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AHUs move into the sustainability spotlight

Air handling units are helping to pave the way to outstanding BREEAM ratings, according to Amanda Sayers
The air handling unit (AHU) has always been the workhorse of the HVAC world. Whilst renewable energy devices and sophisticated control technology scooped awards for reducing carbon footprint of commercial buildings, the AHU kept working away behind the scenes. But in today's environment, where only outstanding levels of sustainability are acceptable for the UK's most prestigious projects, this has all changed.

The latest generation air handling unit is a very different animal. In the move towards significantly reduced energy consumption, the AHU is firmly in the front line, delivering energy savings that were simply not available to system designers just a year ago.

This is due to the fact that the AHU has recently been reinvented from the ground up. After a number of years of incremental improvements, the tweaking and tinkering stopped and every aspect of the design and construction of the AHU was approached with a clean sheet. So what impact has this had? Four key areas have seen significant advances:

· Motor efficiency
· Filter design/performance
· Physical construction
· Single source compatibility

The benefits are perhaps best illustrated by analysing a recent major project.

Five Pancras Square, a key part of the King's Cross Central Development, is a 13-storey, 22,560sq m building which will become the new Camden Council public building. It is the first of its kind to combine a swimming pool, leisure centre, fitness gym, studios, library, customer access centre, café and office accommodation.

The building will boast a number of environmental features including exposed thermal mass, optimisation of daylight factors across the occupied floor plate, solar shading angled to help minimise direct solar gain and the ability to benefit from night purging to naturally cool the building. These passive measures, in conjunction with an energy efficient ventilation system and an innovative modular central cooling system, adaptive control methodologies and connection to the King's Cross Central low carbon district heating network, will result in overall carbon emissions 50 per cent lower than the 2010 Building Regulation target.

To achieve the high efficiency targets, the teams focused their design on new technologies and innovation, selecting products new to the UK market, to suit buildability and better the original energy targets. Main items identified were chillers and chilled water auxiliary equipment, air handling units and exhaust fans, lighting control and luminaires selection, power distribution, metering and sub-section pre-wired and pre-tested off site.

The air system solution for the building, engineered by TROX, includes air handling units, fans (general extract and smoke), VAV & CAV, fan coil units and air terminals. There are 18 AHUs on the project from the recently-launched TROX X-CUBE range. A number of the latest approaches to AHU design can be seen contributing to the sustainability ambitions for the site.

The low specific fan powers have been achieved in TROX X-CUBE AHUs for Five Pancras Square with the plug fans with an IE2 motor, and IE4 permanent magnet motors which ensure high efficiency even under partial loads. The aerodynamically profiled steel or plastic fan blades achieve the highest efficiency with the lowest noise level.

The AHUs also incorporate EUROVENT energy class A minipleat filters which reduce energy consumption considerably. Air filters are, of course, a necessary and compulsory requirement in air conditioning systems, however, the inevitable trade-off when employing particle or particulate filtration is increasing system resistance.

It is estimated that 85 per cent of the costs produced by an air filter are energy costs accrued during operation, but recent developments in filter technology are improving efficiency.

One of the 19 certification programmes run by EUROVENT provides a scheme for independent testing and grading of filters. TROX is one of fourteen manufacturers taking part in the scheme, and the filters incorporated in the X-CUBE design perform at the highest level designated by the standard. These components contribute to the AHU's ability to exceed SFP targets for the unit whilst ensuring effective filtration.

Energy efficiency performance is also assisted by the construction of the AHU cabinet. A seamless foam seal on each panel ensures the low leakage class is maintained, and this type of construction also removes the internal framing which in turn drops the resistance through the unit, further reducing the SFP of the unit. The panels and frames are also fixed using screws rather than permanent fixings, enabling unit sections to be dismantled and reassembled on site should greater access be required.

Prefabrication was high on the agenda from the outset of the project, with both the designer and construction partner identifying this intention from the start. As TROX supplied the full air handling system, opportunities for prefabrication could be optimised, with improved compatibility of equipment throughout the system, for more effective operation and energy efficiency.

The envisaged post-construction BREEAM score for Five Pancras Square is actually expected to be higher than the design-stage score of 93.05 per cent. The team is working towards a revised target of 96% which would make it the most sustainable building in the UK as rated by the BREEAM process. Five Pancras Square was also recently awarded first place in the Other Buildings category of the BREEAM Awards 2013.

// The author is commercial manager of TROX UK //
10 September 2013

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