The LWF Blog
Fire Safety Engineering for Design – Smoke Ventilation – Part 193
July 22, 2024 10:34 amLWF’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 192, LWF discussed tenability criteria by talking about radiation and by starting to discuss visibility in smoke. In part 193, we continue to explore visibility in smoke.
Certain calculations can be made in order to ascertain optical smoke density. For a static homogenous smoke layer, it can be calculated as follows:
Vt is the total volume of smoke (m3), fb is the total mass of fuel burnt (in kg) and Dm is the mass optical density for the fuel concerned (m2 · kg–1). Vt can be calculated from the volume of the smoke layer.
Total mass of fuel burnt is calculated by multiplying the mass burning rate of the fuel (mfuel) by the time (s). This can be found from the heat release rate at steady state:
In this equation, Qsteady is the rate of heat release at steady state (kW), mfuel is the mass burning rate of fuel (kg.s-1) and Hc is the effective calorific value of fire load (kJ.kg-1).
Calorific values for various materials can be found in PD 7974-1:2019 Application of fire safety engineering principles to the design of buildings. Initiation and development of fire within the enclosure of origin (Sub-system 1)
To define visibility in smoke in terms of the furthest distance at which an object can be perceived (S (m)), the following calculation may be used:
The optical density per unit length (m-1), D, and a visibility coefficient, K (m-1).
Light emitting objects (e.g. electric lights) are more easily perceived than other objects of course, imagine a light shining in a smoky area, it would obviously be more visible than a non light-emitting object nearby. The differences are quantified as per the typical visibility co-efficients for wood and plastic based fires:
- For light emitting signs: K = 8
- For light-reflecting signs: K = 3
- For building components in reflected light: K = 3
In part 194 of LWF’s series on fire engineering we will discuss the toxicity of smoke and tenability limits. 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.