Boosting the energy efficiency of fan-
The enormous pay off of variable-air-volume fan coils using high-efficiency EC motors — Barry Trewhitt.
Moving on from constant-volume fan-coil units to variable volume makes the approach to air conditioning more energy efficient — as Barry Trewhitt explains.Fan coils have been one of the most popular terminal air-conditioning systems for the past 40 years. They are a very forgiving system in that there are so many variables such as water flow, water temperature and fan speed that can be adjusted to achieve a comfortable working environment. Over the past few years we have seen the popularity of multi-service chilled beams (MSCB) increase to such an extent in value terms that the market sectors of beams and fan-coil units are similar. Much of this gain is due to the energy credentials of chilled beams and changes to Building Regulations. Whilst the pros and cons of each system are much wider, the focus for fan-coils must be how they can be more energy efficient. With a conventional fan-coil system the motor runs at the same speed irrespective of whether the room needs to be cooled, heated or is at design temperature.
Table 1: The theory and reality of how energy can be saved by variable-air-volume fan-coil units compared with the constant-volume units.
Whilst constant air volume has the advantage of constant sound level and no air dumping from the diffuser, it has the major disadvantage of consuming more energy than is required. EC (electronically commutated) motors are becoming increasingly popular, but instead of running them at a constant speed greater savings will be achieved by adopting VAV principles. If the air volume for each fan coil was reduced from 100% to 80% (a small reduction in air flow and barely discernable to the room occupants) the power consumption of the motor energy would, based on the fan laws, be reduced by 50% — a significant saving in power. If the air volume was reduced from 100% to 60%, the power consumption of the motor would be reduced by nearly 80%. The above theory needed to be confirmed by conducting a project type test in a special BSRIA approved test cell (Fig. 1). Certain project design parameters were established and these are summarised in the table. The results show that the potential pay-off of variable-air-volume fan coils using high-efficiency EC motors is enormous. However, the speed variability of VAV fan coils makes the selection of the grilles and diffusers more critical because they need to work at a range of volumes. That is why it makes good commercial sense to adopt a systems approach to match the grilles and diffusers to the fan coils. Adopting this approach essentially means looking at the overall system in terms of duct design and joint scheduling of fan coils and grilles/diffusers — with a view to offering guarantees on performance.
The effective application of variable-air-volume fan-coil units will require close co-ordination between the fan coil and diffusers if undesirable effects such as the dumping of cold air are to be avoided.
This will take away the concerns of designers about air dumping with variable-air-volume fan-coil units and minimise the risks when designing these types of systems. Starting with the original design, a system approach will ensure the best possible solution is developed. It will also allow for project cost savings by value engineering, off-site fabrication, and pre-commissioning of units. Indeed, I believe that offsite fabrication will become increasingly common as the shortage of skilled site labour starts to bite and as demands grow for better build consistency and efficiency, reduced installation times and increased fast track programming. Factory prefabrication and pre-commissioning offer two big benefits in particular: First, fan-coil controllers can be factory set for design air flow and pressure-independent modulating water valves can be factory set for design water flow. Secondly, construction costs are significantly lower because the construction process is faster, there are fewer trades, and the process is more predictable, of higher quality, less wasteful and safer.
Fig. 1: This dedicated test cell for fan-coil units at Trox has BSRIA approval.
Single-point responsibility also makes performance guarantees more easily available because of the focus on one manufacturer. Variable-air-volume fan coils could give these systems an energy-saving option to chilled beams, particularly with elevated chilled water temperatures. In conclusion, the benefits of variable-air-volume fan-coil units can be summarised as follows.
?• Energy efficient when compared with constant-volume units.
?• Exceed the requirements of the Building Regulations and reduce carbon emissions.
?• High motor efficiency, leading to lower specific fan power
?• Efficient speed control
?• Lower maintenance costs due to longer bearing and motor life. Barry Trewhitt is fan coil product manager with Trox UK