TURBO COMPRESSORS

Centrifugal and Axial flow Compressor

ContentsAdvantages and disadvantages Characteristics shape Blade angle Fans law effects Surge

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AdvantagesIn general Centrifugal Compressors have these characteristics Discharge flow is relatively free of pulsation. Mechanical design permits high through puts, capacity limitation is rarely a problem. Centrifugal Compressors are capable efficient performance over a wide range of pressure and capacities even at constant speed operation. These are relatively small, occupy less space, operate with minimum attention and quieter. Less contamination due to lubricants.SSB

DisadvantagesLess efficient for small volumes. Discharge pressure limitation. Effect of gas density and temperature. Problem of surge phenomenon.

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Centrifugal Force

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Working PrincipleThe working principle is based upon the conversion of dynamic energy into static energy, i.e., from velocity into pressure. The specific compression energy transmitted to the gas by the impeller is called "head." The discharge pressure of a turbo compressor, which is affected by gas inlet conditions, is a function of head.SSB

Main Components of Centrifugal CompressorImpeller The part of centrifugal compressor that moves the gas is the impeller. As the impeller rotates, it moves the gas toward the outer rim of the impeller and its velocity increases. Volute Gas passes from diffuser into the volute. In the volute, the conversion from velocity to pressure continues. Diffuser As the gas leaves impeller, it flows into a passage-way called the diffuser. The diffuser being larger in volume, the velocity of gas decreases and its pressure increases.SSB

Compressor Cassing

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Rotor AssemblyClick to edit Master text styles Second level Third level Fourth level Fifth level

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Stationary Components

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Rotating ComponentsClick to edit Master text styles Second level Third level Fourth level Fifth level

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Gas Flow

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Compression

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Absolute Velocity CAbsolute Velocity of Gas C Its two Components are; A) Relative inlet velocity W ( Parallel to blade) B) Local Blade Velocity U ( Perpendicular to Impeller Radius ) The Combination of absolute, relative and local blade velocities is known as velocity triangle of gasSSB

Blade VelocityClick to edit Master text styles Second level Third level Fourth level Fifth level

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Velocity TriangleTangential Velocity CU The component of absolute velocity C, with the same direction as blade velocity Momentum = Radius of Impeller * Tangential Velocity Work Performed Change in momentum from inlet to discharge of impeller, multiplied by the angular velocity, represents the work performed by impeller blade on unit massSSB

Velocity Triangle

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Blade AngleClick to edit Master text styles Second level Third level Fourth level Fifth level

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Performance Curve

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Performance CurveClick to edit Master text styles Second level Third level Fourth level Fifth level

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Operating Area

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Operating ConditionClick to edit Master text styles Second level Third level Fourth level Fifth level

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Operating Condition

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Stability Range and Pressure Raise to SurgeClick to edit Master text styles Second level Third level Fourth level Fifth level

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Stability Range and Pressure Raise to SurgeStability Range The Volume flow inlet variation that occurs between the operating point and surge limit line at constant speed Pressure Raise to Surge The variation of discharge pressure between the operating point and the surge limit line at constant speedSSB

Turndown Range

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Turndown Range

Turndown Range The variation in inlet volume flow that occurs between the operating point and the surge limit line at the same discharge pressure

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Performance Curve ApplicationClick to edit Master text styles Second level Third level Fourth level Fifth level

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ChokeClick to edit Master text styles Second level Third level Fourth level Fifth level

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What word strikes fear in the heart of centrifugal compressor operators?

SURGE!

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What is Compressor Surge?Compressor surge is an unstable flow pattern in a compressor where the total flow across the airfoil alternately stops, flows backwards, and then flows forward. The surge flow has been defined by some as the flow at which the head flow curve is perfectly flat and below which the head actually decrease.

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SURGING

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Pressure and Flow Variations During Surging

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SurgingThe smaller the angle the longer is the flow path of the gasbetweenimpellertip andthediffuserouter diameter.whenthepath becomeslongenoughthat flowmomentumis dissipatedbythefrictionto thepointwherepressure gainedbydiffusioncauses areversalofflowsurge results.SSB

Figure Inception of surge

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Antisurge Control Loop

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Anti Surge Loop

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SurgeClick to edit Master text styles Second level Third level Fourth level Fifth level

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StallClick to edit Master text styles Second level Third level Fourth level Fifth level

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StallThis usually happens in a low flow stage as it approaches the surge. Stall is caused by the destabilization of the impeller or diffuser flow or by unsteady interaction between the impeller and diffuser

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Off Design ConditionClick to edit Master text styles Second level Third level Fourth level Fifth level

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Off Design Condition: Molecular Weight

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Off Design Condition: Suction PressureClick to edit Master text styles Second level Third level Fourth level Fifth level

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Off Design Condition: Suction Temperature

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Fan lawsWith impeller diameter D held constant: B. Q1/Q2=N1/N2 C. H1/H2=(N1/N2)2 D. BHP1/BHP2=(N1/N2)2 Q = capacity, cfm H = total head, ft ,Bhp = brake horsepower, N = compressor speed, r/min With speed N held constant: E. Q1/Q2=D1/D2 F. H1/H2=(D1/D2)2SSB

G. BHP1/BHP2=(D1/D2)2

Impeller Diameter using inlet volume

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EfficiencyClick to edit Master text styles Second level Third level Fourth level Fifth level

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