The LWF Blog
Fire Safety Engineering for Design – Smoke Ventilation – Part 186
June 3, 2024 11:10 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 185, LWF looked at the area of a reservoir for smoke control systems, as well as discussing reservoir screens and curtains. In part 186, we will discuss replacement air.
A smoke control system relies on the removal of smoke from an area and, importantly, for the smoke to be replaced with air. The replacement air can be supplied by either natural or mechanical means, dependent upon the type of smoke ventilation system installed.
The inlets for air should be located in such a way that the hot smoke layer is not disturbed significantly. This is for two reasons, the smoke layer won’t remain a predictable layer at a certain height if it is moved around by an air inlet, and the inlet air could cool the smoke layer to the point at which it drops, which is detrimental to the process and to the conditions in the area.
The air inlets should ideally be located at a lower level or remotely from the smoke layer base.
The designers of a smoke ventilation system should be aware of the velocity of incoming air. When air is introduced at a high speed through openings used for means of escape, it can impede the evacuation of occupants. High-velocity air inlet at or near the base of the smoke layer can increase layer turbulence and instability.
In the UK, a maximum velocity for inlet air of around 5 m.s-1 across doorways or other openings used for escape is commonly considered appropriate. BS 7346-4 makes recommendations that the upper edge of an inlet opening should be calculated to be at least 1 metre below the smoke layer base, or that the inlet air speed beneath the smoke layer should be less than 1 m.s-1. These precautions will help to ensure the smoke layer is not disturbed or pulled down.
NFPA 92 takes a similar approach by stating that the replacement air velocity should not exceed 1.02 m.s-1 in all instances where the air could come into contact with the plume, unless fire engineering analysis has indicated otherwise.
In part 187 of LWF’s series on fire engineering we will continue to discuss replacement air for smoke ventilation before moving on to the number of extract points required. 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 since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.