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Contents
  1. 1Definition and drop sizes
  2. 2Development and applications
  3. 3How water mist suppresses fire
    1. 3.1Water needed for cooling
  4. 4Spray characteristics
  5. 5System types
  6. 6Design, testing and acceptance
  7. 7Limitations and misconceptions
  8. 8See also
  9. 9Notes
  10. 10References

A water mist system is a fixed fire suppression system that discharges water as a very fine spray. In fire protection, water mist means a spray with no drops larger than 1.0 mm, and with much more of its water in very fine drops than a standard sprinkler produces.[1] Splitting water into fine drops increases the surface available for heat transfer, so each kilogram absorbs heat faster and less water is needed to suppress a fire.[1] Water mist acts mainly by cooling the flame and the hot gases, by diluting the oxygen with steam and by wetting the fuel. Blocking radiation and effects on the flame chemistry play a smaller part.[2]

The technology grew rapidly in the 1990s, driven by the phase-out of halon extinguishing gases and by new rules for sprinklers on passenger ships.[3] It is used both as a halon alternative for flammable liquid hazards in machinery spaces and as a sprinkler equivalent for ordinary combustibles.[2] Unlike sprinklers, water mist systems cannot yet be designed from first principles. Each design has to be linked to full-scale fire tests of the specific manufacturer's equipment, because nozzles that pass the same test can use very different amounts of water.[4][5]

Definition and drop sizes

The term water mist was adopted in the early 1990s by the committee responsible for NFPA 750, the US standard for water mist systems. It separates this technology from fixed water spray systems under NFPA 15 and from sprinkler systems under NFPA 13.[1] Strictly speaking, a fire protection mist is not a true mist. In the scientific sense, a mist lies between an aerosol, with particles of about 5 µm, and a fog, with droplets of 10 to 100 µm. Droplets smaller than about 20 µm stay airborne for a long time.[1] A fire protection mist contains a range of drop sizes, many in the true mist range and some considerably larger.[1]

The advantage of fine drops was known long before the term existed. Research in the 1950s showed good results in cooling and extinguishing diffusion flames with sprays of mean drop diameter below 0.3 mm. Smaller drops have more surface area per unit mass and settle more slowly, so they have more time to absorb heat and evaporate.[1] For decades this knowledge was not applied widely. High operating pressures, small orifices that could become blocked by corrosion products and doubts about maintenance were technical concerns. Standard sprinklers and halon gases were also cheaper alternatives.[1]

A single number such as a mean diameter says little about the spread of drop sizes in a spray. NFPA 750 therefore describes a spray by a curve of cumulative volume against drop diameter.[6] Two points on that curve are commonly quoted. The value Dv0.90 is the diameter below which 90 % of the water volume is contained, and Dv0.50 is the diameter that divides the volume into two equal halves.[6] In measurements on four commercial nozzles, even the coarsest spray carried at least 50 % of its volume in drops smaller than 300 µm, and another spray carried about 80 %.[7]

Development and applications

Three events in the 1980s made fine sprays economically attractive. After an aircraft fire at Manchester in 1984, an international programme developed a cabin spray system. It extended tenable conditions by 7 minutes using about 10 litres of water, but regulators judged the cost per life saved too high to make it mandatory.[3] The Montreal Protocol of 1987 began the phase-out of ozone-depleting halons, which released research funds for alternative agents, water among them.[3]

The third event was a ruling of the International Maritime Organization (IMO). After several fires with many deaths on Scandinavian ferries, it required sprinklers on all ships carrying 35 or more passengers from 1995.[3] Ship designers regard sprinklers as heavy, and extra weight on upper decks can affect stability. Water mist promised the same protection with less water and less weight.

As a result, water mist has two main fields of use. Where flammable liquids are present, or where clean gaseous agents were chosen to avoid water damage, it serves as a halon alternative. Where ordinary cellulosic materials are protected, it serves as an alternative to a sprinkler system.[2] By 2000 the marine sector was the strongest market. Land-based niches included machinery modules, turbine enclosures, metro station plant rooms and clean-room wet benches.[8] A water mist system installed in a London hotel instead of sprinklers offered smaller pipes, which eased retrofitting in a heritage building, and a lower water demand.[8]

Other guidance refers to water mist only briefly. The International Fire Engineering Guidelines (2005) state that the effect of water mist and gaseous systems should be assessed from their design specifications and the relevant standards, and do not cover these calculations themselves.[9] Dutch guidance on car parks with electric vehicles names a sprinkler or water mist system as an effective way to keep a fire limited to a small number of cars.[10]

How water mist suppresses fire

Mawhinney and co-workers describe three primary and two secondary extinguishing mechanisms for water mist. The primary ones are cooling of the gas phase, oxygen depletion with dilution of the fuel vapour, and wetting and cooling of the fuel surface. The secondary ones are attenuation of radiation and kinetic effects on the flame chemistry.[2] All mechanisms usually act to some degree, but their relative importance differs between liquid and solid fuels.[2]

Cross-section of a room with ceiling nozzles spraying fine mist onto a pool fire; labels mark flame cooling, steam diluting oxygen and wetting of the fuel.
The three primary mechanisms of water mist: gas phase cooling, oxygen depletion and dilution by steam, and wetting and cooling of the fuel surface. Radiation attenuation acts as a secondary mechanism. Based on Mawhinney and Back (2002), pp. 4-313 to 4-314.

Gas phase cooling removes heat from the flame and the hot gases as the drops evaporate. Fine drops evaporate faster, so they extract more heat. If the flame temperature falls below a limiting value of about 1,600 K (1,326 °C) for diffusion flames, flame extinction follows.[2] Laboratory work on small counterflow flames showed that water mist can be more effective than Halon 1301 on a mass basis. That holds only if nearly all the water reaches the flame, and in full-scale compartment fires the efficiency is far lower.[2]

When a drop turns into steam, its volume increases by more than three orders of magnitude. Close to the flame, this expansion can disrupt the flow of air into the flame. In a closed room, the steam together with the oxygen consumed by the fire lowers the oxygen concentration of the whole space.[11] If it drops below the limiting oxygen concentration, about 13 % for most hydrocarbon fuels, the fire goes out.[11] In full-scale water mist tests in obstructed machinery spaces, the oxygen concentration at extinction averaged 14.5 % for heptane spray fires and 13.5 % for pool fires.[12]

For solid fuels and for liquids with a flash point above room temperature, wetting and cooling of the fuel surface is often the dominant mechanism. It reduces the release of fuel vapour until the mixture above the surface is too lean to burn.[11] Water mist and steam also reduce the radiant heat reaching nearby objects, which helps to prevent fire spread to unburnt fuel.[11] Fire models such as the Fire Dynamics Simulator therefore calculate the absorption and scattering of thermal radiation by water droplets.[13][14]

The enclosure itself strengthens these effects. The first contact with mist cools the hot, oxygen-poor gases in the upper layer, and the spray drives them down to the seat of the fire.[11] Some manufacturers found that switching the spray off and on again, known as cycling, speeds up extinguishment in enclosures. Liu and co-workers showed that cycling extinguished fires faster and with less water than continuous discharge.[15] Not every effect is favourable. A brief flare-up of the flame has been measured when mist first reaches it.[11]

Water needed for cooling

Wighus related the cooling capacity of a spray to the size of the fire with the spray heat absorption ratio (SHAR). It is the rate at which evaporating water absorbs heat divided by the heat release rate of the fire:[16]

SHAR=Q˙wQ˙f

Symbols
Symbol Description Unit
SHAR spray heat absorption ratio –
Q˙w rate of heat absorbed by evaporating water kW
Q˙f heat release rate of the fire kW

For an unconfined propane flame with optimum application, a ratio as low as 0.3 was enough. Values around 0.6 were found for more realistic machinery space conditions, where small flames survive in shielded corners and cause re-ignition.[16] The ratio is well below 1 because the flame only has to be cooled to its limiting temperature, not to ambient temperature.[2] Heating water from 298 K to 373 K takes 0.32 kJ/g and evaporating it another 2.26 kJ/g, so each gram that turns into steam absorbs 2.58 kJ.[17]

As an example, take the 750 kW pan fire used in tests by the National Research Council Canada in a 60 m³ enclosure.[18] With a SHAR of 0.3, the evaporating water must absorb 0.3 × 750 = 225 kW. Divided by 2.58 kJ/g, this gives about 88 g/s, or 5.3 litres per minute of water that actually evaporates in the flame. With the more realistic ratio of 0.6, the amount doubles to about 175 g/s, or 10.5 litres per minute.[16][17] Typical water mist nozzles deliver 5 to 80 litres per minute each.[19]

Bars of 5.3 and 10.5 l/min evaporated water for a 750 kW fire at SHAR 0.3 and 0.6, beside a band of 5 to 80 l/min per nozzle.
Minimum water that must evaporate in the flame to cool a 750 kW fire, for spray heat absorption ratios of 0.3 and 0.6, compared with the flow of a single water mist nozzle. Based on Mawhinney and Back (2002), pp. 4-313, 4-327 and 4-329, and Tewarson (2002), p. 3-100.

The result is a lower bound, not a design flow. In real systems, the share of the mist that reaches the flame and evaporates there is almost unpredictable and cannot be controlled.[2] The SHAR is therefore mainly of interest as a building block for computer models of suppression.[2] Measured mist concentrations needed around a diffusion flame range from 100 to 200 g/m³, and 50 g/m³ has extinguished heptane flames. In a real machinery space, such local concentrations are extremely difficult to predict or control.[11]

Spray characteristics

A spray is fully described by its drop size distribution, cone angle, discharge velocity, mass flow rate and momentum. Together these govern nozzle spacing and the maximum ceiling height.[6] Listed nozzles usually produce solid cones of 90° or 120°.[6] The mass flow of a nozzle depends on the water pressure and the total area of its orifices.[6]

Drop size alone does not decide which spray is better. In one comparison, a nozzle with about 80 % of its volume in drops below 300 µm discharged only 5 litres per minute. A coarser nozzle discharged 12 kg/min and therefore delivered more fine water in total, about 6 kg/min against 4 kg/min.[7][20] Discharge rate, cone angle and velocity also affect how well the mist mixes with the fire gases.[20]

Spray momentum combines the mass of the drops and of the air they drag along with their velocity. Its direction matters. A spray directed against the rising fire plume penetrates the flame and carries steam to the seat of the fire. A spray moving in the same direction as the plume mixes poorly, and the steam is carried away from the fuel.[6] Mist does not spread through a room as a gas does. Its concentration can fall by more than half within one metre sideways from the spray pattern, so higher-performance systems use high-velocity sprays to mix the mist through the space.[4]

Additives can improve performance. Alkali salts and the surfactants of foam concentrates help to extinguish fires, and inert gases mixed with the mist support oxygen depletion in enclosed spaces.[20] In the United States, the Environmental Protection Agency accepted water mist as a halon alternative in occupied spaces only with potable water or natural seawater. Anyone proposing an additive must show that it poses no health risk.[20]

System types

Water mist systems are classified in three ways: by the way they are applied, by how the spray is produced and by the operating pressure.[18] NFPA 750 distinguishes total compartment application, local application and zoned application systems.[18] A total compartment system has open nozzles throughout the room, all of which discharge when a single control valve opens, in the manner of a deluge system.[18] NUREG-1805 uses the term deluge water mist system for a system whose open nozzles discharge mist simultaneously.[21]

Three room plans: nozzles discharging over the whole room, nozzles around one machine only, and nozzles discharging only in the zone around the fire.
Total compartment application (left), local application (centre) and zoned application (right). Based on Mawhinney and Back (2002), pp. 4-327 to 4-328.

Total compartment systems benefit from the enclosure, which captures heat, confines steam and recirculates oxygen-depleted gases. They are nevertheless not "total flooding" systems in the sense used for gases. A fixed quantity of mist does not put out a fire; the mist must be discharged continuously long enough to gain control. Unlike gaseous systems, which need a tight room, these systems can work with fairly large openings.[18] Their main drawback is a high total water demand, because many nozzles each discharging a little still add up to a lot of water.[18]

A local application system sprays directly onto an object or hazard, for example a diesel engine or a turbine on an open floor. With little confinement of heat or steam, it relies almost entirely on gas phase cooling and wetting of the fuel.[22] A zoned system protects only part of an enclosure, which usually reduces the water flow to a quarter or a third of a total compartment system. A system of individually heat-activated nozzles, as on ships, is in effect a zoned system in which each nozzle acts as detector and valve.[22]

The design of a water mist system for a 9 km cable tunnel in Auckland shows why zoning can be necessary. Opening all nozzles over the full length would have required very large mains and booster pumps. Heat-activated nozzles were not suitable either, because the ventilation air at 5 m/s would carry the heat away from the nozzles nearest the fire.[22] The chosen solution released the system in zones upstream and downstream of the fire. That made reliable detection and location of the fire the critical part of the design.[22]

By spray generation, single-fluid nozzles atomise water alone, either by forcing it through small orifices at high speed or by striking it against a surface. Twin-fluid nozzles add compressed gas as an atomising medium at the nozzle. They can work at low water pressure and have larger orifices that block less easily, but they need separate piping and storage for gas and water.[19] Single-fluid nozzles are more widely used.[19]

By pressure, NFPA 750 defines low-pressure systems below 12 bar, intermediate-pressure systems between 12 and 34 bar and high-pressure systems above 34 bar.[23] Low-pressure systems can follow standard sprinkler practice, with conventional centrifugal fire pumps.[23] High-pressure systems are widely installed on cruise ships and in machinery spaces. They use stainless steel tubing and positive displacement pumps, or gas-driven cylinders whose pressure falls from about 140 bar to 30 to 50 bar over a 10-minute discharge.[24] Mawhinney and Back report that acceptable suppression can be achieved with both high- and low-pressure systems; economic and engineering factors decide which suits an application.[23]

Pressure bars: sprinklers 0.5 to 7 bar, low-pressure mist below 12 bar, intermediate 12 to 34 bar, high-pressure 34 to about 140 bar.
Operating pressure ranges of standard sprinklers and of low-, intermediate- and high-pressure water mist systems. Based on Mawhinney and Back (2002), pp. 4-330 to 4-331.

Design, testing and acceptance

There is no theoretical basis yet for designing the drop sizes, concentrations and distribution that a water mist system must deliver. Design parameters are extrapolated from large-scale tests case by case.[4] NFPA 750 therefore requires every design to be linked to formal fire tests.[5] Testing has shown that performance depends on the specific equipment, especially the nozzle. Two systems that pass the same test may differ in average water flux density by a factor of three.[5] Generic design criteria that do not depend on the equipment, such as a nominal flux density for a given fire scenario, cannot yet be set.[5]

A listed system has had its design criteria validated through fire tests by a recognised laboratory. The engineer must still check that the real application is close to the tested conditions and that the pass criteria match the needs of the owner.[5] The IMO tests for cabins and public spaces require only that the fire is controlled for ten minutes. The IMO machinery space test requires all fires to be extinguished, including small hidden fires in the bilge.[5] A pre-engineered system has been tested for a hazard of limited size, such as a turbine enclosure, and needs no hydraulic calculations if the installer stays within its limits. Extrapolation beyond the listing turns it into an engineered system.[19]

Models can help to bound performance. A quasi steady-state zone model by Back and co-workers predicts the temperature and oxygen concentration in a machinery space from the fire size, room geometry, vent area and water flow. It assumes extinction when the oxygen falls to the limiting value, typically taken as 14 %.[25][26] The model agreed well with tests in rooms of 100 to 3,000 m³, but it cannot distinguish between systems with the same flow and different sprays.[26] Computational fluid dynamics models show promise, yet still lack validated methods for drop transport and extinction.[27]

Acceptance testing is more demanding than for sprinklers. NFPA 750 recommends a full discharge test where possible, especially for gas-driven systems with declining pressure.[28] In one test of a twin-fluid system designed for six discharge cycles, all six discharges could be seen and heard. The pressure records showed that the compressed gas had run out before the end of the fifth cycle.[28]

Limitations and misconceptions

A common misconception is that a water mist system fills a room like a gas. Mist drops settle and deposit on surfaces, and the mist must be generated and distributed continuously.[4][18] Effectiveness falls when the room is very large compared with the fire, because the enclosure effects become weak and the mist must then be projected to the seat of the fire.[18] For the same reason, Dutch guidance notes that water mist is less effective in open car parks, where wind and weather reduce its action.[29]

Some effects of mist can be harmful. Releasing mist into a very hot, tight room can cool the gases so fast that a strong negative pressure develops. At the National Research Council Canada, a wall of a 60 m³ test enclosure deflected inward and tore open along the ceiling line. That happened when mist was injected after a 750 kW pan fire had burnt for 90 seconds.[18] The effect of water sprays on gas explosions is mixed. Some fine-mist tests reduced the overpressure by about 15 %, while coarser deluge sprays in a closed compartment increased it, because the spray made the gas mixture more turbulent.[15][30]

Water mist cannot reach every seat of fire. Dutch guidance on car parks states that neither sprinklers nor mist can extinguish a fire inside the battery pack of an electric vehicle, because the water cannot enter the pack. A system can still limit fire growth and spread to nearby vehicles and structures.[31] In general, an active suppression system acts during the growth phase and prevents the fire from becoming fully developed.[32]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Mawhinney and Back (2002), p. 4-311.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Mawhinney and Back (2002), p. 4-313.
  3. ↑ 3.0 3.1 3.2 3.3 Mawhinney and Back (2002), p. 4-312.
  4. ↑ 4.0 4.1 4.2 4.3 Mawhinney and Back (2002), p. 4-320.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Mawhinney and Back (2002), p. 4-325.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Mawhinney and Back (2002), p. 4-317.
  7. ↑ 7.0 7.1 Mawhinney and Back (2002), p. 4-318.
  8. ↑ 8.0 8.1 Mawhinney and Back (2002), p. 4-335.
  9. ↑ National Research Council of Canada et al. (2005), p. 2.7-8.
  10. ↑ Van der Graaf et al. (2021/2023), p. 36.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 Mawhinney and Back (2002), p. 4-314.
  12. ↑ Beyler (2002b), p. 2-184.
  13. ↑ McGrattan et al. (2026), p. 13.
  14. ↑ McGrattan et al. (2026), p. 74.
  15. ↑ 15.0 15.1 Mawhinney and Back (2002), p. 4-315.
  16. ↑ 16.0 16.1 16.2 Wighus (1995), as cited in Mawhinney and Back (2002), p. 4-313.
  17. ↑ 17.0 17.1 Tewarson (2002), p. 3-100.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 Mawhinney and Back (2002), p. 4-327.
  19. ↑ 19.0 19.1 19.2 19.3 Mawhinney and Back (2002), p. 4-329.
  20. ↑ 20.0 20.1 20.2 20.3 Mawhinney and Back (2002), p. 4-319.
  21. ↑ Iqbal and Salley (2004), p. F-21.
  22. ↑ 22.0 22.1 22.2 22.3 Mawhinney and Back (2002), p. 4-328.
  23. ↑ 23.0 23.1 23.2 Mawhinney and Back (2002), p. 4-330.
  24. ↑ Mawhinney and Back (2002), p. 4-331.
  25. ↑ Mawhinney and Back (2002), p. 4-321.
  26. ↑ 26.0 26.1 Mawhinney and Back (2002), p. 4-322.
  27. ↑ Mawhinney and Back (2002), p. 4-324.
  28. ↑ 28.0 28.1 Mawhinney and Back (2002), p. 4-333.
  29. ↑ Van der Graaf et al. (2021/2023), p. 65.
  30. ↑ Mawhinney and Back (2002), p. 4-316.
  31. ↑ Van der Graaf et al. (2021/2023), p. 37.
  32. ↑ Van Rossum (2027), §4.11.

References

  • Beyler, C. L. (2002b). Flammability limits of premixed and diffusion flames. 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. 2-172–2-187). 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.
  • McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • 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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