter, it can be regenerated repeatedly and has a long service life. It can replace the medium efficiency and even sub high efficiency filter.

Pulse jet bag filter

The dusty gas is introduced into the pulse dust cleaning bag dust collector from the lower vertebral body, and the dust is blocked on the outer surface of the filter bag. The air inlet through the filter bag enters the upper box through the venturi tube and is discharged from the air outlet pipe.

Ash cleaning process:

The controller triggers each exhaust valve regularly and sequentially to connect the back pressure chamber of the pulse valve with the atmosphere (vent). When the pulse valve is opened, the compressed air in the air bag is ejected through the small hole on the injection pipe through the pulse valve (primary air), and the surrounding air (secondary air) is blown into the filter bag through the venturi induction multiple (about 5 ~ 7 times of the primary air), resulting in rapid expansion and vibration of the filter bag, coupled with the reverse purging effect of air flow, Make the dust layer accumulated on the outer surface of the filter bag fall off.

This dust cleaning method is characterized by pulse, so it is called pulse jet bag filter. The injection pressure of compressed air is 500 ~ 700KPA, the injection time is 0.1 ~ 0.2S, and the injection cycle is generally 60 ~ 180s. The pulse injection system is composed of pulse controller, control valve, pulse valve, injection pipe and compressed air bag. According to this principle, based on the traditional pulse bag filter, it has developed into an off-line ash cleaning bag pulse filter. Its features are:

1. When the filtered air flow is stopped, carry out blowing and ash cleaning to completely clean the ash of the filter bag;

2. The pressure of injection air source can be reduced from 0.5 ~ 0.7MPa of on-line pulse dust collector to 0.2 ~ 0.3MPa, so as to save injection energy;

3. Due to the reduced injection frequency, the service life of the filter bag pulse valve diaphragm can be extended, the reliability of the equipment can be increased, and the maintenance time and cost of the equipment can be reduced;

4. Due to the reduction of injection frequency, the filtering effect of the ash layer of the filter bag can be brought into full play, so as to improve the overall dust removal efficiency. When using 500g / m2 needle felt filter bag, the emission concentration is generally below 20mg / Nm3.

Reverse blowing bag filter

The air flow during ash cleaning of this bag type dust collector is opposite to that during normal filtration. It is a reverse air flow mode. The back blowing bag type dust collector is usually divided into several chambers. Each chamber is provided with a separate ash hopper, a dusty gas inlet pipe, a clean gas outlet pipe and a back suction air pipe, which are respectively connected with the air inlet main pipe and the back suction main pipe. A switching valve (primary valve) is set in the clean gas pipe, and a reverse air flow valve (secondary valve) is set in the reverse blowing pipe.

The dust remover that uses the periodic vibrating filter bag of mechanical motion device to remove the dust on the filter bag is called mechanical vibrating bag dust remover. It has two types: one is continuous; The other is intermittent. The difference is: the continuous use of the dust collector divides the dust collector into several compartments, one of which continues to remove dust while the other compartments continue to remove dust; The dust remover used intermittently has only one chamber, and the dust removal will be suspended during dust removal, so the dust removal process is intermittent.

Pulse bag filter

Pulse bag filter is a dust removal equipment developed in the 1950s. It has been widely valued because of its high ash removal efficiency. Over the past few decades, it has developed rapidly, with more and more applications, and the sizes, categories and specifications are relatively complete.

The ash cleaning device of pulse bag filter is composed of pulse valve, spray pipe, air storage bag, inducer, controller and other components. Pulse bag filter generally adopts circular bag, which can be divided into two forms: side air inlet and lower air inlet according to the movement direction of dusty air flow. The dust collector is usually composed of upper box (air purification chamber), middle box, ash hopper, frame and pulse injection device.

Reverse blowing bag filter

Reverse blowing bag dust collector refers to a bag dust collector that uses reverse reverse reverse blowing air flow to remove dust from filter bags.

The back blowing method uses the action of reverse air flow and reverse pressure to compress the filter bag into a star section and shake it to shake off the deposited dust layer. In order to ensure the continuous operation of the dust collector, the multi chamber working system is adopted.

The ash cleaning effect of this ash cleaning method is relatively weak, the vibration is not violent, and the damage effect on the filter cloth is smaller than that of vibration ash cleaning and pulse mode. Therefore, the back blowing method is not only used for textile filter cloth, but also suitable for glass fiber filter cloth.

The anti blowing bag type dust collector is composed of dust collector box, frame, ash hopper, valve (ash unloading valve, anti blowing valve, air volume regulating valve), air duct (air inlet pipe, exhaust pipe, anti blowing pipe), differential pressure system, ladder platform and electric control system. The so-called back blowing ash cleaning is to use the circulating flue gas of the atmosphere or dust removal system for back blowing (suction) air ash cleaning.

Maintenance points of bag filter

Since there are many domestic manufacturers of bag filter and the product quality varies greatly, it is necessary to be familiar with and master the product instructions provided by the manufacturer during operation and maintenance, pay attention to the specific requirements on operation and maintenance management in the instructions, and file and keep the instructions for reference at any time.

Dust properties

The nature of dust has a great influence on the design of bag filter. For some special properties of dust, effective measures should be taken according to the design experience.

(1) Adhesion and cohesion. When the adhesive and agglomerative dust enters the bag dust collector, the dust will become larger after slightly agglomerating. The dust accumulated on the surface of the filter bag can continue to agglomerate in the process of shaking off. The ash removal efficiency and the amount of dust passing through the filter material are also related to the adhesion and agglomerativity of the dust.

Therefore, in the design, for the dust with significant adhesion and cohesion, or almost no adhesion and cohesion, different treatment measures must be taken according to the different types and uses of dust and according to the design experience.

(2) Particle size. The main influence of particle size distribution on bag filter is resistance loss and wear. Fine dust has a great impact on pressure loss, and coarse dust plays a decisive role in wear, but only particles with high dust concentration and high hardness at the inlet have a great impact.

(3) Particle shape. It is generally believed that acicular crystalline particles and flake particles are easy to block the pores of the filter material and reduce the dust removal efficiency. Fibrous particles that can condense into flocs are difficult to fall off from the surface of the filter material if a high filtration speed is adopted. Different filtration speeds are selected according to the particle shape and characteristics in the design.

(4) The density of particles. The bulk density of dust is related to particle size, cohesion and adhesion, as well as the resistance loss and filtration area of bag filter. The smaller the bulk density is, the more difficult it is to remove the ash. Therefore, a lower filtering wind speed should be selected in the design. In addition, the bulk density of dust is very important for selecting the capacity of dust hopper and ash discharge device of dust collector.

(5) Hygroscopicity and deliquescence. Dust with strong hygroscopicity and deliquescence is very easy to absorb moisture and solidify on the surface of the filter material during the operation of the bag filter, or deliquescent into a thick substance in case of water, resulting in difficulty in ash removal, increased equipment resistance, and even affecting the normal operation of the filter. For example, necessary technical measures shall be taken for dust containing strong deliquescent substances such as KCl, MgCl2, NaCl and Cao.

(6) Static electricity. Once the easily charged dust generates static electricity on the filter material, it is not easy to fall off. For the very easily charged dust, technical measures such as anti-static filter material must be adopted to avoid explosion caused by sparks caused by static electricity.

(7) Flammability. Although combustible dust may not always cause explosion, if sparks appear in the process flow in front of the dust collector and can enter the dust collector, explosion-proof measures should be taken, such as adding spark catcher, setting anti explosion door, etc.

Inlet dust concentration

The dust concentration at the inlet is often expressed by the dust mass in the standard volume. For the dust concentration at the inlet, the following considerations shall be taken into account in the design of bag filter

(1) Equipment resistance and ash cleaning cycle. When the dust concentration at the inlet increases, the equipment resistance also increases under the condition of the same filtering area. In order to maintain a certain equipment resistance, the ash cleaning cycle is shortened accordingly;

(2) Wear of filter material and box. Under the condition of high concentration of dust with strong wear, the wear amount is directly proportional to the dust concentration. There should be diversion and wear-resistant treatment technologies at the inlet of the dust collector, such as sintering dust, aluminum oxide powder, silica sand powder, etc;

(3) Pre dust collector and filtering wind speed. When the dust concentration at the inlet is very high, a lower filtering wind speed and a pre dust collector shall be designed. However, if a structural form with primary settlement function is designed, the pre dust collector can also be cancelled;

(4) Ash discharge device. The ash discharge capacity is based on the ability to discharge all collected dust. The amount of dust discharged is equal to the product of the dust concentration difference at the inlet and outlet and the treatment air volume. The capacity design of multi-stage ash discharge device shall be subject to the ash discharge capacity of the lower level greater than that of the upper level.

Dust concentration at outlet

The dust concentration at the outlet must be lower than the specified value of environmental protection regulations and national health standards. The dust concentration at the outlet of bag filter varies according to the structural form, filter material type and dust property of the filter, which is generally between 1 ~ 50mg / m3. In the case of containing harmful substances such as lead and cadmium, the outlet concentration is required to be particularly low. In the design, different bag filter structures and filter materials are selected according to different uses and process characteristics.