Heating and Ventilating

The ubiquitous fan coil has received a tremendous boost from the development of EC motors and clever concepts such as pressure independence. Andrew Sargent explains some key features of this new breed of fan coil systems and how flexible they have become.
Pressure independence means that the fan coil delivers the required air volume irrespective of changes in external pressure. This is an excellent feature because as the filters get dirty and the resistance increases the air volume remains the same even if the duct configuration has changed such that pressures are increased or even reduced the pre-set air volume on the fan coil will be delivered to the respective air diffusers. This pressure independence can be achieved in a variety of ways.

Our own units incorporate a smart card embedded in the motor that contains a three dimensional algorithm which ensures the motor/fan assembly delivers a constant air volume. It mathematically manipulates the fan characteristic into a straight horizontal line as shown in Figure 1.

These air volumes in litres/second would also correspond to a particular voltage which can easily be set either at the fan coil unit or very simply remotely at the BMS. Following on from this it is now possible to pre-set the air volumes in the factory and therefore no commissioning of the unit is required on site. The fan coil in this configuration can be run even if the BMS is not working, which is often the case. So no BMS is required to get the fan coil operating in this pressure independent mode. When the BMS is connected, control of the air volume can then be overtaken by the BMS and if any changes are required the air volume could be adjusted through these controls.

Another way of creating pressure independence is a little more complex and requires a fan in each discharge spigot and the plenum within the fan coil units to be segmented so that there is a separate and isolated air path for each fan and spigot. By the use of additional control each of these air paths can be made pressure independent which then means the fan coil has the benefits as already outlined.

With pressure on energy consumption through the various building regulations, it is important to ensure these terminal units have a high efficiency which is generally measured by Specific Fan Powers in W/l/s. The traditional AC motor has SFPs as high as 0.7 W/l/s, but EC motors can achieve an SFP half of this at 0.35 W/l/s, and some EC motors can be as low as 0.2 W/l/s. This makes the EC fan coil unit one of the most energy efficient terminal air conditioning systems.

Since the load requirements within the building are constantly changing variability within the air conditioning system needs to be introduced through variable speed drives which can save enormous amounts of energy. There is no point in having plant items running at full capacity if the demand in the building is constantly changing. With AC motors variable speed was both difficult and expensive but the situation with EC motors is very different.

A simple reduction in voltage will bring about a proportional reduction in speed and hence air volume. With an EC fan coil unit it is therefore very easy to provide variable air volume and the saving in energy is quite significant. Comparing a conventional AC fan coil with the EC VAV fan coil unit the reduction in motor energy would be a staggering 83 per cent.

As most designers would tell you space for plant is always an issue. The challenge for all those concerned in a project is generally trying to squeeze the equipment into tight places, for example with refurbishments there is generally very limited ceiling void space. This again is where the fan coil unit can be adapted to meet the project requirements with a slimline unit which has been developed to fit in to ceiling voids of around 200mm. Further reduction in space has been achieved for the residential market where fan coils are fitted into cupboards or wardrobes.

Figure 2 shows a new development which actually has split the fan deck and cooling coil into two parts with the coils in the plinth of the cupboard and the fan deck behind the grille at the top of the cupboard. This requires a vertical duct which can be as small as 100mm and therefore the space issues are really not a problem.

Noise levels need to be low and NR25 can be achieved but still producing 1kW of cooling. With horizontally mounted fan decks using 200W EC motors, i.e. very much larger than the conventional 75W EC motor, very low noise levels can be achieved for fan coils mounted in the ceiling voids, i.e. the chassis type. In fact my company has been involved in a project for media rooms where we needed to achieve NR20 in the room and this was done utilising the horizontally mounted fan deck design.

So whatever your project needs fan coils can invariably be adapted to meet all your requirements which makes them the system of choice for virtually all designers.

// The author is general manager of Advanced Air //