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Impulse ventilation systems can be used in car parks not only as an alternative to traditional ducted systems but also – if correctly specified – instead of other fire safety measures. Paul Compton examines the options.
Impulse systems use a number of jet fans – mounted under car park ceilings – to move air and smoke towards an extract point, where a mechanical extract system can remove them. This avoids the need for extensive ductwork in a traditional system, resulting in a more open environment, better CCTV coverage and lower initial and running costs.
The most common objective for these systems is simply to provide equivalent performance to the traditional systems, as defined in Approved Document B (ADB) of the Building Regulations. In this case, the requirements are simple: provide an extraction rate of six air changes per hour (ACH) for day-to-day ventilation and 10 ACH for ventilation in case of fire, using the impulse fans to avoid stagnant areas where fumes and smoke could collect. This is referred to as smoke clearance and, in case of fire, is intended only to assist fire fighters to clear the smoke. It is common practice to actually provide 12 ACH instead of 10, not for any significant improvement in performance, but for the convenient use of two-speed fans at full and at half speed.
Claims have been made that this increase of ACH, coupled with the use of impulse fans, negates the need for sprinklers in car parks. There is no justification for this. Similarly, there is no justification for the argument that sprinklers are detrimental to the performance of impulse systems and that sprinklers should be removed on this basis. If impulse fans were rendered ineffective by sprinklers, the logical conclusion would be that, in situations where sprinklers are required, an impulse system should not be considered and a traditional ductwork system installed instead. However, it is widely known that impulse systems are effective, whether or not sprinklers are installed.
Colt recently carried out demonstrations in an underground car park in Bristol, comparing the performance of a traditional ducted system complying with ADB to an equivalent impulse system. In these demonstrations, the traditional system cleared a smoke logged car park in 40-45 minutes, while the impulse system took 30-35 minutes. So the performance of an impulse system for smoke clearance is equivalent to, or a little better than, the traditional system and therefore meets the requirements of Building Regulations.
Impulse ventilation, however, is not limited to simply meeting the basic requirements of ADB. It can also be used as a smoke control system to assist fire service access, or to protect the means of escape.
Stepping up a gear
In this case, the design requirements are very different. Systems designed for smoke control are also inevitably more complex and more expensive, so there obviously needs to be an added benefit to persuade purchasers to increase their investment. The benefit comes in car parks attached to shopping centres, or to buildings covered by section 20 of the London Building Act. In these, sprinklers are required to limit the spread of fire and the use of a smoke control impulse ventilation system to assist fire service access can be offered as an alternative.
A system designed for smoke clearance ensures that the air and smoke throughout the car park are mixed, and flow generally towards the extract points. This means that the whole car park may become filled with a diluted smoke/air mixture, just as it can with traditional, natural or mechanical ventilation.
An impulse system designed to assist fire service access will keep part of the car park free of smoke. The control system detects where the fire occurs and starts only those impulse fans and extract fans that will move the smoke directly to the nearest extract point and remove it, without recirculation through the car park. This means that fire fighters can more easily locate the seat of the fire and can approach from upwind to within 10m of the fire in clear air. The resulting speed and efficiency of fire fighting may be considered to make control of the fire satisfactory, without the need for sprinklers. This approach is only practical in fairly large car parks, typically above 2000 square metres per level, and where the fire service attendance time is reasonably quick.
To achieve clear access for fire fighters wherever a fire occurs requires greater extract ventilation rates, together with sophisticated design and control of the system. In a typical large car park, this would involve:
– accurate detection of the location of the fire
– matching of zoning of fire detection zones and impulse ventilation zones
– at least two well separated extract points, each capable of extracting the full ventilation rate needed for successful smoke control
– complex control logic to operate the correct selection of impulse and extract fans, dampers, and so on
– possibly using extract fans outside the smoke zone in reverse to supply part of the inlet air supply.
The extract rate required is not based on a simple air change rate, but is designed to provide a sufficient air flow velocity across the smoke zone, to prevent back flow of smoke and recirculation outside the smoke zone. How the design extract rate compares to 10 ACH varies tremendously from car park to car park, but as a typical example on a recent project, Colt found it necessary to provide approximately 30 ACH. Since each of the two extract points required this flow, the actual installed extract fan capacity needed to be 60 ACH – six times the requirement for a simple smoke clearance system. Nevertheless, the marginal cost of the enhanced impulse ventilation system and the additional space requirements are usually an attractive trade off for the cost and space requirements of a sprinkler system.
For smoke control, validation and demonstration of the system can be a matter of some debate. For many projects, the use of computational fluid dynamics (CFD) or manual calculation may be acceptable, but there are instances where real smoke is required. Sadly, it is not often practical to set fire to a vehicle and simply releasing a few puffs of synthetic smoke demonstrates very little, apart from general air movement.
Scaling, where the fire size is reduced to a level that will minimise damage, is often used in acceptance smoke tests. Where this is used, however, not only the fire but also the impulse fan speed and the extract rate of the system must be scaled down in proportion, to provide a realistic demonstration. For example, to reduce the fire from the 4-8MW that could be expected from a real vehicle fire and which is commonly used for design, to a more practical 0.5-1MW test fire without scaling down the fan speeds and performances, would result in a flawed test.
Colt has recently carried out large-scale testing with the Building Research Establishment (BRE) in Middlesbrough, to both prove the concept of maintaining clear areas in a car park and to validate the CFD modelling approach used.
Specifiers and approving authorities need to be aware of the important differences in design and performance of smoke clearance and smoke control systems, and to ensure that the correct system is specified and installed. It is all too easy for an initial specification to slip from smoke control to smoke clearance during the design and tendering process, potentially leaving a building under-protected.
Currently, there is no published standard for impulse ventilation systems. But BSI is currently drafting BS7346-7 which covers car park ventilation, and which is expected to clarify and regularise the differences between systems. Until this is published, extra care in specification and vigilance in assessing tenders is needed to ensure that the full benefits of impulse ventilation can be safely utilised.
Paul Compton is smoke control manager at Colt Fire. For a copy of two short reports describing the Bristol and Middlesbrough test, contact Alex Howard on 023 9249 1951 or email customer.services@uk.coltgroup.com A DVD of the Bristol tests is also available.