Fans (1)

1. Introduction

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) defines a fan as an “air pump that creates a pressure difference and causes airflow. The impeller does the work on the air, imparting to it both static and kinetic energy, varying proportion depending on the fan type” (ASHRAE 1992).

2. Types of Fans (Bodman and Shelton 1995)

The two general types of fans are axial-flow and centrifugal. With axial-flow fans, the air passes through the fan parallel to the drive shaft. With centrifugal fans, the air makes a right angle turn from the fan inlet to outlet.

2.1 Axial Fan
Axial-flow fans can be subdivided based on construction and performance characteristics.
• Propeller fan – The basic design of propeller fans enhances maintenance to remove dust and dirt accumulations. The fan normally consists of a “flat” frame or housing for mounting, a propellershaped blade, and a drive motor. It may be direct drive with the wheel mounted on the motor shaft or belt driven with the wheel mounted on its own shaft and bearings.
• Tube-axial fans – A tube-axial fan consists of a tube-shaped housing, a propeller-shaped blade, and a drive motor. Vane-axial fans are a variation of tube-axial fans, and are similar in design and application. The major difference is that air straightening vanes are added either in front of or behind the blades. This results in a slightly more efficient fan, capable of somewhat greater static pressures and airflow rates.
2.2 Centrifugal Fans
Often called “squirrel cage” fans, centrifugal fans have an entirely different design (Figure 5). These fans operate on the principle of “throwing” air away from the blade tips. The blades can be forward curved, straight, or backward curved. Centrifugal fans with backward curved blades are generally more efficient than the other two blade configurations. This design is most often used for aeration applications where high airflow rates and high static pressures are required. Centrifugal fans with forward curved blades have somewhat lower static pressure capabilities but tend to be quieter than the other blade designs. Furnace fans typically use a forward curved blade. An advantage of the straight blade design is that with proper design it can be used to handle dirty air or convey materials.

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