Tuesday, December 20, 2011


Fig. 3-3 Pressure and velocity changes
through a centrifugal compressor.
5. Centrifugal flow compressors have a single or double-sided impeller and occasionally a two-stage, single sided impeller is used, as on the Rolls-Royce Dart. The impeller is supported in a casing that also contains a ring of diffuser vanes. If a double-entry impeller is used, the airflow to the _rear side is reversed in direction and a plenum chamber is required. Principles of operation
6. The impeller is rotated at high speed by the turbine and air is continuously induced into the centre of the impeller. Centrifugal action causes it to flow radially outwards along the vanes to the impeller tip, thus accelerating the air and also causing a rise in pressure to occur. The engine intake duct may contain vanes that provide an initial swirl to the air entering the compressor.
Fig. 3-4 Impeller working clearance and
air leakage.

7. The air, on leaving the impeller, passes into the diffuser section where the passages form divergent nozzles that convert most of the kinetic energy into pressure energy, as illustrated in fig. 3-3. In practice, it is usual to design the compressor so that about half of the pressure rise occurs in the impeller and half in the diffuser.

8. To maximize the airflow and pressure rise through the compressor requires the impeller to be rotated at high speed, therefore impellers are designed to operate at tip speeds of up to 1,600 ft. per sec. By operating at such high tip speeds the air velocity from the impeller is increased so that greater energy is available for conversion to pressure. 9. To maintain the efficiency of the compressor, it is necessary to prevent excessive air leakage between the impeller and the casing; this is achieved by keeping their clearances as small as possible (fig. 3- 4).

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