Handbook of air conditioning and refrigeration / Shan K

22.12 STAIRWELL PRESSURIZATION AND
ZONE SMOKE CONTROL
Stairwell Pressurization
A stairwell pressurization system uses fans to pressurize the stairwells to provide a smokefree escape
route for the occupants in case of a building fire. A stairwell pressurization system is a kind of
smoke control system. National and local codes require stairwell pressurization systems in high-rise
buildings. In a stairwell pressurization system, all interior stairwells are pressurized to a minimum
of 0.15 in. WC (37 Pa) and a maximum of 0.35 in. WC (87 Pa) when all stairwell doors are closed.
City of New York Local Code 1979 requires a minimum air supply flow rate of 24,000 cfm
(11,326 L/s) plus 200 cfm (94 L/s) per floor for the stairwell pressurization system. The maximum
allowable pressure difference between the stairwell and the floor space is 0.40 in. WC (100 Pa)
whether stairwell doors are opened or closed. The minimum allowable pressure difference is 0.10
in. WC (25 Pa) when all stairwell doors are closed, or 0.05 in. WC (13 Pa) when any three stairwell
doors are open. An alternative is to maintain at least 0.05 in. WC (13 Pa) or a minimum average air
velocity of 400 fpm (2 m/s) at the stairwell door when any three stairwell doors are opened. The
force required to open a stairwell door must not exceed 25 lb f (111 N) at the doorknob.
A stairwell pressurization system consists of centrifugal or vane-axial fans, a stairwell pressurization
supply duct with several supply air inlets, relief vents, and a control system, as shown in
Fig. 22.7. Outdoor air is extracted directly by the centrifugal fan. It is forced into the supply duct
and then supplied to the stairwell through supply inlets. When the stairwell is pressurized to a pressure
typically 0.10 to 0.40 in. WC (25 to 100 Pa) higher than that of the air outside the stairwell on
various floors across the stairwell doors, the smoke will not enter the stairwell, even an open stairwell
door. Air supplied into the stairwell is discharged through the open stairwell doors, leakage
area around closed stairwell doors, relief vents, or other openings to the rest of the building, and
then is discharged to the outdoors.
If the stairwell is an airtight enclosure or its doors have very small leakage areas, it will be overpressurized
when all stairwell doors are closed. The pressure difference across the stairwell doors
may be greater than 0.3 in. WC (75 Pa). Often, a large force is required to turn the stairwell doorknob
to open it. The total force required to open a stairwell door should not exceed 25 to 30 lb f (111
to 134 N), or it will be too difficult to open stairwell doors during evacuation. Methods of overpressure
relief are discussed later.
The air velocity at the opened stairwell door on the fire floor required to prevent the backflow of
smoke from the fire to the stairwell is called the critical velocity v crit, in fpm (m/s). The outward
flow air velocity from the stairwell through the open door on the fire floor should be greater
than v crit.
Characteristics of Stairwell Pressurization
AIR SYSTEMS: FAN COMBINATION AND SMOKE CONTROL 22.29
Centrifugal or vane-axial fans can be used for stairwell pressurization. Fans can be installed either
at the bottom level of the building (bottom injection) or on the rooftop (top injection). Bottom injection
is preferable because it minimizes the possibility of smoke inhalation and optimizes the
stack effect to assist stairwell pressurization during winter. In any case, the fan intake must be remote
from the smoke exhaust during a building fire. If the fan room for stairwell pressurization is
located on the rooftop, facilities must be provided to minimize the influence of wind pressure on
fan performance.
Multiple injections, in which air is supplied from multiple inlets into the stairwell, provide a
more even pressure distribution along the stairwell than a single injection from the top or bottom.
Typically, each supply inlet serves two or three floors.
Open-tread stairs provide less flow resistance than closed-tread stairs. This difference becomes
more prominent when occupants are walking on the stairs during evacuation. Compartmentalization