Water requirement to extinguish fire ?

[davekardle]

My job is to come up with the water requirement for a supposed
tire fire. Given tyre caloric value 2800KJ/kg
and for water Cp= 4200J/Kg.K and Latent heat: 2260 KJ/kg.
Tyre weight: 5.6KG
Stockpile containing: Half million tyres.


I came up with some ridiculous high numbers but I think they are all wrong.
Do I have all the figures I need to calculate it or is it anything missing?

 

[phinds]

This is just a side issue but I think a firefighter would find the problem statement to be absurd. What would matter is HOW the water is delivered. If a solid high-pressure stream, you've got one situation but if a fine mist over the whole area, quite a different one.

 

[256bits]

You could with calculations determine the minimum amount of water to extinguish a fire.

Fire burns on the outside surface where the temperature has become high enough for vaporization of the fuel. In liquid or solid form the fuel does not burn ( in most cases )

Water acts to lower the temperature of the fuel so as to limit vaporization ( and at times the steam can also deprive the fire of oxygen and will smother the fire but that is not the objective with water). A lot of the water used will be to keep the rest of the fuel cool so that the fire hot spot does not spread and become larger.

some of the water directed at a hot spot will not reach it - ie for example debris may be in the way causing a deflection.

If you have two candles, a birthday candle and a taller dining table candle which one is harder to put out. Does the amount of candle in solid form bear in how hard the candle fire is to put out?

there is no way to tell exactly how much water will be needed - just have lots of water available.

 

[davekardle]

You could with calculations determine the minimum amount of water to extinguish a fire.

Fire burns on the outside surface where the temperature has become high enough for vaporization of the fuel. In liquid or solid form the fuel does not burn ( in most cases )

Water acts to lower the temperature of the fuel so as to limit vaporization ( and at times the steam can also deprive the fire of oxygen and will smother the fire but that is not the objective with water). A lot of the water used will be to keep the rest of the fuel cool so that the fire hot spot does not spread and become larger.

some of the water directed at a hot spot will not reach it - ie for example debris may be in the way causing a deflection.

If you have two candles, a birthday candle and a taller dining table candle which one is harder to put out. Does the amount of candle in solid form bear in how hard the candle fire is to put out?

there is no way to tell exactly how much water will be needed - just have lots of water available.

I've researched Some real tire fire cases ( having over 1/2 million of tires) and saw that the fire brigade claim that using water isn't feasible as millions of cubic meter would be necessary so they just leave the tires burning. The thing is I don't know what to do with these figures my lecturers gave me. ( Calorific values of tyres and specific heat/latent heat of water). Any ideas ?

 

[ChaseRLewis]

I've researched Some real tire fire cases ( having over 1/2 million of tires) and saw that the fire brigade claim that using water isn't feasible as millions of cubic meter would be necessary so they just leave the tires burning. The thing is I don't know what to do with these figures my lecturers gave me. ( Calorific values of tyres and specific heat/latent heat of water). Any ideas ?

This isn't simple, a lower limit might be able to be calculated fairly easily. Essentially if he's giving you a specific heat / latent heat value he's probably wanting you to get the temperature of the tires below the self-ignition temperature of the tires which is ~400 C. Also tires have a high heat capacity, so you probably going to take a lot of water.

I'd figure out an approximate temp for the fire and then use an energy balance to calculate how much water would be needed to sap the energy from the tires that I want to get out of them. (Test is the estimated temp average temp of the tires)

mass of water*specific heat water*(100 C - 25 C) + latent heat of vaporization of water * kg of water - tiremass*heat capacity of tires*(Test - 400) = 0

Also this assumes you are saturating the entire surface area of every tire so no additional reaction takes place. If the reaction continues that adds heat to the energy balance that needs to be taken into account.

assuming water starts at 25 C it will heat to 100 C then vaporize, that should give you a lower bound. Test is the average temperature of the tires, just make some assumption based on the estimated temperature of the fire.

This could be made much, much, much, more complicated but any thought into a more exact answer is impractical imo.

 

[SteamKing]

With half a million tires burning, it's going to take a lot of water (thousands of tons by my back of the envelope calculations).

There may be more effective methods of controlling and ultimately extinguishing a tire fire. IMO, and having no experience as a firefighter, such a fire would be more effectively fought by some type of smothering to remove the oxygen from feeding the flames. This could be as simple as throwing sand over the tires or using certain types of AFFF (aqueous film forming foam).

The US EPA studies suggest that a water supply of 1000-2000 gallons per minute be available for at least 6 hours when fighting a tire fire, depending on the size of the tire stockpile, which is roughly equivalent to a supply of 1500-2000 tonnes of water.