Half-truths can be believable
03/01/2000
by Richard A. Matthews
MYTHS: 1) Positive pressure prevents airborne contamination from entering the cleanroom. 2) Cleanroom HEPA filters never need to be changed. 3) Air showers are 100 percent effective at removing contamination from personnel, equipment and supplies entering the cleanroom. They compensate for poor prep and gowning procedures. 4) The cleanroom will decontaminate itself. REALITY: Humans must constantly control the integrity of clean space.
Most myths have strength because in some ways they are half-truths and therefore have some built-in believability. This is certainly true of the four myths mentioned above, which were submitted by a reader. Let me debunk them one at a time.
1) Positive pressure prevents airborne contamination from entering the cleanroom.
Positive pressure by itself does not prevent contamination from entering a clean area. As mentioned in "Cleanroom velocity not subject to outdated standards" (CleanRooms, March 1999, p. 90), positive pressure is only one of many factors helping to prevent contamination. Equally or more important are the amount of clean air and the direction of this air in preventing contamination. However, the amount of positive pressure (at least 0.5-in. w.g. or 12 Pa) between clean space and adjoining space or between different cleanliness levels of clean space when combined with air flow direction and sometimes air velocity/volume does offer some protection from contamination. Of course, just to compound your thinking, some applications need negative pressure to prevent contamination problems. Every application is unique.
2) Cleanroom HEPA filters never need to be changed.
As for the ultimate life of a HEPA filter, remember none of them is immortal. Sooner or later every HEPA filter needs to be replaced. One of two factors will dictate the need to change a HEPA — either its air volume/velocity capacity will fall off due to high pressure drops caused by dirt loading, or the HEPA will deteriorate because of age. The components of a HEPA are subject to mechanical and chemical attack over time. HEPAs will last a very long time. We recently changed the HEPAs in a cleanroom we installed 13 years ago. Most HEPAs can have a normal service life of 8 to 10 years depending upon their application. It is prudent management to test HEPAs for air flow capacity and leak tightness on a regular basis. By so doing their service life can be determined well before they fail, thereby preserving the integrity of your clean space.
3) Air showers are 100 percent effective at removing contamination from personnel, equipment and supplies entering the cleanroom. They compensate for poor prep and gowning procedures.
 Courtesy of Micron Training International Ltd. |
The almost-40-year-old great air shower debate just goes on and on. The original test evaluation conducted by the U.S. Air Force in 1963 concluded that air showers were 60 to 76 percent effective in removing dust from clothing based upon particle count data measured at 5 microns and larger. There is no question that air showers are somewhat effective at blowing off contamination from people, equipment and supplies, but certainly not 100 percent effective. Again, air showers do provide a means of blowing off potential contaminants, but there is no guarantee that they blow off everything. Time of exposure, direction of air jets, shape of items being blown — all those factors contribute to the supposed air shower effectiveness. More importantly, there is absolutely no substitute for proper preparation and pre-cleaning of equipment and supplies before they enter a cleanroom. Of course, proper gowning is an absolute necessity before personnel enter a cleanroom. The type of gowning is determined by the air cleanliness class of the cleanspace and the process therein. For guidance on proper gowning refer to IEST-RP-CC003.2 available from the Institute of Environmental Sciences & Technology.
4) The cleanroom will decontaminate itself.
Only people can decontaminate a cleanroom. The cleanroom cannot do that for itself. Why? The fixed airflow pattern in a cleanroom can only maintain a level of stability. It cannot overcome changing conditions within the clean space. These changing conditions are brought on by process equipment, raw materials, product flow, changing arrangement of material flow and personnel movement. Cleanrooms are dynamic environments. They require constant housekeeping effort to be decontaminated properly. There is simply no other way. Good cleanroom environments require constant human attention and involvement.
As you can see, myths do tend to have some ring of truth, but, alas, they are never complete truths. Proper employee training and education is the key to overcoming myths. This education is a two-way street of knowledge going from management to employees and employees to management. Speak up when you see fictitious myths in action.
Richard A. Matthews is founder of Filtration Technology Inc. (Greensboro, NC) and president of Micron Video International. He is chairman of the International Organization for Standardization Technical Committee ISO/TC209 "Cleanrooms and associated clean environments," and past president of the Institute of Environmental Sciences and Technology. He is on the CleanRooms Editorial Advisory Board.