Centrifugal fan
A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid.
As shown in the animated figure, the hub turns on a driveshaft mounted in bearings in the fan housing.
[13] In 1827, Edwin A. Stevens of Bordentown, New Jersey, installed a fan for blowing air into the boilers of the steamship North America.
[14] Similarly, in 1832, the Swedish-American engineer John Ericsson used a centrifugal fan as blower on the steamship Corsair.
Belt slippage can reduce the fan wheel speed by several hundred revolutions per minute (RPM).
The amount of air moved is non-linear with the motor speed, and must be individually balanced for each fan installation.
Typically this is done at time of install by testing and balancing contractors, although some modern systems directly monitor airflow with instruments near the outlet, and can use the feedback to vary the motor speed.
They are capable of providing lower air flow with a higher increase in static pressure compared to a vane axial fan.
Larger backward-inclined/-curved blowers have blades whose backward curvatures mimic that of an airfoil cross section, but both designs provide good operating efficiency with relatively economical construction techniques.
These types of blowers are designed to handle gas streams with low to moderate particulate loadings [citation needed].
Backward curved wheels are often heavier than corresponding forward-curved equivalents, as they run at higher speeds and require stronger construction.
As a result, the kinetic energy of gas is measured as pressure because of the system resistance offered by the casing and duct.
A diagram called a velocity triangle helps us in determining the flow geometry at the entry and exit of a blade.
[citation needed] Typically due to the higher pressures involved blowers and compressors have much sturdier builds than fans.
Ratings found in centrifugal fan performance tables and curves are based on standard air SCFM.
Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and power.
Air density corrections must account for centrifugal fans that are specified for continuous operation at higher temperatures.
[28] 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.
Centrifugal fans suffer efficiency losses in both stationary and moving parts, increasing the energy input required for a given level of airflow performance.
Friction and flow separation cause impeller blade losses since there is change in incidence angle.
Leakage of some air and disturbance in the main flow field is caused due to the clearance provided between the rotating periphery of the impeller and the casing at the entry.
Flow from the impeller or diffuser expands in the volute, which has a larger cross section leading to the formation of eddy, which in turn reduces pressure head.
