Smoke shafts are simple, yet effective, ventilation systems that help maintain tenable conditions in common escape routes in the event of a fire. Mechanically ventilated smoke shafts are now a widely used solution, yet there is still an air of mystery surrounding their design and application. Simon Plummer, national fire safety manager at FläktGroup, discusses mechanical smoke-shaft design, explains how they are typically approached, and discusses solutions which have simplified the concept and streamlined installation.
Smoke 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 builders’ 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, ADB requires a 1.5m² 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 builders’ 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.
By the book
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 3, 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 considered.
The FläktGroup approach
FläktGroup is committed to demystifying mechanical smoke-shaft design by drawing on years of data, providing advice and guidance and developing new solutions.
When smoke shafts were first adopted, each situation was, in 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 to assist in designing systems, especially for residential buildings where one lobby is very similar to another. At FläktGroup we have data from dozens of models and have designed a matrix that can develop appropriate extract
rates for buildings.
And, drawing upon our extensive experience in smoke ventilation, we recently launched an innovative ‘all in one’ solution for fire safety in residential buildings, further helping to demystify the concept.
Our Smoke Shaft Vent incorporates energy-efficient, ErP-compliant extract fans, which are mounted on a skid frame and connected to the riser to extract smoke. Our Smoke Shaft Vent system is customisable to meet the individual ventilation and sizing requirements of individual buildings, and also comes in modular, preassembled parts including the fans, shaft interface ducting and controls, mounted on a fabricated skid and ready to fit into position on site. These are all assembled off site at our Colchester factory, eliminating the additional labour requirements to install the system.
The rooftop plant can be fully installed after only three 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. Project-specific CFD Analysis is not required for our solution as our database of projects covers all typical installations and is ratified by LABC Approval of the system. However, for peace-of-mind, CFD Analysis can be provided as part of the modular package.
For added efficiency, the Smoke Shaft Vent can provide on-demand daytime ventilation, extracting hot air from the corridors and stairwells within a building during summer months – helping to solve the problem in residential buildings where energy centres provide the heating and heat-distribution pipework runs through corridors. Obviously, any day-to-day ventilation functions are overridden should a fire arise.
Where to start
The installation of smoke shafts should be undertaken by a competent contractor who understands the working relationship of each installed element. Prior to handover, the commissioning process needs to be able to prove the effectiveness of the system in a variety of test operation scenarios, in accordance with agreed ‘cause and effect’. The latest standard on smoke control states that: ‘smoke control equipment should only be maintained by a competent person with specialist knowledge of smoke control systems, adequate access to spares and sufficient information regarding the system’. Installation and maintenance can be carried out by FläktGroup, or by our network of approved engineers.
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 FläktGroup are making it more straightforward for building owners to benefit from this type of technology whilst safeguarding fire-escape routes for occupants.
For more information, go to www.flaktgroup.com