The LWF Blog
Fire Safety Engineering for Design – Pressurisation Systems – Part 204
October 7, 2024 10:24 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 203, LWF discussed the use of pressurisation systems and compensated systems. In part 204, we continue to discuss planning for the use of compensated systems.
Changing conditions coupled with the use of a compensated system for pressurisation mean that modulating the supply air flow is a necessity. This is usually achieved by providing a pressure sensor in the relevant area to link to an inverter controlling the fan. In this instance, the engineer in question should ensure that the highest and lowest air flow requirements do not exceed the manufacturer’s recommended ranges for inverter-controlled fans. Commonly, the lowest air-flow rate is limited to 40% of the total air flow provided, e.g. 10 m3.s-1 fan may be inverted to a lower limit of 4 m3.s-1.
In cases where natural overpressure relief is provided, the following equation can be used to calculate the area.
Where Qc is the air supply used to calculate overpressure relief vent (m3 · s–1), Qfr is the air supply needed to provide the required air flow through an open door into the fire room (m3 · s–1) and Qp is the air supply to the stair or lobby needed to satisfy the pressure differential requirement (m3 · s–1).
In cases where overpressure relief is provided by natural ventilation, the following equation should be used:
Where Qc is the air flow into a pressurised space (m3 · s–1), P is the pressure (Pa) and Apv is the area of air/pressure relief (m2).
It is necessary to provide a sufficient air release path to adjoining areas of a pressurised space, as otherwise, over time the adjoining space in question will reach the same pressure as the pressurised space and result in smoke and combustion gases flowing from the adjoining space into the pressurised one.
Where the fire is originating in the adjoining space, it is possible that ventilation will be provided by window breakage, but it cannot be relied upon. It may also be the case that the adjoining space is a corridor which doesn’t have ventilation and is otherwise smoke-logged.
In part 205 of LWF’s series on fire engineering we will continue to discuss the air release path and leakage. 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.