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...

Flame Indicators
Firefighters' attention is often drawn to flames like a moth to a candle. However, this is only one of many fire behavior indicators. Visible flames may provide an indication of the size of the fire (i.e., fire showing from one window vs. fire showing from all windows on the floor). The size or extent of the fire may also be indicated by the effect (or lack of effect) of fire streams on flaming combustion (Figure 6).

Size and Location
Location of the flames may provide important information. If flames are visible from outside the structure, where are they coming from? It is important to connect this information with building factors such as compartmentation. Is fire showing from a single window due to compartmentation or simply because that is the only window that has failed? Are the flames pushing from inside a compartment or is smoke igniting and burning outside?

While working inside the building, what is the flame height? Are the flames impinging on the ceiling and bending to travel horizontally? Do you observe flames in the hot gas layer (i.e., ghosting, rollover)? Fire development speeds considerably after flames in the plume of hot gases reach the ceiling and begin to travel horizontally in the ceiling jet. Flames in the hot gas layer or development of rollover are an important indicator of imminent flashover.

With flame indicators, it is not just what you see that is important. What you do not see is equally important. Remember that the low oxygen concentration in backdraft conditions may preclude flaming combustion (at least in that compartment). However, conditions can vary widely from compartment to compartment (void spaces are compartments too!) and you may have visible flames from the exterior, but quite different conditions inside the building.

As with other fire behavior indicators, change over time is an important indication of fire development or progress towards control. This is particularly true with flaming combustion. Once fire control operations have started, firefighters and fire officers must evaluate the effect of fire streams. Failure of water application to reduce the size of the fire indicates that either the flow rate is inadequate, the application point is ineffective, or both.

Flame Color
Flame color is largely dependent on the type of fuel involved and the extent to which the fuel and oxygen are mixed. For example, in a candle flame, the air and fuel mix as combustion occurs resulting in incomplete combustion and a bright yellow flame (diffusion flame). With a propane torch, the fuel and air mix prior to combustion, resulting in more complete combustion and a blue flame (premixed flame). Because there are several influences on flame color, it is important to interpret this information in context with other fire behavior indicators. Organic materials (natural or synthetic) will tend to burn with light yellow to reddish orange color depending on oxygen concentration as illustrated in Figure 7. If organic fuel gas or vapor is premixed with air, flame color will be bluish. A bright white flame is usually indicative of high temperature such as that generated by burning metal (i.e., magnesium). One other influence on color will be flame contact with other materials. For example, flame impinging on copper will have a blue green color.

Study and Discussion Questions
Use the information presented in this article to answer the following questions:

  1. What is the difference between heat and temperature?
  2. What are the two categories of heat indicators?
  3. How do your observations of air track provide an indication of the temperature inside a compartment or building?
  4. A thermal imaging camera (TIC) can be a valuable tool on the fireground. How should you use the TIC as part of size-up?
  5. What other heat indicators might you observe from the exterior of the structure?
  6. How can your hoseline be used to evaluate temperature conditions prior to and following entry?
  7. Why might your perception of temperature and temperature change while working inside a poor indicator of the thermal hazards encountered in the fire environment?
  8. What are the limitations of thermal sensors provided on personal alert safety system (PASS) devices?
  9. What are the basic categories of fire behavior indicators associated with flames?
  10. What can observation of flames showing from the exterior of a structure tell us about conditions on the interior?
  11. What flame indicators observed on the interior warn of imminent flashover?
  12. What factors influence flame color and how might this information be useful as a fire behavior indicator?