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Re-inventing the fan coil

Fan coils have a proven track record for reliability and flexibility. Despite this longevity, Andrew Saxon of Biddle Air Systems believes they still have a key role to play in the drive towards energy efficiency
WITH the implementation of the revised Part L of the Building Regulations the onus is very much on the building services industry to reduce energy consumption.

The new Parts L2A and L2B require significant savings over the current Part L. But increased insulation and sophisticated control and monitoring equipment come at a price. These costs will have to be met by the client, who will demand a rapid return on investment through savings in running costs.

Fan coils, acknowledged as a cost-effective, flexible and reliable means of air conditioning offices, are the choice of many. Variable speed fans draw unconditioned air over a cooling/heating coil to vary the volume of air passing across the coil. Fan speed is controlled by taking tappings directly off the internal windings of the motor or, more commonly, by connecting to a transformer.

The unit is installed to operate at a specific air volume (or fan speed) and noise level. In reality this air volume is not always required, and so the fan coil will be less efficient than it need be. Controls more in tune with the demands of the building are being implemented in many cases with building management systems able to provide ever-closer control.

Measuring efficiency

Parts L2A and L2B state fan coil units must not have a specific fan power exceeding 0.8W/ls-1.

Specific fan power (SFP) is a measure efficiency, being the amount of energy consumed in moving each litre-per-second of air. The rating weighted average is calculated by adding the product of the power supplied (Pmains) and the specific fan power (SFPfcu) for each fan coil unit in the installation, and dividing by the sum of the power supplied (Pmains) for all the fan coil units in the installation. This can be expressed as in Table 2.

It is worth noting that, because the majority of fan coil units in this installation have low specific fan powers, the SFP (rating weighted average) of this installation is below 0.8W/ls-1, despite there being one small fan coil unit with a high individual specific fan power.

A typical fan coil uses either one 2-pole AC motor to drive two or three fans on a fan deck, or a number of 2-pole AC fan motors each driving an individual external rotor motor (pod) fan. These are not energy-efficient having a specific fan power of 0.9W/ls-1 or higher.

More enlightened manufacturers, while still using AC fans because of economic constraints, recognise that 4-pole AC motors are much more efficient, operating between 0.5W/ls-1 and 0.75W/ls-1.

Much work is afoot to develop more efficient motors. DC motors are more efficient but perceived as unaffordable. A less expensive alternative being investigated is electronically commutated (EC) motors, which use permanent magnets and convert AC to DC power. Claims that these motors use up to 70% less energy than typical AC motors are being made.

Evaluating the capital and running costs of these alternatives will surely become common. An example can be seen in Table 1.

While it is vital that new energy-efficient fan motor technologies are developed, it must not be assumed that they will always provide the most cost-effective solution. In many cases, existing technologies may be the most affordable way of complying with the regulations.

I would suggest that, before air conditioning system designers and their clients get too caught up in the myths about fan coils being inefficient and the need for new technologies, their attention should be drawn to 4-pole AC fan motors. These have already been used by manufacturers such as Biddle in fan coils for a number of years. Fan coils are operating in thousands of buildings across the UK and there really is no reason why they should not continue to be used to provide air conditioning while conserving power.

www.biddle-air.co.uk
1 February 2008

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