Kidde VM-1 Manual page 19

Stairwell pressurization systems
Stairwell pressurization systems are built with the intent
of keeping stairs clear of smoke in order to assist in the
evacuation of occupants. Stairwell pressurization
systems are commonly dedicated smoke control
systems. Activation of stairwell systems can be by
automatic or manual means.
Stairwell pressurization systems can be from a single
injection point into the tower or multiple injection points.
Single injection systems are commonly used for eight or
fewer stories. Multiple injection systems (see Figure 15
below) provide several supply inlets in the stairwell.
Compartmentation of the stairwell can also be used in a
pressurized design to maintain stair tenability.
Pressurization systems may operate throughout the fire
event, offering refuge for firefighters as they enter or
leave the fire floor.
Figure 15: Compartmentation of a pressurized stairwell
Stairwell
compartments
A fire in a multiple story building will develop a positive
pressure in the fire area until ventilation occurs, often
due to the opening of a door or the failure of window
glass. The positive pressures developed by a fire can
enter a stair as occupants leave the fire floor and reduce
VM-1 Smoke Management Application Guide
Chapter 1: Fire geometry and smoke movement in buildings
the usefulness of the stair for escape. The design
objective of achieving a higher pressure in the stair than
is found on the fire floor is usually achieved by a single
dedicated fan in the stairwell.
Life safety and fire codes require stairwells to be isolated
from the building they serve, making the use of shared
building HVAC systems unlikely or prohibited. Dedicated
HVAC systems for stairwell pressurization systems are
also used with modulating dampers controlled by static
sensors at each doorway or at selected points in a
stairwell.
For pressurizing a stairwell, the smoke control system
designer must define the number of doors expected to
be open at any one time and design air flows which
compensate for the open doors. If more than the
expected or design number of doors is opened, the
pressure in the stairwell may drop below that of the fire
floor and smoke will be able to enter the tower.
Power requirements for smoke control system operation
must consider the total number of systems or zones in
operation. For example, if there are two stairwells with
pressurization, they will both operate in a fire event and
power must be available for both tower systems. If a
smoke control zone on the fire floor will also operate,
then the three separate smoke control systems must be
powered and operable from the FSCS.
Automatic operation of one of a building's fire alarm
systems should cause all stair pressurization fans to
start. Where an engineering and life safety analysis
determines that the configuration of the building is such
that only certain stairs need pressurization, programming
of the smoke control system will need to be tailored to
various fire scenarios.
A smoke detector should be provided in the air supply to
the pressurized stairwell. Smoke drawn into the stairwell
from the exterior of the building will be detected and fans
will then shut down. Detectors selected for fan flow
monitoring should be within the air velocity ranges
specified in the detector's installation sheet.
The firefighter's smoke control station (FSCS) must
contain a manual override, to be operated by an
authorized person, to restart fans should they shut down
due to the operation of smoke detectors installed in the
stairwell. The authorized person may determine that a
lesser hazard exists from smoke entering the fan than
smoke migrating into the tower from the fire floor and
override fan shutdown based upon exterior smoke entry.
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