Adhesion in Gas Cleanup Filters

Filtration and Adhesion. In the cleanup of dust-containing process gases or air, the particles suspended in the gas stream pass through a porous filter medium and stick to the surfaces of this material. The dust precipitate thus accumulated becomes, in turn, the filter medium for the subsequent particles. As the deposit builds up, the porosity of the medium decreases, hindering the free flow of gas. At some point this deposit must be removed. 

Thus we see that adhesion is manifested at two stages in the filtration process, first, in the retention of particles when they come into contact with the filter element so that they are not carried away when additional amounts of gas pass through the filter and, second, in regenerating a plugged filter. 

The relationship between adhesive properties of the dust and the filter material is often the specific factor governing the selection of a particular type of filter fabric or other material, the method of regeneration, and the optimal conditions for filter operation. 

In order to clarify the role of adhesion in the filtration process, let us examine the deposition of particles on isolated cylindrical fibers placed in an aerosol stream. They way in which the dust deposit is formed on individual cylindrical fibers of a permeable filter barrier, with a flow velocity of 1 m/sec, is shown in Fig. XII. 1. The clearly visible local side growths of lead and zinc oxide dust particles (particle size approximately 1 jim) are directed at an angle of 110-120° to the flow axis. When more aerosol passes through the filter, the growths may meet, forming a continuous layer that acts as a secondary filter medium [322]. 

We must emphasize the difference in nature of adhesion to dust-free (fresh) and dusty filters, particularly in the case of highly disperse dusts. The duststopping capability of a filter increases very markedly as it picks up dust a secondary porous filter barrier formed by particles of precipitated dust in the 

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