What is Gas Turbine? & classification of gas turbine

Definition

A type of internal combustion (IC) engine is known as a gas turbine a turbine to generate power by burning a mixture of air and fuel.

Classification of gas turbine:

Gas turbines are classified as:

1. On the basis of combustion process:

a) Constant pressure gas turbine.

b) Constant volume or explosion gas turbine.

2. On the basis of path of working substance or Cycle of operation:

a) Open cycle gas turbine

b) Closed cycle gas turbine

3. On the basis of action of expanding gases:

a) Impulse gas turbine

b) Impulse reaction gas turbine

4. On the basis of direction of flow:

a) Axial flow

b) Radial flow

5. On the basis of working cycle:

a) Brayton (P = Constant)

b) Atkinsons (V= Constant)

Applications :

(1) Turbojet engines

(2) Supercharging

(3) Marine field

(4) Railway engines

(5) Electric power generation

(6) Industry

Fuel used :

1. Coal,

2. Producer gas,

3. Blast Furnace gas,

4. Diesel,

5. Paraffin,

6. Oil and pulverized coal

1) Open Cycle Gas Turbine:

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• The schematic arrangement of an open cycle gas turbine as shown in figure.

• Combustion, a compressor, and an open cycle gas turbine are the basic components of chamber and a turbine which drives the generator & compressor.

* In this turbine, the air is first sucked from the atmosphere & then compressed isentropically & then passed into the combustion chamber

.• By burning the fuel, heat is added to the compressed air in the combustion chamber.

*The hot gases from the combustion chamber are then passed to the turbine where it expands doing mechanical work. • The compressor and other accessories are driven by some of the power generated by the turbine, while the remaining is used for power.

* The compressor receives fresh air, and the turbine exhausts gases into the atmosphere.exhausted into the atmosphere.

• An open cycle gas turbine is also called a continuous gas turbine as the combustion of fuel takes place continuously.

2) Closed Cycle Gas Turbine:

• The schematic arrangement of a closed cycle as shown in figure.

* A simple closed cycle gas turbine consists of a compressor, combustion chamber and a turbine which drives the generator & compressor, & a cooling chamber.

• In this turbine, the air is first sucked from the atmosphere & then compressed isentropically & then passed into the heating chamber.

*Heat is added to the compressed air in the heating chamber by some external source.

• The hot gases from the heating chamber are then passed to the turbine where it expands doing mechanical work.

• The compressor and other accessories are driven by some of the power generated by the turbine, while the remaining is used for power.

* Gases coming out of the turbine are passed to the cooling chamber where it is cooled at constant pressure with the help of circulating water to its original temperature.

• Now the air is made to flow into the compressor again & thus cycle is repeated.

Difference between closed cycle & open cycle gas turbine:

Open cycle gas turbine	Close cycle gas turbine
1. The compressed air is heated in heatingchamber.	1. The compressed air is heated in combustion chamber.
2. As the gas is heated by an external source, hence the amount of gas remains same thought the cycle	2. The products of combustion are get mixed up in the heated air hence same gas doesn’t remain in cycle.
3. The gas after turbine is passed into the cooling chamber.	3. The gas after turbine is exhausted into the atmosphere.
4. The working fluid is circulated continuously.	4. The working fluid is replacedcontinuously.
5. Any fluid with better thermodynamic properties can be used.	5. Only air is used as the working fluid.
6. The turbine blades do not wear away earlier, as the enclosed gas does not get contaminated while flowing through heating chamber.	6. The turbine blades wear away earlier, as the air from atmosphere get contaminated while flowing through combustion chamber.
7. The mass of installation per Kwatt is more	7. The mass of installation per Kwatt is less
8. Maintenance cost is high	8. Maintenance cost is low

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Differentiate between Gas Turbine and I. C. Engine:

Sr. No.	Parameters	Gas Turbine	I.C. Engine
1.	Mechanical Efficiency	High due to absence of reciprocating parts	Low due to large number of reciprocating parts
2.	Starting Trouble	Starting of gas turbine is difficult and needs complex arrangements	Starting of I. C. Engine is simple
3.	Weight to power ratio	The weight of gas turbine per kW power developed is low since the working pressures are low requiring lighter construction	The weight of LC. engine per kW power developed is high since the working pressures are high requiring heavy construction
4.	Part load thermal efficiency	Part load thermal efficiency is poor and it is less efficient	They are efficient and part load thermal efficiency is high

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Methods to improve thermal efficiency :

1. Regeneration

2. Improving turbine output: this can be done by

(a) Reheating

(b) Increasing the value of maximum cycle temp.

3. Reducing compressor input By

(a) Intercooling

(b)By lowering inlet temp to compressor

(d)Water injection at inlet to compressor

1. Regeneration:

• The temperature of the exhaust gas leaving the turbine is higher than the temperature of the air leaving the compressor.

• The air leaving the compressor can be heated by the hot exhaust gases in a counter-flow heat exchanger (a regenerator or recuperator). This process is called regeneration.

* The heated air is passed into the combustion chamber & part of it is employed to burn the fuel.

• Some heat is added to the air in regenerator thus saving fuel consumption & hence thermal efficiency increases.

2. Improving turbine output: this can be done by

(a) Reheating

(b) Increasing the value of maximum cycle temp.

(a) Reheating:

• The turbine expands completely in two or more stages, with reheating occurring after each stage.

* In this arrangement, the air is first compressed in the compressor, passed into the combustion chamber &then to the high pressure turbine.

• The hot gases from HPT are passed to another combustion chamber for reheating & then reheated hot gases are passed to the low pressure turbine.

3. Reducing compressor input By

(a) Intercooling

(b) By lowering inlet temp to compressor

(d) Water injection at inlet to compressor

(a) Intercooling :

• By intercooling the air between the compressor stages, compressor work is reduced.

• In this arrangement, the air is first compressed in the low pressure compressor. As a result of this compression, the pressure & temperature of the air is increased.

• The air is now sent to an intercooler, which returns the compressed air to its original temperature while maintaining constant pressure.

* After that, the compressed air is once again compressed in high pressure compressor.

• Now the compressed air is passed to the combustion chamber.

* By burning the fuel, heat is added to the compressed air in the combustion chamber.

• The hot gases from the combustion chamber are then passed to the turbine.