Smoke shafts are now the most commonly employed smoke control measure for high rise buildings such as hotels, offices and apartment blocks, being more widely adopted than automatic opening vents and pressurisation systems.
The term ‘smoke shaft’ is commonly used to describe quite simple ventilation systems installed in the lobbies of tall buildings to maintain tenable conditions in the escape routes that would usually be used in the event of a fire. A vertical builders’ work duct that rises through the building would typically be used to extract smoke from the lobbies, with each one having a damper connected to the builders’ work duct.
Smoke shafts originated from BRE research presented in the 2002 report Smoke Shafts Protecting Fire Fighting Shafts, Their Performance and Design. This specifically looked at firefighting shafts and proposed natural ventilation (commonly known as the BRE Shaft) – which relies on the buoyancy of hot smoke and the inlet of fresh air to extract smoke in the event of a fire.
Mechanical smoke shafts
However, in order to reduce the space required (BRE require a 1.2 to 2.5m² shaft rising through the building for natural flows, where-as a mechanical smoke extract shaft can be applied with a 0.6m² shaft), mechanically ventilated shafts have been developed and accepted to provide both firefighting access and means of escape protection. They are ideal for regular multi-storey buildings up to 20 storeys in height, and are particularly suitable if space constraints prevent the use of simpler solutions.
However, whereas guidance for natural smoke shafts can be found in the Building Regulations (paragraph 2.26 of the Approved Document B), mechanical smoke shafts do not yet appear in the regulations, and are treated as a fire safety engineered solution. So, although they are a very common solution and are actually very simple extract systems, there is still an air of mystery surrounding their design and application.
With no single common standard applying to mechanical smoke shafts, they are typically approached using the appropriate parts of several related documents; Approved Document B of the Building Regulations 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.
In a natural shaft, the head of the shaft is terminated with an automatic opening ventilator. Mechanical shafts use extract fans, 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.
For buildings with a storey over 18 metres high, firefighting access would also need to be taken into account. This would usually mean that the system is designed to cope with the door to the fire room being open to the lobby, representing firefighting conditions. In practise this simply means a higher extract volume flow rate for mechanical systems. Typically the required conditions within the lobby would be based on the tenability criteria in PD7974 Part 6:
- Visibility (5m for small enclosure and 10 m for large enclosure – extended travel distance would require a 10m visibility)
- Temperature (smoke temperature is less than 120oC – some say 60oC in a moist environment)
There is also a requirement by London Fire Brigade that the lobby/corridor returns to a smoke-free environment within two minutes of the last occupant’s escape through the stair before the onset of firefighting. BS 7974 recommends design fire sizes for a range of applications.
Design, installation and maintenance
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. More complex systems – for example those using twin shafts with reversible fans – would require the services of a suitable qualified fire engineer and a specialist smoke contractor.
The automatic opening ventilator above the stairwell is used to provide replacement air for the smoke shaft. There is a risk of lobby depressurisation when using mechanical extract in confined spaces like residential buildings, which could make it difficult to open exit doors from the lobby. Common methods to overcome this have been pressure sensing fan control, or reverse hanging the stair/lobby door.
A provisional design can be created in a matter of minutes by selecting modular components; builders’ work shaft, lobby vent, roof extract unit (duty and standby fans) and a control system including automatic changeover between the fans when necessary
The installation of smoke shafts should be undertaken by a competent contractor who understands the working relationship of each installed element of the shaft system. 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 Regulatory Reform (Fire Safety) Order 2005 (RRO) dictates that a building’s ‘responsible person’ (generally the building owner, manager or FM) has to ensure proper operational service and maintenance of smoke control systems. Smoke shafts are obviously life-critical aspects of a building’s operation, so correct service and maintenance is vital. 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’.
Smoke Shaft Vent
Drawing upon our extensive experience in smoke shaft ventilation, Fläkt Woods has recently launched an innovative ‘all in one’ solution for fire safety in residential buildings.
The Smoke Shaft Vent incorporates high efficiency extract fans, which are mounted on a roof and connected to the ducting to extract smoke. An automatic opening ventilator is mounted at the top of the stairwell and at each lobby leading to the smoke riser, with a control system that provides automatic operation of the ventilation by interface with the fire alarm system or smoke detectors.
Our Smoke Shaft Vent system is not only custom-designed to meet the individual ventilation and sizing requirements for each building, but also comes in preassembled parts, ready to fit into position on site, including the fans, shaft interface ducting and controls mounted on a fabricated skid assembly off site from our Colchester factory – eliminating the additional labour requirements to install the system.
The rooftop plant can be fully installed after only three palletised lifts for fan assembly, riser duct assembly and the head-of-stair vent. For even further ease, the system comes with a pre-programmed HMI, which is also configurable on site.
All of the custom designed systems are configured using an easy-selection tool, making our expertise accessible to specifiers, building owners and installers.
Importantly, our mechanical Smoke Shaft Vent system not only provides a simple-to-install fire engineered solution using best practice from the latest UK Fire Safety Standards, but it also incorporates extract fans selected to meet ErP criteria, ensuring energy efficient operation.
For added efficiency, the Smoke Shaft Vent can provide on-demand daytime ventilation, extracting hot air from the corridors and stairwells within the building during summer months.
Heat build-up in corridors, particularly in residential buildings with energy centres providing the heating and heat distribution pipework running through corridors, can be problematic, and a side benefit of smoke control systems is that they can be used to dissipate some of this heat, though any day-to-day ventilation functions must obviously be overridden in emergency conditions should a fire arise.
There are various approaches available – from running the smoke fans at low speed and opening the smoke lobby dampers proportionately to adding smaller environmental fans and dedicated dampers above the ceiling – but they are limited by the outside air temperature, so the reduction of corridor temperature cannot always be guaranteed.
Adding a temperature control function (as with our Smoke Shaft Vent) can ensure the ventilators are opened in a predetermined sequence to evacuate heat when temperatures become excessive. With our Smoke Shaft Vent the smoke fans are inverter controlled and run at low speed to deliver a notional air change rate within the lobby – typically four air changes. An automatic rain sensing control prevents the stairwell ventilator opening in poor conditions.
Smoke shafts have become the preferred solution for smoke control in high rise buildings, with mechanical options being ideal where space is limited.
For more information, go to www.flaktwoods.co.uk
