As part of the regeneration of the city, a mixed use retail and leisure centre in Leicester is undergoing an extension and redevelopment. Conor Logan examines the smoke control strategy being deployed.
The first smoke control system of its type in the UK is being installed at The Shires shopping centre in Leicester, which is undergoing refurbishment as part of the council’s strategy to regenerate the city. Designed as a seamless extension of the existing site, the mixed use development will more than double the amount of retail space. The development will incorporate Leicester’s first multiplex cinema and contemporary city-style accommodation, with open streets and squares centred around a retail core, and a cafe and restaurant quarter.
The development has employed a wide range of technologies, including smoke control and day-to-day ventilation, firefighting shafts, and mechanical and natural smoke extraction. Colt is providing an innovative smoke control system for the car parks and connecting tunnels, with the design proven by CFD modelling and adhering to recently published guidance.
A key factor in Colt’s involvement in this project was its adherence to standards set out in the HEVAC Guide, CFD Modelling for Car Park Ventilation Systems, which supports BS 7346-7 Components for Smoke and Heat Control Systems – Part 7: Code of practice on functional recommendations and calculation methods for smoke and heat control systems for covered car parks.
Longitudinal approach
A road tunnel – approximately 125m long with the road sloping down towards a central horizontal section – has been built to provide an exit route from the car park with traffic flowing in one direction and passing under the main retail development. This setup, however, prevents the use of a central extract system along the length of the tunnel. Due to the one-way movement of vehicles, there is an increased risk that an incident in the tunnel would block the flow of traffic. This means that those trapped behind it would most probably have to evacuate on foot, while those ahead of it could, with control of the traffic management system, continue to drive out of the tunnel. A ‘longitudinal’ approach was therefore chosen, using Colt Cyclone fans to oppose smoke flowing upstream and direct it towards the tunnel exit, preventing it from affecting the evacuation of the occupants whose vehicles are trapped. The tunnel smoke control system is designed to take account of the following conditions:
– Initially one, and then two burning cars with a peak heat output of 8MW in accordance with BS 7346 Part 7 for unsprinklered premises
– An opposing wind speed of 8m/s, taken from the CIBSE Guide J as the maximum wind speed, which is not exceeded 95% of the time, in the Midlands area of the UK.
The tunnel resistance to take into account is a length of 125m, a typical area of 19.2m2, and a greatly varying cross sectional area with sharp changes in profile, causing additional tunnel wall friction.
Design calculations were carried out to establish the required air velocity necessary to overcome the momentum of the heat and smoke from the fire, keeping smoke moving in one direction only and preventing ‘backlayering’. A safety factor was then added to make an allowance for fan redundancy, resulting in a total thrust requirement for the tunnel, which was met using Colt Cyclone induction fans. These are available in 100N and 50N sizes and for this project a combination of both were used – sixteen in total. The 50N unit, which is only 250mm deep, was particularly useful at the bottom section of the tunnel, where the headroom was severely limited.
In ‘fire’ mode the ventilation system is designed to move smoke in the same direction as the traffic flow. This means that vehicles downstream of the fire can continue driving, in association with synchronised traffic control signals, and leave the tunnel. Occupants of vehicles trapped upstream of the fire are protected from the smoke so that they can be evacuated safely on foot as necessary. Firefighters attending the scene can approach the fire in relatively smoke-free conditions within approximately 10m of the fire location in the tunnel, as required by BS 7346 Part 7.
CFD results
Once positioning of the fans was decided on, CFD modelling of the structure was carried out to verify the validity of the ventilation system. Each model was run with the opposing wind speed of 8m/s. A box was created around the portals to allow the external wind pressure to be applied.
The first modelling exercise was carried out with a 4MW fire in the central section and all Cyclone fans operating correctly. Figure 2 is a view at the exit portal showing that the thrust from the fans is able to overcome the pressure of the opposing wind, resulting in a net airflow out of the tunnel. Figure 3 shows the visibility around the fire – downstream, conditions are smoke logged as expected, but upstream they are considerably better with the smoke being less dense and controlled within 10m of the fire. The model was then re-run to examine the effect of a bigger fire representing a larger vehicle, such as a people carrier, or a fire spreading from the first car to an adjacent vehicle. The fire was modelled using a t2 fire growing at a fast rate, peaking at a maximum of 8MW at 413 seconds from the start of the fire. All fans were running and the opposing wind was again modelled.
The sequence of images making up Figure 4 shows the spread of smoke from the start of the fire at intervals of 60, 90 and 120 seconds. It takes between 90 and 120 seconds before the downstream section becomes smoke logged. The final figure shows the smoke after 628.5 seconds, when the fire has reached an 8MW steady condition but with the system dealing very well with the increased fire load.
The results of the CFD modelling show that, in the event of a fire in the tunnel, the system is capable of holding the smoke to within 10m of the fire position while overcoming:
– an opposing 8m/s wind profile
– the radial jet smoke profile from an 8MW fire
– the resistance of the tunnel surface
– steps in the tunnel section.
Vehicle emissions control
In normal operating conditions, the ventilation system will be controlled by carbon monoxide (CO) sensors mounted in the tunnel. When CO levels are minimal the fans will run at low speed. If levels increase the fan speed will adjust accordingly.
In an adjacent service yard, Colt was able to reduce the number of fans originally proposed with a smaller number of more powerful Cyclone fans, to provide a nominal 6 ACH (fume) and 10 ACH (smoke) air change rate, pushing fumes and smoke towards a mechanical extract system.
The new mall and the individual stores are protected by a variety of means. Firefighting access points have the benefit of two BRE natural shaft systems and one Colt mechanical firefighting shaft , which was chosen since a traditional BRE shaft – with its requirements of a 3m2 area – would have exceeded the available space. The Colt shaft uses mechanical ventilation to extract smoke and requires a shaft area of only 0.6m2. In the main mall natural ventilators have been incorporated into the facade, hidden behind a louvre screen. Over in the existing mall area the original smoke extract system was upgraded to one using both powered and natural ventilation. In the apartments adjoining the shopping centre, smoke clearance for common corridors and staircases was provided via natural smoke shafts and window actuators.
The use of CFD modelling is becoming much more common, particularly for proving complex designs and also for uncovering any design snags, preventing problems in construction and reducing risk for building occupants. As with any computer programme or model, however, the validity of the results is only as good as the quality of the programme, its user, and the input data. Colt wholly supports the intentions of the new HEVAC Guide for CFD modelling in car parks and hopes that it will raise the standard of computer modelling within the smoke control market, helping to eliminate poor, inadequate or even misleading modelling.
Conor Logan, BSc (Hons) AMIMechE is principal design engineer for Colt International.
As part of the regeneration of the city, a mixed use retail and leisure centre in Leicester is undergoing an extension and redevelopment. Conor Logan examines the smoke control strategy being deployed.
The first smoke control system of its type in the UK is being installed at The Shires shopping centre in Leicester, which is undergoing refurbishment as part of the council’s strategy to regenerate the city. Designed as a seamless extension of the existing site, the mixed use development will more than double the amount of retail space. The development will incorporate Leicester’s first multiplex cinema and contemporary city-style accommodation, with open streets and squares centred around a retail core, and a cafe and restaurant quarter.
The development has employed a wide range of technologies, including smoke control and day-to-day ventilation, firefighting shafts, and mechanical and natural smoke extraction. Colt is providing an innovative smoke control system for the car parks and connecting tunnels, with the design proven by CFD modelling and adhering to recently published guidance.
A key factor in Colt’s involvement in this project was its adherence to standards set out in the HEVAC Guide, CFD Modelling for Car Park Ventilation Systems, which supports BS 7346-7 Components for Smoke and Heat Control Systems – Part 7: Code of practice on functional recommendations and calculation methods for smoke and heat control systems for covered car parks.
Longitudinal approach
A road tunnel – approximately 125m long with the road sloping down towards a central horizontal section – has been built to provide an exit route from the car park with traffic flowing in one direction and passing under the main retail development. This setup, however, prevents the use of a central extract system along the length of the tunnel. Due to the one-way movement of vehicles, there is an increased risk that an incident in the tunnel would block the flow of traffic. This means that those trapped behind it would most probably have to evacuate on foot, while those ahead of it could, with control of the traffic management system, continue to drive out of the tunnel. A ‘longitudinal’ approach was therefore chosen, using Colt Cyclone fans to oppose smoke flowing upstream and direct it towards the tunnel exit, preventing it from affecting the evacuation of the occupants whose vehicles are trapped. The tunnel smoke control system is designed to take account of the following conditions:
• Initially one, and then two burning cars with a peak heat output of 8MW in accordance with BS 7346 Part 7 for unsprinklered premises
• An opposing wind speed of 8m/s, taken from the CIBSE Guide J as the maximum wind speed, which is not exceeded 95% of the time, in the Midlands area of the UK.
The tunnel resistance to take into account is a length of 125m, a typical area of 19.2m2, and a greatly varying cross sectional area with sharp changes in profile, causing additional tunnel wall friction.
Design calculations were carried out to establish the required air velocity necessary to overcome the momentum of the heat and smoke from the fire, keeping smoke moving in one direction only and preventing ‘backlayering’. A safety factor was then added to make an allowance for fan redundancy, resulting in a total thrust requirement for the tunnel, which was met using Colt Cyclone induction fans. These are available in 100N and 50N sizes and for this project a combination of both were used – sixteen in total. The 50N unit, which is only 250mm deep, was particularly useful at the bottom section of the tunnel, where the headroom was severely limited.
In ‘fire’ mode the ventilation system is designed to move smoke in the same direction as the traffic flow. This means that vehicles downstream of the fire can continue driving, in association with synchronised traffic control signals, and leave the tunnel. Occupants of vehicles trapped upstream of the fire are protected from the smoke so that they can be evacuated safely on foot as necessary. Firefighters attending the scene can approach the fire in relatively smoke-free conditions within approximately 10m of the fire location in the tunnel, as required by BS 7346 Part 7.
CFD results
Once positioning of the fans was decided on, CFD modelling of the structure was carried out to verify the validity of the ventilation system. Each model was run with the opposing wind speed of 8m/s. A box was created around the portals to allow the external wind pressure to be applied.
The first modelling exercise was carried out with a 4MW fire in the central section and all Cyclone fans operating correctly. Figure 2 is a view at the exit portal showing that the thrust from the fans is able to overcome the pressure of the opposing wind, resulting in a net airflow out of the tunnel. Figure 3 shows the visibility around the fire – downstream, conditions are smoke logged as expected, but upstream they are considerably better with the smoke being less dense and controlled within 10m of the fire. The model was then re-run to examine the effect of a bigger fire representing a larger vehicle, such as a people carrier, or a fire spreading from the first car to an adjacent vehicle. The fire was modelled using a t2 fire growing at a fast rate, peaking at a maximum of 8MW at 413 seconds from the start of the fire. All fans were running and the opposing wind was again modelled.
The sequence of images making up Figure 4 shows the spread of smoke from the start of the fire at intervals of 60, 90 and 120 seconds. It takes between 90 and 120 seconds before the downstream section becomes smoke logged. The final figure shows the smoke after 628.5 seconds, when the fire has reached an 8MW steady condition but with the system dealing very well with the increased fire load.
The results of the CFD modelling show that, in the event of a fire in the tunnel, the system is capable of holding the smoke to within 10m of the fire position while overcoming:
• an opposing 8m/s wind profile
• the radial jet smoke profile from an 8MW fire
• the resistance of the tunnel surface
• steps in the tunnel section.
Vehicle emissions control
In normal operating conditions, the ventilation system will be controlled by carbon monoxide (CO) sensors mounted in the tunnel. When CO levels are minimal the fans will run at low speed. If levels increase the fan speed will adjust accordingly.
In an adjacent service yard, Colt was able to reduce the number of fans originally proposed with a smaller number of more powerful Cyclone fans, to provide a nominal 6 ACH (fume) and 10 ACH (smoke) air change rate, pushing fumes and smoke towards a mechanical extract system.
The new mall and the individual stores are protected by a variety of means. Firefighting access points have the benefit of two BRE natural shaft systems and one Colt mechanical firefighting shaft , which was chosen since a traditional BRE shaft – with its requirements of a 3m2 area – would have exceeded the available space. The Colt shaft uses mechanical ventilation to extract smoke and requires a shaft area of only 0.6m2. In the main mall natural ventilators have been incorporated into the facade, hidden behind a louvre screen. Over in the existing mall area the original smoke extract system was upgraded to one using both powered and natural ventilation. In the apartments adjoining the shopping centre, smoke clearance for common corridors and staircases was provided via natural smoke shafts and window actuators.
The use of CFD modelling is becoming much more common, particularly for proving complex designs and also for uncovering any design snags, preventing problems in construction and reducing risk for building occupants. As with any computer programme or model, however, the validity of the results is only as good as the quality of the programme, its user, and the input data. Colt wholly supports the intentions of the new HEVAC Guide for CFD modelling in car parks and hopes that it will raise the standard of computer modelling within the smoke control market, helping to eliminate poor, inadequate or even misleading modelling.
Conor Logan, BSc (Hons) AMIMechE is principal design engineer for Colt International.
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