DIESEL, GAS TURBINE & COMBINED CYCLE POWER PLANTS UNIT III

POWER PLANT ENGINEERING
S.BALAMURUGAN - M.E
ASSISTANT PROFESSOR
MECHANICAL ENGINEERING
AAA COLLEGE OF ENGINEERING & TECHNOLOGY
UNIT 2 – DIESEL, GAS TURBINE AND
COMBINED CYCLE POWER PLANTS

• A Generating station in which diesel
engine is used as prime mover for
generation of electrical energy is
known as diesel power station .
• Diesel power plant produce power
in the range of 2 – 50 MW .
• Diesel Power stations are favored
where demand of power is less
sufficient quantity of coal and water
is not available
• They are used as standby set for
continuity of supply such as
hospitals, radio stations, cinema
theatres, etc.
DIESEL ENGINE POWER PLANT
ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET

ADVANTAGES
• The design and layout of the plant are quiet simple.
• It occupies less space.
• It can be located at any place.
• It can be started quickly and can pick up load in a short time.
• It requires less quantity of water for cooling.
• The overall cost is much less than that of steam power plant of
same capacity.
• The thermal efficiency of the plant is higher.
• It requires less operating staff.
DISADVANTAGES
• The plant has higher running costs as the fuel that is used is costly.
• The plant does not work satisfactory under overload conditions for a longer
period.
• The plant can only generate small power.
• The cost of lubrication is generally high.
• The maintenance cost is also high.

APPLICATIONS
• They are used as central stations for small power supplies.
• They can be used as standby plants to hydrodynamic plants and steam power
plants In case of an emergency.
• They can be used as peak load plants in combination with thermal or hydro plants.
• They are quiet suitable for mobile power generations and are widely used in
transportation systems such as automobile, railways, ships.

GENERAL LAYOUT

ESSENTIAL ELEMENTS OF DIESEL POWER PLANT
• Starting system
• Air Intake System
• Fuel supply system
• Exhaust system
• Cooling system
• Lubrication system
• Governing system

STARTING SYSTEM
• 1. Starting by Auxiliary engine (petrol driven)
• Clutch is disengaged, auxiliary engine started by hand or electric motor,
once reach the optimal stage, clutch is engaged.
2. Use of Electric motors or Self Starters – Battery – Electric motor is coupled
with flywheel
3. Compressed air system
• The function of this system is to start the engine from cold by supplying
compressed air at about 17 bar from an air tank that is admitted into few
cylinders making them work like reciprocating air motors to run the engine
shaft
• Fuel is then injected into remaining cylinders and ignited in the normal way
causing the engine to start

ENGINE SYSTEM
• Generally classified as Two stroke engines and 4 stroke engines.
• In diesel engine air is admitted into the cylinder and is compressed.
• At the end of the compression stroke fuel is injected.
• The burning gases expand and does work on the piston.
• The gases are then exhausted from the cylinder.

AIR INTAKE SYSTEM
• The air intake system conveys fresh air through pipes or ducts to
• (i) the air intake manifold
• (ii) the supercharger inlet of a supercharged engine.
• Air filter is used to remove the dust from air which is taken by the engine.
• The super charger is used to increase the pressure of air supplied.
• Air is first drawn through the filter to catch dirt particles that may cause
excessive wear in cylinders.
• Filters may be of two types
• Dry type (paper, cloth etc.)
• Wet type (oil impingement type where oil helps to catch particles )

FUEL SUPPLY SYSTEM
• It includes storage tank, fuel pump, fuel transfer pump, strainers and
heaters.
• Pump draws diesel from storage tank through the filter to day tank.
• The day tank is usually placed high so the diesel flows to engine under
gravity.
• Diesel is filtered before being injected into the engine by the fuel injection
pump.

FUEL SUPPLY SYSTEM
This System performs the following function
• Filter the fuel.
• Meter the correct quantity of the fuel to be injected.
• Time the injection process.
• Regulate the fuel supply.
• Distribute the atomized fuel properly in the combustion chamber.

Classification of solid injection systems
Common rail injection system: The system is named after the shared high-
pressure (100 to 200 bars)reservoir (common rail) that supplies all the
cylinders with fuel. With conventional diesel injection systems, the fuel
pressure has to be generated individually for each injection. With the common
rail system, however, pressure generation and injection are separate, meaning
that the fuel is constantly available at the required pressure for injection.
•Individual pump injection system:
Distributor system:

CRDI

Individual pump Injection System

Individual pump Injection System
The schematic is shown in fig.
An individual pump or pump
cylinder connects directly to
each fuel nozzle.
Metering and injection timing
controlled by individual
pumps.
Nozzle contains a delivery
valve actuated by the fuel
pressure.

Distributor System
The schematic is shown here.
The fuel is metered at a central
point.
A pump meters, pressurizes and
times the fuel injection.
Fuel is distributed to cylinders in
correct firing order by cam
operated poppet valves which
admit fuel to nozzles.

EXHAUST SYSTEM
• This includes the silencers and connecting ducts.
• The exhaust gases coming out of the engine is noisy.
• Silencer is provided to reduce the noise.
• Exhaust pipe leading out of the building in should be short in length with
minimum number of bends to provide as low a pressure loss as possible.
• Each engine should have its independent exhaust system.
• Where possible exhaust heat recovery should be made to improve plant
thermal efficiency. Eg. Air heating, steam generation in diesel steam power
plant etc.

COOLING SYSTEM
• The cooling system consists of a water source, pump and cooling towers. The pump circulates
water through cylinder and head jacket. The water takes away heat from the engine and it
becomes hot. The hot water is cooled by cooling towers and re circulated for cooling.
• The temperature of the burning fuel inside the engine cylinder is in the order of 2750deg
Celsius. In order to lower this temperature water is circulated around the engine.
• Above 65°C, the lubricating oil will begin to evaporate.
• The water envelopes(water jacket) the engine. The heat from the cylinder, piston, combustion
chamber etc., is carried by the circulating water.
• The hot water leaving the jacket is passed through the heat exchanger
• The heat from the heat exchanger is carried away by the raw water circulated through the heat
exchanger and is cooled in the cooling system.
• Only 30% of heat – Useful Work
• 40% of heat – exhaust gas
• 1-1.5% of heat – Lubricating oil
• Remaining 30 % heat

COOLING SYSTEM
Liquid cooling:
• In this method, the cylinder
walls and head are provided
with jackets through which
the cooling liquid can
circulate.
• The heat is transferred from
the cylinder walls to the liquid
by convection and
conduction.
• The liquid gets heated during
its passage through the
cooling jackets and is itself
cooled by means of an air
cooled radiator system.

COOLING SYSTEM

COOLING SYSTEM
There are two methods of cooling I.C. engines:
1.Air cooling.
2. Liquid cooling
Air cooling:
• In this method, heat is carried away by
the air flowing over and around the
cylinder.
• Fins are added on the cylinder which
provide additional mass of material for
conduction as well as additional area for
convection and radiation modes of heat
transfer

LUBRICATION SYSTEM
The following are the important functions of a lubrication system:
• LUBRICATION: To keep parts sliding freely past
each other, reducing friction and wear.
• COOLING: To keep surfaces cool by taking away
part of the heat caused by friction.
• CLEANING: To keep the bearings and piston rings
clean.
• SEALING: To form a good seal B/W the piston
rings and cylinder walls.
• REDUCING NOISE: to reduce the noise of the
engine by absorbing vibration.

LUBRICATION SYSTEM
• It includes the oil pumps, oil tanks, filters, coolers and connecting pipes.
• The purpose of the lubrication system is to reduce the wear of the
engine moving parts, cool the engine
• Part of the cylinder such as piston , shafts , valves must be lubricated.
The lubricant is cooled before recirculation.
• Lubrication oil starts evaporating when the temperature inside the
cylinder exceeds 70deg

GOVERNING SYSTEM
• The function of the
governing system is
to maintain the
speed of the engine
• This is done
generally by varying
fuel supply to the
engine according to
load.
• It is achieved with
use of governors.

SUPERCHARGING
The apparatus used to increase the air density is called
supercharger. It is similar to a compressor(centrifugal
type), which provides greater mass of charge with same
piston displacement.
• The purpose of supercharging is to raise the volumetric
efficiency above that value which can be obtained by
normal aspiration.
• The engine is an air pump, increasing the air
consumption permits greater quantity of fuel to be
added, and results in greater potential output.
• The power output is almost directly proportional to the
air consumption.

SUPERCHARGING
Three methods to increase the air consumption are
1. Increasing the piston displacement: leads to more size and weight, cooling
problems
2. Running the engine at higher speeds leads to mechanical wear and tear.
3. Increasing the density of the charge, so that greater mass of charge is
introduced in same volume.
The supercharger produces following effects:
1. Provides better mixing of air fuel mixture due to turbulent effect of
supercharger.
2. The temperature of charge is raised as it is compressed, resulting in higher
temperature within the cylinder, so better vaporization of fuel.
3. Power required to run the supercharger is obtained from engine

DIESEL CYCLE

A diesel engine has a compression ratio of 15 and heat addition at
constant pressure takes place at 6% of stroke. Find the air standard
efficiency of the engine. Take γ of air as 1.4.

The stroke and cylinder diameter of a compression ignition engine are 250
mm and 150 mm respectively. If the clearance volume is 0.0004 m3 and fuel
injection takes place at constant pressure for 5% of the stroke. Determine the
efficiency of the engine. Assume the engine working on the diesel cycle.
Length of stroke L = 250 mm = 0.25 m
Molecules of neon,
a monatomic gas,
have only one
atom. Molecules of
diatomic gases,
such as hydrogen,
have two atoms

TURBINES: Machines to extract fluid power
from flowing fluids
Steam Turbine Water Turbines Gas TurbinesWind Turbines
Aircraft Engines
Power Generation
•High Pressure, High Temperature gas
•Generated inside the engine
•Expands through a specially designed TURBINE
Wind turbine VS Gas Turbine

GAS TURBINES
• Invented in 1930 by Frank Whittle, Patented in 1934
• First used for aircraft propulsion in 1942 on Me262 by Germans during
second world war
• Currently most of the aircrafts and ships use GT engines
• Used for power generation
• Manufacturers: General Electric, Pratt &Whitney, SNECMA, Rolls
Royce, Honeywell, Siemens – Westinghouse, Alstom
• Indian take: Kaveri Engine by GTRE, Bangalore(gas turbine research
establishment) (DRDO)
• A Gas turbine is a Turbo-machine and basically similar to steam turbine
regarding its working principle
• The first turbine to produce useful work was probably a wind mill,
where no compression and no combustion exist
• Now a days gas turbine include a compression process and
combustion process.

GAS TURBINE - PRINCIPLE OF OPERATION
• Intake
– Slow down incoming air
– Remove distortions
• Compressor
– Dynamically Compress air
• Combustor
– Heat addition through chemical
reaction
• Turbine
– Run the compressor
• Nozzle/ Free Turbine
– Generation of thrust power/shaft
power
Limitations
– Not self starting
– Higher rotor speeds
– low plant efficiency
– Low efficiency at part loads
– Thermal efficiency = 20 – 30%, (steam
PP 40%), combined plants 45%
Aviation & Marine - Light weight,
not requiring cooling water & fit
into the overall shape of the
structure
Power Generation – Simplicity, Lack
of cooling water, needs quick
installation, quick starting.
Oil & Gas industry – Cheap Fuel &
low installation cost.

Advantages and Disadvantages
• Great power-to-weight ratio
compared to reciprocating
engines.
• Smaller than their
reciprocating counterparts of
the same power.
• Lower installations &
maintenance costs.
• Less vibrations due to perfect
balancing
• Poor quality fuels can be used
• The ignition & Lubrication
systems are simpler.
• Special metals & alloys are required for
different components of the plants.
• Use of Nickel Chromium alloy, the
manufacture of the blades is difficult &
costly.
• Special cooling methods are required
for cooling the turbine blades.
• High temperature in Blades (1000K) &
Combustion chamber (2500K) &
Centrifugal force leads to short life of
this components
• For the same output, gas turbine
produces five times exhaust gases than
I.C engines
• Poor thermal efficiency at Part loads, as
the quantity of air remains same
irrespective of load, & output is reduced
by reducing the quantity of fuel
supplied.

BRAYTON CYCLE: IDEAL CYCLE FOR GAS-TURBINE ENGINES
Gas turbines usually operate on an open cycle
Air at ambient conditions is drawn into the compressor, where its temperature and pressure
are raised. The high pressure air proceeds into the combustion chamber, where the fuel is
burned at constant pressure.
• The high-temperature gases then
enter the turbine where they expand to
atmospheric pressure while producing
power output.
• Some of the output power is used to
drive the compressor.
• The exhaust gases leaving the turbine
are thrown out (not re-circulated),
causing the cycle to be classified as
an open cycle.

• The open gas-turbine cycle can be
modelled as a closed cycle, using the
air-standard assumptions.
• The compression and expansion
processes remain the same, but the
combustion process is replaced by a
constant-pressure heat addition
process from an external source.
• The exhaust process is replaced by a
constant-pressure heat rejection
process to the ambient air.
Closed Cycle Model
AIR-STANDARD ASSUMPTIONS
• Air is the working fluid, it obeys the perfect gas law, pV = mRT
• The engine operates in a closed cycle. The cylinder is filled with constant amount of
working medium & the same fluid is used repeatedly, mass remains constant.
• The compression & expansion processes are assumed to be adiabatic
• The values of specific heat (Cp & Cv) of the working fluid remains constant
• Combustion replaced by Constant pressure heat addition, Exhaust replaced by Heat
rejection process.

37
COMPARE OPEN CYCLE AND CLOSED CYCLE GAS TURBINES
OPEN CYCLE:
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. Open cycle plants occupy comparatively little space.
4. Open-cycle gas turbine power plant, except those having an intercooler,
does not require cooling water.
5. The part load efficiency of the open cycle plant decreases rapidly as
the considerable percentage of power developed by the turbine is used to
drive the compressor.
6. The open-cycle gas turbine plant has high air rate compared to the other
cycles, therefore, it results in increased loss of heat in the exhaust gases.

38
COMPARE OPEN CYCLE AND CLOSED CYCLE GAS TURBINES
CLOSED CYCLE:
1. The machine can be smaller and cheaper than the machine used to
develop the same power using open cycle plant.
2. The closed cycle avoids erosion of the turbine blades due to the
contaminated gases and fouling of compressor blades due to dust. Therefore,
it is practically free from deterioration of efficiency in service.
3. The need for filtration of the incoming air which is a severe problem in open
cycle plant is completely eliminated.
4. The maintenance cost is low and reliability is high due to longer useful life.
5. The system is dependent on external means as considerable quantity of
cooling water is required in the pre-cooler.
6. The response to the load variations is poor compared to the open-cycle
plant

Emission in Gas Turbines
•Lower emission compared to all conventional methods (except nuclear)
•Regulations require further reduction in emission levels

The ideal cycle that the working fluid undergoes in the
closed loop is the Brayton cycle. It is made up of four
internally reversible processes:
1-2 Isentropic compression;
2-3 Constant-pressure heat addition;
3-4 Isentropic expansion;
4-1 Constant-pressure heat rejection.
Compressor isentropic Efficiency
ηcomp.=
𝑊𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛 𝑖𝑠𝑒𝑛𝑡𝑟𝑜𝑝𝑖𝑐 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛
𝐴𝑐𝑡𝑢𝑎𝑙 𝑤𝑜𝑟𝑘 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
=
𝑇2
−𝑇1
𝑇2
′−𝑇1
Turbine isentropic Efficiency
ηturbine.= 𝐴𝑐𝑡𝑢𝑎𝑙 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡
𝐼𝑠𝑒𝑛𝑡𝑟𝑜𝑝𝑖𝑐 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡
=
𝑇3
−𝑇4
′
𝑇3
−𝑇4
THE BRAYTON CYCLE

BRAYTON CYCLE
Heat Addition (Combustion Chamber) = m Cp (T3 – T2)
Heat Rejected (Cooler) = m Cp (T4 – T1)
Efficiency η =
𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒
𝐻𝑒𝑎𝑡 𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛
=
𝐻𝑒𝑎𝑡𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛−𝐻𝑒𝑎𝑡 𝑅𝑒𝑗𝑒𝑐𝑡𝑒𝑑
𝐻𝑒𝑎𝑡 𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛
=
m Cp (T3 – T2) −m Cp (T4 – T1)
m Cp (T3 – T2)
= 1 -
(T4 – T1)
(T3 – T2)
Isentropic process (1-2 & 3-4)
𝑇2
𝑇1
=
𝑃2
𝑃1
𝛾 −1
𝛾 𝑇3
𝑇4
=
𝑃3
𝑃4
𝛾 −1
𝛾
=
𝑃2
𝑃1
𝛾 −1
𝛾
T2 = T1 𝑟 𝑝
𝛾 −1
𝛾 T3 = T4 𝑟 𝑝
𝛾 −1
𝛾
Air Standard Efficiency η = 1 -
(T4 – T1)
(T4 𝑟 𝑝
𝛾 −1
𝛾 –T1 𝑟 𝑝
𝛾 −1
𝛾 )
= 1 -
1
𝒓 𝒑
𝜸 −𝟏
𝜸
Work ratio =
𝑊 𝑛𝑒𝑡
𝑊𝑡𝑢𝑟𝑏𝑖𝑛𝑒
=
−𝑊𝑐𝑜𝑚𝑝𝑟𝑒
= 1 -
(T2 – T1)
(T3 – T4)
= 1 -
𝑻 𝟏
𝑻 𝟑
𝑟 𝑝
𝛾 −1
𝛾
rp = P2 / P1

The air enters the compressor of an open cycle constant pressure gas turbine at a
pressure of 1 bar and temperature of 20°C. the pressure of the air after
compression is 4 bar. The isentropic efficiencies of compressor & turbines are 80%
& 85% respectively. The air-fuel ratio used is 90:1. If flow rate of air is 3 kg/s, Find
(i) Power Developed (ii) Thermal Efficiency of the cycle. Assume Cp = 1.0 kJ/kg K
and Ƴ = 1.4 for air & gases. Calorific value of fuel = 41800 kJ/kg.
GIVEN – P1 = 1 bar, T1 = 20°C = 20+273 = 293 K, P2 = 4 bar, ηcomp = 80%, ηcomp = 85%
Air –fuel ratio = 90:1 , Air flow rate ma = 3 kg/s, C.V = 41800 kJ/kg
Power Developed, P = Net work, Wnet = Wturbine - Wcompressor
𝑇2
𝑇1
=
𝑃2
𝑃1
𝛾 −1
𝛾
=
4
1
1.4 −1
1.4
= 1.486 , T2 = 293 × 1.486 = 435.4 K
ηcomp =
𝑇2
−𝑇1
𝑇2
′−𝑇1
, 0.8 =
435.4 −239
𝑇2
′−293
, 𝑻 𝟐
′= 471 K
Heat supplied by fuel = Heat taken by burning gases
mf × C.V = (ma + mf ) Cp (𝑻 𝟑 − 𝑻 𝟐′)
C.V = (
ma
mf
+ 1) Cp (𝑻 𝟑 − 𝟒𝟕𝟏), T3 =
41800
91
+ 471 = 930 K = T3

Problem continued….
𝑻 𝟑
𝑻 𝟒
=
𝑷 𝟑
𝑷 𝟒
𝜸 −𝟏
𝜸
=
𝑷 𝟐
𝑷 𝟏
𝜸 −𝟏
𝜸
=
𝟒
𝟏
𝟏.𝟒 −𝟏
𝟏.𝟒
= 1.486, T4 =
𝟗𝟑𝟎
𝟏.𝟒𝟖𝟔
= 625.8 K
ηturbine =
𝑻 𝟑
−𝑻 𝟒
′
𝑻 𝟑
−𝑻 𝟒
, 0.85 =
𝟗𝟑𝟎 −𝑻 𝟒
′
𝟗𝟑𝟎−𝟔𝟐𝟓.𝟖
, 𝑻 𝟒
′ = 670.6 K
Wturbine = mg × Cp × (T3 – T4’) mg – mass of hot gases formed
Wturbine = (
𝟗𝟎+𝟏
𝟗𝟎
)× 1.0 × (930 – 670.6)
= 262.28 kJ/kg of air
Wcompressor = ma × Cp × (T2’– T1) = 1 × 1.0 × (471 – 293) = 178 kJ/kg of air
Wnet = Wturbine – Wcompressor = 262.28 – 178 = 84.28 kJ/kg of air
Thermal Efficiency = ηthermal =
𝑾𝒐𝒓𝒌 𝒐𝒖𝒕𝒑𝒖𝒕
𝑯𝒆𝒂𝒕 𝑺𝒖𝒑𝒑𝒍𝒊𝒆𝒅
=
𝟖𝟒.𝟐𝟖
*
𝟏
𝟗𝟎
𝟒𝟏𝟖𝟎𝟎+
= 0.1814 = 18.14%
Power Developed = P = 84.28 × 3 = 252.84 kW

45
The thermal efficiency of an ideal Brayton cycle
depends on the pressure ratio, rp of the gas turbine
and the specific heat ratio, γ of the working fluid.
The thermal efficiency increases with both of these
parameters, which is also the case for actual gas
turbines.
A plot of thermal efficiency versus the pressure ratio
is shown in fig. for the case of γ =1.4.
Pressure ratio – It is the ratio of cycle’s highest to its
lowest pressure, (Compressor outlet to compressor
inlet)
PARAMETERS AFFECTING THERMAL EFFICIENCY

46
The early gas turbines (1940s to 1959s) found only limited use despite their
versatility and their ability to burn a variety of fuels, because its thermal efficiency
was only about 17%. Efforts to improve the cycle efficiency are concentrated in
three areas:
1. Increasing the turbine inlet (or firing) temperatures.
The turbine inlet temperatures have increased steadily from about 540°C (1000°F)
in the 1940s to 1425°C (2600°F) and even higher today.
2. Increasing the efficiencies of turbo-machinery components (turbines,
compressors).
The advent of computers and advanced techniques for computer-aided design
made it possible to design these components aerodynamically with minimal losses.
3. Adding modifications to the basic cycle (intercooling, regeneration or
recuperation, and reheating).
The simple-cycle efficiencies of early gas turbines were practically doubled by
incorporating intercooling, regeneration (or recuperation), and reheating.
IMPROVEMENTS OF GAS TURBINE’S PERFORMANCE

METHODS TO IMPROVE THERMAL EFFICIENCY - INTERCOOLING
• Compressor utilize major percentage of power developed by turbine
• The work required by compressor can be reduced by compressing the air in two stages &
incorporate intercooler between the compressor.
• Work input (with intercooling) = Cp (T2’ – T1) + Cp (T4’ – T3)
• Work input (with out intercooling) = Cp (TL’ – T1) = Cp (T2’ – T1) + Cp (TL’ – T2’)
• Work input with intercooling < Work input without Intercooling
• Work ratio = Net work output / Gross work output =
𝑾𝒐𝒓𝒌 𝒐𝒇 𝑬𝒙𝒑𝒂𝒏𝒔𝒊𝒐𝒏 −𝑾𝒐𝒓𝒌 𝒐𝒇 𝑪𝒐𝒎𝒑𝒓𝒆𝒔𝒔𝒊𝒐𝒏
𝑾𝒐𝒓𝒌 𝒐𝒇 𝑬𝒙𝒑𝒂𝒏𝒔𝒊𝒐𝒏

METHODS TO IMPROVE THERMAL EFFICIENCY - REHEATING
• The output of a gas turbine can be increased by expanding the gases in two stages with a
reheater between the two turbine
• High pressure turbine output = Work input required for compressor
• Net work output of L.P. Turbine ( With Reheating) = Cp(T5 – T6’)
• Net work output of L.P. Turbine ( Without Reheating) = Cp(T4’ – TL’)
• Increases the Net work output
Net work output of L.P. Turbine ( With Reheating) > Net work output of L.P. Turbine ( Without Reheating)
• Reduces the Thermal Efficiency, Heat supplied = Cp(T3 – T2’) + Cp(T5 – T4’)

METHODS TO IMPROVE THERMAL EFFICIENCY - REGENERATION
• Exhaust gases from a gas turbine carry a large quantity of heat . Higher than the ambient
temperature.
• This can be used to heat the air coming from the compressor, thereby reducing the mass of
fuel supplied in combustion chamber.
• 2’ – 3 = Heat flow into the compressed air from Heat exchanger
• 3 – 4 = Heat taken from Combustion chamber, 6 – Temperature of Exhaust gas
• Effectiveness =
𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑒𝑛𝑡𝑕𝑎𝑙𝑝𝑦 𝑝𝑒𝑟 𝑘𝑔 𝑜𝑓 𝑎𝑖𝑟
𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑖𝑛𝑐𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑒𝑛𝑡𝑕𝑎𝑙𝑝𝑦 𝑝𝑒𝑟 𝑘𝑔 𝑜𝑓 𝑎𝑖𝑟
=
𝑇3
−𝑇2
′
𝑇5
′−𝑇2
′

Brayton Cycle With Regeneration
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.
Note:
The use of a regenerator
is recommended only
when the turbine exhaust
temperature is higher than
the compressor exit
temperature.

Inlet system Collects and directs air into the gas turbine. Often, an air cleaner and silencer are
part of the inlet system. It is designated for a minimum pressure drop while maximizing clean
airflow into the gas turbine.
Compressor Provides compression, and, thus, increases the air density for the combustion
process. The higher the compression ratio, the higher the total gas turbine efficiency . Low
compressor efficiencies result in high compressor discharge temperatures, therefore, lower gas
turbine output power.

52
GAS TURBINE COMPONENTS
Combustor Adds heat energy to the airflow. The output power of the gas turbine is
directly proportional to the combustor firing temperature; i.e., the combustor is
designed to increase the air temperature up to the material limits of the gas turbine
while maintaining a reasonable pressure drop.
Gas Producer Turbine Expands the air and absorbs just enough energy from the
flow to drive the compressor. The higher the gas producer discharge temperature
and pressure, the more energy is available to drive the power turbine, therefore,
creating shaft work.
Power Turbine Converts the remaining flow energy from the gas producer into
useful shaft output work. The higher the temperature difference across the power
turbine, the more shaft output power is available.
Exhaust System Directs exhaust flow away from the gas turbine inlet. Often a
silencer is part of the exhaust system. Similar to the inlet system, the exhaust
system is designed for minimum pressure losses.

Intercooling and reheating always decreases thermal efficiency unless are accompanied by
regeneration.
Therefore, in gas turbine power plants, intercooling and reheating are always used in conjunction with
regeneration.
The work input to a two-stage compressor is minimized
when equal pressure ratios are maintained across each
stage. This procedure also maximizes the turbine work
output.
The net work output of a
gas-turbine cycle can be
increased by either:
a) decreasing the
compressor work, or
b) increasing the turbine
work, or
c) both.
The work output of a turbine can be increased by expanding
the gas in stages and reheating it in between, utilizing a
multistage expansion with reheating.

COMBINED GAS TURBINES & STEAM POWER PLANTS
The combination gas turbine – steam cycles aim at utilizing the heat of exhaust gases
from the gas turbine, improves the overall efficiency.

COMBINED GAS TURBINES & DIESEL POWER PLANTS
Improve Efficiency
More suitable for Rapid start & shut
down
Less cooling water requirement
It gives high ratio of power output to
occupied ground space
ADVANTAGESOF COMBINEDCYCLE

STARTING OF A GAS TURBINE
• Auxiliary power source required till the plants own compressor inducts air & compresses it to a
pressure such that expansion from reasonable temperature will develop enough power to
sustain operation. IC Engine, Steam turbine, Auxiliary Electric motor, Another Gas turbine
PROCEDURE
• Run the unit
• Actuate the combustion ignition system & inject the fuel. The fuel flow is controlled to obtain the
necessary warm-up
• Adjust the speed & voltage & synchronize the alternator
• Build up the load on the alternator by governor gear control
SHUT DOWN OF A GAS TURBINE
• It occurs very quickly after the fuel is cut off from the combustion chamber.
• Rapid Shut down – Minimizing the Chilling effect – (Cold air through Hot Turbine)
• To avoid Thermal Kinking of shaft - Starting device operated at reduced speed to ensure
symmetrical cooling of rotor.

DIESEL, GAS TURBINE & COMBINED CYCLE POWER PLANTS UNIT III

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DIESEL, GAS TURBINE & COMBINED CYCLE POWER PLANTS UNIT III