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The following forces affect smoke movement:
Stack Effect:
When the outside air temperature is colder than the temperature inside a building, there is often an
upward movement of air within the building. This air movement is most noticeable in stairwells,
elevator shafts, electrical risers, or mail chutes, and is referred to as “Normal Stack Effect.” This
phenomenon is most noticeable in tall buildings during winter, but can occur in a single story
building as well.
When the outside air temperature is warmer than the temperature inside a building, there is often a
downward movement of air within the building. This air movement is referred to as “Reverse
Stack Effect.”
In a building with normal stack effect, the existing air currents can move smoke a considerable
distance from the fire origin. If the fire is below the neutral plane of the shaft (i.e., an elevation
where the hydrostatic pressure inside the shaft equals the hydrostatic pressure outside the shaft),
smoke moves with the building air into and up the shaft. Once above the neutral plane, smoke
flows out of the shaft into the upper floors of the building.
Buoyancy:
Smoke from a high temperature fire has buoyancy due to its reduced density. In a building with
leakage in the ceiling of the fire room, this buoyancy-induced pressure can produce smoke
movement. In a fire room with only one opening to the building, air flows into the room while hot
smoke flows out of the room. If the fire room has open doors or windows, the pressure difference
across these openings is negligible because of the large flow areas involved. However, for a
tightly-sealed room, the pressure differences due to expansion may be important.
Wind:
Wind can also have a pronounced influence on smoke movement within a building. The effect of
wind velocity on the air movement within a well-constructed building is minimal. However, the
effects of wind can become important for loosely-constructed buildings or buildings with open
doors or windows.
Frequently in fires, a window breaks. If the window breaks on the side of the building away from
the wind (the leeward side), the negative pressure caused by the wind vents the smoke from the
fire room. This can greatly reduce smoke movement within the building. However, if the broken
window is on the windward side of the building, the positive pressure of the wind can force the
smoke throughout the fire floor, and possibly to other floors as well. This event can endanger lives
and hamper firefighters as well. Wind induced pressures can be quite large and can easily
dominate building air movement.
Continued on next page
Controlling Smoke Movement, Continued
Causes of
Smoke Movement
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