GAS TURBINES: Classification of Gas Turbines, Merits and Demerits of Gas Turbines over I.C. engines and Energy Cycle for a Simple-Cycle Gas Turbine

GAS TURBINES

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 

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 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.

Merits and Demerits of Gas Turbines over I.C. engines

Merits of Gas Turbines 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.

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  over I.C. engines

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

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

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

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

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

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.
Simple gas turbine plant




Refer to Fig. simple gas turbine plant consists of the following part

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.

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.

Energy Cycle for a Simple-Cycle Gas Turbine

shows an energy-flow diagram for a simple-cycle gas turbine, the description of which is given below :

Energy flow diagram for gas-turbine unit
Energy flow diagram for gas-turbine unit





— The air brings in minute amount of energy (measured above 0°C).

— Compressor adds considerable amount of energy.

— Fuel carries major input to cycle.

— Sum of fuel and compressed-air energy leaves combustor to enter turbine.

— In turbine smallest part of entering energy goes to useful output, largest part leaves in exhaust.

Shaft energy to drive compressor is about twice as much as the useful shaft output.

Actually the shaft energy keeps circulating in the cycle as long as the turbine runs. The important comparison is the size of the output with the fuel input. For the simple-cycle gas turbine the output may run about 20% of the fuel input for certain pressure and temperature conditions at turbine inlet. This means 80% of the fuel energy is wasted. While the 20% thermal efficiency is not too bad, it can be improved by including additional heat recovery apparatus.

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