American Coatings Association


Industry News, Science and Technology, and Market Insight

Clean Air

by Clifford K. Schoff, Schoff Associates

Air is necessary for life and compressed air is necessary for spraying paint, but both ambient and compressed air can cause a lot of mischief. Incoming air in paint plants and paint shops may bring in dust, dirt, insects, and other contaminants that can affect the quality of paint and painted ware as well as the health and safety of workers. Therefore, air is filtered wherever possible. Painting outdoors or in facilities without spray booths can be difficult because of the lack of control of air quality. Wherever painting occurs, compressed air used in spraying must be clean with no measurable oil content and little or no water or other contaminants.

Air filtration often is taken for granted, yet may or may not be effective. On their roofs or inside on upper levels, paint plants and industrial painting facilities have air houses that contain banks of air filters, usually more than one of many possible types—frame filters, bag filters, panel filters with pleated media, even filters with folded glass fiber media. Air houses and their filters must be inspected on a regular basis (once a week during down time is common), the action noted on a check-off sheet, and any faulty or damaged filters replaced. Things to look for include torn, collapsed and poorly fitting or missing filters, water from roof leaks, and rust stemming from those leaks. Many facilities now have gauges that measure pressure drops between the upstream sides of filters or banks of filters and the downstream sides. Inspection is simple—read the gauge, note the reading, and act on any unusual readings. No drop at all usually means that air is flowing without filtration due to holes, collapse of filter media, or some other filter failure. A large drop likely means that the filter is clogged. Filters may be replaced when a certain pressure drop is reached or on a preset schedule.

Besides the air houses, a plant may have other filters closer to work areas. For example, plenum structures above paint booths may have intermediate levels with filters in openings above the usual blanket filters. Inspecting that area is difficult, but should be done periodically (quarterly or every six months) because of possible non-working filters and build-up of dirt, including rust particles resulting from water leaks. The blanket should be inspected whenever the booth is cleaned. It must fit perfectly with no holes, tears, or pull-away from its frame and must be replaced according to schedule.

Air that reaches a factory floor or spray booth may be clean, but defective processes may manufacture dirt or contamination within the plant. Two examples are excessive sanding and sanding in open areas rather than enclosed booths. In both cases, the dust may well travel throughout the plant.  Other examples include poor housekeeping, faulty pumps or other devices that mist or spatter oil, overly lubricated chains, smoking ovens, and construction occurring in the plant.

Compressed air systems vary widely. New or relatively new plants use oil-free air compressors, and air lines usually are stainless steel. Older plants may still have oil-type compressors and black iron piping. All systems include filtration and drying units. In addition, paint application areas have traps on the air lines to remove oil, water, and dirt.

It is important to test compressed air, particularly at the point of paint application. A simple qualitative test for this is to spray air from a spray gun or air nozzle into cotton wool for at least five minutes, then check and see if any oil, water, or dirt has been caught. Quantitative testing at any point in the system can be done with the Draeger Aerotest Air Tester (Draeger, Inc., Houston, TX). There may be other testers available, but this is the only one that I ever have encountered. Compressed air is run through the device and special Draeger tubes are used to measure oil mist and water vapor levels. The oil mist concentration must be so low as to not be measurable (i.e., less than 1 mg/m3) with a Draeger 6733031 tube. Oil is bad is because it causes craters. The water vapor concentration should not exceed 200 mg/m3, and it is better to be at 100 mg/m3 or below. One reason for keeping water vapor at a low level is that water acts as a carrier for dirt and oil and can contribute to cratering and dirt in finishes. If there is measurable oil in the compressed air, the oil tube will provide the concentration, but cannot identify the oil. If necessary, tubes containing charcoal or silica gel can be used to collect oil and other contaminants so that they can be identified in an Analytical laboratory.

Oil/water/dirt traps attached to the compressed air supply can be useful indicators of possible causes of problems. If a lot of material is being trapped, it is likely that some is getting through and is landing in the paint.

It is very important to control ambient and compressed air quality where paint is being tested, albeit in the QC or technical lab at a paint plant, in a customer factory, or in a third party test lab. Lab housekeeping and cleanliness are equally important. I have seen too many examples of lab contamination (including dirty air) being the source of defects when paint is being tested.  I wonder how many batches of perfectly good paint have been rejected and sent for rework or disposal because of testing under dirty conditions.