Watermist
The term “water mist” refers to fine water sprays in which 90% of the volume spray is in drops with diameters less than 400 microns.
The term “water mist” refers to fine water sprays in which 90% of the volume spray is in drops with diameters less than 400 microns.
The water mist combines the extinguishing advantages of water (like conventional sprinkler) and of gaseous agent.
Hence, the extinguishing abilities of the water are magnified: increasing effect of oxygen displacement and reaction zone cooling.
Reaction zone cooling is a thermal mechanism, where the high heat capacity and heat of vaporization of water are heat sinks that reduce the flame temperature. The thermal conductivity of water vapor also contributes to a smaller degree to reaction zone cooling. Moreover, the droplets can form thermal shield which mitigate the radiant heat.
Oxygen displacement reduces the concentration of oxygen available to sustain the flame, by both global dilution and physical displacement by localized water vapor from mist evaporation.
Hence, these mechanisms are resulting from the evaporation of the water mist in the area surrounding the fire.
Beside this efficiency, the use of water mist in fire suppression, has demonstrated advantages including:
The NFPA 750:2015 standard documents a number of aspects and regulations concerning water mist systems, including categories, testing, design, components and basic stipulations.
Likewise, the APSAD D2 technical documentation is the reference document for France: it defines minimum installation, maintenance and reliability requirements.
Either way, water misting systems should be considered on a case-by-case basis to determine whether the proposed system will meet the fire protection objectives, which should be clearly stated
Water has favorable physical properties for fire suppression. Its heat capacity (4.2 J/g.K) and high latent heat of vaporization (2442 J/g) can absorb a significant quantity of heat from flames and fuels. Water also expands 1700 times when it evaporates to steam, which results in the dilution of the surrounding oxygen and fuel vapors.
With the formation of the fine droplets, the effectiveness of water in fire suppression is further increased due to the significant increase in the surface area of water that is available for heat absorption and evaporation.
|
Droplet size (mm) |
6 |
1 |
0.1 |
|
Total number of droplets |
8.8 x 103 |
1.9 x 106 |
1.9 x 109 |
|
Total surface area (m2) |
1 |
6 |
60 |
Hence, the two main mechanisms (Reaction zone cooling and Oxygen displacement) are magnified with a decreasing of the droplet size.
But, many factors, such as the enclosure effect, dynamic mixing created by water mist discharge, types of water mist systems applied (total or local application) and the use of additives and discharge modes, have important impacts on the effectiveness of water mist in fire suppression.
As we see above, lots of parameters can impact the effectiveness of the water mist, nevertheless, they can be further broadly classied as three main parameters: droplet size distribution, flux density and spray momentum. These three main parameters of water mist not only directly determine the effectiveness of the water mist for fire suppression but also potentially determine the nozzle spacing as well as the ceiling height limitation for a given installation.
In theory, small droplets are more efficient in fire suppression than large droplets, because of their larger total surface area available for evaporation and heat extraction. Hence, NFPA 750 has divided the droplets produced by a water mist system into three classes to distinguish between “coarser” and “finer” droplet sizes within the 1000 microns window. The classifications are: Class 1 mist has 90% of the volume of the spray within drop sizes of 200 microns or less; Class 2 mist has its 90% of volume of 400 microns or less; and Class 3 mist has its 90% of volume value larger than 400 microns.
But, in conclusion, there is no one‑size distribution to fit all fire scenarii (fuels, ventilation, compartment…). Moreover, the performance of water mist with a well‑mixed distribution of fine and coarse droplets is better than that with a uniform droplet size.
Spray Flux Density refers to the amount of water spray in a unit volume (L per minute/m3) or applied to a unit area (L per minute/m2). Without sufficient flux density of water sprays to remove a certain amount of heat from a fire or to cool the fuel below its fire point, the fire can sustain itself by maintaining high flame temperature and high fuel temperature. Since water does not behave like a “true” gaseous agent, it is difficult to establish the “critical concentration” of water droplets required to extinguish a fire (i.e. the minimum total mass of water in droplets per unit volume or per unit area for fire suppression). The amount of mist reaching the fire is determined by many factors. These include the spray momentum and angle, fire size, ventilation conditions and compartment geometry.
The flux density distribution of water mist within a single nozzle spray cone is non-homogeneous. Some types of nozzles for the production of water mist concentrate a high percentage of the water spray into the center of the cone area while other types of nozzles may have less water mist concentration at the center area. When sprays cones from a group of nozzles overlap, the flux densities at any point are also different from those observed with a single nozzle due to the dynamics of spray interaction.
Water spray momentum refers the need to bring as close as possible to the flames, in the heart of the fire, the maximum number of droplets. However, multiple parameters can influence droplet trajectories: These include droplet size and velocity, but also discharge pressure and cone angle. The compartment geometry and the fuel size (=surrounding temperature).
Basically, the spray momentum will gradually decrease, as fine waterdroplets travel through hot gas and the droplet velocity and size are reduced due to gravitational and drag forces on the droplets with the evaporation. That’s why the performance of water mist with a well‑mixed distribution of fine and coarse droplets is better than that with a uniform droplet size.
This is why a study, on a case-by-case basis, by our experts, is necessary to design the most efficient installation.