The LWF Blog
Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 3February 1, 2017 10:39 am
In this blog series for architects and others involved in the building industry, we have been looking at the use of fire and smoke ventilation as a part of a property’s fire protection plan. In Part 2, we established that there were two types of system objective – to protect the area where a fire has started or to protect areas beyond the fire area. In this blog, we are going to look at the design parameters for a system to protect an area where a fire starts.
The aim of the design is to maintain a smoke-free layer with a view to persons being able to negotiate the area. The base of the smoke layer should be set above head height, which would be a minimum of 2m from the floor, however, the anticipated downward radiation intensity should not exceed 2.5kW.m-2, so in real terms, the base of the smoke layer may need to be set higher than 2m to compensate for this.
Realistically, these calculations correspond to a smoke layer temperature of 185oC, so it can be assumed that the smoke layer temperature will not be that high if a fire is controlled by sprinklers.
In deciding upon accurate calculations to ensure a clear layer suitable for means of escape, the temperature of the smoke and extent of the smoke layer must be taken into account, with the limit of 2.5kW.m-2 set on downward radiation.
The area of the reservoir has traditionally been limited to 2000-3000m2, to avoid disproportionate cooling and smoke mixing downwards. In reality, where a reservoir exceeds this size, it can be possible to use a computational fluid dynamics model (CFD) in order to model smoke development and buoyancy, as long as consideration has been made relating to heat loss in the surrounding area structure.
The materials used to enclose a reservoir are also important in that they must be able to tolerate the calculated smoke temperature for the required amount of time. The screens or curtains used should be as impermeable as practically possible, although some leakage at curtain junctions might be impossible to contain and for most applications, this will not prove critical.
The curtain drop, from top to bottom (depth), should be the same as the calculated depth of the smoke layer without additional length being added as overcompensation. However, the pressure of gases on the smoke curtain may cause the smoke curtain to deflect from the vertical towards the horizontal, which can cause the base of the curtain to rise outwards. This can obviously lead to a leakage of gas through the resultant gap if the movement lifts the curtain above the base of the gas layer. This should be taken into account when calculating curtain depth.
In next week’s blog we will be continuing to look at the design of fire and smoke ventilation systems to maintain a smoke free layer in the area where a fire starts. 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.