Given the current trend for energy costs to rise at rates which appear to be almost exponential, air conditioning and ventilation systems are coming under close scrutiny, particularly in terms of efficiencies and coefficients of performance. Add to this the requirements of Building Regulations and the much talked about Energy Using Products Directive, fan efficiencies are a major issue now.
In air conditioning and ventilation systems, fans can account for a significant proportion of the energy consumed. It therefore makes sense to install new equipment and/or retrofit existing equipment with fans having the highest possible efficiency.
Considering Air Handling Units and Air Cooled Condensers / Dry Coolers, the types of fans used are normally centrifugal and axial respectively, and optimisation is possible in both instances.
Air Handling Units (AHU)
In the UK, the AHU fan arrangement for many years has been belt-driven forward curved fans using AC induction motors, using pulley changes, or sometimes inverters, to adjust air flows. A number of changes are in the process of taking place, hastened by rising energy costs, to achieve the highest possible efficiencies. Some or all of the following are being adopted:
Replacing forward curved fans with backward curved fans, realising a significant step change in efficiency.
Replacing belt-drive systems with direct drive systems, realising efficiency gains in the order of 10-15%, and saving on maintenance costs and reducing system contamination with the absence of belt-dust.
All of the benefits above are achieved by using a motorised impeller arrangement with EC motor, but the highest efficiencies are achieved by matching high efficiency backward curved impellers with high efficiency EC motors. A fully optimised solution is then the result, with the added benefit of high efficiencies at reduced speed; the alternatives are:
a non-optimised solution where a high efficiency motor drives an inferior impeller
a non-optimised solution where a high efficiency impeller is driven by an average motor
or worst of all, an inferior impeller driven by an average motor
One further optimisation step is the introduction of centrifugal impellers with aerofoil section blades.
With larger applications where EC motors may not be available or viable, a high efficiency AC motor is normally used, with or without inverter speed control.
Air Cooled Condensers / Dry Coolers Axial fans are the norm for condensers and dry coolers, where relatively high air flows and low to medium pressure development are required. Low noise emissions are synonymous with this type of application. Some condensers / coolers may have as many as sixteen axial fans, and a small energy saving per fan results in significant saving per condenser, or in the case of supermarket chains for example, with hundreds of stores around the country, results in thousands of pounds saved on annual energy bills.
The key once again is the optimisation of the axial impeller and motor. The EC motors with the lowest motor losses and hence the highest electrical efficiencies must be matched to the axial impellers with the highest aerodynamic efficiency to achieve the highest combined efficiency.
Aerofoil section blades, often with winglets at the blades tips, are a feature of high performance axial impellers. However, as well as being designed for maximum aerodynamic performance, the axial impeller must operate at the lowest possible noise levels. Advanced blade designs, some incorporating serrated trailing blade edges, have resulted in surprising reductions in emitted noise levels.
Concluding, to maximise energy savings when operating fans, both the motor and impeller must be matched and optimised; anything else is a compromise.
