Centrifugal and Axial Compressor 2
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Transcript of Centrifugal and Axial Compressor 2
TURBO COMPRESSORS
Centrifugal and Axial flow Compressor
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Contents
Advantages and disadvantages
Characteristics shape
Blade angle
Fans law effects
Surge
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Advantages
In 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.
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Disadvantages
Less 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 Principle
The 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.
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Main Components of Centrifugal Compressor
Impeller
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.
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Compressor Cassing
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Rotor Assembly
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Stationary Components
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Rotating Components
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Gas Flow
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Compression
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Absolute Velocity “C”
Absolute 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 gas
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Blade Velocity
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Velocity Triangle
Tangential 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 mass
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Velocity Triangle
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Blade Angle
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Performance Curve
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Performance Curve
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Operating Area
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Operating Condition
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Operating Condition
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Stability Range and Pressure Raise to Surge
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Stability Range and Pressure Raise to Surge
Stability 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 speed
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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 Application
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Choke
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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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Surging
The smaller the angle the
longer is the flow path of the
gas between impeller tip and
the diffuser outer diameter.
when the path becomes long
enough that flow momentum
is dissipated by the friction
to the point where pressure
gained by diffusion causes a
reversal of flow surge results.
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Figure Inception of surge
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Antisurge Control Loop
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Anti Surge Loop
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Surge
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Stall
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Stall
This 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 Condition
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Off – Design Condition: Molecular Weight
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Off – Design Condition: Suction Pressure
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Off – Design Condition: Suction Temperature
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Fan laws
With 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)2 G. BHP1/BHP2=(D1/D2)2
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Impeller Diameter using inlet volume
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Efficiency