Backward inclined fans are one of the three styles of centrifugal fans commercially available; the other two styles use forward inclined blades and radial blades. All centrifugal fans, regardless of the fan wheel configuration pull air into the fan housing along the axis if the impeller, the air is discharged at 90 degrees to the impeller shaft. Backward inclined fans are capable of producing high pressure as well as high volume and they do it quietly and efficiently. The impeller that holds the blades is located in a scroll shaped housing; the housing directs the air flow as the housing can exit at any angle although most are set so the air exists horizontally. This arrangement converts kinetic energy to static pressure. As well as the outlet being at any angle, the inlet can also be single or double which means the air intake can be from the left or right side of the housing or both if the volume demands are such.
The fan wheel, technically this is the impeller, can be shaped differently based on the application. The primary designs are forward and backward inclined and the blades can be straight, curved or true airfoil shaped. It is the shape of the blade and the configuration that is the basis for the performance and efficiency of the fan. Although the shape is critical for performance the width of the blade as well as the clearance between the blades and the inside of the housing also come into play as does the area of the discharge.
Recent developments have made electronic variable speed motors a practical drive for centrifugal fans. In the past if a single speed motor was coupled directly to the fan shaft there was no way to alter or vary the speed. The traditional method to alter the speed of the impeller was to connect it to a single aped motor through an arrangement of belts; the pulley diameters provided a reduction ratio so the impeller ran at the design speed, not the speed of the motor. These new variable speed drives lessen vibration and certainly cut down on maintenance and belt changes.
Backward inclined fans rotation develop high air pressure and the non-loading power characteristic means that the motor will never overheat causing a failure. Although centrifugal fans are selected for their application the backward inclined style is somewhat heavier than its counterparts making it more costly but the increase in efficiency tends to compensate as the power savings can be considerable.
