Xpelair and AirVent Products. Axial flow fans utilise a propeller type blade to move the air in line with the axis of the motor. They are very efficient at moving large volumes of air through walls or windows into free air.  Centrifugal fans incorporate drum type blades and move the air at 90 degrees to the axis of the motor. They do not achieve the same air volumes as axial fans but generate much higher pressures which make them ideal for ducted installations