User Manual

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Siemens Building Technologies
Cerberus Division
01.2002
6.2 Air change
Air cleanliness class Number of air changes
per hour (approx.)
10,000 15 – 30
1,000 20 – 60
100 70 – 300
10 and 1 600 – 700
To achieve the required air cleanliness class for new products, e.g. 16 Mbit DRAM, pro-
cess-controlled equipment is placed in so-called „mini-environments”. Only in these mini-
environments is the highest air cleanliness class maintained, e.g. class 1 means that in a
cube of air with sides 30 cm long, only one 0.5µm
3
particle is accepted. Due to this feature,
in areas around the mini-environments a lower class can be tolerated without prejudice to
the process.
6.3 Air speed
In order to guarantee the necessary stability, as a rule, unidirectional airflow in clean rooms
requires air velocities of between 0.3 and 0.5m/s.
6.4 Pressure differences, overpressure,
underpressure
6.4.1 Overpressure
In order to prevent the transport of contaminated air from one room to the neighbouring
rooms (so-called cross-contamination) pressure differences are maintained between
neighbouring workrooms. If the room has to be protected from the processes in its sur-
roundings, then overpressure must be maintained in that room.
6.4.2 Underpressure
If, conversely, the surroundings have to protected against the contaminated air released in
the workroom, then underpressure must be maintained in the room against the surround-
ings. The pressure difference between neighbouring rooms of differing air cleanliness is at
least approx. 5 Pa (pascal); in the pharmaceutical industry, however, the authorities de-
mand values of 10 – 15 Pa.
Note: 1 pascal is the pressure exerted vertically and uniformly on an area of 1m
2
by 1 new-
ton. 1 newton is a unit of force that imparts an acceleration of 1 metre per second to a mass
of 1 kilogram.
6.4.3 Room-in-room construction
Overpressure can help to prevent the infiltration of air contamination from outside into a
building to be protected. At higher wind speeds and with the accompanying wind pressure
on the building shell, such infiltration cannot be avoided. It is impossible to make fassades
entirely air-tight.
That is the reason why room-in-room construction is preferred for clean room working
areas, as a result of which infiltration can be intercepted in the corridors surrounding the
clean room areas.