The challenge of over-exposure
Tim Tanner, Product Technical Manager – Ventilation Technology at TROX UK, discusses how air management challenges with exposed ceilings can be overcome.
Exposed ceilings are proving increasingly popular with today’s architects and interior designers, but the cool industrial vibe can prove decidedly uncomfortable for room occupants.
The absence of a ceiling surface creates inevitable challenges in relation to air movement and acoustics, which can lead to unwelcome noise levels, uneven temperatures and draughts.
The problems relate to the way in which air reacts when introduced into the space at ceiling level. In spaces with suspended ceilings, Coanda effect (the tendency of a fluid jet to stay attached to a convex surface) prevents ‘dumping’ of cold air into the room. A typical air distribution approach would see the supply air “attach” to the ceiling because of low-pressure differential between the jet and the ceiling.
This keeps the cooler and denser supply air higher for longer. As it travels, the jet of air expands and mixes with the warmer room air, reducing the temperature differential and the velocity, so when it hits the wall or a jet travelling from the opposite direction and descends into the occupied zone, it is more comfortable for the room occupant.
Standard ceiling diffusers are only designed to work with an adjacent ceiling surface. If the ceiling system is removed, for an exposed ceiling, almost all standard diffusers will be unable to function correctly. The loss of Coanda effect/ceiling attachment will result in near vertical discharge of cooler air into the space, therefore insufficient time for air to mix.
Even with a high discharge velocity and pressure, diffusers may still dump without a ceiling to provide Coanda effect. If the distance from the air discharge to the occupant is not sufficient, the air will not mix, resulting in excessive velocity (high air speed), excessive temperature differential (large difference between room and supply air) and a high risk of draughts.
This would make it difficult to achieve the required comfort conditions. ISO7730 states a with DR of more than 20% is necessary for a CAT B environment.
This need for higher discharge velocities in exposed installations greatly effects the turndown that can be achieved by fan coils and the associated air terminal devices. It is also important to consider the effect of the supply temperature when selecting exposed diffusers where heating is required. Without a ceiling, warm supply air will stratify rather than serving the occupied zone. Overcoming this problem requires low temperature differentials or, preferably, variable geometry diffusers.
The requirement for an exposed diffuser to have a higher discharge velocity has an impact on the possible turndown. When installed within a suspended ceiling, RFD diffusers can turn down to a discharge velocity of 2.0m/s.
Without a suspended ceiling acting as a Coanda surface, the minimum discharge velocity for a RFD is 3.5m/s. If we take a RFD/250 selected at 15Pa, this will give 74l/s, with a ceiling the diffuser can turndown to 30l/s or 41%. When installed exposed the turndown is restricted to 53l/s or 72%. While, this has clear disadvantages, there is the upside of being able to oversize VAV which can mitigate some of the acoustic issues.
Acoustics
The absence of a suspended ceiling, sub-dividing the services zone from the room occupants, has a significant impact on noise levels. Suspended ceilings are typically constructed from metal tiles incorporating acoustic materials that can absorb sound, minimising noise 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 the percentage leaving the outlet, distance to listener, directivity and reverberation time.
Removal of a suspended ceiling can, therefore, cause the noise level of HVAC to increase. For a FCU, this effect can lead to around an 8NR increase to the noise level.
As well as fan coil units, the acoustic effects of other HVAC elements, normally installed behind a false ceiling, need to be considered. This can include common extracts (bell mouths), CAV/VAV and any equipment further upstream that now has a direct sound path to occupants (AHUs).
Air distribution solutions for exposed ceilings
A traditional solution offered to HVAC system designers has been the addition of an extended face plate to diffusers adding a Coanda surface. While this will offer some improvement, air distribution may still be far more ideal.
A more effective approach is to employ the latest generation of swirl diffusers designed with exposed ceilings in mind. These swirl models are designed with 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, models can be provided with a centre punched face for vertical air discharge.
Solutions for heating and cooling
In a room without 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).
Until recently, this had remained a problem for HVAC designers, but the introduction of variable geometry diffusers in recent months has brought an effective solution. They can be 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.
Until recently, bringing the aesthetic impact of exposed ceilings to a building’s interior involved the creation of air movement challenges for those designing ventilation, cooling and heating systems. With the latest generation of air terminal devices, the architect’s interior design intentions can be achieved without compromising the comfort conditions for building occupants.
