Reasonable Air Shower Design
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. We will concentrate largely on the air shower design to ensure it delivers an operational and economic benefit.
Kwang believe that a properly designed air shower should include the following features.
Filtration. It is the guiding premise of cleanroom design and 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 extract the removed contaminant from the environment. The authors suggest the use of ULPA filtration–99.9995 percent at 0.12 microns.
Proper Protocol. The proper protocol in using an air shower weighs greatly on its effectiveness. Although not part of an air shower design, it is a large factor in designing the cleanroom “entry system.” As is commonly accepted, training is of utmost importance in ensuring 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 360 degrees continuously during the air shower cycle to ensure contamination removal is as efficient as possible. For further effectiveness, hands should be placed on head while rotating.
Multiple points of impact. Multiple points of impact during the rotating process will ensure 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 inches 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. As a minimum, airflow should range between 6,000 to 7,500 feet per minute (fpm), or the equivalent of 60 to 90 mph, to ensure that the air is turbulent enough to dislodge surface particulate from a cleanroom garment. There have been studies done which proclaim the advantages of still higher velocities, that of up to 12,000 fpm, and of 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 the tests` 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. In addition, lower velocity air showers (90 to 150 fpm) although effective in preventing the infiltration of particulate into the cleanroom during the entering process, (acting 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 6,000 to 7,500 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. This 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 using a real-time clock and calendar to increase cycle time during the later stages of garment use. Or better yet, an air shower design utilizing particle count technology to control exit would probably produce a benefit which far outweighs the additional cost.
Dwell time. If designed into the air shower control system, dwell time will ensure that contamination removed by the unit is allowed to settle out upon completion of the air showering cycle. This helps prevent 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 completing the air showering cycle to opening the air shower exit door and entering the cleanroom.
Constant purge. The technology is available to constantly purge an air shower during its non-use periods. However, this technology is not yet embraced by either air shower users or manufacturers. Constant purge is the continuous flow of low velocity air–during down time within the air shower–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 manufacturer`s option which should be considered standard, and its cost pales next to its benefit. Optimum effectiveness of constant purge is through vertical laminar airflow in the air shower`s ceiling.
Selection of the proper flooring. Using the proper flooring in an air shower can benefit the control of contamination. Often times larger particles (over 25 microns) 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 conducted utilizing air showers which deliver air from the floor. At all times we suggest an air shower design that uses 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. These are features commonly used in air shower design, but in principal go against good contamination theory. Using door interlocks in an air shower design implies that personnel will not follow protocol training on air shower use. Instead, restrict exits from the air shower until such time as the cycle and dwell times have been elapsed. For practical purposes interlocks should be utilized to ensure protocol compliance; however, they should not be used as a replacement for proper training.