Fire Development shows the time history of a fuel-limited fire. In other words, the fire growth is not limited by a lack of oxygen. As more fuel becomes involved in the fire, the energy level continues to increase until all of the fuel available is burning fully developed stage. Then as the fuel is burned away, the energy level begins to decay known as the decay stage. The key is that oxygen is available to mix with the heated gases fuel to enable the completion of the fire triangle and the generation of energy, which is know as fire (Fire Dynamics. . d. ).
Now lets take look at how the different stages of fire development breaksdown. INCIPIENT STAGE The incipient stage, which is the first stage in fire development, begins when heat, oxygen and a fuel source combine and have a chemical reaction resulting in fire, this is known as the fire tetrahedron. The fire triangle was the fundamental training point for the fire service and safety professions until the 1960s, when Walter Haussler introduced a fourth component.
The fire tetrahedron incorporates this fourth component for combustion to occur the concept states that a continuous chemical chain eaction must occur between the fuel and oxygen in the presence of heat, this new model constructed by Walter Haussler also depicts this. Extinguishment results when any component is removed, which is similar to the fire triangle, with the exception that, if the chemical chain reaction is interrupted, the fire will also be extinguished.
Within these models, heat is a form of energy that is capable of initiating and supporting chemical changes and changes of state This is also known as “ignition” and is usually represented by a very small fire which ften (and hopefully) goes out on its own, before the following stages are reached. Recognizing a fire in this stage provides your best chance at suppression or escape. If it reaches past the ignition point it will develop into the next stage, which is the growth stage. GROWTH STAGE The growth stage is shortly after ignition; a fire plume then develops above the burning fuel.
The gases are generated due to a process that is known as where the structures fire load and oxygen is used as fuel for the fire (Fire Dynamics. n. d. ). During this a Heat Release Rate (HRR) at which heat energy is enerated due to fuel being consumed by the fire this is calculated by the following equation (Gorbett 132-133) In combination with the HRR the ” fire plume identifies the combustion zone, where flaming combustion is occurring, and represents the luminous body of burning gases. Heat from the exothermic chemical reaction causes electrons within involved atoms to move more quickly.
This increase in movement, when coupled with the specific free radicals produced in the reaction, results in emission of energy waves in a frequency visible to humans. Temperatures and the types of chemical radicals roduced in the reaction determine the color observed. Thus, flame color can provide indicators of combustion efficiency; however, it does not serve as a reliable indicator of the fuels involved”(Gorbett 76). For example if you burn a sofa cushion in the center of a room the fire can obtain oxygen from all sides has normal flame height.
If it placed against the wall it has half the space to consume oxygen resulting in moderate flame height. If it is placed it in the corner of a wall it can only consume one forth the amount of space to consume oxygen resulting igher flame height due to the flames seeking more oxygen (Sudbury 2012). Once the plume has formed it becomes buoyant and creates a hot thin layer of gases that gets impinged by a horizontal surface known as a ceiling. The ceiling then forces it to move across horizontally known as ceiling jet.
The ceiling jet is also a very important aspect to sprinkler and detector activation. Ultimately, the time it takes for a sprinkler to activate is based on the temperature, velocity of the heated gases flowing by the sprinkler and/or detector, and the time it akes the detector or sprinkler to reach its activation setting (known as a response time index). Therefore, the detector and sprinkler activation is a function of the heat release rate, the height of the ceiling, and the distance from the axis of the fire plume” (Gorbett 232-233).
There are numerous factors affecting the growth stage including where the fire started, what combustibles are near it, ceiling height and the potential for “thermal layering”. It is during this shortest of the 4 stages when a deadly “flashover” can occur; potentially trapping, injuring or killing firefighters. Flashover is when the temperature in a compartment that results in the simultaneous ignition of all the combustible contents in the space.
Flashover is the transition between the growth and the fully developed fries stage and is not a specific event like ignition. “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 – 11120 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. 7 Btu”(Fire Development in a Compartment – Part II (Firehouse n. d. ).
One method to predict the heat release rate to cause flashover would be the “Babrauskas Method. This method gives us a formula for determining the minimum heat release rate of a fire that can cause a flashover in a given room as a function of the ventilation provided through an opening. Known as the ventilation factor, and colloquially referred to within the fire science community as “A root H,” it is calculated as the area of the opening (Av) times the square root of the height of the pening (Hv) (Babrauskas, 1980).
An approximation of the heat release rate required for flashover to occur from the method of Babrauskas is given in the following equation ” (Gorbett 244). FULLY DEVELOPED The fully developed stage occurs when all the combustible materials in the compartment become involved in fire. In this stage the fire can be controlled by ventilation. Fire fighters can cut holes in a roof of a structure to ventilate the super heated gas trapped in the compartment to help fire suppression efforts. Even though there are many different types of fires they all have any factors that impact their development.
For example, Size, number and arrangement of ventilation openings, volume of the compartment, thermal properties of the compartment enclosure, ceiling height of the compartment, size, composition and location of the fuel package that is first ignited, and availability and locations of additional fuel packages. Once all of the fuel in the compartment has been consumed by the fire then transitions into the decay stage. DECAY STAGE The decay stage is the rate of heat release that begins to decline as the fuel in the compartment is consume.
The stage of fire development within a structure characterized by either a decrease in the fuel load or available oxygen to support combustion, resulting in lower temperatures and lower pressure in the fire area. This is also known as pyrolysis of material. In conclusion fire development has four stages Incipient, growth, fully developed and the decay stage. Fire Fighters knowing these different stages through the study of fire dynamics and what these stages entail will allow them to have the upper hand on the beast by knowing how it develops and thrives.