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Chapter 3 — Fire Behavior
3–2
Chapter 3 Lesson Goal
• After completing this lesson, the student shall be able to summarize physical & chemical changes & reactions that occur with fire & the factors involved in fire development
3–3
Combustion
Rapid oxidation of fuel that produces heat & light
3–4
Matter is…
anything that occupies space & has mass (weight)
3–5
Physical & Chemical Changes of Matter Related to Fire
Physical change
• Water freezing
• Water boiling
Chemical reaction
• Reaction of 2 or more substances to form other compounds
• Oxidation
(Continued)
3–6
Physical & Chemical Changes of Matter Related to Fire
Chemical & physical changes
• Usually involve exchange of energy
• Potential energy released & changed to kinetic energy
• Exothermic reaction – may produce heat, flames & toxic smoke (Heat “exit”)
• Endothermic reaction – absorbs heat energy (Heat “in”)
3–7
DISCUSSION QUESTION
What are some examples of physical & chemical changes of matter?
3–8
Combustion — Modes
Flaming (Fire) Non-flaming (No fire)
Oxidation involves fuel in gas phase
Requires liquid/solid fuels to be converted to gas or vaporized
When heated, liquid/solid fuels give off vapors that burn
Some solid fuels can undergo oxidation at the surface of the fuel
Examples — Burning charcoal or smoldering fabric
3–9
Fire Triangle
Fire Parts
3–10
Fire Tetrahedron
Fire Process
3–11
Heat as Energy
• Heat is a form of energy
• Potential energy — Energy possessed by an object that may be released in the future
• Kinetic energy — Energy possessed by a moving object
3–12
Temperature
• Temperature is a measurement of kinetic energy
When molecules move they produce heat
Faster they move, the more heat produced
• Heat energy moves from objects of higher temperature to those of lower temperature
• Understanding this movement is important
3–13
Measuring energy
• Not possible to measure energy directly
• It is possible to measure the work it does
• Work means increasing temperature of a substance
Example: mercury in a thermometer rises when heated
3–14
Measuring energy
Measured in British thermal units (BTU)
• Amount of heat required raise 1 Lb of water 1 degree Fahrenheit (F)
Measured in calories
• Amount of heat required raise 1 gram of water 1 degree Celsius (C)
3–15
Scales Used to Measure Temperature
• Celsius — Metric
• Fahrenheit — Customary
3–16
Conversion of Energy Into Heat
• Heat is the energy component of tetrahedron
• Fuel is heated = temperature increases
• Starting ignition
• Forms of ignition
3–17
Forms of ignition
Two forms:
• Piloted ignition outside heat source used to ignite material i.e. match
3–18
Forms of ignition
Auto ignition temperature
• Temperature at which a material self-sustains combustion
• Material is heated to ignition temperature
• Auto ignition temperature is always higher than piloted ignition temperature
• Free-burning occurs
3–19
Chemical Heat Energy
• Most common heat source in combustion reactions
• Oxidation almost always results in production of heat
• Self-heating
3–20
Electrical Heat Energy
• Can generate temperatures high enough to ignite any combustible materials near heated area
• Can occur as
Resistance
Overcurrent/overload
Arcing
Sparking
3–21
Mechanical Heat Energy
• Generated by friction or compression
• Movement of 2 surfaces against each other creates heat of friction
• Movement results in heat and/or sparks being generated
• Heat of compression generated when gas compressed
(Continued)
3–22
Mechanical Heat Energy
3–23
DISCUSSION QUESTION
What are some examples of chemical, electrical, & mechanical sources of heat energy?
3–24
Transfer of Heat
• Basic to study of fire behavior
• Affects growth of any fire
• Knowledge helps FFs estimate size of fire before attacking
• Heat moves from warmer objects to cooler objects
(Continued)
3–25
Transfer of Heat
• Rate related to temperature differential of bodies & thermal conductivity of material
• Greater the temperature differences between bodies, greater the transfer rate
• Measured as energy flow over time
3–26
Conduction
• Transfer of heat within a body or to another body by direct contact
• Occurs when a material is heated as a result of direct contact with heat source
• Heat flow depends on several factors
e.g. heat flow in wood is slower than steel
3–27
Convection
• Transfer of heat energy from fluid to solid surface
• Transfer of heat through movement of hot smoke & fire gases
• Flow is from hot fire gases to cooler components
3–28
Radiation
• Transmission of energy as electromagnetic wave without intervening medium
• Distance between heat & surface is biggest factor
(Continued)
3–29
Radiation
• Thermal radiation results from temperature
• Affected by several factors
• Energy travels in straight line at speed of light, i.e. heat from the sun
• Major contributor to flashover
3–30
Heat Transfer in Buildings
Heat can be transferred in buildings by:
• Conduction
• Radiation
• Convection
Heat transfer by radiation
3–31
Passive Agents
• Materials that absorb heat but do not participate in combustion
• Fuel moisture = passive agent
• Relative humidity & fuel moisture
3–32
DISCUSSION QUESTION
What is the impact of high fuel moisture on fire spread?
3–33
Fuel
• Material being oxidized in combustion process
• Reducing agent: reduces or uses oxygen
• Inorganic or organic; organic most common
(Continued)
3–34
Fuel
Organic fuels: contain carbon
• Hydrocarbon-based
Petroleum based & some plastics
• Cellulose-based
Wood & paper
Inorganic fuels: do not contain carbon
Hydrogen & magnesium
3–35
Fuel
Key factors influencing combustion process:
• Physical state of fuel
Gases & liquids burn more readily than solids
• Distribution or orientation of fuel
Solid fuels that are vertical burn more readily than when horizontal
Try this with a piece of paper
3–36
Gaseous Fuel
• Only gases burn
• Easiest to ignite, gases are already in state ready for ignition
• Methane, hydrogen, etc. most dangerous because exists naturally in state required for ignition
• Has mass but no definite shape or volume
3–37
Gaseous Fuel
Vapor density
• The weight of a gas compared to air
• Air is 1
• More than 1 will sink & collect in low points
• Less than 1 will rise
3–38
Liquid Fuel
• Has mass & volume but no definite shape except for flat surface
• Assumes shape of container
• Will flow downhill & pool in low areas
• Must be vaporized in order to burn
3–39
Liquid Fuel
Specific Gravity
• The weight of liquid compared to water
• Water is 1
• More than 1 will sink
• Less than 1 will float
3–40
Liquid Fuel Characteristics
• Flash point flashes & goes out
• Fire point continues to burn
• Surface area: how much surface area is exposed to the atmosphere
The greater the surface area, the more vapors produced
3–41
Liquid Fuel Characteristics
(Continued)
Flash point
3–42
Liquid Fuel Characteristics
Non-polar Solvents
• Hydrocarbons, gasoline, diesel
• Does not mix w/ water
Non-miscible, non-soluble
• Lighter than water
3–43
Liquid Fuel Characteristics
Polar Solvents
• Alcohols, acetone, esters
• Readily mixes w/ water
Miscible/soluble
3–44
Liquid Fuel Characteristics
Firefighting considerations - Liquids lighter than water
• Presents a significant challenge when using water
• Volume of liquid increases as water is applied
Potentially spreading the burning fuel
3–45
Liquid Fuel Characteristics
Firefighting considerations - Water-soluble liquids
• Presents a problem in that some water-based extinguishing agents mix w/ the burning liquid
Makes them ineffective
3–46
Solid Fuel
• Definite size & shape
• React differently when exposed to heat
(Continued)
3–47
Solid Fuel
• Pyrolysis evolves solid fuel into fuel gases/vapors
• As it is heated, begins to decompose below 400°F (204°C), giving off combustible vapors
(Continued)
3–48
Solid Fuel
• Commonly the primary fuel
• Surface-to-mass ratio — Primary consideration in ease or difficulty of lighting
(Continued)
3–49
Solid Fuel
• Proximity/orientation of solid fuel relative to source of heat affects the way it burns
3–50
Solid Fuel
• Synthetic fuels such as plastic produce large amounts of toxic gases when burned
• Toxic effect of smoke is not the result of any one fire gas
• Fire gases can be deadly to FFs
3–51
Heat of Combustion/Heat Release Rate
• Heat of combustion — Total amount of energy released when a specific amount of fuel is oxidized
Usually expressed in kilojoules/gram (kJ/g)
• Heat release rate (HRR) — Energy released per unit of time as fuel burns
Usually expressed in kilowatts (kW)
3–52
Oxygen
• In air, is the primary oxidizing agent in most fires
• Air consists of 21% oxygen
3–53
Oxygen Concentrations
• At normal ambient temperatures, materials can ignite/burn at concentrations as low as 14 percent
• When limited, flaming combustion may diminish; combustion will continue in surface or smoldering mode
(Continued)
3–54
Oxygen Concentrations
• At high ambient temperatures, flaming combustion may continue at much lower oxygen concentrations
• Surface combustion can continue at extremely low oxygen concentrations
(Continued)
3–55
Oxygen Concentrations
• When higher than normal, materials burn faster than normal
• Fires in oxygen-enriched atmospheres are difficult to extinguish & present a potential safety hazard
• Flammable/explosive range — Range of concentrations of fuel vapor & air
Flammable Range
• Expressed as a percentage of fuel/air mixture that will ignite or explode
• UEL & LEL: Upper & lower explosive limits
• UFL & LFL: Upper & lower flammable limits
Terms mean the same thing
3–56
Flammable Range
• Flammable limits can change based on temperature
• Higher temperatures can increase the flammable ranges
• Lower temperatures can decrease the flammable ranges
3–57
3–58
Flammable Range
Product Flammable Range
Methane 5% - 15%
Propane 2.1% - 9.5%
Carbon Monoxide 12% - 75%
Gasoline 1.4% - 7.4%
Diesel 1.3% - 6%
Ethanol 3.3% - 19%
Methanol 6% - 35%
3–59
Self-Sustained Chemical Reaction
• Very complex
• Example: Combustion of methane & oxygen(Continued)
3–60
Flaming Combustion
• Sufficient heat causes fuel/oxygen to form free radicals, initiates self-sustained chemical reaction
• Fire burns until fuel/oxygen exhausted or extinguishing agent applied
• Agents may deprive process of fuel, oxygen, sufficient heat for reaction
3–61
Surface Combustion
• Without flames
Example: charcoal
• Distinctly different from flaming combustion
• Cannot be extinguished by chemical flame inhibition
Dry chemical will not work
• Must be extinguished by removing fuel, heat, or oxygen
3–62
Surface Combustion
• Without flames
Example: charcoal
• Distinctly different from flaming combustion
• Cannot be extinguished by chemical flame inhibition
Dry chemical will not work
• Must be extinguished by working removing fuel, heat, or oxygen
3–63
General Products of Combustion Include Heat, Smoke, Light
• Heat, smoke impact FFs most
• Heat generated during fire helps spread fire
• Lack of protection from heat may cause burns & other health issues
• Toxic smoke causes most fire deaths
3–64
Common Products of Combustion
• Carbon monoxide – biggest killer of people in house fires
• Most common product of combustion encountered in structure fires
• Chemical asphyxiant
• Flammable
3–65
Common Products of Combustion
Hydrogen cyanide (HCN)
• Commonly encountered in smoke
• Acts as a chemical asphyxiant
• Byproduct of the combustion of polyurethane foam, carpet & wool
3–66
Common Products of Combustion
Carbon dioxide (CO2)
• Product of complete combustion of organic materials
• Acts as a simple asphyxiant by displacing oxygen
• Also acts as a respiratory stimulant, increasing respiratory rate
3–67
Hazards to Firefighters
• Toxic effects of smoke inhalation not result of any one gas
• Smoke contains a wide range of irritating substances that can be deadly
• FFs must use SCBA when operating in smoke
3–68
Flame
• Visible, luminous (glowing) body of a burning gas
• Becomes hotter, less luminous when burning gas mixes with proper amounts of oxygen
• Product of combustion
3–69
Class A Fires
• Involve ordinary combustible materials
• Primary method of extinguishment is cooling to reduce temperature of fuel to slow or stop release of pyrolysis products
3–70
Class B Fires
Flammable/combustible liquids & gases
• Gas fires are extinguished by cutting off gas supply
• Liquids are extinguished with foam and/or dry chemical agents
3–71
Class C Fires
• Involve energized electrical equipment
• Typical sources — Household appliances, computers, electric motors
• Actual fuel usually insulation on wiring or lubricants
(Continued)
3–72
Class C Fires
• When possible, de-energize electrical equipment before extinguishing
• Any extinguishing agent used before de-energizing must not conduct electricity
3–73
Class D Fires
• Involve combustible metals
• Powdered materials most hazardous
• In right concentrations, metal dust can cause powerful explosions
• High temperature of burning metals makes water & other extinguishing agents ineffective & dangerous
(Burning Magnesium
3–74
Class D Fires
• No single agent effectively controls Class D fires
• Materials may be in a variety of facilities
• Caution urged when extinguishing — Can react violently to water & may produce toxic smoke/vapors
3–75
Class K Fires
• Involve oils & greases
• Require extinguishing agent specifically formulated for materials involved
• Agents use saponification to turn fats & oils into soapy foam that extinguishes fire
3–76
Fire Development in a Compartment
• Compartment — Closed room or space within building
• Walls, ceiling, floor absorb some radiant heat produced by fire
• Radiant heat energy not absorbed is reflected back, increasing temperature of fuel & rate of combustion
(Continued)
3–77
Fire Development in a Compartment
• Hot smoke/air becomes more buoyant
• Upon contact with cooler materials, heat is conducted, raising the temperature
• Heat transfer: Process where heat is absorbed (transferred) from one body to another
• Raises temperature of all materials
(Continued)
3–78
Fire Development in a Compartment
• Heat always goes from warmer to colder
Until both bodies are the same temperature
• As nearby fuel is heated, begins to pyrolize, causing fire extension
(Continued)
3–79
Fire Development in a Compartment
3–80
Fire Develops in Stages
• Incipient
• Growth
• Fully Developed
• Decay
Flashover
3–81
Incipient Stage
• Ignition — Point when the 3 elements of fire triangle come together & combustion occurs
• Once combustion begins, development is largely dependent on characteristics & configuration of fuel involved
(Continued)
3–82
Incipient Stage
• Fire has not yet influenced environment to a significant extent
• Temperature only slightly above ambient, concentration of products of combustion low
(Continued)
3–83
Incipient Stage
• Occupants can safely escape from compartment & fire could be safely extinguished with portable extinguisher or small hoseline
• Transition from incipient to growth stage can occur quite quickly
3–84
Growth Stage
• Fire begins to influence environment within compartment
• Fire influenced by location of fire, layout of the compartment & amount of ventilation
(Continued)
3–85
Growth Stage
• Thermal layering — Tendency of gases to form into layers according to temperature
• Hot gases rise
• Cooler gases sink
• Also known as thermal balance & heat stratification
(Continued)
3–86
Growth Stage
Isolated flames — As fire moves through growth stage, pockets of flames may be observed moving through hot gas layer above neutral plane
3–87
Growth Stage
Rollover – Where unburned gases accumulate at the top of the compartment & ignites
3–88
Growth Stage
• Rollover may precede flashover
• Flashover – rapid transition of fire growth to a fully developed compartment fire
Everything in the compartment ignites at once
Temperatures of flashover: 900° – 1200°F (483°-649°C)
3–89
Growth Stage
Just prior to flashover:
• Temperatures are rapidly increasing
• Additional fuel packages are becoming involved
• Fuel packages are giving off combustible gases
Flashover
3–90
Flashover Video
3–91
Growth Stage
Factors determining flashover:
• Does not occur in every compartment fire
• Fuel must have sufficient heat energy to develop flashover conditions
• Ventilation — A developing fire must have sufficient oxygen
3–92
Fully Developed Stage
• Occurs when all combustible materials in compartment are burning
(Continued)
3–93
Fully Developed Stage
• Burning fuels in compartment release maximum amount of heat possible for available fuel & ventilation, producing large volumes of fire gases
• Fire is ventilation controlled
3–94
Decay Stage
• Fire will decay as fuel is consumed or if oxygen concentration falls to point where flaming combustion can no longer be supported
• Decay due to reduced oxygen concentration can follow much different path if ventilation profile of compartment changes
(Continued)
3–95
Decay Stage
• Consumption of fuel
• Limited ventilation
• Backdraft is produced in the decay stage
3–96
Backdraft Video
Decay Stage - Backdraft
3–97
Decay Stage - Backdraft
Explosion that occurs when oxygen is suddenly admitted to a confined area that is very hot & filled with combustible vapors
3–98
Backdraft Recognition
• Little or no fire
• Smoke exits in puffs
• Smoke stained windows
• Inward drawing smoke
• Grayish, yellow smoke
• Confined fire area
3–99
3–100
Fuel Type
• Impacts both amount of heat released & time over which combustion occurs
• Mass & surface area are most fundamental fuel characteristics influencing development in compartment fire
3–101
Availability/Location of Additional Fuel
Factors that influence fire development
• Configuration of building
• Contents
• Construction
• Location of fire in relation to uninvolved fuel
Which one will have the fastest fire spread?
3–102
Compartment Volume & Ceiling Height
• All other things being equal, a fire in a large compartment will develop more slowly than one in a small compartment
• The large volume of air will support the development of a larger fire before ventilation becomes the limiting factor
3–103
Ventilation
• Influences how fire develops
• Preexisting ventilation is the actual & potential ventilation of a structure
• Consider potential openings that could change the ventilation profile
Size, number, & arrangement of existing & potential ventilation openings
3–104
Thermal Properties of Enclosure
• Include insulation, heat reflectivity, retention, conductivity
• When compartment well-insulated, less heat lost; more heat remains to increase temperature & speed combustion reaction
(Continued)
3–105
Thermal Properties of Enclosure
• Surfaces that reflect heat return it to the combustion reaction & increase its speed
• Some materials act as heat sink & retain heat energy
• Other materials conduct heat readily & spread fire
3–106
Ambient Conditions
• Less significant factor inside structure
• High humidity/cold temperatures can impede natural movement of smoke
• Strong winds significantly influence fire behavior
Wind Direction?
3–107
Impact of Changing Conditions
• Structure fires can be dynamic
• Factors influencing fire development can change as fire extends from one compartment to another
• Changes in ventilation likely most significant factors in changing behavior
3–108
Temperature Reduction
• One of the most common methods of fire control/extinguishment
• Depends on reducing temperature of fuel to point of insufficient vapor to burn
• Solid fuels, liquid fuels with high flash points can be extinguished by cooling (Continued)
3–109
Temperature Reduction
• Use of water is most effective method for extinguishment of smoldering fires
• Enough water must be applied to absorb heat generated by combustion
• Cooling with water cannot reduce vapor production enough to extinguish fires in low flash point flammable liquids/gases
(Continued)
3–110
Temperature Reduction
• Water can be used to control burning gases/reduce temperature of products of combustion above neutral plane
• Water absorbs significant heat as temperature raised, but has greatest effect when vaporized into steam
3–111
Fuel Removal
• Effectively extinguishes any fire
• Simplest method is to allow a fire to burn until all fuel consumed
3–112
Oxygen Exclusion
• Reduces fire’s growth & may totally extinguish over time
• Limiting fire’s air supply can be highly effective fire control action
3–113
Chemical Flame Inhibition
• Extinguishing agents such as halon-repalcements & dry chemicals interrupt combustion reaction, stop flame production
• Effective on gas, liquid fuels because they must flame to burn
• Does not easily extinguish surface mode fires
3–114
Summary
• Many people believe that fire is unpredictable
• There is no unpredictable fire behavior
• Our ability to predict what will happen in the fire environment is hampered by limited information, time pressure, & our level of fire behavior knowledge
(Continued)
3–115
Summary
• FFs need to understand the combustion process & how fire behaves in different materials/different environments
• FFs need to know how fires are classified so that they can select & apply the most appropriate extinguishing agent
(Continued)
3–116
Summary
Most importantly, FFs need to have an understanding of fire behavior that permits them to:
• Recognize developing fire conditions
• Be able to respond safely & effectively to deal w/ the hazards presented by the fire