388
11 Power Gas Turbines
An opposite effect occurs with series connection of two expansions with the
same isentropic efficiency (Fig. 11.12, right):
The isentropic efficiency of the entire expansion is greater:
This follows from the divergence of the isobars: 2s
3ss
2
3s
h
h
h
h
−
<
−
.
Part of the energy dissipation of the first expansion is recovered with the second
expansion. On the other hand more work is required for a recompression from the
final point 3.
Series connection of compressor stages with equal isentropic efficiency results
in an isentropic efficiency of the entire compression lower than the isentropic efficiency of the components. For the isentropic re-expansion efficiency, the result is
higher efficiency of the series connection. With series connection of turbine stages
the result is also higher isentropic efficiency of the series connection. So, isentropic
efficiency is unsuitable for the study of the effects of changing the pressure ratio in
gas turbine cycles.
2
3
1
2
s1
s2
1
2s
2
3s
h h
h h
.
h h
h h
h
h
−
−
=
=
=
−
−
1
3
1
2
2
3
s
1
3ss
1
2s
2s
3ss
h h
h h h h .
h h
h h
h
h
h
−
− + −
=
=
−
−
+
−
Fig. 11.12 Series connection of compressions and expansions