The LWF Blog
Fire Safety Engineering for Design – Types of Water Mist System – Part 302
September 1, 2026 8:43 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 301, LWF looked at the types of water mist system, concentrating on single fluid systems operating at 70 bar or above. In part 302, we discuss twin fluid water mist systems.
Water mist is not a single technology. Different systems use different methods to produce the droplets that provide fire suppression, and the distinction becomes particularly interesting when comparing conventional single-fluid systems with twin-fluid, or hybrid, technology.
Twin-fluid systems use two separate media, normally water and compressed air or inert gas. Rather than relying solely on hydraulic pressure to atomise the water, the two fluids are brought together at the nozzle. The gas provides additional atomisation energy, producing a very fine mist. Industry literature describes twin-fluid systems operating at around 6–8 bar and producing droplets in the region of 10–50 microns, although the actual droplet distribution depends on the nozzle and operating conditions.
One important engineering insight is that droplet size is not determined by water pressure alone. A conventional high-pressure water mist system creates fine droplets by using hydraulic pressure to drive water through specially designed nozzles. FM’s water mist standard, Class 5560, covers the testing and certification of water mist systems used for fire control or extinguishment, with performance demonstrated through defined component, system and fire testing.
Twin-fluid technology takes a different route. Water can be delivered at a comparatively modest pressure while compressed gas assists the atomisation process at the nozzle. Published laboratory studies on twin-fluid atomisation, including peer-reviewed work in fire safety and spray atomisation journals, have examined nitrogen-assisted nozzle systems operating at relatively low water and gas pressures. One widely cited study published in Experimental Thermal and Fluid Science reports mean droplet diameters in the tens of microns and shows a consistent trend: as assist-gas pressure increases, droplet size decreases while jet velocity increases.
The significance of very small droplets is their large surface area relative to their volume. When exposed to the heat of a fire, these droplets can absorb heat rapidly and evaporate. Water mist therefore suppresses fire through several mechanisms, including flame and fuel cooling, attenuation of radiant heat and displacement of oxygen by water vapour.
Twin-fluid water mist and hybrid fire-extinguishing systems are related, but they are not necessarily the same thing. In a twin-fluid water mist system, the second fluid can primarily provide the energy needed to atomise the water. In a hybrid system, both water and inert gas are considered extinguishing agents and contribute to the suppression process.
The distinction between twin-fluid and hybrid systems is reflected in the approval standards. FM 5560 covers water mist systems, while FM 5580 covers Hybrid (Water and Inert Gas) Fire Extinguishing Systems. FM 5580 was first published in 2012 and establishes performance and design requirements for hybrid systems. Because the technology is highly application-specific, approval is linked to defined hazards and tested configurations rather than being a universal approval for every possible fire scenario.
An application-specific approach is critical when considering flammable-liquid fires. Hybrid systems can be designed and tested for Class B hazards, including liquid-fuel fire scenarios, but the precise permitted application depends on the individual system approval and listing. FM’s current guidance states that hybrid systems are special protection systems for specific hazards and that their effectiveness must be demonstrated through fire testing representative of the intended application.
The same principle explains why it would be misleading to describe an ultra-fine twin-fluid mist as automatically suitable for every type of combustible. Droplet size alone does not establish fire-extinguishing capability. A system must be evaluated against the fuel, enclosure, fire geometry, ventilation, discharge arrangement and other characteristics of the intended hazard.
The comparison with conventional water mist is therefore less about which technology produces the smallest droplets and more about how those droplets are produced and what the complete system has been tested and approved to protect. A high-pressure single-fluid system can produce fine water mist through hydraulic energy, while a twin-fluid system uses a second fluid to assist atomisation at substantially lower water pressures.
For designers and specifiers, that distinction matters. The headline figure for operating pressure can tell only part of the story. The nozzle, water flow, gas flow, droplet distribution and, most importantly, the system’s fire-test evidence all determine whether a particular technology is appropriate for the hazard.
In part 303 of LWF’s series on fire engineering we will begin to discuss system configuration. 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.