Typical static pressure and power requirements
In the case of centrifugal fans, the hub-to-tip ratios (ratio of inner-to-outer impeller diameter), the tip angles (angle at which forward or backward curved blades are curved at the blade tip – at the base the blades are always oriented in the direction of flow), and the blade width determines the pressure developed by the fan.
Forward curved fans have large hub-to-tip ratio compared to backward curved fans and produce lower pressure.
At both design and off-design points, backward curved fans provide the most stable operation.
Also, the power required by most backward curved fans will decrease at flow higher than design values
A similar effect can be obtained by using inlet guide vanes instead of replacing the impeller with different tip angles.
Radial fans are simple in construction and are preferable for high-pressure applications.
Axial-flow fans produce lower pressure than centrifugal fans, and exhibit a dip in pressure before reaching the peak pressure point.
Propeller-type fans are capable of high-flow rates at low pressures.
Tube-axial fans have medium pressure, high flow capability and are not equipped with guide vanes.
Vane-axial fans are equipped with inlet or outlet guide vanes, and are characterized by high pressure, medium flow-rate capabilities.
Performance is also dependant on the fan enclosure and duct design.
Spiral housing designs with inducers, diffuses are more efficient as compared to square housings.
Inlet and outlet conditions (whirl and turbulence created by grills, dampers, etc.) can significantly alter fan performance curves from the provided by the manufacturer (which are developed under controlled conditions).
Bends and elbows in the inlet or outlet ducting can change the velocity of air, thereby changing fan characteristics (the pressure drop in these elements is attributed to the system resistance).
All these factors, termed as System Effect Factors, should, therefore, be carefully evaluated during fan selection since they would modify the fan performance curve.
