The controversy over the use of air showers in the contamination control process has been ongoing for many years. The effectiveness of an Air Shower entry system relates directly to its proper design and use. In recent years tests have been conducted and articles written which show air showers to be 35% to 90% efficient in the removal of contamination dependent upon the size of the particle,the design of the air shower, garment type, garment procedures,shower utilization technique, cycle time, and cleanroom classification. Within the following paper attention will be paid to the opinions of both the supporters and critics of air showers as well as results of published test reports, and those features which should be considered in a properly designed air shower entry system.
A conventional air shower should only be incorporated within the design of an engineered cleanroom “entry system.” For the purposes of our discussions, . “entry system” should be defined as a well designed garmenting area incorporating proper class identification, garment storage, and garmenting procedures, with the overall goal being the prevention of particulate entering the cleanroomon garments, or as a result of the garmenting process. An Air shower is defined as an isolated chamber equipped with a self contained blower and motor, interlocking doors, hepa/ulpa filtration, and a recirculating exhaust system. The features, functions, and benefits of an air shower entry system depend largely on its proper design and use. This paper concentrates
largely on the air shower and its proper design to insure it delivers an operational and economic benefit.
The arguments of the supporters and critics of the use of air showers, as well as recently published test reports will be examined. The required features of a properly designed air shower will be discussed along with the operational application of an air shower, within a properly designed”entry system”, and the proper use of the air shower to enhance effectiveness. The ultimate goal being to extract maximum value from its installation.
What are the features required in a proper air shower design?
Let us not lose site of the fact that an air shower, if incorporated, should be part of an engineered” entry system” and is not designed as a watch dog to compensate for poor protocol, but as a tool to control contamination levels within the cleanroom, just as the garment itself should be considered. At all times we must remember, the
garment is the tool within the cleanroom which comes in closest contact, and in contact most often, to the product. Features which the authors believe should be part of a properly designed air shower are:
Filtration which is the guiding premise of cleanroom design, should not be overlooked in air shower design. It should not be assumed that air showers recirculate clean air, therefore they do not require filtration themselves. Air showers, as cleanrooms, should follow the basic concept of filtering and moving air, to both remove contamination from the garment and extracting the removed contamination from the environment. The authors suggest the use of ulpa filtration 99.9995 % efficient @ .12 micron.
Proper protocol in using an air shower weighs greatly on its effectiveness. Although not part of air shower design, it is a large factor in designing the cleanroom “entry system.” As is commonly accepted, training is of utmost importance in insuring reduced contamination levels in cleanrooms, additionally it is of utmost importance in extracting maximum effectiveness from an air shower. Proper protocol suggests personnel should be trained to rotate continuously 360 degrees during the air shower cycle to insure contamination removal is as efficient as possible. For further effectiveness, hands should be placed on head while rotating.
Multiple points of contact during the rotating process will insure the garment is agitated during the act of air showering, thus creating the “pulsating” effect which will dislodge particulate. The nozzles which deliver the air, within the air shower, should be 3/4″ to 11/2″ in diameter, be distributed evenly throughout the walls and
ceiling of the air shower, and directed toward the marked spot where personnel should be rotating. As a rule of thumb, air should be delivered through between 20 to 26 nozzles in a single person chamber.
Airflow should as a minimum range between 6000 to 7500 feet per minute (fpm), or the equivalent to 60 to 90 miles per hour, to insure turbulent enough air to dislodge surface particulate from a clean room garment. There have been studies done which proclaim the advantages of still higher velocities, that of up to 12,000 fpm, and or lower velocity, as low as 90 to 150 fpm which approach laminar airflow design levels. As neither of these alternative tests have been documented, the authors could not evaluate their effectiveness; however, it is their opinion that airflow upwards of 12,000 fpm may be of substantial speed to actually impregnate particles on cleanroom garments and low velocity air showers (90 to 150 fpm) although effective in preventing the infiltration of particulate into the cleanroom during the entering process (acting as more as an air lock than an air shower) will not dislodge particulate which has settled on the garment. It should be noted that the 6000 to 7500 fpm rate is suggested as part of a design which incorporates multiple points of impact and is widely accepted by air shower manufacturers.
Cycle time is considered to be the most critical aspect of air shower effectiveness. Studies have suggested that a minimum of 20 seconds is required to properly remove contamination from garments. More intriguingly, our own studies tend to indicate garments in the second and third day of use require longer air shower cycle times to remove contamination. These findings suggest a “smart” air shower design utilizing a real time clock and calendar to increase cycle time during the later stages of garment use, or better still an air shower design utilizing particle count technology to control exit, would most probably produce a benefit which far outweighs the additional cost.
Dwell time designed into the air shower control system will insure that contamination removed by the unit is allowed to settle out upon completion of the air showering cycle, preventing removed contamination from being swept into the cleanroom with the turbulence caused during the door opening/entry process. Dwell time is defined as the period of time between the completion of the air showering cycle, to the opening of the air shower and door and entry into the cleanroom.
Constant purge of an air shower during non-use periods is technology available, but not yet embraced by either air shower users or manufacturers. Constant purge is the continuous flow of low velocity air within the air shower, during down time, preventing settling of contamination. This settled contamination often gets swept into the cleanroom as personnel open and close air shower doors and walk through the contamination. Constant purge is a manufacturers option which should be c0nsidered standard, and which cost pales next to its benefit. Optimum effectiveness of constant purge is through vertica1laminar airflow in the air shower’s ceiling.
Selection of the proper flooring within an air shower can benefit the control of contamination. Often larger particles (over 25 micron) settle out of the airstream due to their size and weight. When possible, an air shower should be designed to sit on a raised access floor or utilize its own raised (grated) floor, with clean out pan, to allow contamination to settle out. At times height restrictions, handicapped access, and vibration issues require air showers to be utilized with non-raised floors. With this type of design we suggest the use of a permanent type sticky flooring which will control particle migration. It should be noted that several air shower tests have been control particle migration. It should be noted that several air shower tests have been conducted utilizing air showers which deliver air from the floor. At all times we suggest an air shower design which utilizes return air at or in the floor to utilize gravity as an aid to particle removal from the chamber. This design will alleviate the
possibility of constant re-agitating of settled particles.
Door Interlocks are a design feature commonly utilized in air shower design, but in principal go against good contamination theory. Utilizing interlocks accepts the idea that personnel will not follow protocol training on air shower use, instead restricting exit from the air shower until such time as the cycle and dwell times have been elapsed. Practicality suggests that interlocks should be utilized to insure protocol compliance; however, they should not be utilized as a replacement for proper training.
The Psychological barrier
The integration of an air shower in the design of an “entry system” cannot really be discussed without addressing the psychological barrier effect it delivers. Although it cannot be quantified, and a dollar value can’t be put on it, it should be a factor in the evaluation process. The idea of passing through a chamber specifically designed to remove contamination, which you may bring into the room, enforces the mind set that protection of the product from personnel is a significant concern. As always is the case with intrinsic value it is difficult to debate either side of the argument.
Air Tunnel Technology
One of the most significant criticisms of air shower use is the time delay, at shift change, getting personnel through the shower and working; however, the delay in entry far outweighs the detriment fifty (50) people, entering a cleanroom simultaneously, could have on particle counts. Staggered breaks and shift hours have
become part of cleanroom operating parameters to minimize square footage allotted to gown rooms. Air tunnel technology is evolving to help reduce the delays. Multiple person air showers are becoming more prevalent, and even tunnels designed to shower personnel appropriately, while walking their length, are being considered and utilized in state-of-the-art cleanroom design. When designing multiple person air showers, a minimum of four (4) feet in length should be designed for each person targeted to utilize the shower, per cycle. In a walk through tunnel design, optimum length for effectiveness is thirty-two (32) feet to achieve acceptable contamination removal. Dimensions less than the above stated criteria will reduce air shower
effectiveness and should be evaluated for cost benefit. In this instance a complete “entry system” technology concept should be utilized to achieve maximum effectiveness of gown room space and cost.
In reviewing the air shower controversy, the first thing that came to mind was what is the controversy? The most vocal critics W. Whyte, University of Glasgow and S. Hoenig, University of Arizona, agree air showers have a degree of efficiency, they question the cost effectiveness and the percentage of effectiveness. In his Microcontamination article Whyte states “It is recognized that air showers can remove particles from the surface of clothing; however it has not been demonstrated that this surface particle removal is important. ” Hoenig agreed~ in a debate at CLEANROOMS ’95 WEST, that air showers are effective on larger particles of 5 micron or more, but went on to say those particles did not concern him in today’s technology. Most
technologists seem to agree that, personnel are the largest contributors to contamination in cleanrooms, and air showers contribute to contamination control. It also seems evident they don’t generate contamination. The only question seems to be, how much do they contribute and what is their contribution worth? If they remove
that one particle that could destroy a semiconductor chip or contaminate a pharmaceutical batch, how much is that worth? Are we playing the particle dilution game, or trying to keep every particle possible from reaching our process? $4,000,000 worth of semiconductor chips fit in an average briefcase, what is that one
particle worth? The perception that air showers are costly or not cost effective may not hold up under this type of logic. Further, in the total scheme of cleanroom cost, their inclusion in room design may not amount to a significant percentage. If they are viewed as the tool that they can be, to control contamination, the cost justification may not be so hard, especially if they can be utilized to .incorporate less frequent garment changes. Cost to benefit ratio needs to be considered on a case by case basis.
Most tests reviewed, both supporting and not supporting the use’ of air showers, are flawed due to the preconceived position of the party performing the tests. None prove conclusively that air showers are either effective or ineffective. The time may be here f9r the parties on each side of the fence to get together to create a uniform test criteria for air showers. This could standardize air shower comparison and quite possibly put the controversy to rest. As a minimum it might create recognized design parameters, get all sides on the same page, and allow the product to be developed to enhance effectiveness utilizing such technology as fluctuating cycle time, dependent upon the garment use cycle, or the next step which would be particle count monitoring to control exit.
Controversy can be positive. It spurs thought, conversation, and debate. But at a point, both sides should sit down and search for common ground.