Handbook of air conditioning and refrigeration / Shan K

Stair and Shaft Vents
Zone Smoke Control
AIR SYSTEMS: FAN COMBINATION AND SMOKE CONTROL 22.31
stairwell doors was lower than 0.25 in. WC (65 Pa) in summer and winter. During nonfire conditions,
pressure differences were between 0.05 and 0.1 in. WC (13 and 25 Pa) across the stairwell
doors if there was a vent in the external wall on the second floor. When a building fire was simulated
on the second floor, there was no smoke backflow from the fire floor through the stairwell
door when the stairwell doors on the first and second floors were opened and the vent on the second
floor external wall was also opened. Smoke backflow occurred when stairwell doors on the first,
second, and third floors were open.
Overpressure Relief Vents. A relief vent is typically an assembly in which a fire damper is connected
in series with a barometric damper. The barometric damper is normally closed. If the stairwell
is pressurized above a predetermined limit, the vents open and relieve air to various building floors. A
counterweight in the barometric damper sets the maximum pressure limit. Fire dampers are normally
closed, and open when the stairwell pressurization system is turned on in case of a building fire.
According to the experimental results of Tamura (1990b), the performance of the overpressure
relief vents at a supply flow rate of 28,000 cfm (13,210 L/s) was slightly better than the performance
of a ground-floor exit door at a supply flow rate of 17,800 cfm (8400 L/s). If supply flow
rates are the same, the difference is further reduced.
Fan Bypass. When the stairwell is overpressurized, a pressure sensor located inside the stairwell
signals a feedback control to open a fan bypass damper so that part of the supply air returns to the
centrifugal fan inlet, as shown in Fig. 22.7a. The air volume flow supplied to the stairwell is reduced
until the pressure at the pressure sensor drops below a preset value. If the pressure sensor is
located on the rooftop or outdoors, it should be shielded from the influence of wind.
Variable-Speed or Controllable-Pitch Fan. When excessive pressure is detected in the stairwell,
a controller actuates an adjustable-frequency variable-speed drive to reduce the speed of a centrifugal
fan or to vary the blade pitch of a vane-axial fan to maintain the required pressure inside the
stairwell at the pressure sensor. The test results in Tamura (1990b) showed that both fan bypass and
variable-speed controls require a response time of more than 5 min. Variable-speed control is
slightly faster. Because reliability is the primary factor in stairwell pressurization control, and because
of its very short operating period, it may not be worthwhile to install an expensive adjustablefrequency
ac inverter to provide variable-speed control.
ASHRAE/IESNA Standard 90.1-1999 mandatorily specifies that stair and elevator shaft vents shall
be equipped with motorized dampers which are automatically closed during normal building operation
and are interlocked to open by fire and smoke control detection systems when required.
Zone smoke control requires that a building be divided into a number of smoke control zones. The
control zones are separated from one another by barriers, such as walls, floors, ceilings, and doors.
In high-rise buildings, each floor is a separate zone, or it may be subdivided into many smoke control
zones, or a smoke control zone consists of more than one floor.
In case of a building fire, the spread of smoke from the zone of fire origin (the smoke zone)
to adjacent zones is limited by pressure differences and airflows. The required pressure differential
depends on the pressure produced by the fire gases, i.e., the temperature of the building fire. To
form a pressure difference between the smoke zone and adjacent control zones, zone smoke
control provides smoke exhaust on the fire floor (smoke zone) by opening the smoke damper
connected to the smoke exhaust duct. At the same time, it supplies outdoor air to the floor or
floors above and below the fire floor (to control zones adjacent to the smoke zone) and pressurizes