Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 5. In comparison with Figure 3, these curves show the effect on the rate of contaminant removal by using a coarser filter medium. Dirt pickup may increase for a while due to more effective filtration; the solids pickup increases the filter medium density after which it decreases as flow rate is also reduced. A, the highest possible flow rate; B, addition of filter aid reduces flow; C, addition of carbon; D, maximum dirt particle removal; E, no flow. double water-flushed seal More suitable for use with slurry tank for chemical or filter aid/carbon addition or backwashing Larger dirt holding and flow capacity from cartridges or surface media Cartridge Filters Cartridges offer both surface and depth-type filtration characteristics, providing various levels of particle retention at different efficiencies (nominal and absolute), manufactured in natural and synthetic (plastic) materials to provide a wide range of chemical resistance, flow rates, and particle retention capacities. Pleated- surface media offer initially higher flow rates, are available with a choice of porosities (usually in the denser range), and are sometimes given an absolute par- ticle-retention rating. Depth-type media are available in 1- to 100-µm particle retention and, because of the variety of porosities available, they are sometimes best suited to handle high- dirt-load conditions. This is a result of the manner in which the depth-type cartridge filter is manufactured. Basically, it consists of a series of layers, which are formed by winding a twisted yarn around a core to form a diamond opening. The fibers, which are stretched across the diamond opening, become the filter media. Succeeding layers lock the previously brushed fibers in place and, since there is the same number of diamond openings on each layer, the openings become larger due to the increase in circumference; other fiber-bonded types also increase density across the depth of the media. During filtration, the larger particles are retained on the outer layers of the car- tridge where the openings are large, whereas the smaller particles are retained selec- tively by the smaller openings on succeeding inner layers. This, then, makes it pos- sible for an individual cartridge to have a dirt-holding capacity equal to 3.5 ft2 of surface filter area of the same density. Cartridges having a 15- to 30-µm retention will often hold 6 to 8 oz of dry solids before replacement is necessary, whereas car- 602

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