Managing Smoke Movement - Simplex 4100U Application Manual

Controlling Smoke Movement,
Causes of
Smoke Movement
Managing
Smoke Movement
Continued
HVAC System:
Before the development of smoke control systems, HVAC systems were shut down when a fire
occurred. This is because an HVAC system frequently transported smoke during building fires.
In the early stages of a fire, HVAC smoke transport can be a good thing. When a fire starts, the
HVAC system can transport the smoke to a location where people can smell it and be alerted to
the fire (although they may not know where the smoke is coming from). However, as the fire
progresses, the HVAC system transports the smoke to every area that it serves, endangering life in
all those places. To make matters worse, the HVAC system also supplies air to the fire, which aids
combustion.
Although shutting down the HVAC system prevents it from supplying air to the fire, this action
does not prevent the movement of smoke through the supply and return air ducts, air shafts, and
other building openings due to stack effect, buoyancy, or wind effect.
"Smoke Movement Management" includes all of the methods that can be used to modify and
control smoke movement for the benefit of the building occupants, firefighters, and for the
reduction of property damage. The use of barriers, smoke vents, and smoke shafts are traditional
methods of smoke management.
Barriers:
The effectiveness of barriers in limiting smoke movement depends on the leakage paths in the
barrier and on the pressure differential across the barrier. Holes where pipes penetrate floors or
walls, cracks around doors, and cracks in walls or between walls and floors are a few of the places
where smoke can leak through a barrier. The pressure differential across these barriers depends on
wind, buoyancy, stack effect, and the HVAC system.
Smoke Vents and Smoke Shafts:
The effectiveness of smoke vents and smoke shafts depends on their distance from the fire, the
buoyancy of the smoke, and the presence of other driving forces. In addition, when smoke is
sprinkler-cooled the effectiveness of smoke vents and smoke shafts is greatly reduced.
Elevator shafts in buildings have often been used as smoke shafts. The obvious problem with this
is that it prevents the elevator from being used for fire evacuation (because of the "piston effect"
of an elevator), and frequently allows the smoke to travel between floors. Specially designed
smoke shafts which have no leakage can be used to prevent the distribution of smoke to fire-free
floors.
In summary, the effectiveness of barriers in a traditional smoke management system is limited to
the extent that the barriers are free of leakage paths. Smoke vents and smoke shafts are limited by
the fact that the smoke must have sufficient buoyancy to overcome other forces that may be
present.
In the last few years, motorized fans have been used to overcome the limitations of the traditional
systems. The systems that employ these motorized fans are called "Smoke Control Systems."
These Smoke Control Systems rely on creating air pressure differences and positive or negative
airflows to limit and control the movement of smoke and other noxious gases.
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