The report of the Micro-Electronics Facility Efficiency Workshop (1995) points out that:
Specifying a high efficiency fan out of a catalog is not enough. Careful design requires looking at all aspects of the fan-motor-housing system. ...[In] an example...an otherwise efficient direct-drive axial fan...had a motor mounting scheme that covered up nearly a third of the fan opening. Likewise, it is not enough to just specify fan efficiency at a single point, it is also important to specify a fan with a large "bulls-eye"; i.e., where the peak efficiency can be maintained over a wide range of static pressure and fan flow rate. ...efficiency plots from one manufacturer of virtually identical models [can be quite different], one with a very small efficiency "bulls-eye" and one with a very large "bulls-eye" for an otherwise identical "80% efficient" fan.
