The LWF Blog
Fire Engineering Design and Risk Assessment – Choice of Smoke Control System – Part 17May 10, 2017 2:17 pm
In this Fire Engineering Design and Risk Assessment blog series for Architects and others in the building design industry, we have been looking at the use of smoke control systems and concept designs. In Part 17, we’re going to talk about meeting design objectives and how the final result will differ depending upon the requirements.
A smoke control system where the design objective is to limit the upper temperature of the smoke but with no requirement for means of escape provision is one where there will not be a need for the smoke to be diluted or for a smoke-free layer to be maintained in order for persons evacuating the building to be able to breathe. The provision of natural or mechanical smoke extraction may be sufficient with requirements such as cost, reliability or aesthetics being met too.
A natural ventilation system used to limit the maximum temperature must be designed and maintained so that they fail safe in the open position, to allow smoke to escape the area. With a mechanical ventilation system, a back-up power source might be a necessary consideration to afford the system additional reliability, although such an improvement would increase the costs involved.
In cases where such a system is being designed for a building which already has an air conditioning system or mechanical ventilation system, it may be that this can be adapted or added to, in order to achieve some or all of the design aims.
Looking now at those systems which are designed to maintain a clear layer below the smoke, to facilitate a means of escape in that compartment. A natural smoke control system is most appropriate for those scenarios where the quantity of smoke is considerable and the smoke is still sufficiently buoyant and there are no adverse wind effects.
In cases where there is the potential for positive wind pressure, or where a cooler non-buoyant smoke condition is important, it might be more useful to use a mechanical system.
When considering a very large and high space, such as an atrium or a mall, it is common to see very high rates of air entrainment into spill plumes, causing increases in mass flow as the plume height increases. This indicates that there may be a point at which a smoke control system becomes economically unviable.
There may be situations where a smoke control system is not required at all. If an area is sufficiently large and fire sizes are relatively small, it could be likely that the space would remain in a tenable state until the Fire Service were able to attend and put out the fire. In some cases, a dilution system is most appropriate.
In Part 18 of this series, we will look in more detail at the use of Dilution Systems and how external wind conditions can affect a ventilation 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 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.