The LWF Blog

Fire Safety Engineering for Design – Fire Growth – Part 90

July 18, 2022 9:19 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 89, LWF looked at spill plumes, otherwise known as flow from an opening. In part 90, we consider accumulated ceiling smoke layers.

A simple zone model might show that smoke rises within a compartment to coalesce in a smoke layer of a uniform depth and temperature and that under that layer is a substantially smoke-free layer. With this in mind, for effective means of escape, life sustainability and visibility of firefighters, it is common to install a smoke control system in order to maintain a minimum height for the smoke-free layer for a specified amount of time.

Smoke filling times

In the case of steady-state smoke control design, entrainment equations may be used to calculate the smoke exhaust required.

In larger spaces, however, the volume of the smoke reservoir is so large that the size itself is a form of smoke control, since any smoke reservoir will take a certain amount of time to become full. The time that will be taken may be calculated in a few different ways:

• A computer program may be used to integrate calculated smoke volumes produced at small time intervals, e.g. available safe egress time
• Various relationships may be integrated mathematically, using simplifying assumptions to ascertain a formula

The second method contains more approximations and will therefore produce a more conservative figure.

Methods of calculating simple smoke filling can be worked out relatively easily. A given fire growth curve – fast, medium or slow – should be subdivided into time elements and entrainment equations are applied to each successive element. The depth of the layer of smoke in the reservoir at the end of each time period can then be taken as the starting point for the next element, calculating the smoke filling the reservoir.

The smoke layer, then, will consist of a number of elemental thin layers. As well as adding the elemental layers, elemental smoke extract may then be subtracted, depending upon which type of smoke control is applied (if any is).

The output of the program can show the following as a function of time:

• Clear layer position
• Average temperatures
• Average visibilities

In part 91 of LWF’s series on fire engineering, we will touch on smoke filling and heat transfer. 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.