| Pressure drop and filter collection efficiency were evaluated experimentally and theoretically for a variety of fibrous filters, as they were loaded with polydisperse mists. Filter performance was evaluated during the transition from clean to saturated conditions. Test aerosols included metalworking fluids and simple, organic liquids to provide a range of physical properties. Face velocities ranged from 5 cm/sec to 150 cm/sec.; As fibrous filters were loaded with liquid aerosols, pressure drop and collection efficiency increased for most conditions studied. Filter pressure drop increased more rapidly when loaded with the same volume of high density and viscosity liquids, when compared to lower density and viscosity liquids. Early in the filter loading process, different liquids produced only minor differences in filter pressure drops. However, as more and more liquid collected within the filter structure, pressure drop differences between loading liquids increased.; Although filter collection efficiency generally increased with loading, there were some notable exceptions. For filters with low initial packing densities and filter materials that readily held static charge, large drops in collection efficiency occurred for all particle sizes at low face velocities. Drops in efficiency were usually temporary in nature and occurred when the pressure drop ratio was less than five. Eventually the collection efficiency stabilized and later began to increase as loading continued. These collection efficiency results were explained in part by a loss of electrostatic charge, increased effective fiber diameter, and flow channeling within the filter. Filters loaded with nonwetting liquids tended to have higher fractional collection efficiencies than filters loaded with other liquids under the same conditions.; Data collected during the experimental portion of this study was used to develop empirical models. The filter pressure drop/thickness and the pressure drop ratio were expressed in terms of the revised packing density, flow, liquid properties, and related parameters. Pressure drop models were based on theories from fluid flow through porous media and granular filtration. It was also demonstrated that in Stoke's regime, the total filter collection efficiency can be expressed in terms of the pressure drop ratio, regardless of the loading liquid. |
