Air movement & ventilation Distribution
Specifiers should opt for quality rather than lower cost for air-terminal devices.
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This usually means calling upon a single company
to supply every part of a ventilation system – typically, either chilled beams or fan coil units, plenum boxes, filters, grilles, louvres and diffusers. In this way, the specifier can benefit from a single source of supply and an integrated system that helps mitigate potential design problems (which could include high room noise levels, poor air distribution and less than satisfactory thermal performance). It is argued that, with a single point of responsibility,
The architect
starts designing his building and by the time we get to our equipment we have one hand tied behind our
backs – Ian Thomas
58
CIBSE Journal May 2010
a system can be guaranteed by the manufacturer, and risk and worry can be removed from that area of the project. But systems engineering also offers another advantage – it enables the designer to find and limit unnecessary costs and avoid over-engineering the system. Some industry insiders have estimated that this ‘value engineering’ approach can lead to 10 to 15 per cent cost savings. Value engineering makes even more sense when
variable air volume (VAV) fan coils are employed that require modulating air flow rates. In this case, it is essential that the diffuser will not only operate at peak design condition, but also at reduced air supply volumes. With VAV, as the cooling requirement is reduced,
the fan speed is varied to reduce the air volume. When varying the air volume, it is important that the velocity isn’t so low that the air ‘dumps’ (in other words, rather than skidding across the ceiling, the air falls towards the room occupants, thus creating draughts). Dumping occurs when cool air does not have enough energy to run along the ceiling and there is a significant
risk of it happening if the air velocity falls below 60 per cent of the maximum. Once dumping starts in a room, the air volume must be boosted significantly to get the air back up onto the ceiling and away from the occupants. In other words, considerable energy must be expended to influence the undesirable room air pattern. That is why it is important to examine the diffuser selection to support VAV fan coils and, preferably, use a device that prevents the diffuser from dumping. So the emphasis is on good selection. Says Thomas: ‘You can pick up a pair of diffusers and
decide they are the same, but the months of testing and proving and acoustic analysis are impossible for people on the outside to see and it is that information that will make the difference between that item working and failing to work in a system.’ l
How to defeat the diffusion confusion with HEVAC
The Air Distribution Group of the HEVAC Association has published a guide to air
distribution called HEVAC General Specification and Product Directory for Air Distribution and Related
Equipment (August 2004). It consists of a ‘General Specification and
Members Product Directory’ covering the full range of products in some detail and can be downloaded free at: www.feta.co.uk/hevac/downloads/
airdistgenspecandpd-aug04.pdf
www.cibsejournal.com
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