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FSE wiki: Write out abbreviations at first use (NFPA, NIST, NRC, WTC, AFFF, PMMA) (written by Claude Opus 5.5)
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Cooling the fuel surface slows down the pyrolysis that feeds the flame with gaseous fuel. When water is applied to a burning solid in air, the solid is cooled by contact with the water, so its rate of pyrolysis falls.<ref name="IS-B15" /> For fuels that give no flammable vapour at room temperature, such as solids and liquids with a high flash point, wetting and cooling of the surface is in many cases the dominant mechanism.<ref name="MB-314">Mawhinney and Back (2002), p. 4-314.</ref> If the vapour and air mixture above the surface falls below the lower flammability limit, the flame goes out.<ref name="MB-314" /> The NRC describes sprinklers in the same terms. The water vaporises and cools the burning materials below their ignition temperature, and nearby materials can no longer heat up to it.<ref name="IS-10-1">Iqbal and Salley (2004), p. 10-1.</ref>
Cooling the fuel surface slows down the pyrolysis that feeds the flame with gaseous fuel. When water is applied to a burning solid in air, the solid is cooled by contact with the water, so its rate of pyrolysis falls.<ref name="IS-B15" /> For fuels that give no flammable vapour at room temperature, such as solids and liquids with a high flash point, wetting and cooling of the surface is in many cases the dominant mechanism.<ref name="MB-314">Mawhinney and Back (2002), p. 4-314.</ref> If the vapour and air mixture above the surface falls below the lower flammability limit, the flame goes out.<ref name="MB-314" /> The NRC describes sprinklers in the same terms. The water vaporises and cools the burning materials below their ignition temperature, and nearby materials can no longer heat up to it.<ref name="IS-10-1">Iqbal and Salley (2004), p. 10-1.</ref>


Tewarson links this mechanism to the fire properties of the fuel. He calculates a [[critical water application rate]] for [[flame extinction]], expressed in grams of water per square metre of burning surface per second.<ref name="TE-148">Tewarson (2002), p. 3-148.</ref> The calculated values assume no water puddle on the surface and perfect efficiency. They are 2.1 to 2.5 g/(m²·s) for polyoxymethylene, PMMA and polyethylene with 25 % chlorine. Polyethylene and polypropylene need 3.5 to 4.1 g/(m²·s), and polystyrene 5.1 g/(m²·s).<ref name="TE-148" /> Materials that do not burn in normal air without an external heat flux have a critical rate of zero.<ref name="TE-148" />
Tewarson links this mechanism to the fire properties of the fuel. He calculates a [[critical water application rate]] for [[flame extinction]], expressed in grams of water per square metre of burning surface per second.<ref name="TE-148">Tewarson (2002), p. 3-148.</ref> The calculated values assume no water puddle on the surface and perfect efficiency. They are 2.1 to 2.5 g/(m²·s) for polyoxymethylene, polymethyl methacrylate (PMMA) and polyethylene with 25 % chlorine. Polyethylene and polypropylene need 3.5 to 4.1 g/(m²·s), and polystyrene 5.1 g/(m²·s).<ref name="TE-148" /> Materials that do not burn in normal air without an external heat flux have a critical rate of zero.<ref name="TE-148" />


=== Cooling the flame ===
=== Cooling the flame ===

Revision as of 23:58, 1 November 2026

Extinguishing mechanisms are the physical and chemical ways in which a fire is put out or suppressed. Four mechanisms are distinguished, and usually several act at the same time. They are physically separating the fuel from the flame, removing or diluting the oxygen, lowering the temperature of the fuel or the flame, and adding chemicals that change the combustion chemistry.[1] A glossary of the US Nuclear Regulatory Commission (NRC) lists the matching methods as cooling, oxygen depletion or removal, inhibition of the chemical reaction and flame removal, or blowout.[2]

The mechanisms follow from the conditions that a fire needs. The fire triangle shows fuel, oxygen and heat; taking away any one of them stops the combustion.[3] The fire tetrahedron adds a fourth element, the chain reaction of combustion, which explains why some agents work without cooling or smothering.[4] Each extinguishing agent uses a mix of mechanisms. Water cools, foam excludes air from the fuel surface, inert gases act primarily by depleting the oxygen, and halons interrupt the chain reaction.[5][6][7][8] The relative importance of each contribution varies with the circumstances.[9]

The fire triangle and the fire tetrahedron

Introduction to Fire Safety Engineering describes combustion with three factors: oxygen, fuel and temperature.[10] Air contains 78 % nitrogen, 21 % oxygen and 1 % other gases by volume. If the oxygen level falls, a fire stops spreading and may eventually go out, depending on the exact oxygen concentration.[10] A fuel mixed with oxygen cannot ignite unless enough heat is added, and the amount depends on the fuel.[10] In the NRC guidance, the fire triangle requires fuel, an oxidising agent and a heat source in the same place at the same time. If any leg of the triangle is removed, the combustion is not sustained.[3]

The triangle leads directly to three classic mechanisms. Typical extinguishment removes the fuel from the fire, limits the oxygen (smothering), or removes the heat (quenching).[4] The NRC adds that the triangle fits most scenarios, with the exception of extinguishment by dry chemicals and halons.[3]

Grant explains that the theory had to be modified as halons became better understood. Halons do not work in any of the three classic ways, but break up the uninhibited chain reaction of the combustion process.[4] The resulting model is called the tetrahedron of fire, with temperature, oxygen, fuel and the chain reaction as its four faces.[8] The NRC glossary defines it as a representation of fire in which fuel, oxidant, heat and chain reactions are the four corners of a tetrahedron.[11]

The tetrahedron is also used in firefighting practice. The Netherlands Institute for Public Safety (NIPV) published guidance on fires in Dutch indoor car parks. For an interior attack, it advises crews to think in terms of the fuel, heat, oxygen and chemical chain reaction tetrahedron.[12] The same guidance asks whether enough cooling capacity is available before an attack from inside or outside is chosen.[12]

Cooling

Cooling removes heat from the fuel or the flame. Water is by far the most common cooling agent, mainly because it is widely available and inexpensive.[13] It also has a high specific heat capacity and a high latent heat of vaporisation.[13] The specific heat of water at room temperature is 4,186 J/(kg·K), about four times that of air at 1,005 J/(kg·K).[14] When water evaporates, it takes extra energy from its surroundings, because every change from liquid to gas absorbs energy.[15]

The heat that a mass of water can absorb follows from the energy needed to heat it to its boiling point plus the energy needed to evaporate it. Introduction to Fire Safety Engineering gives the sensible part as mass times specific heat times temperature rise, and notes that the energy of a phase change must be added.[14] Combined, the heat absorbed is:

E=m(cpΔT+hv)

Symbols
Symbol Description Unit
E heat absorbed by the water kJ
m mass of water kg
cp specific heat of liquid water kJ/(kg·K)
ΔT temperature rise from the initial temperature to the boiling point K
hv heat of vaporisation of water at the boiling point kJ/kg

The relation assumes a constant specific heat and that all of the water evaporates. Water that runs off or drains away absorbs only the sensible part, or less.[14] Tewarson gives the property values: a specific heat of 0.0042 kJ/(g·K) and a heat of vaporisation of 2.26 kJ/g at 373 K.[16]

Worked example

Tewarson calculates the heat needed to gasify water that starts at 298 K (25 °C). Heating the water to 373 K takes 0.0042 × 75 = 0.32 kJ/g, and evaporating it takes 2.26 kJ/g, so the total is 2.58 kJ/g.[16] One kilogram of water, about one litre, therefore absorbs about 2.58 MJ. Almost 88 % of this is absorbed by evaporation rather than by heating. Fleming gives 9,280 Btu for one US gallon of water heated from 21 °C to steam at 100 °C, which is equivalent to about 2.59 MJ per litre.[13]

An average living room sofa can burn at about 2.5 MW.[17] To absorb all of that heat, water would have to evaporate at 2,500 / 2,580 ≈ 0.98 kg/s, or about 59 L/min. If the water were only heated to 100 °C without evaporating, the same task would need about eight times as much water. In practice, much less than the full heat release has to be absorbed, as the section on flame cooling below explains. The calculation does show why evaporation, not heating, gives water most of its cooling power. In Introduction to Fire Safety Engineering, the heat release rate of such an object is set against the cooling capacity that a fire service can deliver at a given flow rate.[18]

Cooling the fuel

Cooling the fuel surface slows down the pyrolysis that feeds the flame with gaseous fuel. When water is applied to a burning solid in air, the solid is cooled by contact with the water, so its rate of pyrolysis falls.[1] For fuels that give no flammable vapour at room temperature, such as solids and liquids with a high flash point, wetting and cooling of the surface is in many cases the dominant mechanism.[19] If the vapour and air mixture above the surface falls below the lower flammability limit, the flame goes out.[19] The NRC describes sprinklers in the same terms. The water vaporises and cools the burning materials below their ignition temperature, and nearby materials can no longer heat up to it.[5]

Tewarson links this mechanism to the fire properties of the fuel. He calculates a critical water application rate for flame extinction, expressed in grams of water per square metre of burning surface per second.[20] The calculated values assume no water puddle on the surface and perfect efficiency. They are 2.1 to 2.5 g/(m²·s) for polyoxymethylene, polymethyl methacrylate (PMMA) and polyethylene with 25 % chlorine. Polyethylene and polypropylene need 3.5 to 4.1 g/(m²·s), and polystyrene 5.1 g/(m²·s).[20] Materials that do not burn in normal air without an external heat flux have a critical rate of zero.[20]

Cooling the flame

Water can also act in the gas phase. Gas phase cooling is the removal of heat from the flame and hot gases by evaporating water, and fine droplets evaporate faster.[21] If the flame temperature falls below the limiting adiabatic flame temperature, about 1,600 K (1,326 °C) for diffusion flames, the flame goes out.[21] Only enough heat has to be removed to bring the flame down to this limit, not to ambient temperature.[21] In Wighus's experiments, the heat absorbed by evaporating water needed to be only 0.3 times the heat release rate for an unconfined propane flame under optimum conditions. For more realistic machinery spaces, ratios of about 0.6 were noted.[21]

A cooler flame also radiates less heat back to the fuel, which further reduces the pyrolysis rate.[21][1] In laboratory counterflow methane flames, water mist and vapour were more effective on a mass basis than Halon 1301, provided they were delivered with nearly 100 % efficiency. In full-scale compartment fires the efficiency is much lower.[21]

Removing or diluting the oxygen

A flame needs a minimum oxygen concentration. For most hydrocarbon fuels, this limiting oxygen concentration is about 13 %.[19] Smothering means limiting the oxygen supply to the fire.[4] In an interior attack on a building fire, NIPV advises anti-ventilation, keeping the structure closed where possible.[12] Introduction to Fire Safety Engineering notes that a fire stops spreading and may go out as the oxygen level falls.[10]

Water can dilute the oxygen as steam. When water droplets evaporate, the volume of the water increases by more than three orders of magnitude.[19] In the flame, this expansion can disrupt the entrainment of air. In a compartment, steam from water hitting the flame, hot gases or hot surfaces can significantly reduce the oxygen concentration.[19] The NRC notes that steam may keep oxygen away from the fire under some confined conditions.[1]

Inert gases work in the same way, but they also act thermally. They lower the flame temperature below the threshold for the combustion reactions, both by reducing the oxygen concentration and by raising the heat capacity of the atmosphere.[22] Adding enough inert gas to reduce the oxygen concentration below 12 % extinguishes flaming fires.[22] Carbon dioxide displaces the normal atmosphere and reduces the oxygen below the 15 % that the NRC report gives as necessary for a diffusion flame.[23]

The NRC report describes the action of carbon dioxide as mainly thermophysical. The reacting gases are kept below the temperature needed to maintain the free radicals that sustain the flame chemistry.[23] For inert gases, the extinguishing concentration is linearly related to the heat capacity of the agent and air mixture.[23]

Separating the fuel from the flame

The third route breaks the contact between fuel, heat and air. Physically separating the combustible substance from the flame is one of the four mechanisms in the NRC list.[1] The simplest form is removing the fuel from the fire.[4] Flame removal, or blowout, is listed in the NRC glossary as a further method of extinguishment.[2]

Firefighting foam is the best known separating agent. It consists of air-filled bubbles formed from a solution of foam concentrate in water, typically 1, 3 or 6 % concentrate.[6] The foam floats on the fuel and forms an air-excluding layer that halts the vaporisation of fuel at the surface. If the entire surface is covered, the fuel vapour is completely suppressed and the fire goes out.[6] Film-forming foams can also produce a vapour-sealing film of surface-active water solution on most hydrocarbon fuels.[6] Low-expansion foams are effective on flat pool fires, but not on three-dimensional fires such as running fuel or pressurised sprays.[6]

Foam combines several mechanisms on a burning pool. It stops radiant heat from the fire reaching the liquid surface and supplying the heat of vaporisation. If the fire point of the liquid is above the foam temperature, the liquid is cooled and its vapour pressure falls.[1] A water-soluble fuel such as alcohol is also diluted by water from the foam.[1] Water alone cannot separate a hydrocarbon fuel from the air. It is denser than most hydrocarbon fuels and does not mix with them, so the fuel floats on the water and may continue to burn and spread.[13]

Chemical inhibition

Chemical inhibition interrupts the chain reaction in the flame rather than removing a leg of the triangle. Halogenated agents act in this way, although the mechanism is not completely understood.[8] The halogen atoms remove the active chemical species that carry the flame chain reaction.[8]

Bromine is much more effective than chlorine or fluorine.[8] In Halon 1301, which is 54 % bromine by weight, it is the bromine radical that acts as the inhibitor.[8] The NRC report states that Halon 1301 suppresses a fire chemically and very quickly, unlike agents that work by removing heat or oxygen.[24]

The replacements for halon show that the boundary between chemical and physical action is not sharp. Halocarbon clean agents extinguish fires by a combination of chemical and physical mechanisms, depending on the compound.[22] Compounds with bromine or iodine scavenge flame radicals in the same way as Halon 1301. Other compounds work mainly by extracting heat from the flame zone, through their heat of vaporisation, their heat capacity and the energy absorbed when they decompose.[22] Because these compounds have little chemical inhibition, they need higher extinguishing concentrations than Halon 1301.[22] The NRC report likewise describes halocarbon agents as acting largely by absorbing heat, with some chemical effect on the flame reactions.[7]

Dry chemical agents also combine mechanisms. They can interact chemically with the flame, coat the fuel surface, cool the flame and block the transfer of radiant heat.[1] The NRC also treats extinguishment by dry chemicals, like that by halons, as an exception to the simple fire triangle.[3]

Combined action of common agents

In practice, no agent relies on a single mechanism, and the relative importance of each mechanism varies with the circumstances.[9] The table summarises the mechanisms that the sources describe for the most common agents. It is based on the descriptions of the individual agents.[1][7][23][21][19][6][22][8]

Main extinguishing mechanisms of common agents
Agent Cooling Oxygen removal or dilution Separation of fuel and flame Chemical inhibition
Water spray fuel and flame steam, in confined conditions – –
Water mist flame and fuel steam, locally and in the compartment – –
Foam cools the liquid – floating, air-excluding layer –
Dry chemical flame – coats the surface yes
Inert gases, carbon dioxide raised heat capacity primary action – –
Halon 1301 and bromine or iodine clean agents – – – primary action
Other halocarbon clean agents heat extraction (primary) important role – minor

Water mist shows how mechanisms interact. Mawhinney and Back name gas phase cooling, oxygen depletion with dilution of the fuel vapour, and wetting and cooling of the fuel surface as primary mechanisms. Radiation attenuation and kinetic effects are secondary.[21] All of these act to some degree, with different weights for liquid and solid fuels.[21] Water vapour between the fuel and the flame absorbs radiant energy, which helps to prevent fire spread to unburnt fuel.[19]

The choice of agent also depends on the class of fire. The NRC table matches agents to fire classes. Water, dry chemicals, foam and some halons are listed for ordinary combustibles (Class A), and carbon dioxide, dry chemicals, halons and foam for flammable liquids and gases (Class B).[9] Water and water-based extinguishers must never be used on burning metals (Class D).[9]

Limits and misconceptions

A common misconception is that the fire triangle explains every extinguishment. The NRC notes that it does not apply to dry chemicals and halons, which act on the chain reaction.[3] Another is that a flame that has gone out means the fire is extinguished. Halon 1301 at normal concentrations of 5 to 6 % instantly extinguishes visible flames on Class A materials. Glowing combustion inside the material may continue and reignite when the enclosure is ventilated.[25]

Some fuels resist the usual mechanisms. Reactive metals such as magnesium, sodium and titanium burn so intensely that they overpower halon.[4] Fuels that contain their own oxidiser, such as gunpowder and cellulose nitrate, also burn freely in a halon atmosphere.[4] Water can be inappropriate for water-reactive materials, because it may produce heat, flammable or toxic gases, or explosions.[13]

Water can even make a flame worse at first. Flare-up of a flame on first contact with water mist has been measured, possibly because the rapid evaporation adds turbulence and speeds up burning.[19] Finally, the mechanisms are understood far better than they can be calculated. The ways in which water suppresses fire have not been characterised well enough for mathematical modelling, so sprinkler design remains empirical.[26] No fundamental explanation of foam performance based on first principles has been developed either.[6] Even where water cannot suppress a fire, its cooling can still protect the structure by containing the fire until other means put it out.[13]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Iqbal and Salley (2004), p. B-15.
  2. ↑ 2.0 2.1 Iqbal and Salley (2004), p. F-23.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Iqbal and Salley (2004), p. 1-5.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Grant (2002), p. 4-152.
  5. ↑ 5.0 5.1 Iqbal and Salley (2004), p. 10-1.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Scheffey (2002), p. 4-88.
  7. ↑ 7.0 7.1 7.2 Iqbal and Salley (2004), p. B-41.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 Grant (2002), p. 4-153.
  9. ↑ 9.0 9.1 9.2 9.3 Iqbal and Salley (2004), p. B-16.
  10. ↑ 10.0 10.1 10.2 10.3 Van Rossum (2027), §4.2.
  11. ↑ Iqbal and Salley (2004), p. F-29.
  12. ↑ 12.0 12.1 12.2 Van der Graaf et al. (2021/2023), p. 63.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 Fleming (2002), p. 4-72.
  14. ↑ 14.0 14.1 14.2 Van Rossum (2027), §3.4.
  15. ↑ Van Rossum (2027), §3.5.
  16. ↑ 16.0 16.1 Tewarson (2002), p. 3-100.
  17. ↑ Van Rossum (2027), §4.5.
  18. ↑ Van Rossum (2027), §8.6.
  19. ↑ 19.0 19.1 19.2 19.3 19.4 19.5 19.6 19.7 Mawhinney and Back (2002), p. 4-314.
  20. ↑ 20.0 20.1 20.2 Tewarson (2002), p. 3-148.
  21. ↑ 21.0 21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 Mawhinney and Back (2002), p. 4-313.
  22. ↑ 22.0 22.1 22.2 22.3 22.4 22.5 DiNenno (2002), p. 4-174.
  23. ↑ 23.0 23.1 23.2 23.3 Iqbal and Salley (2004), p. B-46.
  24. ↑ Iqbal and Salley (2004), p. B-42.
  25. ↑ Iqbal and Salley (2004), p. B-43.
  26. ↑ Fleming (2002), p. 4-73.

References

  • DiNenno, P. J. (2002). Halon replacement clean agent total flooding systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-173–4-200). National Fire Protection Association; Society of Fire Protection Engineers.
  • Fleming, R. P. (2002). Automatic sprinkler system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-72–4-87). National Fire Protection Association; Society of Fire Protection Engineers.
  • Grant, C. C. (2002). Halon design calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-149–4-172). National Fire Protection Association; Society of Fire Protection Engineers.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Mawhinney, J. R., & Back, G. G., III. (2002). Water mist fire suppression systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-311–4-337). National Fire Protection Association; Society of Fire Protection Engineers.
  • Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-88–4-122). National Fire Protection Association; Society of Fire Protection Engineers.
  • Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-82–3-161). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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