Ventilation Principles, Part III Backdraft Backdraft: The introduction of oxygen to a confined area that is pressurized with heated-flammable gases, that are deficient in oxygen, that results in an explosive force of significant intensity. Unlike flashover (see below), potential backdraft conditions can generally be identified by several key warning signs. However, before considering the identification and method of minimizing or eliminating a backdraft, let's review some basic theory to understand the why of a backdraft. If a free-burning fire is transferred to a confined building and allowed to burn, the fire initially has the proper ratio of fuel, heat, and oxygen, which is about 21 per cent in air. This is referred to as the first phase of a fire. As this process continues, an interesting chain of events is initiated. As the quantity of hot gases and smoke in the room increases, the available oxygen necessary for combustion is reduced. As the oxygen content is reduced below 21 per cent, the burning rate will decrease and the production of smoke, heat, and fire gases will increase. The increased smoke and heat will fill the interior of the room and try to exhaust through any available openings. Additionally, the oxygen content will be reduced to about 17 per cent, levels of carbon monoxide and other products of combustion will increase, and temperatures may exceed 1,300-degrees F. This is referred to as the second phase of a fire. At this point however, another condition is rapidly becoming significant. As the fire continues to burn, the ratio of heat, fuel, and oxygen has become unbalanced, with levels of oxygen approaching 13-15 per cent, which is not adequate to support a free burning fire. When the percentage of oxygen reaches 13-15 per cent, the fire will smolder due to insufficient oxygen and is referred to as the third phase of a fire. Simultaneously, with the fire smoldering and the subsequent reduction of oxygen in the room, the fire gases and products of smoke have been heated and have also expanded. In fact, the volumetric expansion of gases and products of smoke can increase up to a factor of three, creating a significant pressure inside the structure. This internal pressure will force smoke from all cracks or other available openings. At this stage, the room is pressurized, flammable carbon monoxide is abundant, temperatures are high, and oxygen levels are low. Obviously, we have a prime opportunity for a backdraft if oxygen is suddenly introduced through an open door or window. Now that we have reviewed the why of a backdraft, consider the following trademarks of a potential backdraft:
Once a potential backdraft has been identified, how can it be minimized or eliminated? The hazard of a backdraft can be eliminated by vertical ventilation over the involved area. This will reduce the internal temperatures and exhaust smoke and fire gases from the uppermost portion of a contaminated area to the exterior of a structure. The vertically exhausting superheated smoke and fire gases will not allow the introduction of a fresh supply of oxygen to the oxygen deprived fire. Conversely, if horizontal ventilation is utilized, fresh oxygen can be introduced to the oxygen deprived fire. Although the preceding question and answer are simple, practical fireground application may prove difficult due to limited experience, training, and the fact that suppression personnel can be very competitive. Some attack personnel find it difficult to stand at the doorway of an involved structure with a loaded hose line and wait for additional personnel to create an appropriate ventilation opening. However, to reduce or eliminate the chance of an apparent backdraft, a coordinated operation must be initiated between suppression personnel with hose lines, and ventilation personnel ventilating above a fire. If ventilation above a fire precedes entry into the structure, heat, smoke and fire gases will be reduced and so will the potential for a backdraft. The success of this operation is dependent on the following factors:
Flashover: "The sudden ignition of exposed combustible surfaces and/or combustible gases in an involved area that results in a sudden and intense rise in temperature." Although the definitions of flashover and backdraft (see above) may initially appear similar, flashover is substantially different from a backdraft due to the omission of the following three subtle principles:
As a fire burns in a building, the heat and products of combustion rise to the highest available point then begin to bank down. As this process continues, another chain of events is simultaneously taking place. As heat and other products of combustion collect at the highest possible point, their temperature will increase. This rising temperature will also increase the temperature of adjacent materials which assists heated gases and products of combustion in radiating additional heat to exposed combustible materials. If this process continues, a flashover can occur with sudden rapidity when combustible fire gases, smoke, and/or contents within the fire area reach their ignition temperature. Currently, there are two viewpoints on the cause of a flashover. One viewpoint states that as the contents within a room are heated to their ignition temperature from radiated heat, a flashover of these combustible materials will occur when their ignition temperature is reached. The other viewpoint states that combustible smoke and fire gases will initially flashover when their ignition temperature is reached, and can then cause exposed contents to also suddenly flashover as their ignition temperature is abruptly elevated to their ignition temperature. Although both of these viewpoints have merit, current video tapes of tests and actual fires indicate that a flashover normally begins with the sudden ignition of combustible smoke and fire gases which then can result in the exposed contents also flashing over. This process is dependent on the following factors:
Consider a typical scenario:
This fire is responsible for heat, gases and products of combustion rising to the highest point, then banking down as illustrated. As the fire continues to burn, heat and smoke will also continue to bank down. The density of the smoke will increase as temperatures in the upper portion of the room and radiated heat continue to rise. Up until this point, the attack personnel have been able to stand erect as they approach the fire. This is due to moderate temperatures, their safety equipment, and natural ventilation from the open door. However, if the fire continues to burn, the attack personnel may encounter three distinct temperature zones as follows:
At this point, let's assume that the smoke and fire gases at the upper portion of the room have reached their ignition temperature and flash over, creating a sudden and intense rise in temperature and a loss of visibility within the entire room due to increased smoke and heat levels. Tests have shown that this type of flashover can result in the following conditions:
Obviously, these conditions are immediately untenable for suppression personnel and may result in injury or death. Consider the following conditions present in this scenario:
These three principles are confronted on a daily basis but may not be recognized as a potential hazard. Most fireground personnel are familiar with the term backdraft, yet may not consider or recognize the possibility of flashover. Consider the conditions that may indicate a potential flashover:
Following are several additional considerations:
It is imperative that personnel engaged in suppression operations be aware of, and constantly monitor their fireground environment as follows:
Chief Mittendorf is the author of Truck Company Operations and Facing the Promotional Interview. To purchase, return to the main page and scroll down to Fire Engineering Books. © Copyright Firenuggets.com 2006 • Click here for Terms and Conditions of Use
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