The core working principle of a bag filter is the cycle of "filtration cleaning dust discharge", which uses the physical interception, inertial collision, diffusion and other effects of the filter bag (filter medium) to separate the dust in the dusty gas, achieving gas purification and dust recovery. The detailed workflow can be divided into the following 5 key steps, which are broken down according to the structural characteristics as follows:

1、 Core premise: Structure and medium foundation

Core components: filter bag (made of organic fibers such as polyester and PPS, or inorganic fibers such as glass fiber, with a surface covered with film or flocking to improve filtration accuracy), box body (divided into dust chamber and clean chamber, separated by flower board), ash hopper (collecting dust), ash cleaning device (pulse valve, air bag, spray pipe, etc.), fan (providing airflow power).

Filtering logic: The filter bag serves as a "filtering barrier" that allows gas to pass through but intercepts dust, which mainly adheres to the outer surface (external filter type, mainstream design) or inner surface (internal filter type, suitable for special scenarios) of the filter bag.

2、 Detailed workflow (closed-loop loop)

1. Entry and pre separation of dusty gases

Under the negative pressure (or positive pressure) of the fan, the dusty gas enters the ash hopper (bottom conical structure) from the inlet of the dust collector:

The velocity of the airflow inside the ash hopper decreases, and due to gravity and airflow diversion, large particles of dust (particle size>50 μ m) or condensed dust clusters directly settle and fall into the bottom of the ash hopper (pre dust removal stage, reducing the pressure of the filter bag filtration);

The remaining gas containing fine dust continues to rise and enter the dusty chamber (the area where the filter bag is located).

2. Filter bag filtration: dust interception and gas purification

When the dusty gas passes through the fiber gaps of the filter bag, dust separation is achieved through multiple mechanisms:

Physical interception: Dust particles with a diameter greater than the fiber gap of the filter bag are directly blocked on the outer surface of the filter bag;

Inertial collision: When the airflow passes around the filter bag fibers, dust particles cannot follow the airflow to turn due to inertia, and collide with the fiber surface and attach;

Diffusion effect: Fine dust (particle size<1 μ m) diffuses due to Brownian motion and is adsorbed upon contact with filter bag fibers;

Screening effect: As filtration progresses, a layer of "initial dust layer" is formed on the surface of the filter bag, which is equivalent to a "secondary filtration membrane", further improving the interception efficiency of fine dust (filtration accuracy can reach 0.1 μ m or more, efficiency ≥ 99.99%).

Purified gas: The clean gas passing through the filter bag fibers enters the clean gas room, collects in the outlet pipeline, and is discharged by the fan (meeting the standard discharge).

3. Resistance rise and dust cleaning trigger

As the filtration time increases, the dust layer on the outer surface of the filter bag (including the initial layer and subsequent attached dust) continues to thicken:

The dust layer will increase the resistance of gas passing through (known as "filtration resistance"). When the resistance rises to the set threshold (usually 1200-1500Pa, which can be monitored by a differential pressure transmitter) or reaches the preset time interval, the dust cleaning controller (PLC) will issue a dust cleaning command to start the dust cleaning process (to avoid excessive resistance causing an increase in fan energy consumption and a decrease in processing air volume).

4. Dust removal process: Dust stripping

According to the design method (mainly pulse jet type), the dust cleaning device performs "offline" or "online" dust cleaning on the filter bag (offline dust cleaning refers to temporarily stopping the filtration of the filter bag in this area for more thorough dust cleaning; online dust cleaning refers to continuous operation without stopping the filtration):

Taking mainstream pulse jet cleaning as an example: the controller triggers the pulse valve, and compressed air (pressure 0.4-0.6MPa) is instantly sprayed into the filter bag through the nozzle of the jet pipe from the air bag (forming high-speed airflow);

The high-speed airflow drives the filter bag to rapidly expand and vibrate, while forming a reverse airflow, completely peeling off the dust layer (including the initial layer) attached to the outer surface of the filter bag (the initial layer will not be completely blown off, leaving a small amount to maintain filtration accuracy).

5. Dust collection and discharge

The stripped dust falls into the bottom of the ash hopper under the action of gravity, and is continuously or intermittently discharged through the ash discharge device below the ash hopper (such as screw conveyor, star unloader) to avoid gas leakage from the ash discharge port and ensure dust removal efficiency. The discharged dust can be recycled (such as industrial raw materials, catalysts) or disposed of in compliance.

3、 Summary of Core Principles

The essence of a bag filter is "physical interception of the filter medium+secondary enhanced filtration of the dust layer", which achieves continuous and stable dust removal effect through the cycle of "filtration resistance rise dust cleaning dust discharge". The key lies in:

• Filter bag material and structure: determines filtration accuracy, temperature and corrosion resistance (such as PPS filter bags for high-temperature flue gas and PTFE filter bags for corrosive flue gas);

• Cleaning method: directly affects the dust stripping effect and the service life of the filter bag (pulse jet cleaning is widely used due to its high efficiency and minimal damage to the filter bag);

Airflow velocity: The filtering wind speed (the speed at which gas passes through the filter bag) should be controlled within a reasonable range (usually 0.8-1.5m/min). High wind speed can easily cause dust penetration and filter bag wear, while low wind speed can result in excessive equipment volume.