Centrifugal fan

(PD) Image: U.S. Environmental Protection Agency
Figure 1: Centrifugal fan components

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

Centrifugal fans can generate pressure rises in the gas stream. They are commonly used to move the air in central heating and cooling systems. They are also used in some air pollution control systems as well as in various industrial processes

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 rotational speed of the fan wheel and the extent to which this speed can be varied. There are three basic types of fan drives.

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.

(PD) Image: U.S. Environmental Protection Agency
Figure 2: Belt-driven centrifugal fan.

Belt drive

As shown in Figure 2, belt driven fans use multiple belts that rotate in a set of sheaves mounted on the motor shaft and the fan wheel shaft. The belts transmit the mechanical energy from the motor to the fan.

The fan wheel rotational speed (expressed in revolutions per minute, abbreviated as RPM) depends upon the ratio of the motor sheave diameter to the diameter of the fan wheel sheave and can be obtained from this equation:

$rpm_{\mathrm {\,fan} }=rpm_{\mathrm {\,motor} }\,{\bigg (}{\frac {\,D_{\mathrm {motor} }}{D_{\mathrm {fan} }}}{\bigg )}$

where:
$rpm_{\mathrm {\,fan} }$	= fan wheel speed in revolutions per minute
$rpm_{\mathrm {\,motor} }$	= motor nameplate speed in revolutions per minute
$D_{\mathrm {motor} }$	= diameter of the motor sheave
$D_{\mathrm {fan} }$	= diameter of the fan wheel sheave

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.

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 hydraulic or magnetic couplings.

Fan dampers

Fan dampers adjustable, movable plates used to control gas flow into and out of the centrifugal fan. They 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 used to control gas flow. Dampers on the inlet side are designed to control gas flow as well as 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.

Fan blades

(PD) Image: U.S. Environmental Protection Agency
Figure 3: Types of centrifugal 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.

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 particulate matter. 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 performance ratings

In the United States, tabulated fan performance ratings are available from fan manufacturers. For each of the manufacturer's fans, such tabulations list how much discharge pressure is created for a given volume of standard air handled by the fan and how much power must be provided by the fan's drive motor. The specific measurement units used in such tabulations are:

• Discharge static pressure created, denoted as (SP), in inches of water which may be denoted either as (in H₂O), (" H₂O) or ("). [Note: 1 inch of water pressure is 248.8 Pa]

• Volume of standard air being handled in cubic feet per minute (CFM). [Note: 1 cubic foot is 0.0283 m³ ]

• The amount of power that must be provided by the fan's drive motor in brake horsepower (BHP). [Note: 1 brake horsepower is 745.7 watts or 745.7 J/s]

Fan manufacturers define standard air as clean, dry air at an absolute pressure of 14.696 psi (101.325 kPa) and a temperature of 70 °F (21 °C), which amounts to air at a standard density of 0.075 lb/ft³ (1.20 kg/m³). Thus, the fan performance rating tables are based on moving standard air at 70 °F and at an absolute pressure of 14.696 psi, and having a density of 0.075 lb/ft³.

A centrifugal fan is a constant volume device that will move the same amount of air at two different temperatures. Thus, a centrifugal fan that moves 2,000 ft³/min (56 m³/min) at 70 °F (21 °C) will also move 2,000 ft³/min at 250 °F (121 °C). However, since a given volume of 250 °F air weighs much less than the same volume of 70 °F air, the fan will create a lower discharge pressure and will require less drive motor power when moving the hotter air.

Thus, for example, when selecting a fan to handle 2,000 ft³/min at 250 °F, an air density correction factor must be applied to select the proper size fan to produce a discharge pressure of say 6 inches of water (1493 Pa). The air density correction factor is defined as the density of air at the standard temperature of 70 °F to the density of air at the non-standard conditions . In this example, since 250 °F air weighs only 75% of 70 °F air, the air density correction factor is 1.33 (i.e., 1.0 ÷ 0.75). Thus, the fan to be selected must be one rated to produce a discharge pressure of 8 inches of water (i.e., 1.33 × 6) when handling 2,000 ft³/min of standard air at 70 °F. If the performance rating table shows such a fan to require 4 brake horsepower, then the operating horsepower would only be 3 horsepower (i.e., 4 ÷ 1.33) when handling the lighter air at 250 °F.

The product catalogs of most fan manufacturers usually contain a tabulation of air density correction factors that takes into account both the temperature of the air to be handled as well as the altitude at which the fan is to operate. Higher altitude locations have lower atmospheric air pressures and therefore lower air densities.

Fan laws

In a steady-state system, the basic fan laws governing the effect of fan wheel rotational speed (RPM) changes on volume (CFM), discharge static pressure (SP) and the drive motor operating horsepower (BHP) are:

${\frac {CFM_{2}}{CFM_{1}}}={\bigg (}{\frac {RPM_{2}}{RPM_{1}}}{\bigg )}$

     ${\frac {SP_{\,2}}{SP_{\,1}}}={\bigg (}{\frac {RPM_{2}}{RPM_{1}}}{\bigg )}^{2}$

${\frac {BHP_{\,2}}{BHP_{\,1}}}={\bigg (}{\frac {RPM_{2}}{RPM_{1}}}{\bigg )}^{3}$

Air Movement and Control Association (AMCA)

The centrifugal fan performance tables provide the fan RPM and power 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.

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 are on the same basis and may be compared.