AIR SYSTEMS
passes into the PT section. Most of the remaining
energy is extracted by the PT rotor, which drives
Air is used for many different functions on
the high-speed, flexible-coupling shaft. The shaft
the GTE. The terms primary airflow and
provides the power for the ship's drive system.
secondary airflow designate the major systems.
The gas exits from the PT through the turbine rear
Figure 1-33 (At the end of this chapter)
frame and passes into the exhaust duct and out
shows the LM2500 GTE airflow. See page 1-34 for
through the ship's exhaust.
an overall view and pages 1-35 through 1-37 for details.
Secondary Airflow
Althoughthe following discussion on air systems is
generic, we are using the LM2500 system discussed
Secondary air is the source of bearing pressure
in chapter 2 as our example. For a better
seal air and bleed air for cooling. Bleed air has
understanding of primary airflow and secondary
numerous other uses. We will not attempt to
airflow, use this figure in conjunction with the
describe them all because each type of engine has
following discussion. You should also use this
its own peculiarities.
figure when studying the bleed air system in
Secondary air is taken from the compressor
chapter 2 of this manual.
ahead of the combusiton stage. Secondary air
Primary Airflow
is bled from various pressure stages on the
The GG compressor draws air from the ship's
compressor due to different pressure requirements
inlet plenum. After being compressed, the primary
at different points in the engine. Occasionally it
air enters the combustion section, where some of
is bled from the combustor outer case. The air
it is mixed with fuel, and the mixture is burned.
is fed internally through passages to bearing
The remainder of the primary air is used for
cavities and seals, and it also cools the GG
centering the flame in the combustor and
turbine and nozzles. On some engines the air is
cooling some parts of the GG turbine. The
piped externally to seals where shafts extend
outside a housing, such as a reduction gear.
primary air becomes part of the hot combustion
gases. Some of the energy in the hot combustion
Seal air is used in the GTE air seals, which are
gas is used to turn the GG turbine rotor, which
of two types: labyrinth/honeycomb, used in the
is coupled to, and turns, the compressor rotor.
sump and turbine areas, and fishmouth, used in
Upon leaving the GG turbine section, the gas
the combustor and turbine midframe (fig. 1-34).
Figure 1-34.--Typical air seals.
1-27
