Fire Development in a Compartment - Part II

When a fire is unconfined, much of the heat produced by the burning fuel escapes through radiation and convection. Think about a pile of wood pallets burning is an open parking lot. While you can feel the radiant heat as you approach the fire, convection moves smoke and hot gases up and away from the burning pallets. What changes when the fire occurs in a compartment?

Other materials in the compartment as well as the walls, ceiling and floor absorb some of the radiant heat produced by the fire. Radiant heat energy that is not absorbed is reflected back, continuing to increase the temperature of the fuel and rate of combustion. Hot smoke and air heated by the fire become more buoyant and rise, on contact with cooler materials such as the ceiling and walls of the compartment; heat is conducted to the cooler materials, raising their temperature. This heat transfer process raises the temperature of all materials in the compartment. As nearby fuel is heated, it begins to pyrolize. Eventually the rate of pyrolysis can reach a point where flaming combustion can be supported and the fire extends. In addition to containing heat energy, fires in compartments are influenced by the ventilation profile. The size of the compartment and the number and size of the openings that can provide a source of oxygen for continued combustion also influence fire development.

Stages of Fire

While the "stages of fire" are been described differently in fire service textbooks the phenomenon of fire development is the same. For our purposes, the stages of fire development in a compartment will be described as incipient, growth, fully developed and decay (see Figure 2). Despite dividing fire development into four "stages" the actual process is continuous with "stages" flowing from one to the next. While it may be possible to clearly define these transitions in the laboratory, in the field it is often difficult to tell when one ends and the next begins.

Incipient: This stage of fire development can be defined in two ways. The simplest definition is a small fire that has not yet significantly impacted the environment inside the compartment (i.e. heat, toxicity, visibility). Occupational Safety and Health Administration (OSHA) regulations dealing with fire protection (OSHA, 1993) identify an incipient fire in terms of risk. This regulation states that a fire which is in the initial or beginning stage and which can be controlled or extinguished by portable fire extinguishers or a small hoseline without the need for protective clothing or breathing apparatus.

Going back to the basics of fire behavior, ignition requires heat, fuel, and oxygen. Once combustion begins, development of an incipient fire is largely dependent on the characteristics and configuration of the fuel involved (fuel controlled fire). Air in the compartment provides adequate oxygen to continue fire development. During this initial phase of fire development, radiant heat warms adjacent fuel and continues the process of pyrolysis. A plume of hot gases and flame rises from the fire and mixes with the cooler air within the room. This transfer of energy begins to increase the overall temperature in the room. As this plume reaches the ceiling, hot gases begin to spread horizontally across the ceiling. Transition beyond the incipient stage is difficult to define in precise terms. However, as flames near the ceiling, the layer of hot gases becomes more clearly defined and increases in volume, the fire has moved beyond its incipient phase and (given adequate oxygen) will continue to grow more quickly. The fire now presents an immediately dangerous to life and health (IDLH) threat and the OSHA requirements for two-in/two-out apply (OSHA, 1993).

Growth: As the fire continues to grow, the rate of energy released by the burning fuel will continue to increase (given adequate oxygen). While considerably more complex, gas temperatures within the compartment may be described as existing in two layers: A hot layer extending down from the ceiling and a cooler layer down towards the floor. In addition to the effects of heat transfer through radiation and convection described earlier, radiation from the hot gas layer also acts to heat the interior surfaces of the compartment and its contents (see Figure 3).

As the volume and temperature of the hot gas layer increases, so to does pressure. Higher pressure in this layer causes it to push down within the compartment and out through openings. The pressure of the cool gas layer is lower, resulting in inward movement of air from outside the compartment. At the point where these two layers meet as the hot gases exit through an opening the pressure is neutral. The interface of the hot and cool gas layers is commonly referred to as the neutral plane.

The fire can continue to grow through flame spread or by ignition of other fuel within the compartment. As flames in the plume reach the ceiling they will bend and begin to extend horizontally. Pyrolysis products and flammable byproducts of incomplete combustion in the hot gas layer will ignite and continue this horizontal extension across the ceiling. This phenomenon is known as rollover and is an indicator of impending flashover.

Flashover is the sudden transition from a developing to fully developed fire. When flashover occurs, there is a rapid transition to a state of total surface involvement of all combustible material within the compartment. Conditions for flashover are defined in a variety of different ways. However, in general the temperature in the compartment must reach 500o - 600o C (932o - 1112o F) or the heat flux (a measure of heat transfer) to the floor of the compartment must reach 15 - 20 kW/m2 (79.25 Btu (min/ft2) - 105.67 Btu (min/ft2). To put heat flux simply, at flashover sufficient heat energy is being transferred to the each square foot of floor to raise the temperature of a pound of water between 79o and 105o F every minute. When flashover occurs, burning gases will push out openings in the compartment (such as a door leading to another room) at a substantial velocity.

Flashover will not always occur. Two interrelated factors have a major influence on fire development within a compartment. First, the fuel must have sufficient heat energy to develop flashover conditions. For example, ignition of several sheets of newspaper in a small metal wastebasket is unlikely to have sufficient heat energy to develop flashover conditions in a room lined with sheetrock. On the other hand, ignition of a couch with polyurethane foam cushions placed in the same room is quite likely to result in flashover. The second factor is ventilation. A developing fire must have sufficient oxygen to reach flashover. In modeling fire development in a hotel room, Birk (as cited in Grimwood, Hartin, McDonough, and Raffel, 2005) determined that closing the door prevented the room from reaching flashover (provided that other openings such as windows remained intact). If insufficient ventilation exists, the fire may enter the growth stage and not reach the peak heat release of a fully developed fire.

Fully Developed: At this post-flashover stage, energy release is at its greatest, but is generally limited by ventilation (more on this in a bit). Unburned gases accumulate at the ceiling level and frequently burn as they leave the compartment, resulting in flames showing from doors or windows. The average gas temperature within a compartment during a fully developed fire ranges from 700o - 1200o C (1292o - 2192o F)

Decay: As the available fuel is consumed, the heat release rate will decline and the fire may return to a fuel controlled state as the available oxygen supply becomes adequate for the rate of combustion.

More detailed discussions of compartment fire development can be found in Enclosure Fire Dynamics (Karlsson & Quintiere, 2000) and An Introduction to Fire Dynamics (Drysdale, 1998).

Fuel Controlled vs. Ventilation Controlled

The distinction between fuel controlled and ventilation controlled is critical to understanding compartment fire behavior. As previously outlined, compartment fires are generally fuel controlled while in the incipient and early growth stage and again as the fire decays and the demand for oxygen is reduced. Of particular interest are the factors that influence development of a fuel controlled fire (see Table 1).

Table 1. Factors Influence Development of a Fuel Controlled Fire

Mass and Surface Area

The greater the surface area for a given mass of fuel, the easier it is for that fuel to be heated to its ignition temperature.

Chemical Content

The chemical makeup of the fuel has a significant impact on the heat released during combustion. For example, the heat of combustion of ordinary combustibles (such as wood and paper) is approximately 8000 Btu/lb (33.494 MJ/Kg). On the other hand, hydrocarbon fuels such as gasoline have a heat of combustion of 20,000 Btu/lb (83.736 MJ/Kg). Many synthetic materials such as plastics have a heat of combustion near that of gasoline.

Fuel Load

The total amount of fuel available for combustion influences total potential heat release.

Fuel Moisture

While not a factor with all types of fuel, water acts as thermal ballast, slowing the process of heating the fuel to its ignition temperature.

Orientation

Orientation in relation to the fire influences how heat is transferred. For example, a wood wall surface is heated by both convection and radiation, where the floor is more likely to be heated by radiant heat alone.

Continuity

Continuity is the proximity of various fuel elements to one another. The closer (or more continuous) the fuel is, the easier and more rapidly fire will extend. Continuity may be either horizontal (i.e. ceiling surface) or vertical (i.e. wall or rack storage).

While a fire is fuel controlled, the rate of heat release and speed of development is limited by fuel characteristics as air within the compartment and the existing ventilation profile provide sufficient oxygen for fire development. However, as the fire grows the demand for oxygen increases, and at some point (based on the vent profile) will exceed what is available. At this point the fire transitions to ventilation control.

When fire development is limited by the ventilation profile of the compartment, changes in ventilation will directly influence fire behavior. Reducing ventilation (i.e. by closing a door) will reduce the rate of heat release and slow fire development. Increasing ventilation (i.e. by opening a door or window) will increase the rate of heat release and speed fire development. Changes in ventilation profile may be fire caused (failure of glass in a window), occupants (leaving a door open), or tactical action by firefighters.

Study and Discussion Questions

Reading about fire behavior is considerably different than experiencing it first hand. Resist the temptation to brush off basic concepts as too simple or elementary or more detailed explanations as too complex. Making a connection between theory and your own experience or the experiences of others is an effective way to learn. Use these questions to focus your thinking on how basic fire behavior theory connects with incidents that you or other members of your crew have responded to.

2. Visit the Photo stories page of Firehouse.com and use incident scene photos to practice identifying if the fire is fuel or ventilation controlled. Discuss what impact making an opening for tactical ventilation and/or entry would have on fire behavior.

Application Activities

2. Discuss your last structure fire from a fire behavior perspective. What were conditions on arrival? Did fire behavior change (other than hopefully going out) during your tactical operations? Why did the fire behave the way that it did? Make a connection to fuel and ventilation control and the stages of fire.

What's Next

In the next article titled "smoke burns" we will begin a more detailed examination of the phenomenon of flashover, backdraft, and smoke explosion. Subsequent articles will connect the theory to practical experience by examining a series of case studies involving extreme fire behavior.

References

  • Occupational Safety and Health Administration (OSHA). (1993). 29 Code of federal regulations, 1910.155 Fire protection. Washington, DC: Author.
  • International Fire Service Training Association. (1998). Essentials of firefighting (4th ed). Stillwater OK: Fire Protection Publications
  • Karlsson, B. & Quintiere, J.G. (2000). Enclosure fire dynamics. Boca Raton, FL: CRC Press.
  • Drysdale, D. (2000). An introduction to fire dynamics. Chichester, England: John Wiley & Sons.
  • Grimwood, P., Hartin, E., McDonough, J., & Raffel, S. (2005). 3D firefighting: Training , techniques, and tactics. Stillwater, OK: Fire Protection Publications.

Loading