The LWF Blog
Fire Safety Engineering for Design – Pressure differential systems – Part 201
September 16, 2024 10:33 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 200, LWF explored cross-ventilation systems by discussing opposed air flow systems. In part 201, we begin to look at pressure differential systems, starting with depressurisation systems.
A depressurisation system works to control smoke by extracting air (or smoke) from the fire-affected area of a building to create a negative air pressure in the space, to a level less than that in the surrounding areas. The pressure differential causes smoke to be held in the area of lesser pressure. In the US, this type of system can be referred to as a zoned smoke control system.
The effect can be multiplied by creating a positive air pressure in the adjoining areas unaffected by the fire, e.g. staircases or zones immediately adjacent to the area of fire origin (to each side and above and below).
Minimum design pressure differences are dependent on local codes of practice, but a design pressure difference of 25 Pa between the depressurised and pressurised space can be sufficient for an unsprinklered room with max. ceiling height of 2.7 metres. In a sprinklered area, this figure could be halved.
BS EN 121016 Smoke and heat control systems – Specification for pressure differential systems. Kits recommends a pressure differential of 50 Pa, regardless of the ceiling height of an area or if sprinklers are present, when doors and windows are closed, 0.75 m · s–1 velocity between pressurised and depressurised spaces when doors etc. are opened for means of escape and 2 m · s–1 for firefighting.
The necessary extract rate may be calculated from the required pressure difference and the assumed leakage area, as follows:
where Q is the air flow into or out of a pressurised space (m3 · s–1), Al is the inherent leakage area from openings and building construction (m2), R = 2 (constant) and P is the pressure (Pa).
On this basis, the required air flow between a pressurised space and an adjoining area (at 0 Pa) with leakage area being only a single 2 m2 door would be 8.3 m3.s-1 in order to maintain a pressure difference of -25Pa in the room of fire origin.
To maintain the pressure of the adjoining space at 0 Pa, inlet vents are required from the adjoining room. This ventilation may be natural or mechanical, but should be sufficient to ensure the pressure in the adjoining room does not drop below that which is required by the relevant codes. In the case of a mechanical ventilation system, this can be achieved by using the Q (m3 · s–1) value to calculate the make-up air needed.
In part 202 of LWF’s series on fire engineering we will discuss other factors, such as the temperature of extraction fans and use of the HVAC system. 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.