The LWF Blog

Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 9

March 14, 2017 12:25 pm

In the last blog of this Fire Engineering Design and Risk Assessment series for architects and others in the building trade with an interest in fire safe design, we looked at opposed air flow and depressurisation as two methods of smoke control. In Part 9, we will continue by discussing slit extract systems and pressurisation.


A slit extract system works by extracting and siphoning off smoke from an area and can be used in addition to an exhaust system, thereby removing the need for a downstand. It can also be found useful at the openings to a room to prevent smoke outflow.


While such a system can be most effective at avoiding smoke entering a designated area, it may not be able to clear a smoke free layer within the area itself and works best in a room with sprinklers and a standard ventilation system throughout.  Extraction should be provided close to the opening of a continuous slit, which may be situated in the ceiling.


Tests on such systems have shown that it is possible a powered exhaust from a slit at right angles to the layer flow can remove smoke completely, provided that the extraction rate is at least 5/3 times the flow in the horizontal layer, flowing towards the slit. Through considering this, an idea of the size of the extract can be gained.


BS EN 12101-6:2005 – Smoke and heat control systems. Specification for pressure differential systems. Kits gives an overview of the design process of Pressurisation. Pressurisation is based upon the principle of raising the pressure of a protected area to a higher level than that of the surrounding spaces. The air will move, therefore, from the protected area to the surrounding spaces.


When implemented in an area, the aim is to keep it clear of smoke to aid escape in case of fire and so is most useful in corridors/stairs leading to an exit, for example. It is achieved through air relief routes, so that air can be vented from the fire area to the outside.


Air is simply injected into the protected areas and allowed to escape through the unpressurised areas to the open air outside the building by automatic means, this can be achieved by natural ventilation or by mechanical extract, subject to bespoke, detailed design.


One potential issue with this system in a building is that if it is leased and the occupant of the building makes changes to the environment, this could impact upon the effectiveness of a pressurisation system. For example, through the installation of partitions, or through the disconnection of the system following false alarms, or simply through it not continuing to work due to lack of maintenance.  For this reason it is essential that any lessees understand the fire protection elements built-in and how they should be maintained.


In Part 10 of this series, we will continue to look at Pressurisation and then talk about Natural Ventilation Systems too. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.


Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.


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