These systems uses fine water mist as the extinguishing agent. More precisely, the relevant European regulation UNI CEN/TS 14972:2008 defines water mist as a water spray for which the Dv0.90, measured on a plane situated 1 m from the nozzle, is less than 1000 µm; it specifies that the Dv0.90 is the value of the typical diameter whereby 90% of the total volume of spray is characterised by drops with a diameter smaller than this value. This type of water spray is obtained using appropriate nozzles supplied with water at high pressure; in reference to the system’s design pressure, the European standard (previously stated) refers to three types of systems:
1. Low pressure systems (design pressure up to 12.5 bar)
2. Medium pressure systems (design pressure between 12.5 and 35 bar)
3. High pressure systems (design pressure higher than 35
The water mist system is highly effective in extinguishing fires, due to the different actions it performs:
1. a thermal cooling action, determined by the enormous absorption of the amount of heat;
2. a mechanical action of reducing the concentration of combustible oxygen, due to the significant quantities of water vapour quickly produced;
3. an action of reducing the thermal power transmitted by irradiation, due to the high quantity of fine particles and vapour dispersed into the air, in addition to the combination of the phenomena of absorption and scattering.