The LWF Blog

Fire Safety Engineering for Design – Smoke Clearance or Purging Systems – Part 196

August 12, 2024 10:40 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 195, LWF discussed noise levels for design values when considering smoke extraction and begin to look at the design of systems for smoke removal. In part 196, we continue to look at smoke clearance/purging systems.

As smoke dilution systems are designed to clear the smoke without maintaining a smoke-free clear layer (mainly because dilution systems tend to be used to clear cold smoke after a fire, not to maintain conditions during a fire), they are often based on a nominal air change rate given in prescriptive guidance. However, they can be based on design times to improve visibility or temperatures within a space, often through computer modelling.

An extract rate of 6 or 10 air changes per hour has been widely adopted as a prescriptive standard to clear smoke, except in atriums where sprinklers are provided (and where fire loads at the atrium base are controlled) when 4 air changes per hour suffice in England and Wales.

An equation to calculate ‘time to improve visibility within a space to a predetermined level’ is as follows:

Where c is the concentration at time t, c₀ is the initial concentration, a is the air changes per minute (or rate of purge) and t is the dilution time in minutes.

The concentrations must be calculated using the same units, although any form of unit appropriate for the particular containment is acceptable.

An area can be considered reasonably safe where the concentration of smoke is not greater than 1% of the concentration at the fire.

Cross-ventilation Systems

A cross-ventilation system is most commonly seen used to dilute or disperse smoke, particularly for firefighting operations. It is based on the flow of air from often opposing vents on the same floor level to provide dilution.

Cross-ventilation can be achieved either using natural or mechanical means.

Natural cross-ventilation in the UK has been based on providing nominally prescribed vent areas based on a percentage of the floor area of the building. At least half of the total of vents should be equally distributed on opposing sides of a space.

In part 197 of LWF’s series on fire engineering we will continue to discuss natural cross-ventilation systems, before beginning to look at mechanical cross-ventilation systems. 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.