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Laminar Flow Units
The
term Laminar Flow as it applies to hoods or clean air equipment can mean
many things. "Laminar Flow" typically means air flowing in one
direction (unidirectional) with very low turbulence. In a horizontal "Clean
Bench" air flows straight out of the hood towards the operator. In
a true Vertical Flow clean bench the air flows directly down onto the
worksurface, then out into the room. Some people will call Class II Biological
Safety Cabinets (BSCs) "Vertical Flow" cabinets although that
is technically incorrect.
The most common reference standard for clean benches is Federal Standard 209-B which covers Clean Room and Work Station Requirements, Controlled Environment. This standard was published in 1973 and contained many useful definitions particularly of Air Cleanliness Classes. Federal Standard 209-B was superceded by 209-C and D. The most recently published standard is FS 209-E, which was officially sunset in November 2001. Federal Standard 209-B is still used today, since it was the last FS 209 to include recommendations for air velocity testing and HEPA filter leak testing in an appendix titled "Nonmandatory Supplemental Guidance Information". On page 18, section 40.3.5, FS 209-B recommends 90 feet per minute average, and uniformity within ± 20 % across the entire area of the air exit. This has been interpreted over the years to mean the air velocity average should be between 72-108 feet per minute. Section 50.1 (a) on page 18 discusses in-place filter testing for HEPA filters and recommends that no penetrations exceed 0.01% of upstream smoke concentration. A more current standard addressing clean air workstations is the Institute of Environmental Sciences and Technology (IEST) Recommended Practice, IEST-RP-CC-002-86 for "Laminar Flow Clean Air Devices". It was published in January 1986.
The Institute of Environmental Sciences and Technology Recommended Practice, IEST-RP-CC-002-86 for "Laminar Flow Clean Air Devices" defines laminar flow equipment as: clean benches; clean work stations; wall and ceiling-hung modules; and other laminar flow clean air devices with self-contained motor-blowers.
Like Biological Safety Cabinets, most manufacturers recommend annual testing and certification. For hospital applications, JCAHO goes along with that for the most part. Pharmaceutical companies under FDA jurisdiction are required to perform semi-annual certification. Some State Boards of Pharmacy also require semi-annual certification. For R&D or electronics manufacturing use annual tests are recommended. IEST-RP-CC-002-86 recommends testing at "regular periodic intervals, at a frequency consistent with location, function, and established guidelines", and "following potentially disruptive events, such as relocation of the device or replacement of the HEPA/ULPA filters".
IEST-RP-CC-002-86 recommends the following tests for certification in the field: Air Flow Velocity; HEPA/ULPA Filter Installation Leak Test; Induction Leak Test/Backstreaming Test (when appropriate); Lighting Level (when appropriate); Noise Level (when appropriate); Vibration (when appropriate). The "when appropriate" clauses recognize the necessary testing differences between a horizontal flow clean air bench and a ceiling-hung laminar flow module that is 8 feet off the floor.
Laminar Flow units can be affected by the same problems as biological safety cabinets, i.e., incorrect location in the work room, in proximity to high traffic areas or doors, room ventilation problems, or building electrical limitations.
