Enclosure fires

The production of
unburnt gases is
crucial to the smoke
gas layer being
able to ignite. If
the unburnt gases
accumulated in
the smoke gas
layer ignite, the
radiation levels will
rise dramatically in
the compartment.
This, in turn, causes
other materials to
ignite. The fi re then
grows quickly and
spreads.
Figure 27. (page
opposite) Black smoke
gases are escaping from
the room, accompanied
by a lot of potential
energy. As we can see,
not everything combusts
outside either.
40
pounds). A more detailed description of the constituents
which can be formed during a fi re is given in the next section,
3.1.2.
Unburnt smoke gases are always formed if combustion
takes place where the oxygen supply is insuffi cient. But even
when the oxygen supply is suffi cient for all the fuel to combust,
there are always some unburnt smoke gases formed.
The combustible products found in smoke gases originate
from:
1. Pyrolysis from materials which are not in contact with the
actual seat of the fi re. As the temperature is very often
high up at ceiling level, combustible ceiling material is
usually pyrolysed.
2. Incomplete combustion from the actual seat of the fi re.
The more incomplete the combustion is, the more combustible
products there are in the smoke gases. The poorer the access to
air, the more incomplete the combustion process is. This then
increases the likelihood of the smoke gas layer igniting.
You should note that some of the potential energy available
in the smoke gas layer is very diffi cult to extract, even when
the smoke gas layer ignites. In the case of soot particles, for
instance, the temperature needs to reach u 1000 °C for the
potential energy to be converted to heat.
That explains why we often see black smoke gases streaming
out of the fi re room, even if the temperature in the room
is high. Soot particles can be recognised from their black colour,
see Figure 27.
We earlier talked about the heat release rate. The amount
of heat energy released during a particular period of time,
such a second, is called the heat release rate and is expressed in
J/s (joules per second) or W (watts). The heat release rate is a
very important concept as it offers us the opportunity to assess
the size of a fi re, which in turn means that we can work
out the scope of the fi refi ghting operation. When working out
the scope of a fi refi ghting operation, the fi re’s heat release rate
is compared with the capability offered by different fi refi ghting
methods and resources.
The heat release rate from a fuel surface is based on a cer-