Power Plant Engineering

GAS TURBINE POWER PLANT 273

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.

9.3.2 Intercoolers and Heat Exchangers

The intercooler is generally used in gas turbine plant when the pressure ratio used is sufficiently
large and the compression is completed with two or more stages. The cooling of compressed air is
generally done with the use of cooling water. A cross-flow type intercooler is generally preferred for
effective heat transfer.

The regenerators, which are commonly used in gas turbine plant, are of two types, recuperator
and regenerator.

In a recuperative type of heat ex-
changer, the air and hot gases are made to flow
in counter direction as the effect of counter-
flow gives high average temperature difference
causing the higher heat flow.

A number of baffles in the path of air-
flow are used to make the air to flow in con-
tact for longer time with heat transfer surface.

The regenerator type heat exchanger
consists of a heat-conducting member that is
exposed alternately to the hot exhaust gases
and the cooler compressed air. It absorbs the
heat from hot gases and gives it up when ex-
posed to the air. The heat, capacity member is
made of a metallic mesh or matrix, which is
rotated slowly (40-60 r.p.m.) and continuously
exposed to hot and cold air.

Inlet Guide Vanes

Fig. 9.6. Axial Flow Air Compressor.

Fig. 9.7. Ritz Regenerative Heat Exchanger.

Exhaust to

Atmosphere

Rotating Mesh Type

Heat Exchanger

Air to Combustion Fixed Casing

Chamber

Air from

Compressor

Exhaust from

gas turbine