Air Pollution Control Devices: Bag Filters

India has committed to reducing air pollution levels. Pollution control boards across the country have more stringent, and industries are taking steps to combat the menace. A bag filter is a pollution control device that helps to tackle air pollution.

What Is A Bag Filter?

Bag filters or fabric filters are air pollution control devices commonly used to tackle air pollution from industrial boilers. The bag filter is used to remove particulates from the flue gas. The different types of bag filters are:-

  • Shaker
  • Reverse Air
  • Pulse-jet

The fabric filter has multiple compartments. Each compartment contains several long vertically supported bags. As the gas passes through the bag material, it separates the particulates from the flue gas. 

Manufacturers use woolen fabric/felted fabric/fiberglass to make the bags. The bag size is usually 305 mm in diameter, and the length varies between 9.1 to 11 mm. Usually, the bags are used for 3 to 5 years. 

Operating Fundamentals Bag Filter

The particle-laden flue gas leaves the boiler and enters the fabric filters. It leaves the bag filter due to the positive suction generated through the ID fan. The gas is distributed to the compartments for cleaning. The outlet plenum collects the clean flue gas from each compartment and directs it towards the chimney via the ID Fan. Each compartment has inlet and outlet dampers allowing the flexibility to isolate it for maintenance or bag cleaning. It has a hopper for gas inflow and a hopper to collect the particulate matter. The bags are closed at one end and connected to tube sheets to allow the flue gas to pass through. The layer of dust that accumulates in the bag filter is called dust cake. The main forces involved in dust collection are impingement by either direct contact or impaction and sieving. Other minor forces that help in the collection are diffusion, electrostatic forces, London Van der Walls forces, and gravity. Once the dust cake layer is formed, it helps in the filtration and improves the efficiency of the bag filter.

The filter needs to be cleaned when pressure drop increases across the bag filter. Equipment providers usually recommend the bag be cleaned after a fixed period or after a set pressure drop. The compartments are cleaned sequentially to remove excess dust cake and reduce pressure drop.

There are two important design parameters in bag filters:-

  • Air/cloth ratio is the volumetric flow rate of the flue gas to the exposed bag area.
  • Drag

Cleaning The Bag Filter

Both reverse air and shaker bag filters are cleaned are inside-the-bag filters with the air flowing from inside the bag to the outside. 

In a reverse air bag filter, the reverse air or reverse gas flow from the outer plenum chamber back into the bag collapses the isolated bag filter and dislodges the dust cake. Once the dust particles are collected, the bag is inflated again. A reverse gas fan is required to support the flow of reverse clean gas. Additional dampers help regulate the flow. Some manufacturers include a sonic horn to help in cleaning.

Shake bag filters have a similar reverse flue gas mechanism where reverse air is fed into the isolated bag filter. Mechanical motions are used to shake the bag to remove accumulated dust cake.

While the pulse-jet bag filter is an outside-the-bag filter with clean gas flows from outside to inside. The ‘Pulse Jet’ technology is considered to be more vigorous. It can be used when the bag is isolated or in service. A pulse of compressed air is directed from the open-top causing shock waves across the length of the bag, causing the dust cake to get dislodged from the outer surface of the bag. Most pulse bag filters have a wire gauge around each bag to protect the bag during filtration. A series of pulse jet pipes are located above the bag each having a solenoid valve. The operator can clean one bag at a time. 

Advantages Of Bag Filter

  • The collection efficiency of bag filters is very high 99%. The fabric filter can handle different particle sizes and a range of grain loading. 
  • Collection efficiency is not affected by the presence of sulfur.
  • Reduced sensitivity to particle size distribution.
  • The filter does not need a high voltage to operate.
  • The fabric filter can be used when the flue gas is inflammable.
  • With a few modifications, it can help in the sub-micron removal of smoke.

We, at Mago Thermal Private Limited, offer a range of air pollution control equipment including bag filters. Our bag filters are carefully designed based on inlet and outlet gas temperature, pressure drop, opacity, and gas velocity. The chemical properties and physical properties of the flue gas are factored in the design. 

Mago Thermal has a professional and experienced team of engineers that design and manufacture air pollution control devices. To know more about bag filters in India, call at +91-9910490700.