Chapter 1Basics of Fire Behavior

Objectives • Describe the difference between fire and

combustion.• Identify and describe the elements of the

fire triangle and fire tetrahedron.• List and describe the different types of fire.

Objectives • List and describe the different stages of

fire.• List and describe forms of heat transfer. • List and describe methods used to

extinguish fires. • List and describe the classes of fire and

their relationship to extinguishing agents.

Introduction • It is necessary to understand fire behavior

as it relates to the hazards present.– Factors affecting hazard fuel load:

• Occupancy and use conditions• Construction materials• Materials present within a building• Storage of items

Introduction • Fire protection systems must be adequate

for conditions of occupancy and use.– Update preplan and response protocols as

needed.– Reevaluate systems as needed.

• Basic understanding of fire chemistry and dynamics is needed to assess fire hazards.

Combustion and Fire • Combustion vs. fire

– Combustion is a chemical reaction involving combustible materials and an oxidizing agent producing heat or energy.

– Fire is a chemical reaction producing energy in the form of heat, light, and flame.

– The difference: • With combustion, the released energy stays in the

reaction to continue it. • With fire, energy is dissipated as light and heat.

Combustion and Fire • The fire triangle

– Illustrates how a fire is started

– All three elements must be present in sufficient quantities for a fire to exist.

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Combustion and Fire • The fire tetrahedron

– Shows that chain reaction is necessary to sustain fire

– Illustrates a fire’s nature once ignition occurs

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Fire and Flame Types • Diffusion flame

– Most natural flaming fires are diffusion fires.– Examples: lighting a match, campfire,

structure fire– Require combustible gas

• Visible flame is a gaseous reaction.

Fire and Flame Types • Smoldering

– No flame, but significant heat– Examples: charcoal on a grill, cigarette

burning in an ashtray– May precede a flaming fire– May occur in the final phase of a fire event

Fire and Flame Types • Spontaneous combustion/self-heating

– No known external source of heat– Oxidation occurs in an environment that limits

heat dissipation, so heat stays in the reaction.– Timing depends on the material and the

environment.

Fire and Flame Types • Premixed flame

– Used to help people get ready for the day or for transportation

• Cooking meals, driving to work, etc.– Gas fuel and air must mix before ignition or

combustion occur.– Knowing flammable limits of gas fuels helps

safely manage incidents.

The Stages of Fire• The incipient stage

occurs when heat, fuel, and oxygen come together and generate more heat than is dissipated.– Uninhibited chain reaction– No need for additional heat

sources (can self-sustain if fuel/oxygen sufficient)

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The Stages of Fire• In the free burning

stage, adequate oxygen must be available to sustain the fire.

• Fire growth is affected by numerous factors:– Oxygen supply– Amount/type of fuel– Container size/structure– Insulation

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The Stages of Fire• In the free burning stage, the size of a structure

and the shape of walls, ceilings, and roofs, also impact fire growth.– Large compartments allow heat to dissipate.– Low ceilings hold heat and reflect radiant heat energy.– Ability of container to retain heat (insulation)

accelerates preheating of fuel.

The Stages of Fire• The flashover stage is

the transition in which room temperature reaches a point at which all surfaces ignite.– Confined spaces:

determined by air supply– Outdoors: determined by

fuel supply

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The Stages of Fire• The smoldering/decay

stage occurs when available fuel is exhausted or oxygen level drops below 16%.

• Burning only continues as glowing combustion or smoldering.

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Forms of Heat Transfer• Conduction

– Transfer of heat from one body of material to another by direct contact

– Rate of transfer depends on efficiency of material as conductor

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Forms of Heat Transfer• Convection

– Transfer of heat from one body of material to another via liquid or gas

– Convection effect is influenced by size and shape of fire compartment. © Jones & Bartlett Learning

Forms of Heat Transfer• Radiation

– Transfer of heat through electromagnetic energy such as light

– Radiant heat travels in a straight line.

– Affects the speed of flashover in structure fires

– Can cause fires to spread

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Methods of ExtinguishingFires

• Cooling– 1 gallon of water absorbs 8000 Btu.– Water must be applied at

sufficient rate and quantity.– Water cooling effectiveness

depends on:• Fuel type• Form of material• Storage arrangement

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Methods of ExtinguishingFires

• Oxygen reduction– Fires are starved by

controlling air intake.• Foam• Carbon dioxide © Jones & Bartlett Learning

Methods of ExtinguishingFires

• Removing or interrupting the fuel supply– Backfires– Foam– Water

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Methods of ExtinguishingFires

• Interrupting the chain reaction– Suppression agents

• Displace oxygen• Decompose or isolate

material from other elements• Change environment so

more energy required for combustion

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Classes of Fire • In the early 1900s, Underwriters

Laboratories created a classification system based on extinguishing agents.

• Extinguishing agents are matched to fire hazards with letters of the alphabet.

Classes of Fire • Class A

– Involve ordinary combustibles

– Suppression:• Water (most effective)• Class A foam

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Classes of Fire • Class B

– Involve flammable and combustible liquids and gases

– Suppression:• Smothering, blanketing• Foam• Carbon dioxide and

other gaseous agents• Dry chemical agents• Halon

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Classes of Fire • Class C

– Involve energized electrical equipment

– Suppression:• Shut off electricity.• Once it is off, the

fire changes to another classification.

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Classes of Fire • Class D

– Involve combustible metals

– Suppression:• Extinguishing

agents must match metal type.

• “Dry powder” agents © Jones & Bartlett Learning

Classes of Fire • Class K

– Involve cooking appliances

– UL 300 addressed failure of suppression systems.

– Suppression:• Wet-chemical agents• Supplemental hand-held

extinguishers © Jones & Bartlett Learning

Summary • The “fire triangle” and “fire tetrahedron”

describe the relationship between the components required to sustain combustion. These models provide a simple illustration of what elements are necessary for fires to occur and show the logic behind the methods used to extinguish fires.

Summary • The four different types of fire are diffusion

flame, smoldering, spontaneous combustion, and premixed flame.

• The type of ignition, the amount of available fuel, the form of fuel, and the amount of oxygen available all play significant roles in fire development.

Summary • The four stages of fire are incipient, free

burning, flashover, and smoldering/decay. • The three methods of heat transfer are

conduction, convection, and radiation.

Summary • There are four basic methods used to

extinguish fires: cooling, oxygen reduction, removing the fuel supply, and interrupting the chain reaction. These are the same basic methods used in the design of fire protection systems.

Summary • A fire classification system helps to

determine the types of extinguishing agents to use, thus reducing the possibility of applying an inappropriate extinguishing agent to a fire.