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.
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.
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
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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