Five factors for effective smoke control
Whilst jet fans can be an extremely effective smoke control tool in the event of a car park fire, there is more to designing a system than including as many fans as possible. James Allen, senior fire design and CFD engineer at Fläkt Woods, explains five key factors to consider when specifying jet fans to ensure escape routes are protected, firefighting operations are supported, and smoke spread is limited in the event of a fire.
When it comes to car park ventilation, jet fans are now a tried and tested option. Since the end of the 20th century, jet fan systems have provided an efficient solution to car parks across the world, and over time have become a must-have for cark park ventilation and fire protection.
At present, there is a heavy reliance on air change rate methods when designing ventilation systems, (10-12 air changes per hour for smoke venting is common practice.) Unfortunately, although this does assist in meeting minimum ventilation requirements, it is not sufficient when it comes to smoke control.
It is vital to ensure that a proposed jet fan system would not cause smoke to spread, and that fire fighters would be able to see a fire (in order to put it out) should a blaze ever develop, but air change rate methods alone are unable to comprehensively answer these needs.
Although there is currently no UK legal requirement for smoke control measures, the British Standard BS7346 part 7, published in 2006 by the UK Smoke Control Association, sets out three design purposes – to assist fire fighters to clear smoke from a car park, to provide smoke free access to fire fighters to a point close to the fire, and to protect means of escape. Whilst the first pertains to air changes, the latter two specifically relate to smoke control.
Significantly, section 10.1.2 of BS7346 part 7 notes that smoke control should be designed so that the extract rate is calculated for the removal of the mass of mixed air and smoke impelled towards exhaust intakes. Yet, at the moment, there is no single definitive guide available to demonstrate how to calculate air and smoke flow rates when jet fans are in operation.
Therefore, when considering a ventilation and smoke control system for an enclosed car park, there are a number of factors which must be considered in order to arrive at an effective solution. With this in mind, we’ve put together a selection of the key considerations required.
Identify air supply and smoke exhaust locations
Firstly, it’s important to ensure air supply and extractor points are not placed in immediate proximity to one another. This could cause short-circuiting, whereby air supplied is immediately extracted.
Provision of sufficient escape routes should also be a consideration. In many cases, smoke shafts are placed near stair cores for cost efficiency, but if this is an extract point it could compromise visibility for the individuals evacuating and fire service personnel entering. With this in mind, it must be ensured that sufficient stair cores are available without neighbouring extract points.
Prepare worst-case fire scenarios
Understanding the worst-case fire scenarios enables the designer to better plan which fans are specified where.
These are determined in part by the geometry of the space, which will have a considerable influence over the possible smoke mass flow and temperatures produced. However, there are several common scenarios to pay attention to.
One such is corner located fires. Fires in such positions tend to produce large quantities of smoke and heightened heat intensity as the corner walls absorb and radiate heat. What’s more, if a fire is in a corner, smoke can only travel away from the corner, which means fire fighters would likely have to approach against the smoke flow.
Another potential scenario is a fire at the furthest distance from an extraction point. This leads to increased smoke production, as there is more air to mix with. Equally, if a fire originates on a zone boundary, smoke has the potential to spread to multiple zones.
A further worst-case is a fire backed by a wall. This can lead to increased energy in the smoke compared to less confined areas. Air entrainment into the smoke plume can cause the plume to lean and attach itself to the wall. In these circumstances, the energy in the smoke layer often occupies a hemispherical shape, which can lead to higher smoke velocities.
Create an initial fan layout and zone strategy
A zone strategy should be created in accordance to guidelines provided by BS 7346 Part 7 and those of the fan manufacturer.
The maximum smoke control zone size is 2000m2 according to BS7346 Part 7, but, in practice zoning must be dictated by car park geometry and size. The smoke control zones should be governed by the location of air supply, and the critical air velocity required for controlling the smoke.
Where possible, best practice is to consider extract shafts portioned every 30-40m, the distance which generally constitutes a smoke control zone. This distance loosely corresponds to recommended travel distances as specified in Approved Document B, so for most car parks this should be a practical proposition.
Once the smoke control zones have been delineated, the initial fan layout can be worked out based on lateral and longitudinal fan positioning advice from the fan manufacturer.
The fan type and model can also be determined at this stage, taking into account the worst-case scenarios that have been identified. For example, potential risk areas such as corners, walls and partial enclosures may need fans with higher thrust compared to less restricted areas.
Ensure flow of smoke and air can be removed at extraction points
To ensure that the flow of smoke and air is effectively removed, the number of jet fans and the mass flow that they generate, plus the total maximum smoke mass flow, must be balanced with the mass flow of air and smoke extracted.
In order to determine this, a set of mass flow balancing calculations must be completed in addition to an estimation of the critical smoke velocity. Contact Fläkt Woods Ltd for a copy of its technical paper #0410 for details.
The results of these calculation sets then decides the extract rate, which must be greater than the highest of either the induced smoke and air mass flow or the critical velocity.
Use modelling methods
Although there is a perception that CFD modelling is high cost, technological advances, both commercial and open source, plus its availability on the cloud mean that the practice is more affordable than ever. Whilst the time investment may seem daunting, the long-term benefits of using CFD modelling more than compensate for the initial setup.
Unfortunately, many projects wait until a point at which the results of the model can mean difficult adjustments to an already meticulously detailed plan, which could vastly outweigh the cost of earlier modelling.
Realistically, smoke control systems should always be CFD modelled to verify smoke control calculations, and if this is done at an early stage, it can save time and money further down the line.
It is crucial that fire engineering consultants and specifiers bear smoke control in mind from the earliest possible stage when designing for a larger car park. By considering the necessary factors at this point, required ventilation rates can be reduced, simultaneously supplying safer systems and providing increased protection for fire fighters and building contents.
For more information, go to www.flaktwoods.co.uk