Centrifugal Fans

  • Air enters the fan axially and discharged perpendicular to fan axis.
  • Some times called blowers or squirrel cage fans
  • high static pressure and ducted systems applications.
  • higher efficiency than axial fans.
  • Motor can be onside air stream provides energy saving in cooling applications.
  • Motors may be overloaded and burn out if fan is operated outside certain pressure range.

Centrifugal fan types are:

  • Airfoil.
  •  Backward Inclined.
  • Forward Curve.
  • Radial Blade or radial tip.
  • In line tubular Centrifugal.

Backward Inclined, Airfoil, Forward Curved, and Radial Tip.

1- Airfoil

  • The highest efficiency of all of the centrifugal fans. up to 83%.
  • The highest speed of the centrifugal fans.
  • Suitable for higher static pressure, up to 14″ (3.5 KPa) .
  • 9 to 16 blades of airfoil curved away from the direction of rotation.
  • Deep blades for efficient expansion with the blade passages.
  • Air leaves the impeller at a velocity less than its tip speed.
  • Not suitable for corrosive and/or erosive environment
  • Suitable for large air handling units 

2- Backward Inclined

  • High efficiency up to 83%.
  • Suitable for higher static pressure, up to 12″ (3.0 KPa).
  • Higher speed than forward curved. More noise.
  • Backward inclined or backward curved blades are single thickness
  • 8 to 16 blades.
  • Deep blades for efficient expansion with the blade passages.
  • Air leaves the impeller at a velocity less than its tip speed.
  • Requires larger shaft.
  • Fan Selection: Working range is from 40% to 80% of full range flow rate. The larger impeller diameter the more efficient for specific flow rate and static pressure. So fan shall be selected by optimizing between efficiency, cost and space availability.

3- Forward Curved

  • Suitable for high air volumes at low static pressure.
  • Less efficiency than airfoil and backward curved fans.
  • The maximum efficiency is 58-67%.
  • Lower cost than airfoil and backward inclined.
  • 24 to 64 shallow blades.
  • Less speed than airfoil and backward curved fans.
  • Air leaves the impeller at velocities greater than the impeller tip speed.
  • Fan Selection: Ideal selection at right side of maximum static pressure. Working range is from 40% to 70% of full range flow rate. Fan is unstable when overloaded. Do not select fan in the pressure curve to the left of peak pressure.
  • Suitable for low pressure applications like domestic furnace, medium size air conditioning units up to 25 tons (87.0Kw).

 

4- Radial, blades:

  • Simple in its design. straight blades.
  • Low efficienc, 69-75% at peak.
  • Higher mechanical strength.
  • Generally 6 to 12 blades.
  • Suitable for low air volume at high static pressure.
  • Typical Applications: High pressure industrial. Radial centrifugal fans are rarely used in HVAC applications.

 

5- Inline tubular Centrifugal

  • Single width airfoil wheel in a cylinder, air is discharged radially, then vanes are used to deflect air parallel with the fan shaft to straighten the flow.
  • The selection range is about 55-80% of wide open volume.
  • Suitable for applications up to 10″ (2.5KPa) Static pressure.
  • Lower efficiency than backward-curved fan.
  • Has straight-through flow.

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