Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 89 of 707

RINSE TANK DESIGN Care should go into the design of rinse tanks so that they may operate as effi- ciently as possible. The overflow weir must be level and full-length to minimize short-circuiting; lacking a level overflow weir, the fresh water added to the tank will take the path of least resistance, rather than mixing thoroughly with the con- tents of the tank. This would be a waste of water, as the overflowing waste- water would then be less concentrated than the tank contents. Equally important to promote good mixing is that the fresh water inlet be brought to the bottom of the tank and distributed via a perforated sparge pipe. As noted in the earlier discussion of counterflow rinsing, it is extremely important that sufficient gravity head exist, and the details of the design be prop- erly executed, to preclude a backwash of dirty water from the initial rinse to the final rinse. MINIMIZING WATER USAGE Once the desired dilution ratio is known, equipment may be installed that will help hold the concentration of contaminants down to the desired level with minimum waste. For example, a timer and solenoid valve can be rigged to deliver a fixed quantity of water to the rinse tank with each load. The timer can be actuated by foot treadle, limit switch, or, in the case of automatic lines, from the machine's control system. If loads vary in frequency and drag-out volume, a better remedy may be to employ a conductivity controller that is adjusted to feed additional rinse water only when contaminant concentration exceeds the acceptable level. It is almost always worthwhile to incorporate flow restrictors, which are sim- ple plumbing devices designed to govern the flow of fresh water to the rinse tanks, holding it to the desired setting despite fluctuations in water pressure. Designing an efficient rinsing system requires an understanding of the prin- ciples, clear thinking regarding the specific goals, and careful attention to the details of the installation. 88

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