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Fig. 3. Triple-stage rinse. tank. This is usually called the drag-in. The main mechanism that carries SALT OUT of the rinse tank is the overflowing rinse water. Some salt is removed from the rinse tank by drag-out too, but this is a minor factor. Figure 1 illustrates a rinse tank in equilibrium. If we assume perfect mixing (about which we'll have more to say later), we can cal- culate what overflow rate is needed in order to hold the concentration in the rinse tank down to a desired value. For example, if we know the drag-in to be 1 volume of plating salt per hour, and we have found that the acceptable concentration in the rinse tank is 0.1%, we can calculate the required overflow rate as follows: SALT IN = 1 vol. of drag-in 100% conc. = 1 unit of salt SALT OUT = SALT IN = 1 unit of salt = vol. out 0.1% allowable conc. Vol. out = 1 unit of salt out/0.1% conc. = 1,000 vol. Required overflow rate = 1,000 vol. — drag-out vol. = 999 vol. Figure 2 shows the same problem being addressed, but with two rinse tanks employed. The overflow rate need only be 30.6 volumes per tank, or 61.2 volumes total, to achieve the same 0.1% concentration in the final rinse. The total rinse- water leaving each tank is 31.6 volumes (30.6 via overflow plus 1 via drag-out). Note that 31.6 is, not coincidentally, the square root of 1,000. If the concentra- tion of the process tank is 31.6 times the concentration in the initial rinse, and the concentration in the initial rinse is 31.6 times the concentration in the final rinse, the concentration of the final rinse is 1/1,000 the concentration of the process tank. If the goal is to reduce the concentration in the final rinse to the required val- ue, with the lowest expenditure of rinse water, the optimum way to allocate the water is to distribute it evenly between the two rinses. But remember our open- ing assertions, this may not in fact be the main goal. When a third rinse is added, as shown in Fig. 3, the required overflow rate is reduced to about 9 volumes per tank (the total amount of rinsewater leaving the 83