VENTILATION 1 CAR PARKS
IMPULSE VENTILATION
In the last 10 years there has been a shift from the predominant use of totally ducted car park ventilation systems to impulse ventilation. Impulse ventilation takes the methods traditionally applied in tunnel ventilation (using the tunnel as the ‘duct’ with a series of high level fans to move the air through the tunnel) to direct air movement through a car park towards the extract fans. It works in conjunction with the
supply and extract points located, typically, at the perimeter of the car park. As the high velocity air moves from the outlet of the units it entrains more air by the Bernoulli effect – this system does not use an array of individual extract grilles to remove the air from the space. Individual variable speed
fan units are suspended from the ceiling of the car park and may be controlled separately to provide localised zoned dilution of contaminants by introducing variable rates of air. The units were based initially on axial flow fans in tubular housings, and these have now been joined by low profile centrifugal fan units that can be mounted in the space between structural members in car parks. In the event of a fire the
impulse ventilation can be used with smoke and heat detectors to accurately pinpoint the source of the fire and then, working as a selective network of fans, direct the smoke most effectively towards the extract position to provide the best opportunity for escape routes and to allow firefighters to access the fire. Apart from physical modelling the only practicable method of predicting the pattern of flow within the car park is through the use of flow modelling software.
Working in conjunction with the supply and extract systems, the direction and flow rate of air may be automatically controlled by impulse systems to provide the most effective removal of smoke from the car park
for example at exits and ramps, the local ventilation rate should be increased to at least 10 air changes per hour. Alternative ventilation strategies can be
used, so long as they are proved to limit the concentration of carbon monoxide to not more than 30ppm averaged over an eight-hour period, and peak concentrations, such as by ramps and exits, to not more than 90ppm for periods not exceeding 15 minutes. The proving of
should not be any major challenges in diluting the vehicle exhaust to maintain appropriate carbon monoxide levels. To both maintain carbon monoxide levels
alternative designs is frequently undertaken by computational fluid dynamics (CFD) analysis. These same restricting carbon monoxide limits are reflected in the requirements recommended as meeting the ventilation needs of the building regulations across the UK. If a mechanical system has been designed to meet the requirements to satisfy the demands of the fire regulations (nominally 10 air changes per hour) there
54 CIBSE Journal May 2011
The use of various design tools can help to meet the whole- life needs of car park ventilation
and minimise operational costs, a naturally ventilated system is the ideal solution. This does not automatically mean the concrete monstrosities that have blighted the skylines of many city centres. The imposing architecture of the Manchester Transport Interchange development uses offset glass panels to obscure the sight of the cars while maintaining adequate
free area so that it benefits from natural ventilation as well as being protected from the weather. In many car parks (subterranean or
enclosed) there is no opportunity to achieve wholly natural ventilation. In such cases mechanical ventilation systems will have traditionally operated at a fixed ventilation rate (typically three to six air changes per
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Flakt Woods
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