1. Pressurization (Smoke Control)
Systems that pressurize an area using mechanical fans are
referred to as smoke control in this chapter and in NFPA Standard
92A. A pressure difference across a barrier can control smoke
movement, as illustrated in Figure 6. 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.
For smoke control analysis, the orifice equation can be used to
estimate the flow through building flow paths:
(8)
where
Q = volumetric airflow rate, m3/s
C = flow coefficient
A = flow area (leakage area), m2
Δp = pressure difference across flow path, Pa
ρ = density of air entering flow path, kg/m3
The flow coefficient depends on the geometry of the flow path, as
well as on turbulence and friction. In the present context, the flow
coefficient is generally 0.6 to 0.7. For ρ = 1.2 kg/m3 and C = 0.65,
Equation (8) can be expressed as
The flow area is frequently the same as the cross-sectional area
of the flow path. A closed door with a crack area of 0.01 m2 and a
pressure difference of 2.5 Pa has an air leakage rate of approximately
0.013 m3/s. If the pressure difference across the door is
increased to 75 Pa, the flow is 0.073 m3/s.
Frequently, in field tests of smoke control systems, pressure differences
across partitions or closed doors have fluctuated by as
much as 5 Pa. These fluctuations have generally been attributed to
wind, although they could have been due to the HVAC system or
some other source. To control smoke movement, the pressure difference
produced by a smoke control system must be sufficiently
large to overcome pressure fluctuations, stack effect, smoke buoyancy,
and wind pressure. However, the pressure difference should
not be so large that the door is difficult to open.