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Ventilation in the balance

The need to balance indoor air quality with energy consumption provides a strong case for using quiet, decentralised air handling units as an alternative to traditional ductwork systems. Lars Fabricius explains
When it comes to maintaining indoor air quality (IAQ), specifiers need to strike the right balance between a number of variables. These include meeting minimum ventilation requirements while responding to local requirements, minimising energy consumption and avoiding intrusive noise
from the system.

In a commercial environment, of course, the traditional approach to ventilation uses centralised air handling plant distributing air through a system of ductwork and grilles.

Flexible decentralised units
However, I would suggest there are now strong reasons for considering decentralised air handling units (AHUs) as a more flexible, energy efficient and responsive solution. There are a number of reasons for this, which are explained in this article.

First, it's important to address the concern that in bringing the fans closer to the people in the space, noise becomes an issue. This has certainly been the case with some local ventilation systems but modern designs of decentralised AHUs now deliver noise levels as low as 35dB at 1m at 100 per cent power, and as low as 30dB at 1m at 90 per cent power. So as well as starting at a low noise level at full power, and given that the decibel scale is logarithmic, this means that a relatively small reduction in power to reduce energy consumption will deliver a very significant noise reduction.

Clearly, in the case of traditional centralised systems noise should not be a problem either. However, the energy that is wasted in overcoming the air resistance of the system - along with heat losses from the distribution system before that heat can be recovered - is a major issue.

With decentralised AHUs, the unit is mounted in the space that is being ventilated - at high wall or floor level - and uses a direct connection to the outside air through an external wall or roof. This means that air resistance is minimal and there is no wasted fan power. The system incorporates a counter-flow heat exchanger to recover up to 85 per cent of the heat from the extract air, while EC fan technology helps to reduce energy consumption.

In eliminating the need for complex and extensive ductwork systems, a decentralised approach also reduces capital and installation costs. Also, as there is no penetration of fire compartments, it is not necessary to install fire dampers or intumescent seals, as would be the case with a traditional ductwork system.

Decentralisation also makes it considerably easier - and more viable - to retrofit an effective ventilation system to an existing building.

Demand controlled ventilation
Decentralised AHUs can be linked to carbon dioxide, occupancy or humidity sensors to provide demand controlled ventilation in response to the most appropriate IAQ metrics in each space. Furthermore, decentralisation helps to avoid the wasted energy associated with over-ventilation.

This is because a traditional ductwork system is typically designed to allow for the maximum usage of the spaces, irrespective of whether there are spaces that do not need ventilation.

Clearly, it is also easier to extend the system at a later stage, simply by introducing additional units. In contrast, extending a traditional ductwork system is disruptive and expensive, and may require upgrade of the central plant as well as to the distribution system.

'Plug and play' cooling units can also be introduced to provide extra cooling in areas that require it - either at the time of installation or at a later date should heat loads increase.

However, decentralised plant does not mean decentralised control, as multiple units can be monitored and controlled from a central PC.

As with any ventilation system, it's important to get the design right and with decentralised AHUs it is possible to design very precisely for each individual space.

The first consideration is whether to use high wall mounted or floor mounted units and this will be dictated by the nature of the space. For example, if people are to be close to the unit, high mounting will help to avoid uncomfortable draughts.

High wall units can be mounted flush to the ceiling or partly recessed and in both cases they provide an even distribution of air across the ceiling, exploiting the Coanda effect, so that the air falls gently into the entire space. The units should be positioned to avoid obstacles on the ceiling, such as light fittings or ceiling beams, and if these can't be avoided a floor mounted system should be considered.

Floor-mounted AHUs can be installed against the wall or within the room and configured to provide mixing with room air or to provide displacement ventilation.

A major benefit of decentralised AHUs is that they provide the inherent flexibility to address all design considerations and tailor the ventilation to each space. In this way, the system is properly balanced with the needs of the people in the space - and at the end of the day that's what building services should be doing.

//The author is managing director of SAV Systems //

25 May 2012

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