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INTRODUCTION—OUTLINE OF SOME DESCRIPTIVE SYSTEMS 9

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6. The components of the gas turbine can be made lighter since the pressures used in it are
   very low, say 5 bar compared with I.C. engine, say 60 bar.


7. In the gas turbine the ignition and lubrication systems are much simpler as compared
   with I.C. engines.


8. Cheaper fuels such as paraffine type, residue oils or powdered coal can be used whereas
   special grade fuels are employed in petrol engine to check knocking or pinking.


9. The exhaust from gas turbine is less polluting comparatively since excess air is used for
   combustion.


10. Because of low specific weight the gas turbines are particularly suitable for use in aircrafts.
    Demerits of gas turbines


11. The thermal efficiency of a simple turbine cycle is low (15 to 20%) as compared with I.C.
    engines (25 to 30%).


12. With wide operating speeds the fuel control is comparatively difficult.


13. Due to higher operating speeds of the turbine, it is imperative to have a speed reduction
    device.


14. It is difficult to start a gas turbine as compared to an I.C. engine.


15. The gas turbine blades need a special cooling system.

1.4.4. A simple gas turbine plant

A gas turbine plant may be defined as one “in which the principal prime-mover is of the

turbine type and the working medium is a permanent gas”.

Refer to Fig. 1.6. A simple gas turbine plant consists of the following :

1. Turbine.


2. A compressor mounted on the same shaft or coupled to the turbine.


3. The combustor.


4. Auxiliaries such as starting device, auxiliary lubrication pump, fuel system, oil system
   and the duct system etc.

Air in Exhaust

CT

Fuel

Condenser

Generator

C = Compressure

T = Turbine

Fig. 1.6. Simple gas turbine plant.

A modified plant may have in addition to above an intercooler, regenerator, a reheater etc.

The working fluid is compressed in a compressor which is generally rotary, multistage

type. Heat energy is added to the compressed fluid in the combustion chamber. This high energy

fluid, at high temperature and pressure, then expands in the turbine unit thereby generating

power. Part of the power generated is consumed in driving the generating compressor and accessories