Cleaning in a solvent offers a level of simplicity and forgiveness not seen in aqueous methods. At one time solvent cleaning was considered mandatory for successful vacuum processing but environmental concerns (VOC and other emissions) and improvements to aqueous systems including drying technology has seen the industry shift to aqueous cleaning as the norm. Today, however, with the advent of vacuum technology, vacuum vapor degreasing has emerged as a viable alternative to aqueous processing.
Solvent cleaning involves three basic steps: wash, rinse and dry. Washing is where the parts are immersed in or placed in contact with a (typically boiling) solvent to assist with the contaminant removal process. The purpose of rinsing is to bring “fresh” or clean solvent in contact with the parts. The aim is to dilute the contaminated solvent present on the surface of the parts from washing. It is important to remember that the rinse solvent must be kept clean. Contaminated solvent is a very common problem and will only reintroduce contaminants back onto the surface. The drying step evaporates the solvent and separates the rinse solvent from the parts.
Traditionally, solvent based degreasers depended on the application of heated solvent vapors in an open tank to accomplish the required cleaning tasks. The concept of heated solvent vapor cleaning is a simple one. Heated solvent vapors are drawn to the cooler surface of the parts to be cleaned. These vapors are actually drawn “ through “ organic contaminates by this temperature differential and in the process solubalize and flush off both organic and inorganic contamination.
The principle behind the vapor degreasing process is that a solvent can be used to dissolve then remove contaminants found on most parts by condensation. The basic idea is as follows: A tank of solvent is set up with a heating coil to bring the solvent to boil. As the solvent evaporates it rises to the fill-line in the chamber, above which is air with a much lower density than the solvent (often cooling coils are installed to reduce the amount of solvent released to the atmosphere). This contains the vaporized solvent in a closed space where the workpiece is placed. The solvent condenses on the more frigid workpiece and the now liquid solvent dissolves the contaminants (oils, greases, dirt, etc.) on the part. With the impurities contained in the liquid beads, the solvent runs off the part. Some systems are designed to capture and reclaim this solvent, making the process much more economical.
Vacuum Vapor Degreasing Technology
Today, vacuum vapor degreasers (Fig. 1) have dramatically improved upon and replaced the traditional open tank method of vapor degreasing by placing the process in a sealed vacuum environment, which employs a programmed sequence of heated vapors and cool solvent sprays. Emissions are virtually eliminated.
This cleaning sequence first allows the solvent vapors to be drawn into all areas of the parts and the load, including difficult to reach locations, such as blind holes, crevices and inverted “cups”. Due to the vacuum nature of the process, there is no air” bubble” in these tight areas to interfere with the solvent vapors reaching all surfaces . As the solvent vapors condense on the part surfaces, the surface temperature rises and the effect of vapor cleaning is reduced. At this point, cool solvent is sprayed onto the parts to lower the surface temperatures and flush the parts. When the spray phase has ended additional heated vapors are again introduced into the cleaning chamber and vapor cleaning is begun again. This sequence can be repeat a number of times.
A typical sequence would three two minute vapor phases and several (2 – 3) thirty second spray phases. This sequence provides a method of applying only freshly distilled solvent to all surfaces that are being cleaned rather than create a cross contaminations situation as is often done in solvent immersion cleaning. Additionally the use of sequenced vapor and spray reduces the volume of solvent that is being sent to distillation. Lower volumes of contaminated cleaning solvent being distilled greatly reduces the amount of energy that is used to distill and condense the solvent resulting in lower initial equipment and operating costs.
The sequenced vapor spray technique is managed by the PLC, and is variable in time and temperature and frequency which means that it is possible to “ custom “ design a cleaning process to the specific part and configuration that is being cleaned. Since cleaning is being done in a sealed chamber, the spray function can be designed to be mild to very aggressive, and since total solvent volume being used are smaller than immersion methods, this technique actually provides more cycle time to be dedicated to actual cleaning rather that chemistry management.
Sequence of Operation
In operation (Fig. 2), the parts to be cleaned and dewatered are placed in the vacuum process chamber, and the following sequence of operation takes place:
1. A vacuum is pulled to about 1 Torr.
2. At this point heated solvent vapors are brought into the cleaning chamber and allowed to condense on the parts. The transfer of heat by solvent vapors is very fast when compared to the transfer of heat by air.
3. As the parts heat, the water on and within the parts being process, flashes off the parts under vacuum and the exiting water-solvent vapor mixture leaving the process chamber is directed to the water management module. As the water flashes off, the parts would tend to cool except for the fact that as the parts begin to cool they attract more heated solvent vapors, thus keeping the parts hot until all the water has been flashed off of and from within the part. At this point the parts are free of water and can rise to the operating temperature of the solvent. With the water removed future steps such as immersion, rotation, etc., can be applied as needed to complete the cleaning of the manufacturing contaminants.
4. After the water removal, the heated part can now be dried of solvent by vacuum since less solvent is present than the original volume of water, the part now holds more heat than the water soaked part and because the latent heat of vaporization of the solvent is much lower than water.
5. After vacuum drying, the chamber is reduced to 1 torr to assure recovery of all solvent.
6. Finally, the solvent can now be separated from the water and be recycled for future use.
Solvent cleaning has a negative connotation in the heat-treating industry primarily due to environmental concerns, safety and cost issues. The emergence of vacuum vapor degreasing in a sealed vessel offers an attractive alternative that takes advantage of the best aspects of solvent cleaning. The size and amount of residual contaminants are reduced while meeting the most stringent cleaning requirements and avoiding the traditional problems of open degreasing systems.
Next Time: More on vacuum vapor degreasing including a comparison between air drying and vacuum drying.
- Wikipedia (www.wikipedia.org)
- Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.
- Gray, Donald and Joseph P. Schuttert, “Removal of Entrained Moisture from Powdered Metal Parts Using High Temperature Solvent and Vacuum” PM2TEC 2003.
- Herring, Daniel H., “It’s Time to Clean Up Our Act!”, Industrial Heating, 2008.
- “Hyperflo Sequenced Solvent Vapor-Spray Cleaning Technique, white paper.
Daniel H. Herring / Tel: (630) 834-3017) /E-mail: [email protected]
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.