Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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serves as a prefilter, and an inner layer, which serves as a trap filter. These handy cartridges are ideal for small filter chambers because of the ease and convenience of quickly replacing a conventional depth tube with the carbon tube when necessary. They may also be used with submersible filter systems, but in this case the flow rate could be greatly reduced. Carbon Canister Fig. 8. Suction or dispersion piping system with strainer and siphon breaker. Drill a hole 2 in. below working solution level as a siphon breaker to prevent solution loss due to unforeseen damage to piping, pump, and so on. Chlorinated polyvinyl chloride with screwed connections offers maximum flexibility and ease in installation and may also be used on the return line by eliminating the strainer and replacing it with a longer length of pipe that is open along the full length. Granular carbon may be used in ready-to-use chambers, each with a number of canisters holding up to 10 lb of granular carbon, and placed in line to the tank. A built-in trap filter eliminates migration of the carbon. Prefiltration ahead of the purification cham- ber will prevent solids from coating the surface of the carbon in the canister, assuring maxi- mum adsorbency. The carbon in the canister can be replaced when its adsorption capacity has been reached. This method of separate purification offers the most flexibility. Any portion or all of the filtrate can be treated as needed by means of a bypass valve after the fil- ter. Bulk Carbon Method Granular or bulk carbon is poured loosely around standard depth-type cartridge filters or sleeves, is poured into specific chambers designed for carbon, or is pumped between the plates or disks of other surface media. Since no filter aid is used, fines breaking off from the piece of carbon will have to be stopped by the surface media. Therefore, an initial recirculation cycle without entering the plating tank or recirculation on the plating tank prior to plating is desirable. This method does not alter the solids-holding capacity of depth-type cartridges, as most of the car- bon will stay on the outer surface layer; however, carbon removal is not easily accomplished. TIPS ON FILTER INSTALLATION Filtration equipment should be installed as close to the plating tank as possible in an area that affords access for servicing. Equipment that is not easy to service will not be attended to as frequently as required, and the ben- efits of filtration will not be maximized. The suction line should always have a larger diameter than the discharge to avoid starving the pump (e.g., 1 in. versus in. or 2 in. versus 1.5 in.) Where it is necessary to install the equipment more than 10 to 20 ft away, check the pump suction capabilities and increase the size of the suction piping (1.5 in. instead of 1 in., or 2 in. instead of 1.5 in.) to offset the pressure loss. Hoses made of rubber or plastic should be checked for compatibility with the different solutions. Strong, hot alkaline and certain acid solutions such as chromium are especially aggressive. The use of chlorinated polyvinyl chloride 607

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