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The interior fire operation techniques of direct attack, indirect attack and gas cooling are often confused with one another in definition and application.

December 28, 2012
By Ian Bolton

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The interior fire operation techniques of direct attack, indirect attack and gas cooling are often confused with one another in definition and application. The way these techniques are defined and used can vary from department to department and from one region to another. Each technique, when used properly and with a good understanding of its capabilities and limitations, can be extremely effective. Conversely, when these techniques are misused, conditions can often deteriorate and become much worse for firefighters and any potential fire victims.

We will explore each technique and look at situations in which it can provide the greatest benefits for firefighters operating on the interior.

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To cool fire gases that are farther away, use a narrow fog pattern with pulses lasting from half a second to several seconds. Photos by John Riddell


 

Gas cooling
When firefighters first arrive at a working fire, flames are often the primary focus of crews during fire-ground size-up. Although flames can help firefighters determine the areas of involvement, the flaming combustion, in many cases, should not be the first concern for firefighters considering interior operations. The first danger to firefighters entering an involved compartment or structure is the potential for ignition of the accumulated fire gases. The danger and ignitability of the fire gases, and the significant contribution of fire gases to fire development, must not be ignored. When firefighters find themselves in the presence of hot fire gases, a strong effort should be made to take control of the environment through gas cooling.

In many parts of world, and particularly in Europe, gas cooling is a bread-and-butter technique used by firefighters. However, in North America, gas cooling has been an area of debate in the fire service. Some firefighters believe gas cooling doesn’t work and if the nozzle is opened before the seat of the fire is found, an enormous amount of steam will be created, which will drop the thermal layer to the floor. This concern is warranted if the fundamentals of gas cooling are not well understood and practised. But, when gas cooling is performed by knowledgeable and well-trained firefighters, it can be extremely effective for controlling the fire environment and helping to minimize the threat of flashover.

One reason gas cooling is not used as often in North America as it is in Europe may be that fire schools and fire departments do not frequently teach the science behind gas cooling or the method for properly applying this skill. Instead, the methods taught end up resulting in a technique that is similar to surface cooling rather than gas cooling. This more frequently taught method involves the projection of large quantities of water in a fairly narrow cone, upward into the super-heated gas layer, for a duration of time that is often too long. When this technique is used, the water stream frequently passes right through the hot gas layer, hitting super-heated surfaces, such as walls and ceilings. This action then commonly results in the production of a large quantity of steam, which adds to the overall volume of gases in the hot gas layer and, in turn, pushes the layer down to the floor. Moreover, when this occurs, there is very little cooling of the fire gases. For this reason, it is very important that firefighters avoid hitting all super-heated surfaces within the compartment they are trying to cool.

An essential element of gas cooling that is not always conveyed to firefighters during training is the scientifically proven contraction effect that takes place when fire gases are cooled. Because of this effect, when water droplets of the fog stream are converted to steam and expand by absorbing the thermal energy of the fire gases, the fire gas layer will decrease in volume. Not only do the fire gases contract when they are cooled, but also they contract at a greater rate than water expands, and this can result in the fire gas layer actually lifting when gas cooling is performed correctly.

The ability of properly projected water droplets to significantly affect the temperature of fire gases has also been well documented. In 2002, the National Research Council of Canada studied the effects of gas cooling based on European techniques. The final report found that when these techniques were used, “the smoke temperature was quickly reduced by 200 to 250 C and then continued to cool.” This temperature reduction is crucial in lowering the chances of the fire gases igniting by reducing the flammability range of the fire gases, as well as keeping the gases well below their auto-ignition temperate.

To gas cool more effectively, water droplets must be placed high into the hot fire gas layer without hitting the compartment boundaries. To achieve this, firefighters may need to adjust the fog angle and the duration of the opening and closing times, also referred to as the pulse. To cool gases close by, firefighters should consider adjusting the fog pattern to as much as a 60-degree cone with short pulses lasting approximately half a second. To cool fire gases farther away, the cone angle should be narrowed to about 30 degrees, with pulses lasting from half a second to several seconds; this allows for better projection and travel distance of the water droplets. 

It is also imperative that gas cooling is done frequently inside the structure.  Firefighters should attempt to cool all fire gases around them and within the reach of stream. The natural flow path that the fire will create has the potential to carry the recently cooled fire gases outside via a doorway or opening, as the developing fire is continuously creating new hot fire gases.

Gas cooling is a skill that takes considerable practice to master. These recommendations serve only as an introduction, as this skill can truly be developed only with the assistance of an experienced and capable fire instructor in a safe and controlled live fire training setting. 

Direct attack
Once interior fire crews have safely advanced and found the developing fire, either a direct or indirect fire attack can be made. Direct fire attack is probably the only technique that is not often questioned and, for the most part, firefighters can agree on its definition. Simply put, direct fire attack is the act of placing water directly on the surfaces on the burning combustibles. By doing this, the temperature of the involved combustibles is lowered below the fire point, stopping further pyrolysis, and inhibiting the chemical chain reaction that is fire by removing the heat. This should always be done at the first available opportunity, as these involved surfaces create the heat, fire gases and dangerous environment that firefighters occupy.

Indirect attack
The indirect fire attack has been around for a very long time. It fact, it was Chief Lloyd Layman of Parkersburg, Va., who is credited for its creation shortly after the Second World War. Even though its roots stem from one source, there is still some confusion over the definition of indirect fire attack. In basic terms, an indirect attack is using the principles of water-to-steam conversion and expansion to knock down a well-developed fire by absorbing heat energy. This is performed by directing a straight or narrow fog toward all super-heated surfaces within the fire compartment until the fire is knocked down.

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Fire gases that are close by should be cooled using a wide fog pattern and short pulses, lasting no more than half a second.


 

When the projected stream makes contact with the super-heated walls and ceilings, most will immediately be converted to steam; the remainder of the stream will be broken into smaller droplets and rebounded to other surfaces, cooling the environment even further, as the droplets are also converted to steam. This method has been found to be extremely effective, but it creates a large volume of steam. This massive steam expansion, in all likelihood, will make the fire compartment very untenable for firefighters, and for this reason it should only be performed from the protection of a doorway or with considerable distance between the firefighters and the fire compartment.

While use of this technique is often necessary to gain control of a fully developed fire, indirect fire attack can also help firefighters prevent potential backdraft situations. By using the protection of a doorway, firefighters can work together to maintain proper door control while performing an attack. For this attack to be successful the door should be opened only enough to project the fire stream inside the involved compartment. Any airflow into the structure can be minimized by reducing the size of the door opening. While the fire stream is flowed, firefighters should aim to hit all fire gases and surfaces possible for between two and five seconds. Following this controlled attack, the door is closed for five to 10 seconds to allow for the steam to expand and cool the environment. This sequence should then be repeated as many times as needed until conditions improve.  Firefighters should watch for indications that the temperature has decreased, steam has filled the entire compartment, and there is a reduction in the air/gas exchange at the doorway.


Ian Bolton has been active in the fire service for 10 years. While working in Australia he achieved instructor level certification in Compartment Fire Behaviour Training (CFBT) and tactical ventilation, and has received advanced training in fire behaviour and ventilation from the Swedish Civil Contingencies Agency in Revinge, Sweden. Ian is currently working as a firefighter and lead fire behaviour instructor for the District of North Vancouver Fire and Rescue Service. He is also studying Fire dynamics in preparation for exams with Institute of Fire Engineers. Contact Ian at ian.bolton@firedynamics.ca


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