Metal Finishing Guide Book

2013

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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. 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 cartridge where the openings are large, whereas the smaller particles are retained selectively by the smaller openings on succeeding inner layers. This, then, makes it possible 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 cartridges of 10 μm down to 1 μm will have a dirt-holding capacity of perhaps 3 oz to less than 0.5 oz. These figures merely indicate that the coarser cartridges have greater dirt-holding capacity, are more economical to use, and can be used longer before replacement. Also, as pointed out earlier, dirt loads vary from tank to tank, and cartridges should be selected according to the individual requirements. A dense cartridge having less dirt-holding capacity will load up more quickly, increasing the pressure differential and, therefore, reducing the flow (Fig. 5). Using coarser cartridges (greater than 30 μm on zinc, for example) that have greater dirt-holding capacity and a longer service life may make it possible to clarify the plating tank more quickly because of the high obtainable flow rate. This will be accomplished at less cost. Usually two cartridges (three on zinc, tin, and cadmium) are recommended for each 100 gal of tank capacity. The pump should provide a pumping rate of at least 100 gph (two tank turnovers per hour) for each cartridge. Usually, a cartridge life of 6 weeks on nickel or 4 weeks on zinc can be expected, with some tanks running as long as 12 weeks; however, much depends upon dirt load, hours of plating, and so on. With cartridges, a higher dirt load can be retained in the filter chamber because of the coarseness of the filter media. Higher flow rates can usually be 640

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