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Sunday, 10 January 2016

Fighting fires with water mist

As a process, a fire involves a chemical reaction between combustible fuel and oxygen from the air. The prerequisites of sustained burning are:
  • Combustible fuel: This can be solid (Class A), liquid (Class B) or gas-phase
  • Oxygen: The ambient air contains about 21 volume-% of oxygen
  • Heat for ignition: Once the fire has got started, the fire itself will generate the heat to sustain burning
  • Uninterrupted chemical chain reaction between the above three elements

To extinguish a fire, at least one of the above elements must be removed:
  •  The fuel supply can in some cases be removed, e.g. by shutting of the fuel supply to an engine. In other cases it cannot be affected
  • The oxygen concentration can be lowered, e.g. by adding an inert (non-reactive) gas into the fire
  • The heat can be removed, e.g. by wetting the combustible surfaces or cooling the flame.
  • The chain reaction can be interrupted by adding a chemical that reacts with unstable sub-products of combustion.

When talking about fire fighting, three different terms are used:
  •  Fire extinguishment: Complete elimination of any flames and smouldering fire. No re-ignition will take place, when the fire fighting is ceased
  • Fire suppression: A sharp reduction in the heat release rate and prevention of re-growth of the fire
  • Fire control: Limitation of fire growth and prevention of structural damage

When talking about fire protection, water mist is divided into three categories:
  • Low pressure: < 12 bar
  • Medium pressure: > 12 bar and < 36 bar
  •  High pressure: > 36 bar

Water uses two major firefighting mechanisms, both related to the evaporation of water:
  • Cooling: When turning into vapour, water absorbs more heat than any other firefighting agent
  •  Inerting: When turning into vapour, the water volume expands 1760 fold, causing the oxygen to be displaced

The droplet size of a conventional sprinkler system exceeds 1 mm. The corresponding size for high pressure water mist, when using special nozzles, is in the order of 0.01 mm (nebula). Hence, with a high pressure water mist system, considerably less; water is needed for the same cooling and inerting efficiency as in conventional water spraying systems. Water in the form of mist offers an additional firefighting benefit that no other fire-fighting agent has, i.e. blocking of radiant heat. The large number of droplets and the large combined surface area effectively absorbs and scatters heat radiation. This prevents the spreading of the fire. This radiant heat blocking is so efficient that people at a distance of only a few meters from the fire feel no heat.

The use of water mist systems in the United Kingdom has increased over the last few years due to a number of reasons, as a technology it is:
  • Non-toxic;
  • No asphyxiation risk;
  • Low cost;
  • Efficient at cooling due to fast vaporisation of the small droplets;
  • High efficiency in suppression;
  • Environmentally friendly;
  • Minimising consequential water damage;

(Reference: High Pressure Water Mist - The Modern Approach in Fire Fighting, Stefan Gordin)

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