How Does DualMIST Work?

Overview of watermist. What is Watermist?

Mist is defined as a large mass of water vapor in the air consisting of fine drops of water, or a fine spray of liquid.

Following the CEN/TS 14972 definition, watermist is a water spray for which the 90% of the total volume of liquid (Dv0. 90) is distributed in droplets with a diameter smaller than 1000 microns at the minimum design operating pressure of the water mist nozzle.

A water mist system uses water mist to control, suppress, or extinguish fires by:

  • Cooling the flames and surrounding gasses through evaporation.

  • Displacing oxygen by evaporation.

  • Diminishing radiant heat by the small water droplets themselves.

The Dualmist low-pressure misting system is a highly effective fire suppression method due to it's range of droplet sizes.

Dualmist utilises it's vast well of engineering knowledge incorporating complex methods to produce the unique nozzles and range in droplet sizes.  These range from small(<1000m) to large (jet spray).  Each of these size ranges play their own part in displacing the fire.

The larger droplets have the ability to penetrate the fire as the smaller droplets act like a gas causing the fire to suffocate and removing large amounts of heat energy particularly in places other suppression systems may struggle to reach (under tables for example).

In the chart above you can see that the range of droplet sizes produced by the Dualmist system is in between that of sprinklers and high pressure systems.

With high pressure systems - a very fine mist/aerosol is produced which is very effective at heat suppression but not so good at high ceiling heights or in open areas as side draft can cause the vapor to be swept sideways away from the fire. With high pressure systems there is little surface wetting due to the droplets evaporating before they reach the fire or nearby heated surfaces.

With sprinkler systems there is a huge amount of surface wetting but very little heat suppression as there is very little mist produced and little to stop the heat spreading across the ceiling cavity by convection which is where most of the heat from the fire ends up. There is also no mechanism to stop heat transfer across the room from radiating burning/hot objects and even worse with sprinklers most of the water quickly ends up on the floor and is then effectively useless from a suppression point of view. The length of time the the water droplets spend in the air is a critical factor in heat suppression.

The fine droplets stay air-born for a prolonged period of time until most eventually evaporate and expand into inert gas (water vapor) displacing the air in the room and thus reducing the percent oxygen levels which in turn reduces the overall burn rate of the fire. Dualmist nozzles use a combination of all the above methods to suppress the fire.

As you can see for a 2.5 height room there is an optimum range of droplet size to produce both effects. For higher height rooms the optimum range droplet size moves upwards accordingly.

How does low pressure watermist work?

 

Low pressure watermist generally produces droplets over a range of sizes described as fine and coarse which are between those usually created by high pressure systems and sprinklers.

 

Most fires expand as the result of the exponentially increasing processes of pyrolysis and oxidation.

 

The pyrolysis process occurs when energy in the form of heat is applied to the surface of a fuel, as at some point the fuel begins to decompose into flammable gases (eg CH4 = methane).

 

The oxidation process occurs when more energy (heat) is released as the free oxygen in the air combines with the flammable gases. This process releases large amounts of additional energy and is what gives the fire the ability to grow ever larger.

 

Water mist is a very effective means of fighting fires as it interferes with both the pyrolysis process and the oxidation process and stops the expansion of the fire - ie the fire is suppressed.

The fight against the oxidation process:

Small drops of water evaporate easily as they have a large surface area per volume of water. When water evaporates it takes large amounts of energy to change state (latent energy) and this energy is removed from the fire. If as much energy can be removed as the fire produces, the fire is under control.

 

Water vapor is also created when water evaporates and the vapor expands in volume by a factor of about 1400.

 

The density of water is 1000 kg/m3

 

The density of water vapor is approximately 0.7kg/m3

 

This vapor acts like an inert gas that does not participate in the oxidation process. Therefore, it helps to reduce the amount of oxygen in the room and the vicinity of the fire, since it replaces the normal atmosphere. The more inert gas kept close to the fire, the better the performance, the oxygen level will be reduced and the fire cannot easily expand.

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