Paper 3
The Measurement of Air Supply
Volumes and Velocities in Cleanrooms
W
Whyte, W M Whyte and G Green
Department
of Mechanical Engineering, James Watt Building North,
University
of Glasgow, Glasgow G12 8QQ, Scotland, UK.
Abstract
Air supply
volumes and velocities in cleanrooms are monitored by airflow measuring
hoods and anemometers but these measuring methods can be inaccurate if
used incorrectly. It is demonstrated in this article that measuring hoods
are accurate if the air supply passes evenly out of the hood, as occurs
when the air volume is measured from a four-way diffuser or no air supply
diffuser. However, when a swirl diffuser was investigated, the measuring
hood gave readings more than 50% greater than the true volume. The reasons
for the inaccuracy, and methods to correct it were established. Vane
anemometers give inaccurate readings at the face of high-efficiency air
supply filters, and it was found that the most accurate reading was found
about 15 cm from the filter face. The number of readings required
across the filter face to obtain an accurate average velocity was
investigated, as was a scanning method using overlapping passes.
Key words: air
supply volumes, air velocities, cleanrooms, anemometer, airflow measuring
hood.
References
Whyte W: (2009). “Cleanroom Technology: the Fundamentals of Design,
Testing and Operation” (second edition), Wiley and Sons, Chichester, UK.
Anonymous: (2006). “Air supply volume
inaccuracies”, The Cleanroom Monitor, 55, April, pp6-7.
ISO 14664-3: (2005). “Cleanrooms and Associated Controlled
Environments – Part 3: Test methods”. International Organization for
Standardization, Geneva, Switzerland.
ISO 7194: (2008). “Measurement of fluid flow in closed conduits -
velocity-area methods of flow measurement in swirling or asymmetric flow
conditions in circular ducts by means of current-meters or Pitot static
tubes”. International Organization for Standardization, Geneva,
Switzerland.
ISO 14664-2: (2000). “Cleanrooms and Associated
Controlled environments – Part 2: Specifications for testing and
monitoring to prove continued compliance with ISO 14644-1”.
International Organization for Standardization, Geneva,
Switzerland.
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