The LWF Blog
Fire Safety Engineering for Design – Water Mist Systems – Part 296
July 20, 2026 9:10 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 295, LWF discussed water mist systems and their performance criteria. In part 296, we continue looking at the performance of water mist systems.
When water mist droplets are exposed to the high temperatures generated by a fire, many of the droplets rapidly vaporise into steam. This process instigates two additional suppression mechanisms alongside direct cooling.
Firstly, steam locally displaces oxygen around the flame front. As the oxygen concentration immediately surrounding the combustion zone falls, the fire becomes less capable of sustaining combustion. Unlike inert gas suppression systems, the oxygen reduction is temporary and confined to the immediate vicinity of the fire.
Secondly, the cloud of water vapour and suspended droplets absorb and scatter thermal radiation. Fire spreads not only through direct flame contact but also by radiative heat transfer to nearby combustible materials. By reducing the amount of radiant heat reaching adjacent surfaces, water mist can help limit fire growth and prevent ignition beyond the seat of the fire.
Droplet size is fundamental to these processes. Fine droplets have a high surface area relative to their volume, enabling rapid heat absorption and evaporation. They are also easily carried into the fire plume by entrainment.
There are potential down-sides to water mist, depending on circumstance. Very fine droplets are more susceptible to ventilation, air movement and the strong thermal currents generated by larger fires. When discharged from significant heights, evaporation or diversion can take place before reaching the fire itself. This is one reason why nozzle design, operating pressure and discharge arrangement are validated through full-scale fire testing rather than determined solely by hydraulic calculations.
The effectiveness of water mist also depends heavily on the type of fire being protected. Class B and Class F fires typically produce high heat release rates, generating sufficient thermal energy to vaporise a large proportion of the discharged water. Under these conditions, cooling, oxygen displacement and radiation attenuation all contribute to fire suppression, allowing appropriately tested systems to extinguish a fire fully. Research has also shown that, within enclosed compartments, a minimum fire size may be necessary before these mechanisms become fully effective.
Class A fires behave differently. Fires involving timber, paper, furnishings or cable insulation generally produce lower heat release rates, meaning less of the discharged water evaporates. As a result, oxygen displacement and radiation attenuation play a smaller role, with suppression relying primarily on cooling the fuel surface and wetting combustible materials. Systems protecting Class A hazards therefore often use higher water application rates and slightly larger droplets to improve penetration into deep-seated fires and prolong cooling.
A water mist system usually uses less water than a comparative sprinkler system, resulting in benefits in reduced water capacity, pipework size and collateral damage.
These differences illustrate why water mist should not be regarded as a single, universally applicable technology. Performance depends upon the interaction between droplet characteristics, nozzle design, enclosure geometry, ventilation and the specific fire hazard. For architects, fire engineers and approving authorities, the selection of a water mist system should therefore be based on demonstrated performance for the intended application. Standards such as BS EN 14972 and NFPA 750 reflect this performance-based approach, requiring systems to be supported by representative full-scale fire testing rather than relying solely on theoretical suppression mechanisms.
In part 297 of LWF’s series on fire engineering we will look at fire test protocols for water mist systems. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.