FIRE BEHAVIOR

Copyright© 2000. Delmar is a division of Thomson Learning.

Objectives 1 of 3

• Describe the chemistry and physics of fire.

• Identify the sources of heat.

• Describe the characteristics of fire.

• Describe the effect of oxygen on fire.

• Define combustion.

Objectives 2 of 3

• Describe vapor pressure and vapor density.• Describe the meaning of flammable and explosive

limits.• Describe the three types of heat transfer.• Describe the significance of the thermal conductivity

of materials.

Objectives 3 of 3

• Describe fuel types and their effects on combustion.• Describe the basis for the theory of fire

extinguishment.• Identify the classes of fire and methods of

extinguishment.• Describe the effects of thermal balance.

Introduction 1 of 2

• Fire has played a major role in the development of society.

• Fire has been a friend and enemy.• Burning, known as combustion, is a chemical

reaction.• It is important to understand the behavior of fire and

what causes it to begin, grow, and spread.

Fire Triangle, Tetrahedron, and Pyramid

Measurements• Measurement is an important part of firefighting.

• Need to know common terms for heat, electricity, volume, length, energy output, concentrations, and weight.

• Understanding measurements and limits for each on the fire ground is important.

Measurements

Chemistry and Physics of Fire 1 of 6

• Universe made of matter.• Matter is never destroyed.• Matter may change form.• All living things are made of cells.• Cells are made up of compounds.• Compounds are made up of molecules.• Molecules are made up of two or more elements.

Chemistry and Physics of Fire 2 of 6

Chemistry and Physics of Fire 3 of 6

• Substances that are or were living are referred to as organic.

• Organic substances contain carbon, hydrogen, and oxygen.

• A non-living substance can be considered organic if it’s made of chemicals that were once alive.

Chemistry and Physics of Fire 4 of 6

• Organic compounds are made of various chemicals.

• Chemicals are made up of a combination of molecules.

• Molecules are joined and separated by bonding actions.

• Combination and separation of molecules and atoms causes oxidation and combustion.

Chemistry and Physics of Fire 5 of 6

• Atoms are made up of electrons, neutrons, & protons.

• Neutrons and protons make up the center of the atom.

• Electrons rotate around the nucleus.

Chemistry and Physics of Fire 6 of 6

• Atoms that lack electron will be quick to link-up and form molecules.

• Substances that are non-living, such as minerals, are referred to as inorganic substances.

• For the most part, only organic materials burn.• A “bond” holds molecules together.

Endothermic Reaction

• When molecules are joined heat is absorbed.

• Known as endothermic reaction.

Exothermic Reaction

• When bonds break, heat is released.

• Known as an exothermic reaction.

Oxidizer

• Oxidizer acts as a catalyst in the breakdown of otherwise stable molecules.

• An oxidizer can pull apart a molecule and break the bond that existed.

• The oxidation process is evident during combustion.

Sources of Heat

• Heat • is an energy source that powers the universe.• can neither be created nor destroyed.• is a physical manifestation of energy as it changes

from one form to another.• comes from four basic sources.

• Chemical• Electrical • Mechanical• Nuclear

Chemical Heat

• Most common source of heat.• Anything that burns does so through an exothermic

reaction.• Pyrolysis is decomposition or transformation of a

compound caused by heat.

Mechanical Heat

• Friction causes heat that can reach levels hot enough to ignite other combustibles.

• Build up of heat from friction often causes machinery fires.

• The source of heat has to be stopped when extinguishing a fire.

Electrical Heat 1 of 2

• Most recognized source of heat.• An electrical current can generate heat.• Electricity is a flow of electrons from an area of

greater concentration to lesser.• Places with large quantities of electrons are negatively

charged.• Places that lack electrons are positively charged.

Electrical heat 2 of 2

• Conductors allow flow of electrons.• One electron jumps onto an atom which releases

another move to the next atom.• Heat generated by conductors varies greatly.• Electrical energy is a heat source.

Flow of Electrons

Nuclear Heat

• Generated the same way as other forms.• Radioactive material is very unstable.• Not much can be done to safely fight a nuclear fire.

Nuclear Energy and Heat

Combustion• Term often confused with fire.• Fire is a self-sustaining process that emits light and heat

as byproducts of chemical reaction.• In combustion the released heat energy is reinvested in

the process, causing continued reaction.• If there is continuous access to fuel, oxygen, and heat,

the growth will accelerate.

Oxygen and its Effect on Combustion

• Air contains about 21 % oxygen.• Oxygen acts as catalyst to combustion.• This chemical reaction is called oxidation.• Oxygen can affect a material’s combustibility.• High concentrations of oxygen can cause some

material to ignite spontaneously.

Vapor Pressure and Vapor Density 1 of 3

• Pressure is a continuous application of force by one body on another body that it is touching.

• Vapor pressure is the amount of pressure exerted by a liquid as it converts to gas and exerts pressure against a confined container.

Vapor Pressure and Vapor Density 2 of 3

• Molecules in a closed container are constantly moving and colliding.

• Some molecules escape into the air and fill the area (diffusion.)

• If the weight of the gas is lighter than air, molecules escape (evaporation.)

• Equilibrium is reached when the amount molecules freed equal those reabsorbed.

Vapor Pressure and Vapor Density 3 of 3

• Air pressure changes at different altitudes.• At sea level, air pressure is 14.7 psi.• Gases that weigh less than the same volume of air will

be lighter and tend to rise.• Gases that weigh greater than air will be heavier and

tend to drop to low lying areas.• Vapor pressure is the force exerted on the sides of a

closed container.

Vapor Density

BLEVE 1 of 3

• A BLEVE occurs when a vessel holding liquid ruptures from pressure being exerted on its sides when the liquid inside boils and the pressure exceeds the container’s ability to hold it.

BLEVE 2 of 3

BLEVE 3 of 3

• Even a non-flammable liquid container can rupture.

• A flammable liquid can create a great fireball.

Ignition

• When a substance heats up, burning gases are liberated.

• When all ingredients of a self-sustaining chemical reaction are present, ignition occurs.

• Ignition is the point when a material is able to sustain combustion from the heat generated by the material itself.

Boiling Point

• All materials exist as a solid, liquid, or gas.

• Pressure and temperature affect the state of matter.

• A change in temperature or pressure may change the form of the substance.

Flammable and Explosive Limits 1 of 2

• Combustible materials need certain requirements to oxidize.

• Different concentrations of oxygen may be required.• Combustion can only take place when a substance is a

gas.• Gases can only ignite when certain concentrations are

present.

Flammable and Explosive Limits 2 of 2

• When a gas concentration falls into a range that it can ignite, it is within its flammable or explosive limit.

• Flammable limits can change with temperature and surrounding conditions.

The Burning Process - Characteristics Of Fire Behavior

• Burning process occurs in defined stages.• Various stages must be recognized and

firefighting tactics adjusted.

Growth Stage• Speed, growth, and size of fire depends on several

factors:• Oxygen supply.• Fuel.• Container size.• Insulation.

Fully Developed Stage

• All contents within the perimeter of the fire’s boundaries are burning.

• The speed and extent of a fully developed stage fire depends on amount of air.

• In an outside fire, the amount of fuel available will determine the size of the fire.

Decay Stage

• When all fuel is consumed, fire will begin to diminish.• The fire will begin to extinguish itself when fuel

exhausted.• These factors will dictate attack strategies.

Four Stages of Fire

Fully Developed Phase - Flashover

Decay Stage of Fire

Modes Of Heat Transfer

• Heat is a by-product of combustion.• Heat transfers its energy through:

• Conduction• Convection• Radiation

Conduction

• Transfer of heat from a hot object.• Transfer can be to another object or another part of the

same object.• Molecules and atoms are excited resulting in the spread

of heat.• Heat is conducted through different materials at

different rates.• Density, time, and application of heat will affect rate of

conduction.

Conduction

Convection

• Air that is hotter than its surroundings rises.• Air that is cooler than its surroundings sinks.• As air is heated molecules push onto one another

seeking space to expand.

Convection

Radiation 1 of 2

• When combustion occurs, light traveling in light waves is produced.

• Light waves range from ultraviolet to infrared.• Electromagnetic forces in lightwaves travel across

vacuums and land on remote objects.

Example: Sun heats earth through radiation.

Radiation 2 of 2

• Radiation is a major contributor to flashover.• Objects in the room reach their ignition temperature

through this process.• Heat has to be kept from developing in order to

prevent the fire from extending.

Radiation

Heat Transfer in Fire

Thermal Conductivity Of Materials

• All matter will conduct heat.• The ability to conduct thermal energy depends on its

density.• The less dense the object is, the more difficult it is for

heat to transfer through it.

Physical State Of Fuels And Effect On Combustion

• Matter is found in one of three states.• Most fuels combust in gaseous state• The physical state of matter can affect combustion.

States of Matter

Solid• Molecules in a solid material are packed closely

together.• When heat is applied, molecules become agitated and

collide with each other.• This causes molecules to break apart.• Heat produced causes additional molecules to break

away, become a gas, and combust.

Liquid

• Burning is dependent on the substance’s ability to place its molecules into suspension.

• Can not burn unless it is in suspension.• Acts as a heat sink and dissipate the introduced heat into

cooler areas.• A rise in temperature leads to a boiling or flashpoint.

Gas

• Primed for combustion.• A ready state that will permit a chemical reaction.

Thermal Balance And Imbalance

• During combustion, heat is liberated.• Thermal balance and imbalance causes smoke to

column and mushroom.• Heated air and smoke rise.• Ascent stops when thermal balance is reached

establishing an equilibrium.• This causes a mushroom effect.

Theory Of Fire Extinguishment

• To stop a fire, remove one of its essential ingredients.• Fire will collapse if missing one ingredient.• A fire will be extinguished, if:

• Temperature is lowered.• Fuel is eliminated.• Oxygen is eliminated.• Chemical flame is repressed.

Classes Of Fire

• Class A

• Class B

• Class C

• Class D

Wrap-Up 1 of 2

• Combustion results from the actual chemical reaction of heat, fuel, and oxygen.

• For the most part organic material will burn.• Heat sources are chemical, mechanical, electrical, and

nuclear.

Wrap-Up 2 of 2

• Burning process occurs in three stages: growth, fully developed, and decay.

• Heat is transferred through conduction, convection, and radiation.

• Fires are classified into different types and are extinguished differently.