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Chapter 3 — Fire Behavior

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1 Chapter 3 — Fire Behavior

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 Combustion Rapid oxidation of fuel that produces heat & light

4 Matter is… anything that occupies space & has mass (weight)

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)

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”)

7 DISCUSSION QUESTION What are some examples of physical & chemical changes of matter?

8 Flaming (Fire) Non-flaming (No fire)
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

9 Fire Triangle Fire Parts

10 Fire Tetrahedron Fire Process

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

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

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

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)

15 Scales Used to Measure Temperature
Celsius — Metric Fahrenheit — Customary

16 Conversion of Energy Into Heat
Heat is the energy component of tetrahedron Fuel is heated = temperature increases Starting ignition Forms of ignition

17 Forms of ignition Two forms:
Piloted ignition outside heat source used to ignite material i.e. match

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

19 Chemical Heat Energy Most common heat source in combustion reactions
Oxidation almost always results in production of heat Self-heating

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

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)

22 Mechanical Heat Energy

23 DISCUSSION QUESTION What are some examples of chemical, electrical, & mechanical sources of heat energy?

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)

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

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

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

28 Radiation Transmission of energy as electromagnetic wave without intervening medium Distance between heat & surface is biggest factor (Continued)

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

30 Heat Transfer in Buildings
Heat can be transferred in buildings by: Conduction Radiation Convection Heat transfer by radiation

31 Passive Agents Materials that absorb heat but do not participate in combustion Fuel moisture = passive agent Relative humidity & fuel moisture

32 DISCUSSION QUESTION What is the impact of high fuel moisture on fire spread?

33 Fuel Material being oxidized in combustion process
Reducing agent: reduces or uses oxygen Inorganic or organic; organic most common (Continued)

34 Fuel Organic fuels: contain carbon Hydrocarbon-based Cellulose-based
Petroleum based & some plastics Cellulose-based Wood & paper Inorganic fuels: do not contain carbon Hydrogen & magnesium

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

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

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

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

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

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

41 Liquid Fuel Characteristics
Flash point (Continued)

42 Liquid Fuel Characteristics
Non-polar Solvents Hydrocarbons, gasoline, diesel Does not mix w/ water Non-miscible, non-soluble Lighter than water

43 Liquid Fuel Characteristics
Polar Solvents Alcohols, acetone, esters Readily mixes w/ water Miscible/soluble

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

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

46 Solid Fuel Definite size & shape
React differently when exposed to heat (Continued)

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)

48 Solid Fuel Commonly the primary fuel
Surface-to-mass ratio — Primary consideration in ease or difficulty of lighting (Continued)

49 Solid Fuel Proximity/orientation of solid fuel relative to source of heat affects the way it burns

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

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)

52 Oxygen In air, is the primary oxidizing agent in most fires
Air consists of 21% oxygen

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)

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)

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

56 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

57 Flammable Range Flammable limits can change based on temperature
Higher temperatures can increase the flammable ranges Lower temperatures can decrease the flammable ranges

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%

59 Self-Sustained Chemical Reaction
Very complex Example: Combustion of methane & oxygen (Continued)

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

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

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

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

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

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

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

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

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

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

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

71 Class C Fires Involve energized electrical equipment
Typical sources — Household appliances, computers, electric motors Actual fuel usually insulation on wiring or lubricants (Continued)

72 Class C Fires When possible, de-energize electrical equipment before extinguishing Any extinguishing agent used before de-energizing must not conduct electricity

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

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

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

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)

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)

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)

79 Fire Development in a Compartment

80 Fire Develops in Stages
Flashover Incipient Growth Fully Developed Decay

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)

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)

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

84 Growth Stage Fire begins to influence environment within compartment
Fire influenced by location of fire, layout of the compartment & amount of ventilation (Continued)

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)

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

87 Growth Stage Rollover – Where unburned gases accumulate at the top of the compartment & ignites

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)

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

90 Flashover Video

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

92 Fully Developed Stage Occurs when all combustible materials in compartment are burning (Continued)

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

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)

95 Decay Stage Consumption of fuel Limited ventilation
Backdraft is produced in the decay stage

96 Backdraft Video

97 Decay Stage - Backdraft
3/25/2017 Decay Stage - Backdraft 3–97

98 Decay Stage - Backdraft
3/25/2017 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

99 Backdraft Recognition
Little or no fire Smoke exits in puffs Smoke stained windows Inward drawing smoke Grayish, yellow smoke Confined fire area

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

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?

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

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

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)

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

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?

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

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)

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)

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

111 Fuel Removal Effectively extinguishes any fire
Simplest method is to allow a fire to burn until all fuel consumed

112 Oxygen Exclusion Reduces fire’s growth & may totally extinguish over time Limiting fire’s air supply can be highly effective fire control action

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

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)

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)

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

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