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Fan coil units: the phoenix that keeps on rising

Dave Cogger, fan coil unit product manager at Caice Acoustic Air Movement, discusses the issues around the new DC/EC fan motor and calculates how the rewards in energy saving can be even greater than was first thought
Fan coil units: the phoenix that keeps on rising
THROUGHOUT their existence, fan coil units have been in vogue, out of favour, flavour of the month, and back again. Well as they say, what goes around comes around, and the fan coils market certainly appear to be booming again.

During the last couple of years, it is probably fair to say most manufacturers had seen a turn down. Figures recently quoted stated that the market had shrunk from around £35million a year to below £30million. But, lo and behold, they are making another come back.

So what's started the latest renaissance?

A lot of it can be attributed to the introduction of the latest revision of Part L of the Building Regulations launched in April 2006.

The document has set new energy efficiency targets for new build and refurbishment projects. The impact on fan coil manufacturers was significant and meant a rethink and a move away from traditional components.

The requirement, set out by the new Part L, was that as a weighted average across a building the specific fan power (sfp) level attributable to fan coil units should not exceed 0.8w/l/s.

Energy saving

As standard, most manufacturers had used predominantly 2pole AC powered fans in their units. The implementation of the new Part L has all but precluded the use of such fans as their typical sfps vary between 0.7 to in excess of 1 w/l/s. So what was the solution?

Two options were available:

· Use a slightly larger 4pole AC motor fan;

· Utilise the same size fan but incorporating the new so called DC/EC motor technology.

The new DC/EC technology operates from a standard 230v/1ph/50Hz AC input but rectifies this input on the motor, to provide DC to power the motor, creating the electronically commutated (EC) fan. This leads to energy savings as the EC motor operates with minimal heat gain, whereas the AC motor fan will generate around 60-80 watts of heat which has to be overcome when the fan coil is in cooling mode. So how do the three fan types compare in normal operation?

Specific fan power is calculated from the equation: Sfp = Power supplied to the fan coil in watts divided by air flow provided by the unit in l/s.

By considering a typical 2 fan, fan coil unit providing 190l/s of air against an external resistance of 30Pa, from the testing undertaken by Caice the following figures were calculated:

(i) 2pole AC motor fan unit powered by 2 off fans energy consumed = 146watts, sfp 0.77 = w/l/s;

(ii) 4pole AC motor fan unit powered by 2 off fans energy consumed = 107watts, sfp 0.55 = w/l/s;

(iii) DC/EC motor fan unit powered by 2 off fans energy consumed = 54watts, sfp = 0.28 w/l/s.

It will be seen that when comparing the new DC/EC fan to its pole AC counterpart, energy saving claims of the order of 50-70% are not unreasonable.

Because the sfps when using DC/EC are considerably less than the 0.8w/l/s limit (typically less than 0.5w/l/s) when undertaking their energy assessment, certain designers are already beginning to use these savings to provide some extra in their overall building
calculations.

However, energy saving in running alone is only half of the story when considering the DC/EC technology.

Traditional AC motor fan driven units adjust fan speed using a stepped transformer and speed switches. On the new style fans, fan speed is controlled via a 0-10v DC signal, which in principle gives infinite speed variation. This control gives even greater flexibility to the FCU in relation to remote BMS facility management.

Control

Fan speed on the DC/EC units is controlled manually by a simple potentiometer; although a remote BMS linked fan speed override is now also a possible option. When considering a typical office, hotel room or meeting room scenario, linked to a BMS there is the potential to remotely ramp up, turn down or even turn off the unit (by setting the fan control voltage to 0volts turns the fan off) in line with actual or intended room occupancy.

By linking the variation in air flow rate between limits to the opening and closing of the waterside valves means the units deliver only the thermal load that is actually required, in addition to providing a quieter environment and avoiding the potential of unwanted drafts.

This gives even greater energy savings when compared with the AC motor units which

usually run at constant volume (based on achieving maximum thermal load) and for a set time period.

It can be envisaged that there will be an initial premium to be paid in the capital cost of producing more energy efficient buildings, with more advanced equipment, to meet the new Building Regulation requirements.

However, certainly from a fan coil prospective, by using the new generation of EC units the basic running costs will be greatly reduced and with the incorporation of the greater flexibility indicated this will further be reduced.

It can be envisaged that as the units based on EC technology become more prevalent and more into common use so this flexibility will be utilised in many other energy saving ways.
1 February 2007

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