5 centrifugal compressors and axial compressors

7/28/2019 5 Centrifugal Compressors and Axial Compressors

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TURBOMACHINESChapter 5

CENTRIFUGAL compressors

& axial flow compressors

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Introduction

Compressor is a device used to produce large

pressure rise ranging from 2.5 to 10 bar or more. A single stage compressor generally produce a

pressure rise up to 4 bar.

In general centrifugal compressors may be knownas fan, blower, supercharger etc, depending upon

the need to be served.

Fans are low pressure compressors and blowers aremedium pressure compressors.

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Elements of a centrifugal compressor

It mainly consists of (i) inlet casing with the

converging nozzle (ii) the impeller (iii) the diffuserand (iv) the outlet casing.

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Variation of pressure and velocity

Fig below shows the variation of pressure and

velocity in a centrifugal compressor.

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Velocity triangle at the Eye Hub and Tip

In ideal condition the fluid enters the eye section radially

with no whirl component.

The velocity remains constant from hub to tip of the eye.

The tangential velocities of the impeller at the hub (root)

and the tip of the eye are calculated based on the

corresponding hub and tip diameters of the eyerespectively.

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Different vane shape

The impellers may be classified depending on the

exit angle 2 into (i) Backward curved vanes, (ii)Radial vanes and (iii) Forward curved blades.

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The above equation isknown as H-Q characteristiccurve for the centrifugal fan,

blower and compressor. The value of K1 represents

the K.E of the fluid movingin the tangential tip speed

of the impeller and theconstant K2 represents theslope the H-Q curve whichmay be positive, zero or

negative for fixed value of2.

The H-Q relationship can beobtained as in fig.

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Where Cd is the total drag coefficient.

Total loss in the system,

Actual head produced can therefore be obtained bydeducing these losses from ideal head developed by

the machine, i.e.,

Fig shows the actual characteristic of C.Fcompressor,

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Slip and Slip Co-efficient

It is assumed that the velocities are constant over

the cross sectional area.

But in actual practice this assumption is not correct

as shown if fig,

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Surging

The phenomenon of momentary fluctuations

in head and discharge due to unsteady flow,

flow reversal and vibration at low flow rate is

called surging.

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Pre-rotation or Pre-whirl

We know that the

velocity at inlet havemore effect on Mach

number at inlet.

The relative velocity atthe inlet should be

minimum, which

reduces the Mach

number for a given eye

tip diameter.

For a fixed eye tip

diameter the Mach

number can be reducedby providing pre-whirl at

the inlet using guide

vanes.

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Diffuser

Diffuser is used in the centrifugal compressor to

convert large kinetic energy of the fluid exitingfrom the impeller to useful fluid or pressure

energy.

Diffuser may be (i) Vaneless type (ii) Vaned type

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Axial flow compressor

It is essentially an axial flow turbine driven in the

reverse direction except that in order to achieve asufficiently high efficiency, it is necessary to design

blades by taking extreme care.

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The general velocity triangles at inlet and outlet are

as shown in fig.

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If the reaction is not equal to 50% then the velocity

triangles are unsymmetric and then design is on

whether absolute velocity of air at rotor inlet or

outlet is axial.

The velocity triangles for the two cases are as

shown in fig.

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Work done & efficiencies of compressor

The actual work is given

by

The ideal work

Then the static-to-static

efficiency is defined as

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Work Done Factor Fig shows the axial velocity distributions in the first and last

stage of a multistage axial compressor.

The degree of distortion in the axial velocity distributions

will depend on the number of the stages.

On account of this, the axial velocity in the hub and tip

regions is much less than the mean value, whereas in thecentral region its value is higher than the mean.

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Consider the velocity triangle shown in fig.

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Form Eulers equation,

In terms of blade angles, w.r.t tangential direction is given

by

From geometry,

Therefore,

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5 centrifugal compressors and axial compressors

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