The intelligent use of water High-pressure water mist For a fire to survive, it relies on the presence of the three elements of the ‘fire triangle’: oxygen, heat and combustible material. The removal of any one of these elements will extinguish a fire. A high-pressure water mist system goes further. It attacks two elements of the fire triangle: oxygen and heat. Oxygen Heat The very small droplets in a high-pressure water mist system quickly absorb so much energy that the droplets evaporate and transform from water to steam, because of the high surface area relative to the small mass of water. This means that each droplet will expand more than 1,700 times, when getting close to the combustible material, whereby oxygen and combustible gasses will be displaced from the fire, meaning that the combusting process will increasingly lack oxygen. To fight a fire, a traditional sprinkler system spreads water droplets over a given area, which absorb heat to cool the room. Due to their large size and relatively small surface, the main part of the droplets will not absorb enough energy to evaporate, and they quickly fall to the floor as water. The result is a limited cooling effect. By contrast, high-pressure water mist consists of very small droplets, which fall more slowly. Water mist droplets have a large surface area relative to their mass and, during their slow descent towards the floor, they absorb much more energy. A great amount of the water will follow the saturation line and evaporate, meaning that water mist absorbs much more energy from the surroundings and thus the fire. That’s why high-pressure water mist cools more efficiently per litre of water: up to seven times better than can be obtained with one litre of water used in a traditional sprinkler system. 20 vol % O2 Temp. °C 100 O 2 O2 16 VOL % O2 10 378 2635 Energy absorption KJ/kg Water mist cooling effect per kg water Traditional sprinkler cooling effect per kg water Conclusion The uniqueness of water mist is that it combines the suppression effect of gas and traditional sprinkler systems. As well as removing the oxygen like a gas system, it simultaneously cools the fire like a traditional sprinkler. The cooling effect additionally lowers the risk of re-ignition. SEM-SAFE® by Danfoss · Marine & Offshore 3
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