Airflow Regulation vs. Reality

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?


Federal Standard 209



Section 40.2 - Laminar Flow Rooms


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-D 1988

Federal Standard 209-E 1992


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


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


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


Thermal Anemometer

• Single point

• Turbulent air flow velocity varies thru time

• Time constant

– Moving average results in more stable reading


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


Thermal Probe Orientation Error

Kanomax Literature


Instrument Calibration

• Instruments calibrated in ideal conditions

– Laminar to near laminar airflow

• Cleanrooms are turbulent flow


Laminar to Turbulent Flow

Laminar

Flow

Turbulent Flow


Work Height Flow Vectors

• Iso-axial readings only under filters / screens

• Airflow is not vertical at work height readings

– Flat surfaces

• Vial accumulation table


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 = 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




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


Normal Distribution Curve


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%


0% 4% 8% 12% 16%

Hot Wire Exact Locations

Flow Hood

Velgrid 6" 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


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



Airflow Patterns at 40 fpm vs. 90 fpm


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


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


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?

Documents

Airflow Regulation vs. Reality