Chapter 01
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Transcript of Chapter 01
© 2008 Delmar, Cengage Learning
Basics of Fire Behavior
Chapter 1
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
fires
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
Figure 1-8 Schematic of a diffusion flame
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
Figure 1-9 Common examples of a smoldering fire
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
Figure 1-10 Examples of fuel arrays prone to spontaneous combustion
© 2008 Delmar, Cengage Learning
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
100%
• Changes in atmospheric pressure can alter flammable limit range
• Knowing flammable limits is important to managing an incident
© 2008 Delmar, Cengage Learning
Figure 1-11 Examples of premixed flames
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
fires
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
Figure 1-18 Convection. As air is heated, its molecules become excited and seek more space
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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
© 2008 Delmar, Cengage Learning
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