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chemistry of fire. fire. Definition It is a chemical chain reaction which takes place with the evolution of heat and light. Fire triangle. Oxygen. Fuel. FIRE. Heat. Essentials of Fire. OXYGEN SOURCE. HEAT SOURCES. To Reach Ignition Temp .
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fire Definition It is a chemical chain reaction which takes place with the evolution of heat and light.
Oxygen Fuel FIRE Heat Essentials of Fire
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
Principles of Fire Extinction 1. Starvation Removal of un-burnt material from fire area
Principles of Fire Extinction 2. Smothering Cutting off the supply of oxygen from fire area
Principles of Fire Extinction 3. Cooling Removal of heat from the burning material/fire area
Mono-ammonium phosphate NH3 & H2O Ortho-Phosphoric Acid Meta-Phosphoric Acid Flame Steam Principles of Fire Extinction 4. Breaking of chain reaction It is a chemical process Fig: Extinguishing mechanism of multipurpose mono-ammonium phosphate dry chemical
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.
Three progressives stages of burning Incipient or beginning phase Free burning phase Smoldering phase
Oxygen Rich Atmosphere Oxygen Deficient Atmosphere
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
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
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
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.
Conduction Highly Excited Less Agitated 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.
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.
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
1. Combustion (heat & light) 2. Combustion (without heat & light) Other chemical reactions which produce 3. Combustion ( without heat)
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
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.
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.
CHEMICAL REACTIONS EXOTHERMIC REACTION When heat is liberated in chemical reaction is known as Exothermic Reaction. Ex : N2+3H2=2NH3+ H(-24Kcal) C+O2=CO2+ H (-94.5 K cal) Bond Being Broken Heat Liberated
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
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
Ex: Paper SPECIFIC SURFACE Ex: Card Board 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: Wood Block BULK FUEL : Solids with specific surface of 0.04 to 2 Sq. cm/gm. Bulk Fuel requires burning Kindling for ignition.
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.
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
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
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. < 1 floats on water • S.G. > 1 sinks in water
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. >1) Hazards • Choking, suffocation, death • Distant ignition possible
Vapor Pressure Hazards • Pressure + weakened container container breaks • Breakup can be very explosive IMPORTANT DEFINITIONS Vapour Pressure : It is the pressure exerted by the vapour of the liquid at any given temperature.
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
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.
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.
Gas orVapor 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.
