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Both NFPA 101, Life Safety Code, and NFPA 90A,
Standard for the Installation of Air-Conditioning and
Ventilating Systems, recognize that smoke control may
be either active or passive.
The passive approach recognizes the long-standing
compartmentation concept, which requires that fans shut
down and fire/smoke dampers in ductwork close under
fire conditions. The active approach, which applies
NFPA 92A criteria, utilizes the building's heating,
ventilating, and air conditioning (HVAC) systems to
create differential pressures to prevent smoke migration
from the fire area and to exhaust the products of
combustion to the outside. Active smoke control systems
use passive barrier components to create zones or
areas for effective smoke movement as an essential
component.
Products of combustion
Fire
As a fire burns, it:
•
Generates heat
•
Changes major portions of the burning material or
fuel from its original chemical composition to other
compounds which include carbon dioxide, carbon
monoxide, and water
•
Transports a portion of the unburned fuel as soot or
other material that may or may not have undergone
chemical change
The fire triangle, used to explain the components that
make up fire, is important in understanding smoke
control systems. The oxygen leg of the triangle is always
present and will allow combustion to take place. The
heat leg of the triangle, which presents the ignition
source, is limited or controlled in most built
environments. Smoke control systems designed to
protect people from the effects of fire are installed in
environments with low or ordinary hazard contents in the
protected space. What there is to burn (the fuel leg) will
dictate to a large degree the kinds of fire that can be
expected in an area. The size, location, and character of
the fans and other components in an engineered smoke
control system must consider the fuel loading for an
area.
VM-1 Smoke Management Application Guide
Chapter 1: Fire geometry and smoke movement in buildings
The nature of the fuel only affects the quantity of smoke
produced in relation to the size of the fire and depends
upon what is burning and the rate at which it is burning.
Evaluating and limiting what there is to burn helps in the
determination of what kinds of smoke will be produced
for a given fire or area.
Figure 1: The fire triangle
Smoke
Smoke produced in a fire varies from fire to fire and over
time in the same fire. In examining smoke development,
the constituent parts of smoke will therefore fluctuate.
The plume of hot gases above a fire has many parts that
can be placed into one of three general groups:
•
Hot vapors and gases given off by the burning
material
•
Unburned decomposition and condensation matter
(may be light colored to black and sooty)
•
A quantity of air heated by the fire and entrained in
the rising plume
The cloud surrounding most fires and called smoke
consists of a well-mixed combination of these three
groups and will contain gases, vapors, and dispersed
solid particles.
The volume of smoke produced, its density, and its
toxicity depend upon the material that is burning and its
geometry. The nature of the fuel only affects the quantity
of smoke produced in as far as the size of the fire
depends on what is burning and the rate at which it is
burning.
Smoke movement
Smoke can behave very differently in tall buildings when
compared to low buildings. In low buildings, the
influences of the fire, including heat, convective
3
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