- Describe the methods by which a high pressure water-mist fire suppression system extinguishes fire, stating why it is more effective than a low pressure sprinkler system.(10)
High Pressure Water-Mist Fire Suppression System
Methods of extinguishing fire:
- Cooling effect – the very fine water droplets (mist) produced at high pressure (typically >80 bar) have a huge total surface area, which absorbs heat rapidly, reducing the fire temperature below ignition point.
- Oxygen displacement – droplets vaporise almost instantly into steam, displacing oxygen around the flame zone and reducing the oxygen concentration below that required to support combustion.
- Radiant heat attenuation – the mist cloud absorbs and scatters radiant heat, preventing flame spread and protecting nearby equipment and personnel.
- Localised wetting – although the droplets are extremely fine, sufficient water deposits on hot surfaces to prevent re-ignition.
Why it is more effective than a low-pressure sprinkler system
- Smaller droplet size – high pressure mist produces droplets typically <100 microns, creating a much greater surface area-to-volume ratio, enhancing both cooling and oxygen displacement.
- Lower water consumption – requires around 70–90% less water than traditional sprinklers, reducing water damage to machinery, insulation, and electrical equipment.
- Rapid steam generation – mist vaporises almost instantly, whereas sprinklers produce larger droplets that take longer to convert into steam, giving slower suppression.
- Minimal flooding risk – reduced water use means less risk of free surface effect or damage to ship’s stability compared with low pressure systems.
- Better penetration – high velocity mist penetrates machinery casings and shielded spaces more effectively than low-pressure sprinklers.
- Environmentally safe – no toxic by-products (unlike CO₂ flooding systems) and no contamination of stores/foodstuffs, which can occur with large volumes of sprinkler water.