The LWF Blog

Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 8

March 8, 2017 12:02 pm

In this Fire Engineering Design and Risk Assessment blog series for Architects and others involved in building design and construction, we have recently been looking at fire and smoke ventilation. Part 7 looked at systems in car parks before moving on to begin talking about how systems should be designed to protect escape routes and firefighting shafts in a building. In Part 8, we’re going to discuss methods such as opposed air flow and depressurisation.


Opposed Air Flow is a method of smoke control based on producing an air flow back towards the area of the building where a fire has started. The result of such a working system is that the smoke emitting from a fire cannot leave the area due to the air velocity. As this system has been widely used in the U.S., more details on how the system can be applied can be found in NFPA 92B and the SFPE Handbook.


The suitability of the Opposed Air Flow system depends on variables such as the type of building, size of opening and the potential temperature of the smoke. For example, where it is to be used in cases of a large opening, the smoke temperature should be relatively low, as is the case where the fuel load is low or a sprinkler system is controlling fire growth.


Another method of fire and smoke ventilation not seen commonly in the UK is Depressurisation. This is a system whereby both air and smoke are extracted from the area of the building where the fire is contained, which reduces the pressure in the space to a lower level than the adjacent areas of the building.


This difference in pressure inhibits movement of the smoke in the original area. In some scenarios, the adjacent areas are subject to pressurisation, enhancing the effect.


Mainly seen in practice in the U.S. and Australia, such systems are most useful in situations where the fire-affected part of the building is adjacent to a large space and depressurisation can be carried out through use of the air handling/conditioning systems in the building. This would mean that in case of fire, the air handling system would switch to extract only on the floor of fire origin and to supply mode on the other floors. In order for such a method to be successful, the potential for leakage must be relatively small, but the benefit to owners of a suitable building is that the air system is already in place.


In addition to the standard equipment, such a system may need fire rated fans and fire-resistant ductwork to be installed, although these requirements and others will be based upon scenarios such as potential size of the fire, heat loss and smoke dilution within the duct system.


In Part 9 of this series, we’re going to move on to look at Slit Extract Systems and Pressurisation. 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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