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For further information,
please contact:
Paul Mason
+44 (0)1206 222 668
paul.mason@flaktwoods.com
James Allen
+44 (0)1206 222 557
james.allen@flaktwoods.com
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To the untrained eye, a car park is merely somewhere to leave a vehicle and one is much like any other. To the architect, builder, engineer or approving authority, however, car parks are not all the same and each has its own specific ventilation requirements.
In England and Wales, there are three main reference documents of interest. These are available for general use to assist with the design and approval process, giving functional requirements and recommendations for best practice design:
- Building Regulations 2000 Ventilation Approved Document F.
- Building Regulations 2000 Fire Safety Approved Document B Volume 2.
- BS 7346 Part 7 (Code of practice on functional recommendations and calculation methods for smoke and heat control systems for covered car parks).
Building regulation requirements for Scotland are broadly similar, but are covered under separate Approved Documents K and E.
The guidelines within Approved Document F address the need to prevent build-up of harmful pollutants from vehicle exhausts, where a variety of different by-products are formed from the combustion of petrol, diesel or alternative fuels. Prevailing environmental conditions, particularly within inner city environments, can also impact the conditions within a car park and should ideally be considered where possible.
The main pollutants that are damaging to health are carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons and fine particles. These pollutants can be removed effectively from an enclosed car park, given the correct design approach.
Approved Document B and BS 7346 Part 7 mainly focus on the ventilation of fires within enclosed spaces, where efficient removal of heat and toxic fumes is the primary objective. A system designed to meet this requirement, without any specific need to address tenability criteria within the car park, is by definition referred to as smoke clearance or purging. This normally involves using main extract fans to provide an air change rate (typically 10 air changes per hour). The distribution of air through the space is either achieved using ductwork or a series of fans positioned close to the ceiling, such as jet fans.
The use of jet fans in car parks is now widely accepted as an alternative to traditional ducted systems, having the major advantage of controlling the direction of smoke travel, in addition to removal of heat and toxic fumes from a fire.
BS 7346 Part 7 gives recommendations for specific tenability criteria for smoke control systems including, but not limited to, recommended design fire sizes, supply air velocity criteria, smoke visibility criteria and operational recommendations. There are two types of smoke control system as described within BS 7346 Part 7. The first aims to achieve tenable conditions for the fire brigade to enter the car park, locate and tackle the fire and the second is to assist means of escape for car park occupants.
The Fläkt Woods range includes both Jet Thrust and Induction Thrust products, two fan technologies suitable for delivering a complete solution for car park ventilation. Both products require minimal ductwork, avoiding problems such as system leakage and higher system pressures sometimes found within more traditional systems.
The use of fans in place of ductwork can deliver significant savings and free up large amounts of space for other services. This can help to reduce overall building costs and increase efficiency elsewhere in the building through a reduction in the installed length of other services.
There is a physical phenomenon that makes both technologies particularly effective, differentiating them from traditional ducted systems. This is the efficient transfer of momentum from one point to another as air moves through the car park towards the extract point, or a number of extract points depending on the layout of car park and system objective. The momentum transfer is created by the high velocity air at the discharge of the fan, which varies depending on choice of product.
Figure 1 shows the typical jet profile from a 50N Jet Thrust Fan, compared to the jet profile in Figure 2 of an Induction Thrust Fan of equivalent thrust. The numerical models were developed using commercially available Computational Fluid Dynamics (CFD) software and data from in-house laboratory tests. The figures also show how air is entrained from low to high level into the high velocity jet, providing air movement at all heights within the car park.
Induction Thrust Fans are based on centrifugal fan technology and have a shallow profile, making them suitable for car parks where significant height restrictions apply. They are most suited to large car parks requiring a unidirectional system for pollution and clearance of smoke.
Figure 1: Typical velocity profile, in plan view, of 50N Fläkt Woods Jet Thrust Fan
Jet Thrust Fans are based on in-line axial fan technology, giving the additional benefit of reversibility. This is achieved through use of specially designed truly symmetrical impellers providing efficient flow through the fan in both directions. These fans offer flexibility when switching between pollution and smoke ventilation, making them ideally suited to smoke control applications.
Figure 2: Typical velocity profile, in plan view, of 50N Fläkt Woods Induction Thrust Fan
In addition to manual calculations, Fläkt Woods offers in-house Computational Fluid Dynamics (CFD). The importance of accuracy in the input data is recognised and backed by extensive research and testing. With the correct approach, CFD can be used to simulate accurately the environmental conditions within a car park and demonstrate that the ventilation objectives will be achieved prior to installation. Visualisation of variables such as air speed, velocity, pressure, temperature, visibility and smoke spread can be made.
The ability to combine the differing capabilities of Jet Thrust Fans and Induction Thrust Fans means that a complete solution for car park ventilation can be realised whatever the car park geometry.
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