WHAT IS FIRE ? – DEFINITION

Fire is a chemical reaction, in which substance (fuel) combines with oxygen. The reaction is exothermic and usually associated with emission of Heat, light and smoke.

 

1. Combustion Process

An understanding of the basic principles of combustion or fire, causes and sources of ignition, fire growth and fire spread is necessary for understanding the principles of fire control and extinguishment.

Combustion usually involves an exothermic chemical reaction between a substance or fuel and oxygen. Unlike slow oxidation, a combustion reaction   occurs so rapidly that heat is generated faster than it is dissipated, causing a marked increase of temperature, even up to a few hundreds of degrees. Very often, the temperature reaches so high that visible light or flame is generated.

 

Elements of Fire:

Ü Fuel

Ü Air (Oxygen)

Ü  Heat

Ü  Chain Reaction

Fire Tetrahedron GIF Fuel, Oxygen, Heat, Chain Reaction
Fire Tetrahedron GIF

Type of Fuel:

          1. Liquid Fuels

          2. Gaseous Fuels

          3. Solid Fuels

 

But Solid and liquid can’t burn directly. It must have to vaporise and make a ideal mixture for burning with Oxygen.

Combustible Liquid / Solid    +   Heat   ------->   (at surface) Vapors            

Vapors     +    Oxygen   +      Heat     -------->        Fire

 

2-A. Triangle and tetrahedron of fire.

One way of discussing fire or combustion is in terms of the triangle of fire or combustion. It has been seen that for combustion to occur three factors in proper proportion are essential; Energy (heat, Pressure, Chemical Reaction), oxygen (or air) and a fuel. Fire or combustion will continue as long as these three factors are present. Removal of one of them leads to the collapse of the triangle and the combustion process stops.

Triangle of Fire presents the Smouldering mode of fire or combustion.

         

Fire scientists have now found that there is a fourth constituent in all flaming fires which play a vital part in the fire growth and sustenance. This is the unbroken or uninhibited chain reaction. Thus, as per modern concept, each of its four sides representing one of the four basic requirements: fuel, Energy, oxygen and unbroken or uninhibited chain reaction. This last factor comes into play only in flaming mode of combustion which is normally applicable in the case of some Combustiblesolids, inflammable liquids and flammable gases.


Nature of flame

A flame is the cloud of free radicals.

The burning of most materials produces a flame. A flame front stemming from a local ignition source is established in a flammable medium. A form of chemical reaction is set-up in the layer of gas adjacent to this source with the result that heat and what are called chain carriers. Pass into the next layer of gas and continue the cycle of the operations there, rather like runners in a relay race. Chain carriers are believed to be atoms or part of molecules, known as free radicals and these are extremely reactive Combustion, therefore, is a type of chain-reaction.


ØThe flame temperature is very important because the rate of a key combustion reaction (H+O2 = OH+O) is very sensitive to temperature.

ØA small decrease in temperature causes a disproportionately large decrease in the rate of the reaction.

ØA single H atom, when introduced into an H2-O2 mixture at an elevated temperature will be transformed in a fraction of a millisecond to form 2 molecules of H2O and 3 new H atoms.

ØEach of these H atoms can immediately initiate the same sequence, resulting in a branching chain reaction, which continues until the reactants are consumed. The remaining H, O and OH species recombine according to the reaction.

H+O = OH and H+OH = H2O

 

Similar chain reactions occur in flames of any H containing fuel. H is present in the vast majority of combustibles except for metals and pure carbon. H atoms or other active species (radicals) may also be removed from the flame by purely chemical means, which is by an extinguishing agent capable of chemical inhibition

2-C. THE EFFECTS OF FIRE

A burning substance produces a number of chemical reactions. These reactions produce flames, heat, smoke, and number of gases and other combustion products. The gases and combustion products will reduce the amount of oxygen available for breathing. All of these effects are vitally important for fire fighter. Fire fighter must be prepared to protect them self against Effect of fire.


FLAME, HEAT, AND SMOKE

            Personnel must be protected from the flames, heat, and smoke to avoid injuries or loss of life. Before you enter a compartment or area where there is a fire, you need to be dressed out properly. You must tuck your pants into your socks, button the collar on your shirt, and put on a helmet. Wear any other protective clothing prescribed by current directives. If you are a Branch man, you will also need to wear protective gloves and an breathing apparatus (BA). The flames and heat from a fire can be intense. However, if you are dressed out properly and maintain adequate distance, you can minimize your chances of getting burned. The smoke will make it hard to see and breathe. However, you can cope with these problems by wearing a BA and a headlamp.


3. CLASSIFICATION OF FIRE 

as per BIS-15683-2006Para No. 3


Fire Extinguishment - Principles

•       Starvation           - By eliminating the Combustible Fuel.

•       Smothering         - By cutting off the Oxygen   Supply.

•       De energizes        - By cooling to a point below Ignition Temperature.

•       Inhibition of Chain Reaction - Breaking the Chain Reaction

 

STARVATION

•       Combustion will cease, if un-burnt fuel is removed by physical or mechanical means.

•       Electrical equipment- Electrical current supply cut off.

•       Gas Fires -stop the flow of gas.

•       Storage Tanks – Transfer to another tank. 

•       Water curtain-blocks the passage of heat to the fuel. 

SMOTHERING

•       Oxygen is removed or prevented from reaching the combustible material.

•       This can be done by-

o    blanketing

o    diluted by an inert gas

o    dispersion by non-combustible powder

o    blanket of foam

De energizes (COOLING)

•       Heat is removed by cooling with water.

•       Cooling of the burning material reduces the Temperature of material.

•       If the temperature of material is below Ignition Temperature combustion ceases.

INHIBITION OF CHAIN REACTION

•       This can be achieved by some vaporizing liquids or dry chemical powder.

•       DCP captures these free radicals and slow down chain carriers. Due to breaking the chain reaction fire will no more.