Filtration is dependent on the porous material characteristics, the surface area available for filtration and the process conditions of the application. Filtration with porous sintered metals is best applied to gas-solid or liquid-solid separations. The filtration properties are very different for gas and liquids since for the same filter media, the filter efficiency in a gas stream will be higher than for a liquid stream. Sintered metal filters can also be used for gas-liquid separations such as removing water from air due to surface tension differences. Filters are specified and compared by their desired performance characteristics which include high permeability, low pressure drop, retention efficiency of specified particle sizes, particle loading capacity and resistance to blinding. The desire to have high permeability must be balanced with the minimum mechanical properties required for filtration. As the porosity is reduced, the density of a filter increases resulting in higher strength and lower permeability. Predicting filter performance in an application is difficult since there are many process variables. Process variables such as particles size, particle shape, compressibility, composition, particle concentration, fluid flow rate, viscosity, vibration, service temperature and pressure must be specified, so actual filter testing is recommended.
Filtration occurs by both Surface and depth type mechanisms to separate solids from a process stream. Surface filtration is characterized by the formation of a layer of particles on the upstream surface of porous media due to sieving and bridging of the particles. As the filter cake forms, the layer of particles begins to filter the process stream and the porous media acts as more of a support. Wire mesh screens are examples of surface type filters which are generally used to retain particles coarser than 140 micrometers. Finer filtration retention can be accomplished with screens, but the higher pressures required to drive the fluid often pushes the finer particles into the screen openings and causes pore blinding. Surface filters can be easily cleaned by reversing the flow direction if properly sized. Depth filtration is a more complex form of filtration than surface filtration since it is characterized by the retention of particles within the interconnected pore structure. Particles are retained by a variety of mechanisms Sieving mechanisms also occur in depth filters depending on the size and distribution of the contaminants.
For filtration requirements less than 140 micrometers, porous sintered materials offer a combination of surface and depth filtration that can withstand the higher operating pressures associated with depth type filters. The permeability decreases and the corresponding pressure drop increase across the filter as the thickness of the media is increased. Filtration ratings can be determined from standardized testing under tightly controlled conditions in order to compare various filter grades and materials. Standardized filter efficiency tests using glass beads, test dusts or salt particles will more accurately determine filter efficiency ratings.