# TURBOMACHINES Chapter 5 CENTRIFUGAL compressors & axial flow compressors 1.

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TURBOMACHINES Chapter 5 CENTRIFUGAL compressors & axial flow compressors

TURBOMACHINES

Chapter 5 CENTRIFUGAL compressors & axial flow compressors

1Introduction 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 known as fan, blower, supercharger etc, depending upon the need to be served.Fans are low pressure compressors and blowers are medium pressure compressors.

2Elements of a centrifugal compressorIt mainly consists of (i) inlet casing with the converging nozzle (ii) the impeller (iii) the diffuser and (iv) the outlet casing.

3Variation of pressure and velocityFig below shows the variation of pressure and velocity in a centrifugal compressor.

4Velocity triangle at the Eye Hub and TipIn 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 eye respectively.

5Different vane shapeThe impellers may be classified depending on the exit angle 2 into (i) Backward curved vanes, (ii) Radial vanes and (iii) Forward curved blades.

6The above equation is known as H-Q characteristic curve for the centrifugal fan, blower and compressor.The value of K1 represents the K.E of the fluid moving in the tangential tip speed of the impeller and the constant K2 represents the slope the H-Q curve which may be positive, zero or negative for fixed value of 2.The H-Q relationship can be obtained as in fig.

7Where Cd is the total drag coefficient. Total loss in the system,Actual head produced can therefore be obtained by deducing these losses from ideal head developed by the machine, i.e.,Fig shows the actual characteristic of C.F compressor,

8Slip and Slip Co-efficientIt 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,

9Surging 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-whirlWe know that the velocity at inlet have more effect on Mach number at inlet.The relative velocity at the 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 reduced by providing pre-whirl at the inlet using guide vanes.

11Diffuser Diffuser is used in the centrifugal compressor to convert large kinetic energy of the fluid exiting from the impeller to useful fluid or pressure energy.

Diffuser may be (i) Vaneless type (ii) Vaned type12Axial flow compressorIt is essentially an axial flow turbine driven in the reverse direction except that in order to achieve a sufficiently high efficiency, it is necessary to design blades by taking extreme care.

13The general velocity triangles at inlet and outlet are as shown in fig.

14If 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.

1515Work done & efficiencies of compressorThe actual work is given by

The ideal work

Then the static-to-static efficiency is defined as

16Work Done FactorFig 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 the central region its value is higher than the mean.

17Consider the velocity triangle shown in fig.

18Form Eulers equation,

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

From geometry,

Therefore,

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