Removing smoke and mirrors from mechanical ventilation shaft systems

In high rise properties such as apartment blocks, mechanically ventilated smoke shafts are now being more widely adopted. However, because there isn’t a single common standard for mechanical smoke shafts, there is still an air of mystery surrounding their design and application. Ross Barritt-Mehta demystifies mechanical smoke shaft design, explains how they are typically approached, and discusses solutions which have simplified the concept and streamlined installation.

n high-rise buildings, mechanical smoke shafts are becoming an increasingly popular option because they take up less space compared to natural ventilation systems. They offer a simple, effective way to help protect fire escape routes, yet many people are still left confused about their design, specification and compliance requirements.

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, 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 builders’ work duct with sheet metal ducting. An automatic opening ventilator is mounted at the top of the adjacent stairwell. The complete set-up is controlled by an addressable system that interfaces with fire alarms or smoke detectors, and automatically operates the ventilation components in the event of a fire.

Despite the simplicity of mechanical ventilation smoke shaft systems, much confusion still exists around their design and application. This is primarily because 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 document ‘Guidance on Smoke Control to Common Escape Routes in Apartment Buildings’, published in 2012, offers a comprehensive guide to smoke shafts in residential buildings. For buildings with a storey over 18 metres high, 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 systems, especially for residential buildings where one lobby is very similar to another. At Fläkt Woods we have data from dozens of models and have designed a matrix that can develop appropriate extract rates for buildings. That’s why our smoke shaft ventilation system doesn’t need project-specific CFD analysis and is ratified by LABC approval for all typical installations. However, for peace of mind, CFD Analysis can be provided as part of our package.

By modularising the design of our system, we have further demystified the concept of mechanical smoke ventilation. It is customisable to meet the individual ventilation and sizing requirements of individual buildings, and also comes in modular, preassembled parts, ready to fit into position on site, including the energy efficient, ErP compliant extract fans which are connected to the riser to extract smoke. The shaft interface ducting and controls are also mounted on a fabricated skid. 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.

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 – which is a particular 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.

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 Fläkt Woods, or by our network of approved engineers.

Smoke shafts are becoming the preferred solution for smoke control and are an ideal option where space is limited. Years of use means that there is now a lot of data for specifiers to draw on. This information has informed the design of standardised modular solutions, which means the specification and installation of mechanical smoke shaft systems are now straightforward for building owners.

For more information please go to www.flaktwoods.co.uk