Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 85 of 707

Fig. 4. Counterflow rinse. tank is 10 volumes, the cube root of 1,000). If a fourth rinse is used, the required flow rate is about 4.6 volumes per tank. Thus, multiple rinses are gainfully employed where high dilution ratios are required, as for example in the last rinse before drying. Where minimal rinsing is required, for example between a soak clean and electroclean process, it may prove difficult to justify even two rinses. COUNTERFLOW RINSING Figure 4 illustrates the use of a technique called "counterflow rinsing," whereby the water overflowing from the final rinse is used as the input source for the ini- tial rinse. Because the same rinsewater is used twice, this method cuts the required water flow almost in half (compared with Fig. 2), to about 31.1 volumes total. Solving the formulas for counterflow rinsing is a little more complicated because the rinsewater flowing back from the final rinse to the initial rinse carries some salt with it. Looking at the initial rinse: SALT IN = 1 vol. of drag-in 100% conc. + counterflow vol. conc. of final rinse SALT OUT = (1 vol. of drag-over + overflow vol.) conc. of initial rinse Looking at the final rinse: SALT IN = 1 vol. of drag-over conc. of initial rinse SALT OUT = (1 vol. of drag-out + counterflow vol.) conc. of final rinse The concentration in each rinse thus depends on the concentration in the oth- er rinse. The traditional way of finding the concentrations involves using special 84

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