Centrifugal fan

  A centrifugal fan (also squirrel-cage fan, as it looks like a hamster wheel) is a mechanical device for moving air or other gases. It has a fan wheel composed of a number of fan blades mounted around a hub. As shown in Figure 1, the hub turns on a driveshaft that passes through the fan housing. The gas enters from the side of the fan wheel, turns 90 degrees and accelerates due to centrifugal force as it flows over the fan blades and exits the fan housing.^[1]

Centrifugal fans can generate pressure rises in the gas stream. Accordingly, they are well-suited for industrial processes and air pollution control systems.

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Fan components

The major components of a typical centrifugal fan include the fan wheel, fan housing, drive mechanism, and inlet and/or outlet dampers.

Types of drive mechanisms

The fan drive determines the speed of the fan wheel and the extent to which this speed can be varied. There are three basic types of fan drives.^[1]

Direct drive

The fan wheel can be linked directly to the shaft of an electric motor. This means that the fan wheel speed is identical to the motor's rotational speed. With this type of fan drive mechanism, the fan speed cannot be varied unless the motor speed is adjustable.

Belt drive

  Belt driven fans use multiple belts that rotate in a set of sheaves mounted on the motor shaft and the fan wheel shaft.

This type of drive mechanism is depicted in figure 2. The belts transmit the mechanical energy from the motor to the fan.

The fan wheel speed depends upon the ratio of the diameter of the motor sheave to the diameter of the fan wheel sheave and can be obtained from this equation:^[1]

Fan wheel speeds in belt-driven fans are fixed unless the belts slip. Belt slippage can reduce the fan wheel speed several hundred revolutions per minute (rpm).

Variable drive

Variable drive fans use hydraulic or magnetic couplings (between the fan wheel shaft and the motor shaft) that allow control of the fan wheel speed independent of the motor speed. The fan speed controls are often integrated into automated systems to maintain the desired fan wheel speed.^[1]

An alternate method of varying the fan speed is by use of an electronic variable-speed drive which controls the speed of the motor driving the fan. This offers better overall energy efficiency at reduced speeds than mechanical couplings.

Fan dampers

Fan dampers are used to control gas flow into and out of the centrifugal fan. The may be installed on the inlet side or on the outlet side of the fan, or both. Dampers on the outlet side impose a flow resistance that is used to control gas flow. Dampers on the inlet side are designed to control gas flow and to change how the gas enters the fan wheel.

Inlet dampers reduce fan energy usage due to their ability to affect the airflow pattern into the fan.^[1]

Fan blades

The fan wheel consists of a hub on which a number of fan blades are attached. The fan blades on the hub can be arranged in three different ways: forward-curved, backward-curved or radial.^[1]

Forward-curved blades

Forward-curved blades, as in Figure 3(a), use blades that curve in the direction of the fan wheel's rotation. These are especially sensitive to particulates. Forward-curved blades are for high flow, low pressure applications.

Backward-curved blades

Backward-curved blades, as in Figure 3(b), use blades that curve against the direction of the fan wheel's rotation. These types of fan wheels are used in fans designed to handle gas streams with relatively low particulate loadings because they are prone to solids build-up. Backward-curved fans are more energy efficient than radial blade fans. Backward curved blades are used for high pressure, low flow applications.

Straight radial blades

Radial fan blades, as in Figure 3(c), extend straight out from the hub. A radial blade fan wheel is often used on particulate-laden gas streams because it is the least sensitive to solids build-up on the blades.

Centrifugal fan ratings

Ratings found in centrifugal fan performance tables and curves are based on standard air SCFM. Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds mass per cubic foot (1.2kg/m³), with the barometric pressure at sea level of 29.92 inches of mercury (1013.25hPa) and a temperature of 70°F (21°C). Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and brake horsepower. The volume of air will not be affected in a given system because a fan will move the same amount of air regardless of the air density.

If a centrifugal fan is to operate at a non-standard density, then corrections must be made to static pressure and brake horsepower. At higher than standard elevation (sea level) and higher than standard temperature, air density is lower than standard density. Centrifugal fans that are specified for continuous operation at higher temperatures need to be selected taking into account air density corrections. Again, a centrifugal fan is a constant volume device that will move the same amount of air at two different temperatures.

If, for example, a centrifugal fan moves 1,000 CFM (28.317m³/min.) at 70ºF it will also move 1,000 CFM at 200ºF (93ºC). Centrifugal fan air volume delivered by the centrifugal fan is not affected by density. However, since the 200°F air weighs much less than the 70ºF air, the centrifugal fan will create less static pressure and will require less brake horsepower. Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and brake horsepower. When a centrifugal fan is specified for a given CFM and static pressure at conditions other than standard, an air density correction factor must be applied to select the proper size fan to meet the new condition. Since 200°F air weighs only 80% of 70°F air, the centrifugal fan will create less pressure. To get the actual pressure required at 200°F, the designer would have to multiply the pressure at standard conditions by an air density correction factor of 1.25 (i.e., 1.0 / 0.8) to get the system to operate correctly. To get the actual horsepower at 200°F, the designer would have to divide the brake horsepower at standard conditions by the air density correction factor.

Air Movement and Control Association (AMCA)

The centrifugal fan performance tables provide the fan RPM and brake horsepower requirements for the given CFM and static pressure at standard air density. When the centrifugal fan performance is not at standard conditions, the performance must be converted to standard conditions before entering the performance tables. Centrifugal fans rated by the Air Movement and Control Association (AMCA) are tested in laboratories with test setups that simulate installations that are typical for that type of fan. Usually they are tested and rated as one of four standard installation types as designated in AMCA Standard 210.^[2]

AMCA Standard 210 defines uniform methods for conducting laboratory tests on housed fans to determine airflow rate, pressure, power and efficiency, at a given speed of rotation. The purpose of AMCA Standard 210 is to define exact procedures and conditions of fan testing so that ratings provided by various manufacturers (ebmpapst etc) are on the same basis and may be compared. For this reason, fans must be rated in SCFM.

See also

• Fan (mechanical)
• Ducted fan
• Standard temperature and pressure
• Wind turbine

References