Metal Finishing Guide Book


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Fig. 4. Comparison of filtered volumes for 100 gal of solution after 5 min of filtration at respective turnover rates. It has also been found that the effective life of surface filters may often be tripled by doubling the surface. By increasing the dirt-holding capacity and reducing the frequency of filter servicing and replacement, the cost of filtration on a per month or per year basis is substantially reduced. TYPES OF FILTER SYSTEMS After estimating the dirt load and determining the flow rate and filtration frequency required, a choice of filter method and medium must now be made. The most common types of filters used in the plating industry are discussed below. These filters may be placed inside or outside the tank. In-Tank Considerations: Tank space Motors located over fumes Limited size of filter (less service life of media if used on pump suction) Out-of-Tank Considerations: Remote possibility for easy service Employ sealless magnetically coupled pumps or direct-drive with single or 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 particle-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, 639

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