An introduction to Mechanical Smoke Ventilation

24 August 2017

Although flames are ultimately the most dangerous element of a fire, statistics show that 80% of victims who die in a fire are killed by smoke. Here, Axair Fans explain the goals and standards relating to smoke control systems.

Axair Sybilo ATEX certified jet smoke extraction fan

We can look at the risks associated with smoke by looking at four subsections;
* Toxicity; including lack of oxygen, inhalation of toxic gases and suffocating gases.
* Heat; including internal and external burns from direct contact with or inhalation of smoke.
* Irritation; including respiratory infections, skin and eye irritations.
* Visibility; including difficulty locating the source of fire and escape routes.

Goals:
The goals of a smoke control system will inevitably look to address these risks by increasing visibility, reduce heat and avoid suffocation. Various factors including building geometry, control activation and other types of fire safety systems need to be considered when designing the smoke control system.
Even so, there will always be common goals for ventilation to facilitate safety in the event of a fire; to guarantee the safe evacuation of people, allow access for the fire brigade, to control the temperature in the building, to protect goods and to extract smoke during and after a fire.

Standards:
Various European Standards are in place and are important to note when designing a smoke control system. These include EN12101-1 Specification for smoke barriers, EN12101-2 Specification for natural smoke and heat extraction and EN12101-3:2015 Specification for mechanical ventilators. Specifically regarding mechanical ventilators and how Axair can directly contribute to the system components, there are two types of fans suitable for installation within the system; those for use inside the fire danger zone and those for use outside of the danger zone.
System design will depend on the type of building being specified, whether a single floor industrial building, multi floor building, car parks or pressurisation of stair in multi storey buildings. Alongside our supplier partner, Casals, Axair can advise on the correct method of smoke taking the type of building, the risk associated with each and the ventilation goals to be achieved through the system.
Control Systems Goals:
Smoke Buoyancy: The goal is to have two layers (or zones) in the vertical plane. One with smoke below the ceiling (known as the smoke zone) and a second without smoke (below the smoke zone, called the smoke free zone). The existence of this smoke free zone allows the evacuation of people and access for the fire brigade. The height and temperature of this smoke free zone has to be controlled.
Smoke Extraction: The goal is to have a system able to extract smoke generated during a fire. The system has to work during and after a fire in order to clear smoke from the building. It is mainly designed to achieve a set number of air changes per hour or an extraction rate related to other parameters (i.e. an airflow rate per number of cars in carparks). It can also be used for smoke extraction after the fire, where a suppression system is also installed (sprinklers, water mist system).
Horizontal Ventilation: This system is used in areas that do not have high enough ceilings to use a smoke buoyancy based system and which might have one or two of its other dimensions (width and length) bigger than the ceiling height. (e.g. tunnels and some types of carpark). As it is not possible to create two vertical zones, the goal is to create two horizontal zones, so that there are parts of the building free of smoke.
Pressurisation of Evacuation Routes: The goal here is to set a pressure differential in a protected area or route, to keep the smoke away from that route. This is achieved by maintaining a higher pressure in the escape route than the smoke affected area.
For more information on our induction fans for smoke control visit our website:
www.axair-fans.co.uk