The LWF Blog
Fire Engineering Design and Risk Assessment – Water Mist Systems – Part 13
July 26, 2018 12:59 pmIn LWF’s Fire Engineering blog series for architects and others in the building design business, we have been looking at fire suppression systems and part 12 consisted of an introduction to water mist systems. In part 13, we look at how water mist systems work to suppress fire.
A water mist system works somewhat in the same way as a sprinkler system, with some important exceptions. As might be imagined, water mist systems deliver water in very small droplets which together form a mist. In order for water mist to be effective against a fire the droplets are expelled at high momentum, so they cannot be blown off course by air currents.
The water mist cools and smothers the flames in steam and oxygen dilution and helps stop the spread of fire by providing a barrier against radiant heat transfer. Smoke particles are cooled and suppressed so that they cannot spread in the air so easily. The effects of a water mist system mean that both evacuation and firefighting can be undertaken in a safer environment.
The very small droplets of water provide a high surface area, relative to the amount of water, which allows for high heat absorption and high surface contact with smoke particles. As the water droplets weigh very little, it enables them to stay buoyant for longer and while airborne they are able to continue to absorb heat from the fire.
As well as being small, the individual droplets are also low mass, so they can be converted to steam, a process which allows them to absorb a measurable amount of heat. Water requires 4.18 kJ of heat to raise the temperature of 1kg of water from 1 degree Celsius to 100 and a further 2.26 kJ to convert it to steam.
Each water droplet undergoes a volumetric expansion of 1620:1, which displaces air away from the flame front where combustion and evaporation take place.
One of the most attractive features of a water mist system is how little water is actually used when compared to other water-based fire suppression systems. The less water is used to subdue a fire, the less damage and clean up there is likely to be post-fire.
Although water mist systems are not recommended for fires involving live electrical equipment, (it is assumed that will be shut down when a fire is detected), it has been shown that there is little to no electrical transmission until a water layer is formed, which can be some time. In cases where sensitive electrical equipment may be a consideration, de-ionised water can be used to reduce conductivity.
Water mist systems should not be considered for outdoor use, or where there may be high air movements and while the small water droplets can be drawn in by combustion air flows, they are not suitable for permeating small openings or equipment in the way that a gaseous system would.
In part 14 of this series, LWF will look at water mist system configurations. 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.
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.