Centrifugal fan - AbsoluteAstronomy.com

A centrifugal fan is a mechanical device for moving air or other gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

es. It has a fan

Fan (mechanical)

A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...

wheel

Wheel

A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

composed of a number of fan blade

Blade

A blade is that portion of a tool, weapon, or machine with a cutting edge and/or a pointed tip that is designed to cut and/or puncture, stab, slash, chop, slice, thrust, or scrape animate or inanimate surfaces or materials...

s, or ribs, mounted around a hub. As shown in Figure 1, the hub turns on a driveshaft

Driveshaft

A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...

that passes through the fan housing. The gas enters from the side of the fan wheel

Wheel

, turns 90 degrees and accelerates due to centrifugal force

Centrifugal force

Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

as it flows over the fan blades and exits the fan housing.
Centrifugal fans can generate pressure increases in the gas stream. Accordingly, they are well-suited for industrial

Industry

Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...

processes and air pollution

Air pollution

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

control systems. They are also common in central heating/cooling systems.

The centrifugal fan was invented by Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n military engineer Alexander Sablukov

Alexander Sablukov

Alexander Alexandrovich Sablukov was a Russian Lieutenant General, engineer and inventor. Sablukov is credited with invention of the centrifugal fan and contribution to the development of centrifugal pump.-References:*...

in 1832, and found its usage both in the Russian light industry (such as sugar making) and abroad.

Fan components

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

Damper

A damper is a device that deadens, restrains, or depresses.Damper may refer to:* Dashpot, a type of hydraulic or mechanical damper,* Shock absorber , a mechanical device designed to dissipate kinetic energy...

Types of drive mechanisms

The fan drive determines the speed of the fan wheel (impeller) 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

Electric motor

An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

. This means that the fan wheel speed is identical to the motor's rotation

Rotation

A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

al speed. With this type of fan drive mechanism, the fan speed cannot be varied unless the motor speed is adjustable.
Often the motor has several electric wires. Air conditioning will then automatically provide faster speed because colder air is more dense.

Belt drive

A set of sheave

Sheave

A sheave is a wheel or roller with a groove along its edge for holding a belt, rope or cable. When hung between two supports and equipped with a belt, rope or cable, one or more sheaves make up a pulley. The words sheave and pulley are sometimes used interchangeably.A sheave can also refer to a...

s are mounted on the motor shaft and the fan wheel shaft, and a belt transmits the mechanical energy from the motor to the fan.

The fan wheel speed depends upon the ratio

Ratio

In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...

of the diameter of the motor sheave to the diameter of the fan wheel sheave and can be obtained from this equation:

where:
	= fan wheel speed, revolutions per minute
	= motor nameplate speed, revolutions per minute
	= diameter of the motor sheave
	= 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 mechanical couplings.

Bearings

Bearings are an important part of a fan.

Sleeve-ring oil bearings are used extensively in fans. Some sleeve-ring bearings may be water-cooled. Water-cooled Sleeve bearings are used when hot gases are being moved by the fan. Heat is conducted through the shaft and into the oil which must be cooled to prevent overheating of the bearing.

Since lower speed fans which have a bearings in tough-to-get-to spots, Grease-packed anti-friction bearings are used.

Fan dampers and Vanes

Fan dampers are 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 that is used to control gas flow. Dampers on the inlet side (inlet vanes) are designed to control gas flow by changing the amount of gas or air admitted to the fan inlet.

Inlet dampers (Inlet vanes) reduce fan energy usage due to their ability to affect the airflow pattern into the fan.

Fan ribs

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 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. The backward curvature mimics that of an airfoil cross section and provides good operating efficiency with relatively economical construction techniques. These types of fan wheels are used in fans designed to handle gas streams with low to moderate particulate loadings . They can be easily fitted with wear protection but certain blade curvatures can be prone to solids build-up.

Backward curved fans can have a high range of specific speeds but are most often used for medium specific speed applications-- high pressure, medium flow applications.

Backward-curved fans are much more energy efficient than radial blade fans and so, for high power applications may be a suitable alternative to the lower cost radial bladed fan.

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, but it is often characterized by greater noise output. High speeds, low volumes, and high pressures are common with radial fans, and are often used in vacuum cleaner

Vacuum cleaner

A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...

s, pneumatic material conveying systems, and similar processes.

Centrifugal fan ratings

Ratings found in centrifugal fan performance tables and curves are based on standard air SCFM

SCFM

Standard cubic feet per minute is the volumetric flow rate of a gas corrected to "standardized" conditions of temperature and pressure. It is equivalent to the molar flow rate by the ideal gas law. Conversion of SCFM to mass flow requires knowledge of the mixture averaged gas molecular weight. ...

. Fan manufacturers define standard air as clean, dry air with a density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

of 0.075 pounds mass per cubic foot (1.2 kg/m³), with the barometric pressure at sea level of 29.92 inches of mercury (101.325 kPa) and a temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

of 70 °F (21 °C). Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and power

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

.

At higher than standard elevation (sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

) and higher than standard temperature, air density is lower than standard density. Air density corrections need to be taken into account for centrifugal fans that are specified for continuous operation at higher temperatures. The centrifugal fan will displace a constant volume of air in a given system regardless of the air density.

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 (93.3 °C) air weighs only 80% of 70 °F (21.1 °C) air, the centrifugal fan will create less pressure and require less power. To get the actual pressure required at 200 °F (93.3 °C), 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 power at 200 °F (93.3 °C), the designer would have to divide the power 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 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

Air Movement and Control Association

The Air Movement and Control Association International, Inc. is a nonprofit organization that creates standards for, tests, and rates Heating, Ventilating, and Air Conditioning equipment...

(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. For this reason, fans must be rated in SCFM.