Fine mist filtration by wet filters—II: Efficiency of fibrous filters

Abstract

Stainless steel fibrous filters of 4–22 μm wire diameter have been studied as collectors for dense mists of approx. 1 μm particles. The liquid collected in the filter in the steady state when equilibrium has been reached with the aerosol stream is found to increase the filter collection efficiency, sometimes very greatly, under most conditions studied. Only in the diffusional collection regime at very low fibre volume fractions (packing densities) or large fibre diameters does collected liquid diminish the efficiency. These findings are explained in terms of the effect of the liquid distribution on the collection mechanisms. In particular, collected liquid usually increases the efficiency of all significant single fibre collection mechanisms by increasing the effective packing density of the fibres, i.e. by adding to the distortion of a single fibre's flow field caused by neighbouring fibres. An existing model for dry fibrous filters has been modified to take account of the liquid distribution and provides good quantitative prediction of the relative collection efficiency of wet and dry filters having packing densities over 0.04.