User Manual

31
Siemens Switzerland Ltd
Building Technologies Division
4.2.2. Gas
Gases are fast, highly efficient, electrically non-conductive and clean. Their properties make them particularly
suitable for rooms with high value content. Some types of extinguishing gas rely on the removal of oxygen, while
others rely on chemical inhibition of the combustion process:
Natural gases (removal of
oxygen)
The following natural gases are suitable for extinguishing purposes:
• Carbon dioxide (CO
2
• Citrogen (N
)
2
• Argon (Ar)
)
Mixtures of these gases are also commercially available:
e.g. Inergen (52% N2, 40% Ar, 8% CO2) or Argonite (50% N2 , 50% Ar).
Automatic extinguishing systems using such gases (or gas mixtures) rely on the
partial displacement of air (and consequently oxygen) from the protected area.
The concentration of oxygen in air is 20.8 vol.-% and reducing this concentration
to below 13% will stop the combustion process for most combustible materials.
When CO2 is not used then the remaining oxygen concentration (generally
between 10 and 13 vol.-%) is not life threatening. However, this is not the case if
CO2 is used; as a concentration of as little as 5 vol.-% of CO2 gas can be life
threatening. This has nothing to do with the reduced oxygen content of the air,
but is purely due to the toxicity of the CO2 itself. As concentrations of up to 50%
may be reached in some extinguishing systems, it would be fatal for anyone
remaining in the gas-flooded area.
Chemical extinguishing
gases (inhibition of
combustion process)
Due to their chemical properties, extinguishing gases such as Novec™ 1230 and
HFC227ea inhibit the combustion process by extracting energy from the fire. On
being released into the atmosphere, these gases form an extinguishing mixture.
A correctly controlled concentration of this mixture extracts heat from the fire
and cools the burning material down to the point where the fire is extinguished.
For HFC227ea and Novec™ 1230, the concentration level used in practice is
below the NOAEL value (No Observed Adverse Effect Level). It does not therefore
pose a health threat to people present in the protected area. However, the area
should always be evacuated before the gas is released.
4.2.3. Foam
Blanketing the burning surface of a combustible material (either solid or liquid) with foam effectively separates it
from the surrounding air, depriving the fire of the necessary oxygen. The cooling effect of the foam also
contributes to the extinguishing process. Foam is particularly suitable for use in fire extinguishers.
4.2.4. Chemical powder
Extinguishing powder relies on interrupting the combustion process by interfering with the chemical reaction and
the associated extraction of energy. Powder is mainly used in fire extinguishers. One disadvantage of this type of
extinguisher is that it leaves a very fine residue which is difficult to remove and is highly corrosive. The powder is
also incompatible with electrical and electronic equipment.