b Fire Behavior

7/28/2019 b Fire Behavior

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FIRE SAFETY



• It is the condition of being safe from risk or

danger of fire.

• The quality or state of not presenting or

involving risk or danger of fire.



Fire Behavior



Fire has been both a help and a

hindrance to mankind throughout history.

Fire has heated our homes, cooked our food,and helped us to become technologically

advanced. Fire, in its hostile mode, has also

endangered us for as long as we have usedit.



FIRE

• Is a chemical reaction. It is the rapid oxidation of a fuel producing heat and light.

• It is an oxidation taking place with a rate rapid enoughto produce heat and light.

Is a rapid, self-sustaining oxidation processaccompanied by the evolution of heat and light of varying intensity.



Triangle of Fire

For many years, the fire triangle

(oxygen, fuel and heat) was used to teachthe components of fire. While this simple

example is useful, it is NOT technically

correct.



Fire Tetrahedron

Each component of the tetrahedron must

be in place for combustion to occur.Remove one of the four components and

combustion will not occur. If ignition has

already occurred, the fire is extinguished

when one of the components is removed

from the reaction.



OXYGEN (Oxidizing Agent)

Oxidizing agents are those materials

that yield oxygen or other oxidizing gases

during the course of a chemical reaction.Oxidizers are not themselves combustible,

but they support combustion when

combined with a fuel.



FUEL

Fuel is the material or substance being

oxidized or burned in the combustion process.



HEAT

Heat is the energy component of the fire

tetrahedron. When heat comes into contact

with a fuel, the energy supports the

combustion reaction.



SELF-SUSTAINED CHEMICAL

REACTIONCombustion is a complex reaction

that requires a fuel (in the gaseous or

vapor state), an oxidizer, and a heat

energy to come together in a very

specific way. Once flaming combustionor fire occurs, it can only continue when

enough heat energy is produced to cause

the continued development of fuel vapors

or gases. Scientists call this type of reaction a “chain reaction”. A chain

reaction is a series of reactions that occur

in sequence with the result of each

individual reaction being added to the

rest.



Fire Development

When the four components of the fire

tetrahedron come together, ignition occurs.

For a fire to grow beyond the first materialignited, heat must be transmitted beyond the

first material to additional fuel packages.



Stages of Fire

• Ignition

• Growth

• Flashover

• Fully developed

• Decay



IGNITION

Ignition describes the period when the

four elements of the fire tetrahedron come

together and combustion begins



GROWTH

Shortly after ignition, a fire plume

begins to form above the burning fuel. As

the plume develops, it begins to draw or entrain air from the surrounding space into

the column.



FLASHOVER

Flashover is the transition between thegrowth and the fully developed fire stages

and is not a specific event such as ignition.During flashover, conditions in thecompartment change very rapidly as the firechanges from one that is dominated by the

burning of the materials first ignited to onethat involves all of the exposed combustible surfaces within the compartment.



FULLY DEVELOPED

The fully developed fire stage occurs

when all combustible materials in thecompartment are involved in the fire.



DECAY

As the fire consumes the available fuel

in the compartment, the rate of heat released begins to decline.



MODES OF HEAT TRANSFER



Heat is by-product of combustion that is

of significant importance to the firefighter.It is heat that causes fire to sustain its

combustion and, more important, to extend.

When heat given off as a product of

combustion is exposed to an unheated

substance, certain changes occur that can

make the new substance a contributing

factor in extending a fire.



CONDUCTION

When a hot object transfers its heat,conduction has taken place. The transfer could beto another object or to another portion of the sameobject. As we have discovered and will beconstantly reinforced about, combustion occurs onthe molecular level. When an object heats up, theatoms become agitated and begin to collide with

one another. A chain reaction of molecules andatoms, like a wave energy, occurs and causes theagitated molecules to pass the heat energy to areasof non-heat.



CONVECTION

Air that is hotter than its surroundings rises.

Air that is cooler than its surroundings sinks. Air

is made up of many molecules floating aboutfreely. Even so, it still has weight. Some

molecules are made up of the same element. For

example, oxygen in its natural state will combine

with another oxygen atom to form a stable oxygenmolecule. In a given volume, air at a given

temperature will have the same density.



When heated, as in conduction theory,the molecules become agitated and begin to

collide with one another. In the process, themolecules are demanding more space toaccommodate the vibrations and they push

into one another as they seek that space.When that happens, the density of a givenvolume is reduced and it weighs less.Because it weighs less, it rises until it

reaches equilibrium-the level at which theweight is the same as the surroundingatmosphere.



RADIATION

The last form of heat transfer occurs by

radiation. As we have already seen, heatenergy can be transmitted directly when

molecules collide with one another and

cause the waves of heat energy to travel.



Conduction

Convection

Radiation

Flame Plume

Examples of heat transfer in fire



SPECIAL CONSIDERATIONS



Flameover / Rollover

The terms flameover and rollover describe a

condition where flames move through or across

the unburned gases during a fires’ progression.Flameover is distinguished from flashover by its

involvement of only the fire gases and not the

surfaces of other fuel packages within a

compartment. This condition may occur during thegrowth stage as the hot-gas layer forms at the

ceiling of the compartment.



Thermal Layering of Gases

The thermal layering of gases is the

tendency of gases to form into layers

according to temperature. Other termssometimes used to describe this tendency

are heat stratification and thermal balance.

The hot gases tend to be in the top layer,while the cooler gases form the lower

layers.



Backdraft

Firefighters operating at fires in the buildingmust use care when opening a building to gainentry or to provide horizontal ventilation (openingdoors or windows). As the fire grows in a

compartment, large volumes of hot, unburned firegases can collect in unventilated spaces. Thesegases may be at or above their ignitiontemperature but have insufficient oxygen available

to actually ignite. Any action during thefirefighting operations that allows air to mix thesehot gases can result in an explosive ignition called

backdraft.



PRODUCTS OF COMBUSTION



Products of Combustion

1. Heat

2. Light3. Smoke

4. Toxic gases



FIRE EXTINGUISHMENT

THEORYFire is extinguished by limiting or interrupting

one or more of the essential elements in the

combustion process (fire tetrahedron). A fire may be extinguished by:

a.) Reducing its Temperature

b.) Removal of available Fuel

c.) Exclusion of Oxygen

d.) Inhibition of Self-Sustained Chemical

Chain Reaction.



end

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