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ENGINEERING MANUAL OF AUTOMATIC CONTROL
SMOKE MANAGEMENT FUNDAMENTALS
178
The door widths in Table 2 apply only for doors that are
hinged at one side. For other arrangements, door sizes, or for
hardware other than knobs (e.g., panic hardware), refer to
calculation procedures furnished in Design of Smoke Control
Systems for Buildings published by ASHRAE
3
.
AIRFLOW
is most commonly used to stop smoke movement through open
doorways and corridors. Figure 4 illustrates a system with
relatively high velocity to prevent backflow of smoke through
an open doorway. Figure 5 illustrates a system with relatively
low velocity which allows backflow of smoke. The magnitude
of the velocity of the airflow required to prevent backflow
depends on the energy release rate of the fire. Since this can
vary, the velocity should be regulated to prevent oxygen from
being fed to the fire. The fact that doors are sometimes left
open during evacuation of a building, allowing smoke to flow
through, should be taken into account in designing the smoke
control system. This is done by designing and testing the system
with one or more doors open.
M13024
DILUTED
SMOKE
RELATIVELY
HIGH AIR
VELOCITY
Fig. 4. High Air Velocity Preventing Backflow
of Smoke Through an Open Doorway.
M13025
SMOKE
RELATIVELY
LOW AIR
VELOCITY
SMOKE
BACKFLOW
Fig. 5. Low Air Velocity Allowing Backflow
of Smoke through an Open Doorway.
PURGING
Because fires produce large quantities of smoke, purging cannot
ensure breathable air in a space while a fire is in progress. After a
fire, purging is necessary to allow firefighters to verify that the
fire is totally extinguished. Traditionally, firefighters have opened
doors and windows to purge an area. Where this is not possible,
the HVAC system can be designed to have a purge mode.
The principle of dilution can be applied to zones where smoke
has entered and is being purged. Purging dilutes the contaminated
air and can continue until the level of obscuration is reduced and
the space is reasonably safe to enter. The following equation allows
determining a concentration of contaminant in a compartment
after purging for a given length of time:
3
C = C0 x e–at
Where:
C = concentration of contaminant at time, t
C0 = initial concentration of contaminant
a = purging rate in number of air changes per
minute
t = time after doors close in minutes
e = constant, approximately 2.718
Care must be taken in the use of this equation because of the
nonuniformity of the smoke. Buoyancy is likely to cause greater
concentration of smoke near the ceiling. Therefore,
consideration of the locations of supply and exhaust registers
is important to effective purging.
CONTROL APPLICATIONS
Figure 6 illustrates a smoke control system with detectors,
an initiating panel, and a communications bus to an alarm
processor and remote control panels in appropriate areas of the
building. A configuration similar to this will meet the
requirements of UL 864, Standard for Control Units for Fire-
Protective Signalling Systems, and comply with NFPA 92A
recommended practice for smoke control systems. The remote
control panels position dampers and operate fans to contain or
exhaust smoke, depending on the requirements of the various
areas in the building. The system can have an operator’s control
console for the building personnel and an FSCS from which to
view the status of and override the smoke control system. The
system requires a means of verifying operation, such as
differential pressure or airflow proving devices, for each control
sequence. An uninterruptible power supply (UPS) is optional
but recommended.