CENTRIFUGAL COMPRESSOR IN GAS TURBINES

 Centrifugal compressors used in gas turbine plants are most often of the single-stage type and much more rarely have two stages.  They are advantageous over axial compressors in the following aspect.

• Have an appreciably shorter length, Since the pressure ratio attainable in a single stage is B=4.5.(High pressure ratio per stage)

• They are simpler in design and easy to manufacture thus leads to lower cost.

• Under all operating conditions, more reliable in operation.

• Centrifugal compressor is rugged in construction and is less susceptible to the effects of deposits left on the flow passages by the air.

• They are less sensitive to the fouling of the flow path

• Maintains good efficiency over a wide range of operations. (Their efficiency drops down less strongly in off rated-regimes.  With a low volume discharge of air, a centrifugal compressor often turns out to be more efficient, since the efficiency of an axial compressor reduces substantially in such cases owing to increasing losses associated with the low height of blading.)

Though the centrifugal compressor is having so many advantages over the axial compressors it is also having following drawbacks with respect to the latter,

• When high-pressure ratios are required, it is normal practice to use a centrifugal compressor in conjunction with axial flow compressors or using two stage centrifugal compressors. However, the latter arrangement requires a complicated duct arrangement between the stages due to increase in losses. Two stages are used in some smaller units.

• Having larger cross sectional areas for the same capacity.

Principle of Operation:

The principle components of the centrifugal compressor are the stationary casing which contains the rotating impeller and a device to convert kinetic energy into pressure energy. Process of converting kinetic energy is known as diffusion and consequently that part is known as the diffuser. The impeller may be single or double-sided but the fundamental theory is the same for both. The double-sided impeller was required in early aero-engines because of the relatively small flow capacity of the centrifugal compressor for a given overall diameter. The impeller rotates with the shaft at high speed and imparts kinetic energy (high velocity) to the air with some static pressure rise. The diffuser is stationary and converts the velocity of air leaving the impeller into static pressure.  
Air is sucked into impeller eye, whirled round at high speed by the vanes of the impeller disc, and driven away by centrifugal force. At any point in the flow of air through the impeller, the centripetal acceleration is obtained by a pressure head, so that the static pressure of the air increases from the eye to the tip of the impeller. The remainder of the pressure rise, is obtained in the diffuser, where the very high velocity of air leaving the impeller tip is reduced to somewhere in the region of the velocity with which the air enters the impeller eye. It should be appreciated that friction in the diffuser will cause some loss in “stagnation” pressure. Modern practice is to design the compressor in such a way that about half the pressure rise occurs in the impeller and half in the diffuser.

It will be appreciated owing to the action of the vanes in carrying the air around with the impeller; there will be a slightly higher static pressure on the forward face of a vane than on the trailing face. The air will thus tend to flow round the edges of the vanes in the clearance space between the impeller and casing. This naturally results in a loss of efficiency, and the clearance must be kept as small as possible. A shroud attached to the vanes would eliminate such a loss, but the manufacturing difficulties are vastly increased and there would be a disc friction or ‘windage’ loss associated with the shroud. Although shrouds have been used on superchargers, they are not used on impellers for gas turbines.
The impellers of modern centrifugal compressors operate with very high tip speeds resulting in very high stress levels. Generally, the back-swept curved vanes are desirable for compressors of high pressure ratio, but for many years designers were forced to use radial vanes to straighten out under the action of the considerable centrifugal force involved, setting up undesirable bending stresses in the vanes. Modern methods of stress analysis combined with stronger materials, however, now permit backswept vanes to be used in high-performance compressors.
Diffuser simply converts one type of energy into another type hence no work is done on the air in the diffuser. Therefore, the energy absorbed by the compressor will be determined by the condition of the air at the inlet and outlet of the impeller.