Chapter 4

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22 févr. 2014 (il y a 3 années et 3 mois)

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Chapter 4

Fire Behavior

Introduction


Since ancient times, fires has been
one of the most important life
-
sustaining components


Fire has played a major role as a tool
in the development of society


Sometimes an ally, sometimes an
enemy


Much has been learned about fire in
the last thirty years

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Fire Defined


Fire

is burning


Burning is
combustion


Combustion is a chemical reaction


Deflagration

is combustion at a rate
below the speed of sound


Detonation

is combustion above the
speed of sound


Fire tetrahedron

is the assembly of
heat, fuel, oxygen, and a self
-
sustaining chemical reaction

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Figure 4
-
2 The old and new ways of visualizing the
combustion process: the fire triangle and the fire tetrahedron.

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Figure 4
-
3 An exothermic reaction.

Heat


For a fire to begin, fuels need to be
heated


Chemical heat: various chemicals
react with each other


Mechanical heat: friction of two
materials rubbing against each other


Electricity most recognized source of
heat


Nuclear heat comes from unstable
radioactive materials

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Figure 4
-
5 Heat from friction can be produced whenever
any rubbing or compression occurs.

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Figure 4
-
6 Electricity is simply a flow of electrons from a
place where there are many to a place where they are
lacking.

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Figure 4
-
7 The process of creating nuclear energy and heat.

Fuel


Material consumed by the combustion
process


Molecules in a solid are packed closely
together


Liquid’s ability to burn depends on
substance's ability to place its molecules
into suspension


Gaseous fuels are in a state of suspension


Primed for combustion


Fuel must be at a certain temperature
before combustion can take place

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Figure 4
-
8 States of matter. Solid materials have dense
arrangements of molecules whereas gaseous molecules are
more free
-
flowing.

Oxygen


Acts as a catalyst for the combustion
process


Oxidation is the process of oxygen
bonding to other elements and
compounds


Oxidation affects the process of
combustion


Chemical reaction accelerated with
an abundance of oxygen

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Figure 4
-
11 The speed of combustion is affected by the amount
of oxygen present.

Chemical Chain Reaction


When heat, fuel, and oxygen combine
to start combustion, a chemical chain
reaction forms


Scientists discovered certain
chemicals introduced into burning
process can stop flaming


Foundation for understanding how
fires begin is grounded in the fire
tetrahedron

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Fire Growth


Once a fire begins, it will grow in a
self
-
sustaining manner


The heat being released will be
transferred to other fuels


The firefighter who understands fire
growth can better predict fire behavior

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Modes of Heat Transfer


Heat is a by
-
product of combustion


Conduction is the transfer of heat
through a solid object


Convection is the transfer of heat
through air and liquid currents


Radiation is the transfer of heat
through invisible lightwaves

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Figure 4
-
12 Examples of heat transfer in fire.

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Figure 4
-
14 A natural fire plume in an unrestricted space.

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Figure 4
-
16 (A) Radiation, single heat source. (B) Radiation,
multiple heat sources.

(A)

(B)

Fire Phases


Burning occurs in clearly defined
phases


Ignition phase: substance begins to
heat up, liberates gases that can burn


Growth phase: fire begins to grow
from the point of ignition


Fully developed phase: all contents
within perimeter of fire's boundaries
are burning


Decay stage: all fuel consumed and
the fire diminishes in size

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Figure 4
-
17 The four phases of fire.

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Figure 4
-
19 Full involvement of a structure is an example of a
fully
-
developed
-
phase fire.

Products of Combustion


Matter is neither lost nor gained; it
changes form


By
-
products of burning process are
heat, light, and smoke


Heat causes dehydration, heat
exhaustion, and burns


Light can cause damage to eyesight


Smoke is the most dangerous
product of combustion

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Figure 4
-
21 The products of combustion are deadly
and include light, heat, and smoke.

Specific Fire Characteristics

and Events


Liquid, electrical, and metal fires
have certain characteristics


Several unique fire events can occur
within a compartment or structure


Learning to “read smoke” can help
predict fire behavior

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Liquid Fires


Liquid fuels must vaporize


Vaporization

is the process in which
liquids are converted to gas or vapor


Most liquid fires do not mix well with
water


Typically extinguished by coating the
liquid with an agent such as foam


Liquid heated to a high pressure in a
closed vessel can explode


BLEVE (boiling liquid expanding vapor
explosion)

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Figure 4
-
23 A boiling liquid vapor explosion (BLEVE).

Electrical Fires


Class C fire: electrical energy is
creating heat


Control of a Class C fires begins with
control of electricity


Fire control efforts are aimed toward
Class A or B materials


Use of water can present extreme
shock hazards while fire is still
electrically energized

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Metal Fires


Class D metal fire is a chemical
reaction fire


In some cases, the presence of
water will cause a violent reaction


Control of Class D fires can be quite
difficult or amazingly simple


Shape, size, amount, and type of
metal burning will dictate the best
extinguishment method

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Hostile Fire Events


Any uncontrolled fire in a building is
a hostile fire event


Hostile fire event
: unique fire
phenomenon


Flashover


Backdraft


Rapid fire spread

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Reading Smoke at
Structural Fires


Firefighter applies understanding of basic
fire behavior by reading smoke


Helps discover clues about the location of the
fire


Smoke volume is an indicator of the
amount of fuels that are “off
-
gassing”


Smoke velocity is an indicator of pressure
that has built up within the building


Smoke density is indicative of the amount
of fuel that is laden within the smoke

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Figure 4
-
24 Smoke leaving a building has four attributes:
volume, velocity, density, and color.
(Photo by Keith
Muratori from FIREGROUNDIMAGES.com)

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Figure 4
-
25 Turbulent smoke flow is a warning sign of
impending flashover. The brown smoke issuing from the eves
indicates that the roof trusses have been heated

a collapse
warning sign.
(Photo by Keith Muratori from
FIREGROUNDIMAGES .com)

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Table 4
-
3 Reading Smoke Shortcuts

Lessons Learned


Firefighter survival and fire attack
effectiveness is dependent on the
understanding of fire dynamics


Ingredients needed for combustion


Heat sources


Fuels


Fire phases


Special considerations


Reading smoke

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