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Heat and Flammability


Heat is a form of energy, It is measured in calories or B. T. u’s.

Units of measurement: Calorie

The amount of heat required to raise the temperature of one gram of water by one degree centigrade is equal to one calorie:

BTU (British-thermal Unit):

The amount of heat requires to the temperature of one pound of water by one dogu Fahrenheit is equal to one B. T. U. c.

Effects of heat:

  • It raises temperature.
  • It changes color. Heat expands all matter.
  • It provides thermocouple effect.
  • It changes the physical state of matter.


The degree of hotness or coldness is called temperature.

Units of Measurement:

  • Celsius (°C)
  • Fahrenheit (°F)
  • Kelvin (K)

Relation among °C, °F and K:

°C = (F – 32) 5/9

°F = Cx 9/5+ 32

K= C- 273.

Specific Heat:

Imagine a stool of steel and a stool of wood are lying in Sun for a few hours. the steel stool gets too hot as compared to the wooden stool why? This is because of the difference in the specific heat values of steel and that of woods requires less heat to raise its temperature to a certain degree, than the wood. Specific heat is the amount of heat require to rise. The temperature of one gram of any substance by one degree centigrade.

Latent Heat:

When substance is heated its physical state changes at a certain temperature, from solid to liquid and liquid to gas (vapour). During the period of time the change of state is taking place the temperature remains-constant. for example water begins to change to vapour or gas at 100°C. This is the boiling point of water. This temperature remains constant at 100°C till the entire quantity of water is converted to vapour. The heat given to water during this period of time is hidden as there is no rise in temperature. Hence the quantity of heat given to each gram of water to; convert it from liquid state to vapour state called as the Latent Heat of vaporization of water. This is approximately540 calories per gram of water.



It is the ability of a combustible or flammable matter to burn which is called the flammability or combustibility of that matter. Wood, paper, textiles, kerosene, petrol, LPG, Acetylene will catch fire when heated sufficiently in the presence of sufficient air or oxygen. Hence, these are known as flammable or combustible. On the other hand, water, sand, iron etc. do not burn when heated. Hence these are called Non – Flammable or Incombustible.

Flammable and in combustible:

The word inflammable is not the antonym (opposite) of it means the same. Whereas the word in combustible is the antony (opposite) of the word combustible. Flammable Mixture: The mixture of flammable vapour and air which can catch fire on the application of a source of ignition is called “Flammable Mixture”.

Limits of Flammability:

All mixtures of flammable vapour and air do not necessarily catch fire even in the presence of a source of ignition. If the percentage of flammable vapour less than the minimum required, the mixture is Non-Flammable. Similarly if the percentage of flammable vapour 1 air more than the maximum, the mixture turns Non-Flammable, Hence there are two limits of flammability: Lower limit of flammability & Upper limit of flammability.

Lower limit of flammability:

The minimum percentage of flammable vapour in air which can catch fire called as the lower limit of flammability of that fuel.

Upper limit of flammability:

The maximum percentage of flammable vapour in air which can catch fire in the presence of a source of ignition called as the Upper Limit of Flammability of that fuel. These limits vary for different fuels.

Lean Flammable Mixture:

The mixtures of flammable vapour and air containing the concentration of flammable vapour close to the Lower or Upper limits of flammability are “L ean Flammable Mixture”. the meaning of Lean here is Weak.

Lean and Rich flammable limit

Rich Flammable Mixture:

The mixture of flammable vapour and air containing the concentration of flammable vapour dose to the mean (average) value of Lower and Upper limit values called as “Rich Flammable Mixture” the meaning of Rich here is strong.

In combustibility:

The substances which do not burn when heated up to a temperature of 750°Care called incombustible.

Transmission of Heat:

Most fires start propagates and extinguish due to the transfer of heat from source to the fuel or its removal from the heat. There are three ways by which the heat travels. They are:


In conduction heat travels through the molecules of the substance without the movement of the molecules themselves. Solids generally heated in this way Good or Bad conductors of heat. The ability to conduct heat varies from material. There are materials such as iron rod. Copper wire which conducts heat with much ease. Known as good conductors of heat.


Only liquids and gases use this way for transfer of heat. Liquids and gases are termed as “Fluids” also. When the fluid heated, the heated molecules of fuels expand and hence become light and move upward. To take their place the cold molecules come in. These too move up to make place for the cold molecules. This process continues in circulation till a uniform temperature reached throughout the fluid. This way of heat transfer by the actual movement of the molecules called as Convection. In fire situation in’ a building convection currents can convey hot gases, produced by combustion, upwards through stairways open lift shafts etc., thereby spreading fire to the upper floors. In turn fresh air drawn ill through doors or windows which help fire to burn vigorously.


In this way beat travels without any help of the medium. Radiated approximately. It travels in straight lines and known as radiant heat. This process heat transfer called as “Radiation”.

Flash Point:

The minimum temperature, at which the rate of vaporization of fuel is just sufficient to produce a momentary flash upon the application of ignition source and in the presence of air, known as Flash Point.

Fire Point:

The minimum temperature at which ‘the heat radiated back from the burning vapour is capable of producing sufficient vapour to enable combustion to continue.

Ignition Temperature:

The minimum temperature at which a fuel will ignite without the help of ignition source. It is also called Spontaneous Ignition Temperature or Auto Ignition Temperature.