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ISAT 413 - Module IV

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ISAT 413 - Module IV: Combustion and Power Generation Topic 4: Gas and Steam Cycles, Steam Turbines Conversion of Thermal Energy Thermodynamic Power Cycles – PowerPoint PPT presentation

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Title: ISAT 413 - Module IV


1
ISAT 413 - Module IV Combustion and Power
Generation Topic 4 Gas and Steam Cycles,
Steam Turbines
  • Conversion of Thermal Energy
  • Thermodynamic Power Cycles
  • Internal-Combustion Engines and Engine Cycles
  • Engine Performance
  • External-Combustion Systems
  • Vapor-Power Cycles
  • Combined Cycles
  • Steam Turbines

2
  • Conversion of Thermal Energy
  • Almost all of the mechanical energy produced
    today is produced from the conversion of thermal
    energy in some sort of heat engine.
  • The operation of all heat-engine cycles can
    usually be approximated by an ideal thermodynamic
    power cycle of some kind.
  • A basic understanding of these cycles can often
    show the power engineer how to improve the
    operation and performance of the system.

3
Thermodynamic Power Cycles
  • For a thermodynamic heat-engine cycle, the figure
    of merit is called the thermal efficiency, or
    hth. The desired energy output is the net work
    output of the cycle and the energy that costs is
    the heat added from the high-temperature heat
    sources.
  • Another important parameter of any heat-engine
    cycle is the specific work, w, which is the net
    work output per pound of working fluid in the
    cycle. It is also equal to the area enclosed by
    the cycle diagram when it is plotted on either a
    P-v or T-s diagram, providing the mass flow rate
    of the working fluid is the same throughout the
    cycle and the processes are reversible.

4
P-? and T-s Diagrams of Power Cycles
The area under the heat addition process on a T-s
diagram is a geometric measure of the total heat
supplied during the cycle qin, and the area under
the heat rejection process is a measure of the
total heat rejected qout. The difference between
these two (the area enclosed by the cyclic
curve) is the net heat transfer, which is also
the net work produced during the cycle.
5
Reversible Heat-Engine Cycles
  • The second law of thermodynamics states that it
    is impossible to construct a heat engine or to
    develop a power cycle that has a thermal
    efficiency of 100. This means that at least part
    of the thermal energy transferred to a power
    cycle must be transferred to a low-temperature
    sink.
  • There are four phenomena that render any
    thermodynamic process irreversible. They are
  • Friction
  • Unrestrained expansion
  • Mixing of different substances
  • Transfer of heat across a finite temperature
    difference

6
Categorize Cycles
  • Thermodynamic cycles can be divided into two
    general categories Power cycles and
    refrigeration cycles.
  • Thermodynamic cycles can also be categorized as
    gas cycles or vapor cycles, depending upon the
    phase of the working fluid.
  • Thermodynamic cycles can be categorized yet
    another way closed and open cycles.
  • Heat engines are categorized as internal or
    external combustion engines.

7
Air-Standard Assumptions
  • To reduce the analysis of an actual gas power
    cycle to a manageable level, we utilize the
    following approximations, commonly know as the
    air-standard assumptions
  • The working fluid is air, which continuously
    circulates in a closed loop and always behaves as
    an ideal gas.
  • All the processes that make up the cycle are
    internally reversible.
  • The combustion process is replaced by a
    heat-addition process from an external source.
  • The exhaust process is replaced by a heat
    rejection process that restores the working fluid
    to its initial state.

8
Air-Standard Cycle
Another assumption that is often utilized to
simplify the analysis even more is that the air
has constant specific heats whose values are
determined at room temperature (25oC, or 77oF).
When this assumption is utilized, the
air-standard assumptions are called the
cold-air-standard assumptions. A cycle for which
the air-standard assumptions are applicable is
frequently referred to as an air-standard
cycle. The air-standard assumptions stated above
provide considerable simplification in the
analysis without significantly deviating from the
actual cycles. The simplified model enables us to
study qualitatively the influence of major
parameters on the performance of the actual
engines.
9
Mean Effective Pressure
The ratio of the maximum volume formed in the
cylinder to the minimum (clearance) volume is
called the compression ratio of the engine.
Notice that the compression ratio is a volume
ratio and should not be confused with the
pressure ratio. Mean effective pressure (MEP) is
a fictitious pressure that, if it acted on the
piston during the entire power stroke, would
produce the same amount of net work as that
produced during the actual cycle.
10
Three Ideal Power Cycles
  • Three ideal power cycles are completely
    reversible power cycles, called externally
    reversible power cycles. These three ideal cycles
    are the Carnot cycle, the Ericsson cycle, and the
    Stirling Cycle.

11
Three Ideal Power Cycles
  • The Carnot cycle is an externally reversible
    power cycle and is sometimes referred to as the
    optimum power cycle in thermodynamic textbooks.
    It is composed of two reversible isothermal
    processes and two reversible adiabatic
    (isentropic) processes.
  • The Ericsson power cycle is another heat-engine
    cycle that is completely reversible or
    externally reversible. It is composed of two
    reversible isothermal processes and two
    reversible isobaric processes (with regenerator).
  • The Stirling cycle is also an externally
    reversible heat-engine cycle and is the only one
    of the three ideal power cycles that has seen
    considerable practical application. It is
    composed of two reversible isothermal processes
    and two reversible isometric (constant volume)
    processes.

12
Carnot Cycle and Its Value in Engineering
The Carnot cycle is composed of four totally
reversible processes isothermal heat addition,
isentropic expansion, isothermal heat rejection,
and isentropic compression (as shown in the P-?
diagram at right). The Carnot cycle can be
executed in a closed system (a piston-cylinder
device) or a steady-flow system (utilizing two
turbines and two compressors), and either a gas
or vapor can be used as the working fluid.
13
Limit of TH and TL in a Carnot Cycle
Thermal efficiency increases with an increase in
the average temperature at which heat is supplied
to the system or with a decrease in the average
temperature at which heat is rejected from the
system.
The highest temperature in the cycle is limited
by the maximum temperature that the components of
the heat engine, such as the piston or turbine
blades, can withstand. The lowest temperature is
limited by the temperature of the cooling medium
utilized in the cycle such as a lake, a river, or
atmospheric air.
14
Internal-Combustion Engine Cycles
  • Internal-combustion (IC) engines cannot operate
    on an ideal reversible heat-engine cycle but they
    can be approximated by internally reversible
    cycles in which all the processes are reversible
    except the heat-addition and heat-rejection
    processes.
  • In general, IC engines are more polluting than
    external-combustion (EC) engines because of the
    formation of nitrogen oxides, carbon dioxide, and
    unburned hydrocarbons.
  • The Otto cycle is the basic thermodynamic power
    cycle for the spark-ignition (SI),
    internal-combustion engine.

15
Otto Cycle The ideal Cycle for Spark-Ignition
Engines
Figures below show the actual and ideal cycles in
spark-ignition (SI) engines and their P-?
diagrams.
16
Ideal Otto Cycle
The thermodynamic analysis of the actual
four-stroke or two-stroke cycles can be
simplified significantly if the air-standard
assumptions are utilized. The T-s diagram of the
Otto cycle is given in the figure at left.
The ideal Otto cycle consists of four internally
reversible processes 1?2 Isentropic
compression 2?3 Constant volume heat
addition 3?4 Isentropic expansion 4?1 Constant
volume heat rejection
17
Thermal Efficiency of an Otto Cycle
The Otto cycle is executed in a closed system,
and disregarding the changes in kinetic and
potential energies, we have
18
Engine Knock and thermal Efficiency of an Engine
The thermal efficiency of the ideal Otto cycle
increases with both the compression ratio and the
specific heat ratio.
  • When high compression ratios are used, the
    temperature of the air-fuel mixture rises above
    the autoignition temperature produces an audible
    noise, which is called engine knock. (antiknock,
    tetraethyl lead? ? unleaded gas)
  • For a given compression ratio, an ideal Otto
    cycle using a monatomic gas (such as argon or
    helium, k 1.667) as the working fluid will have
    the highest thermal efficiency.

19
Example IV-4.1 The Ideal Otto Cycle
An ideal Otto cycle has a compression ratio of 8.
At the beginning of the compression process, the
air is at 100 kPa and 17oC, and 800 kJ/kg of heat
is transferred to air during the constant-volume
heat-addition process. Accounting for the
variation of specific heats of air with
temperature,
determine a) the maximum temperature and pressure
that occur during the cycle, b) the net work
output, c) the thermal efficiency, and d) the
mean effective pressure for the cycle. ltAnswers
a) 1575.1 K, 4.345 MPa, b) 418.17 kJ/kg, c)
52.3, d) 574.4 kPagt Solution
20
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24
Diesel Cycle The Ideal Cycle for
Compression-Ignition Engines
The diesel cycle is the ideal cycle for CI
(Compression-Ignition) reciprocating engines. The
CI engine first proposed by Rudolph Diesel in the
1890s, is very similar to the SI engine,
differing mainly in the method of initiating
combustion. In SI engines (also known as gasoline
engines), the air-fuel mixture is compressed to a
temperature that is below the autoignition
temperature of the fuel, and the combustion
process is initiated by firing a spark plug. In
CI engines (also known as diesel engines), the
air is compressed to a temperature that is above
the autoignition temperature of the fuel, and
combustion starts on contact as the fuel is
injected into this hot air. Therefore, the spark
plug and carburetor are replaced by a fuel
injector in diesel engines.
25
Ideal Cycle for CI Engines (continued)
In diesel engines, ONLY air is compressed during
the compression stroke, eliminating the
possibility of autoignition. Therefore, diesel
engines can be designed to operate at much higher
compression ratios, typically between 12 and
24. The fuel injection process in diesel engines
starts when the piston approaches TDC and
continues during the first part of the power
stroke. Therefore, the combustion process in
these engines takes place over a longer interval.
Because of this longer duration, the combustion
process in the ideal Diesel cycle is approximated
as a constant-pressure heat-addition process. In
fact, this is the ONLY process where the Otto and
the Diesel cycles differ.
26
Ideal Cycle for CI Engines (continued)
27
Thermal efficiency of Ideal Diesel Cycle
Under the cold-air-standard assumptions, the
efficiency of a Diesel cycle differs from the
efficiency of Otto cycle by the quantity in the
brackets. (See Slide 26)
The quantity in the brackets is always greater
than 1. Therefore, hth,Otto gt hth, Diesel when
both cycles operate on the same compression
ratio. Also the cuttoff ratio, rc decreases, the
efficiency of the Diesel cycle increases. (See
figure at right)
28
Internal-Combustion Engines
The two basic types of ignition or firing systems
are the four-stroke-cycle engines, commonly
called four-cycle engines, and the
two-stroke-cycle engines, commonly called
two-cycle engines. The four-cycle engines has a
number of advantages over the usual two-cycle
engine, including better fuel economy, better
lubrication, and easier cooling. The two-cycle
engine has a number of advantages, including
fewer moving parts, lighter weight, and smoother
operation. Some two-cycle engines have valves and
separate lubrication systems.
29
Cylinder Arrangements for Reciprocating Engines
Figure below shows schematic diagrams of some of
the different cylinder arrangements for
reciprocating engines.
30
  • Vertical in-line engine is commonly used today
    in four- and six-cylinder automobile engines.
  • The V-engine is commonly employed in
    eight-cylinder (V-8) and some six-cylinder (V-6)
    automobile engines.
  • The horizontal engine is essentially a V-engine
    with 180o between the opposed cylinders. This
    system was used as the four-cylinder, air-cooled
    engine that powered the Volkswagon bug.
  • The opposed-piston engine consists of two
    pistons, two crankshafts, and one cylinder. The
    two crankshafts are geared together to assure
    synchronization. These opposed-piston systems are
    often employed in large diesel engines.

31
  • The delta engine is composed of three
    opposed-piston cylinders connected in a delta
    arrangement. These systems have found application
    in the petroleum industry.
  • The radial engine is composed of a ring of
    cylinders in one plane. One piston rod, the
    master rod, is connected to the single crank on
    the crankshaft and all the other piston rods are
    connected to the master rod. Radial engines have
    a high power-to-weight ratio and were commonly
    employed in large aircraft before the advent of
    the turbojet engine.
  • When the term rotary engine is used today, it
    implies something other than a radial engine with
    a stationary crank.

32
Engine Performance
There are several performance factors that are
common to all engines and prime movers. One of
the main operating parameters of interest is the
actual output of the engine. The brake horsepower
(Bhp) is the power delivered to the driveshaft
dynamometer. The brake horsepower is usually
measured by determining the reaction force on the
dynamometer and using the following equation
Where F is the net reaction force of the
dynamometer, in lbf, R is the radius arm, in ft,
and Nd is the angular velocity of the
dynamometer, in rpm.
33
Horsepower
For a particular engine, the relationship between
the mean effective pressure (mep) and the power
is
Where C is the number of cylinders in the engine,
Ne is the rpm of the engine, and z is equal to 1
for a two-stroke-cycle engine and 2 for a
four-stroke-cycle engine.
34
Brake Thermal Efficiency
The brake thermal efficiency of an engine, hth,
unlike power plants, is usually based on the
lower heating value (LHV) of the fuel. The
relationship between efficiency and the brake
specific fuel consumption (Bsfc) is
Note that the brake specific fuel consumption
(Bsfc) of an engine is a measure of the fuel
economy and is normally expressed in units of
mass of fuel consumed per unit energy output.
35
External-Combustion Systems
External-combustion power systems have several
advantages over internal-combustion systems. In
general, they are less polluting. The primary
pollutants from internal-combustion engines are
unburned hydrocarbons, carbon monoxide, and
oxides of nitrogen. In external-combustion
engines, the CHx and CO can be drastically
reduced by carrying out the combustion with
excess air and the NOx production can be markedly
reduced by lowering the combustion temperature.
By burning the fuel with excess air, more energy
is released per pound of fuel. There are three
general ideal external-combustion engine cycles,
the Stirling and Brayton are ideal gas-power, and
vapor power cycles.
36
Brayton Cycle The Ideal Cycle for Gas-Turbine
Engines
The Brayton cycle was first proposed by George
Brayton for use in the reciprocating oil-burning
engine that he developed around 1870.
Fresh 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 resulting high-temperature gases
then enter the turbine, where they expand to the
atmospheric pressure, thus producing power. (An
open cycle.)
37
Brayton Cycle (continued)
The open gas-turbine cycle can be modeled as a
closed cycle, as shown in the figure below, by
utilizing the air-standard assumptions.
The ideal cycle that the working fluid undergoes
in this closed loop is the Brayton cycle, which
is made up of four internally reversible
processes 1?2 Isentropic compression (in a
compressor) 2?3 Constant pressure heat
addition 3?4 Isentropic expansion (in a
turbine) 4?1 Constant pressure heat rejection
38
T-s Diagram of Ideal Brayton Cycle
Notice that all four processes of the Brayton
cycle are executed in steady-flow devices (as
shown in the figure on the previous slide, T-s
diagram at the right), and the energy balance for
the ideal Brayton cycle can be expressed, on a
unit-mass basis, as
39
P-? Diagram and ?th of Ideal Brayton Cycle
Then the thermal efficiency of the ideal Brayton
cycle under the cold-air-standard assumptions
becomes
40
Thermal Efficiency of the Ideal Brayton Cycle
Under the cold-air-standard assumptions, the
thermal efficiency of an ideal Brayton cycle
increases with both the specific heat ratio of
the working fluid (if different from air) and its
pressure ratio (as shown in the figure at right)
of the isentropic compression process.
The highest temperature in the cycle occurs at
the end of the combustion process, and it is
limited by the maximum temperature that the
turbine blades can withstand. This also limits
the pressure ratios that can be used in the cycle.
41
Net Work of the Brayton Cycle
For a fixed turbine inlet temperature T3, the net
work output per cycle increases with the pressure
ratio, reaches a maximum, and then starts to
decrease, as shown in the figure at right.
Therefore, there should be a compromise between
the pressure ratio and the net work output. In
most common designs, the pressure ratio of gas
turbines ranges from about 11 to 16.
42
The Back Work Ratio
A power plant with a high back work ratio
requires a larger turbine to provide the
additional power requirements of the compressor.
Therefore, the turbine used in gas-turbine power
plants are larger than those used in steam power
plants of the same net power output.
In gas-turbine power plants, the ratio of the
compressor work to the turbine work, called the
back work ratio, is very high. Usually more than
half of the turbine work output is used to drive
the compressor.
The two major application areas of gas-turbine
engines are aircraft propulsion and electric
power generation.
43
Development of Gas Turbines
  • The efforts to improve the cycle efficiency
    concentrated in three areas
  • Increasing the turbine inlet (or firing)
    temperature (high NOx!?) which can be achieve by
    the development of new materials and the
    innovative cooling techniques.
  • Increasing the efficiencies of turbo-machinery
    components.
  • Adding modifications to the basic cycle such as
    incorporating intercooling, regeneration, and
    reheating techniques.

A more recent gas turbine manufactured by GE use
1425oC turbine inlet temperature, 282 MW, and
39.5 efficiency in the simple-cycle mode.
44
Deviation of Actual Gas-Turbine Cycles from
Idealized Ones
The deviation of actual compressor and turbine
behavior from the idealized isentropic behavior
can be accurately accounted for by utilizing the
isentropic efficiencies of the turbine and
compressor defined as (equations at bottom
right). Where states 2a and 4a are the actual
exit states of the compressor and the turbine,
respectively, and 2s and 4s are the corresponding
states for isentropic case.
45
The Brayton Cycle with Regeneration
In gas-turbine engines, the temperature of the
exhaust gas leaving the turbine is often
considerably higher than the temperature of the
air leaving the compressor. Therefore, the
high-pressure air leaving the compressor can be
heated by transferring heat to it from the hot
exhaust gases in a counter-flow heat exchanger,
which is also known as a regenerator or a
recuperator (as shown in the figure above.)
46
T-s Diagram of a Brayton Cycle with Regeneration
The thermal efficiency of the Brayton cycle
increases as a result of regeneration since the
portion of energy of the exhaust gases that is
normally rejected to the surroundings is now used
to preheat the air entering the combustion
chamber.
47
Thermal Efficiency of the Ideal Brayton Cycle
with and without Regeneration
The use of a regenerator with a very high
effectiveness (0.85 in practice) cannot be
justified economically unless the savings from
the fuel costs exceed the additional expense
involved. Under the cold-air-standard
assumptions, the thermal efficiency of an ideal
Brayton cycle with regeneration is shown at
right, which operates most effectively at lower
rp and (T1/T3) ratios.
48
Rankine Cycle The Ideal Cycle for Vapor Power
Cycles
Many of the impracticalities associated with the
Carnot cycle can be eliminated by superheating
the steam in the boiler and condensing it
completely in the condenser, as shown
schematically on a T-s diagram in the figure (on
next slide). The cycle that results is the
Rankine cycle, which is the ideal cycle for vapor
power plants.
49
Rankine Cycle (continued)
The Ideal Rankine cycle does not involve any
internal irreversibilities and consists of the
following four processes
1?2 Isentropic compression in a
pump 2?3 Constant pressure heat addition in a
boiler (steam generator) 3?4 Isentropic
expansion in a turbine 4?1 Constant pressure
heat rejection in a condenser (water or dry air
cooling)
50
Energy Analysis of the Ideal Rankine Cycle
All four processes that make up the Rankine cycle
can be analyzed as steady-flow processes. The
steady-flow energy equation per unit mass of
steam reduces to
The boiler and the condenser do not involve any
work, and the pump and the turbine are assumed to
be isentropic, thus,
51
Thermal Efficiency of the Ideal Rankine Cycle
Thermal efficiency of the ideal Rankine cycle is
determined from
The conversion efficiency of power plants in the
United States is often expressed in terms of heat
rate, which is the amount of heat supplied, in
Btu, to generate 1 kWh of electricity.
For example, a heat rate of 11,363 Btu/kWh is
equivalent to 30 percent thermal efficiency, the
smaller the heat rate, the greater the efficiency.
52
Deviation of Actual Vapor Power Cycle from
Idealized Ones
53
Things to be Considered in Evaluating the
Performance of Actual Power Cycle
  • The irreversibilities occurring within the pump
    and the turbine.
  • Fluid friction causes pressure drops in the
    boiler, the condenser, and the piping between
    various components.
  • Heat loss from the steam to the surroundings.
  • Heat losses occur at the bearings between the
    moving parts as a result of friction.
  • Steam that leaks out during the cycle and air
    that leaks into the condenser.
  • Power consumed by the auxiliary equipment such
    as fans that supply air to the furnace.

54
How Can We Increase the Efficiency of the Rankine
Cycle?
The basic idea behind all the modifications to
increase the thermal efficiency of the power
cycle is the same Increase the average
temperature at which heat is transferred to the
working fluid in the boiler, or decrease the
average temperature at which heat is rejected
from the working fluid in the condenser. That is,
the average fluid temperature should be as high
as possible during heat addition and as low as
possible during heat rejection. There are three
ways of accomplishing this for the simple ideal
Rankine cycle.
55
1. Lowering the Condenser Pressure
The colored area on the T-s diagram represent the
increase in net work output as a result of
lowering the condenser pressure from P4 to P4.
The heat requirement also increase (represented
by the area under curve 2-2), but this increase
is very small. Thus the overall effect of
lowering the condenser pressure is an increase in
the thermal efficiency of the cycle.
For effective heat transfer (DT 10oC), the
pressure must be above ? kPa for a condenser to
be cooled by a nearby river at 15oC. (The
drawbacks are air leak and moisture content.)
56
2. Superheating the Steam to High Temperatures
The colored area on this diagram represents the
increase in the net work. The total area under
the process curve 3-3 represents the increase in
the heat input. The overall effect is an increase
in thermal efficiency. Superheating the steam to
higher temperatures has another very desirable
effect It decreases the moisture content of the
steam at the turbine exit.
The temperature to which steam can be superheated
is limited, however, by metallurgical
considerations. Presently the highest steam
temperature allowed is about 620oC. Ceramics!
57
3. Increasing the Boiler Pressure
Another way of increasing the average temperature
during the heat-addition process is to increase
the operating pressure of the boiler, which
automatically increase the temperature at which
boiling takes place. The effect of increasing the
boiler pressure on the performance of vapor power
cycle is illustrated on a T-s diagram in the
figure at right.
The undesirable side effect as shown in the
diagram above can be corrected by reheating the
steam. Usually nuclear plant (hth 34) is lower
than fossil-fuel plant (40) for safety reason.
58
The Ideal Reheat Rankine Cycle
How can we take advantage of the increased
efficiencies at high boiler pressures without
facing the problem of excessive moisture at the
final stages of the turbine?
  • Superheat the steam to a very high temperature
    Not to over the metallurgical unsafe levels.
  • Expand the steam in the turbine in two stages,
    and reheat it in between.

59
The T-s Diagram of Ideal Reheat Rankine Cycle
The T-s diagram of the ideal reheat Rankine cycle
is shown in the figure below. The total heat
input and the total turbine work output for a
reheat cycle become
  • The incorporation of the single reheat in a
    modern power plant improve the cycle efficiency
    by 4 to 5.
  • The use of more than two reheat stages is not
    practical.
  • If we had materials that could withstand
    sufficiently high temperatures, there would be no
    need for the reheat cycle.

60
Example IV-4.2 The Ideal Reheat Rankine Cycle
Consider a team power plant operating on the
ideal reheat Rankine cycle. Steam enters the
high-pressure turbine at 15 MPa and 600oC and is
condensed in the condenser at a pressure of 10
kPa. If the moisture content of the steam at the
exit of the low-pressure turbine is not to exceed
10.4 percent, determine
(a) the pressure at which the steam should be
reheated and (b) the thermal efficiency of the
cycle. Assuming the steam is reheated to the
inlet temperature of the high-pressure turbine.
ltAnswers (a) 4.0 MPa, (b) 45.0gt Solution
61
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63
Ideal Regenerative Rankine Cycle with Open
Feedwater Heater
Another way of increasing the thermal efficiency
of the Rankine cycle is by regeneration. During a
regeneration process, liquid water (feedwater)
leaving the pump is heated by some steam bled off
the turbine at some intermediate pressure in
devices called feedwater heaters.
64
Ideal Regenerative Rankine Cycle with Closed
Feedwater Heater
The two streams are mixed in open feedwater
heaters, and the mixture leaves as a saturated
liquid at the heater pressure. In closed
feedwater heaters, heat is transferred from the
steam to the feedwater without mixing.
65
A Steam Power Plant with One Open and Three
Closed Feedwater Heaters
66
An Ideal Cogeneration Plant
The production of more than one useful form of
energy (such as process heat and electric power)
from the same energy source is called
cogeneration. Cogeneration plants produce
electric power while meeting the process heat
requirements of certain industrial processes.
This way, more of the energy transferred to the
fluid in the boiler is utilized for a useful
purpose. The fraction of energy that is used for
either process heat or power generation is called
the utilization factor of the cogeneration plant.
67
More Ways to Increase Power plant Thermal
Efficiency
The overall thermal efficiency of a power plant
can be increased by using binary cycles or
combined cycles. A binary cycle is composed of
two separate cycles, one at high temperatures
(topping cycle) and the other at relatively low
temperatures. The most common combined cycle is
the gas-steam combined cycle where a gas-turbine
cycle operates at the high-temperature range and
a steam-turbine cycle at the low-temperature
range. Steam is heated by the high-temperature
exhaust gases leaving the gas turbine. Combined
cycles have a higher thermal efficiency than the
steam- or gas-turbine cycles operating alone.
68
Mercury-Water Binary Vapor Cycle
69
Combined Gas-Steam Power Plant
70
  • Steam Turbines
  • The turbine is a device that converts the stored
    mechanical energy in a fluid into rotational
    mechanical energy. There are several different
    types, including steam, gas, water, and wind
    turbines.
  • There are several ways to classify steam
    turbines
  • With respect to the purpose of the turbine
  • Central-station units which are used to drive
    electrical generators at synchronous speed.
  • Superposed or topping steam turbines are
    high-pressure turbines that are installed in
    older, low-pressure steam systems to improve the
    overall efficiency of the power plant.
  • Mechanical-drive turbines to power large draft
    fans.

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  • According to the exhaust or back pressure of the
    unit
  • Either condensing or noncondensing. In the
    noncondensing turbine, the turbine-exhaust
    pressure is above or equal to atmospheric
    pressure and the system can operate with or
    without a condenser, If there is no condenser,
    this system will require continuous water
  • According to the method of steam injection or
    extraction from the turbine
  • Bleeder or extraction turbines are used where
    turbine steam is removed partway through the
    turbine for process use or for feedwater heating.
  • Reheat turbines are used in the reheat
    vapor-power cycles.
  • Extraction-induction turbines have ports for both
    the extraction and injection of steam at
    intermediate points in the turbine.

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Turbine Blading
There are two basic types of turbine balding,
impulse and reaction. Two different types of
impulse staging and a typical reaction stage are
shown in the figure at right
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Energy Transferred to the Moving Blades
The velocity vectors in the tangential and axial
directions of the turbine rotor are shown in the
figure below. The force on the moving blade, Fb
is equal to mat, or letting m_dot represent the
steam flow rate through the blade, Then, the
energy transferred to the moving blades Pb is
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Blade Efficiency
The performance of a given blade is given by the
blade efficiency, which is defined as the
fraction of the kinetic power of the inlet steam
that is transferred to the blade, or
The blade efficiency and blade power are maximum
when V2 is a minimum and this occurs when V2,r is
zero and V2 is equal to V2,a. When there is no
friction, V1,t Vb,opt, i.e.,
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