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Chapter 1: Fire geometry and smoke movement in buildings
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The HVAC system supplied air to the room of origin
and thus had the potential to help accelerate the fire.
An HVAC system may aid in the detection of fire in its
early stages when area smoke detection is not provided.
The HVAC system can transport smoke from an
unoccupied area to one where smoke detection or
occupants are present and can then alert others of the
fire.
Once fire is detected, HVAC systems installed in
accordance with NFPA 90A and utilizing an internal
smoke detector will shut down fans and dampers or
provide a special smoke control mode. NFPA 90A-3-4
contains damper shutdown provisions. NFPA 90A-4-4
contains provisions for smoke detectors when area
detectors are not used in air distribution systems:
•
Downstream of air filters and ahead of any branch
connections in air supply systems having a capacity
greater than 2000 cfm (944 L/s)
•
At each story prior to the connection to a common
return and prior to any recirculation or fresh air inlet
connections in air return systems having a capacity
greater than 15,000 cfm (7,080 L/s) and serving
more than one story
Note:
See NFPA 72 for guidance on installing smoke
detectors used in smoke control systems.
If neither of the NFPA 90A steps are taken, the HVAC
system will transport smoke to every area that a system
serves; putting occupants in peril, damaging property,
and possibly inhibiting fire fighting.
Shutting down fans does not prevent smoke movement
through supply and return air ducts, air shafts, and other
building openings due to stack effect, buoyancy, and
wind. Installation of smoke dampers for use when the
system is shut down will help inhibit smoke movement in
this case. Again, NFPA 90A contains damper
requirements that are referenced by building and fire
codes, standards, or guidelines used in the design and
installation of smoke management systems.
Additional contributing factors
Thermal expansion:
In addition to stack effect,
buoyancy, and HVAC factors, the energy released by a
fire can cause smoke movement due to thermal
expansion.
6
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 consideration for tightly
sealed compartments where fires can occur.
It is possible for the volume of smoke to almost triple in
size when temperatures over 1,000°F (538°C) 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.
Elevator piston effect:
be significant contributors to smoke movement in a
building when no control measures are in place.
The downward movement of an elevator car in a shaft
produces 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 pressure increase tends to
move air out of the elevator shaft and into the floors. The
temporary pressure decrease tends to move air from the
floors and into the elevator shaft. See Figure 5 on page
7.
Pressure differences, due to the piston effect, are
greater in single car elevator shafts as compared to
multiple car shafts. In a multiple car shaft there is usually
more room to the left and right of the moving car to allow
for pressure relief.
VM-1 Smoke Management Application Guide
Vertical shafts for elevators can
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