Residential HVAC


Clearing the air on smoke shafts


Ross Barritt-Mehta, business development manager for fire safety at Fläkt Woods, argues that despite an air of mystery surrounding their design and application, mechanically ventilated smoke shafts are actually very straightforward extract systems to specify and install for tall buildings


S Ross Barritt-Mehta


Mechanically ventilated smoke shafts are particularly suitable if space or architectural restrictions prevent the use of simpler solutions


moke shafts originated from BRE research presented in the 2002 report ‘Smoke Shafts Protecting Fire Fighting Shafts, Their Performance and Design’. Commonly known as the


BRE Shaft, this specifically looked at firefighting shafts and proposed natural ventilation, which relies on the buoyancy of hot smoke and the inlet of fresh air to extract smoke in the event of a fire. A vertical builder’s work duct that rises through the property would typically be used to extract smoke from the lobbies, with each one having a damper connected to the duct. However, BRE requires a 1.2 to 2.5m² shaft rising through the building for natural flows. So, in order to reduce the space required, mechanically ventilated smoke shafts – which can be applied with a smaller, 0.6m² shaft – have been developed. Mechanically ventilated smoke shafts are particularly suitable if space constraints or architectural restrictions prevent the use of simpler solutions, or if the owner wants to increase the building’s lettable area. In a natural shaft, the head of the shaft is terminated with an automatic opening ventilator. In comparison, mechanical smoke shafts use extract fans, which are mounted on the roof and connected to the builder’s work duct with sheet metal ducting. An automatic opening ventilator is mounted at the top of the adjacent stairwell, and the complete system is controlled by an addressable system that provides automatic operation of the ventilation system by interface with the fire alarm system or smoke detectors. Despite the popularity of mechanically ventilated smoke


shafts, they do not yet appear in the Building Regulations, and are treated as a fire safety ‘engineered solution’ (in comparison, guidance for natural smoke shafts can be found in paragraph 2.26 of Approved Document B of the Building Regulations). Since there isn’t a single common standard applying to mechanical smoke shafts, they are typically approached using the appropriate parts of several related documents. Approved Document B is applied to the stairwell ventilators, lobby ventilators, system triggering method and ventilator free area measurement; European Standard 12101 Parts 6,7,9 and 10 are referenced for fans, ducts, control equipment and power supplies, and PD 7974-6:2004 is used to identify acceptable conditions for the escape of occupants of buildings. In addition, the Smoke Control Association’s ‘Guidance on Smoke


Control to Common Escape Routes in Apartment Buildings’, published in 2015, offers a comprehensive guide to smoke shafts in residential buildings. If the floor of a building’s highest storey is 18m from ground level or higher, firefighting access also needs to be taken into account. When smoke shafts were first adopted, each situation was, in


36 July 2017


effect, a new scenario. Therefore, Computational Fluid Dynamics (CFD) was essential to ascertain the volume flow rate required to maintain the design conditions within the lobby. However, after years of common use, a bank of data exists and


can be applied consistently to ensure that appropriate extract rates are achieved, especially for residential buildings where one lobby is very similar to another. This means that project-specific CFD analysis is not required for Fläkt Woods’ standardised modular smoke shaft ventilation system, as calculations from our extensive database of projects cover all typical installations, and are ratified by LABC approval. However, we can provide CFD Analysis for peace-of-mind. For added efficiency, our system comes in modular,


preassembled parts. It is customisable to meet the individual ventilation and sizing requirements, and is delivered ready to fit into position on site, including the fans, shaft interface ducting and controls, mounted on a fabricated skid. The rooftop plant can be fully installed after only two palletised lifts. The other modular elements (controls, lobby vents and sensors) are simple to install with a plug and play operation protocol, making commissioning simple and reliable. The modular system is configured using an easy-selection tool, making our expertise accessible to specifiers, building owners and installers. Although mechanical smoke shafts have been regarded by


many as a fire engineered solution requiring complex modelling, it is time to demystify their design and application. By drawing on data from years of experience across many similar projects, and developing innovative solutions, companies like Fläkt Woods are making it more straightforward for building owners to benefit from this type of technology whilst safeguarding fire escape routes for occupants.


Mechanical smoke shaft skid


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