Fabric Filter Components

Fabric filtration is one of the most common air pollution control technique used to collect particulate matter. Two basic types of filters are disposable and non disposable.

Disposable filters are similar to those used in a home heating or air conditioning system, They can be constructed as mats or s depth filters (12 inches or more). Mat Filters are usually made using fiber glass bats with a thin metal plate on the outside of the filter used for structural reinforcement. Depth filters are generally constructed using fiber glass, fiber glass filter paper or some other inert materiel such as fine steel fibers to form a deep mesh. The filters are very efficient (99.99%) for the collection of particles less than 1 micro meter in diameter but must be replaced when they become loaded with particulate matter (when the pressure drop across the filter exeeds design specifications). Depth filters are widely useful for the collection of toxic dust materials.

 Non disposable fabric filters of a fabric material (nylon, fiberglass, or other). These filters are commonly used to clean dirty exhaust gas streams from industrial processes. The particles are retained on the fabric material, while the cleaned gas passes through the material.

The collected particles are then removed from the filter by one of the following cleaning mechanism (1) shaking (2) reversing the air flow, or (3) pulses of air. The removed particle are temporary stored in a collection hopper until they are disposed of on are reused in the process.

Non disposable fabric filter systems are used many different industrial process to control particulate emissions.

 A fabric filter consists of the following components

1. Bags, fabric and support

2. Housing or shell

3. Collection hopper

4. Discharge device

5.Fiber cleaning devices

6.Fan

                      Typical fabric filter system

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Particle Behavior

Like any other physical entities, particle behave in fairly predictable ways, governed by the law of physics. particle from the industrial source generally float along in the gas stream. If we put something in their path they would bump in to it and under the right circumstances. stay there. That's what a filter is designed to do. but a filter by definition. has tiny pores to allow the gas molecules to flow through it. these molecules create a continuous stream around the fibers in the filter. small particle could easily be carried with the gas stream through the pores and into atmosphere. But larger particle don't have a chance. because of their greater inertia. they can't make the turn around the fiber. instead they keep going straight ahead until they impact on the fibers surface. we call this behavior impaction (see fig.1)

Medium sized particle have less inertia. actually they tend to start going around the fiber with the gas stream, but they can't quite make it.so instead of hitting the fiber head on, they end up grazing it on the side or being intercepted, this works too as a collection mechanism and we call this direct interception. (see fig 2).

Impaction and direct interception account for almost 99% collection of the particles greater than 1 micrometer in aerodynamic diameter in fabric filter system.( Bethea 1978). This tells you that fabric filters are pretty good air pollution collection devices for particles of this size.

Fabric filters can also collect very small particles. less than 1 micrometer in aerodynamic diameter. you would think this size particle would be carried right along with gas stream. in fact, these particles are so small, they just sort of bounce around and deflect slightly when they are struck by gas molecules. this individual or random motion causes them to be distributed throughout the fluid(gas) and is known as Brownian motion or Brownian diffusion. (see fig 3). The particle may have a different velocity than the gas stream and at some point could come in contact with the fiber and be collected.

Fig 1. Impaction

Fig 2. Direct Interception

Fig. 3. Diffusion 

Particle can also be collected because of other properties that occur in the gas stream. Relatively large particles may be overcome by the force of gravity and settle in the collection hopper.This force is particularly important when dust laden gas enters the baghouse through a hopper inlet where the large particles fall out of the gas stream before actually hitting the filters(bag). Particle can agglomerate or grow in size and then be more easily collected by the fibers. These larger particles are easier to filter from the gas stream.

Some particles have a small electrostatic charge and can be attracted to a material of opposite charge. Electrostatic charge could, on the other hand, have the reverse effect if the charges of the particles and fiber are the same. In this case the particles would tend to repel from being collected on the fabric.Electrostatic charges can be particularly useful for the capture of particles in the sub micron range. The use of a selected fiber material or specially coated material may enhance particle capture (Frederick 1974). Different materials will develop electrostatic charges of varying degree and sign.

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Fabric filtration design and baghouse component

Many industries today face that their exhaust gas contain too many particles. Baghouse are used in many industries because they are better efficiency in any sized particle. Fabric filters take advantages that the particle size is larger than gas molecule. Therefor the all particle collected by the membrane of fabric filter and release free gas to atmosphere. This collected particle on membrane will remove by usind jet puse of mechanical shaking. this particle collects at the hopper.

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