How Biddle Air Systems developed its quieter fan-coil unit

FCU
Less noise by design — Biddle Air Systems has tackled the noise issues of fan-coil units at source. The split intakes have baffles to turn the air through 90° and act as noise attenuators.
Noisy fan-coil units can be the bane of an air-conditioning system, and it is not always easy to reduce that noise. Now Biddle Air Systems has tackled the problem from first principles.One of the biggest concerns about fan-coil units installed in ceiling voids, which is where the vast majority are, is noise. A designer aiming to achieve, for example, NR35 in the room below typically tackles the problem by specifying a larger unit than is necessary to achieve the cooling/heating duty and running the fans at a lower speed, leading to reduced air velocity and lower noise levels. Not content with this simple, but, no doubt, effective approach to achieving lower noise levels, Biddle Air Systems, which targets its range of fan-coil units at the specifier market, sought to gain a greater understanding of the nature of noise generation in fan-coil units. Engineering manager Mike Price was assigned the task. He started with the generally accepted knowledge that much of the noise heard in a room is generated at the air intake and travels through the coil, fan chamber and diffuser. Though the coil provides some attenuation, the effect is very small. SRL In seeking to acquire a greater understanding of the problem Mike Price was a regular visit to Sound Research Laboratories in Suffolk for six months. Andrew Saxon, Biddle’s marketing manager, recalls the day he came back from SRL believing he had the solution to reducing noise levels generated at the air intake of a fan-coil unit. He had been working with liquid flow in pipes and discovered that when a straight run branches off into two tees, the noise from the feeder pipe is, like the water, shared between each tee — a reduction of 3 dB in noise. That information led Mike Price to think that the airflow through a fan-coil unit should be encouraged to change direction — a perception that was indicated as reasonable by a simulation project. Conventional fan-coil units have a single, large air intake opposite the discharge. Most of the noise generated at the intake passes straight through the fan-coil unit. Mike Price’s solution was to have two air intakes, one on each corner of the back of the fan-coil unit and the discharge on the opposite end. Air from the ceiling plenum is directed into the fan-coil unit by guide vanes on each inlet, turns through 90° to be drawn through the coil by fans on the discharge side of the coil and is then discharged into the room. These guide vanes also act as noise baffles. Dramatic Andrew Saxon describes the effect on noise levels as ‘dramatic’. Noise measurements in the space using standard SRL methodology showed a 3 dB(A) reduction compared with a conventional fan-coil unit. That represents a halving of noise levels. Achieving those lower noise levels has a slight penalty in length and width, but because units are inherently quieter, it could well not be necessary to specify a larger unit to reduce noise levels. Andrew Saxon suggests that being able to specify smaller units more than offsets the price penalty incurred by having to specify a larger size of a unit that is inherently noisier. Testing To give confidence to specifiers, Biddle has invested over £25 000 in comprehensive noise testing at SRL. The company has also patented the design of the air inlets in both the UK and Europe. These fan-coil units have not been submitted for Eurovent approval because the test requires an air temperature onto the coil at is about 4 K higher than the UK norm. of 23 to 24°C. Having achieved a quieter fan-coil unit, Biddle has incorporated a host of other features to enhance product quality and simplify maintenance. The coil takes a lesson from those used in air-handling units in being of counterflow design rather than crossflow. The coil is mounted vertically, and a more uniform temperature is achieved — improving heat transfer by up to 20%. A counterflow coil is not left- or right-handed, which has significant installation implications. Biddle has stayed with waterside control, despite misgivings from some consultants. To provide a wide range of fan speeds, a transformer with 36 tappings is provided, from which three main speeds can be selected, each with three fine adjustments. Speed groups are selected by two rocker switches. Good-quality panel filters to EU3 standard with cardboard frames are used. For ease of maintenance, two or three small filters are used, rather than a single large one. The benefit is that filters can be easily changed by removing just one ceiling tile. Quarter-turn captive fasteners provide easy access to the filters. The high-quality ABS plastic drain tray will not corrode, and it can be steam cleaned. A stainless-steel drain tray is offered for those consultants that consider this a better option. Improved Fan decks have been improved, following discussions with fan manufacturers. More bracketing is used, a higher-temperature bearing lubricant provides longer life, and motors are quieter and have ventilation slots to keep down their temperature. This new range of fan-coil units is called Powerbreez. It has been the subject of a ‘sshhh...keep you cool’ advertising campaign since the start of this year — though not in Modern Building Services since our first issue was published in April. However, we are pleased to be the among the first to bring the news to the industry. From the viewpoint of consultants and Biddle, the high specification of the Powerbreez range makes spec-busting more difficult — a fact that Biddle sees as crucial to growing sales of fan-coil units made in its Nuneaton factory in the next few years. sales@biddle-air.co.uk
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