TURBO MACHINES
Steam and Gas Turbines
Eng. R. L. Nkumbwa MSc, REng.
Copperbelt University, ST
©August, 2010
Intended Contents
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Gas Turbine
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The Power Cycle for a Jet Engine
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Steam Turbine
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Joule (UK) or Brayton (US) Cycle
Rankine or Vapor Cycle
Combined Cycles
Gas Turbine Engine Pressure Ratio
“Classical Thermodynamics is the only
physical theory of universal content which
…. within the framework of its basic
notions, will never be toppled.”
Albert Einstein
3
Global Energy Crisis!!
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Do we have an Energy, Enthalpy or an Entropy Crisis?
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Freeman Dyson explains Entropy as a "measure
of disorder in a physical system".
Another useful thermodynamic quantity in the
context of energy conversion is Enthalpy, which
is used to quantify the thermal energy content of
hot steam.
The energy available from a thermal power
system depends on the temperature and
pressure of the hot steam entering the turbine.
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Gas Turbines
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So, what are Gas Turbines?
Popularly called the “GT”
Copperbelt Energy Corporation (CEC) has
Rolls-Royce Gas Turbines at Luano Station
along Chingola Road.
Gas Turbine Principle
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A gas turbine is a rotary machine, similar in
principle to a steam turbine.
In an open cycle gas turbine working on Joule
cycle, the air is compressed in a rotary
compressor and passed into a combustion
chamber where fuel is burnt, the products of
combustion are then made to impinge over rings
of turbine blades with high velocity and work is
produced.
7
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Sadi Carnot, Reflection On The Motive Power of Heat And On
Machines Fitted To Develop That Power, 1824.
“Nature, in providing us with combustibles on all
sides, has given us the power to produce, at all times
and in all places, heat and the impelling power which
is the result of it.
To develop this power, to appropriate it to our uses,
is the objective of heat engines. The study of these
engines is of greatest interest, their importance is
enormous, their use is continually increasing, and
they seem destined to produce a great revolution in
the civilized world.”
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Nicolas Léonard Sadi Carnot
(1796-1832) “French Army”
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10
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Carnot Animation
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http://www.cs.sbcc.net/~physics/flash/heaten
gines/Carnot%20cycle.html
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Carnot Animation
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http://www.cs.sbcc.net/~physics/flash/heaten
gines/Carnot%20cycle.html
Carnot Animation
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http://www.cs.sbcc.net/~physics/flash/heaten
gines/Carnot%20cycle.html
So, What is a Heat Engine?
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Real Engines
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Heat Engines in the real world are constrained
by various factors.
In commercial power plants the work needed to
turn the generator is supplied by a device called
turbine, which has curved blades at the
circumference and the working fluid is air and
water at ambient temperature (in case of
hydroelectric plants) or high temperature
(in steam cycle plants).
Real Engines
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Most commercial power plants use steam or hot
gas supplied by combustion of fuels to propel the
turbine.
Efficiencies of steam cycle plants are low, but the
quantity of working fluid is less.
There are different types of cycles that can be
employed for extracted work from steam and
other working fluids.
What is Work?
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Efficiency depends on the heat input and work
output.
The Work Output can mean different things to
different folks…
Work to turn a shaft …. electric generator, ship
propulsion, helicopter rotor,
Turboprop engine, Abrams A1A tank, Automotive
propulsion, …
Thrust efficiency @ NASA, etc
What Drives this AV-8B Harrier Attack
Fighter Jet?
17
Harrier Fighter Jet
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Harrier Vertical Takeoff
Gulf War / Iraq / Afghanistan
US Marines / UK Royal Air Force / Italy –
Spain
Harrier Fighter Jet
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Power Plant:
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One Rolls-Royce Pegasus 105 vectored-thrust
turbofan.
Thrust = 21,750 lb
Max payload for vertical take off ~9,000 lb
Max payload for short take off ~17,000 lb
Max speed ~ Mach 0.98
What is the Motive Power?
20
Gas Turbine
21
Gas Turbine Characteristics
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Relatively Small
Light Weight
Balanced Operation
No Oil
More Reliable
Gas Turbines Timeline…
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1791 John Barber patent but nothing more
1872 -04 Stolze, no result
1882 -03 Aegidius Elling 11-44 hp
1901 -06 Charles Lemale patent, Rateau design
1906-08 Hans Holzwarth Brown Boveri
1936 Noack (Stodola ) at Brown Boveri
1939 Escher -Wyss, closed cycle with He
1939 Ganz-Jendrassik
1930 -39 Frank Whittle patented with Han von Ohain
1936 -39 Hans von Ohain First Flight
Steam Turbine Timeline…
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1848 Francis first turbine
1849 James Francis improved Howdturbine
1874 Francis turbine with variable guide vanes
1880s Modern pumps by Massachusetts pump in
USA
1880 Peltons free jet turbine
1905 Föttingers torque converter
1913 Victor Kaplans propeller turbine
Gas Turbine Heat Source
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Liquid petrol
Gas
Coal
Residuals
Gasified coal
Nuclear
Bio, renewable
All other
Problems Associated with Turbines
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Turbine erosion
Turbine corrosion
Fuel oxidizer
Stochiometric temperature
Turbine Elementary Components
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Compressor
Turbine
Combustor
Heat exchanger
Gear
Flow divider
Flow unifier
Nozzle
Gas Turbine Layouts
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Single shaft
Free load turbine
Two spool
Three spool
Separate units
Variable geometry
Several combustors and heat exchangers
Gas Turbine vs. Steam Turbines
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Direct heating
Higher max temp
Cooling possible
Liquid compression
Closed system
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Liquid compression
Closed system
Turbine Cycle Improvements
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Reduce compression work
Increase expansion work
Reduce outlet heat loss
Improve thermodynamics
Inter-cooling
Reheat
Heat exchanger
Gas Turbine Applications
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Jet Engines
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Straight jet
Turbofan
Turboprop
Helicopter
Pump units
Compressor units
Naval or Marine Engines
Gas Turbine Applications
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Power Generation
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Not for Land transportation
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Peak load
Auxiliary Power Unit (APU)
Base load CC, CHP
Trucks
Trains
Gas Turbine Operation
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Military jets 500-5000 h
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Civil jets 5000-20000 h
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Stationary GT >100000 h
Gas Turbine Sizes
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0.5 - 10 MW vehicle
40% Simple cycle
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20 - 100 MW mobile unit
60%Combined cycle
Brayton Cycle
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The Brayton cycle models power systems
based on Gas Turbines.
When hot gas is used to drive turbine to
generate work, the energy conversion process is
much simpler because the working fluid (gas) is
directly heated without need for a large boiler as
in case of steam cycle plants.
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Brayton Cycle
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Brayton Closed Cycle Operation
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Brayton Open Cycle Operation
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Components and States in a Brayton
Combustion Gas Turbine cycle.
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Brayton Combustion Gas Turbine cycle
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Brayton Combustion Gas Turbine cycle
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The Brayton cycle (or Joule cycle) represents the
operation of a gas turbine engine.
The cycle consists of four processes, as shown
in Figure above of an engine:
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a - b Adiabatic, quasi-static (or reversible)
compression in the inlet and compressor;
b - c Constant pressure fuel combustion (idealized as
constant pressure heat addition);
Brayton Combustion Gas Turbine cycle
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c - d Adiabatic, quasi-static (or reversible) expansion
in the turbine and exhaust nozzle, with which we take
some work out of the air and use it to drive the
compressor, and take the remaining work out and use
it to accelerate fluid for jet propulsion, or to turn a
generator for electrical power generation;
d - a Cool the air at constant pressure back to its initial
condition.
Brayton Components
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Temperature Entropy Diagram of Ideal
Brayton Cycle
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Brayton Cycle
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Air is first compressed in a compressor and then
heated in a combustion chamber fired by cleaner
fuel like natural gas.
The working fluid in this case would be a mixture
of air and the combustion products (carbon
dioxide, water vapor and nitrous oxide).
Dirtier fuels like coal cannot be used in this
cycle. As in the case of steam cycle, the hot gas
is directed through a nozzle to drive the gas
turbine blades that turns a generator to produce
work or electricity.
Brayton Cycle
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At the turbine exit, the gas has to be cooled to a
temperature of around 550 degree Celsius,
which is still hot but not sufficient to efficiently
extract additional work in the turbine.
At most gas turbine plants the exhaust gas is
directly vented into the atmosphere.
Simple gas plants and airplane engines are
common examples of Brayton cycle in an open
cycle arrangement.
Brayton Thermodynamic Efficiency
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The thermodynamic efficiency of a Brayton cycle
can be defined using enthalpy changes
between various points inside a gas plant as:
Brayton Thermodynamic Efficiency
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Alternatively, the efficiency of a Brayton cycle
can also be expressed in terms of pressure
ratios and the thermodynamic properties of
air and combustion products.
Brayton efficiency equations says that for a
high cycle efficiency, the pressure ratio of
the cycle should be increased.
See figure below the effects of increased
pressure
Methods to Improve Efficiency
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The efficiency of a Brayton engine can be
improved in the following manners:
Intercooling, wherein the working fluid passes through a first stage
of compressors, then a cooler, then a second stage of compressors before
entering the combustion chamber. While this requires an increase in the fuel
consumption of the combustion chamber, this allows for a reduction in the
specific volume of the fluid entering the second stage of compressors, with
an attendant decrease in the amount of work needed for the compression
stage overall. There is also an increase in the maximum feasible pressure
ratio due to reduced compressor discharge temperature for a given amount
of compression, improving overall efficiency.
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Methods to Improve Efficiency
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Regeneration, wherein the still-warm postturbine fluid is passed through a heat exchanger
to pre-heat the fluid just entering the combustion
chamber. This directly offsets fuel consumption
for the same operating conditions improving
efficiency; it also results in less power lost as
waste heat.
Methods to Improve Efficiency
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A Brayton engine also forms half of the
combined cycle system, which combines with
a Rankine engine to further increase overall
efficiency.
Cogeneration systems make use of the waste
heat from Brayton engines, typically for hot
water production or space heating.
Methods to Increase Power
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The power output of a Brayton engine can be
improved in the following manners:
Reheat, wherein the working fluid—in most cases air—expands
through a series of turbines, then is passed through a second
combustion chamber before expanding to ambient pressure
through a final set of turbines. This has the advantage of increasing
the power output possible for a given compression ratio without
exceeding any metallurgical constraints (typically about 1000°C).
The use of an afterburner for jet aircraft engines can also be
referred to as reheat, it is a different process in that the reheated air
is expanded through a thrust nozzle rather than a turbine.
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Methods to Increase Power
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The metallurgical constraints are somewhat alleviated enabling
much higher reheat temperatures (about 2000°C).
The use of reheat is most often used to improve the specific
power (per throughput of air) and is usually associated with a
reduction in efficiency, this is most pronounced with the use of
afterburners due to the extreme amounts of extra fuel used.
Trend of Brayton cycle thermal efficiency
with compressor pressure ratio
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Basic Gas Generator
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Basic Gas Generator
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Shaft could provide power take off for electric
generator, ship propulsion, etc.
Compressor and turbine could be axial or
radial.
Altitude Vs. Mach Number
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Turbojet Systems
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Turbojet Systems
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Output of gas turbine passed through exhaust
nozzle and used entirely for thrust.
High subsonic and supersonic.
Developed by the British - Wiggins Co., Rolls
Royce, Parsons, Bristol Engines, ABB,
SIEMENS
Turbojet Systems
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And the Germans - Junkers, BMW,
Messerschmidt in 1930’s and 1940’s.
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General Electric (GE) started US production in
1942 in Massachusetts, today with a production
facilities in Angola and Egypt
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Original designs from Frank Whittle who had a
centrifugal compressor.
So, who is Sir Frank Whittle?
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Sir Frank Whittle
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Air Commodore, OM, KBE, CB, FRS, Hon FRAeS
Born in Earlsdon, CoventryNRL, United Kingdom
on 1st June 1907 – Died 9th August 1996
Was a British Royal Air Force (RAF) Officer.
Sharing credit with Germany's Dr. Hans von
Ohain for independently Inventing the Jet
Engine,
He is hailed as a Father of Jet Propulsion.
Whittle W2/700 Turbojet Engine - 1943
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Whittle’s Contributions…
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Frank Whittle, Royal Air Force, patented a jet
engine in 1930.
Axial PLUS centrifugal compression, 2-stage
turbine.
Free vortex turbine blade design.
Host of mechanical and thermal challenges, not
too reliable.
Whittle considered high temperature ceramic
blades.
Real Engineers! See Real Engines!
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Eng. Nkumbwa visits Sir Frank Whittle birth
place and invention centre in Rugby,
Coventry – UK.
See Living evidence below…
Sir Frank Whittle
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MIT Video Lecture on Sir Frank Whittle Jet
Engine Invention in Coventry, England
Engineer Nkumbwa visits Sir Frank Whittle
birth place in Rugby – Coventry, England
2006.
Turbojet Operation
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Output of gas turbine passed through exhaust
nozzle and used entirely for thrust.
High subsonic and supersonic.
Developed by the British (Wiggins Co., Rolls
Royce, Parsons, Bristol Engines)
And the Germans (Junkers, BMW,
Messerschmidt) in 1930’s and 1940’s.
GE started US production in 1942 in
Massachusetts.
T-s Block Diagram for a TBJET
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Turbofan
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Exhaust passes through additional turbine stage
to power a ducted fan that accelerates
A large stream of air passing around the core.
Typically flow through the outer part of the fan is
5-6 times that flowing through the engine core
(compressor).
Thrust from both hot gases leaving the nozzle
and from the cold by-pass flow.
Bypass ratio = flow through fan / Flow through
engine Type values: 0 -10
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Turbofan Operation
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Turbofan Operation
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T-s Diagram for a Turbofan
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Turboprop Operation
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Turboprop Operation
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Actually developed prior to the turbofan.
Rate of airflow through the prop may be 25-30
times that through the engine.
Gear box necessary for both lower prop speed
and higher turbine speed.
At higher speeds, tip losses considerable.
Also called a Turbo shaft engine to power
helicopter or marine.
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Ramjet Engine
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Ramjet Engines
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Air-breathing engine similar to a turbojet but
without the mechanical compressor or turbine.
Compression is accomplished entirely by ram air
and it is thus sensitive to vehicle forward speed.
No thrust at rest.
Subsonic combustion, interior air slowed to
subsonic speed for combustion.
Mach number: 3~6
Scramjet Engines
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Scramjet Engines
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Supersonic combustion ramjet. The flow
through the combustor is still supersonic.
“Strange and tricky” combustion dynamics.
Typically hydrogen fuels.
Mach number >5+
Combined Ramjet and Scramjet Engine
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Rocket Engine
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Internal forces to Create Thrust
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Thrust Force Analysis
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The thrust from a jet engine is not as easy to
assess as a rocket.
Thrust and drag forces occur throughout the
engine.
Del-P across compressor stages helps push
engine forward.
Internal Pressure & Thrust Distribution
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Single Shaft Turbo Engine
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Power to Weight Ratio
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Look at the following Case Study for the
“Proflight Zambia Airlines” which operates
between Lusaka and Copperbelt
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Gas Turbines Engine Manufacturers
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General Electric (GE) - Ecomagination
Rolls-Royce
Platt & Whitney (PW)
ABB
SIEMENS
Gas Turbine Engine Pressure Ratios
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See the next slide
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Gas Turbines Advantages
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They operate at high temperatures.
They can and are capable of meeting peak
load demands.
They are compact and easy to operate, and
take advantage of aerospace propulsion
applications.
They operate at relatively low pressures.
Gas Turbines Advantages…
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Many installations burn natural gas, or in dual
fuel mode burning NG and/or oil.
They do not handle wet gases, and are not as
vulnerable to corrosion as steam turbines.
Combustion Gas Turbines do not require heat
transfer equipment on the low-temperature side,
and no coolant either
Gas Turbines Limitations…
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They have relatively low efficiency since their
maximum temperature is limited by material.
Their efficiency is low because of the high
compressor work, and low efficiency of
compressors.
Gas Turbines Limitations
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Open cycle turbines are limited by the high
exhaust temperature, which limits the turbine
work.
They cannot be used with “dirty” fuels, such
as coal, since sulfur oxides can damage their
blades.
Steam or Rankine Cycle
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Steam, or Vapor Rankine Cycles overcome some
of these limitations, and hence have been very
popular in electric power generation.
All steam cycle plants are modeled using what is
known as Rankine cycle.
Steam cycle can be constructed using closed and
open cycles.
Open cycle require dumping exit steam into
environment and so is restricted to 100 degree
Celsius.
Rankine Cycle Plants
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Modern steam cycle plants consist of a boiler
which generates steam using the energy
provided by coal, oil, gas or nuclear fuels.
In addition they have a feed water pump that
pumps the fully condensed steam exiting the
condenser to the boiler at high pressure.
Rankine Efficiency
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1
Efficiencies are very low in such arrangement.
Modern steam plants are based on a closed
cycle arrangement that uses a condenser to cool
the waste steam, an innovation that was
originally conceived and designed by James
Watt.
Rankine Efficiency
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2
Here overall efficiency is the electrical output
from the turbine generator minus the energy
needed to operate the feed water pump and
frictional losses in the turbine generator.
The steam cycle efficiency in this case is defined
by the following equation:
Rankine Efficiency
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3
Modern steam plants achieve steam cycle
efficiencies of 30-45%.
It is possible to raise efficiencies by maximizing
the inlet steam enthalpy, but these are limited
due to engineering constraints.
Components of a Rankine Cycle
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4
T-s Diagram of Open Rankine Cycle
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Rankine Power Cycles Operation
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A schematic of the components of a Rankine
cycle is shown in Figure below.
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The cycle is shown on P-v, T-s, and h-s
coordinates.
Rankine p-v Coordinates
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Rankine T-s Coordinates
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Rankine h-s Coordinates
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Rankine Cycle Process
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The processes in the Rankine cycle are as
follows:
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a b: Heat added at constant temperature T2 (constant
pressure), with transition of liquid to vapor
c d: Liquid-vapor mixture condensed at temperature
T1 by extracting heat.
d e: Cold liquid at initial temperatureT1 is pressurized
reversibly to a high pressure by a pump. In this, the
volume changes slightly.
e a: Reversible constant pressure heating in a boiler
to temperature T2
Rankine Cycle Process
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1
In the Rankine cycle, the mean temperature
at which heat is supplied is less than the
maximum temperature, T2 , so that the
efficiency is less than that of a Carnot cycle
working between the same maximum and
minimum temperatures.
Rankine Cycle Process
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2
The heat absorption takes place at constant
pressure over eab, but only the part ab is
isothermal.
The heat rejected occurs over cd; this is at both
constant temperature and pressure.
Rankine Cycles Advantages…
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3
Fuel flexible, works well with coal (closed
cycle).
High efficiency, low pumping power.
Lower flow rate (latent enthalpy).
Run at low T (works with geothermal and
solar), but high p.
Rankine Cycles Advantages
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Works well with nuclear energy:
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4
Pressurized : T = 350 C
Boiling : T = 400-500 C
Gas Cooled R: T = 600-800 C
High Temperature GR T > 800 C
Rankine Cycle Disadvantages
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High inertia, good for base load but not for
load following.
Require cooling, big condensers, .. Water …
Find bellow the Efficiency levels of Current
Power plants
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Combined Cycles
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Recent years have seen a growth of plants
employing Rankine and Brayton cycles.
Such plants are called combined cycle plants.
Rather than venting the hot exhaust gas from the
Brayton cycle into the atmosphere there are
plant designs that use this heat for other
purposes using heat recovery steam generators.
This has resulted in overall efficiencies
reaching as high as 50 to 55%.
Combined Cycle
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Combined cycles take advantage of high T gas
turbine exhaust. Combined cycle efficiency:
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0
Wrap up…
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Any more worries…
God help u…