The LWF Blog
Fire Safety Engineering for Design – Natural Ventilation Systems – Part 206
October 21, 2024 10:52 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 205, LWF talked about the air release path and leakage for ventilation systems. In part 206, we begin to look at natural ventilation systems and the three types available.
There are multiple types of natural ventilation systems available in buildings and they can be largely divided into three headings:
- Smoke clearance – see detail below
- Smoke control – consisting of high-level vents with low-level inlet and are designed to maintain conditions as determined by the designer, e.g. maintain tenability or fix smoke layer height.
- Smoke shafts – designed to protect adjoining spaces, e.g. residential corridors or firefighting shafts.
Smoke clearance systems are usually dilution or cross-flow and should be designed taking into account the relevant fire size and other factors such as mass flow rate, volume flow rate and smoke temperature.
Where the size of the vents isn’t known, they can be calculated as follows:
The equation can also be arranged to allow the vent area to be calculated in terms of Q, but should only be used for very large inlet areas:
where Av is the throat area of the ventilator (m2), msmoke is the mass flow rate of smoke (kg · s–1), T is the smoke temperature (K), Cv is the coefficient of discharge (dimensionless), g = 9.81 (m · s–2), d is the smoke layer depth (m), p0 is the ambient air density (kg · m–3), i is the excess temperature (°C), T0 is the ambient air temperature (K), Ai is the total area of all inlets (m2) and Ci is the entry coefficient for inlets (dimensionless).
The coefficient of discharge (Cv) is provided by the manufacturer but typically taken as 0.6. If inlet air is provided by smoke ventilators in adjacent smoke reservoirs, then Ci would be the same as Cv. Ambient air density is usually considered as 1.2 kg · m-3. Excess temperature is simply the smoke layer temperature in degrees Celsius minus the ambient temperature.
In part 207 of LWF’s series on fire engineering we will discuss natural smoke shafts and their use in residential buildings and firefighting shafts. 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.