By Michael Reick
The principles of fire dynamics and fire ventilation are important for fighting fires in buildings. The balance between rescue operations and fire extinguishment is a tactical necessity, and it is directly linked to these principles. The ability to estimate the burning regime and perform proper fire ventilation is essential for safe operations on the fireground, and it is the basis of controlling the spread of smoke and the flow path. This can lead to efficient rescuing for civilians and to safe and fast extinguishment.
Firefighters need to know the difference between developed fires in large and small compartments and understand fuel-controlled, ventilation-controlled, and underventilated fires. Realizing which burning regime is present is essential to understanding if the actions being taken are influencing the fire’s development. A knowledge of aggressive ventilation, defensive ventilation, and non-ventilation is needed to understand the different approaches. The principles and limitations of different strategies are the basics for incident commanders’ decisions. Looking from a worldwide perspective, these topics should be addressed in relation to what firefighters need to know at their individual positions.
Fires in the open air and fires with a limited size in large compartments develop according to the available fuel load. As there is enough air (and oxygen) available, they are called to be fuel controlled. When comparing fires in large compartments to fires in the open air, we have to consider the huge amount of smoke released by the fire. Even in large compartments like industrial buildings, this will lead to a smoke-filled compartment, and a knowledge of the basic laws of physics is needed to understand the flow and spread of smoke in such a situation. The expansion of heated gases and the buoyancy force will lead to an upward flow of smoke that will mix with air and fill a compartment from top to bottom.
A closer look into some physical concepts helps to understand how smoke spreads in large compartments, looking at flows of smoke and mixing processes between smoke and air. Some basic knowledge will also help firefighters to understand how smoke exhaust systems are designed to support the work of firefighters. In these fires, the fire’s development is not dependent on the increased ventilation caused by technical means or the fire service.
(1) A camp fire is an example of a fuel-controlled fire. The amount of smoke is often underestimated.
(2) A fire in a large compartment. Smoke production, smoke spread, and smoke mixing with air will fill up the compartment in a fairly short time. The fire’s development is not dependent on enforced ventilation of the compartment.
In compartments of limited size in relation to the size of the fire, the concentration of oxygen will decrease because of the burn process. A descending concentration of oxygen will lead to a ventilation-controlled fire.
(3) A fire in a small compartment will immediately go into decay phase if ventilation is limited–in this case, if the door to the furnace is closed.
To sustain a constantly progressing fire in a limited compartment, some gas exchange between the compartment and the outside has to occur. This necessitates that smoke be released from the compartment and that fresh air flows into the compartment. If these flows are to go in opposite directions through the same opening, a bidirectional flow must be established. This will always result in some kind of mixing process between smoke and fresh air. In normal building situations, if this has to occur through doors and windows, this gas exchange will often limit the heat release rate from the fire.
Figure 1. The flows of smoke and fresh air in a building fire. Illustrated are bidirectional and unidirectional flows.
Controlling Smoke Spread
If smoke is released to the outside of buildings, at least this does not increase the hazard of smoke spread into escape routes in the buildings. Fire crews should always avoid making the situation worse for civilians in the buildings. Therefore, prevent smoke from spreading into escape routes and especially into stairways. Keeping every stairway some free is a better option than fire crews trying to use one stairway for evacuation and one stairway for attack operations because it is difficult to communicate this plan to all of the trapped civilians.
The smoke spread into the rest of the building is one thing. But, the flow of fresh air into a fire compartment also has an important influence on the fire, especially if the fire is ventilation controlled. Looking at the fire research from the past decade, it is obvious that many firefighters underestimated this relationship. It has been shown that a fire can grow from the size of a portion of the room to a fully developed room-and-contents fire is only a few minutes. All the emphasis on controlling the door or controlling the flow path shows the importance of this research.
For all these reasons, fire departments in Europe perform flow path control have been using smoke-blocking devices extensively more than 10 years. Besides the influence on the burning regime and the limitation of smoke spread into escape routes, we do this also to separate the fire hazard from potential victims and to reduce damage. This has become a standard operational procedure especially in multistory buildings in several countries worldwide.
Figure 2. A smoke-blocking device installed in the entrance door to a burning compartment can essentially improve the prevention of smoke spread into escape routes in a multistory building.
For decades, similar applications have been used in mines and on vessels. The blocking of openings is especially essential in these circumstances. Vessels are in some ways similar to large buildings: You can’t evacuate them while you fight a fire inside the construction, so you have to be careful about smoke spread and not harming people.
With thousands of documented uses in real fires and tens of thousands of devices in the fire services, especially in Europe but also in America and Asia, there is a great range of experience for this application also in building fires. Photos 4-5 are from Austria. The following movie on youtube provides more information on this incident:
Quad view movie: https://www.youtube.com/watch?v=Cv5H7agDXVI
Single view movie: https://www.youtube.com/watch?v=914dY02JhPM
(4-5) A smoke-blocking device installed in the entrance door to a burning compartment can essentially decrease smoke spreading into a building in a multistory building.
In the United States, quite a few fire departments use these devices. Their aim might be door control, but they also enhance ventilation when used in combination with a fan as illustrated in photo 6.
(6) Enhanced ventilation combined with the use of a smoke-blocking device in the door frame and a fan with limited distance because of the width of the corridor. (The photo is from a real wind-impacted fire incident in a high-rise building).
The Kill the Flashover Project
As a result of all this discussion, the Kill the Flashover Project also used these devices in multiple fire tests. Among them were many applications that compared the ability to control the door with a curtain and controlling the door with staffing as well as a wind-impacted fire scenario. The comparison between controlling the door with a curtain or staffing is illustrated in photos 7-8 and also in this youtube footage:
(7-8) Kill the Flashover Project (KTF) comparison of the results of tests in which the door was controlled with staffing (photo 7) and a smoke blocking device (photo 8). June 2016, Loveland, CO.
Regarding only the flow of smoke and fresh air, it is obvious from photos 7-8 that the total blocking of the upper door area with a curtain is much more effective compared to door control with staffing because of the gap between the door and the door frame if a hose is stretched through the door. The wind-impacted scenario can be seen at: https://www.youtube.com/watch?v=EePmk3ThKB8.
(9). The KTF burn setup with a fire in the AD corner room and a strong wind facing the AD corner of the building. June 2016, Loveland, CO.
After ignition of the fire, the simulated wind forced the smoke out of the building mainly at B side. After the window in the AD corner failed, the amount of smoke leaving the building at the B side increased tremendously. Still, flames developed in the direction opposite of the wind, indicating that the fire is still searching for oxygen. Because of this wind-impacted situation, there is no temperature layering in the fire compartment any more. Extremely high temperatures are documented in the fire room in this situation.
(10-11) A wind-impacted fire before and after the window failed. The flames out of the window are in the direction opposite to the simulated wind on the AD corner of the test building.
Fire ventilation is obviously a key factor for safe and effective firefighting operations.
Regional Fire Chief Michael Reick, Fire Dept. Göppingen/Germany
MICHAEL REICK is the regional fire chief for the county of Göppingen, Germany. He has 12 years of experience as a volunteer firefighter and an additional 18 years as a regional fire commander. He was a research engineer and the leader of the Fire Research Laboratory at the University of Stuttgart.