The LWF Blog

Fire Safety Engineering for Design – Mechanical Ventilation – Part 209

November 11, 2024 11:23 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 208, LWF discussed natural smoke shafts for firefighting shafts and considered wind overpressures. In part 209, we begin to look at mechanical ventilation.

A mechanical smoke ventilation system is one that is designed to extract smoke from a given area or building, through the use of fan systems. The design may also be ducted, depending upon the requirements of the environment.

Appropriate equations will be used to calculate the relevant fire size, type of smoke plume or flow and the resulting mass flow rate for the proposed fire, in relation to the area under assessment.

The volume flow rate of the smoke may calculated and should be less than or equal to the rate of extract proposed.

A mechanical smoke ventilation system is usually used in conjunction with a smoke shaft and is of particular use where space may be an issue or where natural airflow is insufficient to allow the use of a natural ventilation system.

A slot extraction system (also known as a slit extraction) may be used to prevent smoke flowing across room openings and into adjoining spaces. They may be employed to supplement an exhaust system and negate the need for a downstand or channelling screen, where employed over the length of the flow path.

A slot extract system is designed to prevent smoke entering an adjoining area, but it does not necessarily guarantee a clear layer within the original space. For this reason, slot extracts may be supplemented with additional mechanical ventilation from the room of fire origin.

The extraction process should be provided very close to the opening from a continuous slot, commonly situated at the plane of the ceiling or at a very high level (because smoke rises).

The powered exhaust from a slot at right angles to a layer flow can wholly prevent smoke passing the slot, provided the extraction rate at the slot is at least 5/3 times the air flow in the horizontal layer flowing towards the slit. Therefore, the general method of sizing such extracts may be commenced as follows:

• Calculate the flow rate of gases approaching the opening or gap
• Multiply the mass flow by 1.667 (5/3)
• Use the convective heat flux of the smoke layer to calculate the volume extract required (taking into account sprinkler cooling if appropriate).

 

In part 210 of LWF’s series on fire engineering we will look at residential smoke ventilation. 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.