Chapter 3 Fire Behavior. 3–23–2 Chapter 3 Lesson Goal After completing this lesson, the student shall be able to summarize physical & chemical changes.

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