Gas Turbine: Classification and Operation

In this article we will discuss about:- 1. Definition of Gas Turbine 2. Classification of Gas Turbine 3. Comparison of Gas Turbine with Reciprocating IC Engines 4. Operation.

Definition of Gas Turbine:

Gas turbine is a rotary type of IC engine. It has replaced cylinder piston type reciprocating internal combustion. Gas turbine is a simple way of power-producing system. The unique features of a gas turbine include the compression of atmospheric air and heat addition through the combustion of fuel process in a combustion chamber known as combustor.

A gas turbine consists of a combustion chamber in which liquid fuel is burnt along with compressed air. The gas is produced at high temperature and pressure. The burnt gas is expanded through turbine. The flow of gas takes place through blade passage where kinetic energy is absorbed and thus rotation is obtained at the shaft. The principle remains the same as in the case of steam turbine.

Gas turbine now-a-days has become the most important way of electrical power generation with compact unit. The thermal efficiency of such unit has been found quite low in the range of 20-30%. The thermal efficiency, however, can be improved with certain modifications in the gas turbine cycle plant. A gas turbine can be used for aviation, power generation, marine propulsion, oil and gas field, etc.

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Gas turbines are used in the jet engines of aircrafts. In this plant, the turbine supplies power for the operation of compressor only. The exhaust gases from the turbine are expanded in the exhaust cone or nozzle to obtain a very high kinetic energy, the resulting reaction force propelling the aircraft.

Both compressor and turbine are mounted on a single shaft. Thus, the power used for the operation of compressor is supplied by the gas turbine. The gas turbines are used as a turbocharger in IC engines. They have limited application in marine propulsion.

These turbines can be operated by oil and gas where large quantities of oil and gas reserves are available and are cheaper in those areas. A gas turbine is mostly used for power generation because of its compactness, simplicity, lack of cooling water installation, and quick installation and starting. A gas turbine plant is very compact in comparison to a steam plant.

Following are the limitations of a gas turbine:

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(a) The overall efficiency of the gas turbine plant is very low.

(b) Gas turbine rotor speed is found very high.

(d) The weight-to-power ratio of gas turbine is low.

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(e) Self-starting of gas turbine is not possible.

Classification of Gas Turbine:

A gas turbine can be operated on either closed cycle or open cycle principle.

i. Closed Cycle Gas Turbine:

In case of a closed cycle gas turbine, a fixed mass of working substance is allowed to flow inside the cycle. The working substance, i.e., air or gases, is confined inside the plant and it never leaves the plant.

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Hence, the gas turbine is said to be a closed cycle. Figure 4.36 shows the system diagram of a simple closed cycle gas turbine plant. It consists of a compressor, heater, gas turbine, and a cooler. The compressor shaft and turbine shaft are coupled for the transfer of power. The working substance is compressed by the compressor.

The high-pressure and high-temperature gas coming out from the compressor is heated by an external source of heat which further increases the temperature of the gas. The gas is then supplied to the gas turbine where the expansion of gas takes place up to a lower pressure. The flow of gas takes place through the blade passage where kinetic energy is absorbed.

Thus, the rotation of the shaft is obtained and power is developed at the turbine shaft. The low-pressure gas is then exhausted from turbine and enters into the cooler where cooling of the gas is done by circulating coolant. The cooled gas at low temperature and pressure again enters into the compressor and the process is repeated over and over again.

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ii. Open Cycle Gas Turbine:

Figure 4.37 shows the cycle of operation of an open cycle gas turbine. A simple open cycle gas turbine plant consists of an air compressor, combustion chamber, and a gas turbine.

Initially, the plant is started with the help of an auxiliary engine or electric motor. Atmospheric air is drawn in the compressor and compressed to a high pressure, and relatively high temperature, and is then supplied to a combustion chamber or combustor in which liquid or gaseous fuel is injected into the compressed air stream and the fuel is ignited.

The resulting high-pressure and high-temperature product of combustion is then passed to the turbine and is expanded to a lower pressure and finally discharged to the atmosphere through exhaust pipe.

The shaft starts rotating by the flow of gas. The turbine shaft is directly coupled with compressor and excess power is produced by the turbine which is used for the purpose of operating auxiliaries. Again fresh air is sucked into the compressor and thus the cycle is repeated (Fig. 4.38). In almost all the fields, open cycle gas turbine plants are used.

Cyclic operation is represented on T-S plot.

1-2: Isentropic compression

1-2′: Actual compression

2′-3: Heat supplied in combustor

3-4: Isentropic expansion

3-4′: Actual expansion

4-1: Heat rejected

Comparison of Gas Turbine with Reciprocating IC Engines:

Advantages of Gas Turbine over Reciprocating IC Engines:

Gas turbines:

i. The mechanical efficiency of gas turbines is very high in the range of 90-95%. Power development is based on rotary components.

ii. The overall weight of gas turbine per unit power produced is less.

iii. Gas turbine can produce speed as high as 40,000 rpm.

iv. Gas turbine does not require a flywheel as torque produced is continuous and uniform.

v. Work developed by a gas turbine per kilogram weight of air is more.

vi. The components of gas turbine can be made lighter because the pressure developed in this case is quite low, about 4-6 bar.

vii. The ignition system is much simple.

viii. The lubrication system is much simpler.

ix. Cheaper fuel such as paraffin can be used.

x. The exhaust gases from gas turbines are less polluting since air is used (A/F = 90)

xi. They are much suitable for use in aircrafts due to low specific weight.

IC engines:

The mechanical efficiency of IC engines is low in the range of 80-85% as IC engines have many reciprocating parts.

ii. The overall weight of IC engine per unit power produced is quite high.

iii. Such a high speed is not possible in IC engines.

iv. The use of flywheel is a must in IC engine due to reciprocating parts in motion.

v. Work developed by IC engine per kilogram weight of air is less.

vi. In IC engines pressure developed is more, about 60 bar. Hence, heavy parts are required to withstand high pressure.

vii. The ignition system is comparatively complex.

viii. The lubrication system is quite complicated.

ix. Special grade fuels are needed for better performance.

x. The exhaust gases from IC engines are more polluting.

xi. They are not suitable for aircrafts due to high specific weight.

Disadvantages of Gas Turbine over Reciprocating IC Engines:

(a) Thermal efficiency is quite low as compared to IC engines.

(b) Due to high operating speed, there is a need to have a speed reduction device.

(d) It is difficult to start a gas turbine as compared to IC engine.

(e) The gas turbine blades need to have a special cooling arrangement due to excess heat produced.

(f) The manufacturing of blade profile is difficult and costly due to high temperature produced. Costly and tough material such as nickel-chromium is used.

(g) Because of the high temperature produced and high centrifugal forces, the life of blades and combustion chamber is short.

Operation of a Gas Turbine (Brayton Cycle):

The theoretical cycle used for the operation of a gas turbine is known as Brayton cycle. Both open and closed cycle plants are operated on Brayton cycle.

Following are the assumptions made:

i. The compression and expansion of a working substance are reversible adiabatic.

ii. There is no variation in the specific heat of working substance.

iii. Pressure losses throughout the cycle are neglected.

iv. Heat losses are neglected.

The processes involved are (Fig. 4.39):