Depth filters are the variety of filters that use a porous filtration medium to retain particles throughout the medium, rather than just on the surface of the medium. These filters are commonly used when the fluid to be filtered contains a high load of particles because, relative to other types of filters, they can retain a large mass of particles before becoming clogged. Depth filtration, as its name implies, involves the separation of a suspended particle or liquid droplet from its carrying fluid within the depth (thickness) of the filter medium.
The depth filter cartridge, in general is a cylindrical shape fitted into a cylindrical housing. This cartridge may either be an actual, self-supporting cylinder of thick porous material, or a similar shape created by the laying down of a thick layer of porous material on a cylindrical core, or the quite different format created by clamping together a set of lenticular discs, each made from two circular layers of thick material, bonded together at their outside edge.
The substance from which such cartridges may be made includes a very wide range of materials, both natural and synthetic. These may be natural or artificial fibres or filaments. Increasingly the important modern varieties of nonwoven materials made from extruded thermoplastics are being applied directly from the extrusion process onto a take-up core, to make a layer of any desired thickness.
The cartridge may also be made from a continuous multi-filament yarn wound at an angle around a core, to give a spirally wound medium. Alternatively, thick sheets of material, woven, nonwoven or bonded, may be cut, wrapped around a core and joined at the long edge.
However the depth filter medium is made, there is considerable scope for building in to the resultant filter element the required filtration efficiency, flow resistance and dirt-holding capacity, by the proper choice of basic fibre size and shape.
Many cartridges claim to be depth filters. But to be a true depth filter, a cartridge must be able to retain contaminants throughout the entire cross section of the filter. A graded density allows larger particles to be trapped near the surface while smaller particles penetrate the larger open areas to be trapped near the center of the filter in the smaller openings. Without graded density, a so-called “depth” filter will operate in practice only as a surface filter.