CENTRIFUGAL FANS are strong and versatile rotating machines that move air or other gases in many industrial applications.

The centrifugal fan consists of a wheel with several blades (impeller) set to rotate in a casing of spiral form.
The gas passes through the casing inlet in axial direction then it is turned through a right angle and flows radially outwards between the blades .
The pressure and velocity of the gas are both increased under the action of centrifugal force. The scroll form of the casing serves to convert the kinetic energy into potential (static pressure) energy.

Impellers are normally fabricated from steel plate in qualities depending on the intended application and can be of single or double inlet
The side plate at the gas inlet side can be either conical or flat, parallel in this case to the central plate.
In order to avoid turbulences, a precisely machined inlet ring is fixed to the gas entrance. This ring also assures the correct gap with the inlet conical duct.
 

The absorbed power and efficiency of a fan strongly depends on the design of the wheel and blades.

Centrifugal fans may be classified into three basic types according to blade configuration:

1. Forward curved blades, the concave blade faces the direction of rotation (1).
   Theoretically, this geometry produces the highest pressure rise for a given tangential speed. Nevertheless, losses in the transformation of the gas kinetic energy are important and can produce a relatively poor efficiency.
   
2. Radial blade (paddle wheel), perpendicular to the direction of the rotation (2).
   Efficiency is not high and they are employed when low flow and medium pressure increments are required.
   The ability of this type to adapt to harsh environment answers for their use in applications that handle streams with high level of particulate where auto-cleaning is valued.
   
   
3. Backward curved blades, inclined away from the direction of the rotation.
   Blades can be flat single thickness (3), curved single thickness (4) and (5) or curved airfoil (6).
   For a given pressure and diameter these fans have to rotate at higher speed but, due to the shape of the channels between the blades, they work with an almost optimum efficiency .
   The horsepower characteristic curve of these fans is not -overloading, that is, the horsepower increases to a maximum as airflow increases and then drops again as airflow goes to maximum.

Centrifugal fans are most of times moved by electric drives. Occasionally other drives as steam or gas turbines can be employed. Turbines can also be installed conjointly with the electric drive in a way that permits one or other engine to be used at convenience.

The GRUBER standard of fan-drive arrangements for an overhung impeller .

M arrangement
The impeller is directly mounted on the shaft of the electric motor.
The fan is small in size, power is limited and the fan turning speed corresponds to the motor speed.

TV arrangement
Fan and drive speeds are different.
The fan wheel is overhung on a shaft driven through pulleys and belts by a motor mounted on tensioning rails at floor level.

TVM arrangement 
Monoblock assembly in which the motor and the fan casing have a common base. Power transmission is made through pulleys and belts. It is used when fan speed must be different to primary drive speed.

D arrangement 
In this case there is a direct drive connection made by means of an elastic coupling. The wheel is overhung on a shaft that turns on a rugged monoblock support or on independent bearings. It is recommended for continuous service and hot gases.

Some of the most typical GRUBER fan-drive arrangements when the impeller is mounted between shaft bearings.

BD arrangement
Double inlet fan mounted between bearings on a passing shaft. Fan and drive are arranged on an integral supporting structure equipped with vibration isolator blocks.
This is an optimum disposition for fans that must be installed on structures exposed to vibrations. The antivibration blocks decrease the la amplitude of the dynamic forces caused by the rotation.

EPD arrangement
Single inlet fan mounted between bearings on a passing shaft. The assembly is placed on an elevated structure made by concrete pedestals.
This is a non complex and very reliable arrangement. The bearing blocks are placed on pedestals of reduced height and the drive sits on a metal frame or directly over the concrete. The casing is anchored to the floor by means of its own frame.

BDs arrangement
Double inlet fan furnished with an integral frame that give stiffness to the motor-fan assembly. This frame is anchored to a concrete structure in which is lodged the lower part of the fan casing.

 Blast direction designation direction of the rotation

The rotation direction and fan blast position should be chosen according to the kind of installation in which the fan is integrated. It is the customer who sets these constraints .
The reference blast direction is marked in the adjoining figure as "GD360" and it is related to a wheel turning clockwise when the fan is viewed from the drive side. In this reference. Arrangement the gas flows upwards in vertical direction
All the other standard blast directions are shown in the figure. We can also supply fans with the blast directed otherwise.