The use of water mist systems as a modern sustainable technology in the firefighting industry has grown significantly over the last two decades. Water mist systems offer significant advantages over conventional fire protection systems – they use less water than traditional sprinkler systems, cause less damage to high-value tangible assets, and are also a cost-effective and environmentally friendly firefighting solution.
And yet, when it comes to fire extinguishing based on the water as an extinguishing agent, we initially think of traditional sprinkler systems.
Sprinkler technology is stable and reliable and has proven its effectiveness in practice for over a hundred years. Sprinkler systems can provide protection throughout the building, but they do not provide the „three-dimensional” protection as a new water mist technology would, and are usually designed only to control fire, protecting the building’s structure and people’s safety. In addition, the response time of sprinklers in case of fire is relatively long, and the water generated through the sprinklers is a significant amount.
Significant water consumption – this is the main disadvantage of the sprinkler technology. It not only leads to unnecessary waste of water resources and complex water supply infrastructure but increases the risk of property and equipment damage more than the fire itself. Sprinkler technology has long been standardized and the industry prohibits water use optimization. This naturally leads to the development of water mist technology as a sustainable alternative with many improved performance indicators for efficiency, safety, environmental friendliness and feasibility.
WHAT IS WATER MIST?
Spraying water in the form of very fine micro-sized droplets forms a water mist with a large volume and the process is called water „atomization”. Water mist is determined by droplet size of less than one millimeter (1 mm). According to the international standards in the firefighting industry, water mist represents atomized water, in the volume of which 99% (NFPA) or 90% (CEN) of water droplets are smaller than 1 mm (1000 microns). This value is calculated for a minimum operating pressure and a distance of one meter from the nozzles. The size of the droplets is a result of the water pressure and the type of spray nozzle used. Three main types of water mist can be defined – with droplet sizes from 10 to 200 microns, from 200 to 350 and over 350 microns. And respectively there is three main types of water mist systems – low, middle and hight pressure (Fig.1).
WHAT IS WATER MIST FIRE EXTINGUISHING TECHNOLOGY??
Water mist firefighting technology is a relatively new technology originally developed for marine vessels, where low water consumption is an essential requirement to the fire system.
The new technology shares many similarities with the well-known sprinkler systems. Both technologies use water as an extinguishing agent, a water distribution pipe network, zone valves and spray nozzles mounted on ceiling or wall. In contrast, however, water mist technology is based on the use of a much smaller amount of water, composed of many more droplets of much smaller sizes. The main reason being the difference in the working pressure used by these two technologies. While sprinkler systems typically operate at pressure up to 12 bar, water mist systems operate within a wide range of up to 200 bar. Thus, the sprinkler system produces droplets with a typical diameter of 1 mm and more, while the water mist systems produce water droplets with a typical diameter of 0.1 – 0.3 mm.
Why does the droplets size matter?
When the droplets are smaller, the heat transfer rate is higher and therefore the result is faster cooling and extinguishing the fire. Moreover, the smaller droplets fall slower from ceiling to floor, stay longer in the air and therefore there is more time to interact with the heat and smoke from the fire. This makes the water mist technology very effective in firefighting. Water mist systems typically consume up to 90% less water than sprinkler systems, resulting in minimal damage after extinguishing and significantly simplified infrastructure for water transportation and storage such as tanks, pumps and pipes.
HOW DOES WATER MIST FIRE EXTINGUISHING TECHNOLOGY WORK??
To explain the operation of water mist systems, it is necessary to clarify how fires occur. For a fire to occur, we need to have four elements in place at the same time: combustible material / substance, heat, oxygen, and chemical reaction (Fig.2).
The first three elements are often called the „fire triangle”. The addition of the fourth element (chemical reaction) forms the so-called „fire tetrahedron”. Removing any of these elements will result in extinguishing the fire while interrupting or weakening one of these elements will lead to fire control or suppression. Water mist takes away one or more elements of the fire tetrahedron.
How does water mist work?
Water mist „copes” with fire by reducing heat, displacing oxygen and controlling the fuel source. The key to the effectiveness of water mist technology is the design of the nozzles together with the established pressure criteria to create a mist of very small water droplets. The small droplets absorb heat more rapidly than the larger droplets due to the larger surface to mass ratio. Water in the form of mist of very small droplets has a larger volume and a larger surface area than the same amount of water composed of larger droplets. The result is rapid heat absorption and rapid evaporation leading to rapid cooling and displacing of oxygen.
When water droplets absorb enough heat, they evaporate quickly, creating a local inert medium with a large volume. Thus, the oxygen is taken away from the combustion zone, disrupting the fire tetrahedron. On the other hand, small water droplets remain in the air longer and fill the space, saturating the air. The result is evaporation and expulsion of the oxygen up to the moment when it is no longer possible to reignite. The water mist also helps to pre-wet and block heat transfer to adjacent flammable materials / substances, reducing the risk of fire spreading and growing.
This „three-dimensional” extinguishing mechanism – cooling, oxygen depletion and heat transfer blocking, determines the high extinguishing speed and high extinguishing efficiency of water mist technology and also the outstanding performance of the systems based on this technology. This means that water mist can be used to fight fires in places where ordinary water would not be the most suited or most efficient extinguishing agent.
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