2. Gas turbine
• A gas turbine is basically a constant
flow engine. The fuel is burned and
serves to add heat to compressed air
which then expands through the blades
of a turbine.
• Use the same impulse and reaction
principle as in steam turbine
3. Gas turbine
• Use in aircraft locomotive and stationary power generation
• Only 1/3 the power produced is useful for work, the rest used
to keep itself (driving the compressor) running.
• Gas turbines have been used in small sizes to produce power
for portable and standby generators, fire pumps, and
compressors.
• Large units are used for electricity generation in stationary
plants.
6. Principle of operations Open cycle
• The simple gas turbine operates by drawing air into
a compressor, compressing it and then discharging
the air into a combustion chamber.
• Fuel is added and burned, heating the air further.
• This hot air is then expanded through a turbine,
providing available power. Air is then exhausted to
atmosphere (open cycle).
• Not self starting; must use starting motor to rotate to
20 to 30% before fuel is turned on
7. Close cycle
• The air used to drive the turbine is in a
close cycle. It is separated from the
combustion air.
• Exhaust from turbine goes to the intake
of the compressor. It absorbs heat from
the furnace and expanded in the
turbine blade.
9. • The turbine and compressor are on the
same shaft so that the compressor obtains
its drive from the turbine.
• About two-thirds of this power is used
to drive the compressor, but the
remainder is available to drive a load or
provide thrust.
10. Efficiency
• The efficiency of such a machine depends
primarily upon the temperature to which the
air can be raised before entering the turbine.
• The output power, or power rating, of the
machine, depends upon the mass of hot gas
flowing through the gas turbine in a given
time per period.
• A gas turbine output increases when using
air of maximum density.
11. Common terms
• "simple" - it has no heat exchanger or regenerators.
• "open-cycle" - the air used to drive the turbine is
drawn from the atmosphere and returned to the
atmosphere after use.
• "closed-cycle" would use the same air again and
again.
• "single shaft" - is self-explanatory, there is only one
shaft. The gas turbine and the compressor are
coupled together mechanically on this shaft.
12. Gas Turbine Characteristics
• Gas turbines have certain
characteristics which distinguish them
from other types of prime movers,
some of these have proved to be
advantages and some disadvantages.
13. High Power to Weight Ratio
• Compared to a steam plant, a gas turbine is
much smaller and lighter for the same power.
This is a considerable advantage since the
building housing a gas turbine can be much
smaller and needs only light foundations.
Construction cost is lower.
14. Low Installed Cost
• The capital cost of a gas turbine is
lower per watt of output than a steam
plant.
15. Low Maintenance and Operating
Cost
• The absence of reciprocating motion
minimizes wear of moving parts.
Lubricating oil consumption is low.
Because of gas turbine simplicity,
many auxiliaries required in a steam
plant are eliminated.
16. Minimum Cooling Water
• Gas turbines require a minimum of
cooling water and can be operated
economically with a closed circulating
water system using air cooled heat
exchangers. This makes gas turbines
ideal for use wherever water is scarce
or unobtainable.
17. Rapid Start-Up and Loading
• A gas turbine can be started almost
instantaneously (Power up in 20 minutes). It
does not require slow and elaborate
preparations as does a steam turbine supplied
by a steam generator.
• The gas turbine is ideal for standby duties, such
as for fire pumps and emergency electricity
generators where instant starting is essential, or
where remote operation by telemetry is required.
18. Disadvantage
• Low Thermal Efficiency
• This is the major drawback of the gas
turbine. The thermal efficiency of the simple,
open-cycle gas turbine is considerably below
that of a steam plant, a diesel engine or a
gasoline engine.
• Auxiliary equipment must be fitted, such as
regenerators, in order to make the gas
turbine competitive.
19. High Noise Level
• Unless special precautions are taken to
baffle the exhaust, the gas turbine is a
noisy machine.
20. Pollution
• NOx – run at high temperature thus
produce NOx
• (NOx is produced around 1500 -1600°C)
22. Objective #3 Regeneration
• The purpose of the regenerator on the gas turbine is to
improve the cycle efficiency by recovering some of the
heat which would otherwise pass to waste with the
exhaust gases.
• The regenerator is placed in the air flow after the
compressor and before the combustion chamber.
• The compressor works most effectively with cold air. The
heat recovered from the exhaust gas reduces the fuel-quantity
required to produce the same load.
24. Combined Steam - Gas turbine Plants
• Gas turbines can be used in
conjunction with steam boilers in
various ways.
• Co-generation
• Waste heat boiler
• Combined cycle
25. Cogeneration
• pressurize the boiler furnace with the air leaving the
compressor and then pass the hot gases from the
boiler through a gas turbine to drive the compressor
and produce mechanical drive for electricity
generation.
• In this type of arrangement the gas turbine
contributes to the total plant capacity and the overall
plant economy is improved.
• The high furnace pressure at which a boiler of this
type operates permits a reduction in boiler size.
27. Gas Turbine Combined with a Steam Plant
• The high energy content of the gas
turbine's exhaust may be used in several
ways to increase the overall efficiency of a
power plant.
• The most common example is a waste
heat boiler where the turbine exhaust
gases are used to heat water to supply
steam for some other purpose.
28. Combined plant
• Another method uses the exhaust gases,
which are at a temperature of about 480ºC
and contain considerable excess oxygen,
as preheated air for a boiler furnace.
• Fuel may be used in conjunction with the
hot air to further burn the remaining
excess oxygen to produce steam.
29. • By employing a combined cycle, the thermal
efficiency can be raised to 60% and the design
retains several advantages over a straight steam
turbine plant.
• A combined cycle plant can be started up and
shut down rapidly.
• Several gas turbines can be used with one
steam turbine to produce more power, and
higher efficiencies can be maintained across a
wider range of loads (for example, 25% to 100%
range).
30. Combined cycle advantages
• Faster start-up: full load can be obtained
in 30 minutes.
• Lower cooling water requirements, by as
much as 67%.
• Lower atmospheric pollution from oxides
of nitrogen (NOX).
• Lower cost of investment per kilowatt
installed.
31. Objective 4 Gas turbine operation
• Turbine capacity is depended on the
density of air.
• Any change in atmospheric conditions that
affect the temperature and pressure of air
will affect the performance of the gas
turbine.
• The mass of inlet air flowing through the
compressor and, after combustion,
through the turbine, is highly dependent
on altitude and air temperature.
32. • Plant capacity will be higher in cold
weather as the density of the air will
increase, allowing more air into the
turbine, increasing mass flow.
• Changes in atmospheric pressure will
similarly affect turbine output.
• A turbine operating at sea level will have
higher density air (more mass) at its inlet
than one operating in higher altitude.
33. Gas turbine starting
• Manual – small plant
• Semi-automatic – operator starts
auxiliary and automatic control starts
the turbine
34. Starting
• Automatic – program starting
• engine starts to rotate the system;
when the speed is ~30% of max rpm,
the combustor will ignite
• when the rpm is 50% of max, started
engine shut off
• if the system does not reach operating
speed in a certain time, abort start up.
35. Load control
• Governor senses load-shaft speed to
adjust fuel to combustor to control the
shaft speed.
• Small unit uses fly ball governor; big
units employ more sophisticated
governors.
• Over speed trip – same as in steam
turbine.
36. Auxiliary systems
• Gas turbine is not self starting. It
requires electric or engine driven
starter. Speed has to be 30% of
maximum before it can be self-support.
• Lubrication – could use pump driven
by the gas turbine shaft. Auxiliary oil
supply is needed for cooling the
bearings in case of failure.
37. Blade cleaning
• Blade cleaning system is used to get rid of
foreign material deposited on blades.
Medium used could be detergent.
• In extreme case, crushed walnut shells
mixed with air could be used!
38. Fire protection
• Pre-charged halon or CO2 cylinders are
located in various parts of the system and
can discharge automatically.
• halon breaks down into corrosive and toxic
byproducts when it is exposed to surface
temperatures above 482°C (900°F).
• Water mist could also be used to for
protection
39. Turbine aging
• Loss of 0.5% per 1000 hours of service
or 4.4% per year is expected.
40. Environmental Considerations
• The exhaust gases of all engines contain some
constituents that are harmful in the atmosphere. If these
constituents are discharged in large enough quantities
for a long enough period of time then they will violate Air
Quality Standards.
• Generally, gas turbines produce trace amounts of carbon
monoxide and sulphur dioxide, but they can be
significant generators of oxides of nitrogen (NOX) and
waste heat.
• The operator should maintain proper combustion in the
turbine and use any equipment required to meet Air
Quality Standards.
41. Environmental Considerations 2
• Noise pollution by the gas turbine can be a
problem, but it may be overcome or
reduced by acoustic insulation and
exhaust silencers.
• Operators should pay special attention to
hearing protection regulations when
entering buildings housing gas turbines in
operation.
