Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Figure 6. Spray rinsing compared to immersion rinsing. approximately 5 minutes to drop the concentration from 5% to 2.5%. The main point is that a single rinse tank is relatively ineffective at providing critical rins- ing. More importantly, increasing the flow rate in a rinse tank does not neces- sarily improve rinsing unless extremely high and costly flow rates are used. By taking immersion rinse flow rates one step further, Figure 4 shows the same rinsing example as shown in Figure 3, with the exception that every 10 minutes an additional load of dragout chemical is added. Note that this causes the rinse tank concentration to rise to very high concentrations very quickly, regard- less of the flow rates used. This is another reason that counterflow rinsing is so effective. The concentration of the dragout chemistry between the first rinse tank and the second rinse tank drops dramatically. Thus, the effective dilution rate due to water flow is much faster, as shown in Figure 5. The final method of reducing rinse water volumes but still obtaining excellent rinsing is by spray rinsing. This method is somewhat limited by the geometry of the parts being rinsed in that complex geometric shapes are difficult to thoroughly rinse with an automatic spray system. In a manual line, the operator can over- come the geometry problem of a part by manually spraying the part areas that are difficult to rinse by a normal battery of spray nozzles. Figure 6 compares a spray rinse to an immersion rinse. There are two major advantages to spray rinsing over immersion rinsing. First, the water hitting the parts is always clean—unlike water in an immersion tank which always contains some residual contamination. Second, a spray rinse needs to be running water only when parts are being rinsed. The rest of the time there is no water use, which is both a cost and environmental savings. A third and lesser advantage to spray rinsing would be in the case of parts requiring a heated rinse. In-line demand heaters can be used to provide hot water as needed during the spray cycle rather than having to continuously heat an immersion rinse tank. The spray system in Figure 6 illustrates the water savings associated with spray rinsing compared to immersion rinsing. The left illustration in Figure 6 is 567

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