Fans and Blowers

INTRODUCTION

The words “fan” and “blower” are frequently used interchangeably. In HVAC/R terminology, the term fan may refer to any device used to create artificial currents of air. A blower is generally considered to be a particular type of fan–specifically, a motor-driven centrifugal fan that delivers air to the conditioned space under pressure.

There are two basic types of fans used in HVAC/R applications–axial and centrifugal. They are classified according to the direction of air flow through the impeller (the rotating blades that push the air). In an axial fan, air flow is parallel to the shaft. Figure 1 shows an exploded view of a typical axial fan. In a centrifugal fan, air flow is radial through the wheel, and therefore perpendicular to the shaft. Figure 2 shows an exploded view of the component parts of a centrifugal fan.

As stated previously, an axial fan gets its name from the fact that the direction of air flow is parallel to the impeller’s axis of rotation. Axial fans can handle large volumes of air at low static pressures. They operate at high speeds, and can be noisy. Axial fans are divided into three main groups–propeller, tubeaxial, and vaneaxial.

A propeller fan consists of two or more blades rotating within a ring or frame. Propeller fans are often used as exhaust fans and condenser fans. They normally are not attached to duct systems. A tubeaxial fan is simply a propeller fan mounted in a tube or cylinder. Tubeaxial fans are more efficient than propeller fans, but they create a spiral discharge pattern. A vaneaxial fan is housed in a cylinder like a tube-axial fan, but the vaneaxial fan includes stationary guide vanes designed to remove much of the “swirl” from the discharge air, thus improving the fan’s performance. Note that Figure 1 shows a vaneaxial fan. Vaneaxial fans are more efficient than tubeaxial fans, and can reach higher pressures.

CENTRIFUGAL FANS

A centrifugal fan is similar to a centrifugal pump, except that it “pumps” air instead of water. As shown in Figure 2, the flow of air as it enters the inlet is parallel to the axis of the impeller shaft. Air is redirected within the impeller wheel, and is moved outward by the centrifugal force created by the rotating blades. The scroll combines the air that passes out between the blades into a single, large airstream.

Centrifugal fans are sometimes called “squirrel cage” fans because of the shape of the impeller wheel. They are widely used in HVAC/R applications to distribute conditioned air through ductwork. There are several types of centrifugal fans, categorized according to how the blades are fixed to the impeller. The three most common impeller designs are radial, backward-curved, and forward-curved.

As shown in Figure 3 A, the blades of a radial centrifugal impeller extend straight outward from a central hub, parallel to the radii of the wheel. Centrifugal fans with radial blades provide high pressures at high speeds. They are sometimes used as cooling fans in electric motors.

The blades of a backward-curved centrifugal impeller are inclined in the direction opposite rotation. As shown in Figure 3 B, the blades actually may be either straight or curved. Centrifugal fans with backward-curved blades are generally more efficient than those with forward-curved blades. Backward-curved blade fans rotate at about twice the speed of forward-curved blade fans, and are therefore noisier.

As shown in Figure 3 C, the blades of a forward-curved centrifugal impeller are inclined in the direction of rotation. The forward-curved impeller usually has a larger number of smaller blades than either the backward-curved impeller or the radial impeller. The lower speeds of forward-curved blade fans minimize shaft and bearing size. As a result, centrifugal fans with forward-curved blades can be made smaller, lighter, and less costly. There is, however, a sacrifice in efficiency, which means increased power. Also, although fans with forward-curved blades have a wide operating range, a risk of overloading the motor exists if static pressure decreases. (Overloading is normally not a problem with backward-curved blade fans).

MIXED-FLOW FANS

As the name implies, a mixed-flow fan utilizes principles of both axial and centrifugal impeller designs. Visualize a propeller mounted in the inlet of a centrifugal impeller. (In practice, the design is less obvious.) Air flow in a mixed-flow fan is primarily parallel to the axis of rotation, but the shape of the blade introduces a radial flow component (generally small) at the discharge. The impeller may be mounted in either a cylindrical or volute housing. (A volute is the outer casing that contains the spinning rotor in a centrifugal device.)

FAN CLASSIFICATIONS

Most fans are rated according to industry standards developed by ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) and AMCA (Air Movement and Control Association). Fan manufacturers publish rating tables to show the speeds and static pressures that a particular fan is capable of producing. If the specifications exceed certain given conditions, the “class” of the fan changes. The class indicates the structural limitations of the wheels, bearings, and housing. Under the most recent AMCA standards, there are three classes of centrifugal fans. (There used to be four.) Figure 4 shows how fan classification standards are designated for single-width centrifugal fans with backward-curved blades. To qualify as meeting the requirements of a specific class (Class I, II, or III), a fan must be capable of operating safely at every point of rating on or below the “minimum performance” limit for that class.

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