By Mike McKenna
Are you thinking about heat transfer in two-dimensions in a three-dimensional dynamic fire situation? Many firefighters believe that the main threat to them from radiant heat is from the fire itself. While the fire itself is the greatest source of radiant heat that you see, it is not the greatest threat confronted when you are fighting an interior structural fire.
When your hood becomes saturated, it is time to re-evaluate your situation. In a perfect world, it would be protected from heat. But the hood is at an interface — there are several overlapping elements coming together that only partially protect and block heat. A portion of the hood is obviously under the helmet. And it is also protected from the top by the brims of the helmet that can deflect away some radiant heat. Further protection is afforded from the sides by the interface of the upright collar and the SCBA facepiece interface.
It is important to remember that the greatest threat from radiant heat is not from what is in front of you, but what is over your head. Radiant heat over your head travels straight down to the floor. The safest way to protect yourself from radiant heat is distance. Unfortunately, you can only get so low to the floor.
Our high school science teachers attempted to drive concepts such as "potential energy" into our brains. As high school students we scoffed at their efforts and thought that we would never use it in the real world and as rookies we were all vulnerable to the "moth-candle" syndrome. We went after the fire head-on and in many cases disregarded recognition of our surroundings.
As fire burns, the fire plume continuously launches heated products of combustion into the overhead. This overhead becomes thicker and thicker with smoke and products of combustion. The density of the atmosphere overhead becomes higher and higher. As that pressure increases, the thermal conductivity becomes more efficient.
The thermal conductivity over your head is very efficient at transmitting heat across the entire overhead. Firefighters often ask about convection, but convection is simply a subset of conduction. Remember, convection is a moving current of conductive particles — which means that the overhead is more efficient at moving the heat between the walls and ceiling. This heat is moving in all directions.
Now, think of that three-dimensional space between the four walls and the ceiling as potential energy. The kinetic energy being released is moving from the ceiling to the floor. What is between the ceiling and the floor? You — the firefighter! You are in the center of a high heat environment.
PPE becomes saturated with heat at different rates and when the firefighter is crawling across the floor, their back is primarily exposed to the radiant heat. The SCBA harness and cylinder cover much of the back, but the shoulders are vulnerable to radiant heat. The shoulders are at an increased risk because of the compression from SCBA straps.
NFPA 1971 recognized this and set performance criteria to help protect the firefighter with increased protection. Now let's look back to the hood and consider this scenario: the firefighter is moving around below the bottom of the smoke and the visual heat is overhead. However, the radiant heat is pounding the floor. It preheats the surfaces and ignites them. The area below the smoke is very hot and the firefighter is surrounded by heat and that heat is hitting him from every direction and saturating the PPE. Even with high total thermal protection values, the garments are becoming saturated.
You need to begin to think about the fire environment in a practical, three-dimensional concept with the energy above the bottom of the smoke as the potential energy. The amount of potential energy in that three-dimensional area is staggering and the potential energy above that line will make the firefighter's environment untenable from heat saturation. The further you can get below the bottom of the smoke, the longer you will have before the garments become saturated.
Let's think about PPE — the hood is two layers of knit material and is partially protected. In order for the hood to become saturated to the point where the firefighter thinks that the hood is missing is significant. It may not always be dangerous, but it is significant.