ENERGY SAVING EQUIPMENT
Diffusing the conditions
Mike Gosling, product technical manager–grilles, diffusers and louvres at Trox UK, looks at the effective selection of diffusers to optimise comfort conditions and energy efficiency.
W
hile all diffusers are capable of delivering conditioned air into the space, installing the wrong design can lead to draughts, ‘dumping’ of cold air, or stagnation/inadequate circulation. Each of these problems is sure to result in complaints from uncomfortable room occupants. In addition, incorrect diffuser selection can be an obstacle to energy efficiency. With so many different air terminal devices on offer, however, it can be difficult to identify the most effective diffuser for the job. This article reviews the strengths and weaknesses of the most widely encountered diffuser designs to provide practical guidelines for more effective air terminal device selection. The requirements for comfort conditions are defined in BS EN ISO 7730. To achieve these criteria it is necessary to control air velocity, turbulence intensity and temperature differential, whilst taking into account the varying requirements of different parts of the building. This is where the wide range of diffuser types comes into play. Some important questions to ask when specifying diffusers might include: ■ Does the application involve high induction rates and/or a high number of air changes?
■ How far does the air need to travel to reach the occupied zone?
■ Does conditioned air need to be directed across the ceiling space before entering the occupied zone?
Swirl diffusers
Although available in various shapes and sizes, all swirl diffusers share a similar core design. A series of linear slots –
32 August 2016
aligned radially around the centre of the diffuser face– divide the supply air into a number of high velocity jets. The increase in supply jet surface area and the cyclonic pattern of air discharge – which forces the air jets to travel in a rotating manner around the diffuser centre – enables the diffuser to induce large quantities of room air. This rapidly dissipates the velocity and temperature differentials between the jet and the ambient air, minimising the distance at which high velocities and/or draughts exist. In some designs, the discharge slots are subdivided along their length to further enhance the induction rate.
Swirl diffusers are therefore ideal for high air change applications – up to 30ACH – and for delivering large temperature differential air at high volumes. They can also be used for extracting air. Typical applications might include commercial offices, retail spaces, restaurants, atriums and so on. They are fine for normal or low ceiling heights. However, if you are specifying for ceiling heights above 3.8m, or for exposed or free-hanging ceilings, then an alternative diffuser design will suit you better.
Slot diffusers
Slot diffusers can be selected or set to provide horizontal diffusion in one or two directions across a flat ceiling surface utilising ceiling effect. They feature elongated outlets consisting of single or multiple slots with an aspect ratio >25:1. As the supply jet entrains room air, it expands in the vertical plane.
When selecting a slot diffuser, it is important to take the ceiling height into consideration, as this must be sufficient
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