The LWF Blog

Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 4

February 8, 2017 11:43 am

In LWF’s blog series for architects and other parties involved in the building industry, we have been looking at the use of fire and smoke ventilation as a part of a property’s fire protection plan. In Part 3, we began to lay out the requirements for a design to maintain a smoke free layer in a system to protect an area where a fire starts. In this blog, we will continue from where we left off, looking at replacement air.


Where smoke is removed from the atmosphere, it should be replaced with air, which can be supplied mechanically or by natural means. The velocity of the incoming must be limited to 5 m.s-1 across doorways, which is standard in guidance for certain types of buildings, e.g. shopping centres. The replacement air should have a maximum velocity of 5 m.s-1 when it reaches the bottom of the smoke layer.


In practice, even though inlets are incorporated into the design, replacement air will find its way via the path of least resistance which may not be through the intended pathway. Façade leakage rates should therefore be taken into account as a part of the design, both because of the effectiveness of the working design and due to environmental requirements of the building regulations.


Where the design calls for mechanical smoke removal systems, replacement air should be based upon volume balance rather than mass balance, but a natural smoke removal system design requires that replacement air should be based on mass balance instead. It should be noted that a powered inlet should not be used with a powered exhaust in smaller spaces as this can lead to the creation of a pressure ‘balance point’ which changes as the fire size does – this can have a detrimental effect on the forces acting on the fire escape doors.


When considering the number of extract points, the potential for ‘plug holing’ must be taken into account. This can occur where the smoke layer is shallow and a high extract velocity at any single point can lead to air being extracted from below the smoke layer, instead of extracting the smoke itself. To avoid this, several extract points might be necessary at a lower velocity.


Where a plenum chamber is installed above a ceiling as a part of the design, the amount and size of extract points in the ceiling should be based on the design pressure differential between the plenum chamber and smoke layer, and on the flow impedance of the extract points.


In addition, where suspended ceilings are in use below the level of the smoke extract points, they should be at least ¼ open and distributed evenly so that they do not interfere in any significant way with the flow of smoke.


In next week’s blog we will be looking at the design of systems to dilute the smoke to a tenable condition. 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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