Fire Suppression Properties of Very Fine Water Mist

Water mist is one of the best nongaseous fire suppression replacement agents for Halon 1301 on a mass basis. In general, small drops are more effective than larger drops, but the highly complex nature of fires defies a simple generalization. Drop size is important but so are drop transport and evaporation behavior. Understanding each of these is critical to the design of water mist fire suppression systems. Water spray systems like sprinklers in many buildings produce very large drops, generally above a millimeter in diameter. These systems ineffectively deliver much more water than is usually needed. Water mist systems that use low-pressure nozzles typically produce drops between 200 microns and 1 mm. The production of fine water mist drops below 200 microns requires high-pressure nozzles. NRL studies in laboratory flames show that drops smaller than 40 microns are more effective than Halon 1301 on a mass basis at extinguishing methaneair and propaneair non-premixed flames. Experimental and computer fire modeling studies indicate that effectiveness continues to increase with decreasing drop size. The case looks especially attractive for very fine water mist with drop diameters less than 10 microns.