Fire Physics

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Fire Physics Fire Chemistry

• Fire is a rapid oxidation process with the
  evolution of light and heat with varying
  intensities.
• In order for a fire to initiate or exist, three
  components must be present
• fuel,
• heat
• oxygen.
• Removal of any one of the three will result in
  the extinguishment of the fire.

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Fire Physics Fire Chemistry

• These three components should always be present
  in the investigators thought process when he
  examines a fire scene.

Is there enough AIR, HEAT, FUEL, to cause what I
see? If NO then why?
FUEL
OXYGEN
HEAT
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Fire Physics Fire Chemistry

• We have learned that in order for a fuel to
  combine with oxygen it is chemically broken down
  into smaller units.
• This reaction is the fourth component called the
  uninhibited chain reaction.
• It yields energy or products that cause other
  reactions and is self-sustaining.

FUEL
OXYGEN
Uninhibited Chemical Chain Reaction
HEAT
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FUELS

• A fuel can easily be defined as any substance
  that will combust.
• Types
• All matter can exist in one of three phases
• solid,
• liquid,
• and gas.
• Matter can undergo changes from one form to
  another.

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FUELS

• A fuel can easily be defined as any substance
  that will combust.
• Types
• Solid or liquid materials do not burn.
• For combustion to take place solids and liquids
  must be heated sufficiently to produce
  combustible vapors.
• It is the combustible vapors that actually burn.

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FUELS

• Solids
• Have definite volume and shape.
• In order for combustion to occur, sufficient heat
  must be present for the solid to liquefy and then
  vaporize into the gaseous state.
• During oxidation, it is the gaseous form that is
  capable of combining with oxygen.

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FUELS

• Solids
• In order for a solid to burn, it must reach its
  ignition temperature.
• Terms to learn
• Ignition Temperature
• Pyrolysis
• Heat of Combustion

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FUELS

• Liquids
• Assumes the shape of its container and may
  diffuse.
• It has a definite volume and may be compressed
  slightly.
• Like a solid, in order for combustion to occur,
  sufficient heat must be present to vaporize it
  into the gaseous state.

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FUELS

• Liquids
• Terms to learn

• Viscosity
• Vapor pressure
• Boiling Point
• Flash Point
• Fire Point

• Flammable Limits
• Vapor Density
• Miscible
• Immiscible
• Specific Gravity

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FUELS

• Gases
• Have no definite shape or volume and assumes the
  shape and volume of its container.
• A gas will spread and eventually equalize its
  distribution throughout a fixed room or
  container.
• Combustion in this state needs no heating and
  only requires the proper mixture of oxygen and an
  ignition source.

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What is burning?
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FUELS

• Fuel load
• The total quantity of combustible contents of a
  building, space, or fire area, including interior
  finish and trim expressed in heat units or the
  equivalent weight in wood.
• It was normally expressed in terms of kJ or kg of
  fuel per square meter of floor area.

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FUELS

• Fuel load
• The fire growth rate is controlled by
• Physical and Chemical Properties of the Fuel
• Fuel condition
• Fuel configuration
• Compartmentation/ventilation

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FUELS

• Fuel condition and geometry
• Moisture content of Fuel and its equilibrium
  point.
• A fuel when exposed to air will become in
  equilibrium with the air and have the same
  moisture content as the air.
• The amount of exposure and age would also effect
  its wetness or dryness.
• This will effect the fuels ignition temperature
  and the rate that it burns.

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FUELS

• Fuel condition and geometry
• Moisture content of Fuel and its equilibrium
  point.
• Generally when the moisture content is above 15,
  ignition is rather difficult, even when it is
  exposed to heat for a prolonged period of time.
• Equilibrium fuel wetness vs. humidity
• Dry fuels burn readily
• Wet fuels must first have excess moisture
  evaporated before it can be raised to its
  ignition temperature

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FUELS

• Fuel condition and geometry
• Configuration of Fuel
• The combustion of solid fuels is more complex
  than the combustion of a liquid or gas.
• Fuels take on many shapes and can be divided into
  many forms.

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FUELS

• Fuel condition and geometry
• Configuration of Fuel
• The severity and the duration of the fire will be
  dependant upon
• Arrangement and/or Placement
• How a fuel is normally configured in a structure
  will determine how it will burn and how it will
  affect other fuels present in the structure.

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FUELS

• Fuel condition and geometry
• Physical characteristics
• A fuel must be present in a suitable condition to
  be ignited.
• An example would be a heavy petroleum distillate
  spread on a floor will not easily ignite, however
  by placing a wick in it, the wick will easily
  ignite and draw the fuel to the flame.

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FUELS

• Fuel condition and geometry
• Physical characteristics
• Amount
• The total amount of fuel present must be balanced
  with the amount of air in the area in order to
  continue the burning process.
• Level of ignition on the fuel, size of ignition
  source relative to mass/surface area of fuel
• The location of ignition on the fuel will
  determine the rate of fire development.

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FUELS

• Fuel condition and geometry
• Physical characteristics
• An example would be a fire starting on the arm
  rest of a chair verses starting at the bottom of
  the chair.
• As fire burns upward it will progress faster if
  the fuel is above it, especially in the earlier
  stages.

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An example would be a fire starting on the arm
rest of a chair verses starting at the bottom of
the chair.
As fire burns upward it will progress faster if
the fuel is above it, especially in the earlier
stages.
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FUELS

• Fuel condition and geometry
• Physical properties
• Density
• The measure of a substances mass per unit
  volume.
• This effects the fuels ability to transfer and
  spread heat energy.
• Low density materials burn faster than high
  density materials.

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FUELS

• Fuel condition and geometry
• Physical properties
• Thermal conductivity
• Conducts heat readily
• How a material will conduct heat determines how
  rapid the flame will spread across the surface of
  a material.

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FUELS

• Fuel condition and geometry
• Physical properties
• Heat capacity
• Is the amount of heat energy required to change a
  materials temperature by one degree.
• This effects the time required to transfer heat.

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FUELS

• Fuel condition and geometry
• Physical properties
• Heat loss
• If the thermal insulation between two materials
  is increased, the flame spread rate will increase
  since less energy would be lost from the burning
  area.

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FUELS

• Fuel condition and geometry
• Physical properties
• Heat of Combustion
• The energy released by the fire per unit mass of
  fuel burned.

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FUELS
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FUELS

• Fuel condition and geometry
• Heat release rate (HRR or )
• During the early stages of a fire, the rate of
  fire spread is determined by the heat release
  rate of a burning fuel.
• This rate is expressed in terms of kJ/second or
  kilowatts.

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FUELS

• Fuel condition and geometry
• Heat release rate (HRR or )
• An example could best be illustrated by burning
  wood shavings and a block of wood of the same
  weight.
• The wood shavings would ignite and burn much
  faster than the wooden block.

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FUELS

• Power Usage of Electrical Devices compared with
  Fire Heat Release Rates
• Item Approx. Power Usage/Peak Heat Release
  Rate
• Burning cigarette 5 W
• Standard A Light Bulbs 15 to 200 W
• Burning match 80 W
• Coffee maker, hair dryer, toaster 500 to 1500 W
  or
• 0.5 to 1.5 kW 
• Burning Coffee Maker 40 kW
• Small Trash Can, Trash Bag Fires 50 to 300 kW 
• Burning Upholstered Chair 80 kW to 2.5 MW 
• Burning Upholstered Sofa 3,000 kW or 3 MW 
• Burning Christmas Tree 1.6 MW to 5.2 MW 

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FUELS
 
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OXYGEN

• In order for a fire to occur, a rapid oxidation
  reaction must take place.
• A fuel must be present and mix with the oxygen
  and produce heat energy.
• A fire is an exothermic reaction because it
  produces heat.

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OXYGEN

• In air
• The air we breathe contains approximately 21
  oxygen.
• This is normally considered an ample amount of
  oxygen to begin reacting with the fuel.
• If the level of oxygen source drops below 15,
  the burning process slows and may self extinguish
  or smolder.
• This is called an oxygen controlled or
  ventilation limited fire and is dependent upon
  finding another source of oxygen.

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OXYGEN

• In air
• If a fuel is present in an oxygen-enriched
  atmosphere, the reaction will become more
  vigorous and the combustion process will be
  accelerated.
• This enriched environment may be as simple as a
  person blowing on a fire to an area where medical
  oxygen is present.

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OXYGEN

• Flammable Limits
• Mixtures of flammable gases or vapors with air
  will combust only when they are within particular
  ranges of concentration.
• The ratio of the gas or vapor to air is called
  its flammable or explosive limit.

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OXYGEN

• Flammable Limits
• Flammable limits are divided into 3 areas
• lower flammable limit,
• upper flammable limit and
• ideal or stoichiometric.

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OXYGEN

• Flammable Limits
• If there is more air than gas or vapor then it is
  considered being in the lower flammable limit or
  sometimes called fuel controlled.
• When there is more gas or vapor than air, then it
  is considered as being in the upper flammable
  limit or sometimes called air controlled.

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OXYGEN

• Flammable Limits
• The ideal or stoichiometric area is where the
  fuel (vapor or gas) is in balance with the air.
• It is rare to see this occur in most fires except
  in certain types of gas fires.
• Increases in temperature and pressure will reduce
  the lower limit and increase the upper limit
  making the ideal or stoichiometric area broader.

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Flammable (Explosive) Limits
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Flammable (Explosive) Limits
Increase in temperature pressure
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OXYGEN

• Flammable Limits
• In fires involving most flammable vapors and
  gases, they will be present in a given
  compartment, however it is rare for the gas or
  vapor to have the same fuel to air ratio
  throughout every area of the compartment.
• Thus when it is ignited, some areas will be too
  rich, some too lean, and others near ideal.
• In the too rich and too lean areas there will be
  less burning and evidence of the gas or vapor
  will remain.

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OXYGEN

• Flammable Limits
• In the ideal areas the investigator will be able
  to track the progression of the gas or vapor.
• Many times if not extinguished quickly the too
  rich area will receive more air from ventilation
  and ignite once a balance with the air has
  occurred.

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OXYGEN

• Other sources of oxygen
• Certain other fuels, specifically materials that
  are synthetic or a few natural chemicals, contain
  enough oxygen to maintain decomposition or even
  partial combustion in the absence of additional
  oxygen in the air.

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HEAT

• Heat is the energy possessed by a fuel due to its
  molecular activity.
• This heat energy has to exceed the minimum level
  of the fuel to release fuel vapors and cause
  ignition.
• The measurement of intensity is called is
  temperature.
• Its heating rate is measured as kilojoules.

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HEAT

• Heat Sources
• Heat is needed to start the chemical reaction.
• That heat source can be provided by a wide
  variety of means.

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HEAT

• Heat Sources
• Some heat sources are designed and intended to
  produce heat, such a stove or a heater.
• Some sources of heat for ignition may result from
  a malfunction, such as an overheated motor or
  electrical arcing.

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HEAT

• Heat Sources
• Chemical Produced as the result of rapid
  oxidation.
• Mechanical Produced by rubbing objects
  together, friction.
• Electrical Produced from overcurrent, arcing,
  shorting or other electrical malfunctions.

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HEAT

• Heat Sources
• Compressed Gas The molecular activity of a gas
  is greatly increased when it is heated.
• Natural Lightning, solar
• Nuclear Heat energy is produced when atomic
  particles are split and fused.

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Uninhibited Chemical Chain Reactions

• Description
• As a fuel is heated pyrolysis, a chemical
  decomposition of matter, occurs in the material.
• This action may take place in the absence of
  oxygen and vapors released may include both
  combustible and non-combustible gases.

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Uninhibited Chemical Chain Reactions

• Description
• Once oxygen begins mixing with these gases they
  form other chemical mixtures.
• These gases will only burn when the fuel to air
  ratio is within certain limits.
• This is a critical time in the fire process in
  that the resultant gases must be within these
  limits in order to continue.

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Uninhibited Chemical Chain Reactions

• Description
• As the process continues, the heat produced will
  continue to heat the fuel, producing more vapors.
• These vapors again chemically break down into
  smaller particles to mix with the oxygen, then
  burn, and produce heat that continues to heat the
  fuel and so on.
• This cycle will continue until all the available
  fuel and/or oxidant has been consumed or until
  the flame has been extinguished.

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Uninhibited Chemical Chain Reactions
FUEL
PYROLYSIS
Heat input
Oxidations Stops
2
1
      LIFE CYCLE OF FIRE
3
AIR
Critical Time
6
Ignition Continuity
4
Proper Proportioning
5
Mixing
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Factors (THINGS) Affecting Fire

• Ventilation
• The air/oxygen content and leakage in a structure
  will control the amount of burning until a change
  in ventilation occurs.
• This change can be from any opening that will
  allow entry and/or exit of air.
• It may be natural, mechanical, accidental or
  intentional.

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Factors (THINGS) Affecting Fire

• Ventilation
• The direction of the airflow will cause the fire
  to spread in that direction.
• Most of the time this direction is the same as
  the outside airflow that is caused by atmospheric
  conditions (wind).

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Factors Affecting Fire

• Ventilation
• There are three basic factors that control
  ventilation
• General Conditions
• Such as open/closed of doors, windows, and
  vents.
• In addition, the general condition of the
  structure, type of materials and construction
  techniques would determine the amount of air
  leakage into or out of a building.

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Factors Affecting Fire

• Ventilation
• There are three basic factors that control
  ventilation
• Conditions that the fire creates as it
  intensifies and moves through the structure.
• Many times these openings allow a fresh source of
  air to the fire and will intensify the fire in
  that area

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Factors Affecting Fire

• Ventilation
• There are three basic factors that control
  ventilation
• Fire fighters can create ventilation changes to
  the fire by creating additional openings and
  sometimes by pushing the firewith hose streams.

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Factors Affecting Fire

• Weather
• Temperature, humidity and air/oxygen content
  movement (wind) varies every day and the norms
  are different in every part of the world.
• Even the temperatures that people maintain in
  their structures are different.
• Temperature or heat energy initially starts the
  fire
• Intensifies the fire,
• Is the means by which fire spreads and,
• Produces greatest barrier to extinguishment.

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Factors Affecting Fire

• Weather
• Temperatures effect ignition temperature and
  vapor to air ratios.
• Humidity norms are different in every part of the
  world, and will effect the ignition temperature
  and the rate that the fuel burns.

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Factors Affecting Fire

• Weather
• Air movement outside verses inside determines
  fire flow through a structure unless tampered
  with.
• In normal ambient conditions the oxygen content
  is approximately 21.
• The fire scene must be evaluated to see if
  anything could affect this percentage.

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Factors Affecting Fire

• Weather
• To sustain the fire, it must have at least 15.
  Qualify ..
• Is or was anything present to make this
  percentage vary?
• Is or was there anything that increases or
  decreases air velocity?
• At what point during the progress of the fire did
  either or both of the above occur?
• Is there any substance present in the fuel that
  through decomposition or burning produces
  additional oxygen?

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Factors Affecting Fire

• Structure geometry
• Once the fire has started in a compartment, such
  as a room or building, the fires development
  will be determined by compartments configuration
  and construction.

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Factors Affecting Fire

• Structure geometry
• There are three primary effects that this will
  have on the fire
• ventilation,
• ability to retain heat,
• and the additional fuel load of the compartment.

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Factors Affecting Fire

• Human factors
• Every person has unique daily living habits that
  are a product of their environment, heritage and
  personal preferences.
• Examples are housekeeping, choice and arrangement
  of furnishings, and the interior environmental
  conditions.
• Even if there were two identical structures, the
  building habitants would have different
  furnishings in different places, and different
  configurations of doors and windows, and many
  other differences that effect the progress of a
  fire.

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Factors Affecting Fire

• Human factors
• In addition, humans will react differently to a
  fire.
• Once the fire becomes observable, people will do
  things that impact the fire development.
• Changing ventilation
• Leaving a door open when leaving
• Breaking a window
• Spreading the Fire
• Moving the fire source (pan fire, DC case,
  Christmas tree)
• Attempts to control the fire