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A controlled response to demand fluctuations

The controllability of fan coil units has evolved, to ensure they only use energy when required and can better respond to fluctuations in demand. Andrew Saxon discusses the most recent advances
With a track record for being reliable, cost-effective and flexible, fan coil units (FCUs) have been used to air condition thousands of commercial buildings across the UK since the 1970s. However, there was clearly a need for the technology to evolve.

Drawing unconditioned air over a cooling/heating coil, traditional FCUs deliver a constant air volume, which is set in accordance with maximum requirements. However, ia constant air volume isn't always required; internal environments can be affected by changing weather patterns, building occupancy levels and any increase/ decrease in heat-generating office equipment such as PCs and printers.

Though cooling and heating output is adjusted via four-port water control valves, the constant water flow rate required means the pump always needs 100 per cent of pump design power.

And, of course, in today's energy conscious world the installation of inefficient products isn't acceptable, or permitted.

The revised Part L2A and Part L2B documents rate the efficiency of FCUs in relation to their specific fan power or SPF (a measure of power in Watts required to create an airflow of one litre per second), stating that this should not exceed 0.6W/ls-1 when measured as the rating-weighted average of the installation (calculated by adding the product of the power supplied and the SFP for each fan coil unit in the installation, and dividing by the sum of the power supplied for all the fan coil units in the installation).

Traditionally FCUs have incorporated two-pole ac fan motors, which have a SFP of 0.9W/ls-1 or higher. And, though four-pole ac fan motors are more efficient, having SFPs ranging between 0.5 and 0.75W/ls-1, this still delivers a constant air volume. And, because in truth we know the key to any significant reduction in energy usage is having a variable air volume that can be easily controlled, development has focused on this area.

A more preferable approach to meeting the requirements of Part L2 A and B is therefore the modern electronically commutated (ec) dc motor, which has really given FCUs a new lease of life.

EC FCUs operate with slower fan speeds, which makes them more efficient, but the real benefit is that the air volume can be altered in line with cooling/heating demands. This has led to claims that they use up to 70 per cent less energy than typical ac motors. In addition, they can be coupled with intelligent controls, programmers and building management systems.

Alongside this, the increased use of two-port water control valves means the water flow rate is varied and in turn the pump can run at variable speeds, making significant energy reductions.

However, when a number of FCUs are fitted with two-port valves, some will be opening at the same time as others are closing, meaning the system will struggle to maintain stable control. To reduce these problems, two-port valves are combined with a differential pressure control valve (DPCV) or, better still, a pressure independent (PI) dynamic flow control valve which combines a DPCV, a two-port valve and a balancing valve all in one assembly, reducing installation work and most importantly giving a valve authority of one.

Two-port PI valves allow design water flow rates to be quickly and easily adjusted or pre-set from a remote location in relation to heating/cooling demands, without the need for specialist skills.

As well as being more efficient and controllable, the fan coil controls available today enable quicker and easier commissioning and maintenance, and have allowed more intelligent, open-minded, innovative and flexible approaches to be developed.

For example, vast amounts of time can be saved by using a manifold system, where all the valves for a group of around six terminal units are housed in a single, insulated manifold box, rather than individual valve assemblies being installed at each terminal unit.

And, if a manifold system fitted with PICVs is combined with the single station balancing (SSB) technique, further benefits can be achieved. With conventional proportional balancing the commissioning engineer needs to balance the entire system before problem circuits can be identified, and again after they are solved. This is a lengthy, disruptive process which involves gaining access to numerous ceiling voids.

The innovative SSB technique, on the other hand, uses a 'subtraction' method to identify problem valves, and it is highly appropriate for variable water volume systems. It is based on knowing the design flow rates for each individual valve and therefore the total flow rate for that fan coil group. Assuming all valves are functioning correctly, isolating each valve will have a predictable effect on the total flow rate of the remaining valves. Where an unexpected result is observed, the valve causing the problem will be identified by a process of elimination.

Then there is wireless commissioning - if the valve characteristic and design flow is known then wireless control of the valve actuator can set the position required for the design flow rate. With this approach, there is no need to access ceiling voids and a considerable amount of time can be saved. It is also easy to alter the valve position if occupancy levels change or if the FCUs are reused elsewhere.

The need for controllable, energy efficient FCUs has resulted in the development of variable air volume products which, in turn, has paved the way for innovative commissioning and maintenance. We feel certain these advancements have secured the future of the FCU and will enable the technology to continue to be used in many commercial premises across the UK.

// The author is marketing manager at Biddle Air Systems //
1 February 2012


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