Blog: Lifting the lid on mechanical smoke shaft design

Although it’s been fifteen years since research was first published on the performance and design of smoke shafts, a lot of mystery still surrounds how mechanically ventilated systems should be devised. Ross Barritt-Mehta, Business Development Manager for Fire Safety at Fläkt Woods, argues why this no longer needs to be the case.

Smoke shafts originated from BRE research presented in the 2002 report ‘Smoke Shafts Protecting Firefighting 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 riser through the property would typically be used to exhaust smoke from the lobbies, connected by a fire damper.

However, BRE requires a 1.5 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 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 riser with sheet metal ducting. An automatic opening ventilator is mounted at the top of the adjacent stairwell, and the complete system interfaces with the fire alarm system or local smoke detectors, so that it operates automatically in the event of a fire.

In high-rise buildings, mechanical smoke shafts are becoming an increasingly popular option because they take up less space compared to natural ventilation systems. Despite this, many people are still left confused about their design, specification and compliance requirements. This is primarily because mechanical smoke shafts 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 measurements; 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 2012, 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 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 these systems, especially for residential buildings where one lobby is very similar to another. In fact, project-specific CFD Analysis is not required for Fläkt Woods’ smoke shaft vent system. Because its design draws on information from a large database of projects covering all typical installations, it is ratified by LABC Approval. However, for peace-of-mind, CFD Analysis can be provided.

Because our smoke shaft vent system comes in a modular design, it can be customised to meet individual ventilation and sizing requirements. We preassemble the different components including the energy efficient, ErP compliant extract fans, shaft interface ducting and controls at our Colchester factory. These are all delivered onto site mounted on a fabricated skid, ready to fit into position, eliminating the additional labour requirements to install the system.

Mechanically ventilated smoke shafts are increasingly being used in high-rise properties such as apartment and office blocks. Years of data is now available to inform the design of modular solutions, making it easier for building owners and consultants to specify and install a suitable system that will protect common escape routes and maintain tenable conditions for firefighting access during a fire.

Ross Barritt-Mehta is business development manager for Fire Safety at Fläkt Woods