Exclusive: Turning down the heat
Iain Hoey
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Pablo Boj, fire department consultant for STB, gives a brief review on gas cooling and surface cooling
My professional career in the field of firefighting, since I joined the fire department in 2002, has been fundamentally driven by the gas cooling technique and the doctrine of efficient use of water whose fundamental premise was to make the most of the cooling capacity of water.
However, in recent years, this doctrine has started to raise certain doubts for me. Through my investigations of incidents that occurred during firefighting, I have observed problems with the application of the technique.
These problems stem from the lack of sufficient training and the limitations inherent to the training settings.
The training is usually conducted under very specific conditions, without enough variation, which often fails to mimic the diversity of real fire scenarios, both in terms of the fire load and the geometry of enclosures.
Europe vs America
At the end of 2021 I had the opportunity to participate in a meeting between European and American instructors promoted by Elkhart Brass in Pensacola (USA) under the motto “There is nothing foreign in flowing water”, coinciding with the High-Rise Operation Conference organized by County Fire Tactics.
The main idea was to share experiences and knowledge, starting from the different approaches used in firefighting on both sides of the Atlantic.
An equivalent meeting was held on the European side at the end of 2022 coinciding with the High-Rise Operation Tactics in which we also had the opportunity to put into practice the different techniques in live fire scenarios over several days.
Europeans used an approach based on a reading of the conditions, door control and progression through pulsations on the gas layer with flow rates between 230 and 500 Lpm with pressure nozzles with a 6-bar reference.
In this way, the gas layer is controlled, allowing a safe progression until reaching a position that allows us to attack the base of the fire.
On the other side, Americans, once the door was open, progressed by projecting a flow rate between 550 and 800 Lpm, at a pressure of 3.5 bar, with an O pattern of solid and uninterrupted jet (“move and flow” variant) in order to reach the largest possible surface (walls and ceiling) and generating hydraulic ventilation in the lower plane towards the fire accompanying the firefighters and forcing hot gases to exit through the upper plane to the outside towards the available outlets.
Steam generation
The basis of the gas cooling methodology is the application of very short pulses with an open cone, with relatively low flow rates, whose objective is to expose the maximum volume of water to the heat transfer in the gas layer so that the contraction due to cooling compensates steam generation.
In the case of the surface cooling methodology, the idea is to apply large amounts of water to all surfaces, so that hot gases from the base of a fire cool down when they come in contact with the entirely cold surface.
In gas cooling, pulsations are directed to cool the gas layer, and if they reach walls, they vaporise easily without having a very significant impact on the energy accumulated in the total mass of the facing, since the volume of water is reduced, so that the surface can easily recover a high temperature.
However, in the surface cooling technique, through a water application with high flow rates over large areas, there is more mass of water through which transferring the energy from the facing, achieving a greater reduction in the energy level and surface temperature, and consequently, a lower relative steam generation.
Another interesting point is the cooling effect of gases that occurs when a solid jet hits the ceiling and breaks up, causing a movement of fluids.
On the other hand, by leaving the door open and forcing the hydraulic ventilation in the lower plane with the nozzle, excess of steam and hot gases has a guaranteed exit through the upper plane, with a lower risk for the firefighter.
In the live fire exercises, my personal impression is that thermal stress was significantly lower by using this technique.
From my point of view, one of the weak points lies in the open door, since in the case of residential buildings the products of combustion can affect floors and occupants that are above the fire, when these gases do not have another better way out, although being a very aggressive and forceful technique, it quickly reduces the temperature and toxicity of the gases and the extinction time is shortened.
Training
One of the most interesting conclusions I obtained is that the surface cooling technique is simpler, and therefore, safer for the firefighter.
The gas cooling technique requires a relatively high level of training, the firefighter must carry out regular practices in handling of the nozzle, since a bad application can cause water to reach the walls and generate steam excesses and pressure and caudal requirements are more specific.
In addition, range and penetration capacity of the pulsations are limited, as well as its use in large volumes.
Unfortunately, firefighters do not always have the necessary training hours, or they have to distribute them among the wide range of disciplines that we currently require them.
In contrast, the surface cooling technique is less demanding from the point of view of handling the nozzle, the projection of water is as deep as the enclosure, which keeps sources of heat away from the firefighters and it is also relatively easy for water to reach the base of the fire by hitting door frames, even when out of sight in an adjacent room.
My intention with this article is not to suggest whether one technique is better than another, but rather to assume that there are other valid firefighting approaches, and that the mere fact of understanding how they work is already useful in itself and serves as exercise to examine the strengths and weaknesses of the techniques and variants that each one uses in their interventions.
