Firefighting

Fire fighting is the knowledge and techniques required to extinguish a fire.

Historically, fire scientists created a graphical representation detailing the three elements fire needs to start (fire triangle). However, in recent years, one more point has been added to the triangle, creating the fire tetrahedron. This fourth point represents a chemical chain reaction needed to sustain fire. The three elements needed for the initial start of combustion (a kind of oxidation) are:

combustible matter (fuel): paper, wood, gasoline, oil, gas, etc.;
a combustive agent, i.e. an oxidant, usually the oxygen of the air;
activation energy: heat, spark (electricity), etc.

(See also Combustion). To extinguish a fire, it is necessary to remove at least the fuel or the combustive agent (once it is started, the fire produces its own energy). Once the fire is suppressed, it is necessary to reduce the temperature of the surrounding objects so the fire does not start again.

Risks of a fire

The first risk that comes to the mind is the heat. Even if a person is not "in" the flames, he/she can be burned by the infrared radiations, the contact with a hot object, by the hot gases (heated air, but particularly water vapour produced by the spraying), and by the smoke (hot particles), which are indeed the most dangerous thing. The firefighters are equipped with personal protective equipment (PPE) that includes fire-resistant clothing and helmets that slow down the diffusion of the heat towards the skin.

The primary risk to people in a fire is inhalation (breathing). A vast majority of the victims in fires die from smoke inhalation, not from burns. These risks include :

fire using up the oxygen from the air, so there is no more oxygen to breathe;
the fire producing poisonous gases;
the fire producing smoke (fine particles) that can burn inside the lungs.

For example, the plastics inside a car can generate 200,000 m³ of smoke at a rate of 20 to 30 cubic meters per second. For this reason, firefighters carry breathing apparatus (SCBA).

The heat can make pressurised gas cylinders and tanks explode or BLEVE, as well as some chemical products such as ammonium nitrate fertilizers. These explosions can cause physical trauma or potentially serious injuries caused by shrapnel, or from the blast.

There are two additional risks inside a building:

the vision can be obscured by the smoke: you can not see where you go and can thus fall down or get lost;
the building can collapse.

Means to extinguish a fire

Suppressing the fuel and the energy

The first method is to remove fuel for the fire such as cutting of the gas and moving combustible objects from the path of the fire. When the activation energy is still present, it is also useful to switch it off; this will not stop a fire, but will help in controlling a starting fire and will prevent a new fire from occuring.

The first action is thus to "cut off the energies" such as the gas and power supply and switch off the working machines (motors). It is also important to turn off ventilation and air conditioning that can dangerously change the behaviour of the fire.

Reconnaissance and reading the fire

The first step of the operations is a reconnaissance to search for the origin of the fire (which is not obvious for an inside fire, especially when there are no witnesses), and spot the specific risks and the possible casualties. Any fire occurring outside may not require reconnaissance; on the opposite, a fire in a cellar or an underground car park may require a long reconnaissance to spot the seat of the fire with only a few centimeters of visibility.

The "reading" of the fire is the analysis by the firefighters of the forewarnings of a thermal accident (flashover, backdraft, smoke explosion), which is performed during the reconnaissance and the fire suppression maneuvers. The main signs are:

the research of the hot zones with the hand (protected by a glove!), especially touching a door before opening it;
the presence of soot on the windows, which usually means the combustion is incomplete and thus there is a "lack" in combustive agent (air);
the smoke goes in and out from the door frame, as if the fire breathes, which usually means a "lack" in combustive agent (air);
spraying water on the ceiling with a short pulse of a diffused spray (e.g. cone with an opening angle of 60°) to test the heat of the smoke;
- when the temperature is moderate, the water falls down in drops with a sound of rain;
- when the temperature is high, it vaporises with a hiss.

Use of water

Often, the main way to extinguish a fire is using water. The water has two roles:

in contact with the fire, it vaporizes, and this vapour pushes the oxygen away (the volume of water vapour is 1,700 times bigger than liquid water); the fire has no combustive agent anymore;
the vapourisation of water absorbs the heat; it cools the smoke, air, walls, objects, etc. and prevents an extension of the fire.

The extinction is thus a combination of "asphyxia" and cooling. The flame itself is suppressed by asphyxia, but the cooling is the most important element to master a fire in a closed area.

Open air fire

Outside, the seat of the fire is sprayed with a straight spray: the cooling effect follows directly the "asphyxia" by vapor, it thus reduces the amount of water required. A straight spray is used so the water arrives massively to the seat is is not vapourised before. The spray may also have a mechanical effect: it can disperse the combustible product and thus prevent the fire to start again.

The fire is always fed with air, but the risk is limited for the people, except for wildfires or bushfires where they can be surrounded by the flames. But there might be a big risk of expansion.

It is a surface (object) that is sprayed; for this reason, the strategy is sometimes called two-dimensional attack or 2D attack.

It might be necessary to protect specific devices (house, gas tank) against infrared radiation, and thus to use a diffused spray between the fire and the object.

Breathing apparatus is often required as there is still the risk of breathing in smoke or poisonous gases.

Closed volume fire

Until the 1970s, the fires were attacked while they declined, so the same strategy as the open air fires was effective. In recent times, fires are now attacked in their development phase as:

the firefighters arrive sooner;
thermal insulation of the houses confines the heat;
modern materials, especially the polymers, produce a lot more heat than traditional materials (wood, plaster, stone, bricks, etc.).

Additionally, in these conditions, there is a greater risk of backdraft and of flashover.

The direct spray of the fire seat with a straight spray can have dramatic consequences: the water pushes air in front of it, so the fire is overfed before the water reaches it. This activation of the fire, and the mixing of the gases produced by the water flow, can create a flashover.

The most important is not the flames, but the control of the fire, i.e. the cooling of the smokes that can spread and start distant fires, and that endanger the life of the firefighters and casualties. The volume must be cooled before the seat is treated. This strategy is thus called three-dimensional attack, or 3D attack.

The first who proposed the use of a diffused spray was Chief Lloyd Layman of Parkersburg, West Virginia Fire Department, at the Fire Department Instructor's Conference (FDIC) in 1950 held in Memphis, Tennessee.

The ceiling is first sprayed with short pulses of a diffused spray:

it cools the smoke, thus the smoke is less likely to start a fire when it moves away;
the pressure of the gas drops when it cools (law of the ideal gases), thus it also reduces the mobility of the smoke and avoids a "backfire" of water vapour;
it creates an inert "water vapour sky", and thus prevent the roll-over (rolls of flames on the ceiling created by the burning of the hot gases).

It is necessary to make only short pulses, otherwise the spraying modifies the equilibrium (gas stratification) and mixes the gases: the hot gases (initially at the ceiling) move around the room and the temperature rises at the ground, which is dangerous for firefighters. An alternative is to cool all the atmosphere by spraying the whole atmosphere, e.g. drawing letters in the air ("pencilling").

The modern methods for an urban fire dictate the use of a massive initial water flow, e.g. 500 L/min for each fire hose. The aim is to absorb the maximum of heat at the beginning to stop the expansion of the sinister, and to produce a contraction of the smoke. When the flow is too small, the cooling is not sufficient, and the vapour that is produced can burn the firefighters (the drop of pressure is too small and the vapor is pushed back). Although it may seem paradoxical, the use of a strong flow with an efficient fire hose and an efficient strategy (diffused sprayed, small droplets) requires a smaller amount of water: once the temperature is lowered, only a limited amount of water is necessary to suppress the fire seat with a straight spray. For a living room of 50 m² (60 square yards), the required amount of water is estimated to 60 L (15 gallons).

French fire-fighters used an alternative method in the 1970s: they sprayed water on the hot walls to create a water vapour atmosphere and asphyxiate the fire. This method is no longer used because it was risky: the pressure created pushed the hot gases and vapour towards the firefighters, causing severe burns, and pushed the hot gases in other rooms where it could start a new fire.

Asphyxiating a fire

In some cases, the use of water is undesirable:

some chemical products react with water and produce poisonous gases, or even burn in contact with water (e.g. sodium);
some products float on water, e.g. hydrocarbon (gasoline, oil, alcohol, etc.); a burning layer can then spread and extend;
in case of a pressurised gas tank, it is necessary to avoid heat shocks that may damage the tank: the resulting decompression may give a BLEVE.

It is then necessary to asphyxiate the fire. This can be done by two methods:

some chemical products react with the fuel and stop the combustion;
a layer of water-based foam is projected on the product by the fire hose, to separate the fuel from the combustion agentin the air (usually oxygen).

Ventilation or isolation of the fire

One of the main risks of a fire is the smoke: it carries heat and poisonous gases, and obscures vision. In the case of a fire in a closed location (building), two different strategies may be used: the isolation of the fire, or positive pressure ventilation.

The isolation, or anti-ventilation, consists of closing all the openings to prevent the air from coming in and the smoke from going out. As the smoke is confined, this makes rescue operations easier, and prevents the extension of the fire. But this also confines the heat and the gases produced by pyrolysis, giving a risk of backdraft if ever some air gets in, e.g. when opening a door to spray the fire.

The positive pressure ventilation (PPV) consists of using a fan to create excessive pressure in a part of the building; this pressure will push the smoke and the heat away, and thus secure the rescue and fire fighting operations. It is necessary to have an exit for the smoke, to know the building very well to predict where the smoke will go, and to wedge the doors so they will stay opened and will not slam. The main risk of this method is to activate the fire, or even to create a flashover, e.g. if the smoke and the heat accumulate in a dead end.

Individual action

A starting fire is easy to extinguish: a thimbleful of water can extinguish a match, a bucket of water can extinguish a fire created by a match after one minute; but after a few minutes, tons of water are required. It is thus important to know how to fight a starting fire, but also to know that once it is started, the most effective action is to warn people to evacuate the building (if necessary) and call for help; any other action would be dangerous and harmful as it would delay the evacuation and the arrival of the firefighters.

In case of a fire starting on your house:

Fire of a pan or deep fryer:
1. cut of the gas or the power supply;
2. cover the pan or the fryer with a lid;
3. place the cloth around the lid to make it damp.
In other type of fires: use a fire extinguisher;
otherwise try to suffocate the fire with a blanket or soil, sand, or spray water (except in liquid fat fires).
When a persons clothes are on fire, the person will usually panic and run; the wind created by the movement will activate the fire. It is necessary to tell the person to "stop, drop and roll" on the ground (or to force him/her to do so), and to roll him/her in a cloth when available. You should not use a fire extinguisher because the chemical agent may harm them.
NEVER try to put a wet cloth on any part of your body to protect it from fire. The water from the cloth will heat up and could potentially create steam and burn the skin where the cloth is placed.

When the fire cannot be fought, it is necessary

in case of a building, to calmly warn the occupants to avoid panic (e.g. use the fire alarm), and to evacuate the building; in case of a vehicle, help the people to get out; this may necessitate an emergency movement;
call for help (e.g. dial 9-1-1 in North America, 1-1-2 in the European Community, 9-9-9 in the United Kingdom or 0-0-0 in Australia);
when it is not possible to get out (e.g.smoke fills the corridor): to get in a room and close the door, put clothes (possibly wet) under the door, spray the door with water when possible, and warn the firefighters by waving sign at the window; kneel or lay down to get fresh air.

During the evacuation, it is important:

not to use an elevator;
not to go in the smoke: it is easy to get lost, and smoke causes major damages (especially burns inside the lungs and asphyxia);
always go towards the exit.

Appendix : Calculation of the amount of water required to suppress a fire in a closed volume

In the case of a closed volume, it is easy to compute the amount of water. Indeed, when the volume is tight, the air can not come in; and the air is necessary for the combustion, the oxygen O₂ (pure air contains 21% of O₂). Whatever the amount of fuel available (wood, paper, cloth), the combustion will stop when the air becomes "thin", i.e. when there is less than 15% oxygen.

This gives:

The amount of water required to make the atmosphere inert, i.e. to prevent the pyrolysis gases to burn; this is the "volume computation";
The amount of water required to cool the smoke, the atmosphere; this is the "thermal computation".

These computations are only valid when considering a diffused spray. The diffused spray is not possible in the case of a high ceiling: the spray is short and does not reach the upper layers of atmosphere. For this reason, the computations are not valid for big volumes such as barns or warehouses: a warehouse of 1,000 m² (1,200 square yards) and 10 m high (33 ft) represents 10,000 m³. Additionally, a warehouse or a barn can barely be considered as a closed volume, a great amount of water vapour can go away and will not replace the atmosphere of the building.

Volume computation

The fire needs air; if the water vapour pushes all the air away, the fuel can not burn any longer. But the replacement of all the air by water vapour is harmful for firefighters and other people still in the building: the water vapour carries much more heat than the air at the same temperature (one can be burnt by water vapour at 100 °C (212 °F) above a boiling saucepan, whereas it is possible to put an arm in an oven at 270 °C (520 °F) without damage as long as one does not touch the walls of the oven). This amount of water is thus a maximum amount and should never be reached.

The minimal amount of water that could be used is the amount required to dilute the air so there is less than 15% oxygen: below this concentration, the fire can not burn. This minimal amount will be called optimal amount.

The amount that is really used should be between these two values. Any additional water would just run on the floor and cause water damage but would not contribute to the fire suppression.

Let us call:

V_r the volume of the room,
V_v the volume of vapour required,
V_w the volume of liquid water to create the V_v volume of vapour,

then for an air at 500 °C (773 K, 932 °F, best case concerning the volume, probable case at the beginning of the operation), we haveTemplate:Fn

V_{v}=3571\cdot V_{w}

and for a temperature of 100 °C (373 K, 212 °F, worst cas concerning the volume, probable case when the fire is suppressed and the temperature is lowered):Template:Fn

V_{v}=1723\cdot V_{w}

For the maximum volume, we have:

V_{v}=V_{r}

considering a temperature of 100 °C. To compute the optimal volume (dilution of oxygen from 21 to 15%), we haveTemplate:Fn

V_{v}=0.286\cdot V_{r}

for a temperature of 500 °C. The table below show some results, for rooms with a height of 2.70 m (8 ft 10 in).

Area of the room	Volume of the room V_r	Amount of liquid water V_w
Amount of water required to suppress the fire volume computation
Area of the room	Volume of the room V_r	maximum	optimal
25 m² (30 yd²)	675 m³	39 L (9.4 gal)	5.4 L (1.3 gal)
50 m² (60 yd²)	135 m³	78 L (19 gal)	11 L (2.7 gal)
70 m² (84 yd²)	189 m³	110 L (26 gal)	15 L (3.6 gal)

Note that the formulas give the results in cubic meters; they are then translated in liters.

Of course, a room is never really closed, gases can go in (fresh air) and out (hot gases and water vapour) so the computations can not be exact.

Notes: Template:Fnb indeed, the mass of one mole of water weights 18 g, a liter (0.001 m³) represents one kilogram i.e. 55.6 moles, and at 500 °C (773 K), 55.6 moles of an ideal gas at the atmospheric pressur represents a volume of 3.57 m³.; Template:Fnb same as above with a temperature of 100 °C (373 K), one liter of liquid water produces 1.723 m³ of vapour; Template:Fnb we considere that only V_r - V_v of the original room atmosphere remains (V_v have been replaced by water vapour). This atmosphere contains less than 21% of oxygen (some was used by the fire), so the remaining amount of oxygen represents less than 0.21·(V_r-V_v). The concentration of oxygen is thus less than 0.21·(V_r-V_v)/V_r, and we want this fraction to be 0.15 (15%)

Thermal computation

In the case of a fire in a closed volume, the first concern is to lower the temperature. In the worst case, we can considere that it is necessary to absorb all the heat produced by the fire (in fact, only a part of this heat must be absorbed to allow the extinction, not the whole). The heat is transferred to the smoke, walls, ceiling, floor, and a part of it goes away with the smoke by ventilation or through the wall when the insulation is weak. The most critical point is to absorb the heat of the smoke inside the room, and to lower the temperature but certainly not to set it back to 20 °C (68 °F). The computation made with this hypothesis is thus the calculation of a maximum, the amount that is really required is smaller.

If the room is totally airtight, the fire will stop spontaneoulsly when the concentration in oxygen is less than 15%. The volume of oxygen used for this is 0.06·V_l.Template:Fn

A cubic meter of oxygen combined with a fuel typically produces 4,800 k cal, i.e. 20 M J.Template:Fn The rise in temperature from 20 to 100 °C (68 to 212 °F) and the vapourisation of one liter water absorbs 539,000 kcal (2,260 MJ).

The volume of water V_w' that is required to absorb the heat is thus:Template:Fn

V_{e}'=0.00053\cdot V_{l}

Area of the room	Volume of the room V_l	Amount of liquid water V_w'
Amount of water required to suppress the fire thermal computation
25 m² (30 yd²)	67.5 m³	36 L (8.6 gal)
50 m² (60 yd²)	135 m³	72 L (17 gal)
70 m² (84 yd²)	189 m³	100 L (24 gal)

Note that the formula give the result in cubic meter; it is then translated in liter for the table.

Notes: Template:Fnb the concentration of oxygen dropped from 21% to 15%, the volume of oxygen involved represents 21-15 = 6% of the volume of the room; Template:Fnb for example, the combustion of 1 m³ of methane requires 2 m³ of pure O₂ and generates 35.6 MJ ; 1 m³ of O₂ thus contributes to the creation of 17.8 MJ (4,250 kcal);; Template:Fnb V_w'·2260 = 0.06·V_r·20 in megajoules, thus V_w' = 5.31·10^-4·V_r ;
V_w'·539000 = 0.06·V_r·4800 in kilocalories, thus V_w' = 5.34·10^-4·V_r ;
the difference of 0.6% between the values is due to the approximations, and is negligible

Conclusion

Let us compare the calculated values:

Area of the room	Height of the room	Amount of water
Amount of water required to suppress the fire comparison of computations
		Volume computation		Thermal computation
		Maximum	Optimal	Thermal computation
25 m² (30 yd²)	2.7 m (8 ft 10 in)	39 L (9.4 gal)	5.4 L (1.3 gal)	36 L (8.6 gal)
50 m² (60 yd²)	2.7 m (8 ft 10 in)	78 L (19 gal)	11 L (2.7 gal)	72 L (17 gal)
70 m² (84 yd²)	2.7 m (8 ft 10 in)	110 L (26 gal)	15 L (3.6 gal)	100 L (24 gal)

We can see that both computations give close values. This means that the amount of water required to cool the smoke is sufficient to make the atmosphere inert, and thus to suppress the fire.

External links

Firetactics.com
Firefighter Training Services
Fire.gov: Building and Fire Research Laboratory (NIST)
Firefighting flow rates
ops4.com:Fire Fighting Articles
Wiiversity:School of Fire and Emergency Management