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Four fi re scenarios:<br />
1. The fi re has<br />
spontaneously<br />
gone out<br />
2. The fi re resumes<br />
its development<br />
3. The smoke gases<br />
auto-ignite<br />
4. A backdraught<br />
occurs<br />
144<br />
6.7 Summary<br />
We have seen that there are at least four different scenarios<br />
which can occur, where we face a ventilation-controlled situation<br />
and air is being supplied to the fi re room: 1. The fi re<br />
spontaneously goes out, 2. The fi re resume its development, 3.<br />
The smoke gases auto-ignite, 4. A backdraught occurs.<br />
Scenarios 1 and 2 are the most common, but scenario 4 is<br />
by far the most dangerous. This is why it has been described in<br />
the most detail.<br />
A backdraught occurs when a quantity of unburnt gases<br />
have accumulated in the smoke gas layer. If a door is opened,<br />
for instance, a current of air will be sucked into the fi re room.<br />
This creates a well-mixed area, which can ignite if there is any<br />
ignition source present. This then creates a fl ame front, which<br />
spreads out through the opening, producing a fi re ball. This<br />
process can be said, therefore, to go through the following<br />
stages:<br />
• Accumulation of unburnt smoke gases.<br />
• Infl ow of air current.<br />
• Mixing of air and smoke gases.<br />
• Ignition of premixed area.<br />
• Turbulent defl agration.<br />
• Creation of fi re ball outside the fi re room.<br />
The premixed region is roughly just as large for a door opening<br />
as for a window opening. The mixing process occurs when<br />
the current passes by a sharp edge on its way into the room.<br />
The mixing process can also occur if BA fi refi ghters are standing<br />
in the open doorway.<br />
If the air current reaches the back wall the premixed area<br />
will become much larger. If ignition occurs in this situation<br />
there will be a much larger increase in pressure than if ignition<br />
occurs when the air current is on its way into the room.<br />
A third scenario arises if the air current has left the room a<br />
long time before ignition occurs. In this case, there may still be<br />
combustible gases above the doorframe. In most cases, ignition<br />
will not cause any powerful backdraught, which is quite<br />
simply due to the fact that the combustible gaseous mass is so<br />
small. In larger premises, particularly where the distance be-
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