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Variability is key to effective air distribution

Variable air volume fan coil systems have a big impact on effective room air movement, as Andrew Sargent explains
Buildings have always had variable heating and cooling requirements according to the climatic conditions and occupation of the building. Since this is a constantly changing situation, there has always been the need for variability. With AC motors variable speed was generally a complicated and expensive arrangement, and sometimes difficult to achieve. Now, with the advent of EC motor technology, we have seen most drives, e.g. pumps, use EC motors which have the benefit of increased efficiency and variable speed is much easier to achieve.

Until recently EC motors were not available in the sizes required for fan coils but that situation changed a few years ago with the introduction of small ec motors from Europe and the USA. Combining this with the pressure to reduce carbon emissions meant that ec motors were utilised and promoted by fan coil manufacturers since the specific fan power could be reduced, typically from .8 to .35 w/l/s.

Energy savings made
It was also found that by varying the air volume rather than have the fan coil running at constant speed, irrespective of the climatic and occupational parameters, significant energy savings could be made. With VAV fan coils, varying the air volume from 100 to 60% has been recommended by most suppliers and, at 60 per cent, the power drawn by the motor is .6³ which equals .216 or 21.6%, i.e. nearly an 80 per cent reduction in power.

By using variable air volume on fan coils, the energy savings can be significant, with specific fan powers as low as .15W/l/s.

So what about the impact on room air movement and air distribution? Virtually all grilles and diffusers are fixed area devices and are selected to operate at their maximum efficiency at 100 per cent air volumes. As the air flow is reduced, there comes a point where the air can detach from the ceiling and create a down flow which can cause draughts and complaints. This is termed 'dumping'. It can be avoided with good air distribution. If selected correctly, most diffusers can operate down to around 60 per cent of rated air flow before there is any risk of dumping.

One significant danger is that, once the air does detach from the ceiling, there is what is known as the hysteresis effect. This means that the air volume needs to be increased significantly to destroy the dumping air distribution patterns and get the air to re-attach to the ceiling.

Consultants recognise that the air distribution in VAV systems is a very delicate balance and the trend has been to make the VAV fan coil supplier also responsible for the air distribution. Because of this, most projects have a mock-up test where the air distribution at the reduced air volumes can be fully assessed and measured.

Recognised test facilities crucial
The picture, right, shows the R & D facility where we conduct room air tests and mock-ups. It is imperative that proper and recognised test facilities are used to conduct these tests since the consequences of poor air distribution within the room can lead to significant complaints about the whole air conditioning system.

The diffuser mounted in the ceiling is generally the only air conditioning component seen and, compared to the rest of the system, is a relatively low cost item. If treated as a commodity product and procured purely on cost grounds to fill a gap in the ceiling then this can lead to poor room air movement since it has not been designed as an integral part of the fan coil system.

So this low cost item has a very important role to play in making sure that high quality room air conditions exist. As mentioned earlier, with the increasing popularity of variable air volume ec fan coil units it is essential there is a single point of responsibility for both the fan coil and diffuser. In this way, the performance of the whole fan coil system can be underwritten and guaranteed by the specialist manufacturer.

// The author is general manager of Advanced Air //
16 July 2012


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