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8 ENGINEERING THERMODYNAMICS

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turbine represents perhaps the most satisfactory way of producing very large quantities of power

in a self-contained and compact unit. The gas turbine may have a future use in conjunction with

the oil engine. For smaller gas turbine units, the inefficiencies in compression and expansion

processes become greater and to improve the thermal efficiency it is necessary to use a heat

exchanger. In order that a small gas turbine may compete for economy with the small oil engine or

petrol engine it is necessary that a compact effective heat exchanger be used in the gas turbine

cycle. The thermal efficiency of the gas turbine alone is still quite modest 20 to 30% compared with

that of a modern steam turbine plant 38 to 40%. It is possible to construct combined plants whose

efficiencies are of order of 45% or more. Higher efficiencies might be attained in future.

The following are the major fields of application of gas turbines :

1. Aviation


2. Power generation


3. Oil and gas industry


4. Marine propulsion.
   The efficiency of a gas turbine is not the criteria for the choice of this plant. A gas turbine is
   used in aviation and marine fields because it is self-contained, light weight, not requiring cooling
   water and generally fits into the overall shape of the structure. It is selected for power generation
   because of its simplicity, lack of cooling water, needs quick installation and quick starting. It is
   used in oil and gas industry because of cheaper supply of fuel and low installation cost.
   The gas turbines have the following limitations : (i) They are not self-starting ; (ii) Low
   efficiencies at part loads ; (iii) Non-reversibility ; (iv) Higher rotor speeds ; and (v) Overall effi-
   ciency of the plant is low.

1.4.2. Classification of gas turbines

The gas turbines are mainly divided into two groups :

1. Constant pressure combustion gas turbine :
   (a) Open cycle constant pressure gas turbine
   (b) Closed cycle constant pressure gas turbine.


2. Constant volume combustion gas turbine.
   In almost all the fields open cycle gas turbine plants are used. Closed cycle plants were
   introduced at one stage because of their ability to burn cheap fuel. In between their progress
   remained slow because of availability of cheap oil and natural gas. Because of rising oil prices, now
   again, the attention is being paid to closed cycle plants.

1.4.3. Merits and demerits of gas turbines

Merits over I.C. engines :

1. The mechanical efficiency of a gas turbine (95%) is quite high as compared with I.C.
   engine (85%) since the I.C. engine has a large many sliding parts.


2. A gas turbine does not require a flywheel as the torque on the shaft is continuous and
   uniform. Whereas a flywheel is a must in case of an I.C. engine.


3. The weight of gas turbine per H.P. developed is less than that of an I.C. engine.


4. The gas turbine can be driven at a very high speeds (40,000 r.p.m.) whereas this is not
   possible with I.C. engines.


5. The work developed by a gas turbine per kg of air is more as compared to an I.C. engine.
   This is due to the fact that gases can be expanded upto atmospheric pressure in case of
   a gas turbine whereas in an I.C. engine expansion upto atmospheric pressure is not
   possible.