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Tactical Ventilation
Ch pt r
Chapter 5 - Forced Ventilation
1 General
Ventilation efforts should be in concert with existing atmospheric conditions, taking advantage of
natural ventilation whenever possible. However. in
some situations, natural ventilation may be inadequate and may have to be supplemented or
replaced by forced ventilation to provide a tenable
atmosphere and to facilitate rescue operations.
Forced ventilation refers to the use of fans, blowers, water sprays or other mechanical devices to
create or redirect the flow of air inside the building
so that the fire gases are forced out of the building.
If forced ventilation is used to accelerate the
effects of natural ventilation, it must be remembered that all the effects, both good and bad, may
be accelerated. For this reason, it is essential that
•
it requires the use of a mechanical device, a
power source and additional firefighters;
•
it can increase the intensity of a fire and lead
to unwanted fire and smoke spread if incorrectlyapplied;
•
to clear large compartments, it requires a very
large fan, or a number of smaller fans;
•
it can take time to set up;
•
in defensive ventilation operations, the limited airflow available means that systematic
room-by-room clearance is necessary.
The main techniques of forced ventilation are:
•
Positive Pressure Ventilation (PPV). PPV can
be achieved by forcing air into a building
using a fan. The effect of this will be to
increase the pressure inside, relative to
atmospheric pressure. PPV simply refers to
blowing air in through the inlet vent.
•
Negative Pressure Ventilation (NPV). NPV
refers to extracting the smoke and hot gases
from the outlet vent. This will have the effect
of reducing the pressure inside the building,
relative to the atmospheric pressure. It can he
achieved by fans or water sprays.
•
Heating Ventilation and Air Conditioning
Systems (HVAC). Building HVAC systems
can be designed so that, in the event of fire,
they can be used as a smoke control system.
•
Powered Smoke and Heat Exhaust Systems.
Dedicated fans and other devices which, usually triggered automatically, provide a smoke
control system.
the firefighters concerned have a good understanding of the principles of ventilation, and the behaviour of fire, before the use of forced ventilation is
considered.
The main advantages of forced ventilation are:
•
the ventilation objectives. i.e. smoke
removal, restoration of a tenable atmosphere
etc., are achieved more rapidly;
•
it makes horizontal ventilation more effective, so reducing the need for vertical ventilation;
•
it is less susceptible to erratic wind conditions, although it cannot overcome strong
winds;
•
it is a more controllable form of ventilation.
Its disadvantages are:
Tactical Ventilation
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