The objectives of smoke control
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The danger of fumes
Dangers for people
Since the 1980’s, it is known that 80% of the deaths caused by a fire are due to the inhalation of toxic fumes. We can distinguish 3 main dangers. Hazards related to opacity, toxicity and temperature.
Opacity Hazards:
Fumes have a particularity which is their opacity. However, when a person is caught in a fire and visibility is reduced to less than 4 meters, it becomes almost impossible for them to find their way out and find the emergency exits.
In a state of panic, the occupant loses his or her bearings and acts as if he or she is trapped in a confined space, losing all sense of direction. If the person is not removed quickly from this atmosphere, the situation can quickly become fatal.
Loss of visibility at the start of a fire
Toxicity Hazards:
A fire releases about 140 hazardous compounds, some highly concentrated, that can be fatal even after a short exposure. Victims must be rescued within 30 minutes of the fire. Toxic gases fall into two categories: asphyxiating gases (such as cyanides and carbon oxides) that depress the central nervous system, and irritating gases (such as chlorine and its derivatives) that damage the respiratory mucosa. Among the most dangerous compounds for human beings we find :
- Carbon dioxide
- Carbon monoxide
- Nitrogen oxides
In the event of a fire, there is a significant reduction in the amount of oxygen available in the air, which can cause syncope or even death if the oxygen concentration drops below 6%. The fumes from the fire also contain toxic compounds that can cause rapid poisoning. A person in a smoky environment will cough, but this can make the situation worse by increasing their exposure to toxic substances.
In addition, the decreased amount of oxygen in the air can lead to impaired motor coordination and difficulty moving, which can put the victim at risk of death if not evacuated quickly. It is therefore crucial to take precautions to prevent fires and to know how to react in case of emergency.
Temperature-related hazards:
In a fire, the temperature can rise rapidly and reach levels lethal to the lung alveoli above 120°C, causing the victim’s death within minutes. However, the fumes produced have a very low density, which causes them to rise towards the ceiling and accumulate in layers with decreasing temperatures (a phenomenon called “stratification”).
Due to convection and stratification, it is recommended to move closer to the ground during a fire, where temperatures are lower, concentrations of toxic compounds are lower and oxygen levels are higher. In short, it is advisable to get close to the ground to maximize the chances of survival in the event of a fire.
Hazards to property
Smoke plays an important role in the propagation of fires because of its characteristics. During a fire, combustible materials are exposed to high temperatures in hot, oxygen-depleted environments, such as smoke or the interior of the flame. As a result, the materials decompose and produce combustible gases that feed the flames, this phenomenon is called pyrolysis.
In addition, the fumes are also corrosive, containing compounds such as hydrochloric acid, which are hazardous to building structures and property in the area affected by the fire.
The roles and objectives of smoke control
In short, the greatest threat to a person caught in a fire is smoke poisoning. It is therefore crucial to evacuate the smoke from the affected building, which is accomplished through smoke extraction. During a fire, the smoke and heat produced remain trapped inside the building, which can prevent the occupants from safely exiting. The purpose of smoke extraction is to evacuate a portion of the smoke from the fire to create an area of free air below the smoke layer.
The advantages of smoke extraction are multiple: it facilitates evacuation of the occupants by maintaining visibility and fresh air, it limits the spread of the fire by evacuating hot gases and particles, it allows firefighters to access the fire sceneIt helps reduce the risk of building collapse by limiting the temperature rise.
The constraints of smoke control
In order to meet the objectives stated above, several constraints must be respected to ensure effective smoke control. The first constraint consists in limiting the volumes to be cleared of smoke by using compartmentalization through the installation of firewalls and fire doors to reduce the propagation of the fire.
Large sales areas that cannot be compartmentalized will be reduced by creating containment screens to channel fumes to the exhaust systems. Moreover, it will be necessary to respect the stratification phenomenon by avoiding creating turbulences that could send the hot fumes back to the ground. Blowing speeds should never exceed 5 m/s. For an efficient scavenging, the fresh air intakes should be located at the bottom and the smoke exhausts at the top, avoiding any dead zone where a smoke plug could stagnate.
Finally, it will also be important to comply with thermal regulations to ensure optimal thermal comfort without having to resort to permanent openings to the outside.
The constraints of smoke control
There are two main principles for smoke control:
- sweeping
- the hierarchy of pressures.
The sweeping consists in circulating fresh air in the lower part of the volume to be de-smoked and in extracting the smoke in the upper part. This can be done naturally or mechanically depending on the type of drainage.
The pressure hierarchy, on the other hand, consists of establishing a lower pressure in the affected volume than in the adjacent rooms in order to create an equilibrium that prevents the spread of smoke. This method is often used in high-rise buildings and requires mechanical smoke extraction.
The different modes of smoke extraction
Natural smoke control
This is the most common type, which uses smoke vents or remotely controlled openings in the façade to evacuate smoke, and doors or openings to bring in air. The controls must act simultaneously on the exhaust and supply air.
Mechanical/natural smoke control
This configuration is less frequent and requires the use of a fan for the air supply when the volume to be de-scented is semi-buried or otherwise difficult to access. Air inlets must have an air velocity of less than 5 m/s, and smoke exhausts must be made from roof mounted outlets.
Natural/mechanical smoke control
Here, the extraction is carried out by one or more fans, while the air supply is carried out by remote-controlled doors or openings as for natural smoke extraction. The dimensions of the air inlets are calculated taking into account the total extraction flow.
Mechanical/mechanical smoke control
This type of smoke extraction requires both supply and exhaust fans. The air velocity at the supply ports must not exceed 5 m/s to avoid destratification of the fumes. The supply air flow rate must be lower than the exhaust air flow rate to respect the pressure hierarchy, generally up to 0.6 times the exhaust air flow rate.
The concept of smoke control engineering
The decree of March 22, 2004 introduced the engineering of smoke control, which is used when the strict application of the regulations is not possible, as in buildings classified as historical monuments where certain modifications are impossible. This approach aims to simulate the evolution of smoke during a fire and its control by natural and/or mechanical smoke control systems.
Engineering studies must be performed by organizations recognized as competent by the Ministry of the Interior. These studies should include a comprehensive presentation of the assumptions made, simulations showing satisfactory smoke control, and a presentation of the results and conclusions regarding the effectiveness of the recommended smoke control systems.
In 2017, the Good Practice Guide for Smoke Control Engineering Studies was published by the Central Laboratory of the Paris Prefecture. This guide clarifies the roles and responsibilities of the actors, harmonizes the definitions, formalizes the process of smoke control engineering, defines imposed fire scenarios, harmonizes the predefined acceptability criteria and establishes the different methods of smoke control tests on site.
