Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 602 of 707

Fig. 4. Comparison of filtered volumes for 100 gal of solution after 5 min of filtration at respective turnover rates. In modern electroplating, no area that can result in improved quality should be overlooked. The plater can use the principles of high tank turnover and solu- tion velocity to his advantage in his quest for zero rejects. During recent years the flow rate through the filter, or tank turnover as it is referred to, has increased to two or three per hour or higher for most plating solutions (see Table I). This means that 1,000 gallons require a flow rate of at least 2,000 to 3,000 gallons per hour (7.6-11.5 m3/hr); however, platers should recognize the need and employ turnovers of 10 or even 20 times per hour when all solids must be removed (see Fig. 1). Alkaline solutions may require even higher flow rates for more effective solids removal by recirculation. Depending on the filter medium and its retention efficiency, flow rates in the range of 0.5 to 2 gpm (2 to 8 Lpm) per square foot of filter surface area are obtainable. Although 5 gpm per 10-in. (25-cm) cartridge is permissible, flow rates under 1.5 gpm per cartridge offer better economy. In fact, at a given flow rate with a cartridge filter, servicing, cartridge cleaning, or replace- ment can be reduced significantly by increasing the size of the filter. For exam- ple, if the size of the filter was multiplied by four the annual amount of filter cartridges con- sumed would be cut in half and the filter itself would operate unattended for at least four times as long before cartridge cleaning or replacement was necessary. This is an important con- sideration to reduce media consumption. 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 fre- quency 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 601

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