Probably a wind-mill was the first turbine to produce useful work, wherein there is no precompression and no combustion. The characteristic features of a gas turbine as we think of the name today include a compression process and an heat addition (or combustion) process. The gas 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
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 efficiency of the plant is low.
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.
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”.
|Simple gas turbine plant
Refer to Fig. simple gas turbine plant consists of the following part
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.
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 and the rest is utilised in electrical energy. The gas turbines work on open cycle, semiclosed cycle or closed cycle. In order to improve efficiency, compression and expansion of working fluid is carried out in multistages.