Choosing the Right Vapor Degreasing Method
Q: Our facility is exploring adding vapor degreasing as a way to clean parts. We understand there are various chemistries available for this, but are unsure of the benefits of each.
A: Like any equipment purchasing decision, a finishing shop’s choice about the type of parts cleaning unit to implement comes with deliberation by the company management about its needs. Considerations include the type of contaminant to be removed, construction of the parts being cleaned and time limitations for the cleaning process.
A properly designed, operated and maintained vapor degreaser is a reliable and cost-effective cleaning choice for many applications. However, before investing in a vapor degreasing system, a shop must understand the three types of solvent degreasing processes available: monosolvent cleaning, azeotrope cleaning and cosolvent/bisolvent cleaning. Even before that, however, it is helpful to understand how vapor degreasing works.
Vapor degreasing uses solvent immersion combined with vapor rinsing and drying to remove all types of oil, grease, wax, flux and particulate. It is a closed-loop system consisting of a top-loading steel vessel composed of two chambers, both filled with solvent. In one chamber, the solvent is heated to a boil and this generates a vapor cloud that rises to meet two sets of cooling coils. These cooling coils cause the vapors to condense and return to their liquid state. This liquid is then channeled to the second chamber (the rinse chamber).
A hoist lowers a basket containing the contaminated parts through the vapors and into the boil sump first. The basket is then transferred to the rinse sump containing the clean solvent that has been condensed from the vapors. This process is easily programmable and allows for excellent process control and repeatability. The parts come out clean, dry and ready for packaging or further processing.
In the past decade or so, companies have commercialized environmentally acceptable cleaning fluids suitable for vapor degreasing. This means that the speed, convenience and energy savings of this technology is now easily available to engineers everywhere.
The following describes the three types of vapor degreasing options and their chemistries. Note that each has its advantages as well as its drawbacks, but they can all be beneficial, if used in the right application.
This type of vapor degreasing contains only one component and is usually the most cost-effective option because it is easier to manufacture. The operator also does not need to monitor solvent concentrations or worry about what solution to add into the one-component degreaser. And, because the composition remains consistent, it can clean in both liquid and vapor phases, making it a practical option.
However, the monosolvent’s cleaning abilities are limited, and some of today’s most common monosolvents can have flammability or toxicity issues. N-propyl bromide (nPB), trichloroethylene (TCE) and perchloroethylene (PERC) are some of the most popular monosolvents and they can all contribute to safety and environmental problems.
Although monosolvents can be relatively aggressive in cleaning strength, they are best used to remove machining oils and lubricants consisting of hydrocarbon components. High-boiling monosolvents can be used to melt waxes and remove high-boiling contaminants. They also work well on metals, although it is important to monitor the process for acid acceptance if using one of the less stable (chlorinated) monosolvents.
An azeotrope is a blend of two or more components which, when mixed together, behave as if they were one chemical, delivering the benefits of a mixture of different elements with the convenient handling and storage of a single compound. It is a stable mixture that remains together in the liquid and vapor phases, so the structure stays the same throughout the vapor degreasing process.
For example, flammable and nonflammable ingredients can be combined to produce a stable, nonflammable mixture, an important safety benefit. This makes cleaning simpler, safer and more reliable.
Azeotropes use filters and membranes to distill (rather than trap) contamination. They can then be repurified while the contamination is “locked” into the liquid at the bottom of the vapor degreasing machine.
Azeotropes can be modified to obtain physical properties, making the blends useful across a range of applications and on a variety or combination of organic, inorganic and particulate contaminants. This means they work well on most substrates; and fluorinated azeotropes have the added benefit of not requiring acid monitoring.
Because they can be tailored to the contamination, azeotropic mixes typically clean much faster than some monosolvents, reducing the cleaning cycle time by as much as 75% and increasing productivity.
With many regulatory organizations, such as the U.S. Environmental Protection Agency, considering measures to reduce and restrict the uses of substances containing nPB, PERC and TCE because of safety concerns, azeotropic cleaners are becoming known as a viable replacement. Because they can be blended, chemicals such as nPB can be eliminated, giving them much better toxicity profiles.
Although azeotropes cost more per pound than monosolvents, they are still competitive. And the fact they are comprised of safer, more stable chemistries outweighs any cost implications. Additionally, training and process optimization can make cleaning with azeotropes more efficient and cost-effective than monosolvent cleaning.
Cosolvents/bisolvents are the most complex, but are also the most powerful cleaners available for a vapor degreaser. These systems use a nonvolatile cleaner in combination with a volatile rinsing solution. Most of the cleaning is done in the nonvolatile cleaner, which is then rinsed off by the volatile fluid. The benefit of these systems is that the nonvolatile cleaner can be adjusted and modified to fit the specific cleaning requirements. Occasionally, a contamination cannot be managed by a monosolvent or azeotrope because of material compatibility issues, throughput problems or environmental regulations.
A cosolvent may provide the required cleaning power without the use of ingredients under regulatory restriction or solvent-sensitivity issues. For example, it can be formulated to be nonchlorinated or free of volatile organic compounds.
However, there are a few downsides to cosolvent cleaning; the most obvious is the requirement for a separate cleaning tank. This solvent cannot be used in a one-sump vapor degreaser or with vapor-only cleaning, and the extra cleaning cycle results in a reduction in throughput.
Another disadvantage to this chemistry is that it requires monitoring to ensure the concentrations of each component are correct. This necessitates extra training to guarantee that, when necessary, the correct additions are made to the system.
What a cosolvent system can give the user in return is the ability to remove difficult soils, which often requires high temperatures. It delivers almost the same convenience and speed as traditional solvent cleaning, but increases this cleaning with a second chemical, providing the additional cleaning “horsepower” that the application may require.
Making an Educated Choice
Once shop management has determined which vapor degreasing method may be the right fit for an application, it is essential that the method first be tested. Experience and expertise are helpful in optimizing and developing cleaning cycles that will best suit a specific job, so it also is important to consult with a vendor who can test and provide cleaning solutions to meet requirements.
Venesia Hurtubise is a critical cleaning lab technical service chemist with MicroCare Corp.
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