Delivering value engineering for fan-coil units

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4000 fan-coil units for Trox’s largest order to date were made and delivered over a period of 14 weeks. Full-scale testing of the units with associated plenum boxes, flexible ducting and grilles and diffusers enabled the performance to be guaranteed.
TERRY FARTHING explains how applying a system approach to designing and selecting fan-coil units and their associated equipment automatically leads to delivering value engineering.The key to achieving value engineering for air-conditioning systems based on fan-coil units is not to put together components from separate manufacturers but to apply system engineering. That is how Terry Farthing, sales director with Trox UK, encapsulates the approach that his company brought to this sector of the market in August 2004. The Trox approach involves selecting and designing fan-coil units, plenum boxes, diffusers and the short runs of flexible ducting that make up a system so that they work together effectively and efficiently. Terry Farthing asserts quite simply, ‘System engineering and value engineering are the same thing.’ Instead of applying rules of thumb to selecting components so that they work together to deliver the performance required, the Trox approach is based on testing complete assemblies in full-scale test chambers to prove performance. ‘With system engineering,’ explains Terry Farthing, ‘the professional team is dealing with the complete system and can be certain about the performance of each component, so they don’t need to over-engineer. This saves significant costs.’ Developing such systems and subjecting them to full-scale instrumented tests is the key to confidence on site. ‘Ideally,’ says Terry Farthing, ‘solutions developed for every project should be tested individually, which is why our approach is best suited to installations comprising large numbers of fan-coil units.’ The potential for system engineering is demonstrated by a project comprising fan-coil units in the ceiling, with air discharged through slot diffusers. For aesthetic reasons, the architect favoured the slots in the diffusers being as narrow as possible and as few as possible. The inevitable consequence of just a few narrow slots is a high pressure drop, which requires more fan power and which is also noisy. When the initial-concept layout was mocked up in a test room, the pressure drop from the outlet of the fan-coil unit to the discharge from the diffusers was found to be as high as 60 to 70 Pa. The general rule-of-thumb specified by consulting engineers is 30 to 50 Pa. Indeed, the system may well not have satisfied the requirements of the current Part L of the Building Regulations requiring fan power not to exceed 0.8 W/ls. Terry Farthing explains that by testing various configurations, the pressure drop was reduced by well over half to just 25 Pa. That feat was achieved by increasing the size of the slots in the diffusers (but keeping the number the same), increasing the size of the spigot on the plenum box and the flexible ducting to the diffusers. This project involved over 460 fan-coil units. Fans with variable-speed drives had already been specified, so the benefit of the lower pressure drop could be exploited immediately to halve the energy bill — a saving of some £25 000 a year. Delivering the impossible System engineering and prototype testing enabled the seemingly impossible to be delivered for another project, the new BBC HQ at Pacific Quay in Glasgow, incorporating edit suites. The noise specification was NR25, with an even lower NR20 in some areas. Even as the building was being constructed, the design team had started designing a fan-coil system based on a conventional approach — but rapidly came to the conclusion that a radically different approach was needed. With the space available already predetermined, Trox set about developing a solution. The simplistic approach of reducing noise by using more attenuation would simply have increased the pressure drop, leading to higher noise levels — in an ever-increasing spiral. The engineering approach was to develop a fan-coil unit based on an existing design of VAV box, with heating and cooling coils added. A series of testing and modification over seven to 10 days finally achieved NR25, with the diffuser in place. Subsequent design and testing has now enabled Trox to comply with NR20 requirements as well. Terry Farthing explains that this prototype was developed as a speculative investment — and that Trox won the order. Being able to assess performance using the four test chambers that Trox already has enables design safety margins to be tightened up with confidence. Actual pressure losses can be measured and the effect of supply-air temperature and diffuser type on room-air velocity assessed. Terry Farthing states that a relaxation of the NR level from NR35 to, for example, NR38 could offer cost savings without compromising overall comfort. Being able to tighten up a design in this way, while still guaranteeing performance is said to be capable of reducing capital costs by up to 15%. Trox (UK) is at Caxton Way, Thetford, Norfolk IP24 3SQ.
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