chemistry of fire

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chemistry of fire
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fire

• Definition
• It is a chemical chain reaction which takes
  place with the evolution of heat and light.

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Fire triangle
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• Essentials of Fire

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OXYGEN SOURCE
HEAT SOURCES
To Reach Ignition Temp. Open Flame, the
Sun,Hot Surface, Sparks Arcs, Friction,
Chemical Action, Elec. Energy Gas Compression
Approx. 16 Required Normal air contains 21 of
oxygen Some Fuels contains its own oxygen supply
Oxygen
Heat
Fuel
GASES
LIQUIDS
SOLIDS
Natural Gas, Propane, CO, Butane, Hydrogen,
Acetylene,
Gasoline, Kerosene, Turpentine, Alcohol, Paint,
Varnish, Olive oil, Lacquer
Coal, Wood, Paper, Cloth, Wax, Grease, Leather,
Plastic, Sugar, Grain, Hay,
FIRE TRIANGLE
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Fire Tetrahedron
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• Principles of Fire Extinction
• 1. Starvation
• Removal of un-burnt material from fire area

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• Principles of Fire Extinction
• 2. Smothering
• Cutting off the supply of oxygen from fire area

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• Principles of Fire Extinction
• 3. Cooling
• Removal of heat from the burning material/fire
  area

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• Principles of Fire Extinction
• 4. Breaking of chain reaction
• It is a chemical process

Fig Extinguishing mechanism of multipurpose
mono-ammonium phosphate dry chemical
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Phases of Burning

• It depends on the following factors
• i. The amount of time the fire has
  burnt.
• ii. The ventilation characteristics of the
• confining structure
• iii. The amount and type of combustibles
• present.

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Three progressives stages of burning

• Incipient or beginning phase
• Free burning phase
• Smoldering phase

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Oxygen Rich Atmosphere
Oxygen Deficient Atmosphere
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Incipient or beginning phase

• Oxygen plentiful
• Temperature has not built up to high peak
• Thermal updraft rises,
• accumulates at highest point
• Breathing not difficult
• Fire extinguishments
• Direct application of water
• at base of fire.
• Ventilation not a problem
• Little steam production

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Free burning phase

• Fire has involved more fuel
• Oxygen supply is being depleted
• Heat accumulated at upper areas
• Breathing difficult Masks recommended
• Fire extinguishments is reaching
• the area of major involvement
• Ventilation Not a definite need
• Good steam production

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Smoldering phase

• Oxygen supply not equal to demands of fire
• Temperature throughout building is very high
• Normal breathing is not possible
• Oxygen deficiency may cause back-draft
• Fire extinguishments indirect method
• Ventilation A must
• Maximum steam production from water fog

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Smoldering phase
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Modes of spread of fire
Conduction It occurs only in solids i.e.
metallic objects. Convection It occurs both in
liquids gases Radiation It is neither
conduction nor convection. These are heated rays
emanating from the hot object.
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Conduction

• Heat energy is passed on
• from one molecule to the
• next
• The molecule vibrates
• above its mean position
• Pass on heat energy by
• colliding with their
• neighbors
• The ability to conduct
• heat varies between
• materials.

Highly Excited
Less Agitated
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Conduction
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conduction
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Convection

• When a liquid or gas heated,
• it expands
• It becomes less dense
• The lighter fluid arises being
• displaced by colder and
• therefore denser fluid
• In turn becomes heated and
• so a circulation is set up
• Heat energy is carried
• throughout the fluid by
• actual movements of
• molecules.

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Fig Fire Plume
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Radiation

• It does not involved
• any contact between
• bodies
• It is independent of
• any material in the
• intervening space
• These are the heated
• rays, travels in
• straight line in all
• directions with same
• intensity

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• Other chemical reactions which produce

1. Combustion (heat light)
2. Combustion (without heat light)
3. Combustion ( without heat)
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Rusting of iron
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Types of combustion

• Slow or incipient combustion- in which the amount
  of heat light emitted is feeble.
• ii. Rapid or active combustion- in which a
  considerable amount of heat light is emitted
  within a short time

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Types of combustion

• iii. Deflagration - which takes place with a
  considerable rapidity, evolving heat light
• iv. Explosion - is a very rapid combustion with
  a loud noise within an extremely short time with
  generation of very high pressure temperature.

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Rate of Combustion

• A rate of combustion or the spread of fire would
  depends on
• The area of solid/liquid in contact with air.
• The amount of heat generated to raise the
• temperature of un-burnt portion.
• The ability of materials to conduct heat away.
• Atmospheric humidity.
• Wind velocity.
• Temperature.
• Atmospheric pressure.

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CHEMICAL REACTIONS

• EXOTHERMIC REACTION
• When heat is liberated in chemical reaction is
  known as Exothermic Reaction.
• Ex
• N23H22NH3 H(-24Kcal)
• CO2CO2 H (-94.5 K cal)

Bond Being Broken
Heat Liberated
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CHEMICAL REACTIONS

• ENDOTHERMIC REACTION
• When heat is absorbed from the reacting
  substances is known as Endothermic Reaction.
• Ex
• 2C H2 C2H2 H (54 Kcal)
• 3O2 2O3 H (69 Kcal)

Bond Being Formed
Heat Absorbed
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SPECIFIC SURFACE

• It is the surface area in square centimeter per
  gram of that solid substance.
• Liquids gases have no specific surface, these
  two take the shape of container.
• On the basis of specific surface and fire
  susceptibility, all combustible solids classified
  into three
• Tinder
• Kindling
• Bulk fuel

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SPECIFIC SURFACE
Ex Paper

• TINDER
• Solids with specific surface of more than 20 Sq.
  cm/gm.
• Tinder can be ignited by match stick.
• KINDLING
• Solids with specific surface of 2 to 20 Sq.cm/gm.
• Kindling requires a burning tinder for ignition.

Ex Card Board
Ex Wood Block
BULK FUEL Solids with specific surface of
0.04 to 2 Sq. cm/gm. Bulk Fuel requires burning
Kindling for ignition.
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IMPORTANT DEFINITIONS

• Flash Point It is the lowest temperature at
  which an inflammable substance gives off
  sufficient vapours, so as to form a momentary
  flash on application of a pilot flame.
• Fire Point It is the lowest temperature at
  which the heat from the combustion of burning
  vapours is capable of producing sufficient
  vapours to enable combustion to continue.
• The Fire Point is generally above the Flash Point.

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IMPORTANT DEFINITIONS

• Ignition Temperature It is the lowest
  temperature at which spontaneous combustion can
  takes place without application of an external
  heat.
• Auto Ignition Point Refers to the temperature
  to which a substance must reach, before it
  ignite, in the absence of flame, but in presence
  of air.
• Spontaneous Combustion It occurs as a result of
  heat generated by the reacting substances without
  any external heat.
• e.g. Hot glycerine Potassium Permagnate

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IMPORTANT DEFINITIONS

• Density
• The Density of substance is its mass per unit
  volume. Unit of Density(D) is Kg/m3 or gms/cm3
• Ex
• Water - 1000 Kg/m3 or 1 gm/cm3
• Mercury - 13.6 gm/cm3

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IMPORTANT DEFINITIONS

• Relative Density or Specific Gravity
• It is a ratio of the mass of any volume of a
  substance to the mass of an equal volume of
  water.

Specific gravity, or relative density Material
density to the density of water S.G. lt 1 floats
on water S.G. gt 1 sinks in water
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IMPORTANT DEFINITIONS

• Vapour Density It is the ratio of the mass of
  a given volume of the vapour to the mass of an
  equal volume of air under the same temperature
  pressure.

• Dense Vapor (V.D. gt1) Hazards
• Choking, suffocation, death
• Distant ignition possible

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IMPORTANT DEFINITIONS

• Vapour Pressure It is the pressure exerted by
  the vapour of the liquid at any given temperature.

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IMPORTANT DEFINITIONS

• Latent Heat It is the amount of heat energy
  required to change the state of a matter without
  raising the temperature.
• Latent Heat of Vapourization It is the heat
  energy which is absorbed by the liquid at its
  boiling point, to convert from its liquid state
  to gaseous state, without raising the
  temperature.
• Ex Water - 2260000 Joules/Kg
• Latent Heat of Fusion It is the heat energy
  which is required to change the state from solid
  to liquid at melting point of substance without
  raising the temperature.
• Ex Water - 336000 Joules/Kg

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IMPORTANT DEFINITIONS

• Thermal Capacity The thermal capacity or heat
  capacity of a body is the heat required to raise
  its temperature by 1 degree Centigrade.
• Ex Water - 4.2 KJ/Kg/degree Centrigrade
• Heat Energy can be transferred from a place of
  higher temperature to one at lower temperature.
  When heat is added to a body the temperature
  rises.
• Specific Heat It is the ratio of thermal
  capacity of a substance to that of water.
• Material with low specific heat will heat up
  more rapidly in fire conditions.

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IMPORTANT DEFINITIONS

• Thermal Expansion of Solids
• When a solid is heated, it expands in length,
  breadth thickness.
• Solids which are homogeneous expand uniformly.
• Co-efficient of Linear Expansion
• The amount with which unit length of substance
  expands when its temperature is raised by 1
  degree Centigrade is called the Co-efficient of
  Linear Expansion of the substance.
• Ex Steel - 0.000012 per degree Centigrade.

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IMPORTANT DEFINITIONS

• Co-efficient of Cubical Expansion
• Solid - it is 3 times the co-efficient of linear
  expansion.
• Liquid - only the co-efficient of volume
  expansion is applicable.
• Gases - can be measured either as an increase in
  volume at constant pressure or as the increase in
  pressure at constant volume.

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Any Questions
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Any Questions
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THE END