Fans, energetyka, wentylatory
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//-->MECH7350 Rotating Machinery11. Fans11. FANS(This section is taken mainly from Black and Veatch)11.1 Types of FansFans are used in electricity generation to supply or exhaust air or flue gas. They increase thepressure of a flow stream to offset the pressure losses that result from system resistance.Centrifugal fans move air or gas perpendicular to the impeller shaft. Axial fans move air orgas parallel to the impeller shaft.11.1.2 Centrifugal FansFigures 11.1 and 11.2 show the construction and components of centrifugal fans. Centrifugalfan blades are mounted in an impeller that rotates within a spiral housing. They are designedwith either one or two inlets, and normally run at constant speed with flow and pressurecontrolled with inlet dampers.Fig. 11.1 Double-inlet centrifugal fan (from Black and Veatch).11-1MECH7350 Rotating Machinery11. FansThe performance of centrifugal fans is highly dependent on the type of blades used; backwardcurved, straight or radial tip. This is shown in Fig. 11.3.Fig. 11.2 Centrifugal fan components (from Black and Veatch).Fig. 11.3 Fan blade types and performance curves (from Black and Veatch).11-2MECH7350 Rotating Machinery11. FansBackward curved blades have an aerofoil shape and are most widely used in power plantoperations. Their advantages are:••••Highest efficiency, over 90 percentVery stable operationLow noiseCapacity for high speed operationThey should be avoided where large or adhesive particles are present.11.1.3 Axial FansSingle-stage (one rotor and one set of blades) axial fans are typically used in a forced draftsituation on a balanced draft steam generator. When axial fans are designed for induced draftservice, the higher pressure requirements normally dictate use of a two-stage fan such as inFig. 11.4. Axial fans are driven by single-speed or two-speed motors, with flow and outputpressure controlled by varying blade pitch with a hydraulically actuated mechanism. Axialfans can maintain higher efficiencies at various steam generator loads than can constant-speed centrifugal fans controlled with inlet dampers.Fig. 11.4 Two-stage axial fan (from Black and Veatch).Use of lightweight fan blade materials such as aluminium or magnesium reduces the strengthrequirements of the fan hub and the fan blade thrust bearing loads. This reduces equipment11-3MECH7350 Rotating Machinery11. Fanscosts but in erosive situations blade protection might be needed, such as a hardened steel stripon the leading edge.11.2 System ResistanceA loss of pressure occurs when gas is forced through a duct system. This loss of pressure iscalled system resistance, and is composed of two parts; friction losses and dynamic losses.Friction losses mainly occur at the walls of the duct system and can be quantified by thefollowing empirical equation:∆PfrictionwherefρV2L=Dg(11.1)∆Pfriction= frictional pressure lossf= a dimensionless friction factorρ=gas densityV= gas velocityL= duct lengthD= duct diameterg =gravitational constantDynamic losses occur at changes of direction in gas flow and at sudden duct enlargementsand contractions. They are sometimes called velocity pressure losses. Provided that the gasflow is turbulent, as is the case for nearly all combustion air and flue gas handling systemsand equipment, dynamic losses can be empirically quantified by the following equation:∆Pdynamicwhere1ρKV2=2g(11.2)K= system constant based on geometry of duct and determined experimentally.Then∆P= ∆Pfriction+ ∆Pdynamic(11.3)When the system geometry is not modified (i.e. whenf, L, D, gandKare held constant) onlychanges in gas density and flow rate cause the system resistance to change. Once thepressure test has been made at a known gas flow rate and density, the system resistance can11-4MECH7350 Rotating Machinery11. Fansbe predicted for any flow condition. If the gas density is not changed, system resistance canbe plotted against flowQwhich is proportional toV.11.3 Fan PerformanceFans are used to provide the pressure necessary to overcome system resistance.Fanperformance characteristics are developed from test data. A typical fan performance curve(in American units; in. wg = inches water gauge, acfm = actual cubic feet per minute) for acentrifugal fan operating at a given speed and gas density is shown in Fig. 11.5.Fig. 11.5 Typical centrifugal fan performance curve (from Black and Veatch).Once the fan is installed into a ductwork system, the intersection of the system resistancecurve and the fan characteristic curve defines the system operating point, as shown in Fig.11.6.Fig. 11.6 Typical installed centrifugal fan performance curve (from Black and Veatch).11-5
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