Air handlers have become specialised products, but there is really only one thing on everybody's mind; energy efficiency. Linden Shuttleworth explains the points to look out for to get the most energy efficient plant.
An important development in air handling units has been the more effective use of controllers to optimise their performance.

The typical controller is a complete digital system for most common AHU configurations, including single zone, variable air volume, multi-zone, and dual duct.
Ideal for standalone operation, it can communicate with a management system providing all point and control information to the rest of the network. It can also be configured as a generic loop controller for unique applications.

Controllers often feature a range of intelligent electrical components that make them ideal for field installation. Sensors send the temperature and humidity readings back to the controller every few seconds. Using this information, it automatically calculates the amount of air and heat needed, along with the degree of recirculation that can be permitted.

Another advance in recent years is the advent of the twin wheel. Once the reserve of all but the most expensive kit, they are now fitted as standard, by several manufacturers.

When compared to traditional cross-plate heat exchangers or runaround coils, thermal wheels are now the leading component in a trend towards heat recovery. They are particularly effective for handling large air volumes where higher efficiencies are required.

Thermal wheels have the capacity to turn a standard-efficiency air handling unit into a high-efficiency unit, with the consequent energy savings and reduced carbon emissions that implies. Thermal wheels can recover about 85 per cent of heat from ventilation air, transferring it to incoming fresh air, which then needs minimal additional heating to reach the required temperature for the building.

Given the strict legislative requirements for energy efficiency, it is hard to imagine AHUs in the future being installed without some form of integral heat recovery. Thermal wheels do the job more efficiently than most.

With more organisations choosing to refurbish their existing buildings rather than move to new premises, on-site assembly is a critical factor for many projects. Here, features such as push-fit frames and durable plastic sealing strips instead of silicon can introduce time savings on installation.

There has been a change in recent years from belt-drive centrifugal fan solutions to the direct-drive centrifugal fan (often referred to as a plug fan).

This enables the AHU footprint to be reduced and decreases maintenance (with no belts to re-tension and replace and no belt residue to clean up). There is also the likelihood of reduced downstream filtration requirements (and the ensuing opportunity to reduce the fan by one size).

An inverter also has an important part to play in maximising energy savings of AHUs.
The use of a frequency inverter provides control opportunities not available with belt-drive systems and, when the cost comparison is made (including capital cost, installation, balancing, commissioning and lifetime operating costs), the inverter-driven, direct-drive, plug-fan solution becomes very attractive.

The final 'must have' feature to maximise the effectiveness of a system is an integral cooler. In business and office spaces, lighting, apparatus and people produce a significant heat load. Supplementary cooling is therefore necessary; and cooling can be achieved very economically using an integral cooler.

Integral coolers replace other components, such as liquid coolers and external condensers and take up less space. As they are included into the air handling unit, all components are located in the plant room, avoiding outdoor or roof installation. Because the appearance of a building is not changed, renovation work is easier, and noisy condenser fans are not necessary.

• Linden Shuttleworth is UK sales manager for Flakt System Products