Airshower Controversy Continues
By Lisa A. Coleman
Santa Clara, CA–The final vote count at the CleanRooms `95 West “Great Air Shower Debate” suggests that the controversy over airshower effectiveness is anything but resolved. While 64 attendees voted in favor of the case for air shower effectiveness, an equal number voted against. The deciding voter wrote on the ballot: “inconclusive from debate,” and failed to cast a yea or nay. Although more than 300 people attended the debate, only 129–about 43 percent–cast a vote.
Actually more of a roundtable discussion than a formal debate, the discussion focused on the question: Are air showers effective? Speaking for air shower effectiveness were Tim Loughran, national sales manager of CRP (Ronkonkoma, NY) and Mike Shea, president, Scientific Air Systems (Tigard, OR). They argued that air showers are an effective “first line of defense” and a psychological barrier against particulates entering the cleanroom from worker garments. Presenting the case for the ineffectiveness of air showers were Professor Bill Whyte of the University of Glasgow and secretary of the Scottish Society of Contamination Control (Glasgow, Scotland), and Dr. Stuart Hoenig, professor emeritus at the University of Arizona (Tucson, AZ), who both argued that air showers are essentially “a waste of time and money” and ineffective in removing small particulates.
Opening the debate, Whyte cited the results of an experiment he conducted on the effect of an air shower on the number of particles in a room. However, the experiment used an air shower more powerful than any currently available. Instead of jets of air, a 10-horsepower fan moved an entire mass of air from one side of the tunnel to the other, bathing the whole garment in an average of about 14 meters per second of air. His experiment involved three people moving through a dispersal chamber, or a body box, and through an air shower five times. The number of particles in the cleanroom dispersed by one person was measured. The person then either went through the air shower or walked around the room and back into the body box or dispersal chamber. Whyte`s statistical analysis of the test results showed that over a period of 16 days there was no significant difference between dispersion rates, whether a person went through the air shower or not.
While not denying that jets of air will remove particles from clothing, Whyte maintained his tests showed that 50 percent of 5-micron particles will be removed from clothing. The bigger the particles, the more that will be removed; the smaller, the fewer are removed. Whyte observed that there are fewer particles on the surface of clothing because they get removed in a cleanroom laundry. Also, he maintained that about 95 percent of the particles that get into cleanroom air come not from the outer surfaces of the garment, but from personnel wearing the garment, through the necks and enclosures.
Says Whyte: “It would seem, from a theoretical basis at least, that an air shower is unlikely to make much of a contribution, if any, to reducing contamination in cleanrooms, and our experiments have shown that there is little or no reduction in particles. If you had an infinite amount of money and time, of course, you could put one in–it could certainly do no harm. But, it`s my observation that if you have an air shower, it`s much, much slower for people to get in and out of the cleanroom. And it would seem from a cost effective point of view, that if you wish to spend money on contamination control, it would be better spent on devices or means other than air showers.”
Defending airshowers, CRP`s Loughran retorted, “I`m not sure I can contradict Mr. Whyte`s tests, but I believe that a common sense approach shows that it (an air shower) is going to be effective. How effective? We just don`t know.” Loughran added that air showers are a cleanroom tool: “As any other tool in a cleanroom, why not use a protocol that will make that tool effective. Air showers are an added `defense line.`”
The University of Arizona`s Dr. Hoenig cited test results from a garment laundering study at his university. He suggested there was enough evidence to prove that laundering does not effectively remove one to five micron particles from garments worn by cleanroom workers. The particulate is still there when cleanroom personnel don the garments and enter an airshower. Hoenig added that since many cleanroom workers do not use the air shower correctly–sometimes people tilt their heads to avoid the air blowing over their faces–“they make the air shower even more inefficient than it is normally.”
In defense of air showers, Scientific Air Systems` Shea said he was surprised that the debate continues to rage on. “It`s necessary, however, to restate the importance of air showers because we sometimes forget specifically what they are used for. As much as the removal of particles from garments, we also need to understand the importance of having an entry barrier, so that as people enter the cleanroom–sometimes in large numbers during shift changes–particles in the gowning change area stay there.”
Shea cited studies confirming that significant amounts of particulates are removed by air showers from cleanroom garments.
To determine the amount of particulate removed from garments during a single shower cycle, each cycle on an SAS 2000 Airshower was run for approximately 30 seconds. At the end of the cycle, the shower was run in “constant purge” mode for the same period of time. Particles were measured in increments of 0.1 0.2 0.3, 0.5, 0.7 and 1µm. Tests showed that the number of particles removed from the first ten-second cycle to the next was significant, especially in the smaller particle sizes. He also pointed out that an air shower must provide sufficient agitation of the garment in a very limited amount of time to ensure removal of particulate. “A well-designed air shower entry system certainly provides a first-line of defense within a cleanroom environment. Remember, the garment is perhaps the one piece of equipment in a cleanroom that comes closest to the product the most frequently. We should do everything we can to remove whatever amount of particulate we can. We certainly have enough of a first-line of defense in our current air shower technology.” n