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Chapter 1: Fire geometry and smoke movement in buildings
movement, and fire pressures, can be the major factors
that cause smoke movement. Tall buildings have the
effects found in small buildings combined with smoke
and heat movement by convection and radiation
upwards. Accepted engineering approaches to smoke
removal and venting practices reflect these influences.
A major cause of fire spread across the floor of a
building is heat radiated downwards from the layer of hot
gases beneath the ceiling. Roof venting will limit fire
spread because it limits the spread of hot gases under
the roof. In the alternative, if the major cause of fire
spread is due to flame progressing sideward, at floor
level and through readily combustible material, roof
venting will less readily limit fire spread. Roof venting,
addressed in NFPA 204, Standard for Smoke and Heat
Venting, will only slow sideward movement because it
limits the extent to which heat is radiated downward and
is only one factor in the sideward development of a fire.
All fires produce smoke and the movement of smoke
follows the same pattern as the overall air movement
within a building. Very simply, a smoke control system
needs to be able to inhibit the flow of smoke within a
building.
Smoke movement is determined by two central factors in
a fire. These are:
•
Smoke's buoyancy due to the entrainment of hot
gases which are less dense than the surrounding air
•
Normal air movement inside a building, which may
have nothing to do with the fire, but which can carry
smoke around a building in a positive way
The magnitude of these two smoke-moving factors will
depend upon particular circumstances and will vary
throughout a building. In general, the smoke closer to
the fire poses the greatest risk. The movement caused
by the smoke's mobility is due to pressure differentials
developed by the:
•
Expansion of the gases as they are heated by the
fire
•
Difference in density of the hot gases above the
flames
•
Cooler air which surrounds the fire
Air movement in a building in nonfire conditions can be
caused by three separate factors: stack effect, wind
load, or HVAC (mechanical) systems. In a fire, these
same factors are equally influential.
4
Stack effect
The stack effect is the pressure differential due to the air
inside a building being at a different temperature from
the air outside the building. Stack effect will cause the air
inside the building to move upwards or downwards,
depending upon whether the air inside the building is
warmer or cooler than the air outside the building.
Air within a building has a tendency to rise because it is
warmer and less dense than the outside air. The taller a
building is and the greater the temperature differences
between the building interior and exterior are, the greater
the tendency for air to rise in the building's shafts. See
Figure 2.
Figure 2: Normal stack effect
Outside
65°F (18.3°C)
Neutral
plane
The opposite is true when the outside temperature is
warmer than the temperature inside the building, causing
a downward movement of air within building shafts. This
is referred to as reverse stack effect.
In a building with reverse stack effect, only relatively cool
smoke will follow the downward tendency of air into a
shaft. If a smoldering fire occurs on a floor above the
neutral plane during a reverse stack effect condition, the
smoke will travel into and down the shaft and deposit
VM-1 Smoke Management Application Guide
Inside
75°F (23.9 C)
°
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