HOW THE DUALMIST NOZZLES WORK
The key to understanding the beauty of how the Dualmist System works and also how low pressure watermist works in general is to try to understand the why the droplet size range is so important.
In the chart below 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.
Conversely 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. With sprinklers 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. Please take a moment to study the below chart as it is of critical importance in terms of understanding how the system works.
As shown, the dualmist system produces the range of droplet sizes in between both.
This means the system produces both mist for heat suppression and coarser droplets for surface and wall wetting. Some of these larger droplets spray high up onto the walls and then run down the walls keeping them cool. They also have the benefit of helping to distribute the smaller droplets away from the nozzle head. The fine droplets stay airborn 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. The 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.
OVERVIEW OF WATERMIST
What is Watermist?
According to the NFPA definition, water mist is a water spray for which 99% of the total volume of liquid (Dv0.99) is distributed in droplets with a diameter less than 1000 microns at the minimum design operating pressure of the water spray nozzle.
Droplet size was considered in the early stage of water fog as an important tool for categorizing water mist, as people thought that droplet size and fire fighting performance were correlated. Therefore, older standards and codes differentiated water mist systems based on the droplet size in the system and required droplet sizes to be measured.
This is not the case in up-to-date codes and standards as it has long been proven that only large scale fire tests, and not droplet size, will show whether a system can cope with a certain risk or not.
The reason people thought there was a correlation was because the water mist showed remarkable results in machinery enclosures where tiny water droplets could easily evaporate and turn into inert gas (water vapor) due to the heat created by the big fires and extinguish any fires like gas. It is a fast and efficient system.
However, empirical testing evidence shows that a mixture of many different sized droplets can do the same in enclosures and provide even better results in deep-seated Class A fires. For the same reason, water mist systems will always be chosen based on positive test and approval results, and not just droplet size.
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.
The Fight against the pyrolysis process :
Water droplets are distributed from the nozzles at high speed and this can be used to put out the fire directly. In fact, what happens is that the energy source is removed from the surface of the fuel, so no additional flammable gases can be created.
Part of the distributed water will cover the surface of the nearby hot surfaces and prevent the heating of these surfaces which hinders the creation of additional flammable gases. The more water is distributed over the fire and nearby hot surfaces, the more efficient it becomes. The fire cannot take hold and will eventually run out of fuel.
Why are low pressure water mist systems effective even though they use less water?
Through specially designed nozzles, the system distributes water mist droplets at high speed and with a wide range of droplet sizes ranging from almost invisible droplets to large droplets. The wide variety of droplets helps fight fire in many different ways at the same time. Larger droplets penetrate the fire column and end up on the fuel surface where they prevent flammable gases from being created. The medium-sized droplets penetrate the fire column and evaporate within the fire creating a large quantity of inert gases where it is needed. The small droplets evaporate on the outside of the fire creating a cool atmosphere that ensures that the fire does not spread.
The water mist system allows multiple fire fighting methods at the same time, thus optimizing the use of water to the maximum.
Advantages over sprinklers
– Use less water which benefits green technology (typically around 50% – 70% savings)
– Higher working pressure means pressure drop from height is less significant
– Higher K-Factor means means pressure drop from nozzle activation is less significant
– More attractive aesthetics (innovative designs)
– Reduced weight of the systems
– May be more effective in hydrocarbon fire performance
– Less water damage
– Smaller pipe dimensions
– Smaller water tank required thus reduced space needed
Advantages over High Pressure Watermist
– Less dangerous working pressure (5-16 bar vs 100-300 bar)
– Larger waterways (hole size for low pressure typically 0.5 - 2.0mm vs high pressure: 0.1-0.2mm)
– Less need for filtration and less risk of filter clogging due to larger filter apertures.
– Uses cheaper system components (thin walls versus thick walls, CPVC plastic pipes and galvanized pipes can be used in low pressure systems)
– Easier and cheaper to install and maintain