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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.
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).
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
with the compressor casing. The vanes may be
swept back, but for ease of manufacture straight
Compressors
21
Fig. 3-3 Pressure and velocity changes
through a centrifugal compressor.
Fig. 3-4 Impeller working clearance and
air leakage.
radial vanes are usually employed. To ease the air
from axial flow in the entry duct on to the rotating
impeller, the vanes in the centre of the impeller are
curved in the direction of rotation. The curved
sections may be integral with the radial vanes or
formed separately for easier and more accurate
manufacture.
Diffusers
12. The diffuser assembly may be an integral part of
the compressor casing or a separately attached
assembly. In each instance it consists of a number of
vanes formed tangential to the impeller. The vane
passages are divergent to convert the kinetic energy
into pressure energy and the inner edges of the
vanes are in line with the direction of the resultant
airflow from the impeller (fig. 3-6). The clearance
between the impeller and the diffuser is an important
factor, as too small a clearance will set up
aerodynamic buffeting impulses that could be
transferred to the impeller and create an unsteady
airflow and vibration.
THE AXIAL FLOW COMPRESSOR
13. An axial flow compressor (fig. 3-7 and fig. 3-8)
consists of one or more rotor assemblies that carry
blades of airfoil section. These assemblies are
mounted between bearings in the casings which
incorporate the stator vanes. The compressor is a
multi-stage unit as the amount of pressure increase
by each stage is small; a stage consists of a row of
rotating blades followed by a row of stator vanes.
Where several stages of compression operate in
series on one shaft it becomes necessary to vary the
stator vane angle to enable the compressor to
operate effectively at speeds below the design
condition. As the pressure ratio is increased the
incorporation of variable stator vanes ensures that
the airflow is directed onto the succeeding stage of
rotor blades at an acceptable angle, ref. para. 30,
Airflow Control.
14. From the front to the rear of the compressor, i.e.
from the low to the high pressure end, there is a
gradual reduction of the air annulus area between
Compressors
22
Fig. 3-5 Typical impellers for centrifugal
compressors.
Fig. 3-6 Airflow at entry to diffuser.
Compressors
23
Fig. 3-7 Typical axial flow compressors.
the rotor shaft and the stator casing. This is
necessary to maintain a near constant air axial
velocity as the density increases through the length
of the compressor. The convergence of the air
annulus is achieved by the tapering of the casing or
rotor. A combination of both is also possible, with the
arrangement being influenced by manufacturing
problems and other mechanical design factors.
15. A single-spool compressor (fig. 3-7) consists of
one rotor assembly and stators with as many stages
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