Fire ventilation

Pressure differences created by the fi re
a. inhibited thermal expansion
b. thermal buoyancy force
All these pressures are of the same magnitude, which means
that even very small changes in one pressure can infl uence
the fl ow of fi re gases signifi cantly.
During different types of fi res in different types of buildings,
with different conditions, one or more of the types
of pressure differences described above can dominate to a
greater or lesser degree. A commanding offi cer about to
make a decision about fi re ventilation, must therefore picture
for himself which of the pressure differences are the
most dominant.
In very high buildings with shafts the differences in temperature
between the outside and inside air will cause large
pressure differences. These pressure differences can be considerable,
especially during cold or very warm weather.
The wind can have a very large effect on the spread of
fi re gases, both inside buildings and to other buildings,
especially in certain geographic locations or at high elevations
(high buildings).
Comfort ventilation can in certain types of buildings,
above all those with open or closed supply and exhaust air
systems, cause problems with the spread of fi re gases, especially
if the ventilation system stops working.
Fires in buildings create pressure differences as a result
of inhibited thermal expansion, i.e. the hot gases expands,
and also as a result of the thermal buoyancy force, i.e. hot
air rises upwards. In the case of fi res where the intensity
develops rapidly, or fi res that spread quickly, these pressure
differences taken together can be considerable.
If the boundary of a fi re compartment is defective, or if
the structure is weakened as a result for example of the
impact of the fi re, the pressure differences that arise can
further weaken the boundary of the fi re compartment or
the structure, especially during very intense or rapid fi re
scenarios or at high temperatures.
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