The filtration and dust removal process of a bag filter is mainly completed by dust removal bags, which are carried out by the actual filtration layer. The actual filtration layer consists of a filtration layer composed of a collection unit and a loose and breathable dust layer formed by the deposition of dust particles on the filter material. Its capture principle is the same as that of wet dust collectors and electrostatic precipitators, which use some external force to disrupt the stability of dust particles in the airflow and promote their separation. At the beginning of dust removal, relying on interception, inertia force, diffusion, and the electrostatic effect of fibers, smoke particles deposit on the fibers on the surface of the yarn and in the fuzzing process. There is no smoke or dust deposition in the middle of the fibers inside the twisted yarn, but it passes through the clean air.
The basic requirements for the entire dust removal process for the filter layer are: to meet the gas flow rate passing through the unit filtration area, to maintain a resistance value not greater than a certain predetermined value, and to meet the requirements for structural strength, dust removal efficiency, etc. In practical applications, one of its important properties should be fully utilized, which is that filtering is done slowly, and flat bags that occupy less space than round bags should be selected as much as possible.
We generally use two forms of bag filters, namely the back blowing bag filter and the pulse bag filter. In the 1980s, China started with the development of large-diameter direct flow valves and further developed bag pulse technology, which has now been widely applied to the purification of aluminum electrolysis flue gas. Pulse bag dust collectors have gradually replaced anti blowing bag dust collectors.
Due to the limitations of the bag length of high-voltage pulse bag filters in China, which cannot exceed 2.6m, as well as the small diameter, large quantity, and high maintenance of pulse valves, their range was restricted. Therefore, most of the exhaust gas purification equipment for aluminum electrolysis at that time used anti blowing bag filters. This type of dust collector is widely used due to its advantages such as large processing capacity, simple structure, and the ability to perform offline maintenance in separate rooms. Practical operation has shown that it is feasible to use a back blowing bag filter in the aluminum electrolysis flue gas purification system.
Oil and water in the gas can cause trouble for the pulse system and affect the cleaning of filter bags. Therefore, it is required to have a de oiling and dehydration device in front of the pulse valve, and attention should be paid to anti freezing in cold regions during winter, so that oil-free compressors can be used for gas supply. There are two places where pulse valves are prone to malfunction: solenoid valves and diaphragms. Although there are already high-quality solenoid valves and diaphragms in China, there are still many counterfeit and inferior products with substandard quality. Special attention should be paid when selecting solenoid valves and processing diaphragms.
Pay attention to the selection of filter material and the sewing of the bag, which should be processed by the accessory manufacturer and sprayed with plastic on the surface. Although the cost is higher at once, it can prolong the service life of the filter bag.
The aging of the bag filter bag is mainly caused by the following reasons, and it is necessary to investigate the cause, take measures, and replace the dust removal filter bag.
1. Hardening and shrinkage due to abnormal high temperature;
2. Reacting with vapors of acids, bases, or solvents;
3. Reacts with water.
4. The filter cloth should not be hung too loose or too tight, as too loose can easily accumulate dust, and too tight can easily cause damage.
5. New technology and old cloth bags should not be mixed together to avoid different damage times affecting the normal operation of dust removal equipment.
6. The replaced cloth bag should be blown clean with compressed air first, and then checked for any holes. If there are holes, they should be repaired and left for replacement. If the cloth bag is covered in dust, rinse it with water, let it dry, and leave it for replacement.
In the design of bag filters, small dust collectors can handle air volumes of only a few cubic meters per hour, while large and medium-sized dust collectors can handle air volumes of up to one million cubic meters per hour. So determining the processing air volume is an important factor. In general, the size of a bag filter is directly proportional to the processing air volume. The design considerations are as follows:
1. The air volume unit is expressed in m3/min or m3/h, but attention should be paid to the location where the dust collector is used and the temperature of the flue gas. High temperature gases often contain a large amount of moisture, so the air volume is not expressed as dry air but as humid air, with moisture expressed as a volume fraction.
2. Because the performance of a bag filter depends on the actual filtering speed of humid air, if the processing temperature of the bag filter has been determined and the gas cooling is carried out by dilution method, the processing air volume of the bag filter at this temperature also needs to be calculated with the dilution air volume. When calculating the required filtration area, the filtration rate is the actual filtration rate.
To adapt to changes in dust sources, it is necessary to add several backup air volumes above the normal air volume in the design of the dust collector, so as to design the bag filter according to high air volume. If the bag filter operates beyond the specified processing air volume and filtration speed, its pressure loss will increase significantly, the filter cloth may become clogged, the dust removal efficiency will also decrease, and it may even become the cause of a sharp increase in other fault frequencies. However, if the backup air volume is too large, it will increase the investment and operating costs of the bag filter.
4. Due to changes in the temperature of the dust source, the processing air volume of the bag filter also changes accordingly. But the maximum air volume should not be calculated based on dust source misoperation and occasional malfunctions.
Once the air volume is determined, the filtration area can be determined based on the determined filtration rate. The filtration speed varies greatly depending on the form of the bag filter, the type of filter cloth, and the production operation process. The size of the filtration rate can be estimated based on experience by consulting relevant materials or similar production processes.
