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1、Cen trifugal fanA cen trifugal fan (also squirrel-cage fan as it looks like a hams ter wheel) is a mecha ni cal device for moving air or gases. It has a fan wheel comp osed of a nu mber of fan blades, or ribs, moun ted around a hub. As show n in Figure 1, the hub turns on a driveshaft that p asses t
2、hrough the fan hous ing. The gas en ters 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 in 申Fari WKFigure 1Cen trifugal fans can gen erate p ressure rises in the gas stream. Accordi ngly, they are well
3、-suited for in dustrial p rocesses and air p olluti on con trol systems. They are also com mon in cen tral heati ng/cooli ng systems.1. Fan componentsThe major components of a typ ical cen trifugal fan in clude the fan wheel, fan hous ing, drive mecha ni sm, and inlet an d/or outlet damp ers.2. Type
4、s of drive mecha ni smsThe fan drive determ ines the sp eed of the fan wheel (i mp eller) and the exte nt to which this sp eed can be varied. There are three basic types of fan drivk 2.1Direct driveThe fan wheel can be lin ked directly to the shaft of an electrie motor. This means that the fan wheel
5、 sp eed is ide ntical to the motors rotatio nal sp eed. With this type of fan drive mecha ni sm, the fan sp eed cannot be varied uni ess the motor sp eed is adjustable.2.2.1 Bel t drive4评亓1麻牯Figure 2: Cen trifugal fan with a belt driveBelt drive n fans use mult iple belts that rotate in a set of she
6、aves moun ted on themotor shaft and the fan wheel shaft.This type of drive mecha nism is depi cted in figure 2. The belts tran smit themecha ni cal en ergy from the motor to the fan.The fan wheel sp eed depends upon the ratio of the diameter of the motor sheave to the diameter of the fan wheel sheav
7、e and can be obta ined from this equation: where:rpm fan=fan wheel sp eed, revoluti ons per minuterpm motor 二motor n ame pl ate sp eed, revoluti ons per minuteDmotor二diameter of the motor sheaveDfan=diameter of the fan wheel sheaveFan wheel sp eeds in belt-drive n fans are fixed uni ess the belts sl
8、i p. Belt sli pp agecan reduce the fan wheel sp eed several hun dred revoluti ons per minute (rpm).2.2Variable driveVariable drive fans use hydraulic or magn etic coup li ngs (betwee n the fan wheelshaft and the motor shaft) that allow con trol of the fan wheel sp eed independent of themotor sp eed.
9、 The fan sp eed con trols are ofte n in tegrated into automated systems to mai ntai n the desired fan wheel sp eed八An alter nate method of vary ing the fan sp eed is by use of an electr onic variable-s peeddrive which con trols the sp eed of the motor driv ing the fan. This offers better overallen e
10、rgy efficie ncy at reduced sp eeds tha n mecha ni cal coup li 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 imp ose a flow resista nee t
11、hat is used to con trol gas flow. Dampers on the 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 en ergy usage due to their ability to affect the airflow p attern into the fan 里3. Backward-curved bladesBackward-curved blades, as
12、 in Figure 3(b), use blades that curve aga inst the directi on of the fan wheels rotati on. The backward curvature mimics that of an airfoil cross secti on and pro vides good op erati ng efficie ncy with relatively econo mical con struct ion tech niq ues. These types of fan wheels are used in fans d
13、esig ned to han dle gas streams with low to moderate p articulate load in gs. They can be easily fitted with wear p rotecti on but certa in blade curvatures can be prone to solids build-up.Backward curved fans can have a high range of sp ecific sp eeds but are most ofte n used for medium sp ecific s
14、p eed app licati on s- high p ressure, medium flow app licati ons.Backward-curved fans are much more en ergy efficie nt tha n radial blade fans and so, for high horse po wer app licatio ns may be a suitable alter native to the lower cost radial bladed fan.4. Straight radial bladesRadial fan blades,
15、as in Figure 3(c), exte nd straight out from the hub. A radial blade fan wheel is ofte n used on p articulate-lade n gas streams because it is the least sen sitive to solids build-up on the blades, but it is ofte n characterized by greater no ise out put. High sp eeds, low volumes, and high p ressur
16、es are com mon with radial fans, and are ofte n used in vacuum clea ners, pn eumatic material con vey ing systems, and similar p rocesses.rCurwedIDBoJnnl OWVHJ Fdi jIFigure 35. Cen trifugal fan rati ngsRati ngs found in cen trifugal fan p erforma nee tables and curves are based on sta ndard airSCFM. Fan manu facturers defi ne sta ndard air as clea n, dry air with a den sity of
