Due to its adverse effect on the ozone layer, halon is being phased out in accordance with the Montreal Protocol. As a result, other methods of putting out fires must be found which would help protect the environment, be harmless to humans as well as being equipment friendly.

For fire to break out, three conditions are needed: flammable material must be present, oxygen is required to feed the fire, and heat is required. If any one of these criteria is removed, a fire will not start, or, if already burning, will go out.

The International Maritime Organisation (IMO) has drawn up a set of rules which call on shipping companies to introduce fire-protection measures on passengers vessels transporting more than 36 people. Frequently equipped with halon systems, this sector has been at the forefront of efforts to find a suitable replacement fire protection measure.

Water has always been considered a cheap, available and suitable substance for putting out fires. Its disadvantages are that being used in a traditional sprinkler system, large volumes of water are required, leading to possible equipment damage and a substantial weight problem in any form of transport. In the search for a suitable alternative, water mist was rediscovered. Paradoxically, it was supplanted by halon in its day.

The high pressure water mist process developed by Danfoss is environmentally friendly, can replace halon in most applications, causes little or no equipment damage, has low weight and requires little space. Even small quantities of water can extinguish fires, including electrical fires and those involving hydrocarbons.

The system consists of nozzles developed for high-pressure water atomizing and a high pressure Nessie pump, which is compact, maintenance-free, and lubricated by water. For pressure regulation, there are pressostats, pressure transmitters and overflow valves.

In the event of a fire, one or more nozzle heads will be activated and the pilot pressure will drop. The pressostat records the pressure drop and activates the pump via a control panel.

The water pressure is built up to an 80bar to 100bar setting which creates at the nozzles a water mist with water droplets of less than 100mm, which are jetted into the fire at great speed. Due to atomizing, the water surface is very large and the water evaporates at an explosive rate. The transition from water to steam takes place by a 1700-fold expansion during which the oxygen is displaced from the site of the fire and the fire is simultaneously cooled.

In the event of a fire starting in an engine room, or an oil fire, the water mist will both cool and be emulgated with the oil, thus preventing evaporation of the oil and post-ignition.

The process is undergoing a resurgence and is being considered for use on aircraft, and as protection on the wagons that carry goods vehicles on trains travelling through the Channel tunnel.

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Nessie pumps, valves and water mist nozzles together create a water mist with droplets of less than 100 micro m at the nozzles