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1、Centrifugal fanA centrifugal fan (also squirrel-cage fan , as it looks like a hamster wheel) is a mecha ni cal device for moving air or gases. It has a fan wheel composed of a nu mber of fan blades, or ribs, moun ted around a hub. As show n in Figure 1, the hub tur ns on a driveshaft that passes thr
2、ough the fan housing. The gas enters from the side of the fan wheel, turns 90 degrees and accelerates due to cen trifugal force as it flows over the fan blades and exits the fan hous inFigure 1Cen trifugal fans can gen erate pressure rises in the gas stream. Accordi ngly, they are well-suited for in
3、 dustrial processes and air polluti on con trol systems. They are also com mon in cen tral heati ng/cooli ng systems.1. Fan comp onentsThe major comp onents of a typical cen trifugal fan in clude the fan wheel, fan hous ing, drive mecha ni sm, and inlet an d/or outlet dampers.2. Types of drive mecha
4、 ni smsThe 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 drivSk 2.1Direct driveThe fan wheel can be linked directly to the shaft of an electric motor. This means that the fan wheel speed is identical to
5、 the motors rotational speed. With this type of fan drive mecha ni sm, the fan speed cannot be varied uni ess the motor speed is adjustable.2.2.1 Belt driveFigure 2: Cen trifugal fan with a belt driveBelt driven fans use multiple belts that rotate in a set of sheaves mounted on the motor shaft and t
6、he fan wheel shaft.This type of drive mechanism is depicted in figure 2. The belts transmit the mecha ni cal en ergy from the motor to the fan.The fan wheel speed depe nds upon the ratio of the diameter of the motor sheave to the diameter of the fan wheel sheave and can be obtained from this equatio
7、n：(niotcfrwhere:rpmfan:=fan wheel speed, revolutio ns per minuterpm motor :=motor n ameplate speed, revoluti ons per minuteDmotor=diameter of the motor sheaveDfan=diameter of the fan wheel sheaveFan wheel speeds in belt-drive n fans are fixed uni ess the belts slip. Belt slippage can reduce the fan
8、wheel speed several hun dred revoluti ons per minute (rpm).2.2Variable driveVariable drive fans use hydraulic or magn etic coupli ngs (betwee n the fan wheel shaft and the motor shaft) that allow con trol of the fan wheel speed in depe ndent of the motor speed. The fan speed con trols are ofte n in
9、tegrated into automated systems to mai ntai n the desired fan wheel speedsAn alter nate method of vary ing the fan speed is by use of an electr onic variable-speed drive which controls the speed of the motor driving the fan. This offers better overall en ergy efficie ncy at reduced speeds tha n mech
10、a ni cal coupli ngs.2.3 Fan dampersFan dampers are used to con trol gas flow into and out of the cen trifugal fan. They may be in stalled 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
11、inlet side are desig ned to con trol gas flow and to cha nge how the gas en ters the fan wheel.Inlet dampers reduce fan energy usage due to their ability to affect the airflow pattern into the fan 里3. Backward-curved bladesBackward-curved blades, as in Figure 3(b), use blades that curve aga inst the
12、 directi on of the fan wheels rotati on. The backward curvature mimics that of an airfoil cross secti on and provides good operati ng efficie ncy with relatively econo mical con struct ion tech niq ues. These types of fan wheels are used in fans desig ned to han dle gas streams with low to moderate
13、particulate load in gs. They can be easily fitted with wear protecti on but certa in blade curvatures can be prone to solids build-up.Backward curved fans can have a high range of specific speeds but are most ofte n used for medium specific speed applicati on s- high pressure, medium flow applicati
14、ons.Backward-curved fans are much more en ergy efficie nt tha n radial blade fans and so, for high horsepower applicatio ns may be a suitable alter native to the lower cost radial bladed fan.4. Straight radial bladesRadial fan blades, as in Figure 3(c), exte nd straight out from the hub. A radial bl
15、ade fan wheel is ofte n used on particulate-lade n 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 com mon with radial fans, and are ofte n used in vacuum clea ners
16、, pn eumatic material con vey ing systems, and similar processes.(a/ Fcrviard Curved(Jb) BzK-kwardl Curvel屯R 尺耳日Figure 35. Cen trifugal fan rati ngsRati ngs found in cen trifugal fan performa nee tables and curves are based on sta ndard air SCFM. Fan manu facturers defi ne sta ndard air as clea n, dry air with a den sity of 0.075 pounds mass per cubic foot (1.2kg/n)3with the barome
