AIR CONDITIONING, COOLING & VENTILATION
Exposed ceilings: new solutions to air management challenges
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The increasingly popular architectural trend of utilising exposed ceilings has proved a challenge for those tasked with designing the HVAC system. While it is true that the lack of a flat ceiling surface makes air management more challenging, an intelligently designed system can rise to the challenge and create comfortable spaces without sacrificing the vision of the interior designer. Tim Tanner, product technical manager – ventilation technology, TROX UK explains
o what needs to be taken into account when designing for an open ceiling room? There are two key aspects that must be
considered early in the process - effective air management and acoustic performance. In spaces with suspended ceilings, the Coanda effect prevents ‘dumping’ of cold air into the room. Due to the low-pressure differential between the jet and the ceiling, basic aerodynamic principles kick in and supply air is, in effect, attached to the ceiling upon discharge. The resulting effect is a descent into the room at a manageable velocity, which means air of the desired temperature is delivered into the zone without a feeling of gusts or draughts. See Figures 1 and 2. Without the flat and continuous
surface of a suspended ceiling, however, the Coanda effect is no longer utilisable, leading to near vertical discharge of air into the space, resulting in high temperature differential, draughts and a less comfortable experience for the occupants. As well as the problems associated with cooling, the absence of a ceiling surface can also cause problems when heating a space. Warm supply air will tend to stratify rather than serving the occupied zone, making the heating of a space less effective and energy-efficient.
So how can these issues be overcome?
It is possible, of course, to consider slot diffusers for exposed ceiling applications. These are not ideal for use in the middle of a room, but they could still be utilised in certain places with the correct conditions, such as on the perimeter, or when discharging air at a 45° angle. When discharging at an angle, slot diffusers have to be installed at a higher level than standard slot diffusers, however, and have a lower flowrate capacity to ensure comfortable conditions within the space.
However, there are a number of diffusers such as those designed by TROX specifically for exposed installation, that can provide a more effective alternative. The Tellus swirl diffuser is one option. It works differently to most diffusers on the market, as it has a gap between the diffuser face and the plenum which allows for the air to discharge horizontally and form a ‘Coanda jet’. The air flowing through the swirl pattern face connects to this jet, increasing the throw distance. For high rooms, Tellus can be provided with a centre punched face for vertical air discharge. Another option is the RFD swirl diffuser. This has an increased diameter discharge nozzle (Dx Face), which ensures the diffuser works well in an exposed application without requiring the high discharge velocities and pressures.
Air management for heating and cooling In terms of heating, when there is not a false ceiling the warm air is free to stratify. This means the
space above the diffusers will be heated before the occupied zone, wasting energy, slowing the rate temperature change, and possibly creating discomfort (due to high temperature gradients between head and ankle).
The answer to this, and other
problems, is the use of variable geometry diffusers. The RFD-V, for example, is designed with a thermal actuator and an internal mechanism to manage air more effectively. An adjustable internal sleeve enables the unit to move between horizontal and vertical air discharge, depending on whether the system needs to cool or warm the space. During cooling the air discharge is horizontal and radial. As the supply temperature increases the discharge pattern will switch automatically to vertical, directing warmer air more effectively into the occupied space. Once the supply temperature decreases, the air discharge switches back to horizontal once more. These changes to air discharge are controlled by an integral thermal actuator, within the diffuser which, in turn, controls the internal sleeve. The supply air to room air temperature difference may range from -10 to +15K. By facilitating the ability to move automatically between cooling and heating, a variable geometry diffuser enables the comfort conditions for the occupied space to be managed much more effectively.
Considering acoustics
With the correct approach, with appropriate attention paid to equipment choices and placement, high levels of comfort are now available for these challenging spaces. When designing the system, however, it is also important to consider the acoustic variables. Suspended ceilings are typically constructed from metal tiles incorporating acoustic materials that can absorb sound, minimising noise
8 BUILDING SERVICES & ENVIRONMENTAL ENGINEER FEBRUARY 2024
and reducing the reverberation time within the occupied space. Without these sound-absorbing materials in the vicinity of building services, ambient noise can be expected to increase. In addition, the hard surfaces of an exposed ceiling accentuate echo effects, amplifying sound generated by room occupants. This, in turn, can cause occupants to speak louder, to be heard over the ambient noise, leading to an escalation of the problem. To assess the acoustic requirements for a room with exposed services, a direct noise contribution therefore needs to be calculated. This includes assessment for:
• % leaving outlet • Distance to listener • Directivity • Reverberation time
Be it for a retrofit or a new build, designers need not fear HVAC problems stemming from exposed ceilings, as long as the diffuser selected for the application is capable of overcoming the specific air management and acoustic challenges that these present.
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