The criteria used during the selection of fans for use in the refrigeration and air conditioning market have moved around in recent years and, for many, high energy efficiency is currently at the top of the wish list, says Les Fish of Ziehl-Abegg
THERE are choices to be made in terms of the type of electric motors which may be fitted to direct drive fans, and the method of speed control to be used. In broad terms, the motor options can be thought of as AC (alternating current) motors and EC (electronically commutated) motors.
Regarding speed control of AC motors, the choices in essence are: by voltage control, either transformer-based or electronic (wave-clipping)-based, and by frequency inversion (or Variable Frequency Drive (VFD)), often using an input signal such as 0-10Vdc to control the fan speed.
For EC motors, speed control is an inherent feature, also typically using an input signal such as 0-10Vdc to control the fan speed.
In recent times, EC motor driven fans have been promoted by some to be the panacea for all fan applications. However, while EC fans do indeed offer the best electrical efficiency, the associated total lifetime costs, (comprising initial capital outlay and operating costs for the lifetime of the fans), and the payback period are very often overlooked.
Compared with AC fans with VFD control, EC fans are relatively expensive to buy, especially when the relatively small efficiency difference between the two at part load conditions is taken into account.
Ziehl-Abegg offers EC speed controlled fans with integral controls, as well as VFD controlled AC fans with either internal inverter, or as standard AC fans with external inverter. This allows each application to be examined individually, and the most cost-effective solution to be determined.
An important factor to consider is how many fans are fitted to the equipment and, in the case of air-cooled condensers, dry-coolers etc, the number may range from one to 18 or beyond. Analysis by Ziehl-Abegg shows that, when the number of fans per unit is three or less, fans with integral speed control are economically viable, and the choice then is between integral EC speed control or integral VFD control.
Operational hours per annum and load profile then determine if EC or integral VFD are most suitable.
Once the number of fans per unit exceeds three, serious consideration should be given to using standard AC fans with all pole sinusoidal filter equipped external inverter speed control, enabling the fans to be operated reliably in parallel. This choice provides the benefits of using relatively inexpensive standard AC fans, with low replacement cost (should this be necessary) plus the need for only one or possibly two suitably sized VFDs rather than one per fan.
Added to these benefits is the fact that the efficiencies are only marginally lower than the EC alternative.
Relative input power
The diagram illustrates the relative input power of EC and VFD speed controlled fans compared with fan speed. Both offer significant energy consumption benefits over voltage-based speed control but it is clear that the difference in energy consumption between EC and VFD controlled fans reduces as fan speed reduces, such that at 50% fan speed, the difference estimates to 3-4%.
This has to be compared with a significant price differential, with EC fans costing somewhat more than AC fans, even when the VFD is included.
In conclusion, the most cost-effective solution may be one of several, and making the assumption that EC is the answer to every situation can lead to poor value for money and extended payback periods.