Sintered nonwoven stainless steel fiber filter medium .1. Dependence of filtration properties on fiber layers

The effect of fiber layers in sintered nonwoven stainless steel fiber filter media on filtration properties such as air permeability, pore size distribution and latex bead filtration efficiency has been investigated. The number of fiber layers through the medium thickness were directly proportional to the medium basis weight. At a constant porosity of 70%, when the basis weight of the medium made from fibers of the same size increased from 4.12 oz/yd(2) to 25.38 oz/yd(2), fiber layers increased from 6 to 34, air permeability at the pressure of 0.5'' H2O decreased non-linearly from 322.7 to 30.6 ft/min, and mean flow pore size decreased from 29.8 to 15.4 mu m. Latex bead diameter also at 98% filtration efficiency, defined as micron rating for the filter medium,(4) decreased from 36.0 to 16.0 mu m with this basis weight increase. The decrease of pore size in the fiber medium resulted from the position and orientation offset of filtration layers through the medium thickness. Individual fiber in each fiber layer was randomly distributed to form irregularly shaped pores and the pores in adjacent filtration layers (= fiber layers - 1) offset each other to result in a decrease in pore size through the thickness. Therefore, permeability of a fiber medium decreased nonlinearly as fiber layers increased. This non-linear decrease in the air permeability was due to pore size differences among the fiber filter media. In the medium with more fiber layers, pores were smaller and more tortuous. As a result, it captured rigid latex beads more efficiently resulting in higher filtration efficiencies or smaller micron ratings.