Bag filter

Bag filter (also known as bag filter or filter dust collector) is a dry and efficient dust removal equipment that uses porous fiber filter material to physically filter and separate dusty flue gas. It is currently the mainstream core equipment for achieving ultra-low emission of flue gas dust in industrial fields such as coal-fired power plants, steel, coking, and cement. It is also an essential environmental protection facility for coal transportation systems and boiler flue gas treatment. Its capture efficiency for PM2.5 ultrafine dust can reach over 99.99%, and the export dust concentration can be stably controlled below 5mg/m ³ for a long time, fully meeting the ultra-low emission limit requirements of GB 16297-2023 "Comprehensive Emission Standards for Air Pollutants".

1、 Core working principle
The core operation of the equipment is divided into two stages: filtration and dust removal, and pulse cleaning. The entire process can be achieved through a PLC system to achieve fully automatic unmanned operation:

1. During the filtration and dust removal stage, the dusty flue gas enters the dust collector through the air inlet and is evenly distributed by the guide plate before entering the conical ash hopper. Large particles of dust settle directly into the ash hopper due to gravity inertia; Fine dust enters the middle box filter chamber with the airflow, and when the dusty gas passes through the filter bag, it is trapped on the outer surface of the filter bag through five comprehensive effects: screening, inertial collision, fiber interception, Brownian diffusion, and electrostatic adsorption, forming a stable initial layer of dust (the core layer that ensures filtration accuracy); The purified clean gas passes through the filter bag and enters the clean air chamber of the upper box. Finally, it is sent to the subsequent desulfurization system or chimney for standard discharge through the air outlet by the induced draft fan.

2. During the pulse cleaning stage, as the dust layer on the surface of the filter bag thickens, the pressure difference between the inlet and outlet of the equipment continues to rise. When the set threshold (conventional 1200-1500Pa) is reached, the PLC control system automatically starts the cleaning program. Compressed air is sprayed at an instantaneous high speed of ≤ 0.1s through an electromagnetic pulse valve, inducing several times the amount of air injected into the filter bag, causing the filter bag to rapidly expand and vibrate from the bag mouth to the bottom, shaking off surface dust; After the dust falls into the ash hopper, it is continuously discharged through the ash unloading/conveying device. The mainstream models adopt a single room offline ash cleaning mode. During ash cleaning, the corresponding chamber lift valve is closed, cutting off the flue gas passage, and the other chambers are filtered normally, which does not affect the 24-hour continuous operation of the whole machine, and the ash cleaning effect is more thorough.

2、 Classification and adaptation scenarios of mainstream models
In industrial applications, the cleaning method is the core classification basis, and different models are adapted to differentiated working conditions. Among them, the long bag low-pressure pulse bag dust collector is the absolute mainstream selection in the thermal power industry, and the core classification is as follows:

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3、 Core structure composition and technical requirements for key components
Compliant with GB/T 6719-2023 "Technical Requirements for Bag Dust Collectors", the core of the whole machine is divided into seven functional units, and the technical requirements for key components are as follows:

1. The main load-bearing structure of the pressure bearing assembly equipment is divided into three parts: the upper box (clean air chamber), the middle box (filter chamber), and the lower box (conical ash hopper). It is welded with Q235B carbon steel, with a designed compressive strength of ≥ 5000Pa and a body air leakage rate of ≤ 2%. Epoxy zinc rich anti-corrosion treatment is applied to the inner wall, and high-temperature resistant anti-corrosion coating is used for high-temperature working conditions; Ash hopper cone angle ≥ 60 °, to avoid dust bridging and material blockage, equipped with arch breaking air cannon/vibrator, maintenance manhole, and level gauge. The core key component flower board (separating the filter chamber and the clean air chamber, used to fix the filter bag cage) has a flatness of ≤ 1/1000, a hole diameter tolerance of ≤ 0.5mm, and a filter bag spacing of ≥ 50mm, ensuring reliable sealing of the filter bag installation and no risk of wind or dust leakage.

2. The core functional components of the filtration core unit (filter bag+bag cage) equipment determine the dust removal efficiency and operational stability.

1. Filter bag: PPS needle punched felt and PTFE coated filter material are commonly used in thermal power plants, with a weight of 500~550g/㎡. The diameter of the filter bag is usually φ 130/φ 160mm, with a length of 6~8m. The upper end is sealed and fixed with an elastic expansion ring and a flower plate. The sewing process uses a three needle machine double line sewing, with a seam strength of ≥ 185N and a dynamic filtration efficiency of ≥ 99.5% (for particles with a particle size ≥ 1 μ m).

2. Bag cage: made of carbon steel/stainless steel welded together, with a surface treated with organic silicon spray for corrosion prevention. The number of vertical bars is ≥ 8 (for a diameter of 130mm bag), and the spacing between the ring bars is ≤ 200mm. There are no burrs or welding scars, and the gap between the bag and the filter bag is uniform, supporting the filter bag to prevent collapse and avoiding wear on the filter bag.

3. The core power unit for the stable operation of the pulse cleaning system equipment consists of an electromagnetic pulse valve, a spray pipe, a compressed air storage tank, an oil-water separator, and a PLC control system. The pulse valve adopts a right angle/submerged structure, with a working life of ≥ 1 million times and a response time of ≤ 0.1s. The coaxiality deviation between the spray pipe nozzle and the center of the filter bag is ≤ 2mm, ensuring accurate coverage of the entire filter bag by the cleaning airflow; The control system supports three cleaning modes: timed, fixed resistance, and manual. The new pressure difference adaptive adjustment function can dynamically optimize the cleaning cycle based on operating resistance, achieving energy-saving operation.

4. The inlet and outlet air and airflow distribution system consists of an inlet and outlet main pipe, branch pipes, guide plates, and rectifier grilles. Through CFD flow field simulation optimization design, the uniformity of flue gas distribution in each compartment is guaranteed to be ≥ 95%, avoiding local high flue gas flow velocity from washing the filter bag. At the same time, it strengthens the pre settling of large particle dust and reduces the load on the filter bag.

5. The lifting valve and offline cleaning unit are standard for the offline cleaning machine, which adopts a thin plate type double sealed lifting valve structure. The sealing is reliable and the response speed is fast. During cleaning, it can be isolated in a single room, achieving offline operation and avoiding secondary adsorption of dust during cleaning. The cleaning efficiency is improved by more than 30%.

6. The ash unloading and conveying system consists of a star shaped unloader (ash valve) and a screw conveyor/scraper conveyor. The star shaped unloader has an air locking function to avoid secondary dust flying due to air leakage. It is equipped with frequency conversion control and can dynamically adjust the ash unloading frequency according to the material level of the ash hopper.

7. The safety monitoring and auxiliary system is equipped with online monitoring devices for inlet and outlet pressure difference, temperature, and material level, as well as supporting maintenance platforms, ladders, lighting, and observation windows; Supporting explosion-proof valves, anti-static devices, and fire extinguishing systems for flammable and explosive working conditions; The high-temperature working conditions are equipped with accident sprinkler cooling system and over temperature alarm device to ensure the safe operation of the equipment.

4、 Core technical parameters and key selection indicators
Core technical parameters of mainstream long bag low-voltage pulse models

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Selection of core control indicators

1. Filtration wind speed: a core parameter that determines equipment specifications and operational stability. For working conditions with fine dust particle size, high temperature and humidity, high concentration, and strong viscosity, the filtration wind speed should be selected as ≤ 1.0m/min; Under normal temperature drying, low concentration, and large particle dust conditions, the maximum speed should not exceed 1.5m/min.

2. Net filtration area: When selecting, the filtration area of the offline ash cleaning chamber should be deducted to ensure that the net filtration air speed of the remaining chambers does not exceed the design limit during single chamber ash cleaning, avoiding a sudden increase in equipment resistance.

3. Adaptability to working conditions: When selecting, it is necessary to fully consider the flue gas temperature, humidity, acid-base corrosiveness, dust abrasion, and flammable and explosive characteristics, and match the corresponding filter material, material, and safety protection system.

5、 Core Filter Material Selection Guide (Operating Condition Adaptation Core)
The selection of filter material directly determines the service life and dust removal effect of the filter bag. The core is matched according to the flue gas temperature and working conditions. The mainstream selection is as follows:

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Core principle of selection: The temperature resistance level of the filter material should be at least 10 ℃ higher than the actual maximum temperature of the flue gas, and the instantaneous temperature resistance should be 5 ℃ higher; For high humidity and high viscosity working conditions, priority should be given to using membrane filter media and waterproof and oil proof filter media; Anti static filter material must be selected for flammable and explosive working conditions.

6、 Core advantages of the product

1. The dust removal efficiency is extremely high, perfectly adapted to ultra-low emissions. The ability to capture PM2.5 ultrafine dust far exceeds that of electrostatic precipitators. The dust removal efficiency is stable at ≥ 99.99%, and the export dust concentration can be controlled below 5mg/m ³ for a long time without being affected by dust specific resistance. It is suitable for complex coal types such as high ash low-quality coal and high sulfur coal, fully meeting the national ultra-low emission environmental protection requirements.

2. Strong adaptability to working conditions, stable and reliable operation, able to adapt to working conditions with large fluctuations in smoke volume and dust concentration. The modular structure of the compartment can achieve non-stop online maintenance. The service life of the filter bag can usually reach 3-5 years, and the service life of high-quality PTFE filter material can reach more than 8 years; The whole machine has no complex high-speed moving parts, low failure rate, and can achieve 24-hour continuous stable operation.

3. Energy saving and efficient, with low lifecycle cost, in compliance with the GB 37484-2023 "Energy Efficiency Limits for Dust Collectors" standard. The operating resistance of Class 1 energy-efficient models is less than 800Pa, which can significantly reduce the power consumption of induced draft fans; The standardization level of vulnerable parts is high, and the replacement of components such as filter bags and pulse valves is convenient. The fully automatic dust cleaning system does not require dedicated personnel to be on duty, and the operation and maintenance costs are much lower than other dust removal equipment.

4. Environmental protection without secondary pollution, strong collaborative governance ability, dry dust removal process, no wastewater generation, fully enclosed structure without dust overflow, in line with environmental protection requirements; It can synergistically remove some pollutants such as sulfur trioxide, heavy metals, and acidic aerosols in flue gas, and seamlessly integrate with desulfurization and denitrification systems to build a complete flue gas treatment system.

5. With a high degree of intelligence, it is suitable for the construction of smart factories, supporting pressure differential adaptive dust cleaning, online fault diagnosis, filter bag damage detection, and real-time uploading of operating data. It can seamlessly connect to the power plant DCS/SIS system through industrial protocols such as MODBUS and PROFIBUS, achieving remote fully automatic control and adapting to the construction needs of smart power plants and unmanned workshops.