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