Fog nozzles

You may perceive more and more in recent times, a progressive loss of knowledge about what should be the basis for expertise in firefighting. The wide range of needs during the interventions leads to have to focus on countless different equipment. Some devices are actually complicated; their real potential is considerable, but we should wonder about their usability. Beyond these considerations, a cornerstone problem still remains: firefighters are required to have an excellent dexterity in using basic equipment. We usually take it for granted that they are familiar with the nozzle. On the basis of my personal experience as  instructor, I can assure you that the firemen that can claim to be able to use a branch “blindly” are very few. In this case, saying “with your eyes closed” is not just a way of saying; in our case is a specific need linked to our activity.

The type of fire that we face involves that sometimes, we should make an aggressive entry into buildings completely filled with smoke. During gas cooling it is quite common to be in conditions of zero visibility or nearly so. That is because it is essential to get acquainted with this tool, so you can use it at the maximum even in case of low visibility. Being the sight the most developed sense, or at least the one that allows faster familiarization, in order to use an equipment without being able to see, it is necessary to commit much more time. To facilitate this task, it is important that the type of branch in use at your department would be standardized. Once you find the model that suits your own, you should utilize it in every engine.

Having said that we have to explain our firefighters why is better use that one rather than other. Our goal, as instructors, is to give the right tool with the right justification. We can not say you have to do this only because I tell you!

What we will do is to know better the fog nozzle. Its distribution among the various fire departments started by a couple of decades ago and except for a few cases, it has replaced the solid bore nozzle. You might think that it is recently developed. However, if you study his developing you will be astonished to discover it is more than 100 years old. Its production took place as a result of mechanization and the consequent spread of petroleum products. In fact, it was realized that these nozzles were more efficient than a solid bore in fighting class B fires. The U.S. Coast Guard Fire Fighting School realized the first test campaign between 1943 and 1946. More than 20 experiments were conducted on board a WWII Liberty transport ship. At the purpose they used the engine room. On the bottom of it (about 170mq), they poured between 5000 and 7000 gallons of fuel oil (19000-26000 litres). The fire was left to burn freely for 30 minutes to ensure the worst possible conditions. During these tests were used some different types of fog nozzles, with a flow rate between 435 and 635 lpm at a pressure of 6.9 bar. The branches were inserted from the ceiling of the room on fire avoiding as much as possible the intake of air. The thermocouples recorded a constant drop temperature until get below the ignition temperature of the fuel oil (ca 93 ° C).  The director of the school’s, chief Layman, was amazed that the burning surface oil was shut off without the direct application of water on the fuel on fire. He realized that he had created a new method of attack the fire, that was called “indirect attack “. After numerous studies he came to the conclusion that:

  • The rapid production of steam within a confined space creates a violent disruption inside the confined space ;
  • Each volume of steam created inside a confined space occupies an equivalent space to that atmospheric space.[1]

The importance of these experiments lies in the fact that, for the first time, it was shown that the fog nozzle could be profitably used by adopting the indirect attack. In fact, the insertion of water (in the form of finely divided particles) into a highly heated atmosphere results in in rapid generation of steam that is allowed to distribute it throughout the space. Actually this is a kind of chain reaction. Some drop of water will change from liquid to vapor, this create a disturbance that push the others unchanged drops further where the same effect will happen.

These trials were the first of many other tests and experiments. In the following years were counted dozens of trials. The turning point in the spread of this methodology occurred in 1950 during the Fire Department Instructor’s Conference in Memphis. Chief Layman gave a lecture titled “little drops of water”. As a result of this event were published a few books that made it possible to significantly increase the number of firefighters aware of this method of attack fires.

As can be seen from what was previously written, the use of this type of nozzle goes back several decades ago, both the U.S. and in Europe (early 80s). One wonders why in Italy we always come later…Anyway,being useless dwell on the past, let us focus on what we can do to fill any gaps or deficiencies of knowledge.

Firstly, let us see why it is important to use the right nozzle to be effective if we wish to do some gas cooling during the progression within a building filled by smoke. The variables that affect the ability to extract heat from the superheated smoke are:

  • The mass of the extinguishing agent used;
  • How big are the contact surface with the fluid that is meant to cool;
  • The velocity of the extinguishing agent;
  • The contact time between the extinguishing agent and the fluid.

The laboratory tests indicate that the best results are achieved for drops of diameter circa 0.2-0.5 mm with a flow of approximately 130 lpm and a cone opening of the branch circa 60-90 °. Operating in this way, you will maximize all the parameters listed above. In order to be effective the drop must have a sufficiently long lifetime, before falling to the ground due to gravity, to allow it to pass through the smoke layer and transform from liquid to vapour. It should not be too large as to make them go through all the smoke and hit the ceiling or beyond sidewalls. Furthermore, the droplet size must have the greatest exposure surface, allowing in fact absorbing energy from the smoke. The last parameter is obtained dividing the drops in the finest way possible. As you may see, we will reach the best results balancing the size of the droplets. Not too small, not too big. Therefore, we need the proper dimension.

The droplets undergo two main effects. They fall and evaporate.[2]

Droplets fall

Two forces affect our droplets during their travel through the smoke: air resistance and gravity. The gravity is influenced from the mass of the droplet. Air resistance is influenced from the surface.

Gravity→d3

Air resistance→d2

For this reason it will happen that:

A large droplet fall fast;

A small droplet fall slowly.

 

Droplets evaporate

The rate of evaporation of the droplet is proportional to his volume.

The heat that is supposed to be absorb is proportional to the outer shell.

Evaporation→d3

Heating→d2

For this reason it will happen that:

A large droplet evaporates slowly;

A small droplet evaporates fast.

 

The two effects (fall and evaporation) work together.

 

Size of droplets Temperature of smoke Survivability Type of branch
0,01 mm 600°C A few centimetres at the most High pressure
2-3 mm 600°C Tens of meters Low pressure solid bore
0,2-0,4 mm 600°C A few decimetres Fog nozzle

 

 lance

Too small                                     Proper                                                 Too big

 [3]

 

The fact that water evaporates involves a series of subsequent events. Firstly, the smoke cools down because it has transferred energy to heat the water. As a result, the smoke will be reduced in volume. At the same time, however, its volume increases due to the steam that has been introduced. Which of these two factors will prevail depend on where the water evaporates. The energy used to evaporate the water can be withdrawn from the smoke or the surfaces of the room. If the water evaporates into the smoke layer, the energy it uses to switch the status is taken directly from smoking. The smoke is then cooled resulting in a consequent reduction in volume, which compensates the volume of steam introduced. In this case, there will be a contraction of the final volume of the smoke of about 20%.[4]

When the water evaporates because it cools the surface of the fuel or the side walls, the energy used comes from heated surfaces. The resulting increase in volume of the smoke, as there is no contraction of it, will be approximately 50%.

A rule of thumb states: when approximately 70% of the water evaporates on the surface and the remaining 30% in the smoke (smoke temperature of 600 ° C), the two effects cancel each other and the initial volume of smoke remains unchanged.[5]

This last scenario would be probably the best solution, because the internal crew will not be affect by the hot steam (only surfaces cooling) and air will not enter (it means oxygen). In fact the reduction of volume of smoke and the following decrease of pressure (only smoke cooling) will claim the entrance of air from outside. Sometimes it is useful (more visibility and a more sustainability at the bottom) but it will certain increase the HRR. Which option will be the best is a decision that has to be taken from the crew inside.

The operator must use the branch in such a way as to achieve the goal. The opening angle of the cone and the inclination in respect to the layer of smoke are just as important as regulation of the flow or pressure of use.

This allows us realize how important is training in the use of such equipment. This consideration brings us back to the beginning; the equipment must be selected according to the needs of interventional and then must be known into the tiniest detail from all the operational staff. The worst thing you can do is take it for granted that all know them and that they use those ones rather than others because “we have always done so!”

 

[1] The safe and effective use of fog nozzle. John E. Bertrand John D. Wiseman;

[2] Stefan Särdqvist’s lecture, IFIM 2013 SRTC Skovde;

[3] Offensiver Löschangriff Jan Südmersen

[4] Water and other extinguishing agents. Stefan Särdqvist Raddinings Verket;

[5] Water and other extinguishing agents op cit.

2 thoughts on “Fog nozzles

  1. I enjoyed your article and found it spot on for technical data. Please review my own and let me know your thoughts on the other side of the issue.
    Fraternally..

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