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Applications 19
identify this configuration as ‘ generator-drives’ and list them separate
from mechanical drive units. The single-spool gas turbine with the
output shaft as an extension of the main shaft is conceptually identical
to the turboprop used in fixed wing aircraft applications.
A single spool-split output shaft gas turbine (sometimes referred
to as a split-shaft mechanical drive gas turbine) is a single-spool gas
turbine driving a free power turbine as shown in Figure 2-9. The compressor/turbine
component shaft is not physically connected to the
power output ( power turbine) shaft, but is coupled aerodynamically.
This aerodynamic coupling (also referred to as a liquid coupling) is
advantageous in that starts are easier (cooler) on the turbine components.
This is due to the fact that the gas turbine attains self-sustaining
operation before it “picks-up” the load of the driven equipment.
In fact, the gas turbine can operate at this low idle speed without
the driven equipment even rotating. In this configuration the power
turbine output shaft runs at speeds that can be very different from
the gas generator speed. This configuration is most often seen in process
compressor and pump drives. However, it is also used in electric
generator drive application.
One of the advantages of this arrangement is that the power
turbine can be designed to operate at the same speed as the driven
equipment. Therefore, for generator drive applications the power
turbines operate at either 3,000 or 3,600 rpm (in order to match 50
cycle or 60 cycle generators). For centrifugal compressor and pump
applications, speeds in the 4,000 to 6,000 rpm range are common.
Matching the speeds of the driver and driven equipment eliminates
the need for a gearbox. Gearbox losses are typically in the 2% to 4%
range of power output, therefore, eliminating the gearbox constitutes
Figure 2-9. Sketch of a single spool gas turbine with a hot end drive
power turbine.