Water Mist is defined within NFPA 750 as a water spray where 99% of the water produced is distributed by droplets that are smaller than 1000 microns in diameter at the required design pressure at the water mist discharge nozzle.
Systems are typically categorised into high and low pressure. High pressure systems are defined as those which will operate at pressures greater than 35 bar, but typically from 70 - 130 bar. Low pressure systems typically operate at pressures up to 12 bar.
Both system types can be provided as either wet pipe or deluge systems. Wet pipe systems will operate in the same way as a standard sprinkler system utilising nozzles that incorporate a frangible bulb element that will rupture and activate the system once the pre-determined temperature is reached. Deluge or dry pipe systems will typically operate from electronic detection systems which will then provide a signal to activate a cylinder valve or pump.
The two main mechanisms employed by water mist for the control and extinguishment of fire are by vapour generation and surface cooling of the fire itself. When water vaporises it displaces air to over 1,600 times its original volume, thereby depleting Oxygen within the protected enclosure or local to the fire. As the water mist droplets are smaller in diameter than conventional sprinkler droplets they present a greater surface area for cooling the surface of the fire which therefore absorbs heat faster. The table below demonstrates the surface area available in relation to droplet size.
Consequently, water mist systems will extinguish a fire by improved cooling and blockage of radiant heat but with reduced water demand when compared to traditional sprinkler systems.
For further information on any water mist system please contact your local office.
Northern Office: 0845 0540 516 / Southern Office: 0845 0540 518