The use of micronized spray water in firefighting.

Water is the oldest and most common fire-extinguishing agent known to mankind. This is due, first of all, to its availability and prevalence, as well as its high fire-extinguishing capacity. The high efficiency of water as an extinguishing agent is determined by the cooling effect that water has, getting on the focus of the flame, as well as localization of the flame, limiting the access of oxidant (oxygen molecules of air) to the burning zone due to the formation of vapor space during the evaporation of water.

Obviously, an important factor determining the efficiency of water use in a fire is the method of its supply to the source of combustion. The widely known concepts viz. compact and atomized water supply by their very names define the physical essence of one or another method. Each of these methods has its own positive and negative qualities.

For example, a compact jet is used when it is necessary to deliver water over long distances when only the kinetic energy of the compact jet can solve this problem. In this case, the amount of water spent directly on extinguishing meaning the coefficient of water use is quite low, which is due to water hitting only some parts of the burning surface, when most of the water flowing down remains unused to extinguish the fire, causing significant damage to property, which is most often the case when extinguishing multifloored buildings.

Sprayed water is largely devoid of this disadvantage and the coefficient of water use is significantly higher. This is due to the fact that the droplets form a fairly uniform aerosol suspension in the volume of the protected premises, covering simultaneously not only the fire zone but also adjacent surfaces. Given that the fundamental mechanism of water extinguishing is evaporation, it is obvious that an important parameter determining the effectiveness is the size of the droplets. This is due to the fact that reducing the diameter of the drop leads to an increase in the surface area per unit mass of water viz. the specific surface of the drop, which in turn increases the intensity of evaporation in the hearth of the fire and, thus, increases the effectiveness of fire extinguishing. It follows from the foregoing that to increase the efficiency of water, it is advisable to achieve as much as possible dispersion of water (meaning its grinding) making the droplet size commensurate with the droplets of fog viz. of 10 - 50 microns. However, as numerous experiments show, the effectiveness of such mists (so called "water fog") in extinguishing even small fires is close to zero. This is explained by the fact that when considering the process of evaporation of droplets we ideally assumed that the droplets have reached their target which was the source of combustion and directly affected the flame, while the process of droplet delivery, consisting in overcoming the thermal barrier and convective flows from the seat of the fire to a large extent is a limiting factor in extinguishing with atomized water.

As a result of numerous studies and attempts to achieve positive results on extinguishing by sprayed water, some optimum drop size was obtained which is 100...150...200 microns at which the greatest effect on extinguishing by pure water is achieved. Therefore, it seems most interesting to consider the existing technical methods and means of extinguishing with fine spray water (FWS) which refers to the above-mentioned dispersion of water droplets.

According to their functional purpose FWS extinguishing means are divided into mobile and stationary. Mobile units include backpack type "IFEX" (Germany). The unit uses the principle of discrete pulse delivery of a fixed dose of water (not more than 1 liter) under the influence of a high-pressure air charge. The range of water supply does not exceed 16 m, the average particle size is up to 200 microns.

The use of spray barrels from these units with a large supply of water and air made it possible to create mobile rapid-response complexes based on motorcycles and automobiles. The physical principles inherent in the backpack systems discussed above have been developed in larger units designed for use on heavy vehicles, ships, and helicopters.

The water flow rate in such trunks is 0.3-0.6 l/s at a pressure of 100-200 atm, for which purpose special high-pressure pumps are used which are able to provide the required parameters. Such systems include in particular the developments of German companies "Fogtec Fire Protection" and "Jomos Brandschutz". At the high enough efficiency of the presented trunks, they have significant disadvantages, conditioned by expensive precision technology of element base of pumps and trunks and as a result, high requirements to the quality of water used. Stationary fine-spray water fire-extinguishing installations were known quite a long time ago and were widely used for the ship's protection (in particular, the "Grinnell" systems).

Companies "LPG" (Spain) and "Forgtec Fire Protection" (Germany) operate on a similar principle. The main disadvantage of the last three systems are stringent requirements for water purity and, as a consequence, expensive equipment installation. At the same time, a sufficiently fine atomization of water, accompanied by short pulse duration and high velocity of droplets provides a high intensity of water supply, resulting in a reduction of water consumption required for fire suppression and losses due to the impact of water on protected objects.

?Modular systems by the nature of the effect on the fire are closest to the volumetric fire-extinguishing means, not being such, but belong to the local-volumetric, that is, protecting the limited space in the area of location of the sprayers.

Comparison of extinguishing techniques.

According to the classification, there are Aggregate Fire Extinguishing Units (AFEU), aggregate and modular types with Low Pressure (LP) and High Pressure (HP) sprayers, which differ, besides the operating pressure, and water flow rate.

ONR CEN/TS 14972:2011. Ortsfeste Brandbekampfungsanlagen - Feinspruh Loschanlagen // Planung und Einbau; Deutsche Fassung, Belgium, Brussel, Europaisches Komitee für Normung, 2011, S. 9.

NFPA 750. Standard on Water Mist Fire Protection Systems. - Las Vegas, An International Codes and Standards Organization, National Fire Protection Association, 2015, 88 p.

According to researchers from Finland, they have developed an HP sprayer for 30 minutes is “spreads” 380 liters of water (pressure of about 10 MPa), and the traditional LP sprayer for the same time 3600 liters. Thus, the first conclusion is inevitable: water consumption in systems with low pressure is about 10 times higher than in systems with high pressure.

The main difference between LP and HP atomizers is the size of water particles that are formed at the outlet of the atomizer. In sprayers, HP at a pressure of 7-12 MPa is, above all, a fine stream of water droplets smaller than 150 microns, in fact - from 50 to 100 microns. Developers of low-pressure fire-extinguishing systems operate with an average droplet size of 2 mm, comparing them with 0.05 mm droplets in high-pressure systems.

COMPARATIVE EVALUATION OF WATER PARTICLE SIZES IN LP AND HP ATOMIZERS

The main difference between LP and HP sprayers is the size of the water droplets emerging from the atomizer. In sprayers HP at a pressure of 7-12 MPa the fine stream of water droplets is smaller than 150 microns, in fact from 50 to 100 microns. Developers of LP fire-extinguishing systems operate with an average droplet size of 2 mm, comparing them with 0.05 mm droplets in HP systems.

If we theoretically atomize 1 liter of water to uniform particles of 2 and 0.05 mm, we obtain the following number of droplets: 240 000 and 15 300 000 000. Since water evaporates from the surface, the intensity of evaporation during firefighting depends more on the total free surface area of the droplets than on the number of droplets. The total lateral surface for low and high-water pressure equals 3 and 120 m2 respectively, i.e. increases 40 times. Thus, a huge number of droplets and increased by a factor of tens of times the evaporation surface in the fire-extinguishing systems of HP FWS significantly increases the rate of heat absorption in the combustion zone and the intensity of displacement of oxygen from it, and actively shields the thermal radiation.

WATER FLOW RATE FROM THE ATOMIZER HP

This parameter is very important for such a device: the higher the pressure in the system, the higher the flow rate. At expulsion velocity higher than 100-150 m/s we should take into account the additional powerful aerodynamic crushing factor of water flow, which does not exist at gravitational expulsion in the case of LP nebulizers, i.e. in the end a fast-flying fog turns out. Fine water particles, having good permeability, contribute to the distribution of FWS throughout the space, even "flowing" over obstacles, reminding by nature of distribution in space of gas (quasi-gaz). Such ability of the flying fog is more consistent with the volumetric method of fire extinguishing.

BENEFITS OF USING WATER MIST WHEN EXTINGUISHING A FIRE

- effectively performs smoke suppression;

- finely dispersed water shields heat radiation and can be used to protect the firefighter as well as material assets on the fire;

- atomized water cools the highly heated metal surfaces of supporting structures more evenly, which eliminates their local deformation, loss of stability, and destruction;

- The low electrical conductivity of water mist makes it possible to use it as an effective extinguishing agent on electrical installations under voltage.

?Especially effective is the use of fire-extinguishing systems FWS HP in the early stages of fire detection, in confined spaces, as well as on objects that do not allow secondary damage from the fire (excessive water spillage). In accordance with the recommendations of the international and European standards, research of foreign colleagues, as well as from the accumulated experience the most effective use of FWS HP to extinguish class A, B, and E fires was recorded in the following places:

- In cable tunnels of power plants (NPPs) and substations, industrial and public buildings (tunnels, ducts, basements, shafts, floors, double floors, galleries, chambers used for laying electric cables);

- In-city cable collectors and tunnels;

- In electrical installations with voltages of up to 35000 V;

- Premises for storage of combustible materials or non-combustible materials in combustible packaging;

- In above-ground and underground premises and facilities of subways and underground high-speed streetcars;

- In motor vehicle tunnels;

- Warehouses;

- In the storage rooms of libraries and archives.