Ventilation in for more than
just fire survival. Ventilation for houses
is also useful for removing bad odors,
like the smell of the 32nd bag of burnt
popcorn. In order to ventilate to either
remove heat or odors, we can use multiple
techniques. We'll consider several major
distinctions8.
Passive Ventilation In passive ventilation, we
simply provide openings for air to pass
through and allow ambient conditions to
drive the ventilation of the structure
(i.e. wind speeds).
In passive ventilation, opening a
single window or door on the windward side
of the building would be inefficient as
that tends to pressurize the building as
air would be pushed in and the only way to
escape would be through the same opening
against the air that's being pushed in.
However, if we open a window on the
downwind side as well, we would create a
very good airflow. However, since the
window and door are likely to be
significantly different in size, you might
want to secure loose papers near the
window as air will exit much faster
through the window than the windspeed
outside.
To rapidly passively ventilate,
opening as many windows and doors as
possible is best. However, if we don't
want to open all the windows and doors.
Active Ventilation
Say that we are impatient and don't
want to wait for the breeze to gently
remove the popcorn scent. Or the day has a
significant mosquito population and you
want to minimize their time to invade your
home. We can use devices like fans to
speed up the process.
In fire operations when
smoke removal is necessary, active
ventilation by either positive pressure or
negative pressure fans can be done.
A positive pressure fan works by
accelerating air towards the window or
door, creating a high pressure area. Since
air moves from high pressure to low
pressure, the air will move from the
opening with the positive pressure fan
through the structure to exit.
Setup and
flow paths using a positive
pressure fan. Note that positive
pressure ventilation is only
truely effective if there is
another opening through which
air can escape the structure,
much like only opening a single
window on the windward side of a
building is an ineffective
passive ventilation method.
Image from
http://www.geltechsolutions.com.
Image links to a further
discussion of positive pressure
ventilation, for the curious!
A negative pressure fan
works by accelerating air away from the
window, making a low pressure area in the
window and essentially 'pulling' air from
the structure.
Another way of
mechanically ventilating a structure
is hydraulic ventilation. Certain
types of fire nozzles called fog
nozzles create a negative pressure
differential. By pointing one of these
nozzles out of a window while in a
smoke filled compartment, a strong
draft gets created and smoke exits
rapidly through the window with the
water stream. In the video
below, hydraulic venting can be seen
starting at 1:20.
Horizontal and Vertical
Ventilation Other terms sometimes used
when discussing ventilation are horizontal
and vertical ventilation. Horizontal
ventilation we've already somewhat
discussed: it's when all of the
ventilation openings are on the same
story. Vertical ventilation involves
creating an opening on a higher level.
In a fire, hot gasses tend to rise.
This means that if the goal is to remove
hot gases (and hopefully remove heat
although most fire science is now leading
us to believe that ventilation in and of
itself is not nearly as effective at
reducing temperatures as putting water on
the fire), creating a ventilation opening
on a roof will be most effective at
letting the hottest gas escape.
However, if the fire is only on the
first story, creating a roof opening will
create a flow path going from any openings
on the fire level through the structure
and up to the roof, which would likely
cause rapid fire spread.
