Cooling, Starvation, Smothering, Chain breaking mechanism With Water Merits And Demerits full explained (chapter-2 Unit-1)

Fire Extinguishing Modes

Fire can be extinguished by employing one or more of the following techniques.

1. Cooling:- This means removal of heat this can be achieved by the use of water. Generally 'A' class fire can be extinguished by this method.

1. Starvation:- Removal of fuel. this can be achieved by cutting out the fuel supply to the fire. This can be achieved by physical removal of fuel e.g. pumping out the fuel from the tank on fire or isolating the supply valve etc.

1. Smothering:- Removal of oxygen (air) or diluting the same to blow the minimum requirement. This can be achieved by by applying an inert gas such as CO2 or nitrogen or dry chemical powder or halons etc.

1. Chain breaking mechanism:- This can be achieved by the application of halogenated hydrocarbons or dry chemical powder which distribute the chain reaction between free free radicals of fuel and atmospheric air.

Water:-

Merits:- 

• it is a good solvent. It dissolves and washes many products of combustion such as ash so that the the seat of the fire can be reached.

• it's viscosity change with temperature is small. It can be easily pumped and conducted in horse and pipes with ease.

• it has high latent heat. The latent heat of vaporization of water is 540 calories per gram. Hence it can absorb tremendous amount of heat when converting from boiling liquid to steam.

• its expansion with heat is high. It expands 1600 times at 1000 C (steam). Thus It is a very good smothering agent.

• it has high surface tension:- the high surface tension (72 dynes per centimetre) enable it to issue in a consolidated stream from the so called 'fog' nozzle or spraying device.

• its density is reasonably high. The stream from a nozzle serves penetration purpose.

• it has high molecular stability. It does not break down or dissociate to any appreciable amount and temperature approaching 16500 C which are lighter than the ordinary flame temperature.

• it is cheap and abundantly available.

Demerits:- 

• freezing:- it freezes at sub zero temperatures. Thus cannot be pumped and used in such situations.

• high surface tension:- it restricts water from penetrating and wetting various tightly packed combustible where this action is vitally needed for Deep seated fire extinction.

• low viscosity:- it runs off quickly from non horizontal surfaces living a very thin coating of heat removing liquid.

• high friction:- it gives high friction loss in its transportation through pipes and hoses. Thus the pressure drops while passing through pipes.

• high density:- it readily sinks into most liquid fuels without providing the expected cooling.

• good conductor of electricity:- it cannot be used for live electrical fires.

• metal reactive:- it reacts with several metals such as magnesium, sodium and Aluminium with the production of hydrogen and evolution of heat. Hence it cannot be used on metal fires.