The LWF Blog

Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 6

February 24, 2017 12:33 pm

In this blog series for Architects and others who work in the construction industry and have a responsibility for fire safety standards, we have been looking recently at ventilation and smoke control. In Part 6 we will discuss car park ventilation, which is as per the guidance BS 7346 Part 7.


In the case of underground car parks, where natural ventilation is not available, it is usual to provide a mechanical smoke extraction system which operates at 10 air changes/hour. Temperature rated ducts form part of the system to dilute smoke and heat within an enclosed area.


The system works so the extract rate is calculated to suit the car park layout and results in clear and smoke-free access to the Fire Service close to the fire location, which is classed as within 10m of any fire within the car park.


Where a ducting system isn’t appropriate or an alternative is sought, it is possible to install an impulse fan system. These systems are effective through the use of high velocity fans which forces air movement in a particular direction, towards extract fan intake points. Impulse fans also reduce the temperature in the compartment where the fire has started, in addition to providing the necessary smoke-free zones. They are best sited so that they meet the requirements for standard CO ventilation too.


In practice, large car parks are separated into zones (measuring not more than 2000m2 normally) and each zone is fitted with the necessary amount of impulse fans, but at least two. When a fire ignites and the fire alarm is sounded in that zone, the fans become activated and direct the smoke towards the extract point.


As with the mechanical smoke extraction system, the aim is to provide a smoke-free area within 10m of the fire base, so that the Fire Service can access the site to put out the fire. The system should also keep all other zones clear of smoke.


The fire detection system should be fully addressable to enable to the Fire Service to ascertain the zone where the fire is located and any zones which are smoke filled.


When considering means of escape provision, all escape routes should be kept clear of smoke for the time needed for all persons inside the car park to escape the structure to outside air and safety. This calculation should discount any exits which are within the affected zone.


In Part 7, we will give an overview of the benefits of an impulse jet fan system before moving on to discuss the design of those systems to protect escape routes and firefighting shafts. 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.



Share this post