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GAS TURBINE
INTRODUCTION
In a gas turbine, the air is obtained from the atmosphere and compressed in an air compressor.
The compressed air is then passed into the combustion chamber, where it is heated considerably.
The hot air is then made to flow over the moving blades of the gas turbine, which imparts
rotational motion to the runner.
During this process, the air gets expanded and finally it is exhausted into the atmosphere.
A major part of the power developed by the turbine is consumed for driving th; compressor (which
supplies compressed air to the combustion chamber). The remaining power is utilised for doing
some external work.
Comparison OF GAS TURBINES AND IC.ENGINES
S. No Gas turbines I.C engines
The mass of gas turbine per kW The mass of an IC. engine per kW deveoped is
1.
developed is less more
The installation and running cost is
2. The installation and running cost is more.
less.
3. Its efficiency is higher. Its efficiency is less.
The balancing of a gas turbine is
4. The balancing of an IC. engine is not perfect.
perfect.
The torque produced is uniform. Thus The torque produced is not uniform. Thus
5.
no flywheel is required. flywheel is necessary.
The lubrication and ignition systems The lubrication and ignition systems are
6.
are simple. difficult.
7. It can be driven at a very high speed. It can not be driven at a very high speed.
The pressures used are very low
8. The pressures used are high (above 60 bar).
(about 5 bar)
The exhaust of a gas turbine is free
9. The exhaust of an I.C. engine is more polluting.
from smok and less polluting.
10. They are very suitable for air crafts. They are less suitable for air crafts.
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Comparison Of Gas Turbines And SteamTurbines
S. No Gas turbines Steam turbines
The important components are The important components are steam boiler and
1.
compressor and combustion chamber. accessories.
The mass of gas turbine per kW The mass of steam turbine per kW developed is
2.
developed is less, more.
3. It requires less space for installation. It requires more space for idstallation.
4. The instaitation and running cost is less. The installation and running cost is more
The starting ofgas turbine is very easy The starting of steam turbine is difficult and takes
5.
and quick long time.
Its control, with the changing load Its control, with the changing load conditions, is
6.
conditions, is easy. difficult.
A gas turbine does not depend on water
7. A steam turbine depends on water supply.
supply
Open cycle gas turbine
In this type of gas turbine liquid (or) gaseous fuels are used for power generation. The basic
components are shown in figure above.
Initially, atmospheric air is allowed to pass through rotary compressor in which its Pressure and
temperature is increased, isentropic ally.
Then this compressed air is passed through combustion chamber in which fuel is injected for
combustion purpose. After combustion of fuel in combustion chamber the heat is added under
constant pressure condition the temperature of compressed air is further increased.
Now high pressure and temperature gases are expanded in gas turbine which is helpful to run the
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gas turbine or blades (generally of reaction type)
This gas turbine is directly connected to electric generator to produce electricity and finally
exhausted into the atmosphere.
This type of gas turbine works on open cycle because here working fluid is used only once. After
single use it is thrown into atmosphere.
Here inlet and outlet both the ends are open to atmosphere hence termed as open cycle gas turbine.
It is also called as continuous combustion gas turbine
Advantages:
1. Warm-up time: Once the turbine is brought up to the rated speed by the starting motor and
the fuel is ignited, the gas turbine will be accelerated from cold start to full load without warm-up
time.
2. Low weight and size: The weight in kg per kW developed is less.
3. Fuels: Almost any hydrocarbon fuel from high-octane gasoline to heavy diesel oils can be used in
the combustion chamber.
4. Open cycle plants occupies less space compared to close cycle plants.
5. The stipulation of a quick start and take-up of load frequently are the points in favor of open cycle
plant when the plant is used as peak load plant.
6. Component or auxiliary refinements can usually be varied in open cycle gas turbine plant to improve
the thermal efficiency and can give the most economical overall cost for the plant load factors and
other operating conditions envisaged.
7. Open cycle gas turbine power plant, except those having an intercooler, does not need cooling
water. Therefore, the plant is independent of cooling medium and becomes self-contained.
Disadvantages:
1. The part load efficiency of the open cycle gas turbine plant decreases rapidly as the
considerable percentage of power developed by the turbine is used for driving the compressor.
2. The system is sensitive to the component efficiency; particularly that of compressor. The open
cycle gas turbine plant is sensitive to changes in the atmospheric air temperature, pressure and
humidity.
3. The open cycle plant has high air rate compared to the closed cycle plants, therefore,
it results in increased loss of heat in the exhaust gases and large diameter duct work is
needed.
4. It is essential that the dust should be prevented from entering into the compressor to decrease
erosion and depositions on the blades and passages of the compressor and turbine. So damages
their profile. The deposition of the carbon and ash content on the turbine blades is not at all
desirable as it reduces the overall efficiency of the open cycle gas turbine plant.
In the closed cycle gas turbine, compressed air leaves the compressor and passes via the heat exchanger
through the air heater. In the air heater there are tubes (not shown) through which the compressed air passes.
The air is therefore further heated in the heater. This hot high pressure air then passes through the blade
rings. Whilst passing over the rotor blades, the air is continuously expanding, its pressure energy being
converted into kinetic energy, which in turn, is absorbed by the turbine motor.
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Advantages:
Use of higher pressure throughout the cycle which is useful for reduce size of plant.
No outside air is used for compressing so there is no problem of dust and dirt.
Also there is no need of filtration of incoming air.
Any type of fuel can be used for combustion purpose.
Disadvantages:
Weight of system is high compared to open cycle.
Large amount of water is required for cooling in cooler.
System should be air tight when working substance other than air is used.
If load on system increases then performance of system is poor.
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DIFFERENCE BETWEEN OPEN AND CLOSED GAS TURBINE:
S. No Closed cycle gas turbine Open cycle gas turbine
1. Combustion of fuel is external Combustion of fuel is internal.
Gas from turbine is passed into cooling
2. Gas from turbine is exhausted to atmosphere.
chamber.
3. Any type of fluid is used. Only air can be used.
4. Turbine blades cannot be contaminated. Turbine blades get contaminated.
5. Working fluid circulated continuously. Working fluid replaced continuously.
6. Mass of installation per KW is more. Mass of installation per KW is less.
7. Heat exchanger is used. Heat exchanger is not used.
8. This system required more space. This system required less space.
Since exhaust is cooled by circulating water, Since turbine exhaust is discharged into
9. it is best suited fo stationary installation, atmosphere, it is best suited for moving
marine use. Vehicle like Aircraft.
10. Maintenance cost is high. Maintenance cost is low.
PERFORMANCE CALCULATION OF OPEN AND CLOSED GAS TURBINE CYCLE:
1-2 Process: Adiabatic compression process
Compressor Work (WC) = mcp(T2 − T1) 𝐤𝐉
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2-3 Process: Constant pressure heat addition γ−1 γ−1
γ
γ = rp
QS = mcp(T3 − T2) 𝐤𝐉 )
S P
2 2
= (
T1 P1
3-4 Process: Adiabatic Expansion process
γ−1 γ−1
Turbine Work (WT) = mcp (T3 − T4) 𝐤𝐉 γ−1
T P γ P2 γ γ
3 3 ) = ) = rp
= ( P1
T4 P4
4-1 Process: Constant pressure Heat Rejection
QR = mcp(T4 − T1) 𝐤𝐉
Net Work done
Wnet = Qs − QR kJ
Thermal Efficiency
Work done mcp(T3 − T2) − mcp(T3 − T4)
Wnet T3 − T4
𝟏
= 𝟏− 𝗒−𝟏
(𝐫𝐩 ) 𝗒
Turbine Work Ratio
WT T1 γ−1
W = = 1− (r )
R Wnet
REGENERATIVE GAS TURBINE CYCLE
In this method, a regenerator (heat exchanger) is used for utilising heat of exhaust gases from turbine,
in pre-heating the compressed air before it enters the combustion chamber. The preheating of the
compressed air reduces the fuel consumption and consequently improves the thermal efficiency.
Regeneration is shown in fig. As a result of regeneration, compressed air is preheated and exhaust gases
are
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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) – a process called regeneration
The thermal efficiency of the Brayton cycle increases due to regeneration since less fuel is used
for the same work output
Regeneration process involves the installation of a heat exchanger in the gas turbine
cycle. The heat- exchanger is also known as the recuperator. This heat exchanger is used to
extract the heat from the exhaust gas . This exhaust gas is used to heat the compressed air.
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This compressed and pre-heated air then enters the combustors. When the heat exchanger is
well designed, the effectiveness is high and pressure drops are minimal. And when these heat
exchangers are used an improvement in the efficiency is noticed. Regenerated Gas turbines
can improve the efficiency more than 5 % . Regenerated Gas Turbine work even more
effectively in the improved part load applications.
GAS TURBINE WITH INTERCOOLING
We have already discussed that a major portion of the power developed by the gas turbine is
utilized by the compressor. It can be reduced by compressing the air in two stages with an
intercooler between the two. This improves the efficiency of the gas turbine. The schematic
arrangement of a closed cycle gas turbine with an intercooler is shown.
In this arrangement, first of all, the air is compressed in the first compressor, known as
low pressure (L.P.) compressor. We know that as a result of this compression, the pressure and
temperature of the air is increased.
Now the air is passed to an intercooler which reduces the temperature of the compressed
air to its original temperature, but keeping the pressure constant.
The process 1-2 shows heating of the air in heating chamber at constant pressure.
The process 2-3 shows isentropic expansion of air in the turbine.
The process 3-4 shows cooling of the air in the cooling chamber at constant pressure.
The process 4-5 shows compression of air in the L.P. compressor.
The process 5-6 shows cooling of the air in the intercooler at constant pressure.
Finally, the process 6-I shows compression of air in the H.P. compressor
The work required to compress air depends upon its temperature during compression. The
efficiency of gas turbine is improved by adopting multi-stage compression with intercooling
in between two stages as it reduces the work required to compress the air.
After that, the compressed air is once again compressed in the second compressor known
as high pressure (H.P.) compressor.
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Now the compressed air is passed through the heating chamber and then through the
turbine. Finally, the air is cooled in the cooling chamber and again passed into the low pressure
compressor as shown.
The process of intercooling the air in two stages of compression is shown on T-s diagram in Fig.
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