Page 31 - The Jet Engine

Compressors

THE CENTRIFUGAL FLOW COMPRESSOR per sec. By operating at such high tip speeds the air
velocity from the impeller is increased so that greater
5. Centrifugal flow compressors have a single or energy is available for conversion to pressure.
double-sided impeller and occasionally a two-stage,
single sided impeller is used, as on the Rolls-Royce 9. To maintain the efficiency of the compressor, it is
Dart. The impeller is supported in a casing that also necessary to prevent excessive air leakage between
contains a ring of diffuser vanes. If a double-entry the impeller and the casing; this is achieved by
impeller is used, the airflow to the _rear side is keeping their clearances as small as possible (fig. 3-
reversed in direction and a plenum chamber is 4).
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.

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.

Fig. 3-4 Impeller working clearance and
air leakage.

Construction
10. The construction of the compressor centres
around the impeller, diffuser and air intake system.
The impeller shaft rotates in ball and roller bearings
and is either common to the turbine shaft or split in
the centre and connected by a coupling, which is
usually designed for ease of detachment.

Impellers
11. The impeller consists of a .forged, disc with
integral, radially disposed vanes on one or both sides
(fig. 3-5) forming convergent passages in conjunction
Fig. 3-3 Pressure and velocity changes with the compressor casing. The vanes may be
through a centrifugal compressor. swept back, but for ease of manufacture straight

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