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Although fabric filters have high dust collection efficiency, they have the following issues: (1) the pressure losses are significant, (2) the collection efficiency for sub-micron particles is low, and (3) there are blowing leaks during initial operation. The electrostatically aided fabric filter is a fabric filter that deals with these issues by providing an electrostatic charger at the stage before the fabric filter.
As shown in Fig. 1, the electrostatically aided fabric filter has an electrostatic pre-charger installed in the stage before the conventional fabric filter. The electrostatic charger consists of a discharge electrode and a grounded electrode. The dust in the gas is momentarily charged by the corona discharge from the discharge electrode, which causes the dust to agglomerate to form large particles, which are collected on the filter cloth in the subsequent stage.
The dust that has become large due to electrostatic agglomeration is collected in its coarse form on the surface of the filter cloth. The state of the dust collected on the surface of the filter cloth is shown in Photographs 1 and 2. Photograph 1 shows the dust layer on the surface of the filter cloth that was trapped without being charged by the electrostatic charger, and Photograph 2 shows the dust layer on the surface of the filter cloth that was charged to a high voltage by the electrostatic charger. We can see that the dust is trapped in its coarse form because it has been agglomerated by the electrostatic charge.
The electrostatic agglomeration effect of the electrostatic charger causes dust to be collected on the surface of the filter cloth in its coarse form, which produces the following effects: (1) the pressure losses in the primary adsorption layer are reduced, (2) pressure losses are reduced for the same particle dust load, (3) the sub-micron particle collection efficiency is improved, and (4) the blowing leaks during initial operation are reduced, and so on.
Fig. 1: Principles of electrostatically aided fabric filters
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Photograph 1: Dust layer on the surface of the filter cloth when there is no electrostatic charge
Photograph 2: Dust layer on the surface of the filter cloth when there is a high-voltage charge
The charged dust is collected on the surface of the filter cloth, and forms a primary layer. Further, because the subsequent charged dust is agglomerated and repelled by the static charge of the primary layer, it is collected on the filter cloth in its coarse form.
As the packing density of the dust layer on the surface of the filter cloth is low, the pressure loss of the primary layer of the electrostatically aided fabric filter is about 20% less than that of conventional fabric filters. Also, for the same particle load, the pressure loss is about 30% less than that of a conventional fabric filter. Therefore, the exhaust fans consume less electricity, and we can make the fans smaller.
By reducing the pressure loss, we can increase the time intervals for cleaning the dust collected on the filter cloth, which means that we can reduce the number of dust removal operations, which gives the filter cloth longer life.
Figure: Particle dust load and pressure loss
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Electrostatically charging the fine particles promotes agglomeration into coarse particles, so using electrostatic forces to collect charged fine particles on the dust layer makes it possible to improve the collection efficiency of sub-micron particles, which are difficult to collect.
As shown in the figure, electrostatic charging makes it possible to significantly improve the collection efficiency to 93% for sub-micron particles with diameters of 1 µm or less.
Figure: Improving collection performance by charging
During the time when the gas starts to pass through the fabric filter until the primary layer of dust is formed on the filter cloth, there is a problem with fine dust through and leaking.
With the electrostatically aided fabric filter the dust is electrostatically charged, so the straight fine dust is collected by the filter cloth starting from the time the gas initially passes through, and the dust leaks are drastically reduced.
Figure: Blowing leaks and charging effect