PRINCIPLES OF OPERATION.--The com-
pressor draws in the air at the hub of the impeller
and accelerates it radially by centrifugal force
through the impeller. It leaves the impeller at a
high velocity and a low pressure and flows
through the diffuser (fig. 1-10, view A). The
diffuser converts the high-velocity, LP air to low-
velocity, HP air. The compressor manifold diverts
the flow of air from the diffuser (an integral part
of the manifold) into the combustion chambers.
CONSTRUCTION. --In a centrifugal com-
pressor the manifold has one outlet port for each
combustion chamber. The outlet ports are bolted
to an outlet elbow on the manifold (fig. 1-10,
view A). The outlet ports ensure that the same
amount of air is delivered to each combustion
chamber. Each outlet port elbow contains from
two to four turning vanes to change the airflow
from radial to axial flow and to reduce air
pressure losses by presenting a smooth turning
surface.
The impeller is usually made from a forged
aluminum alloy that is heat-treated, machined,
and smoothed for minimum flow restriction and
turbulence. Some types of impellers are made
from a single forging, while in other types the
inducer vanes are separate pieces that are welded
in place.
Centrifugal compressors may achieve effi-
ciencies of 80 to 84 percent at pressure ratios of
2.5:1 to 4:1 and efficiencies of 76 to 81 percent
at pressure ratios of 4:1 to 10:1.
Some advantages of centrifugal compressors
are as follows:
Rugged, simple in design
Relatively light in weight
Develop high-pressure ratio per stage
Some disadvantages of centrifugal compressors
are as follows:
Figure 1-10.--Centrifugal compressors. A. Single entry.
B. Dual entry.
Large frontal area
L o w e r efficiency than axial-flow
Dual-entry impellers are smaller in diameter
compressors
and rotate at higher speeds to ensure a sufficient
Difficulty in using two or more stages due
airflow. Most modern GTEs use the dual-entry
to the air loss that occurs between stages
compressor to reduce engine diameter. Because
and seals
the air must enter the engine at almost right angles
to the engine axis, a plenum chamber is required
Axial-Flow Compressors
for dual-entry compressors. The air must
surround the compressor at a positive pressure
The purpose of the axial compressor is the
before entering the compressor to ensure an
same as the centrifugal compressor. They both
undisturbed flow.
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