chapter 01

Basics of Fire Behavior

Chapter 1

Learning Objectives

• Describe the difference between fire and combustion

• Describe the fire triangle and fire tetrahedron• List and describe the classes of fire and their

relationship to extinguishing agents• List and describe the different types of fire

Learning Objectives (continued)

• List and describe the different stages of fire• List and describe methods of heat transfer• List and describe methods used to extinguish

Introduction• Understanding of fire behavior is necessary to

manage a hazard• Fire protection systems address a specific

type and size of hazard• Understanding of fire chemistry is necessary to

assess the fire hazard• Understanding of fire behavior is the basis for

every manual fire attack

Fire and Combustion• Fire is beneficial when correctly managed and

destructive when not managed• Combustion: combustible material and an

oxidizing agent produce heat or energy and other products– Heat must be generated more rapidly than it is

dissipated to continue the process

• Fire produces energy in form of heat, light, and flame– Requires a combustible fuel, heat source, and oxygen

The Fire Triangle• Depicts relationship between fuel, air, and heat• All three elements must be present in sufficient

quantities• Changing proportion determines whether fire will

smolder, burn slowly, burn rapidly• Many methods are used to extinguish a fire

The Fire Tetrahedron• Depicts fire’s nature as

uninhibited, chemical, self-sustaining chain reaction– After ignition occurs

• All four elements must be present – Elimination of any results in

extinguishmentFigure 1-2 The fire tetrahedron

Classes of Fire• Early 1900s Underwriters Laboratories

developed fire categorization– Based on extinguishing agent

• Fires involve different materials– Wrong agent could compound the hazard

• Matches extinguishing agent to fire hazard• Labels bearing the classification on packaging

and literature

Class A Fires and Extinguishing Agents

• Class A fires: ordinary combustibles such as cloth, grain, paper, rubber, wood

• Class A materials release approximately 8000 Btu per pound

• Water is the most effective suppression agent– Absorbs heat; reduces temperature below ignition

temperature– Using water or Class A foam may be better than dry

chemical agents and halon gas mixtures

Class B Fires and Extinguishing Agents

• Class B fires: flammable and combustible liquids and gases

• Class B agents smother or blanket– Interrupt the chemical chain reaction– Keep oxygen away from flammable vapors

• Typical agents: foams, carbon dioxide, dry chemical agents, halons

Class C Fires and Extinguishing Agents

• Class C fires: energized electrical equipment• Shutting off electricity is critical

– May require additional suppression activities

• Only fires involving electrical equipment are class C fires– When electricity is no longer involved, fire is

reclassified

Class D Fires and Extinguishing Agents

• Class D fires: combustible metals• Manufacturing process releases metal dust and

particles– Potentially explosive if airborne

• Class D extinguishing agents are “dry powder” and inert– Reduce oxygen concentration– Extremely hazardous

Class K Fires and Extinguishing Agents

• Class K fire: fires in cooking appliances using combustible oils and fats

• Class K agents extinguish by saponification – Fatty acids in cooking medium react with

extinguishing agent and convert to foam– Soapy foam blankets surface of burning liquid– Cools liquid below autoignition temperature

Other Fire Classification Systems• US and Canada use same classification system• Europe and Australia use different classification

systems• Different parts of the world have different

classification systems• All classification systems are practical

Fire and Flame Types• Four types of fire:

– Diffusion flame– Smoldering– Spontaneous combustion

• Self-heating– Premixed flame

• Each requires fuel, source of heat, and oxygen• Each type of fire has unique characteristics

Diffusion Flame• Flame we see when we light a match• Most natural flame fires are diffusion flames• Result of combustible gas emitted by burning

material mixed with oxygen• Moves into reaction zone on either side of the

flame• Combustion products diffuse from the flame

Figure 1-8 Schematic of a diffusion flame

Smoldering• Example: glow of charcoal in grill• No flame, but heat• Air flows over the material surface, providing

oxygen for combustion• Erupts into flame when sufficient oxygen and

heat generated• May also be final phase of earlier fire event

Figure 1-9 Common examples of a smoldering fire

Spontaneous Combustionand Self-Heating

• Material undergoes oxidation where heat dissipation limited

• Oxidation creates heat that cannot be dissipated• Temperature increases and promotes the

reaction• Material smolders or flames if heat is sufficient

Figure 1-10 Examples of fuel arrays prone to spontaneous combustion

Premixed Flame• Requires gas fuel and air mix to ignite• Gas fuel must be within flammable limits

– Lower and upper limit where mixture ignites– Example: acetylene flammable limit range 2.5% to

• Changes in atmospheric pressure can alter flammable limit range

• Knowing flammable limits is important to managing an incident

Figure 1-11 Examples of premixed flames

The Stages of Fire• Fires develop and evolve in stages:

– Pre-ignition– Ignition– Growth– Fully developed– Decay or smoldering

• Fire suppression methods must be appropriate for the conditions

Pre-ignition Stage• Requires some form of energy transfer

– Usually in the form of heat

• Generates gas from the fuel, which mixes with air

• Fuel, in gas form, and oxygen must be in proper mixture to ignite

Ignition Stage• Three things are necessary to start a fire:

– Heat source– Fuel– Oxygen

• Ignition occurs when more heat generated than dissipated

• After ignition, no additional heat needed to continue the chain reaction

Growth Stage• Adequate oxygen must be available

– May be gaseous or chemical oxidizer

• Amount of oxygen affects speed of growth• Amount, type, form of fuel are significant• Structure size, shape of walls, ceilings and roofs

have significant impact

Fully Developed Stage• Combustible contents of compartment totally in

flame• Flashover: transition between growth stage and

fully developed stage– All combustible contents heat to ignition point and

simultaneously ignite– Leads immediately to full development

• Stage regulated by amount of air and fuel

Decay Stage• Fuel exhausted or oxygen level drops below

16%• Burning in the form of glowing combustion• Continues until:

– All fuel is completely exhausted– Temperature drops below ignition point

• Introduction of air may regenerate the fire

Forms of Heat Transfer• Three types of heat transfer:

– Conduction– Convection– Radiation

• Hostile fires develop by heat transfer• Used every day to cook and heat homes• Key to reducing fire spread and extinguishing

Conduction• Transfer of heat from one body to another by

direct contact• When exposing objects to heat, movement of

molecules increases• Heat energy attempts to move to lower

temperature areas or objects• Rate of heat transfer dependent on efficiency of

material as conductors

Convection• Transfer of heat from one body to another

through a medium such as liquid or gas• Example: automobile radiators• In building fires, heat transfers through the air in

convection currents• Size and shape of fire compartment influences

effect of convection– Hurricane force winds can be generated due to

updraft of convection currents

Figure 1-18 Convection. As air is heated, its molecules become excited and seek more space

Radiation• Transfer of heat through electro-magnetic

energy, such as light• Radiant heat travels in a straight line• Has significant impact on the speed at which

flashover occurs• Radiation is significant cause of fire spread from

building to building

Methods of Extinguishing Fires• Four methods to extinguish fires:

– Cooling– Oxygen reduction– Removing fuel supply– Interrupting the chain reaction

• Same basic methods used in design of extinguishment systems

Cooling• One gallon of water can absorb 8,000 Btus

– Same amount of heat produced by one pound of common combustible material

• Water must be applied at sufficient rate and quantity to reach seat of the fire

• Types of fuel, form, storage arrangement impact effectiveness of water systems

Oxygen Reduction• Control of air intake in area of fire helps reduce

or remove oxygen supply• Foam is a commonly used suppression agent

– Covers leaking fuel so oxygen cannot reach the fuel

• Carbon dioxide is heavier than air and displaces oxygen– Effective in utility vaults

Removing or Interruptingthe Fuel Supply

• Backfires are set in wild land fires to remove fuel ahead of a hostile fire

• Foams prevent liberation of vapors– Provide cooling to spills and leaks

• Waterflow from sprinklers pre-wets adjacent fuels– Makes it difficult for fuel to ignite

Interrupting the Chain Reaction• Fire sustained by chemical reaction with

combustion products• Dry chemical, halon, and other clean agent

suppression products disrupt the chain reaction

Summary• Fire triangle/tetrahedron

– Describe relationship between the components required for combustion

– Used by fire prevention codes to prevent hostile fires from occurring

• Fire classification determines type of extinguishment system to use

• Five types of fire; five stages of fire• Understanding fire behavior is critical

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