The laminar flow clean bench is a work bench or similar enclosure which has its own filtered air supply. The clean bench was developed as an adjunct to clean room technology (the need to protect the work from contamination). In recent years, the use of the clean bench, laminar flow cabinet or laminar flow hood has spread from research and manufacturing to other fields such as aerospace, bioscience, pharmaceutical production and food processing.
Today, laminar flow clean benches are used in a variety of applications throughout medical research laboratories, hospitals, manufacturing facilities and other research and production environments.
Clean Bench Function
The clean bench provides product protection by ensuring that the work in the bench is exposed only to HEPA-filtered air.
The HEPA Filter
A common component in any clean bench is the HEPA filter. The HEPA filter (High Efficiency Particulate Air) removes particulates, generally called aerosols, such as micro-organisms, from the air. The HEPA filter does not remove vapors or gases. (See cross section at right.)
Depending on its quality, a HEPA air filter can trap from 9,997 to 9,999 of every 10,000 particulates of a diameter greater than and less than 0.3 micron. For most industrial uses, the 99.97% performance is acceptable. Pharmacy and laboratory applications, however, require a 99.99% filtration performance level. To satisfy performance standards established by Baker, HEPA filters used in all Baker products are leak-free and rated at the 99.99% level.
Another common attribute in clean bench technology is laminar airflow. The following general definition applies to the application of laminar airflow in this literature.
Generally defined, laminar airflow is airflow in which the entire body of air within a confined area moves in a unidirectional velocity along parallel flow lines.
Technically defined, laminar airflow is the fluid flow in which air velocities are free of macroscopic fluctuations which occur when the Reynolds Number is less than 2000. (The Reynolds Number is the ratio of inertial to viscous forces in a pipe or duct.)
The clean bench or "hood" is a work area with its own HEPA-filtered air supply. Protection is provided by capturing room air, passing the air through a HEPA filter, and directing the filtered air horizontally across the work surface at a constant speed toward the user.
In a conventional laminar flow clean bench, particulates may contaminate the work area when hands or materials (illustrated below) are placed inside the hood. The resulting turbulence creates backwash which allows dirty room air to enter the zone. Particle-free air quality in the laminar flow clean bench is accomplished with precise control of airflow volumes and velocities. Any obstruction will have a significant impact on laminar flow performance. Obstructions include work in process or materials placed inside the hood or on the hood surface.
The Baker EdgeGARD® clean bench design includes patented high velocity return air slots which minimize particulate contamination created by backwash common in conventional clean bench construction. All Baker clean benches and laminar flow cabinets employ a patented cabinet design with high velocity return air slots which are proven effective in significantly reducing the effects of backwash.
In Federal Standard No. 209E (last revised Sept.11,1992) the United States Government provides requirements for three classes of air cleanliness. Classifications are based on particle counts taken at a location within the clean bench which will yield a particle count of air as it approaches the work location. It is important to note that government classifications reflect performance when the bench is "at rest," or free of materials or work activity within the hood.
|Air Cleanliness Classes, Federal Standard No. 209E|
|Class 100||Particle count not to exceed a total of 100 particles per cubic foot of a size 0.5 micron and larger.|
|Class 10,000||Particle count not to exceed a total of 10,000 particles per cubic foot of a size 0.5 micron and larger, or 65 particles per cubic foot of a size 5.0 micron and larger.|
|Class 100,000||Particle count not to exceed a total of 100,000 particles per cubic foot of a size 0.5 micron and larger, or 700 particles per cubic foot of a size 5.0 micron and larger.|
Baker Company tests suggest that neither Class 100 nor Class 10,000 federal standards can be routinely maintained in a conventional laminar flow clean bench when a backwash effect is induced by normal placement of any object within the work area.
In tests conducted by The Baker Company, the Baker EdgeGARD® clean bench, with patented high velocity return air slots, has been proven to reduce backwash. Tests were conducted under conditions at and beyond minimum standards established in Federal Standard No. 209E.
|Baker EdgeGARD® Clean Bench||With High Velocity Return Air Slots Covered To Simulate Conventional Clean Bench||With High Velocity Air Slots Uncovered To Deliver Maximum Protection|
|Operating, Empty (Federal Minimum Standard)||Particle count within Class 100 limits||Particle count within Class 100 limits|
|Operating, With Common Equipment In Hood (Baker Standard)||Particle count exceeds limits of Class 100 and Class 10,000 air quality performance||Particle count within Class 100 limits due to performance of high velocity return air slots|
|Operating, Work Being Performed In Work Area (Baker Standard)||Particle count exceeds limits of Class 100 air quality performance||Particle count within Class 100 limits due to performance of high velocity return air slots|
Results of extensive testing by The Baker Company are illustrated by smoke tests conducted at the front of the hood.