Enclosure fires

If turbulence affects the system the fl ame’s surface gets bigger and S u increases
by a factor b. The equation then becomes:
S f = S u × b × (T f /T i)
3.3 Pressure conditions in open and closed rooms
This section will deal with pressure conditions in both open
and closed rooms. The pressure conditions are instrumental
in controlling how smoke gases and air move through the
openings which are always found in a building. Naturally, the
pressure conditions differ if the room has large openings,
compared to if the room is more or less closed. We are still in
the early fi re development phase. It is actually of no consequence
at all in our example whether the fi re is fuel or ventila-
Equation 10
b varies according to the furnishings, openings, etc. or to the use of a fan. The
normal value lies between 1 and 5. This makes S f even higher.
It should be added that T i is at its highest when the stoichiometric point is
reached. This means that the expansion factor too is at its highest when the stoichiometric
point is reached, even as high as around 8, but then it can drop to
5–6 at the fl ammability limits. The expansion factor in the example is based on
the gases being at ambient temperature. (As smoke gases are, more often than
not, hotter than ambient temperature, the amount of expansion is often lower.)
Extinguishing mechanisms – gas phase effect
The reaction formula in Equation 11 below can be used to describe how a fl ame
goes out. When we extinguish a fl ame with water we are therefore adding water
molecules on both sides of the reaction formula, which means that the fl ame
temperature drops and the fl ame goes out. The reaction is a complete reaction
involving methane (CH 4) and air.
H2O + CH4 + 2O2 + 2 × 79 N2 P CO2 + 2 H2O + 2 × 79 N2 + H2O 21
21
Equation 11
Water (steam) therefore draws energy from the mixture and reduces the temperature
to a level where fl aming combustion is impossible.
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