withstand, the mass weight flow cannot be
and turbine blading through corrosion of the
achieved; the result is a loss of net power available
engine parts.
for work. The plant may be able to produce only
90 to 95 percent of its rated horsepower.
On the other hand, if the ambient temperature
TYPES OF GAS TURBINE ENGINES
were to drop to 0F, the volume of air (mass)
required would decrease. However, the mass
The different types of GTEs all use the same
weight would increase. Since the mass weight is
basic principles already discussed. A GTE is
increased and heat transfer is better at higher
classified by its construction (the type of
pressure, less fuel is needed to increase volume.
compressor, combustor, or shaft system used).
This situation produces quite an efficient power
The compressor may be either centrifugal or
plant. It has a GG speed of 85 to 90 percent and
axial type. The combustor may be annular, can-
annular, or can type. The type of shaft used on
a PT speed of 100 percent. In a constant speed
engine, the differences in temperature will show
a GTE may be either single shaft, split shaft, or
twin spool. These classifications will be discussed
up on exhaust gas temperature. In some cases, it
in the following paragraphs.
will show up on the load the engine will pull. For
instance, on a hot day of 120F, the engine on
a 300-kW generator set may be able to pull only
CLASSIFICATION BY
275 kW. This is due to limitations on exhaust or
COMPRESSOR TYPE
turbine inlet temperature. On a day with 0F
The compressor takes in atmospheric air and
ambient temperature, the same engine will pull
compresses it to a pressure of several atmospheres.
300 kW. It can have an exhaust or turbine inlet
temperature that is more than 100F, lower than
A GTE may be classified by compressor type,
average. Here again, less fuel is needed to increase
based on the direction of the flow of air through
volume and a greater mass weight flow. In turn,
the compressor. The two principal types of
the plant is more efficient.
compressors are the centrifugal flow and the
axial flow. The centrifugal-flow compressor draws
in air at the center or eye of the impeller and
Effect of Compressor Cleanliness
accelerates it around and outward. In the axial-
flow compressor, the air is compressed while
Another factor that will have a great effect on
continuing its original direction of flow (parallel
performance is the condition of the compressor.
A clean compressor is essential to efficiency
to the axis of the compressor rotor).
and reliability. During operation at sea, the
Centrifugal Compressor
compressor takes in a high volume of salt-
contaminated air. Salt buildup is relatively slow
The centrifugal compressor is usually located
in the compressor and will occur more on the
between the accessory section and the combustion
stator vanes and the compressor case than on
section. The basic compressor section consists of
rotating parts. Centrifugal force tends to sling salt
an impeller, diffuser, and compressor manifold.
contaminants off the rotor blades.
Any oil ingested into the engine coats the
The diffuser is bolted to the manifold. Often the
compressor with a film and will rapidly increase
entire assembly is referred to as the diffuser. For
ease of understanding, we will discuss each unit
contamination of the compressor. The film traps
separately.
any dust and other foreign matter suspended in
The impeller may be either single entry or dual
the air. The dust and dirt absorb more oil, which
entry (fig. 1-10). The main differences between
traps more dirt, and so forth. If left unattended,
the single-entry and dual-entry impeller are the
the buildup of contamination (either oil or salt)
size of the impeller and the ducting arrangement.
will lead to a choking of the compressor and a
The single-entry impeller permits convenient
restricted airflow. This restricted airflow will
ducting directly to the inlet vanes. The dual-entry
require the main fuel to schedule more fuel to
impeller uses a more complicated ducting to reach
produce an equivalent horsepower. The combus-
the rear side of the compressor. Single-entry
tion gas temperatures will rise until loss of
impellers are slightly more efficient in receiving
power, and damage to the turbine may result.
air, but they must be of greater diameter to
Contamination, if not controlled, can induce a
provide sufficient airflow. This increases the
surge condition in the compressor during engine
overall diameter of the engine.
start. It will also reduce the life of the compressor
1-10