Chapter 4

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

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 self
-
sustaining chemical reaction

4.
3

4.
4

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

4.
5

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

4.
6

4.
7

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

4.
8

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.
9

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

4.
10

4.
11

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

4.
12

4.
13

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

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

4.
17

Figure 4
-
12 Examples of heat transfer in fire.

4.
18

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

4.
19

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

4.
20

4.
21

Figure 4
-
17 The four phases of fire.

4.
22

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

4.
23

4.
24

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

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

4.
27

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

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

4.
32

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

4.
33

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.
34

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

4.
35