Gas turbine: Working, Diagram, Types, Efficiency [with Pdf]

What is a Gas turbine?

Gas turbine is a continuous internal combustion engine that works on the principle of a Brayton cycle, In this flow of working gas continuously goes through the following processes which are Compression, Combustion, and Expansion.

The gas turbine was founded by John Barber in 1791.

Gas turbines have a lower weight-to-power-developed ratio as compared to other power plants hence they are smaller in size as compared to other power plants.

Why gas turbines are used?

Gas turbines are used because of the following significances:-

1. It is smaller in size.
2. It has a Lower weight per power developed
3. No vibration occurs.
4. It can be used where problems of water availability.
5. Simple in operation.
6. Easy installation.
7. It occupies less space hence we can use it where the unavailability of a large area.
8. Low Maintenance.

Classification of Gas turbine:

There are two types of the gas turbine:-

A) Open cycle gas turbine
B) Closed cycle gas turbine

A) Open cycle gas turbine:

1) Open cycle gas turbine uses atmospheric air as a working gas & It is consists of the three main components that are Axial flow air Compressor, Combustor or Combustion chamber & turbine.

2) The air compressor takes the air from the atmosphere and compresses it to high pressure and temperature.

3) The compressed air from the compressor flows to the combustion chamber, where fuel directly mixes with the high-pressure air. Therefore fuel burning takes place inside the combustion chamber and the high-pressure hot gases flow to the turbine expand into the turbine and release into the atmosphere.

4) Due to high-pressure hot gases strikes onto the turbine, power is obtained at the turbine. This power also uses to drive the compressor, for that compressor and turbine are located onto the same shaft.

Net-work obtained = Wn = Wt – Wc

Where
W_t = Work developed by the turbine
W_c = Work to drive compressor

B) Closed cycle gas turbine:

1) Closed cycle gas turbine is consists of the four main components that are Axial flow Compressor, Heat source, turbine & heat sink.

2) The closed-cycle gas turbine uses gases which having more specific heat & good properties such as helium, and argon gases that has properties better than atmospheric air, these gases are costly which needs a closed system for circulation. Hence this type of turbine knows as a closed-cycle gas turbine

3) As compared to open cycle gas turbine, in a closed cycle extra component is used which is a heat sink. The heat sink is one of the heat exchangers which absorbs the heat from hot gases after expansion from the turbine.

4) Also in this type of gas turbine, a Heating source is provided to heat the gas.

5) The compressor takes the gas from the heat sink and compresses it to high pressure and temperature.

6) The compressed gas from the compressor flows to the heating source, where the heating source increases the temperature of the high-pressure gas, and then this high-pressure hot gas strikes onto the turbine, expands, and flows to the heat sink.

7) Due to high-pressure hot gas strikes onto the turbine, power is obtained at the turbine. This power also uses to drive the compressor, for that compressor and turbine are located onto the same shaft.

5) The low pressure expands gas from the turbine further flows to the heat sink which absorbs the heat from gases and makes it cool then this gases again flows to the compressor.

In such a way, this cycle continues again and again.

Working cycle of Gas turbine:

The gas turbine works on the Brayton cycle.

Brayton cycle consists of two isobaric and two isentropic processes.

Brayton cycle ( PV & TS Diagram):-

Processes:-

1-2:- Isentropic compression, Pressure increses from P1 to P2.
2-3:-  Constant pressure heat addition process.
3-4:- Isentropic / Reversible adiabatic expansion.
4-1:- Constant pressure heat rejection process.

Gas turbine efficiency calculation:

Work to drive compressor = (Wc) = m Cp (T2-T1)

Where
T₁ = Input air/gas temperature
T₂ = Compressed air temperature
C_p = Specific heat of air/gas
m = mass of air/gas

Turbine work = Wt = m Cp ( T3-T4)

Where
T₃ = Temperature of air pressure at the entry of turbine
T₂ = Temperature of air pressure at the exit of the turbine
C_p = Specific heat of air/gas
m = mass of air/gas

Net Work = Wn = Wt – Wc

=  m Cp ( T3-T4) – m Cp (T2-T1)

Heat supplied =  Qs = m cp (T3-T2)

Hence,  

Gas turbine efficiency = `frac{Wn}{Qs}` 

Gas turbine efficiency = `frac{Wt – Wc}{Qs}`

Back work ratio of gas turbine = B.W.R. = `frac{Wc}{Wt}`

Work ratio of gas turbine = W.R. = `frac{Wn}{Wt}`

=  `frac{Wt-Wc}{Wt}`

W.R. = `1-frac{Wc}{Wt}`

Methods to improve the efficiency of the gas turbine:

To improve gas turbine efficiency, there are 3 methods as follows:-

1. Reheating in gas turbine
2. Intercooling in gas turbine
3. Regeneration in gas turbine

Advantages of Gas turbine:

1. It can used where the problem of water unavailability.
2. Less maintenance.
3. Simple operation.
4. Ease of installation.

Applications of Gas turbine:

The gas turbine is used in the following applications:-

1. Marine & transport application
2. In the Aircraft
3. In the Military applications
4. Turbocharging in diesel plants.