10. Measurement of Air Quantities and Pressure Differences

Purpose A cleanroom must have sufficient clean air supplied to dilute and remove the airborne contamination generated within the room. Air Cleanliness: –Turbulently ventilated cleanroom air supply; the more air supplied in a given time, the cleaner the room. –unidirectional cleanroom air supply velocity Test: –Initial testing of the design –Regular intervals check

Air Quantities Instruments: –Hoods: air supply volumes –Anemometers: air velocities Turbulently ventilated rooms – measured within the air conditioning ducts  Pitot-static tube

Measuring air quantities from within a cleanroom Air  air filter (no diffuser)  anemometer at the filter face  average velocity  air volume –Difficulty: the non-uniformity of the air velocity  inaccurate measurement Air  air diffusers  unevenness of air velocities  incorrect air volume Hood: air supply volume  average velocity measured at the exit of the hood  air volume

Anemometers Anemometers: away from the filter of about 30cm (12 inches) Vane Anemometer –Principle: Air supply  turning a vane  frequency  velocity –Accuracy: velocity is less than about 0.2 m/s (40 ft/min), the mechanical friction affects the turning of the vane

Vane Anemometer

Thermal Anemometers Principle: Air passing through the head of the instrument  cooling effect  the air velocity: Fig.10.3 : a bead thermistor ( 有 孔的電熱調節器 ) Low velocities can be measured with this type of apparatus

Differential Pressure Tests The units: –Pascals, inch water gauge are used (12Pa = 0.05 inch water gauge). Pressure difference: 10 or 15 Pa between clean areas –15 Pa is commonly used between a cleanroom and an unclassified room, –10 Pa between two cleanrooms.

Large openings: – problems can occur when trying to achieve a pressure difference between areas connected by large openings, such as a supply tunnel. To achieve the suggested pressure drop : Very large air quantities through the tunnel To accept a lower pressure difference

Apparatus for measuring pressure differences Manometer: –range of pressure difference of 0-60 Pa ( inch water) –inclined manometer; magnehelic gauge; electronic manometer

Inclined manometer works by pressure pushing a liquid up an inclined tube. –small pressure changes in the inclined tube up to a pressure of about 60 Pa. –After that pressure, the tube moves round to the vertical measuring pressure differences can be in the 100 to 500 Pa range.

Methods of checking pressure differences pressure differences between areas –adjusting the pressure differences : extract be reduced to increase the pressure, and increased to decrease it. If manometers are not permanently installed, a tube from a pressure gauge is passed under the door, or through an open by-pass grille or damper into the adjacent area. In some ventilation systems, the pressures within rooms are measured with respect to one reference point. When this type of system is being checked, the pressure difference across a doorway can be calculated by subtracting the two readings of the adjoining spaces.