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Air Conditioning World


Dodging the draft


Mike Gosling, product technical manager, grilles, diffusers and louvres, Trox UK has some tips on specifying the right diffuser for the right application


zones, or stuffiness due to inadequate circulation. However, with so many different diffuser products on the market, it can be difficult to decide which of the many options will perform best in specific applications. There are three main types of diffuser


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for mixed air applications: swirl, slot and jet designs. Each has distinct air flow characteristics. To select the best model for the application you need to consider the number of air changes, whether the air needs to be directed across the ceiling space before entering the occupied zone, and the distance the air needs to travel to reach the occupied zone.


High induction/ high air change rates


In high air change applications (up to 30 ACH), swirl diffusers are often the most appropriate selection as they are ideal for delivering large temperature differential air at high volumes. Featuring a series of linear slots (aligned radially around the centre of the diffuser face), swirl diffusers operate by dividing the supply air into a number of high velocity jets. See Figure 1. The increased surface area for the supply


jets, and the way in which air jets are forced 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 differential between the jet and the ambient air, minimising the distance at which high velocities and/or draughts exist. Typical applications might include


commercial offices, retail spaces, restaurants 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.


18 September 2017


pecifying the wrong air terminal device can lead to problems such as ‘dumping’ of cold air, draughty


Diffusion across the ceiling surface


In applications requiring horizontal diffusion of air, in one or two directions across a flat ceiling surface (utilising ceiling effect), slot diffusers are typically the best solution. These 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, see Figure 2. Ceiling height is an important


consideration when specifying slot diffusers, as this must be sufficient to prevent the conditioned air from entering the occupied zone prematurely. As an approximate guide, the discharge jet expands away from the ceiling at approximately 200 – 250mm per metre throw length. Therefore, for a 4.5m throw, the ceiling height should be a minimum of 2.8m to ensure the discharge jet remains outside of the occupied zone. Slot diffusers are suitable around the perimeters of commercial buildings/ offices to direct cooled air against glazing to offset solar gain. As the air direction of the slot diffuser


can be altered on site, without recommissioning, they are suitable where space is frequently reconfigured, for example, due to office churn. There are two main types of deflection devices in slot diffusers: blade and barrel. These can be specified to suit the application, with barrel-style models recommended for greater induction potential.


Large air volumes and long throws


Finally, jet diffusers are designed specifically for handling large air volumes and long throws,


Fig 1. Trox’s TJN jet nozzle


making them suit buildings with high ceilings, and with large distances between the diffusers and the occupied zone. Jet diffusers are relatively low induction


devices which maintain velocity and throw lengths of between 5m and 30m. When the temperature differential between the supply air and room air changes, the air stream is deflected upwards, (for cold air) downwards (for warm air) or horizontally (isothermal), see Figure 3. As the direction of the air stream can be affected by other factors, such as local convection effects or draughts within the room, jet nozzles are designed so that they can be adjusted to meet requirements for both heating and cooling.


XARTO swirl diffuser from Trox


Frontplates for Trox XARTO swirl


diffusers


www.heatingandventilating.net


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