International Fan Efficiency Test Standard: Missing Elements of the System Efficiency

The performance of a driven fan can be determined either by direct measurement or by calculation. In both methods, the ratio of the fan air power (output power) and the electrical input power determines the efficiency. A direct measurement will include the losses of all the components in a fan system, and the effects of those components on the fan system. The calculation methods may not include all the components necessary to build the final fan assembly. Furthermore, some components can have an adverse aerodynamic impact that is not included in the calculation. This paper discusses those potential missing elements. When the driven fan is a simple arrangement, a calculation will give a similar result to a direct measurement. Where a simple arrangement is of a motor directly connected to the impeller, and the motor is located outside of the air stream. However, the arrangement is not simple, and some components can add losses by causing aerodynamic disturbance. The two calculation methods in fan standards do not include losses due to obstructions caused by the components of the fan. This was recognized by European legislators that added a compensation factor in their calculation, but this is disadvantageous in some fan arrangements and possibly is not enough to take other elements and effects of other fan arrangements into consideration. A more pragmatic approach is required in standards and legislation. They must define when a calculation method is appropriate, where care should be taken and where only a direct measurement should be applied.