Terminal failure
The commoditisation of the market for air terminal devices (ATDs) has had a damaging impact on the performance of an alarming number of otherwise well designed air-conditioning and ventilation systems.
As with many parts of our industry during these financially pressurised times, the unit cost of grilles and diffusers has been forced down and down. Yet there has never been a greater need for ensuring the fine details of a system are right. If corners are cut during the final stage of an installation just to save a few pounds the long-term consequences can be extremely expensive.
End clients, regulators and planning officials are looking for vastly improved energy efficiency from their systems, leading to more insulation and increasingly airtight buildings. The challenge is to make sure that does not lead to poor indoor air quality, which has led to an unprecedented level of innovation in air-conditioning and ventilation design. Building-services engineers have to find a balance between the apparently conflicting demands of Part L of the Building Regulations (which governs energy use), Part F (which calls for high fresh air rates) and Part E (which legislates against disruptive levels of noise from the ventilation system).
Achilles heel
There is a tendency, however, to view ATDs as one area where savings can be made — yet they can be the Achilles heel of the whole design.
More engineers will consider a mixed flow or displacement ventilation approach to reduce energy consumption. With either strategy, there is a clear need to control both the direction and speed of the air when it is introduced into the occupied space.
The mixed approach requires a higher induction rate to ensure effective mixing of the supply air with the existing air in the space, while the displacement or laminar strategy calls for even patterns of relatively slow air to be gradually introduced into the area.
Consulting engineers will spend a lot of expensive design time modelling airflow patterns from their ventilation system, but the performance of the system in reality is totally dependent on the correct specification of the ATDs.
How the airflow is treated in the plenum box behind a diffuser can be absolutely critical to the conditions experienced by building occupants. If the terminal device is not properly designed, the result can be air velocities that are uncontrolled, which can lead to cold air ‘dumping’. Similarly, if there is turbulence behind the diffuser, you will not achieve the distribution patterns intended for the room.
Poorly designed and positioned devices will also end up forcing the air-movement system to work harder in an attempt to improve comfort conditions, so driving up energy costs. And, air speed that is not well controlled will cause unwanted noise, which is often down to poor plenum box design. The speed of the air flowing across the spigot of an air-distribution device should never exceed the velocity of the air as it enters the occupied space.
For a mixed-flow approach, there are a number of options open to the specifier — including sidewall located square or rectangular grilles and nozzles, sidewall mounted linear grilles, as well as ceiling or wall-mounted circular, rectangular or swirl diffusers. The choice is critical because this approach depends on the gradual dilution of stale air by the addition and accurate mixing of fresh air via induction. A properly designed 4-way diffuser able to carry out this task is a complex piece of engineering and should be manufactured from high quality materials. Yet there are products on the marketplace available for less than £20. This is not an area for compromise as it can unbalance the whole air distribution strategy.
The laminar approach requires cool air to be cascaded into the space, while displacement depends on air that is only slightly cooler than the air in the room being gradually mixed with it from a lower level via convection. If the momentum of the air is too great, occupants will experience dramatic temperature differences between ankle and head height. To avoid draughts, air speed should be between 0.13 and 0.18 m/s.
Undermining
Poor positioning of grilles and diffusers is another problem. A 4-way diffuser placed close to a wall will, for obvious reasons, not perform well because it will not be able to diffuse in all four directions. Similarly, if the airflow is not properly managed in the plenum box the air may not emerge from the whole of the grille, so undermining the overall design strategy.
How air moves within occupied areas should be taken into account as part of the lifecycle costing of a project. Full CPD modelling of the area will show optimal airflows, but the designer cannot be certain that they are delivering that result if low-quality air-terminal devices are used.
The industry struggles to gain access to buildings after handover to find out exactly what conditions the occupants are experiencing once they have moved in. Yet it is more important than ever — probably via facilities managers— to establish whether the low-energy strategies we are applying are actually delivering the conditions predicted. If a post-occupancy study does reveal poor air distribution, it might not be your overall strategy, but your ATDs that are the problem.
David Fitzpatrick is sales and marketing director of Ruskin Air Management.
