The Singapore Civil Defence Force (SCDF) in recent years has employed water mist guns to extinguish fires more quickly and with little damage to property. How are water mists able to extinguish fire more effectively than conventional jets of water? What is the Physics behind it?
We know water mist consists of many tiny water droplets. With the volume of water fixed, the combined exposed surface area of the droplets is much larger than the exposed surface area of a jet of water. Therefore, in a fire, the water mists will vaporise much afster than jets of water. With faster vaporisation (i.e. latent heat of vaporisation), more thermal energy from the fire can be removed effectively, helping to put out the fire faster.
At the same time, as the mist vaporises to become steam, its volume expands (by at least 1600 times!) and displaces the air from the surroundings. This deprives the fire of the oxygen it needs to continue burning.
During tests conducted by the SCDF, a fire the size of a three-room HDB (Housing Development Board) flat can be put out with just 36 litre of water in less than three minutes. Whereas conventional water jets require 3600 litre of water and at least 10-15 minutes to put out the fire! Also, water mists are able to absorb the toxic gases released when household materials burn; thus protecting the victims. Since less water is used in this method of fire extinguishing, not only is water saved, the amount of property damaged is also minimised (like furniture and home electrical appliances), thereby facilitating fire investigation later on.
Source: Chew C. et al. (2007). GCE 'O' Level Physics Matters. Marshall Cavendish Education, Singapore.
It is quite clever, is it not?
ReplyDeleteya, the use of mist actually saves our precious water.
ReplyDeleteand the fire get put off much more faster.
It's the knowledge of science!!!
Interesting info, Thanks for the share!
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