The LWF Blog

Fire Safety Engineering for Design – Fire Growth – Part 91

July 25, 2022 11:40 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 90, LWF looked accumulated ceiling smoke layers and referenced smoke filling times. In part 91, we discuss smoke filling in rooms with low-level ventilation openings.

In a room with low-level ventilation openings, smoke does not channel through those openings, instead rising to ceiling level as it would in a room without ventilation openings. Additionally, heat loss to the room surfaces is negated, which would otherwise have resulted in slightly smaller fire development.

The equations for axisymmetric plumes and spill plumes for such rooms can be found in the CIBSE Guide E guidance document.


Room filling with smoke extract from layer

A critical height of the smoke layer may be dictated by the need to keep it above eye level, inside a reservoir, or if otherwise too hot, well above head level.

The need for a clear layer height may be necessary for means of escape purposes and in cases where it would be reached before building occupants have escaped to a place of safety, then extract from the smoke layer should be provided under steady state conditions.

Calculations for this can be found in CIBSE Guide E, as indicated above.

In situations where flames are rising to ceiling height, or where smoke flows out of the wall opening, the above-mentioned calculations would not be suitable and alternative equations should be used.

The simple room-filling model described above does not reference heat transfer to the ceiling and wall surfaces. This is a conservative assumption, because the volume of smoke is overestimated. However, in some situations, low-temperature smoke may fill a large reservoir and cooling can results in a loss of smoke buoyancy, which should be taken into account.

Where no experimental data is available, it should be assumed that cooling effects should be allowed for where the area of the reservoir is greater than 200 m2, and/or the temperature of the average layer is less than 10 K above ambient when calculated.

Suggested equations relating to heat transfer from smoke layer by radiation and stratification, as well as Flame Calculations can also be found in CIBSE Guide E.

In part 92 of LWF’s series on fire engineering, we will begin to look at means of escape and how it should be designed with life safety in mind. 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 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.

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