Chemistryofcombustion min

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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 Essentials of Fire Fuel FIRE Heat Oxygen OXYGEN SOURCE HEAT SOURCES To Reach Ignition Temp Approx 16% Required Normal air contains 21% of oxygen Some Fuels contains its own oxygen supply Oxygen Open Flame, the Sun,Hot Surface, Sparks & Arcs, Friction, Chemical Action, Elec Energy & Gas Compression Heat Fuel GASES Natural Gas, Propane, CO, Butane, Hydrogen, Acetylene, LIQUIDS Gasoline, Kerosene, Turpentine, Alcohol, Paint, Varnish, Olive oil, Lacquer FIRE TRIANGLE SOLIDS Coal, Wood, Paper, Cloth, Wax, Grease, Leather, Plastic, Sugar, Grain, Hay, Fire Tetrahedron Principles of Fire Extinction Starvation Removal of un-burnt material from fire area Principles of Fire Extinction Smothering Cutting off the supply of oxygen from fire area Principles of Fire Extinction Cooling Removal of heat from the burning material/fire area Principles of Fire Extinction Breaking of chain reaction It is a chemical process Mono-ammonium phosphate NH3 & H2O Ortho-Phosphoric Acid Meta-Phosphoric Acid Flame Steam 10 Fig: Extinguishing mechanism of multipurpose mono-ammonium phosphate dry chemical 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 33 SPECIFIC SURFACE Ex: Paper TINDER : Solids with specific surface of more than 20 Sq cm/gm Tinder can be ignited by match stick Ex: Card Board KINDLING : Solids with specific surface of to 20 Kindling requires a burning tinder for ignition Ex: Wood Block BULK FUEL : Solids with specific surface of 0.04 to Sq cm/gm Bulk Fuel requires burning Kindling for 34 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 35 The Fire Point is generally above the Flash 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 36 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 gm/cm3 Mercury - 13.6 gm/cm3 37 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 < floats on water S.G > sinks in water 38 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 39 IMPORTANT DEFINITIONS Vapour Pressure : It is the pressure exerted by the vapour of the liquid at any given temperature ↓ ↓ ↓ ↓ ↓ ↓ ◆ Breakup can be very explosive ↓ ↓ ↓ ↓ ↓ ↓ ◆ Pressure↑ + weakened container → container breaks ↓↓↓↓↓↓↓↓↓↓↓↓↓ Vapor Pressure Hazards 40 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 41 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 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 42 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 degree Centigrade is called the Co-efficient of Linear Expansion of the substance Ex: Steel - 0.000012 per degree Centigrade 43 IMPORTANT DEFINITIONS Co-efficient of Cubical Expansion : Solid - it is 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 Gas or volume at constant pressure or as the increase in Vapor pressure at constant volume 44 Question s 45 Any Questions 46 THE END 47
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