Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Airflow Regulation vs. Reality
Dan Milholland [email protected]
Cleanroom Consulting & Test Equipment
How Did We Get Here
• 90 feet per minute ± 20 % (or 0.45 m/s)
– 72- 108 fpm (0.36 – 0.54 m/s)
• How many of you trying to maintain this daily?
• Do you even know why?
Cleanroom Consulting & Test Equipment
Federal Standard 209
Cleanroom Consulting & Test Equipment
Federal Standard 209
Section 40.2 - Laminar Flow Rooms
Federal Standard 209
Federal Standard 209 Federal Standard 209 1963
Federal Standard 209-A 1966
Federal Standard 209-B 1973
Federal Standard 209-C 1987
Federal Standard 209-D 1988
Federal Standard 209-E 1992
No mention of air velocity from this point forward
Federal Standard 209 1963
Federal Standard 209-A 1966
Federal Standard 209-B 1973
Federal Standard 209-C 1987
Federal Standard 209
Federal Standard 209-D 1988
Federal Standard 209-E 1992
Cleanroom Consulting & Test Equipment
2004 US FDA Aseptic Guidelines
• “..at a velocity sufficient to sweep particles away from the filling / closing operation and maintain unidirectional airflow during operation”
• Thus, airflow visualization tests will determine airflow velocity / patterns
“A velocity from 90 feet per minute is generally
established, with a range of ±20% around the set
point. Higher velocities may be appropriate in
operations generating high levels of particulates.”
‘04 Guidelines Foot Note
90 feet per minute as a “set point”
Plus or Minus 20% 72 -108 feet per minute
Here We Are Today
The Reality of Measuring Airflow In Cleanrooms
Cleanroom Consulting & Test Equipment
Anemometer Principles of Operation
• Velocity pressure
– Single Point
– Multi-point as a Velgrid or Matrix
• Single point thermal resistance
– “Hot wire” anemometer
• Ultrasonic
– Times alternating sound travel between points
– Two or three axis average over 5 -10 cm distance
Cleanroom Consulting & Test Equipment
Velocity Pressure Instruments
• Velgrid or Pitot Tube
• Velocity pressure of 0.45 m/s = 0.124 Pa
– This is only 13 µm wc !!
–Near the lower limits of the instrument
VPPascal = Velocitym / s
1.291( )2
Cleanroom Consulting & Test Equipment
Thermal Anemometer
• Single point
• Turbulent air flow velocity varies thru time
• Time constant
– Moving average results in more stable reading
Cleanroom Consulting & Test Equipment
Isoaxial Measurements
• Probe must be oriented toward airflow
• Applies to both tube array and thermal anemometer
– Must be within ± 22.5° of flow axis
22.5° angle
Cleanroom Consulting & Test Equipment
Thermal Probe Orientation Error
Kanomax Literature
Cleanroom Consulting & Test Equipment
Instrument Calibration
• Instruments calibrated in ideal conditions
– Laminar to near laminar airflow
• Cleanrooms are turbulent flow
Cleanroom Consulting & Test Equipment
Laminar to Turbulent Flow
Laminar
Flow
Turbulent Flow
Cleanroom Consulting & Test Equipment
Work Height Flow Vectors
• Iso-axial readings only under filters / screens
• Airflow is not vertical at work height readings
– Flat surfaces
• Vial accumulation table
Cleanroom Consulting & Test Equipment
Room Design Affects Flow Vectors
• Flow angle deviates toward air returns
– Velocity increases approaching returns
• Curtains or barriers are necessary to maintain vertical airflow at work height
fpm
Cleanroom width = 3.05 m
Cleanroom width = 4.88 m
Cleanroom width = 6.10m
Velocity Contour
1.5 m
2 m
1m
Ceiling Velocity
0.42 m/s
1.5 m
2 m
1m
1.5 m
2 m
1m
Flow angles contour
Cleanroom width = 3.05 m
Cleanroom width = 4.88 m
Cleanroom width = 6.10 m
Ceiling Velocity
0.42 m/s
Degree
1.5 m
2 m
1 m
1.5 m
2 m
1 m
Degree
1.5 m
2 m
1 m
Cleanroom Consulting & Test Equipment
Normal Distribution Curve
Cleanroom Consulting & Test Equipment
Normal Distribution of Data
• Outlier frequency
– 5% of data exceeds 2 Standard Deviations from mean
– < 1% of data exceeds 3 Standard Deviations
• If “0.45 m/s +/- 20%” is absolute range
– Then assume “20%” = 3 Standard Deviations
– 20% ÷ 3 = Relative Std. Dev. of 6.6%
Cleanroom Consulting & Test Equipment
0% 4% 8% 12% 16%
Hot Wire Exact Locations
Flow Hood
Velgrid 6" 2 locations
Velgrid 6" 3 locations
Velgrid 2" 3 locations
Velgrid 2" 2 locations
Hot Wire 6" 8 locations
6.6
Eagleson Institute Airflow
Labs
Averages of 60 Readings
Rel STDev of Lab Data – Instrument and Position
Cleanroom Consulting & Test Equipment
Is Velocity Uniformity Important?
• 3D Anemometer study
– Interface mid line between filters
– Each flow vector determined from 1,800 readings
• 3 axis (x, y, z) at 20 Hz for 30 seconds
• Mock filling line – 600 mm x 4800 mm
– Curtains to 760 mm above the floor
– Filters butted end to end
• Minimum “ceiling grid width”
0.43 m/s 0.43 m/s
0.36 m/s 0.56 m/s
0.20 m/s 0.66 m/s
Cleanroom Consulting & Test Equipment
Cleanroom Consulting & Test Equipment
Airflow Patterns at 40 fpm vs. 90 fpm
Cleanroom Consulting & Test Equipment
Let’s talk “CLEAN”
• ISO 14644 cleanliness Classes 1 through 9 – Pharmaceutical critical areas are ISO 5
– Microelectronics are ISO 2 or ISO 3
• How do they do it? – ULPA filters rather than HEPA filters
• No gasket or gel seal leaks
– Air returns through grated / raised floor
– Gore-Tex gowning serves as a filter media
– Filtered, negative pressure head gear
– Extensive use of isolators
Cleanroom Consulting & Test Equipment
Microelectronics Air Velocity
• Never designed at 0.45 m/s or 0.50 m/s!!
– Turbulence under objects in airflow path
• Ceiling grid
• Tall equipment
– Energy cost prohibitive for 20,000 filter facility
• Typical design velocities of 0.30 m/s to 0.35 m/s
– Ceiling heights are now over 8 m
Cleanroom Consulting & Test Equipment
Conclusion
• Velocity measurements prone to error
– Low velocity pressure
– Turbulent air flow
– Isoaxial sampling errors
• Poor measurement repeatability
• Statistical outliers in a normal distribution
• Too much emphasis is placed on velocity
• Is regulatory “guidance” driving best practices?