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The cartridges with thick media are intended to work by depth filtration, and their contaminant loading capacity is an important feature of their usability. Thick media will often be graded so as to have an increase in density towards the centre of the cartridge, thus improving their depth filtration behaviour. Once the maximum load has been accepted then the cartridge will normally be discarded, unless a high proportion of the collected contaminants can be removed by backflushing.
The thick media cartridges can be formed in a number of ways:
● if sufficiently flexible, the medium can be made as a flat sheet, then cut to the correct size and wrapped around the core, before the two edges are sealed together
● the medium can be made as a sleeve, which is then cut to length and slipped over the core (although some sleeves are self supporting and do not need a core), or
● the basic material of the filter medium can be deposited on the core, to the required thickness, and then finished as necessary to give the required filtration properties.
Whichever of these manufacturing techniques is used, the resultant elements will look roughly the same: a cylindrical layer of porous material between two end-caps (one sealing closed one end of the cartridge, the other with appropriate connections
to carry the filtrate out of the filter), ready to be fitted into a housing of the appropriate size. The cartridges are made in a range of lengths and diameters, mainly interchangeable among the available housing sizes.
Almost every kind of material can be made into the thick medium used on these cartridges, including:
● unbonded fibre, as a felt or a needlefelt (mainly natural fibre, which has the necessary fibre adhesion)
● woven multifilament yarns of natural or synthetic fibre, wound as a sheet with several turns on the core to give a porous medium of the required thickness
● resin (i.e. adhesive) bonded fibre, moulded to shape, in a mass that will need to be cured (to set the resin), employing fibres of cellulose, glass or plastic
● thermally bonded (i.e. sintered) fibre, of plastic, metal, glass or ceramic
● resin-bonded plastic granules
● thermally bonded or sintered granules of plastic, metal or ceramic, and
● foams of metal or ceramic.
An important group of polymeric fibre or filament media is that where the extruded plastic (spunbonded, meltblown, electrospun) is dry-laid directly on to a rotating cylindrical core to make a medium that is as thick as required. This also enables the fibre density to be varied with the depth of the fibre layer, giving a coarse structure at the medium surface, with finer pores towards the centre.