why co2 is used as extinguishing media ???

CO₂ is used because it is denser than "mixed" air, while N₂ is slightly less dense, so the CO₂ tends to form a blanket over the seat of the fire, excluding the O₂.

Here is a useful site for looking up the properties of gasses.

Besides the fact that N₂ is not very effective, there's the safety concern. CO₂ excess causes firemen to breath fast and deep; N₂ excess causes firemen to quit breathing.

In addition to the above, CO₂ is readily stored as a liquid at reasonable pressures and ambient temperatures, vaporising readily when squirted at a fire. This property means that the vessel within which it is stored is quite small and therefore easily carried. Further, the vapour produced during the squirt will be rather cold, more dense than the surrounding air and will smother the flames.

Nitrogen will not readily liquefy at ambient temperatures, so any small vessel carrying enough material to extinguish a fire will be so unweildy that the containing vessel will be far more massive, and therefore far too heavy to lift for the mass of extinguishing material stored. Further, the vapour produced from a nozzle, though cold, would mix readily with the surrounding air, making smothering the flames less effective. Further, the better mix of nitrogen with air maxes it more hazardous for personnel that may be close to the fire, including the operator of the nozzle.

So, although nitrogen is also an effective fire extinguishant, CO2 is a more practical selection for most applications, and probably safer for the user.

What an interesting question!

And here I thought CO₂ had been banned as an extinguisher.

There was a time when fire fighters used what I thought was CO₂ foam to inhibit fires in aircraft who'd experienced gear failure and had to belly in.

My understanding was that the foam prevented sparks and kept the aluminum skin cool while dragging along the runway. And, as it evaporated to a gas, it smothered the fuel of oxygen needed to support combustion.

I recall that they stopped using it because of the increased risk of suffocating passengers.

I'm confused

L.J.

I don't know about CO₂ usage at airports, but usually the foam they use is a synthetic film-forming foam that blankets the jet fuel.