The EC VAV fan coil unit provides benefits over the conventional constant speed AC unit and this new approach is now being used on many projects, says Gerry Butler
With the introduction of ECM electronically commutated motors, variable speed is easily achieved by varying the DC voltage to the motor. This can be achieved through a simple control loop with 0 - 10 volt signal. This means that the fan speed can be varied smoothly over the whole range from maximum speed down to zero. This variable speed feature means that variable air volume (VAV) fan coils have evolved with some very significant savings in energy and consequently carbon emissions.
With VAV fan coils it is prudent to maintain at least 60 per cent rated airflow as a minimum since lower air volumes could lead to air dumping. More on air distribution later.
Since the energy absorbed by the motor is directly proportional to the ratio of the speed cubed significant savings in power can be achieved. For example if the speed was reduced to 80 per cent the saving in energy would be 0.8 x 0.8 x 0.8 = 0.512 or 51.2 per cent. If the speed were reduced to 60 per cent then the reduction in energy would be 0.6 x 0.6 x 0.6 = 0.216 or 21.6 per cent.
Power consumption cut
Through Dynamic Simulation Modelling - DSM - on a Part L 2010 compliant office building, the fan coil was predominantly in the 60 to 70 per cent speed range and only on one or two occasions in the summer did it actually reach 100 per cent speed or air volume. This meant that the annual power consumption and consequently carbon emissions were reduced by an enormous 73 per cent compared with a constant speed fan coil.
There is also a further gain using ECM motors since they are more efficient than the equivalent AC motor. A comparison on Specific Fan Powers would give typical figures for AC of 0.6W/l/s, and for an ECM the figure could be as low as 0.2W/l/s.
This significant improvement for Specific Fan Power would further enhance the saving in energy from 73 per cent mentioned above to a staggering 91 per cent. In other words compared to the conventional constant volume AC fan coil unit the state-of-the-art EC VAV fan coil unit would reduce the energy consumption of the motor by over 90 per cent.
To achieve the VAV the control strategy is shown in Figure 1. Looking first at the cooling, and with reference to the notations, (1) this is the dead band, (2) this is where the cooling coil valve opens and cooling commences. Note that the air volume is down at 60 per cent whilst the cooling coil is progressively increasing its output to a maximum at (3). At this stage the fan speed and hence air volume commences to increase (4) until it reaches a maximum speed and air volume (5). At this point the fan coil is on full cooling and 100 per cent air volume. On the heating cycle the heating valve is opened at (6) and at the same time air volume is increased to 100 per cent. This means that the maximum air volume is achieved as soon as the heating cycle begins. This is to avoid any potential stratification and since the heating cycle lasts for a very short period the energy implications are insignificant.
Fan deck design is another important issue where there are two distinct approaches. Most fan coil manufacturers using European sourced EC motors would replicate the AC fan deck configuration with a number of small centrifugal fans vertically mounted. With larger fan coil units this could mean as many as five individual motors and fans. Manufacturers using motors sourced from the USA have access to much larger ECM motors, i.e. 250W as compared to 75W, and therefore by mounting the fan horizontally single fan and motor assemblies can be used for a wide range of duties. Even on the maximum duties, only two fans are used within the fan deck.
With the application of variable air volume comes the challenge of good air distribution since there is always a risk of dumping if the air volumes, and consequently air velocities, are too low. It is now quite common for the designer to seek a specialist supplier who covers both technologies, i.e. fan coils and air distribution. In this way the team have a single point responsibility for the terminal air conditioning system.
Initially, with ECMs power factor was an issue and designers were looking for power factor correction to be undertaken on each individual fan coil unit.
However, the uncorrected power to the ECMs is current leading and this is in opposition to the typical building power. Most buildings are inductive in nature with current lagging power. This means that with a building that has an inductive nature the ECM would actually neutralise this and act as a power factor correction without any additional electronic controls.
In fact our parent company, Nailor Industries in America, has installed more than two million square feet of office building where this natural power factor correction situation has been exploited.
The ECM motor has integrated software which analyses various operating parameters - speed, torque and power. Once the motor/fan assembly has been characterised to the software the inputted air volume set-point corresponds to a given speed/torque value.
If the external static pressure increases, as in instances of extended ductwork or air filter congestion, then the change in internal parameters is sensed and the fan speed is automatically reset to maintain constant pre-set air volume. The motor/fan assembly performance should not be considered simply as a curve but more as a rectangle providing an almost infinite range of air volume/ external static pressure settings.
BMS interfacing is possible
ECM motors applied to fan coil units can be interfaced with building management systems via digital controller incorporating 0-10 Vdc. (4-20mA) control signal. This enables the discharge air volume to be reset remotely at any point in the linear voltage range to match actual site requirements.
As such the ECM fan coils can be put on a special rig at the end of manufacture and the air volumes pre-set so that the unit is pre-commissioned and no other further commissioning is required apart from a simple balance of air flow at the diffuser if the fan coil is serving more than one diffuser.
// The author is engineering manager at Advanced Air //