The working mechanism of bag filter is that the dust containing flue gas passes through the filter material, and the dust particles are filtered down. The filter material mainly catches the coarse dust by inertial collision, the fine dust by diffusion and screening, and the dust layer of the filter material also has a certain filtering effect.

The design of bag filter involves many factors. This paper introduces the design points of bag filter from the following factors.

1. Treatment air volume

The processing air volume determines the size of the bag filter. Generally, the processing air volume is the working condition air volume. Pay attention to the use place and flue gas temperature of the dust collector in the design. If the flue gas treatment temperature of the bag dust collector has been determined and the gas is cooled by dilution method, the treatment air volume should also consider increasing the amount of diluted air; Considering the process changes in the future, the design index value of air volume should increase the safety factor of 5% ~ 10% on the basis of normal air volume. Otherwise, once the process adjustment increases the air volume in the future, the filtration speed of bag filter will increase, which will increase the resistance of equipment and even shorten the service life of filter bag, which will also be the reason for the sharp rise of other failure frequencies. However, if the safety factor is too large, the investment and operation cost of dust collector will be increased; The filtration wind speed varies greatly due to the form of bag filter, the type and characteristics of filter material. Once the treatment air volume is determined, the necessary filtration area can be determined according to the determined filtration wind speed.

2. Service temperature

The service temperature of bag filter is an important basis for design. The deviation between the service temperature and the design temperature will lead to serious consequences, because the temperature is restricted by the following two conditions: first, the allowable temperature of different filter materials (instantaneous allowable temperature and long-term operation temperature) is strictly limited; Second, in order to prevent condensation, the gas temperature must be kept above the dew point of 20 ℃. For high-temperature gas, it must be cooled below the temperature that the filter material can withstand. There are many cooling methods, such as natural air duct cooling, forced air cooling, water cooling, etc. specifically, it can be designed and selected according to different processes, cooling temperatures, layout and size requirements.

3. Gas composition

Except for special cases, most of the gas treated by the bag filter is ambient air or furnace flue gas. Generally, the design of the bag filter is calculated according to the treated air. The composition of the gas is considered only when the parameters such as density, viscosity and mass heat capacity are related to the calculation of wind power performance and pipeline resistance and the design of cooling device. In many working conditions, the flue gas mostly contains moisture. With the increase of moisture in the flue gas, the equipment resistance and fan energy consumption of bag filter also change. The water content in dusty gas can be determined by actual measurement or calculated according to the material balance of combustion and cooling. The corrosion resistance of the filter material and the filter shell must be considered. In addition, if there is toxic gas in the flue gas, it is generally trace, which has little impact on the performance of the device. However, when dealing with this kind of dusty flue gas, the bag filter must adopt a gas tight structure, and it must be maintained and overhauled regularly to avoid safety accidents caused by toxic gas leakage.

4. Inlet dust concentration

The dust concentration at the inlet is often expressed by the dust mass of the standard volume. The dust concentration at the inlet shall be considered as follows 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.

5. Dust concentration at outlet

The dust concentration at the outlet must be lower than the specified value of environmental protection regulations and health standards. The dust concentration at the outlet of the 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.

6. 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 cohesive dust enters the bag type dust collector, the particles of the dust will become larger after a little agglomeration. 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 cohesion 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 deliquescence 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, anti-static filter material and other technical measures 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 explosion-proof door, etc.

7. Equipment resistance

The so-called equipment resistance refers to the total pressure difference from the inlet to the outlet of the dust collector under the operating state. The pressure loss of bag filter is usually between 1000 ~ 2000Pa, and the pressure loss of pulse bag filter is usually less than 1500pa. Equipment resistance is the main basis for fan selection. There is a certain variation range of allowable pressure loss during equipment operation. During design, the variation allowance of equipment resistance should be considered to determine the fan selection.

8. Equipment withstand voltage

The pressure resistance of the bag filter is determined according to the process requirements and the static pressure of the fan. The design pressure resistance of the equipment must be determined according to the normal use pressure of the bag filter. As a general-purpose bag filter, the pressure resistance of the equipment is 4000 ~ 5000Pa, that of the long bag pulse filter is 6000 ~ 8000pa, and that of the negative pressure air conveying device powered by roots blower is 15 ~ 50kPa. In addition, for some special, such as blast furnace gas dry process pulse bag filter, its design pressure resistance requirement is 0.3MPa or higher, and the equipment is generally designed as a circle to meet the pressure resistance requirements

9. Ash cleaning pressure

Ash cleaning pressure is an important parameter in the design of bag filter. According to the different compressed air pressure used, it is divided into high pressure (0.5-0.7mpa), medium pressure (0.35-0.5mpa), low pressure (0.2-0.35mpa) and ultra-low pressure (below 0.2MPa). The pulse valve is divided into high pressure valve (right angle valve) and low pressure valve (drowning valve) according to the pressure in the air bag. The design and selection of high-pressure or low-pressure pulse valve is determined according to the compressed gas supply conditions and dust removal process conditions. At present, the high-pressure dust removal pressure is more used for small dust collectors, while the low-pressure dust removal pressure is more used for long bag pulse. Different ash cleaning pressure designs and different ash cleaning structures of the injection pipe are selected. The opening size, structural type and height size of the injection pipe and the flower plate of the injection pipe will directly affect the ash cleaning performance of the equipment. Pulse ash cleaning is not very strict with compressed air. The indexes of dust, water and oil can reach grade III and grade V in iso8573-1 respectively. When the compressed air quality is not up to standard, the oil and water content of the air flow entering the dust removal filter bag increases relatively. Once the oil and water enter the filter bag, the filter area of the blocked part will be attached, resulting in the increase of the resistance of the dust collector.

10. Ash cleaning method

(1) Online ash cleaning and offline ash cleaning. Pulse bag filter can adopt two methods: on-line ash cleaning and off-line ash cleaning. Online ash cleaning refers to that the filter bag is still filtered with dusty gas during pulse injection. Online ash cleaning, filtration and ash cleaning have little impact on the system fluctuation, but the ash cleaning is not complete and cannot be overhauled online. Off line ash cleaning means that the interior of the dust collector is divided into several filter chambers, and the air purification chamber of each bag chamber is independently equipped with off-line valve, cylinder, electromagnetic pulse valve and other compressed gas control systems. Before pulse ash cleaning injection for each filter chamber, the bag chamber is first closed through the off-line valve, so that the filter chamber can be cleaned without flue gas filtration. Therefore, the off-line ash cleaning effect is more thorough and can be overhauled online. The disadvantages are the addition of off-line mechanism, high cost Dust removal has an impact on the fluctuation of system flue gas, etc.

How to choose on-line or off-line dust removal should be comprehensively considered according to the dust removal process and user conditions. For small dust collectors with low inlet dust concentration, on-line dust removal is generally adopted, otherwise off-line dust removal is adopted; The large dust remover is generally designed as an off-line structure and has two ash cleaning functions that can be switched off-line and on-line. When the working conditions allow no off-line ash cleaning, it can be switched to on-line ash cleaning. However, whether on-line or off-line ash cleaning, it should have the on-line maintenance function of non-stop through the off-line valve.

(2) Box and jet type. Tubular injection is an orderly injection, in which the injection pipe is set above each row of filter bags, that is, the ash cleaning air flow is injected into each filter bag through the nozzle on the pipe, and the injection aperture is different to ensure the uniform ash cleaning intensity of each filter bag. For large long bag pulse dust collector, the tubular injection structure is much better than box injection. The box type injection is a disorderly injection without injection pipe. The ash cleaning air flow is directly injected into the upper box by the pulse valve and pressurized, and then the energy is transmitted to each filter bag in the chamber to realize ash cleaning; In the box type pulse jet dust remover, the dust removal intensity of each filter bag in different parts is quite different, and the general gas consumption is large, the length of the filter bag is limited, and the dust removal effect depends greatly on the air tightness of the off-line valve. Therefore, the box type jet dust remover is mostly used in small and medium-sized dust removers.

11. Rising speed of air flow

Inside the dust remover, the actual speed at which the dusty gas at the low end of the filter bag can rise is the rising speed of the air flow. The rising speed of the air flow has an important influence on the wear of the dust containing gas filtered by the filter bag and the dust separated from the filter bag due to pulse ash cleaning returns to the surface of the dust removal cloth bag with the air flow. The rising speed of air flow is the speed that the flue gas in the dust remover should not exceed. If it reaches or exceeds this speed, the particles in the flue gas will wear out the filter bag or take away the dust, and even lead to large equipment operation resistance.

The filtering of bag filter is divided into internal filtering and external filtering. The former flows from the inside of the filter bag to the outside, and the latter flows from the outside of the filter bag to the inside of the filter bag.

The air flow rising speed of internal filter bag filter is calculated according to the following formula:

Vk = Sa?Vc/S

Where

VK -- air flow rising speed of dust collector, M / min;

SA -- filtering area of single filter bag, m2;

VC -- filtration speed, M / min;

S -- sectional area of filter bag opening, M2.

The air flow rising speed of external filter bag filter is calculated according to the following formula:

Vk =Qv/(SA-nS)

Where

VK -- air flow rising speed of dust collector, M / min;

QV -- treatment air volume of filter bag room, m3 / min;

SA -- sectional area at the lower part of the filter bag chamber, m2;

N -- number of filter bags in the filter bag room, PCs.

The filtration speed and air flow rising speed shall be kept within a certain range everywhere in the bag filter. If the filtration wind speed is improperly selected or the distribution of compartments is uneven, the service life of the filter bag will be affected. Similarly, the improper selection of air flow rising speed or uneven air flow rising speed of compartments will also affect the service life of the filter bag. Therefore, in the design, it is not only necessary to design a reasonable filtering wind speed and diversion technology to make the air flow evenly distributed, but also to design and select a reasonable air flow rising speed according to the particle size, concentration and working conditions of dust, so as to ensure the extension of the service life of the filter bag.