Power Plant Engineering

9.3 ELEMENTS OF GAS TURBINE POWER PLANT
It is always necessary for the engineers and designers to know about the construction and opera-
tion of the components of gas turbine plants.
9.3.1. COMPRESSORS
The high flow rates of turbines and relatively moderate pressure ratios necessitate the use of
rotary compressors. The types of compressors, which are commonly used, are of two types, centrifugal
and axial flow types.
The centrifugal compressor consists of an impeller (ro-
tating component) and a diffuser (stationary component). The
impeller imparts the high kinetic energy to the air and diffuser
converts the kinetic energy into the pressure energy. The pres-
sure ratio of 2 to 3 is possible with single stage compressor and
it can be increased upto 20 with three-stage compressor. The
compressors may have single or double inlet. The single inlet
compressors are designed to handle the air in the range of 15 to
300 m
3
/min and double inlets are preferred above 300 m
3
/min
capacity. The single inlet centrifugal compressor is shown in Fig.
9.5. The efficiency of centrifugal compressor lies between 80 to
90%. The efficiency of multistage compressor is lower than a
single stage due to the losses.
The axial flow compressor consists of a series of rotor
and stator stages with decreasing diameters along the flow of air.
Air Out
Seroll
Air In
Impeller
Fig. 9.5. Single Stage Single Entry
Centrifugal Compressor.

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The blades are fixed on the rotor and rotors are fixed on the shaft. The stator blades are fixed on the
stator casing. The stator blades guide the air flow to the next rotor stage coming from the previous rotor
stage. The air flows along the axis of the rotor. The kinetic energy is given to the air as it passes through
the rotor and part of it is converted into pressure. The axial flow compressor is shown in Fig. 9.6. The
number of stages required for pressure ratio of 5 is as large as
sixteen or more.
A satisfactory air filter is absolutely necessary for clean-
ing the air before it enters the compressor because it is essential
to maintain the designed profile of the aerofoil blades. The depo-
sition of dust particles on the blade surfaces reduces the efficiency
rapidly.
The advantages of axial flow compressor over centrifugal
compressor are high isentropic efficiency (90-95%), high flow
rate and small weight for the same flow quantity. The axial flow
compressors are very sensitive to the changes in airflow and speed,
which result in rapid drop in efficiency.
In both types of compressors, it has been found that lowering of the inlet air temperature by 15 to
20°C gives almost 25% greater output with an increase of 5% efficiency.