Achieving a specific cleanroom class requires not only clean physical design (non-particulating materials, etc.) but also a flow of clean, filtered air sufficient to both dilute existing particulates and to drive “dirty” air out of the controlled space.
Fan-filter units (FFUs) provide air flow at velocities of about 90 to 110 feet per minute, fast enough to generate laminar flow but slow enough to prevent excessive turbulence.
Assuming a clean physical design, air flow determines the design cleanliness of a cleanroom. Cleanroom classes, ISO standards and general recommendations in terms of air-changes-per-hour are:
The cleanroom class is the number of particles >= 0.5 μm in diameter per cubic foot. For instance, a Class 1,000 cleanroom could contain an average of up to 1,000 particles per cubic foot of space.
The good news is that simply adding FFUs can generally lower the cleanroom class (make it cleaner). That assumes, of course, that the cleanroom has ceiling grid spaces not occupied by lights, FFUs or Power Distribution Modules.
Actual particulate levels are measured by taking a panel of readings with a particle counter at various points throughout the cleanroom.
 |
 |
 |
 |
 |
 |
 |
 |
 |