This series on fan power is divided into two parts. Part 1 goes through the general guidelines and part 2 goes through fan efficiency.
Part 1 – General Guidelines
Much emphasis, when examining ways to reduce energy consumption in HVAC systems, is put on the rejection, absorption, generation and transfer of heat. The distribution of air, whether part of the air conditioning system or just a ventilation or extract system, is often treated as a lower priority, or sometimes overlooked completely.
In many cases however, looking at reducing the power consumed by fans can produce significant savings.
This bulletin examines some of the area in which savings could be made, which include:-
- Reduction in system resistance
- Selection of the type of fan
- Consideration of inverter control
The Theory
The power output of the fan is the volume of air delivered multiplied by the pressure generated by the fan. The power input, and hence energy consumed, is the power required to drive the fan – including the losses through belts, bearings, motors and inverters.
The power output is a system design issue – i.e. the designer has decided on the volume of air required based on, for example, an air change rate or minimum litres/sec per person, and calculated the resistance the fan has to overcome to deliver that volume.
A re-check of the initial design is the starting point in any assessment of potential energy saving. Is it possible, for example, to reduce the volume of air? Also, can the system be redesigned to reduce the resistance against which this volume is being delivered?
Any such changes of reduced volume, because the [ower is proportional to (volume) 3, a reduction in volume of 10% would result in a reduced power input of 27%
Similarly, if the system resistance is reduced, for example by better duct design or filter selection, then a significant reduction in power input would be achieved.
Economics
A typical air handling unit providing 2.8m3 /sec of fresh air against a resistance of 300 Pa, and extracting 2.5 m3/sec, would consume about 2kW of fan power.
If the system operated during normal working hours for 50 weeks of the year, and with a unit cost of electricity of 10 pence per kWhr then the annual energy cost would be £400.
The equivalent CO2 emissions for this example would be 2,000 kg.
If it were possible to reduce the supply and extract volumes to, say, 2.4 and 2.1 m3/sec respectively – i.e. a 15% reduction – then the fan power would be reduced to 1.4 kW.
This would represent a 30% reduction in energy consumption, would save £120 per annum, and would reduce the annual CO2 emissions by 600kg.
Action Points
- Check the original design – have the parameters changed?
- Take flow rates and static pressure measurements – is the system achieving above or below design figures?
- Do the filters match the current application, and are they being replaced/cleaned regularly?
- Is the fan operating on the most efficient part of its curve?
- Consider variable, or two-speed, fan control
- Are the fans belt driven? If yes, consider direct drive.
- Check the ductwork installation for potential high resistance areas, and modify if appropriate.