Reading the Fire: Heat and Flame

While all of us have a commonsense understanding of heat and temperature, these concepts are frequently misunderstood.As discussed in previous articles, fire behavior indicators can be grouped into five general categories: Building, Smoke, Air Track...


While all of us have a commonsense understanding of heat and temperature, these concepts are frequently misunderstood.

As discussed in previous articles, fire behavior indicators can be grouped into five general categories: Building, Smoke, Air Track, Heat, and Flame (Figure 1). A simple mnemonic for remembering the categories is B-SAHF ("be safe"). This article focuses on heat and flame indicators.

Caution!
Flames and heat get quite a bit of attention. Flames showing from a couple of windows are sure to increase a firefighters pulse and we often talk about how "hot" a fire was. It is important to remember that while heat and flame are important fire behavior indicators, they provide only part of the picture. There is also a reason why they are last on the B-SAHF list. Flame and heat indicators must be integrated with Building, Smoke, and Air Track indicators to gain a more complete picture of incident conditions.

Heat and Temperature
While all of us have a commonsense understanding of heat and temperature, these concepts are frequently misunderstood. Heat is a form of energy and temperature is a measure of the average amount of thermal energy in a substance or object. Click here for more information on heat and temperature!

Heat Indicators
Neither heat nor temperature can be observed directly. However, there are indicators of heat and temperature that can provide critical information about current and impending fire behavior. Heat indicators can be divided into two categories, those that can be seen (observed) and those that are felt (tactile) as illustrated in Figure 2.

Observed Indicators
Air track often provides an early heat indicator. Observation of turbulent smoke pushing from the building at high velocity is a reliable indicator of a tremendous amount of heat energy and high temperatures inside the structure.

A thermal imaging camera (TIC) provides a highly effective means for examining the building for heat (Figure 3). Use of a TIC should begin on the exterior and continue during interior operations. Despite the tremendous advantage provided by use of a TIC, it is essential to not become completely dependent on technological information. Integrate thermal image data with visual observations to obtain a more complete picture of temperature conditions.

Other visual indicators include bubbling paint, melting roofing, crazing glass, and condensation of pyrolyzate on windows. Fire stream effects such as evaporation of water from a hot surface (such as a door) or lack of return from a temperature check (brief application of water fog into the hot gas layer to check overhead temperature) also provide an indication of temperature (Figure 4)

Tactile Indicators
Tactile effects include sensing temperature or temperature change. Firefighters may sense temperature and changes in temperature, but this is limited by the extent of thermal protection provided by their protective clothing and focus on the task at hand. Firefighters' protective clothing effectively insulates them from the thermal hazards typically encountered in firefighting. The multiple layers of insulation in the protective ensemble slows (but does not stop) heat transfer. This time lag makes it difficult for the firefighter to appreciate the amount of heat flux that they are being exposed to (Bryner, Madrzykowski, & Stroup, 2005).

Personal alert safety system (PASS) devices may be equipped with a temperature sensing function that provides warning at a specified exposure value when the specified temperature is exceeded for a specified time period (Figure 5). However, National Fire Protection Association 1982 Standard on Personal Alert Safety Systems (PASS) (NFPA, 2007) does not address thermal sensing and there is not standardized test protocol for these types of devices (Bryner, Madrzykowski, & Stroup, 2005). Thermal sensing devices use a temperature response curve to provide warning for long duration exposure to lower temperature and short duration exposure to higher temperature. However, during rapid increases in temperature such as those encountered in flashover or other forms of rapid fire development, adequate early warning to permit egress is unlikely due to limited sensitivity of the sensors (Bryner, Madrzykowski, & Stroup, 2005). While firefighters must be attentive to heat level and temperature change, it is often difficult to perceive these changes quickly enough to react to rapidly developing fire conditions. This reinforces the importance of integrating all the fire behavior indicators in your ongoing size-up and dynamic risk assessment.

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