Power Plant Engineering

272 POWER PLANT ENGINEERING

With the use of nuclear reactor as heating source for gas, the heat exchangers can be eliminated

from the closed cycle plant and the above-mentioned limitation (number of heat exchangers) does not

exist. The power density in the core of a helium cooled fast reactor is a few thousand times higher than

in conventional gas heat exchanger. Thus, units of several thousands of megawatts designed for high gas

pressures can be housed in a single pre-stressed concrete vessel.

A typical closed cycle gas turbine plant using helium as working medium and helium cooled fast

breeder reactor is shown in Fig. 9.3 and corresponding T-s diagram is shown in Fig. 9.4.

A closed cycle gas turbine plant using helium as working medium is much smaller than of a

conventional air-turbine plant of the same output. This is due to the better thermodynamic properties of

helium relative to air and much higher pressures can be used in helium cooled fast reactor system. A

helium-turbine used in closed cycle plant of 335 mW capacity at Switzerland is of 3.7 meter diameter

and 14 meters long. The corresponding dimensions of the 17 mW air turbines at Gelsenkirohen plant are

2.6 meters in diameter and 9 meters long.

It is expected that in future, the combination of fast breeder reactors and gas turbines represent a

very promising solution for future power generation. This is because of high breeding characteristics of

the helium cooled fast reactors, which ensure continuity of low fuel cost while the use of closed cycle

gas turbine plant is expected to reduce the capital investment of the plant.

Cost is also roughly proportional to weight. One can expect much cheaper turbo machinery than

steam plant.

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.