Fire ventilation

The hydrodynamic pressure, i.e. the pressure resulting from the fl ow of the
gas, at point 1 and point 2, will therefore be:
ρv 2
−−−−−
2
The fi rst law of thermodynamics (the principle of the conservation of energy)
says that:
ρ1v 2
1 ρ2v 2
2
P1+ ρ1gh1+ −−−−− = P2+ ρ2gh2+ −−−−−
2 2
With the help of Bernoulli’s equation it is possible to derive and analyse what
the pressure structure looks like in a building, and accordingly also how the
fi re gases will fl ow.
In what follows these pressure differences will be treated
separately. In actual fact several or indeed all of these different
types of pressure differences will arise and act simultaneously,
for which reason a more simple line of reasoning
around the problem will be left to the end of the chapter.
Differences in temperature between
outdoor and indoor air
The air indoors is most often warmer than the air outdoors.
Air that is heated up expands, takes up more space, and has a
lower density than cold air. The pressure inside a building,
where the air is warmer than outside, will therefore be higher
than outside. This pressure strives towards equilibrium with
the surroundings, and therefore the heated air fl ows out from
the building; from the higher pressure inside the building to
the lower pressure outside the building. Since a building is
seldom or never completely tight, the (heated) air will always
be forced out from the building and, at least gradually, be
replaced by cold air fl owing in. If the openings are small, or if
the pressure difference is large in relation to the size of the
Pascal
Pressure is measured
in the international
unit Pascal [Pa]. 1 Pa is
the same as 1 N/m 2 (1
Newton per square
metre) or 10 –5 bar
(0.00001 or ten million
parts per bar). 1 Pa is
approximately equivalent
to the pressure a
standard A4 sheet of
paper exerts on the
surface of the table.
The normal air pressure
is 101325 Pa.
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