Chapter 4
Fire Behavior
Introduction
• Since ancient times, fires has been
one of the most important lifesustaining 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
4.2
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 selfsustaining chemical reaction
4.3
Figure 4-2 The old and new ways of visualizing the
combustion process: the fire triangle and the fire tetrahedron.
4.4
Figure 4-3 An exothermic reaction.
4.5
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
4.6
Figure 4-5 Heat from friction can be produced whenever
any rubbing or compression occurs.
4.7
Figure 4-6 Electricity is simply a flow of electrons from a
place where there are many to a place where they are
lacking.
4.8
Figure 4-7 The process of creating nuclear energy and heat.
4.9
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
4.10
Figure 4-8 States of matter. Solid materials have dense
arrangements of molecules whereas gaseous molecules are
more free-flowing.
4.11
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
4.12
Figure 4-11 The speed of combustion is affected by the amount
of oxygen present.
4.13
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
4.14
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
4.15
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
4.16
Figure 4-12 Examples of heat transfer in fire.
4.17
Figure 4-14 A natural fire plume in an unrestricted space.
4.18
(A)
(B)
Figure 4-16 (A) Radiation, single heat source. (B) Radiation,
multiple heat sources.
4.19
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
4.20
Figure 4-17 The four phases of fire.
4.21
Figure 4-19 Full involvement of a structure is an example of a
fully-developed-phase fire.
4.22
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
4.23
Figure 4-21 The products of combustion are deadly
and include light, heat, and smoke.
4.24
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
4.25
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)
4.26
Figure 4-23 A boiling liquid vapor explosion (BLEVE).
4.27
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
4.28
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
4.29
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
4.30
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
4.31
Figure 4-24 Smoke leaving a building has four attributes:
volume, velocity, density, and color. (Photo by Keith
Muratori from FIREGROUNDIMAGES.com)
4.32
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)
4.33
Table 4-3 Reading Smoke Shortcuts
4.34
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
4.35