Pressurization has been used for much of this century to
protect against the spread of biological and chemical contaminants
in a variety of applications, including hospital operating
rooms and laboratories. In the last two decades,
pressurization also has been used to control against the
spread of smoke due to building fires. Systems using pressurization
produced by mechanical fans are referred to as
smoke control systems in NFPA 92A (NFPA 1993) and in
the ASHRAE smoke control design book.
A pressure difference across a barrier can control smoke
movement, as illustrated in Figure 2. Within the barrier is a
door. The high-pressure side of the door can be either a refuge
area or an egress route. The low-pressure side is exposed
to smoke from a fire. Airflow through the gaps around the
door and through construction cracks prevents smoke infiltration
to the high-pressure side.
Later in this paper several full-scale fire tests of smoke
control systems are discussed. Each of these series of tests
was unique in that different types of smoke control systems
were studied in different kinds of occupancies. However, all
showed that pressure differences can prevent smoke migration
from the low-pressure side to the high-pressure side of a
barrier. This holds for pressure differences as small as 2 Pa
(0.01 in. H20). To be effective, a smoke control system must
produce pressure differences in the desired direction under
fire conditions. Fires increase pressures due to the buoyancy
of hot gas, and building pressures fluctuate due to changes in
barometric pressure, wind, doors opening, doors closing, and
system controls. To account for these and to allow a safety
factor, NFPA 92A indicates that smoke control systems must
maintain the pressure differences listed in Table 2.