J.R. Mawhinney

This paper discusses a number of practical issues relating to the design of water mist fire suppression systems. There is presently a lot of interest among fire safety engineers in using water mist systems as an alternative to halon, on the basis that the mist will act like a gaseous suppression agent to fill all recesses of a compartment. Although there is a growing confidence that water mist systems can successfully extinguish flammable liquid pool fires and high pressure jet fires with small amounts of water, engineering criteria are needed to allow designers to match a water mist system to a range of fire scenarios and compartment types. This paper draws a parallel between the long-established practices for design of standard sprinkler systems, which allow any experienced designer to custom-fit a sprinkler system to a wide variety of fuels and buildings, and the need for similar design principles for the design of water mist systems. Research being carried out by various research agencies has just begun to document the information required to establish general design criteria for mist systems.

Starting with the need to define both the fire hazard and the fire safety objective, this paper presents information on characterizing sprays suitable for water mist systems. It points out the macro-scale effects in large volume compartments cause agglomeration of droplets, so that what starts out as a very fine spray ends up as a much coarser spray. It also points out that the types of nozzles available for producing suitable fine sprays have, at best, fairly high pressure demands and in the case of air-atomizing nozzles, very high compressed air demands. These factors set a practical limit on the size of compartment that can be produced in a cost-effective way by total flooding systems. An alternative to total flooding systems for larger compartments would be zoned piping linked to a sophisticated detection system. Much experimental work will be needed to validate such systems.