Many manufacturing processes use compressed air, especially those in the food and beverage, cosmetics, electronics and pharmaceutical industries. Pneumatics can have several functions in manufacturing: process air, temperature control, cleaning, drying and motive power for equipment. In each case, exhausting air to ambient can create several problems in a production environment.
Direct or indirect contact with production equipment and products may be a contamination issue and a health and safety consideration for workers. In the food and pharmaceutical industries, regulatory compliance is also a factor and in specialty fields such as microelectronics or microfluidics, contamination on the micron scale can ruin processes and components.
Where does oil come from in pneumatic systems? Oil may be introduced as blowby from compressors or may be injected into the delivery stream to lubricate equipment, usually with an FRL assembly. Frequently, if contamination from oil is a concern, manufacturers will specify the use of oil-free compressors for their systems. However, even oil-free compressors are not able to supply truly “oil free” air, as ambient air contains oil in the form of vapor.
Where Does Oil Vapor Come From?
Oil in ambient air? Like many contaminants in the form of very small particles or droplets, oil can remain in suspension in the atmosphere similar to dust and can be deposited on any surface or be attracted to a surface electrostatically. Oil vapor, however, is invisible in ambient air, which means if a manufacturer doesn’t take precautions, then it’s possible for a clean, efficient system to deliver contaminated air to production equipment and into otherwise clean operations.
What do we really mean by “oil” in this context? In terms of compressed air systems, oil vapor in ambient air refers to a combination of hydrocarbons and other volatile organic compounds (VOCs). VOCs easily change state from liquids to gases making them compressible and easy for a pneumatic system to process. Under normal temperatures and pressures, the amounts of these compounds found in ambient air are small, between approximately 0.05mg/m3 and 0.5mg/m3 of air. In certain areas, such as parking lots or urban areas with high levels of air pollution, these amounts can be higher.
Unfortunately, there is no single “oil compound” to test for in order to determine whether there is oil vapor in the ambient air. Instead, oil compound levels can be inferred from air quality testing data and measurements of other compounds, including hydrocarbons, VOCs and compounds known to be hazardous to human health such as NOx, SOx, CO, CO2 and ozone. Most are pollutants and greenhouse gases and many local, national and global organizations monitor levels in ambient air. This is useful for pneumatic system designers that need baseline contamination data. It is also highly variable by location; specific needs may require special attention if the manufacturer is located in a region with high pollution.
Why Oil Vapor is a Problem for Air Compressor Systems
Knowing that there are oil compounds in the ambient air is only half the battle; manufacturers must still understand how this oil vapor behaves in a compressed air system, and how to prevent it from reaching their people and products.
When compressing ambient air into a smaller volume, any contaminants within the air are concentrated—and the higher the pressure a volume of air is compressed to, the greater the concentration of those contaminants will be in the compressed air. Contamination is also a rate problem: the concentration of contaminants is also affected by the amount of air going through a system per unit time. A higher volume of air will contain a higher measure of contaminants.
While a 40-micron filter might be adequate to protect an impact wrench or pneumatic cylinder, 80 percent of airborne particles are 10 microns or smaller. At this size, contaminants are invisible to the human eye; “oil free” air cannot be determined by sight, feel or smell. When aftercooling is used to remove water, the reduction in flow temperature may be just right to condense volatiles farther downstream in the system.
Many manufacturers specify oil-less compressors in the belief that this means the compressed air itself will be free from oils. While these do reduce the amount of oil introduced into the air stream from the compressor lubrication system, for ISO Class 0 or 1 requirements, it’s simply not enough.
To achieve “Technically Oil Free” compressed air in accordance with ISO8573-1 Class 0 or Class 1 for Total Oil requires the inclusion of downstream filtration in order to reduce liquid oil, oil aerosols, and oil vapor. Reducing the oil loading from the compressor is still important to prevent the flow from overwhelming the after-filtration system, but it’s only the beginning of a good program.
Testing for Contaminants in Compressed Air Systems
A strong, comprehensive air quality program begins with awareness of the effect of oil vapor in ambient air, how oil contaminants behave under compression, and how to test for oil contaminants in a compressed air system. Success here removes a major factor in process variability and makes production and quality control easier in manufacturing.
There is a lot more to the issue of truly oil free compressed air in manufacturing. Download this whitepaper from Parker Hannifin to get a detailed look into the testing and identification of ambient air contaminants, ISO compliance, and methods of testing for oil contaminants in compressed air systems.