Fire attack: A case for indiscriminate cooling
Some will argue that indiscriminately cooling the approach path will adversely alter the thermal balance; here’s why it improves safety and survivability
The first engine is stretching a line. Their objective is to start cooling, whether it be the cooling of pyrolyzing surfaces or the indiscriminate cooling of the approach path.
Cooling pyrolyzing surfaces is a well-accepted and well-supported method for cooling the fire compartment.
It is my belief that indiscriminate cooling along the approach path, though highly disputed, is the best option to support the ultimate objective of rescuing trapped occupants by reducing the amount of heat energy available to either grow the fire, impact the rescuers or continue harming occupants.
Indiscriminate cooling on approach to the fire can disrupt thermal layering. However, if there is enough heat along the approach path that crews feel heat and the need to cool it, that is indicative of high levels of radiant heat flux from the convective flow of the gas layer and from wall linings that are radiating excess heat back into the environment.
So, while the application of water has the potential to disrupt the layers, it provides cooling – and cooling is important. I suspect some oppose disrupting these layers due to the reported impact on rescuers and potential occupants.
The rescuers feel the impact of thermal layer disruption due in no small part to the fact that their gear is absorbing radiant heat energy as they move along the approach path.
Because the heat energy can only be transferred from areas of higher energy to areas of lower energy, the gear absorbs heat and is unable to release any of that heat back into the environment.
When the rescuers begin to feel heat, it is an indication that their gear has become saturated and is conducting excess energy to the only available area with less energy, their bodies.
Rescuer safety, victim survivability
A rescuer feeling heat is an indication of danger. Once saturated, any additional heat insult will be immediately transferred to the body resulting in injury.
It is this saturation that gives the experience that “things got worse” when the thermal layers were disrupted. Simply touching the rescuer with your hand would give a similar sensation.
When water is applied to the approach path it lowers the temperature in the path and it lowers the radiant heat output. The magnitude of both of those critical metrics decreases immediately.
The approach path cannot be thought of as a homogenous atmosphere. During the initial fire development, there is heat layering. But there is also a convective flow and almost always an influx of oxygen from somewhere along the approach path, even if entry points are controlled or limited in size.
There is a turbulent flow and mixing. The environment is uneven in all measurable respects.
With the chemical reactions that sustain the fire there are localized temperatures of many thousands of degrees, even in a candle flame. These high temperatures are required to support the presence of the free-radicals necessary in the intermediate steps of the chemical reactions.
Fire building’s micro environments
But the environment where these varied conditions exists is so small – a micro-environment – that it is not be experienced by a person.
Each point along the approach path can be thought of as a micro-environment even though the more salient characteristics of the fire’s behavior are taken as the collective behavior of these micro-environments.
It is plausible that the rescuer operating along the approach path at a discrete point will perceive a given situation to be worsening when the collective condition of the approach path is improving.
Putting this into the context of indiscriminate cooling, I theorize that the effect of the immediate cooling, despite the possibility of localized discomfort, has a net positive effect on conditions along the approach path. That makes it an appropriate tactic to meet the ultimate objective of occupant rescue via rapid compartment cooling.
With respect to potential occupants, the discussion is much more clear. Given an approach path where rescuers feel heat, it is terribly unlikely that any person along that path, exposed that amount of radiant heat energy is likely to be alive.
And should they be alive, the most important action is to reduce or remove the threat. That threat is the heat energy; removing it can only be done effectively by applying water.