Cleaning in batches
07 September 2006
Tom Forsythe of Kyzen looks at a 'by the numbers' approach to low cost of ownership in batch cleaning systems
What does low cost of ownership mean? These days, too many define it simply as outsourcing production to China. Many others agree that it goes to the lowest bidder. This is based on a vast body of scientific data and analysis focused on identifying the critical parameters that drive cost of owning and operating a cleaning process. Those parameters make a list for determining the process with the lowest cost of ownership.
One of the fastest ways to lower cost of ownership is lower operating concentrations. New technologies that operate at 50 percent lower concentration should save money. Let's review why this assertion is true. First, the primary consumption operation in a cleaning process, particularly a batch process is known as drag out. Drag out is the wash solution that leaves the wash tank with your manufactured parts and is rinsed off in the second stage of the cleaning process. The constituents of drag out (cleaning agent, water and the soils that have been introduced combined) correlate 100 percent with the targeted operating concentration of the wash tank. While the quantity of drag out will vary due to the nature of the assemblies, the design of the pallets or baskets and the basic design of the cleaning machine, the constituents of drag out are 100 percent the same as the constituents of the wash tank. Therefore, higher concentration means higher cost given equal cost of the cleaning agents. This is true for all equipment choices, batch or in-line, cabinet or immersion. There is much discussion about how temperature affects operating costs. Focusing on batch systems, the most popular systems fall into two broad categories: cabinet/dishwasher type designs and immersion/ultrasonic type designs. The cabinet/dishwasher style features technology that has advanced dramatically in the past few years, making this style attractive to the low-rate production operations so pervasive in Europe and North America. Of particular importance to the discussion of temperature in the cost of ownership equation is how these machines have evolved and have become more cost efficient. Modern cabinet/dishwasher style machines are routinely rated to run at temperatures as high as 158°F/70°C. Why, when higher temperature creates a higher cost cleaning operation, would the equipment designers incur the expense and design challenges associated with such high temperatures? The cabinet washer designers have focused on key design criteria to eliminate the need for significant ventilation, as it is a well-known science that higher temperature often enhances cleaning, adhesives often being a key exception. While new, modern cleaning agents often are quite effective at low temperatures, overall process enhancements can be achieved if they are run at higher temperatures. Likewise, it is well-known that this temperature equals cost concept if more accurately described as high ventilation at high temperature equal high cost. Minimalising ventilation was the design goal, as this has several benefits:
Less ventilation allows better chemistry containment
Few worker exposure concerns (liquid or vapor)
All but eliminating chemical consumption due to exhaust - Less ventilation allows better heat containment
Efficient use of higher operating temperatures _ potentially shorter cleaning cycles, and typically lower chemistry concentrations (lower drag out costs)
Lower heating energy costs due to heat energy retained in the process
Better process control.
There is another reason that temperature is a key operating parameter. Certain soils, including a high percentage of lead-free soldering materials, are quite responsive to temperature. Because increasing temperature has little to no affect on batch cleaning cost of ownership, evaluating the effect high temperatures will have on cleaning results. Particularly with less effective cleaning materials, high temperatures allow them to provide a reasonably effective if higher cost solution. Fortunately, the solid execution on the operating temperature critical design criterion provides a significant opportunity for these batch systems to effectively clean lead-free material with no cost of ownership penalty. Console Equipment, whether or not equipment with ultrasonic transducers: In these systems, many of the cleaning design elements are dramatically different. First, the cleaning materials are not atomized or sprayed-in-air, but rather the process is run in a liquid volume, generally with some sort of agitation beneath the surface. The agitation can be ultrasonic energy, but also a simple circulating pump system moving the cleaning fluid is often employed. In either case, there is a relatively static volume of fluid. Now back to temperature. Science tells us that if we are considering the effects of temperature on the process losses associated with a relatively static volume of fluid, one must understand exactly the physical properties of the materials on the surface of that fluid volume. This basic science is why one finds cleaning chemistry providers offering products specifically tailored for these immersion processes. Simply because the parts must pass through the surface of this fluid volume upon entering and exiting the cleaning process.
A review of more than 10 leading materials designed for ultrasonic applications reveals most with reported boiling points of 100°C, with several others in a range of 100(-110°C. Once again, basic science is the key to understanding this data. All of the materials evaluated for their boiling points are designed to be mixed with water, substantial amounts of water. Therefore, it is logical that the boiling points of these mostly water solutions are close to the boiling point of water itself. With comparable boiling points, losses due to fluid evaporation will be comparable across the broad range of materials and suppliers. With process temperatures generally in the 123°/50° or so range, evaporative losses are largely restricted to the water added to dilute the product. Little active ingredient is lost to evaporation.
For other than immersion processes, there are many technologies offered for aqueous cleaning that are designed to split or separate. Often these materials leave materials floating on the surface of the wash tank, and rarely a good choice for immersion processes. Hence, cleaning technology providers often have specialty materials (often with a US for ultrasonics moniker) for these immersion processes. These materials d minimize the evaporation losses of the water component; however, this separation characteristic is a fatal flaw for their application in traditional immersion processes. Cleaning Cost of Ownership: Process Mechanical Energy: As previously discussed, there are many mechanical energy options for use in a cleaning process. All use pumps or, in the case of ultrasonics, transducers as the energy source for their equipment. Cleaning Cost of Ownership: Process Time: Time is the process variable that all manufacturing operations understand completely. As with the operating concentration of the cleaning material, it is a classic case of less is more 100 percent of the time. This returns us to the temperature parameter. If higher temperature has no affect on the cost of ownership, and high temperature can help accelerate cleaning at low concentrations, then high temperatures will allow for shorter cleaning cycle times.
In conclusion, if you are cleaning in a modern batch system, designing the lowest cost of ownership cleaning process can be accomplished by the numbers. First, select the cleaning chemistry that allows you to run at the lowest operating concentration. Though do be careful to avoid those that offer half the concentration for double the price. Second, temperature can only help speed up the process (unless you are cleaning adhesives). Third, various options for mechanical energy tend not to be significant cost of ownership drivers in their own right. Fourth, a faster cleaning process can clean more parts per manufacturing hour. That faster process has a lower cost of ownership from both an operating and, more dramatically, from a capital expenditure perspective.
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