842 Gas Turbines
Certain aero engines are already fitted with power turbines and termed
turbo-prop or turbo-shaft machines; these are readily adaptable to industrial
and marine use. Special technical and production techniques are applied to the
redesigned gas turbine to ensure its suitability for sea level operation at high
powers and for the marine environment.
A basic gas generator has one rotating assembly: the compressor and its
turbine coupled together. The characteristics of axial-flow compressors vary
considerably, however, over the operating range from starting to full power. On
some high-compression ratio compressors it is necessary to fit automatic blow-
off valves and to alter the angle of the inlet guide vanes and first stages of sta-
tor blades to ensure efficient operation.
To achieve the necessary stability in larger gas generators, the compressor
is divided into two separate units: the LP and HP compressors, each driven
by its own turbine through co-axial shafts. Each compressor is able to oper-
ate at its own optimum speed, giving flexibility of operation and efficient
compression throughout the running range. Only the HP rotor needs to be turned
during starting, and therefore even the largest gas generator can be started
by battery.
Although such a two-spool compressor is very flexible, it is still necessary
in some cases to adjust the inlet guide vanes to deal with the changing flow of
air entering the compressor. This is effected automatically by pressure sensors
acting upon rams which alter the angle of the guide vanes.
An axial-flow compressor consisting of alternate rows of fixed and rotating
blades draws in atmospheric air through an air intake and forces it through a
convergent duct formed by the compressor casing into an intermediate casing,
where the compressed air is divided into separate flows for combustion and cool-
ing purposes. A typical annular combustion chamber receives about 20 per cent
of the air flow for combustion into which fuel is injected and burned.
Initial ignition is executed by electrical igniters, which are switched off
when combustion becomes self-sustaining. The resulting expanded gas is cooled
by the remainder of the air flow which enters the combustion chamber via slots
and holes to reduce the temperature to an acceptable level for entry into the one
or more axial-flow stages of the turbine. The turbine drives the compressor, to
which it is directly coupled. The remaining high-velocity gases are exhausted
and are available for use in the power turbine.
A power turbine of the correct ‘swallowing’ capacity is required to convert
the gas flow into shaft power (Figure 31.10). Typically of one or more axial-
flow stages, the power turbine may be arranged separately on a base frame
designed to mount the gas generator, to which the turbine is linked by a bel-
lows joint to avoid the need for very accurate alignment and to allow for dif-
ferential expansion. In some cases, however, expansion is allowed for in the
design of the power turbine and the gas generator is mounted directly on to
the inter-turbine duct. The power turbine, usually designed to last the life of
the plant, is surrounded by an exhaust volute which passes the final exhaust to
atmosphere through a stack.