The LWF Blog

Fire Safety Engineering for Design – Noise Levels – Extraction design – Part 195

August 5, 2024 10:47 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 194, LWF talked about the toxicity of smoke and tenability limits. In part 195, we discuss noise levels for design values when considering smoke extraction and begin to look at the design of systems for smoke removal.

Extraction systems to remove smoke from a building, compartment or area can create high noise levels, which should be considered and avoided at the design stage.

High noise levels from a smoke extraction system should be considered in terms of the audibility of the fire alarm system to building occupants, particularly in the case of voice alarm systems which may be giving instructions.

The noise level for an extraction system should be limited to 115 dBa a few seconds of exposure and 92 dBa for the remainder of the exposure time.

Smoke dilution systems are sometimes also known as a purging system. The methodology is based upon diluting the smoke in a space in order that the design criteria are not exceeded for tenability or containment temperature limits.

Smoke dilution which is based upon simple dilution using only the volume of the space only tends to be effective in the largest spaces with relatively small fires. It would be unlikely that sufficient dilution could be maintained to enable tenable conditions for a substantial period without smoke ventilation.

Most smoke dilution systems are designed to extract smoke and provide air inlet points.

Prior to the design of a smoke dilution system, the designer should consider and consult any appropriate local codes and regulations to ensure the resulting system will be suitably acceptable to the authority having jurisdiction.

Smoke clearance systems, also known as purging systems, are also based on dilution and are commonly used to clear cold smoke after a fire has been extinguished. Smoke clearance ventilation is usually undertaken using natural ventilation, but a lack of buoyancy at higher levels means that at heights exceeding 18 metres, mechanical ventilation should be considered.

Post-fire clearance of smoke does not have to take into account maintaining a smoke-free layer to aid evacuation and firefighting efforts, therefore replacement air for dilution systems may be from both high and low levels. Careful design should ensure that the supply and exhaust points are located appropriately, to avoid the supply air being extracted by the exhaust.

In part 196 of LWF’s series on fire engineering we will continue to look at smoke clearance/purging 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.