A centrifugal fan is a mechanical device that is used to move air or gas. A fan wheel mounted on a hub inside a casing facilitates this movement. The fan wheel is rotated by an external force, either an electric motor or an internal combustion engine. The fan consists of blades or ribs arranged around a hollow hub. The air or gas to be moved enters the housing through the side of the fan wheel into the hollow hub and then turns ninety degrees to flow over the fan blades. The rotating fan then causes the air to accelerate by imparting centrifugal force to it, hence the name. The accelerated air or gas then exits the housing.
These fans are capable of moving the air with increased pressure, making them well suited for use in pollution control systems, central cooling or heating systems, and as leaf blowers in inflating structures. In addition, by mounting the fan on a heat sink, coupled with metal fins, high cooling capacity in a small scale can be achieved. There are various types of centrifugal fans and they are classified based on the orientation of the fan blades as well as the drive mechanism of the blade.
Orientation of Fan Blade:
Based on AMCA standard 99-2406-83, centrifugal fans can be classified into three basic blade orientation styles with further sub-classifications based on the type of blade:
Radial BladesStraight Radial Blade
Radial Tip Blade
Open Radial Blade
Backward Inclined Blades
Backward Inclined Flat Blades
Backward Inclined Curved Blades
Backward Inclined Airfoil Blades
Forward Inclined Blades
Forward Inclined Flat Blades
Forward Inclined Curved Blades
Radial Blades
Radial blade fans consist of blades that extend outwards, straight from the hub. These radial fans are characterized by high pressure, high speed, and low volume airflow. These characteristics make them suitable for pneumatic conveying systems as well as vacuum cleaners. This type of blade is less sensitive to solids and usually utilized in applications with particulate-laden gas streams. While the radial tip blades achieve efficiencies up to 83% by reducing the gas turbulence, the open radial blade with overloading power characteristic allows the inclusion of wear plates in case of greater abrasive applications.
Backward-Inclined Blades
The fan blade in this type of fan is inclined backwards against the direction of rotation of the fan. Furthermore, these types of fans are classified into three types: flat blades, curved blades, and airfoil blades. The curved and flat blades are capable of handling gas flows containing fine airborne particles that typically damage the design of an airfoil. The flat blades can also be modified to offer reduced instability as compared to curved blades, and provide efficiencies ranging from 77 to 80%. The airfoil blades on the other hand provide better operating efficiencies ranging from 84 to 91%. These blades are typically used in systems with low or moderate loadings. The curved fans with greater specific speed are more efficient than radial blades and generally utilized for medium flow, high-pressure applications. All the three backward inclined blades have non-overloading characteristics. Lower emission of noise and greater stability in parallel running make these backward inclined blade fans a better choice for most applications.
Forward-Inclined Blades
In this fan type, the blades are oriented such that they lean forward in the same direction as rotation of the fan. These fans can be further classified as flat blade and curved blade fans based on the geometry of the blades. Forward curved blades are capable of delivering greater static pressure and air volume than the airfoil backward blade. The forward curved blade fans operate at lower speeds and are particularly suitable as small fans for a ventilation system. While backward curved blades are more efficient, forward curved blade exhibits a "self-cleaning" property, a fact that has led to their use in applications with high dust concentrations, or to systems with air conveyed material.
Drive Mechanisms:
The drive of the centrifugal fan resolves the fan wheel's speed as well as its variation limit. Fans can be classified based on their drive mechanism as direct drive, belt drive, and variable drive.
Direct Drive
In this type of fan, the wheel is linked to the shaft of the motor directly, ensuring that the speed of the wheel is same as the rotational speed of the motor. In this way, the fan speed can be adjusted by controlling the motor speed. This type of drive mechanism results in a compact fan system with a steady RPM.
Belt Drive
In this type of fan, a group of sheaves is mounted on the motor shaft and the fan wheel shaft. A belt, through which the power is transmitted, connects both the sheave groups. The ratio of the motor sheave's diameter to the wheels' diameter determines the speed of the fan wheel. Using this type of drive mechanism ensures a fixed fan speed until and unless the belt slips.
Variable Drive
This type of drive mechanism typically uses magnetic or hydraulic couplings to control the fan speed. This type of drive allows the user to control the fan speed independent of the speed of the motor.
Ron Bargman, president of Zycon.com, has been fascinated and involved with the engineering and manufacturing processes required to turn ideas into products for over 30 years. Mr. Bargman is a regular contributor of manufacturing theme articles, and his rich industry history provides insight into manufacturing and engineering events and changes that are timely, poignant, and relevant. Through Zycon, he is able to transfer his passion for the industry by assisting engineers, designers and inventors find the services, parts and components that they need to succeed.
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