Cleanrooms have evolved into two major types and they are differentiated by their method of ventilation. These are turbulently ventilated and unidirectional flow cleanrooms. Turbulently ventilated cleanrooms are also known as ‘nonunidirectional’. Unidirectional flow cleanrooms were originally known as ‘laminar flow’ cleanrooms. The unidirectional type of cleanroom uses very much more air than the turbulently ventilated type, and gives a superior cleanliness. These two types of cleanroom are illustrated below.
Fig 1. Turbulently Ventilated Cleanroom
Turbulently Ventilated Cleanroom (as shown above)
Figure 1 shows a turbulently ventilated room receiving clean filtered air through air diffuses in the ceiling. This air mixes with the room air and removes airborne contamination through air extracts at the bottom of the walls. The air changes are normally equal to, or greater than, 20 per hour, this being much greater than that used in ordinary rooms, such as in offices. In this style of cleanroom, the contamination generated by people and machinery is mixed and diluted with the supply air and then removed.
Fig 2. Unidirectional Flow Cleanroom
Unidirectional Flow Cleanroom (as shown above)
Figure 2 shows the basic principles of the unidirectional flow room. High efficiency filters are installed across a whole ceiling (or wall in some systems) and these supply air. This air sweeps across the room in a unidirectional way at a speed of around 0.4 m/s (80 ft/min) and exits through the floor, thus removing the airborne contamination from the room. This system uses more air than the turbulently ventilated cleanroom but, because of the directed air movement, it minimises the spread of contamination about the room and sweeps it out through the floor.
Clean air devices, such as unidirectional benches or isolators, are used in both turbulently and unidirectional ventilated cleanrooms. These machines will give a localised supply of filtered air and enhances air conditions where required, e.g. at the area where the product is open to contamination.