Engineered Smoke Control
Systems
Smoke control systems use any and all methods possible to protect from
smoke spread. Doors, fans, sprinklers, dampers, and alarms are unified
into one coordinated system. Coordination of all the smoke control
tactics is typically performed by a fire alarm/smoke control panel. In
most systems, fire fighters have override control from a Fire Fighters’ Smoke
Control System (FSCS) Panel located in a lobby or a protected area.
Overrides and status indication of all equipment are present on the face of the
FSCS or a computer screen display. Figure to the left shows a detail of a
typical override switch and indicator lights on a FSCS panel.
Smoke control tactics
Where
strategy looks at the overall picture, the individual tactics are used to
achieve the goals. The main purpose of this booklet is to explain details of how
these mechanical and electrical systems operate with respect to dampers. The
Fire Marshals, Building Officials, design engineers, and contractors are often
called upon to go beneath the overall operation of a subsystem and look at the
details.
Where
devices and wiring interconnect two disciplines, there is a tendency for those
involved to have only a fuzzy concept of the whole, interrelated design.
Some
system dampers are applied in other ways to control air flow and smoke. Air
Handling Units (AHU) are often shut down if any smoke detector in the area they
serve senses smoke. However, in engineered smoke control systems the fans may
continue to run while the AHU dampers position so that all return air is dumped
outside and only fresh air is brought into the building. For large spaces that
exhaust smoke in case of an event, dampers located on outside walls (with ducts
where appropriate) open to allow outside air to enter to replace air and smoke
pulled out by exhausts.
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Figure 1: Relief damper variation of stairwell pressurization.
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Smoke exhaust or extraction
In
large spaces, there is no way to pressurize the large area to prevent smoke
movement into the space. It is best to exhaust high volumes to remove the
smoke. Atria and large spaces, particularly malls, have exhaust fans to remove
smoke and keep it at least six feet above the occupied levels for 20 minutes to
allow escape. Lower level make-up air dampers open to the outside to admit
fresh air to replace the smoke.
Smoke vents
In
certain warehouse and storage occupancies, smoke vents are prescribed by the
codes. These can be automatic or manually operated. The goal is to remove hot,
buoyant smoke to provide clear air for occupants and fire fighters. When wind
is a potential problem, powered fans are used. These are part of an engineered
system with the switches to operate them located outside the building where the
fire service has quick access. (Section 910. IBC2009.)
Smoke shafts
In
some buildings there are shafts extending the height of the building. Fans are mounted
at the top and closed dampers are mounted in the wall of each floor. In case of
fire, the fan turns on and the damper on the fire floor opens. Smoke is pulled
out of the fire floor. A variation of this is the use of the HVAC ducts to pull
smoke out of a building by sucking with the return air fan and opening the
exhaust damper and closing the return air damper. The HVAC components are not
typically designed for this application and the volume of smoke removed may be
insufficient.
Zoned smoke control.
In
some buildings entire zones or floors are exhausted or pressurized to prevent
smoke migration. The fire zone or floor is placed under a negative pressure,
often by the HVAC return duct damper and fan. The adjacent floors are placed
under a positive pressure to prevent smoke migration. This is a “sandwich
pressurization system.” If all the floors except the fire floor are positive,
the system is known as a “building pressurization system.” Zoned smoke control
was mandatory in high-rise buildings in the legacy codes, but the present IBC
does not require them. They may still be found in some local codes and in
underground buildings which are particularly dangerous since escape paths are
highly restricted. See Below Figure.
Corridor pressurization
If
only the corridors are zone pressurized as above, the system is called a
corridor pressurization system. When smoke fills a corridor, it is very hard to
see exit signs and people become disoriented. A combination of intake and
exhaust fans can clear smoke. Corridor dampers normally provide ventilation air
and exhaust stale air. However, they can be converted to smoke control dampers
very easily. If a fire starts, the floors above and below the fire floor open
their corridor ventilation dampers 100% to pressurize the floors while they
close their return air dampers. This is identical in concept to the floor
pressurization system discussed above. See Figure below.
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| Supply and return ducts in corridor protected by fire & smoke dampers. |
Stairwell pressurization
The
IBC requires that stairwells be designed as smoke proof enclosures. There are
variations allowed by the code for when automatic sprinkles are provided and
some architectural differences. Stairwell pressurization can be accomplished a number of ways. The IBC (IBC.
2012) requires vestibules in unsprinklered buildings. This can be
supplemented with stairwell pressurization. In sprinklered buildings
pressurization alone is allowed. One should consult the IBC for details
of requirements.
One
method uses a constant volume fan capable of pushing air through any stair door
that opens. A barometric damper in the stairwell roof or wall relieves
excessive pressure. See Figure 1. In Figure 2 a combination vestibule
with barometric is shown. There are designs by different fire protection
engineers that use lobbies under positive pressure and others using negative
pressure (IBC method) by exhausting. For the most part these designs do not use
automated dampers in the periphery.
Since
most buildings are sprinklered, pressurization systems alone are more common. A
duct system can be run the height of the stairwell and proportional actuated
dampers located every few floors with local pressure sensors. If a floor
door opens, the damper(s) nearest it modulate(s) open as necessary to maintain
pressure. A certain amount of smoke may enter the stairwell when any door
is opened if there is a lot of pressure behind it. Typically, the
expansion of heated air does provide pressure. It takes some time for the
sensor, controller, and actuator to respond and open the local dampers
further. See Figure 3. The fan may be controlled by a VFD for better
control.
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Figure 2: Vestibule variation and supplemental stairwell
pressurization
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Other
variations are possible and research is incomplete with regards to which is best
in what geometric arrangement of stairs, stack effect, or height of
stairs. One variation is a second fan that turns on when the egress level
door is opened. Then that door does not relieve all the pressure
necessary for the floors. Some research has shown that sufficient
ventilation alone during a fire will keep the stairwell tenable. This
employs a supply fan at the bottom of the stairwell and an exhaust fan at the
top. It can be combined with door pressurization by using variable frequency
drive (VFD) fans.
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| Figure 3: Stairwell pressurization system using proportional damper control |
Stairwells
are built to be smoke proof compartments. The occupants can escape into
the stairwells and be protected from smoke while they escape the
building. When floor doors are opened, smoke must not enter the
stairwell. Since several architectural and control design methods are
used examination of each system is necessary to understand its intent.
Testing using smoke generators helps to ensure the system works as required.
Pressure in the stairwell must be below that which would hinder the opening of
doors.
Elevator lobby pressurization
The
lobbies of elevators can be pressurized to keep smoke from entering. These
lobbies are sometimes areas of refuge and must be kept clear of smoke. The
codes typically require that the elevator lobbies, where pressurized as a smoke
compartment, be kept positive with respect to the occupied spaces. This is
achieved by balancing the air systems to provide more air to the lobbies or by
injecting air with a separate unit.
Special
fire and smoke proportional or three-position actuators can be used to control
the corridor dampers. The dampers must be partially closed for balancing,
however they must reopen 100% to pressurize the floors adjacent to a smoke
floor or to exhaust smoke as quickly as possible. Two speed fan motors or VFD’s
prevent noise due to dampers that must be near closed during normal operation
to avoid imbalance in design flow. Standard balancing dampers would restrict
the full flow when needed. All other floors’ corridor dampers close so that a
higher pressure and more air movement are available for the sandwich floors. Smoke
causes most of the deaths in fires and smoke exhaust or pressurization methods
can constrain it. However, in all of the methods discussed, too much oxygen
cannot be injected and thus feed the fire. When fans are used to pressurize or
add air for smoke removal, smoke detection on the inlet of the fan is used to
avoid injecting smoke if the fire is near the inlet of the fan. Sprinklers are
essential for fire protection. However, they are insufficient for fully
balanced protection in large buildings. A balanced approach between active and
passive measures produces the safest conditions. Compartmentation is the
primary protection method for fire and smoke control. Maintaining the integrity
of walls prevents fire passage and smoke spread. Containment duct and shaft
dampers protect from smoke transport across compartment walls. About 85% of
smoke dampers are used to maintain compartment containment. All means of egress
must be protected – stairwells, elevator hoistways, lobbies, corridors, and
paths to the outside. In addition, dampers are required where ducts penetrate
shaft walls. Shaft dampers are the only way to restrict smoke movement. Air
handling unit shutdown is insufficient alone. Large spaces like atriums,
stages, malls, and stadium seating require smoke exhaust to keep the smoke
layer above the level of the occupants’ heads. Engineered smoke control systems
use mostly pressurization to prevent smoke migration. About 15% of actuated
dampers are installed in them.
In
general, any damper that is part of a smoke control system must be a UL555
(fire) and/or a UL555S (smoke) rated damper. In some cases exceptions are
allowed since the damper is not meant to stop smoke. Examples are outside
make-up air intakes and exhaust dampers on the outside of the building. They
are usually open during an event and do not stop spread of fire and smoke.
Dampers
are required to maintain compartmentation and restrict fire and smoke from
spreading outside of the area of origin. However, full engineered smoke control
systems can actively manage smoke and ensure means of egress for occupants.
Exhausting large spaces with fans removes smoke. Preventing smoke from entering
exit corridors, lobbies, elevators, and stairwells is critical in allowing
escape. Other smoke control methods prevent the spread of smoke in buildings
and along with architectural planning can protect occupants.
