2: Buoyancy; 3: Expansion; 4: Wind; 5: Elevator Piston Effect - Honeywell NOTIFIER SCS Series Operation And Installation Manual

Smoke Movement
In a building with reverse stack effect, only relatively cool smoke will follow the downward tendency of air into the shaft. If a smoldering
fire occurs on a floor above the neutral plane during reverse stack effect conditions, the smoke will travel into and down the shaft and deposit
itself on the floors below the neutral plane. In the case of hot smoke, buoyancy forces can counteract reverse stack effect causing the smoke
to move up the shaft.
2.2.2 Buoyancy
High-temperature smoke has a lower density than cool smoke. Because of this reduced density, it has a greater tendency to rise through the
air and create a buoyant pressure in the smoke zone. The pressure buildup within a compartment due to buoyancy forces moves smoke to the
floor above the fire floor through any leakage paths in the ceiling. The buoyant pressure will also cause smoke movement through any leak-
age paths in the walls or around doors. As smoke travels away from a fire, its temperature drops due to heat transfer and dilution. Therefore,
the effects of buoyancy generally decrease with distance from the fire.
2.2.3 Expansion
In addition to stack effect and buoyancy, the energy released by a fire can cause smoke movement due to expansion. In a fire compartment
with only one opening to the building, air will flow into the compartment and hot smoke will flow out. For a fire compartment with open
doors and windows, the movement of smoke due to expansion is negligible. However, the effects of expansion should be taken into consid-
eration for tightly sealed compartments where fires can occur. It is possible for the volume of smoke to almost triple in size when tempera-
tures of over 1000°F are reached. For tightly sealed compartments, the buildup of pressure resulting from expansion causes smoke
movement through any leakage paths in the walls or around doors.
2.2.4 Wind
Wind can have a dramatic effect on smoke movement. Frequently in fire sit-
uations, a window breaks or is left open in the fire compartment. If the
opening is on the windward side of the building, the wind causes a buildup
of pressure in the fire compartment and forces smoke throughout the floor
and possibly to other floors. Pressures caused by the wind in this condition
can be large and easily dominate smoke movement throughout the building.
If the opening is on the leeward side of the building, the reverse is true. The
negative pressure created by the wind vents the smoke from the fire com-
partment, greatly reducing the smoke movement through the building.
2.2.5 Elevator Piston Effect
The movement of an elevator car in a shaft produces temporary pressure
differences both above and below the moving car. A downward moving
elevator car creates a temporary pressure increase in the area below the
car and a temporary pressure decrease in the area above the car. The
reverse is true for an upward moving elevator car. The temporary pres-
sure increase in the elevator shaft tends to move air into the floors below
the car and the temporary pressure decrease tends to move air from the
floors above into the elevator shaft, as shown in Figure 2.3. Pressure dif-
ferences due to the piston effect are greater in single car elevator shafts
as compared to multiple car shafts because in a multiple car shaft there
is usually more room to the left and right of the moving car to allow for
pressure relief.
SCS Series Manual — P/N 15712:L 7/18/16
Top View of the
Leeward Side
of the Building
Figure 2.2 Wind Effects on a Building
arrows indicate direction
of air movement
Lobby
Downward
Moving
Elevator Car
Elevator
Hoistway
Figure 2.3 Elevator Piston Effect
Concepts of Smoke Control
Windward Side
Building
of the Building
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