fire science chapter 5 fire behavior

flash point

minimum temperature at which a liquid gives off enough vapors to form an ignitable mixture with air near the liquid's surface

thermal layering

outcome of combustion in a confined space in which gases tend to form into layers, according to temperature, with the hottest gases found at the ceiling and the coolest gases at floor level. radiation from the hot gas layer acts to heat the interior surface of the compartment and its contents

ignition

the process of initiating self-sustained combustion (NFPA 921)

oxidizer

any material that readily yields oxygen or other oxidizing gas, or that readily reacts to promote or initiate combustion of combustible materials

matter

anything that occupies space and has mass

upper flammable limit (UFL)

upper limit at which a flammable gas or vapor will ignite; above this limit the gas or vapor is too rich to burn (lacks the proper quantity of oxygen)

unplanned ventilation

wind is an important factor, but unplanned ventilation is often the result of occupant action, fire effects on the building (such as window glazing), or action other than planned, systematic, and coordinated tactical ventilation

Joules (J)

Joules are defined in terms of mechanical energy. It is equal to the energy expended in applying a force of one newton through a distance of one meter. However, it is more useful for firefighters to think about the energy required to increase temperature. 4.2J are required to raise the temperature of one gram of water by one degree C

factors that affect fire development - availability and location of additional fuel

building configuration, construction materials, contents and proximity of the initial fire to these exposed fuel sources.

tactical ventilation

planned, systematic, and coordinated removal of heated air, smoke, gases or other airborne contaminants from a structure, replacing them with cooler and/or fresher air to meet the incident priorities of life safety, incident stabilization and property conservation

potential energy

stored energy possessed by an object that can be released in the future to perform work once released

chemical flame inhibition

extinguishment of a fire by interruption of the chemical chain reaction

polar solvents

flammable liquids that have an attraction for water, much like a positive magnetic pole attracts a negative pole; examples include alcohol, ketone and lacquer

gaseous fuel

gaseous fuels such as methane, hydrogen, and acetylene can be the most dangerous of all fuel types because they are already in the physical state required for ignition.

fire triangle

a model used to explain the elements/conditions necessary for combustion. the sides of the triangle represent heat, oxygen and fuel

fire

a rapid oxidation process, which is a chemical reaction resulting in the evolution of light and heat in varying intensities (NFPA 921)

ceiling jet

a relatively thin layer of flowing hot gases that develops under a horizontal surface (e.g., ceiling) as a result of plume impingement and the flowing gas being forced to move horizontally. (NFPA 921)

factors that affect fire development - compartment volume and ceiling height

all other factors being equal, a fire in a large compartment will develop more slowly than one in a small compartment

flaming combustion

occurs when a gaseous fuel mixes with oxygen in the correct ratio and is heated to ignition temperature.

British thermal unit (Btu)

the amount of heat required to raise the temperature of one pound of water one degree F 1Btu = 1055J

fuel-controlled

a fire with adequate oxygen in which the HRR and growth rates are determined by the characteristics of the fuel, such as quantity and geometry (NFPA 921) example: small fire that is confined to a noncombustible waste basket in a large well-ventilated room

ventilation- controlled

a fire with limited ventilation in which the HRR or growth rate is limited by the amount of oxygen available to the fire (NFPA 921) fire will have a tendency to grow toward ventilation openings

heat

a form of energy characterized by vibration of molecules and capable of initiating and supporting chemical changes and changes of state (NFPA 921)

fuel

a material that will maintain combustion under specified environmental conditions (NFPA 921)

petroleum

aka crude oil. it is a naturally occurring, yellow-to-black liquid found in geological formations beneath the Earth's surface, which is commonly refined into various types of fuels. it consists of hydrocarbons of various molecular weights and other organic compounds.

flammable liquid

any liquid having a flash point below 100F and a vapor pressure not exceeding 40psi absolute. most flammable liquids are lighter than water (s.g less than 1) and will float on its surface

products of combustion

as any fuel burns, its chemical composition changes, producing new substances and releasing energy in the form of heat and light. the products of combustion are often simply described as heat (energy release, thermal energy) and smoke (new substances). thermal energy generated during a fire is one product of combustion that heats adjacent fuels and makes them more susceptible to ignition. as they ignite, the fire spreads. anything can be an adjacent fuel for heat.

growth stage (this stage is only in book)

as the fire transitions from incipient to growth stage, it begins to influence the environment within the compartment and has grown large enough for the compartment configuration and the amount of ventilation to influence it.

upper layer

buoyant layer of hot gases and smoke produced by a fire in a compartment

common oxidizers

calcium hypochlorite - chlorination of water in swimming pools chlorine (gas) - water purification ammonium nitrate (granular solid) - fertilizer hydrogen peroxide (liquid) - industrial bleaching methyl ethyl ketone peroxide - catalyst in plastics manufacturing

energy

capacity to perform work; occurs when a force is applied to an object over a distance or when a chemical, biological, or physical transformation is made in a substance

sources of thermal energy - chemical

chemical energy is the most common source of heat in combustion reactions. when any combustible fuel is in contact with oxygen, the potential for oxidation exists. the oxidation process almost always results in the production of thermal energy.

combustion

chemical process of oxidation that occurs at a rate fast enough to produce heat and usually light in the form of either a glow or flame

oxidation

chemical process that occurs when a substance combines with an oxidizer such as oxygen in the air; a common example is the formation of rust on metal

endothermic reaction

chemical reaction that absorbs thermal energy and heat. converting water from a liquid to a gas (steam) requires the input of energy and is an endothermic reaction

exothermic reaction

chemical reaction that releases thermal energy or heat. fire is an exothermic chemical reaction called combustion that releases energy in the form of heat and sometimes visible light

carbon monoxide (CO)

colorless, odorless, dangerous gas (both toxic and flammable) formed by the incomplete combustion of carbon. it combines with hemoglobin more than 200 times faster than oxygen does, thus decreasing the blood's ability to carry oxygen

fire and combustion

combustion is a chemical reaction while fire is one possible result of combustion. combustion can occur without fire. there are two modes of combustion, flaming and nonflaming. nonflaming combustion occurs at a lower temperature producing a smoldering glow in the material's surface. flaming combustion is commonly referred to as fire because it produces a visible flame above the material's surface

factors that affect fire development - ambient conditions

common, prevailing, and uncontrolled atmospheric conditions. the term may refer to the conditions inside or outside of the structure

solubility

degree to which a solid, liquid or gas dissolves in a solvent (usually water). *miscible - materials that are capable of being mixed in all proportions

sources of thermal energy - electrical

electrical energy can generate temperatures high enough to ignite combustible materials near the heated area. this can occur in several ways: resistance heating, overcurrent or overload, arcing, and sparking

1st phase - incipient stage

first stage in the burning process in a compartment in which the substance being oxidized is producing some heat, but the heat has not spread to other substances nearby. during this phase, the oxygen content of the air has not been significantly reduced (20-21%) and the temperature within the compartment (100F) is not significantly higher than the ambient temperature. at this stage, a plume of hot gases and flame rises from the fire and mixes with the cooler air in the room forming a ceiling jet fire gases include water, CO, CO2, SO2

self-sustained chemical reaction

flaming combustion is one example of a chemical chain reaction. sufficient heat will cause fuel and oxygen to form free radicals and initiate the self-sustained chemical reaction. the fire will continue to burn until the fuel or oxygen is exhausted

flammable (explosive) range

for combustion to occur after a fuel has been converted into a gaseous state, it must be mixed with air (oxidizer) in the proper ratio. the range of concentrations of the fuel vapor and air (oxidizer) is called the flammable range. the flammable range of a fuel is reported using the percent by volume of gas or vapor in air for the lower flammable limit (LFL) and for the upper flammable limit (UFL)

smoke explosion

form of fire gas ignition; the ignition of accumulated flammable products of combustion and air that are within the flammable range

fuel

fuel is the material or substance that is oxidized or burned in the combustion process. most common fuels are organic but they can be inorganic as well, as in hydrogen and Mg. hydrocarbon based fuels include gasoline, fuel oil, and plastics and cellulose based materials such as wood and paper

combustible liquid

liquid having a flash point at or above 100F and below 200F

lower flammable limit (LFL)

lower limit at which a flammable gas or vapor will ignite and support combustion; below this limit the gas or vapor is too lean or thin to burn (lacks the proper quantity of fuel)

specific gravity

mass of a substance compared to the mass of an equal volume of water at a given temperature. a specific gravity less than 1 indicates a substance lighter than water. because the liquid fuel is less dense and will not mix with water, adding water to the liquid may disperse the burning liquid instead of extinguishing it. they should therefore be extinguished with the appropriate foam or chemical agent

backdraft indicators

1. building indicators - fire confined to a single compartment or void space, building contents have a high release rate 2. smoke indicators - optically dense smoke, light colored or black becoming dense gray-yellow; neutral plane rising and lowering similar to a pulsing, breathing movement 3. air flow indicators - high velocity, turbulent smoke discharge, sometimes appearing to pulse or breathe 4. heat indicators - high heat, smoke-stained windows 5. flame indicators - little or no visible flame

hydrocarbon fuel

Petroleum-based organic compound that contains only hydrogen and carbon. lighter than water and do not mix with water

passive agent

materials that absorb heat but do not participate actively in the combustion process. in building construction, one of the most common passive agents is drywall or gypsum board which contains moisture in the form of hydrates. as the drywall is heated, moisture is vaporized, slowing the increase in the temperature of the gypsum board. outside structures, the moisture content of vegetation acts as a passive agent in slowing ground cover fires.

temperature

measure of a material's ability to transfer heat energy to other objects; the greater the energy, the higher the temperature. measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale

fire tetrahedron

model of the four elements/conditions required to have a fire. the four sides represent fuel, heat, oxygen and self-sustaining chemical chain reaction

free radicals

molecular fragments that are highly reactive. as flaming combustion occurs, free radicals are released. these free radicals combine with oxygen or with the elements that form the fuel material producing intermediate combustion products, even more free radicals and increasing the speed of the oxidation reaction. when chemically complex fuels burn, different types of free radicals and intermediate combustion products are created, many of which are also flammable and toxic.

piloted ignition

moment when a mixture of fuel and oxygen encounters an external heat (ignition) source with sufficient heat or thermal energy to start the combustion reaction

nonflaming combustion

occurs when burning is localized on or near the fuel's surface where it is in contact with the oxygen. burning charcoal or smoldering wood or fabric are examples.

rollover

one possible and significant indicator of flashover is rollover. rollover describes a condition where the unburned fire gases that have accumulated at the top of a compartment ignite and flames propagate through the hot gas layer or across the ceiling.

2nd phase - free burning stage

oxygen content 16-19% ceiling temperature ~1300F rapid flame spread probability of flashover thermal balance established

3rd phase - smoldering phase

oxygen content less than 15%, flame may stop temperature throughout the room ~1000F room filled with dense smoke and gases possibility of backdraft -> signs include: 1. smoke under pressure 2. black smoke becoming yellow-gray (the lesser the oxygen content, the darker the smoke) 3. confinement and excessive heat 4. little or no visible flames 5. smoke leaves room in puffs 6. smoke stained glass 7. muffled sounds 8. sudden inrush of air when opening is made

vaporization

physical process that changes a liquid into a gaseous state; the rate of vaporization depends on the substance involved, heat, pressure, and exposed surface area

carbon dioxide (CO2)

product of complete combustion of organic materials that is a colorless, odorless, heavier than air gas that neither supports combustion nor burns; used in portable fire extinguishers as an extinguishing agent to extinguish class B or C fires by smothering or displacing the oxygen.

oxygen exclusion

reducing the oxygen available to the combustion process reduces a fire's growth and may totally extinguish it over time. flooding an area with an inert gas such as CO2 displaces the oxygen and disrupts the combustion process. oxygen can also be separated by some fuels by blanketing them with foam

self-heating

self-heating, a form of oxidation, is a chemical reaction that increases the temperature of a material without the addition of external heat. it is the result of exothermic reactions, occurring spontaneously in some materials under certain conditions, whereby heat is generated at a rate sufficient to raise the temperature of the material (NFPA921)

smoke

smoke is an aerosol comprised of gases, vapor, and solid particulates. it is the product of incomplete combustion. in a structure fire, multiple fuels are involved and limited air supply results in incomplete combustion producing a wide range of products of combustion including toxic and flammable gases, vapors and particulates. there are over 20 irritants in smoke, including hydrogen chloride, formaldehyde, and acrolein. smoke must be treated with the same respect as any other flammable gas because it may burn or explode. firefighters must use SCBA when operating in toxic atmospheres

factors that affect fire development - thermal properties of the compartment

thermal properties include: -insulation - contains heat within the compartment causing a localized increase in the temperature and fire growth -heat reflectivity - increase fire spread through the transfer of radiant heat from wall surfaces to adjacent fuel sources. -retention - maintains temperature by absorbing and releasing large amounts of heat slowly

heat of combustion

total amount of thermal energy (heat) that could be generated by the combustion (oxidation) reaction if a fuel were completely burned. the heat of combustion is measured in Btu's per pound or Megajoules per Kg. -dependent on the chemical content of the fuel many plastics, flammable liquids, and flammable gases contain more potential heat or thermal energy than wood

hydrogen cyanide (HCN)

toxic and flammable substance produced in the combustion of materials containing nitrogen. is also commonly encountered in smoke, although at lower concentrations than CO. HCN is also a chemical asphyxiant, acting to prevent the body from using oxygen at the cellular level. it is a significant byproduct of the combustion of polyurethane foam which is commonly used in furniture and bedding

conduction

transfer of heat through or between solids that are in direct contact. conduction results from increased molecular motion and collisions between the molecules of a substance resulting in the transfer of energy through the substance. heat transfer due to conduction is dependent upon three factors: 1. area being heated 2. temp difference between the heat source and the material being heated 3. thermal conductivity of the heated material thermal conductivity varies; copper conducts heat 7x more readily than steel; steel is ~40x more conductive than concrete; wood shows little thermal conductivity *insulating materials slow the conduction of heat from one solid to another

factors that affect fire development - ventilation

ventilation is the exchange of air inside a building or compartment with air outside the building or compartment. tactical ventilation is intentional and conducted by the fire department. room or compartment fires take two forms: fuel-controlled and ventilation-controlled. changes in ventilation can alter the ventilation flow path, create rapid fire development, and place firefighters in extreme danger. when the fire becomes ventilation-controlled, the fire's HRR will decrease. if windows or door fail at this time, the sudden introduction of fresh air creates a rapid increase in HRR

flame

visible, luminous body of a burning gas emitting radiant energy including light of various colors given off by burning gases and vapors during the combustion process. when a burning gas is mixed with the proper amounts of oxygen, the flame becomes hotter and less luminous. the loss of luminosity is caused by a more complete combustion

temperature reduction

water application in sufficient quantities reduces the temperature of a fuel to a point where it does not produce sufficient vapor to burn. *cooling with water cannot sufficiently reduce vapor production to extinguish fires involving low flash point flammable liquids and gases when water is converted to steam at 212F, it expands ~1700 times

vapor density

weight of a given volume of pure vapor or gas compared to the weight of an equal volume of dry air at the same temperature and pressure. a vapor density less than 1 indicates a vapor lighter than air; a vapor density greater than one indicates a vapor heavier than air

solid fuel

when solid fuels are heated, they begin to decompose and combustible vapors are emitted (pyrolysis). unlike liquids or gases, solids have a definite shape and size. this property significantly affects how easily they ignite. the primary consideration is the surface area of the fuel in proportion to the mass, called the surface-to-mass ratio

flashover

when the combustible materials in the compartment and the gases produced by pyrolysis ignite almost simultaneously; the result is full-room involvement. during flashover, the environment of the room is changing from a two-layer condition (hot on top, cold on bottom) to a single, well mixed hot gas condition from floor to ceiling. whether or not a compartment fire will progress to flashover depends on two things: 1. there must be sufficient fuel and the heat release rate must be sufficient for flashover conditions to develop 2. ventilation. a developing fire must have sufficient oxygen to reach flashover, and a sealed room may not provide enough

flashover indicators

1. building indicators - interior configuration, fuel load, thermal properties, and ventilation 2. smoke indicators - rapidly increasing volume, turbulence, darkening color, optical density, and lowering of the hot gas level 3. air flow indicators - high velocity and turbulence, bi-directional movement with smoke exiting at top of doorway and fresh air moving in at the bottom, or pulsing air movement. 4. heat indicators - rapidly increasing temperatures in the compartment, pyrolysis of contents or fuel packages located away from the fire, darkened windows, or hot surfaces 5. flame indicators - isolated flames in the hot gas layers or near the ceiling

vapor pressure

1. measure of the tendency of a substance to evaporate 2. the pressure at which vapor is in equilibrium with its liquid phase for a given temperature; liquids that have a greater tendency to evaporate have higher vapor pressures for a given temperature *in order for vaporization to occur, the escaping vapors must be at a higher pressure than atmospheric pressure. as liquid is heated, vapor pressure increases along with the rate of vaporization.

four common elements of flashover

1. transition in fire development - flashover represents a transition from the growth stage to the fully developed stage 2. rapidity - although it is not an instantaneous event, flashover happens rapidly, often in a matter of seconds, to spread complete fire involvement within the compartment 3. compartment - there must be an enclosed space such as a single room enclosure 4. ignition of all exposed surfaces - virtually all combustible surfaces in the enclosed space become ignited

energy and fire

fuels have a certain amount of chemical potential energy before they are ignited. different fuels release different amounts of energy over different periods of time. when heat is introduced to a fuel, the molecules within the fuel begin to vibrate. as the heat (thermal energy) increases, these molecules vibrate more and more rapidly. the fuel's kinetic energy is the result of these vibrations in the molecules. in terms of fire behavior, the potential chemical energy of a fuel is converted to thermal energy and released as heat

sources of thermal energy - mechanical

generated by friction or compression. the movement of two surfaces against each other creates heat of friction. this movement results in heat and/or sparks being generated. heat of compression is generated when a gas is compressed

thermal energy (heat)

heat is the kinetic energy transferred from a high-temperature substance to a low-temperature substance. because heat is kinetic energy, it is always in transit from one location (an open flame) to another (an exposed fuel). heat is the thermal kinetic energy needed to release the potential chemical energy in a fuel. heat begins to vibrate the molecules in a fuel. as the molecules break down and release vapors, the vapors can ignite and release thermal energy. this new source of thermal energy begins to heat other, uninvolved fuels converting their energy and spreading the fire.

convection

heat transfer by circulation within a medium such as a gas or a liquid (NFPA 921). in the fire environment, this usually involves a transfer of heat through the movement of hot smoke and fire gases. generally this movement will be upward because the smoke and fire gases are heated and buoyant. convection currents can also move laterally as the result of differences in pressure. firefighters who are working in the flow path of the hot smoke and fire gases will feel the increase in temperature as the velocity and/or turbulence increases causing convective heat transfer

radiation

heat transfer by way of electromagnetic energy (NFPA 921), such as light waves, radio waves, or X rays. the heat of the sun warming our planet is an example. radiant heat can become the dominant mode of heat transfer when the fire grows in size. factors influencing radiant heat transfer include: -nature of the exposed surfaces - dark materials emit and absorb heat more effectively -distance between the heat source and the exposed surfaces -temperature difference between the heat source and exposed surfaces - as the temperature of the heat source increases, the radiant energy increases by a factor to the fourth power radiated heat travels through vacuums and air spaces where conduction or convection would normally be disrupted. however, materials that reflect radiant energy will disrupt the transmission of heat. while flames have high temperature resulting in emission of significant radiant energy, hot smoke in the upper layer can also radiate significant energy.

spontaneous ignition

in order for self-heating to progress to spontaneous ignition, the material must be heated to its autoignition temperature. spontaneous ignition is initiation of combustion of a material by an internal chemical or biological reaction that has produced sufficient heat to ignite the material (NFPA921) the following factors are required for spontaneous ignition to occur: 1. the insulation properties of the material immediately surrounding the fuel must be such that heat cannot dissipate as fast as it is being generated 2. the rate of heat production must be great enough to raise the temperature of the material to its autoignition temperature 3. the available air supply in and around the material being heated must be adequate to support combustion. some common materials that are subject to self-heating include charcoal, linseed oil-soaked rags, straw and manure

heat release rate (HRR)

in terms of fire behavior, power is described as the HRR during combustion. when a fuel is being heated, work is being performed (energy is being transferred). the speed with which this work is occurring, HRR, is the amount of power being generated. HRR is the energy released per unit time as a fire burns and is normally expressed in kilowatts or megawatts. it is directly related to oxygen consumption; typically, the more available oxygen, the higher the HRR

autoignition

initiation of combustion by heat but without a spark or flame (NFPA 921)

pyrolysis zone

stage 1 (less than 392F) - moisture is released as the wood begins to dry; combustible and noncombustible materials are released into the atmosphere although there is insufficient heat to ignite them. stage 2 (392-536F) - the majority of the moisture has been released; charring has begun; the primary compound being released is CO; ignition has yet to occur stage 3 (536-932F) - rapid pyrolysis takes place; combustible compounds are released and ignition can occur; charcoal is formed by the burning process stage 4 (greater than 932F) - free burning exists as the wood material is converted to flammable gases.

fire point

temperature at which a liquid fuel produces sufficient vapors to support combustion once the fuel is ignited. fire point must exceed 5 seconds of burning duration during the test. the fire point is usually a few degrees above the flash point.

heat and temperature

temperature is the measurement of heat. it is the measurement of the average kinetic energy in the particles in a sample of matter

ways firefighters influence fire behavior

temperature reduction fuel removal oxygen exclusion chemical flame inhibition ventilation and fire behavior

pyrolysis

the chemical decomposition of a solid material by heating. pyrolysis often precedes combustion

plume

the column of hot gases, flames and smoke rising above a fire; also called convection column, thermal updraft or thermal column. (NFPA 921)

kinetic energy

the energy possessed by a body because of its motion

backdraft

the explosive burning of heated gases that occurs when oxygen is introduced into a compartment that has a high concentration of flammable gases and a depleted supply of oxygen due to an existing fire

reducing agent

the fuel that is being oxidized or burned during combustion

neutral plane

the level at a compartment opening where the difference in pressure exerted by expansion and buoyancy of hot smoke flowing out of the opening and the inward pressure of cooler, ambient temperature air flowing in through the opening is equal.

autoignition temperature

the lowest temperature at which a combustible material ignites in air without a spark or flame (NFPA 921) the autoignition temperature of a substance is always higher than its piloted ignition temperature

heat flux

the measure of the rate of heat transfer to a surface, expressed in kilowatts/m^2, kilojoules/m^2 x sec, or Btu/ft^2 x sec (NFPA 921) heat can be transferred from one body to another by three mechanisms: conduction, convection, and radiation

oxygen

the primary oxidizing agent in most fires. normally, the air consists of ~21% oxygen. oxidizers are not combustible but they will support or enhance combustion. materials can ignite and burn at oxygen concentrations as low as 14% but when O2 concentration is limited, the flaming combustion will diminish, causing combustion to continue in the non flaming mode which can continue at extremely low oxygen concentrations. when the oxygen concentration is higher than normal, materials exhibit very different burning characteristics. many materials that do not burn at normal oxygen levels will burn readily in oxygen-enriched atmospheres

power

the rate at which the energy is being transferred over time. aka the rate at which energy is converted from one form to another. the SI unit for power is the watt which is equal to one joule per second.

factors that affect fire development - fuel load

the total quantity of combustible contents of a building, space, or fire area, including interior finish and trim, expressed in heat units of the equivalent weight in wood

factors that affect fire development - fuel type

the type of fuel involved in combustion affects the heat release rate (HRR) one of the most fundamental Class A fuel characteristics influencing fire development is surface-to-mass ratio. combustible materials with high surface area-to-mass ratios are much more easily ignited and will burn more quickly than the same substance with less surface area. fires involving class B flammable/combustible liquids follow the same pattern.