The general method of ventilation used in turbulent air flow cleanrooms is similar to that found in
buildings such as offices, schools, malls, manufacturing plants, auditoriums, shops, etc. The air is
supplied by an air conditioning system through diffusers in the ceiling. However, a cleanroom differs
from an ordinary ventilated room in three ways. These are increased air supply, the use of high 
efficiency filters and room pressurization. Figure 1 shows the typical air flow patterns in a turbulent
air flow cleanroom.

The increased air supply is an important aspect of particle control. A typical turbulent air flow
cleanroom would have at least 10 air changes per hour and likely to have between 20 and 60. This
additional air supply is mainly provided to dilute to an acceptable concentration the contamination
produced in the room.
High efficiency filters are used to filter the supply air into a cleanroom to ensure the removal of small
particles. The high efficiency filters used in cleanrooms are installed at the point of air discharge into
the room.
Room pressurization is mainly provided to ensure that untreated air does not pass from dirtier
adjacent areas into the cleanroom. The cleanroom is positively pressurized with respect to these
dirtier areas. This is done by extracting less air from the room than is supplied to it.


Laminar air flow is used when low airborne concentrations of particles or bacteria are required. This
air flow pattern is in one direction, usually horizontal or vertical at a uniform speed of between 60 to

90 ft/min. and throughout the entire space. Figure 2 shows the typical air flow patterns in a laminar air
flow cleanroom.

     The air velocity is sufficient to remove relatively large particles before they settle onto surfaces. Any
contaminant released into the air can therefore be immediately removed by this laminar flow of air,
whereas the turbulent air flow ventilated system relies on mixing and dilution to remove
contamination.
     In an empty room with no obstructions to the airflow, contamination is removed faster by air
velocities much lower than those mentioned above. However in practical situations there are
obstructions and people moving in the space. Obstructions will cause the laminar air flow to be
turned into turbulent air flow around the obstructions. Higher contamination concentrations will be
established in the turbulent areas. Therefore, it has been found that the cleanliness of a laminar air
flow cleanroom is directly proportional to the air velocity. Air changes per unit of time are related to
the volume of the room and are many times greater than those supplied to a turbulent air flow
cleanroom.