Axial Flow Fans
This type has various blade shapes including aerofoil, sickle, paddle, and variable pitch. Axial fans are used for relatively high flow rates and low pressures with flow parallel to the axis of fan. They also have low power input. They are generally selected for simple extraction or cooling applications with very low system resistance, such as moving air from one large space to another, desk fans, ventilation, and condenser cooling in refrigeration.
Typical impeller blade construction types includes paddle, radial, backward curved and forward curved. Characteristics are low flow rates and high pressures with flow perpendicular to blower axis. Air enters around center of the fan and exits around the outside. Centrifugal fans with paddle impellers are used when debris will pass through the system because this configuration prevents any clogging. Fans with backward curved blades produce less air volume than Axial units, but generate considerably more pressure and are the least hungry for power in the centrifugal range. They can also be produced as multistage centrifugal fans to give even higher pressures.
By comparison, centrifugal fans with forward curved blades develop the best airflow and pressure requirement in size by size comparison, although they require extra motor power. Typical applications for centrifugal fans include air handling units, process heating and cooling, electronic cooling and boiler combustion air.
Mixed Flow Fans
These combine the features of Axial and Centrifugal Fans by employing the air movement techniques of both. Air enters the inlet side of the fans and exhausts in an axial route from the outlet side. The pressure development of the mixed flow fan is greater that an Axial fan and is more akin to the Centrifugal fan.
These barrel-shaped fans have curved forward blades that are generally many times longer than any given diameter. Tangential fans tend to scoop the air up and deliver it at relatively low pressure over a long length, ideal for cabinet cooling or process air coverage. Power required is relatively low. The main advantage of Tangential over axial fans is that they provide extremely smooth, laminar airflow over the whole frame width. This makes them suitable for applications such as air conditioning systems, air curtains, computer systems, instrument racks, convector fans, ovens, de-humidifiers and ventilators.