The LWF Blog
Fire Engineering Design and Risk Assessment – Choice of Smoke Control System – Part 18May 18, 2017 12:32 pm
In this Fire Engineering Design and Risk Assessment blog series for architects and others in building design we have been looking at ventilation systems and most recently in Part 17, the maintenance of a clear layer of breathable air beneath the smoke. In Part 18, we will discuss Dilution Systems and their applications before moving on to talk about Wind Overpressures.
There are three main applications for fire and smoke ventilation systems. Temperature controlled systems work to maintain smoke temperature at a level below that which will break the glass and to prevent fire ‘flashover’. A clear layer system will work to extend travel distances or to reduce exit capacity for means of escape. A dilution system helps to extend travel distances in situations where the area is large in comparision to the size of the fire. This is commonplace in a tall atrium, for example.
Dilution Systems helps keep the situation tenable for longer and so is also helpful in demonstrating that a fire in the early stages is smaller, cooler and less hazardous than the hotter smoke of a fire that has grown.
Where a dilution system is designed in conjunction with a natural smoke control system, it is unlikely to be affected by wind overpressures as long as air change rates are sufficient to cause adequate dilution of the smoke.
Natural ventilation has the potential to be affected by external wind conditions. If a ventilator is positioned in such a way that it is exposed to external wind pressures, in a fire situation, the wind could use the vent to blow inside and the pressure could be greater than that of a cooler smoke layer. The result might be that the vent which is designed to extract smoke instead works as an inlet. For this reason, the placement of the ventilator is important, with the ideal being that it is placed in an area of negative wind pressure. This can result in a suction force which will actually assist in extracting smoke from the area.
The impact of wind upon a ventilation system are not just isolated to localised effects either. The wind pressures around the entire building will dictate the flow patterns of smoke inside the building and so impact upon the effectiveness of the extraction procedure. Tall buildings, for example, or those buildings with taller parts can be subject to a positive wind pressure on the upstream section of the lower roof. On large roofs, variations in wind pressures may be substantial and result in adverse inflows, even where the pressures are negative. In addition, large negative pressures at dominant low-level openings can cause a resultant inflow through roof vents in spite of suction at that location.
In Part 19, we will continue to look at the impact of wind overpressures on ventilation systems. . 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.