Cleaning Q&A: Rinsing Following Cleaning
A. The most important question examines the number of rinse stations. Because rinsing efficiency is logarithmic in nature, you gain tremendous efficiencies in water reduction when adding rinse tanks beyond the first tank.
Each rinse tank addition reduces the amount of counter-flowed rinse water by a factor of 8 to 10 if thoroughly mixed. Thorough mixing does not require an air- or pump-agitated rinse tank. Although that can be helpful, the primary goal in rinse tank design is to encourage as much natural mixing as possible. For instance, you would not want to have the rinse tank water inlet positioned close to the outlet. The inlet should be on the opposite side of the outlet. Addition of a baffle in the tank that would drive the new water to the bottom by gravity would be a significant help to ensure good mixing. The case for two or three rinse tanks can be made after considering the expected amount of rinse water reduction, though two rinse tanks may be sufficient. However, a third rinse tank may be necessary if clean process water is very scarce in your manufacturing location or if you reduce water flows sufficiently to achieve a zero-discharge process stream.
The addition of heat may be the second most important question. Heat is usually added to the final rinse tank to assist in flash drying of the part to minimize rust or water spotting. However, this can also affect your decision with the number of rinse tanks. The lower flow for additional rinse tanks may significantly reduce energy usage and maintain a higher temperature in the other rinse tanks. To achieve good drying, especially in the absence of a dryoff oven, you will likely want the rinse tank at about 160°F to 180°F.
I cannot predict the rinse time since that will be a function of several factors, probably the most important being the part geometry. Additionally, the only rinse that matters is the final one. Upstream rinses are intended to remove the majority of the residue from the previous tank.
Originally published in the June 2016 issue.
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