What is a WaterMist System?
What is a WaterMist System?
In this guide from the Sovereign Extinguishing team we discuss water mist systems and how they work.
A water mist system disperses water mist and extinguishes fire using a number of different mechanisms: cooling or heat absorption from a flame; oxygen dilution; absorption of radiated heat and by chemical inhibition of the flame radicals.
Water mist technology was initially developed and then proved for use on ships, but its use has since been applied and extended to many land based applications. When compared to traditional sprinkler systems, the volumes of water discharged by misting systems is greatly reduced (only 10% of the volume of a traditional sprinkler system in most cases). For this reason the technology found favour for use onboard ships, as the reduced water volume safeguards the stability of the ship. Another advantage was the possibility of using salt water to avoid the need to store large quantities of extinguishing agent on-board.
How Do Water Mist Systems Work?
Water is a very stable compound consisting of two Hydrogen atoms and one Oxygen atom, hence its scientific notation as H2O. Both of these individual elements exist as gases at room temperature (Hydrogen and Oxygen have boiling points of -2530C and -1830C respectively). Due to the unique Hydrogen bond that holds the combined H2O molecule together, they combine to form a liquid. This bond also accounts for two very important properties for the cooling mechanism of the fire extinguishment process – a large specific heat and a large latent heat of vaporisation.
Specific heat is defined as the amount of heat required to raise one kilogram of the water by one degree Celsius (10C). If this value is large, it takes more heat (from the thermal energy of the fire flames and core) to increase the water temperature, i.e. the water acts as a very good cooling agent. Cooling dilutes the heat element of the fire triangle that is required to support combustion. The latent heat of vaporisation is the “hidden” heat required to turn to water into steam and again contributes to the cooling effect.
As water expands around 1,700 times in the transition from liquid water to steam, this expansion in volume means that the oxygen available for combustion by the fire is massively reduced – this is another one of the fire-fighting effects of water mist systems.
The cooling and vaporisation effectiveness depends on the size of the water droplet. The maximum size for a droplet for it to be defined as a mist is below 400 microns. Water mist systems are sub-categorised by their droplet size:
- Conventional sprinkler systems operate with a droplet size larger than 1000 microns (1mm) in diameter.
- Water mist – spray droplets comprise of droplets between 100 microns and 400 microns in diameter.
- Water mist – fog droplets comprise of droplets between 10 microns and 100 microns in diameter.
- Water mist – aerosol droplets comprise of droplets less than 10 microns in diameter.
| System Type | Droplet Diameter | Effective Surface Area* | Equivalent number of Droplets* |
| Conventional Sprinkler | 1,000 microns (1mm) | 1 | 1 |
| Water Mist – Spray Droplet | 300 microns | 11 | 37 |
| Water Mist – Spray Droplet | 150 microns | 44 | 296 |
| Water Mist – Fog | 50 microns | 400 | 8000 |
| Water Mist – Aerosol | 10 microns | 10,000 | 1,000,000 |
* Variation of effective surface area and number of droplets per unit volume as a function of droplet diameter.
If the droplet size is small, then there is a greater number of droplets existing in any given volume (see the table above). Additionally, the greater the effective surface area, the greater the cooling effect on a fire and hence the extinguishing effect. Consequently, water mist has over 100 times more efficiency over that of traditional sprinklers, which typically have droplet sizes of about 1 millimetre. In practical terms, much less water is utilised for the equivalent suppression – typically one tenth when compared with traditional sprinklers. For example, in applications as little as a few litres of water are actually required to extinguish a Type B lube oil fire from a diesel generator. This in practical terms means that post discharge clean-up is greatly reduced when compared to traditional sprinklers and should not be of any major concern when using water mist.
Water mist can only contribute to fire extinguishing during discharge, as opposed to gases whereby the extinguishing capacity remains for a pre-determined time after the end of the discharge (Hold Time of the room). Although type testing on high-pressure water mist systems has shown that residual “fog” remains in the air long after discharge is complete.
The application units used for water mist are in litres per minute per m3 (l/min/m3) during the discharge. For gas systems the units used are defined as kilogram per metre cubed (kg/m³) or percentage by volume present in the enclosure at the end of discharge.
Water mist systems designed for total-flood do not require complete integrity for extinguishment to be attained. Openings can be used without detriment to system performance, although large extract systems may need shutdown dampers for additional spaces. In this sense, water mist systems don’t suffer from the need to maintain absolute room integrity as do fixed gas systems. Where an opening is present, a “door” nozzle can be located that produces mist to exclude oxygen entering the room and shield the transfer of radiation.
Production of Water Mist Droplets
The fundamental key to the production of water mist is in the discharge nozzle. The important differences in design of water mist systems are how the water arrives at the nozzle, how the water is pressurised and how the water is made into mist (via atomisation or impingement).
The key to water mist droplet production is the combination of water pressure and discharge nozzle orifice size.
- Open nozzle (dry pipe) systems comprise a mechanical or electrical valve that will allow a flow of water into the distributed pipe work system. The discharge of water normally occurs through all of the nozzles simultaneously.
- Closed nozzle (wet pipe) systems are normally charged with water that is retained behind each nozzle, which is sealed with an integral mechanical heat detector. Discharge is then local to the nozzle that has actuated, much in the same way as a wet sprinkler system.
The water itself can be normal potable water, seawater or distilled water. Fire enhancing additives or antifreeze can be added to some systems if required, and the water can be pressurised cylinder or pump/tank delivered dependent upon the system requirements and running time required in the protected space.
In general, type testing has shown that high-pressure water mist systems (100 bar plus) produce smaller mist droplets, and permit the fog produced to suspend in the air and maintain extinguishing effectiveness over a longer period of time, as well as, achieving gas-like permeability between objects.
The IMO (International Maritime Organisation) adopted design standards and proving tests to validate the use of different water mist systems in the protection of ships. This is partly because the risks inside ships are clearly defined (e.g., machine rooms, cabins). Additionally the risks are clearly specified where each machine room occupies a similar volume, concerns a similar fuel type and has a similar distribution in all ships of a given size. This allows the testing of water mist systems to scale to obtain general guidelines. The tests are carried out in rooms simulating real volumes with actual fuels and objects.
Water Mist System Installation with Sovereign Extinguishing
Here at Sovereign Extinguishing, we specialise in installing and maintaining high-pressure water mist systems for industrial applications across the UK. All fire suppression systems we supply are tailored to your individual requests and requirements.
If you’d like to find out more about water mist systems or would like a quote for installation and maintenance, please don’t hesitate to get in touch today.
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