The LWF Blog

Fire Safety Engineering for Design – Smoke Ventilation – Part 187

June 10, 2024 10:21 am

LWF’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 186, LWF began to discuss replacement air. In part 187, we continue talking about replacement air and start to look at extract points.

While the replacement air for smoke ventilation will be subject to design by fire engineers in order to not disturb the smoke layer or plume of a fire, in practice, it may also from the flow paths of least resistance.  This may include leakage from the façade and other openings. Care needs to be taken when using such paths as a source of replacement air, as the air-tightness of buildings increases due to the environmental requirements of the building regulations.

NFPA 92 recommends that make-up air be designed at 85-95% of the exhaust which does not include leakage through small leakage paths.

The replacement air balance differs depending upon whether mechanical or natural smoke removal systems are utilised. For mechanical, the replacement air requirement should be based on volume balance, not mass balance. For natural, it should be based on mass balance.

When a powered air inlet is used along with a powered smoke exhaust, it may result in changing pressures generated by the developing fire and so thought should be given to the potential impact on the forces acting on fire escape doors etc.

Extract Points

Care should be taken when designing the extracts, depending on design fire size. When a smoke layer may be relatively shallow, a high-extract velocity at any single point may result in plug holing, a situation where air is extracted from beneath the smoke layer, rather than extracting from the smoke layer itself. A remedy may be to provide several extract points, working at a lower velocity.

Volumetric flow rates, distance between extracts and volumetric flow can be calculated by a fire engineer to ensure correct operation of extract points in a given volume.

In part 188 of LWF’s series on fire engineering we will begin to discuss smoke layer depths. 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.

Share this post