Exponential Rinsing

Originally Published June 2017

Question: Why should you consider using multiple rinse tanks? Wouldn’t it be cheaper to build one big rinse tank? Won’t automation and parts handling be easier with one tank? Wouldn’t the cleaning process run faster?

Before we answer this… Let’s review product cleaning during manufacturing. Soil is matter out of place; cleaning is removing matter out of place. Most cleaning processes – industrial cleaning, precision cleaning, and critical cleaning – include wash, rinse, and dry steps. The wash step effectively removes soil from surfaces of the part and to keeps that soil away from the vicinity of the part. The rinse step removes residual soil and cleaning agent; typically you don’t want that cleaning agent any more – so we consider it to be a soil. The dry step removes residual water and/or adsorbed solvent.

And the answer is… Multiple, smaller rinses are better than a single large rinse.

To understand why, let’s look at two scenarios:

1: A single 300 gallon rinse tank is used. Let’s assume that it is 99.67% effective in removing cleaning agent containing residual soil. After rinsing, 0.33% or 3300 parts per million( ppm) of the soil will remain.

2: Three 100 gallon tanks are used sequentially. The same total amount of water is used, but each tank is 1/3 as big as in Scenario 1. Assuming that the rinse efficiency is proportional to the volume of water, three times as much residue will remain after the first rinse. That’s 1% or 10000 ppm of the soil. However, after the second rinse, only 1% of the 10000 ppm soil residue, or 100 ppm remains. The third rinse reduces the residue another factor of 100 to 1 ppm!

Multiple small rinses are more effective than fewer big rinses. Rinsing efficiency increases by the power of the number of rinses. We usually recommend at least two rinses. Frequently three rinses or occasionally even four may be needed depending on cleanliness requirements.

There’s a lot more to rinsing
Without effective rinsing, you more than likely won’t get clean surfaces; designing a great rinsing system takes “cleaning street smarts. The examples above do not include all the variables. In addition to multiple rinse tanks, you need to consider factors like cleaning action, soil loading, bath monitoring, corrosion prevention, and water quality. Stay tuned for more rinsing ideas to improve productivity and product quality. If you’d like to talk about rinsing right away, just contact us.

This article originally appeared in the May/June 2017 issue of Clean Source

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5 Comments

  1. Stephen M Kovach Healthmark Director of education Kovach says:

    thank you , love this i am going to share ,excellent explanation of the process.

  2. I’ve built a hundred cleaning systems with rinse tanks. Here was one effective design:
    * Picture 3 rinse tanks (1), (2), and (3).
    * Parts are immersed in tank (1), then (2), then (3).
    * As parts are lifted out of tank (3), parts are sprayed with clean DI water.
    * The DI water spray over tank (3) falls into tank (3).
    * Because water is being added to tank (3) the water level increases. So design an “overflow” pipe or slot from tank (3) into tank (2).
    * Tank (2) overflows to tank (1)
    * Tank (1) overflows to drain (or some kind of water cleanup system).

    * So parts get cleaner as they move from tank (1) through tank (3) and spray.
    * Water gets dirtier as it overflows the opposite direction from tank (3) to tank (1).
    * You can do the same thing with two tanks.
    * If parts are getting adequately rinsed, just increase the spray flow rate.

    In equilibrium this provides consistency of rinsed parts over time and minimizes water consumption.

  3. Paul Petruna says:

    If the cleaning process is heated (most are) the first rinse may be able to replenish the cleaner tank for evaporation loss thereby saving water (and cleaning chemical). If the second rinse is overflowed to a waste recovery system that can produce “clean” water (such as carbon, IX and/or UV or membrane) perhaps that recovered water can be used to replenish the second (or third) rinses…a win/win either way!

  4. Bob Musselman says:

    Using the first rinse to replenish a cleaning tank is an interesting concept but I’m not sure it’s optimal.

    Every cycle of parts builds up contamination in the clean tank to a point where cleaning becomes ineffective. Many cleaning operations just perform a dump and refill operation when the cleaning tank becomes too dirty, but that results in inconsistent parts cleaning. It’s best to perform a “feed and bleed operation” adding clean water to make up for evaporation and “drag out” (the water carried out with wet parts). An excellent system will monitor surfactant (or other cleaning agent) concentration by monitoring conductivity or some other measurement and injecting new chemical to maintain consistent cleaning agent concentrations. For really dirty parts, a constant flow of clean water with an overflow to drain might be necessary.

    But where does the “clean” water come from? Usually from your DI or other clean water source. Sure it could come from the first rinse tank, but that just provides another variable to account for, and by definition, the 1st rinse tank is the second dirtiest source of water in the system (the clean tank being the dirtiest).

  5. […] B. Kanegsberg and E. Kanegsberg, “Exponential Rinsing,” Clean Source, May/June 2017; https://bfksolutions.com/exponential-rinsing/  […]

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