The fan "system" includes the following:
Centrifugal fan efficiencies range from 50 percent to about 70 percent depending upon blade configuration and vane-axial fan efficiencies range from 80 percent to over 90 percent. However, the energy engineer must keep in mind that all fans add heat to the air stream. The air stream temperature typically rises between three degrees F and six degrees F for inefficient fan types. Also, actual installed efficiency is typically worse than the manufacturer's rating data indicates, dropping the fan's efficiency by 10 to 30 percent in some cases. [Shepard, M. et al., 1995; Micro-Electronics Facility Efficiency Workshop, 1995]
Direct drive fan/motor combinations are the most efficient at 100 percent. Standard V-belt drives are about 93 percent efficient when first installed but drop over time. Synchronous belt systems offer an energy-efficiency increase to about 96 percent for the life of the belt.
Premium efficiency motors that have full-load efficiencies of greater than 94 percent depending upon horsepower are preferable. It is also important to consider the part-load motor efficiency and its operation with a VFD in VAV systems.
Acoustical attenuators, used to reduce a fan's sound, increase the pressure drop of the AHU. Therefore, engineers should consider the disadvantages of high-efficiency but noisy fan that will require acoustical attenuation devices. These devices can negate the energy savings from face velocity reductions.