196
6 Fans and Flow Control Devices
Fig. 6.1 Pathlines in a centrifugal fan
6.2.1 Parts of a Fan
The fans consist of the following parts:
• Impeller: Rotating part that communicates pressure and movement to the air. It
consists of the blades, which may or may not be adjustable, and their support–
normally a shaft.
• Housing or casing: The stationary part, protects the impeller and guides the air in
its route.
• Diffusers and/or evasés: These are the elements of gradually increasing area,
which when coupled to the discharge of the fan transform part of the dynamic
pressure at the outlet into static pressure.
6.2.2 Classification According to the Direction of the Flow
The most normal classification uses the direction of the air driven out of the fan with
regard to the axis of rotation of the blades or vanes.
2 Accordingly, we have:
Centrifugal Fans
The air leaves the fan in a 90°-direction to the shaft (Fig. 6.1). The very word
centrifugal, meaning “away from the centre,” gives us an idea of it. The most common
classification distinguishes centrifugal and axial fans.
3
2 Other classifications consider the type of drive: direct (if impeller or propeller have a common
shaft with the motor) and indirect (via transmission), or the reached pressure (low: <70 Pa, high:
>3000 Pa).
3 In addition to these, there are also cross flow fans (air path normal to the shaft at both the inlet and
the outlet) and helico centrifugal fans (air inlet like in axial fans and air outlet like in centrifugal
fans).
6 Fans and Flow Control Devices
Fig. 6.1 Pathlines in a centrifugal fan
6.2.1 Parts of a Fan
The fans consist of the following parts:
• Impeller: Rotating part that communicates pressure and movement to the air. It
consists of the blades, which may or may not be adjustable, and their support–
normally a shaft.
• Housing or casing: The stationary part, protects the impeller and guides the air in
its route.
• Diffusers and/or evasés: These are the elements of gradually increasing area,
which when coupled to the discharge of the fan transform part of the dynamic
pressure at the outlet into static pressure.
6.2.2 Classification According to the Direction of the Flow
The most normal classification uses the direction of the air driven out of the fan with
regard to the axis of rotation of the blades or vanes.
2 Accordingly, we have:
Centrifugal Fans
The air leaves the fan in a 90°-direction to the shaft (Fig. 6.1). The very word
centrifugal, meaning “away from the centre,” gives us an idea of it. The most common
classification distinguishes centrifugal and axial fans.
3
2 Other classifications consider the type of drive: direct (if impeller or propeller have a common
shaft with the motor) and indirect (via transmission), or the reached pressure (low: <70 Pa, high:
>3000 Pa).
3 In addition to these, there are also cross flow fans (air path normal to the shaft at both the inlet and
the outlet) and helico centrifugal fans (air inlet like in axial fans and air outlet like in centrifugal
fans).