Several points of industrial dust collector design

The working principle of the bag filter: the dust-laden flue gas enters the filter bag, the gas passes through the filter bag, and the dust is blocked on the surface of the filter bag. So as to achieve the filtering effect.

There are many factors involved in the industrial dust collector design of a baghouse. This article tries to summarize all the relevant factors that the author can do, for your reference:

The processing air volume determines the size of the baghouse. In determining the air volume design of the dust collector, in addition to the conventional indicators, additional attention should be paid to the use site of the dust collector and the temperature of the flue gas.

 

The air volume design index value should increase the safety factor of 5% to 10% on the basis of the normal air volume. Otherwise, once the process adjustment increases the air volume in the future, the filtration speed of the bag filter will increase, thereby increasing the resistance of the equipment and even shortening the service life of the filter bag, which is also the reason for the sharp increase in the frequency of other failures. The safety factor should not be too large, otherwise it will increase the investment and operating costs of the dust collector. Once the processing air volume is determined, the necessary filtering area can be determined according to the determined filtering air speed.

If the deviation between the working temperature of the bag filter and the design temperature is too large, the equipment will fail. The working temperature of the dust collector is limited by two conditions:

One is: the maximum allowable temperature of different filter materials (allowable temperature within 15 minutes and operating temperature for 24 hours);

The second is: in order to prevent condensation, the gas temperature must be kept above the dew point of 20 °C.

For high temperature gas, it must be cooled below the temperature that the filter material can withstand (280 degrees). There are many ways to cool dust-laden gas, such as natural air duct cooling, forced air cooling, water cooling, and installing waste heat boilers.

The design of bag filter is generally calculated according to the amount of air after treatment. Only when parameters such as density, viscosity, mass heat capacity, etc. are related to the design, the design of the dust collector can consider the composition of the gas.

In many working conditions, most of the flue gas contains moisture. As the moisture content of flue gas increases, the equipment resistance of the bag filter and the energy consumption of the fan also change. The moisture content of the dust-laden gas can be determined by actual measurements or calculated from the mass balance of combustion and cooling.

Tips (highlight): Whether there is corrosive gas in the flue gas is a factor that must be considered in determining the selection of filter material, dust collector shell material and anti-corrosion. In addition, if there is toxic gas in the flue gas, it is generally a trace amount, which has little effect on the performance of the device. However, when dealing with such dusty flue gas, the bag filter must adopt an air-tight structure, and it must be maintained frequently. Regular maintenance, to avoid the leakage of toxic gas caused by personnel safety accidents.

The dust concentration at the dust collector inlet is usually expressed by the mass of dust in a standard volume.

(1) When the inlet dust concentration increases, the equipment resistance will also increase with the same filter area.

(2) Under the conditions of high concentration dust and strong wear, the inlet of the dust collector should have diversion and wear resistance treatment technology, such as sintering dust, alumina powder, silica sand powder, etc.;

(3) When the inlet dust concentration is very high (>2000g/m3), a lower filtering wind speed and a pre-dust collector can be considered.

(4) The dust removal capacity of the dust collector shall be based on a 110% bonus on the ability to discharge all collected dust. The capacity design of the multi-stage ash discharge device should be based on the ash discharge capacity of the lower layer being greater than the ash discharge capacity of the upper layer.

(1) When the adhesive and agglomerated dust enter the bag dust, the dust becomes larger after being slightly agglomerated.

(2) Fine dust has a great influence on pressure loss, coarse dust plays a decisive role in wear, and particles with high hardness have a major impact on equipment.

(3) Needle-shaped crystal particles and flake-shaped particles are easy to block the pores of the filter material and reduce the dust removal efficiency.

(4) The smaller the bulk density of dust, the more difficult it is to remove dust. Therefore, a lower filtration speed should be selected in the design. In addition, the bulk density of dust is very important in selecting the capacity of the dust collector’s dust removal funnel and ash acceleration device.

(5) The dust with strong hygroscopicity can easily absorb the moisture on the surface of the filter material, and it becomes viscous when it consolidates with water, which makes it difficult to clean the surface of the cloth bag. Necessary technical measures should be taken for dust containing strong pollutants such as KCl, MgCl2, NaCl and Cao.

(6) Slightly charged dust will generate static electricity on the filter bag, which is not easy to fall off. Antistatic (conductive) filter material must be used. And to prevent the explosion of very slightly charged dust caused by static electricity.

(7) Although sparks appear in the process stream before the dust collector, it does not always cause an explosion. However, for sparks that may enter the dust collector, explosion-proof measures should be taken, such as adding igniters and setting explosion-proof doors.

The equipment is a general-purpose bag filter with a pressure of 4000~5000Pa, and a long-bag pulse filter with a pressure of 6000~8000Pa. For blast furnace gas dry pulse filter, the design pressure requirement is 0.3MPa.

The cleaning pressure 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), and the pulse valve is divided into high pressure valve according to the pressure in the air bag (right angle valve) and low pressure valve (submerged valve).

At present, single-chamber dust collectors use high-pressure cleaning pressure, while long-bag multi-chamber pulses use low-pressure cleaning pressure.

Different blowing pipe cleaning structures are selected for different cleaning pressure designs. The size of the opening of the blowing pipe, the structure type, the height of the blowing pipe and the flower plate, etc., will directly affect the cleaning performance of the equipment.

(1) Online cleaning. During the pulse jet process, the filter bag still filters the dusty gas. On the line, cleaning, filtration, and cleaning have little effect on system fluctuations, but incomplete cleaning will cause secondary dust.

(2) Offline cleaning. The dust collector is divided into multiple filter chambers, and each bag chamber is independently equipped with control systems such as offline valves, cylinders, and electromagnetic pulse valves. Before the pulse bag cleaning and injection into each filter chamber, the bag chamber is first closed through the off-line valve, so that the filter chamber can be cleaned without filtering, so as to complete the deashing and cleaning of the bag. The equipment can be designed for on-line maintenance (extraction work, maintenance and repair).

For small dust collectors with low intake air concentration, online dust removal is generally used. Large dust collectors are usually designed as off-line configurations.

(3) Tubular injection. A jet tube is placed on each set of filter bags, with the nozzle facing each filter bag. Make sure every filter bag is sprayed.

(4) Box type blowing. After closing the off-line valve, compressed gas is sprayed into a separate chamber for cleaning; it can ensure that most of the filter bags are sprayed. Can save energy. Small and medium-sized dust collectors can use box-type blowing.

Inside the dust collector, the actual speed at which the dust-laden gas at the low end of the filter bag can rise is the airflow rising speed.
The air rising speed of the bag filter is calculated as follows:

Vk =Qv/(SA-nS)
where Vk is the rising speed of the air flow of the dust collector, m/min;
Qv——The processing air volume of the filter bag room, m3/min;
SA —— the cross-sectional area of the lower part of the filter bag chamber, m2;
n ——the number of filter bags in the filter bag room, pcs.

In the design, it is not only necessary to design reasonable filtration wind speed and diversion technology to make the airflow evenly distributed, but also to design and select a reasonable airflow rising speed according to the particle size, concentration and working conditions of the dust, so as to ensure the service life of the filter bag.