Granular Bed Filters

Abstract

The separation of airborne dust in granular beds takes place in either a “cake” or a “noncake” (deep-bed) filtration mode depending on the region in the filter in which particle deposition actually occurs. During cake filtration, as the name implies, initially deposited dust layers serve as collection media for subsequent filtration and the granules in the bed serve only as a support for the separated dust. The main mechanism of particle separation is sieving: incoming dust particles are retained on the already deposited dust. This results in a significant increase in thickness of the deposited layer as filtration proceeds and is accompanied by a large increase in pressure drop. This in turn causes a compression of the deposited layer and hence results in a higher filtration efficiency as more and more dust accumulates at the surface of the cake. The efficiency of the filter in cake filtration is overwhelmingly a function of the deposited layer’s pore size and increases dramatically with pressure drop. If the dust particle size is larger than the pore size, dust is filtered and the efficiency is very high (practically 100%). However, if the dust is smaller than the open pore size, a cake is not formed and deep-bed filtration takes place.