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Fire Safety & Security Products 03.2009

would occur, now with the extinguishing system already discharged. Deep

seated fires require a cooling time in order to be extinguished. Here inert gases

will also be very efficient. When combined with water, the required cooling ef-

fect is accelerated, the deep seated fire is under control and the risk of re-

ignition is minimized. Inert gas systems with additional water fog are the ideal

combination to extinguish deep seated fires.

4.4.2.2 Chemical agents

In all chemical extinguishing agents, the extinguishing effect is a combination of

several processes: On the one hand, the extinguishing agent molecule is decom-

posed into its constituents, in the hot flame zone. In accordance with physical laws,

this is accompanied by x-fold volume expansion (“x” standing for the number of at-

oms) and thus by a reduction of the local oxygen concentration in the flame zone.

On the other hand the molecule takes energy away from the fire, causing a cooling

effect.

This shows that the extinguishing effect of the so-called chemical agents is largely

physical. Novec™ 1230 molecule is very heavy, consisting of 19 atoms and

HFC227ea molecule consists of eleven atoms. This leads to relatively low extin-

guishing concentrations of chemical extinguishing gases, which is between 7 and 9

percent by volume for HFC227ea and between 5 and 7 per cent by volume for

Novec™ 1230.

In all chemical extinguishing agents containing fluoride atoms, hydrofluoric acid

molecules (HF molecules) are generated in the extinguishing process, due to re-

combination in the flaming zone. This is also true when extinguishing with Novec™

1230 or HFC227ea. Hydrofluoric acid is strongly caustic and corrosive. Therefore

the use of chemical extinguishing agents in areas where large flames are to be ex-

pected at the beginning of the flooding process should be avoided.

Since chemical agents require extinguishing concentrations of only 5% to 9% (de-

pending on the chosen agent and the specific type of risk) the amount of extin-

guishing agent to be flooded into the protected room is lower compared to inert

gases; and as a consequence the overpressure created in the room and the nec-

essary flaps to release this overpressure are also considerably smaller.

Plastic media (X-rays, slides, films, microfilms, etc.) and electronic media (mag-

netic bands, DVD, hard disks, solid state memories, etc.) produce mainly one kind

of fire:

– Smoldering fire: is a slow, low temperature flameless form of combustion. The

use of chemical agents is ideal for this type of applications. Being this fires not

fast growing in size, the production of by-products (i.e. HF) is negligible

The use of chemical agents in applications with risk of open flame fires (surface

fires) is not recommended, since the high production of hydrofluoric acid would re-

sult. HF is strongly caustic and corrosive; and is toxic for people. Therefore the use

of chemical extinguishing agents in areas where large flames are to be expected at

the beginning of the flooding process should be avoided. A fast-reacting fire detec-

tion system immune to deception is indispensable to keep the HF generation as

low as possible.

In Tab. 7 environmental and toxicological parameters are presented for the extin-

guishing agents treated in this paper.