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62 Gas Turbine Handbook: Principles and Practices
Figure 4-11. Velocity diagrams for a centrifugal compressor.
Centrifugal force, applied in this way, is significant in the
development of pressure. Upon exiting the impeller, the air moves
into the diffuser (flow decelerator). The same deceleration of flow
or “diffuser action” that causes pressure build-up in the axial flow
compressor also occurs in the centrifugal compressor. The impeller
is the only means of adding energy to the air and all the work on
the air is done by these elements. The stationary components, such
as guide vanes and diffusers, can only convert velocity energy into
pressure energy (and incur losses). Pressure from the impeller eye
to the impeller outlet is represented by the following:
P m = G
2g c
C 2 2 – C 1 2 + U 2 2 – U 1 2 + W 1 2 – W 2
2
= G
2g c
C 2u U 2 – C 1u U 1
(4-22)
The term C 2 2 – C 1 2 /2g c represents the increase in kinetic energy
contributed to the air by the impeller. The absolute velocity C 1
(entering the impeller) increases in magnitude to C 2
(leaving the
impeller). The increase in kinetic energy of the air stream in the
impeller does not contribute to the pressure increase in the impeller.
However, the kinetic energy does convert to a pressure increase
in the diffuser section. Depending on impeller design, pressure rise
can occur in the impeller in relation to the terms U 2 2 – U 1 2 /2g c
and W 1 2 – W 2 2 /2g c
. The term U 2 2 – U 1 2 /2g c measures the pressure