Basic Documentation

Room Pressurization Control Application Guide
The clean spaces are positively pressurized according to their required level of purity or
cleanliness. The most critical operations are performed in the area that has the highest level
of cleanliness (aseptic area), which also has the highest level of pressurization. In Figure 9,
each plus sign indicates the area's relative level of positive pressurization. The more plus
signs, the higher the positive pressurization. Clean room designs typically use a differential
pressure of about 0.05 inches w.c. (12.4 Pascals) between each different clean room area
classification. Thus, in Figure 9,the aseptic area (+++) would typically be designed to be 0.15
inches w.c. positive with respect to a neutral area such as the Personnel Corridor. The
adjoining Preparation Area would then be maintained at about 0.10 inches w.c., and the entry
and exit airlocks would be maintained at about 0.05 inches w.c. This ensures that airflow
(shown by dotted lines and arrows) will always flow from the most critical and cleanest space
(highest positive pressure) to a lesser clean space.
Airlocks
Four different types of airlocks are often applied to enable entry and exit to clean spaces
depending upon the nature of the clean room's purpose:
• Cascading Pressure Airlock—Figure 10 shows an airlock that is kept at a lower
positive pressure than the adjacent clean space, but at a higher positive pressure
than the corridor. As a result, air from the highly positive pressurized clean space
cascades through the airlock to the area of least cleanliness, which is the Non
Classified Corridor. The same quantity of air is supplied to and exhausted from the
airlock. The FDA prefers this type of airlock when absolute containment of the clean
space is not required.
• Pressure Bubble Airlock—Figure 11 shows an airlock that is kept at a higher
positive pressure than the adjacent spaces. This type of airlock is applied when it is
necessary to separate a bio-contained clean area from other areas. Clean
conditioned supply air is used to create a high positive pressure in the airlock, which
does not have any exhaust provision. The supply airflows to the adjacent areas
through the airlock leakage area that consists primarily of door clearances. This
directional airflow prevents cross contamination between adjacent areas. The
pressure bubble airlock is often used because its positive pressure relationship to the
adjacent areas keeps contaminants from entering the airlock.
• Pressure Sink Airlock—Figure 12 shows an airlock that is kept very negatively
pressurized with respect to all adjacent areas. All of the air that is supplied to the
airlock, plus all air that flows into the airlock from the adjacent areas, is mechanically
exhausted from the airlock. This ensures against cross-contamination between
adjacent areas. Although the pressure sink airlock prevents cross contamination
between adjacent areas, its disadvantage is that unlike the pressure bubble airlock, it
is subject to contamination from the adjacent areas.
34 Siemens Building Technologies, Inc.