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Ensuring best practice in air distribution

Increasingly stringent energy efficiency regulation, including the beefed-up Part L, is stimulating the design of more airtight buildings to minimise heat losses.

At the same time, engineers are increasingly adopting low-energy technologies, such as mixed flow or displacement ventilation, that depend on good control of the direction and speed of the air when it is introduced into the occupied space.

This 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.

The design team might create detailed models of the airflow patterns to be delivered by their planned ventilation system. But the actual performance of the system, once it is handed over, may be radically different to the design intent if the specification of the ATDs is compromised in any way.

How the airflow is treated in the plenum box behind a diffuser can be critical to the conditions experienced by people and equipment in the occupied space.

If the terminal device is not properly designed, it can lead to uncontrolled air velocities and 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 if air speed is not well controlled, it will create noise - this is often down to poor plenum box design.

The air speed 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; and 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 four-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 only slightly cooler air being gradually mixed with the existing air 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.

David Fitzpatrick is sales and marketing director of Ruskin Air Management