Fan coils on an economy drive
Fan coils have been the most popular terminal air system for more than four decades and, although alternative technologies such as chilled beams are placing them under increasing competitive pressure, there is no reason why they should not retain their pole position.
However, remaining first on the grid demands that manufacturers pursue both stringent testing and innovative design in order to boost energy performance and maximise economical operation.
Testing and design
A Formula 1 motor racing analogy illustrates the importance of testing: For an F1 team to be competitive these days, it needs its own highly trained team and exceptional in-house test facilities (including a wind tunnel). The same should apply to fan coil suppliers – only those with well-equipped laboratories can hope to prove the claims they make for their products and develop new ones that meet increasingly stringent performance demands.
Energy performance continues to rise to the top of the industry’s agenda as the pressure to tackle climate change has grown. This has brought the design of products and systems into sharp focus and there are two inter-related areas in particular for fan coils where this has shown up – motor efficiency and variable air volume (VAV) performance.
Fan coil system designers have typically allowed 5 W/m2 for the electric motor power whereas with VAV fan coils we believe this can be reduced to 1.25 W/m2, i.e. a quarter of the traditional fan coil power consumption – a really worthwhile energy saving. Indeed, VAV fan coil units produce a potentially enormous pay-off in terms of efficiency. An example illustrates the point.
With a conventional fan coil system, the motors always run at the same speed irrespective of whether the room needs to be cooled, heated or is at temperature. However, with a VAV system, the motors are not running at 100% all the time. This cuts energy consumption and reduces emissions. On top of this, by varying the motor’s speed to suit the heating or cooling requirement, the motor running cost is reduced and energy is saved.
Generating lower air volumes during periods of reduced room cooling demand also achieves savings in annual demand for energy because the input power is proportional to the cube of the air flow rate. So, for example, if the air volume is reduced to 80% there will be a 50% power saving and a 30% saving in apparent power (taking power factor into consideration).
But the innovation does not stop there. The electrically commutated (EC) motors used in the VAV fan coil are brushless direct current (DC) units that connect directly to the 230V AC mains supply. The AC-to-DC conversion and motor commutation are integrated within the motor body and this ensures increased efficiency which provides lower running temperature and higher life expectancy than a traditional motor. EC microprocessor-based commutation also provides other features including alarm outputs and speed monitoring.
EC motors typically boost motor efficiency from 45 to 80% and therefore achieve lower specific fan power. However, while higher efficiency motors are available for terminal units incorporating fans, the forward curved fan impeller is not particularly efficient so the overall motor and fan efficiency is 37%.
With the revision of the building regulations, we are seeing an increased project demand typically for a mixture of cooling only units and cooling with electric heaters. The proportion typically being 90% cooling only, and 10% cooling with electric heating. This also has major project cost savings due to a hot water boiler and distribution system not being needed.
Speed Monitoring Control PCB
It is becoming increasingly necessary to be able to monitor electric motors for safety or maintenance purposes. Traditionally, pressure switches or more recently current switches have been used for this purpose. Due to the low power consumption of EC motors it is difficult to use current switches and pressure switches.
Trox has therefore developed an EC motor speed monitoring Printed Circuit Board. This monitors the EC motor’s ‘hall effect’ sensor to provide an alarm output when any one of up to four motors is either not running or is below the desired speed. The set point is determined by means of the PCB mounted potentiometer. The PCB can monitor between one and four fans and is therefore suitable for all Trox ‘EC’ fan coil units.
Two changeover relay contacts are provided:
- 16Amp rated contacts for use as part of the heater safety circuit. This ensures the heater is only switched ‘on’ when the fans are running.
- Low current contacts used for connections to BMS controllers for fan status input.
So the potential pay-off of VAV fan coils using high efficiency EC motors is enormous. However, the speed variability of VAV fan coils itself 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.
Essentially, this means looking at the overall system in terms of duct design, joint scheduling of fan coils and grilles/diffusers, with a view to offering guarantees on performance. This will take away the concerns of designers and minimise their risks when designing a fan coil system.
Starting with the original design, a system approach will ensure the best possible solution is developed. This 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 offers two big benefits in particular. Firstly the 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 there is the issue of cost in that construction costs are significantly lower because the construction process is faster, there is a reduction in the number of trades, and the process is more predictable, of higher quality, less wasteful and safer.
On top of this, a single point responsibility makes performance guarantees more easily available because of the focus on one manufacturer. So, there is no reason why fan coil units should not continue to be winners on the heating/cooling circuit just as racing cars are winners on the grand prix circuit. The secret of success in both cases is, of course, picking the right team.
Benefits of VAV Fan Coils
- They are energy efficient when compared with constant volume units.
- They exceed the requirements of the Building Regulations and reduce carbon emissions.
- They have high motor efficiency leading to lower specific fan power.
- They have efficient speed control.
- They have lower maintenance costs due to longer bearing & motor life.