steam turbine design project 2011

8/3/2019 Steam Turbine Design Project 2011

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THERMAL-FLUID SYSTEM DESIGNINGM 427

EES DESIGN AND OPTIMISATION ASSIGNMENT

October 2011

SSCHCHOOLOOL FORFOR MEMECHANICALCHANICALENGINEERINGENGINEERING

Designed by:

AC Du Randt21108293

FacultyFaculty

Engineering


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SUMMARY

This report consists of a Thermodynamic Rankine cycle design for a power station. Thedesign has been done through programming to solve the calculations to gather solutions.After the necessary calculations have been done, the optimisation of the Rankine cycle canbe done to achieve the optimum specific work output and efficiency.

The Rankine cycle design consists of the following:

The Cycle has a maximum steam temperature, Tmax, which can be achieved through

the boiler. The Cycle also has a minimum steam temperature, Tmin, which is determined by the

vacuum pressure and where the power station is situated. There are two pumps for the cycle, the first, a condensate extraction pump and the

second, a boiler feed pump. A total of four turbines are present in this cycle, a high pressure, intermediate

pressure and two low pressure turbines. A condenser to change the phase of the steam to saturated water by using the

cooling water or system plant. Feed water heaters to preheat the water before the boiler.

The design parameters include the following:

The maximum temperature is limited by the metallurgic characteristics of the metalused in the boiler.

The minimum temperature is limited by the location of the power plant. The maximum pressure for the boiler is determined by the critical point of water. The pressures in the different turbines. The different pressures for the steam bleed for feed water heating. The pressures for the boiler feed pump.

The goal with this design is to design and optimise the Rankine cycle for optimum specificwork output as well as the efficiency of the cycle. The cycle will be solved as it is encounteredin the industry with losses and efficiencies of various elements.

The report also includes the discussion between steam turbine driven pump as well aselectrically driven pump, wet and dry cooling systems, coal analysis.

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DECLERATION

I, Armand Charl du Randt (Identity Number: 890817 5149 083), hereby declare that the work

contained in this dissertation is my own work. Some of the information contained in this

dissertation has been gained from various journal articles; text books etc, and has been

referenced accordingly.

________________ ______________ Initial & Name Date

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TABLE OF CONTENTSSummary.................................................................................................................................i

Declaration...............................................................................................................................iiTable of contents....................................................................................................................iiiList of Tables.........................................................................................................................ivList of Figures..........................................................................................................................vList of Symbols and Abbreviations.......................................................................................vi

1 INTRODUCTION...................................................................................1-1

1.1 BACKGROUND..........................................................................................................1-1

1.1.1 THERMODYNAMICS..........................................................................................1-1

1.1.2 MECHANICAL DESIGN.......................................................................................1-2

1.1.3 CONTROL AND INSTUMENTATION..................................................................1-2

1.2 PROBLEM STATEMENT...........................................................................................1-2

1.3 BOUNDARY VALUES AND ASSUMPTIONS FOR IDEAL AND ACTUAL CYCLE. .1-1

1.4 GOAL STATEMENT...................................................................................................1-1

2 TASK .......................................................................................................2-3

2.1 TASK DESCRIPTION.................................................................................................2-3

2.1.1 PRIMARY PROBLEM STATEMENT...................................................................2-3

2.1.2 OUTPUTS............................................................................................................2-32.1.3 METHOD OF WORK...........................................................................................2-3

2.1.3.1 CARNOT CYCLE.............................................................................................2-4

2.1.3.2 RANKINE CYCLE............................................................................................2-5

2.1.3.3 RANKINE CYCLE WITH SUPERHEATING.....................................................2-7

2.1.3.4 RANKINE CYCLE WITH REHEATING............................................................2-8

2.1.3.5 RANKINE CYCLE WITH FEED HEATING.......................................................2-9

3 PRACTICAL SITUATION........................................................................3-1

3.1 ACTUAL RANKINE CYCLE.......................................................................................3-1

3.2 OPTIMISATION..........................................................................................................3-23.2.1 BOILER FEED PRESSURE................................................................................3-2

3.2.2 TEMPERATURE..................................................................................................3-4

3.2.3 REHEAT CYCLE PRESSURE.............................................................................3-5

3.2.4 BLEED STEAM TAPPING POINTS.....................................................................3-7

4 ADDITIONAL..........................................................................................4-10

4.1 MATERIAL SELECTION..........................................................................................4-10

4.2 STEAM VS ELECTRICALLY DRIVEN FEEDPUMP................................................4-11

4.3 WET VS DRY COOLING SYSTEMS........................................................................4-12

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5 CONCLUSION..........................................................................................5-1CONCLUSION

LIST OF TABLESTABLE 1: BOUNDARYVALUESFOR IDEALCYCLE..................................................................................1-1TABLE 2: BOUNDARYVALUESFOR ACTUALCYCLE...............................................................................1-1TABLE 3: CARNOT CYCLE VALUES..................................................................................................2-5TABLE 4: RANKINE CYCLE COMPARISON..........................................................................................2-6TABLE 5: RANKINECYCLEWITHSUPERHEATINGCOMPARISON..................................................................2-7TABLE 6: RANKINE CYCLEWITHFEEDHEATINGCOMPARISON.................................................................2-9TABLE 7: RANKINECYCLEWITHFEEDHEATINGCOMPARISON................................................................2-11TABLE 8: OPTIMISATIONCHANGES...................................................................................................3-9TABLE 9: COMPARISONFOR IDEALAND ACTUAL.................................................................................3-9TABLE 10: PUMPDRIVERSCOMPARISON.........................................................................................4-11TABLE 11: COOLINGSYSTEMCOMPARISON......................................................................................4-12Table 11: Cooling system comparison

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LIST OF FIGURES

FIGURE 1: CARNOT CYCLE...........................................................................................................2-4FIGURE 2: RANKINE CYCLE...........................................................................................................2-5FIGURE 3: RANKINE CYCLE PUMP WORK (ZOOM)..............................................................................2-6FIGURE 4: RANKINE CYCLEWITH SUPERHEATING...............................................................................2-7FIGURE 5: RANKINE CYCLEWITH REHEATING....................................................................................2-8FIGURE 6: RANKINE CYCLEWITHFEEDHEATING...............................................................................2-10FIGURE 7: HP FEEDWATERHEATER...............................................................................................2-10FIGURE 8: DEAERATORCONTACTFEEDWATERHEATER........................................................................2-11FIGURE 9: LP FEEDWATERHEATER...............................................................................................2-11FIGURE 10: ACTUAL RANKINE CYCLE..............................................................................................3-2FIGURE 11: T-SDIAGRAMFOR STEAM.............................................................................................3-2FIGURE 12: EFFICIENCYANDWORKOUTPUTFORFEEDPUMPPRESSURE....................................................3-3FIGURE 13: MAXIMUMTEMPERATURE...............................................................................................3-4

FIGURE 14: EFFICIENCYANDWORKOUTPUTFORREHEATCYCLEPRESSURE...............................................3-5FIGURE 15: DRYNESSFRACTIONFORREHEATCYCLEPRESSURE.............................................................3-6FIGURE 16: PRESSUREFORBLEEDPOINTFOR HP HEATER...................................................................3-7FIGURE 17: PRESSUREBLEEDPOINTFORDEAERATOR..........................................................................3-8FIGURE 18: PRESSUREFORBLEEDPOINTFOR LP HEATER...................................................................3-9FIGURE 19: PRESSURETAPPINGPOINTFORTURBINEDRIVENFEEDPUMP.................................................4-11FIGURE 19: PRESSURETAPPINGPOINTFORTURBINEDRIVENFEEDPUMP

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LIST OF SYMBOLS AND ABBREVIATIONS (NOMENCLATURE)

EES Engineering Equation Solver LP Low PressureIP Intermediate PressureHP High PressureHX Heat Exchanger isn Isentropicopt Optimumtur Turbinep Pumpcond condenser eff Efficiencyact ActualSSC Specific Steam Consumption

Ta Atmospheric TemperatureTmax Maximum TemperaturePa Atmospheric PressureTx Temperature at specific point xPx Pressure at specific point xxx Quality at specific point xsx Entropy generated at specific point xP Pressure loss at specific point Efficiency at specific pointrp Pressure ratioWc Work needed for pumpsWt Work delivered from turbinesqin Heat inputqout Heat rejected

K KelvinkPa PressureoC Degrees Celsius

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School for Mechanical Engineering EES Design Assignment 2011

1 INTRODUCTION

The Rankine cycle is a cycle that converts heat input into work output. The heat is suppliedexternally to a closed loop, with water as working fluid. The Rankine cycle is the most

commonly used for power generation.

The Rankine cycle is sometimes referred to as a practical Carnot cycle, because, when anefficient turbine is used, the T-S diagram begins to resemble the Carnot cycle. The maindifference is that heat addition and rejection are isobaric in the Rankine cycle and isothermalin the theoretical Carnot cycle. A pump is used to pressurize the working fluid received fromthe condenser as a liquid instead of as a gas as it is in the Carnot cycle. All of the energy inpumping the working fluid through the complete cycle is lost, as is most of the energy ofvaporization of the working fluid in the boiler. This energy is lost to the cycle because thecondensation that can take place in the turbine is limited in order to minimize blade erosion;the vaporization energy is rejected from the cycle through the condenser. But pumping theworking fluid through the cycle as a liquid requires a very small fraction of the energy neededto transport it as compared to compressing the working fluid as a gas in a compressor as in

the Carnot cycle.

The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressurereaching super critical levels for the working fluid, the temperature range the cycle canoperate over is quite small: turbine entry temperatures are typically 565C (the creep limit ofstainless steel) and condenser temperatures are around 30C depending on the vacuumpressure in the condenser and the temperature of the surroundings. This gives a theoreticalCarnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coal-fired power station.

1.1 BACKGROUND

In the designing process of the Rankine cycle, the background knowledge in otherapplications is also important and needs to be looked at for the design. The wide field ofmechanical engineering can be divided into the following focus areas:

1.1.1THERMODYNAMICS

The fields included in this section are:

Fluid mechanics Thermodynamics Thermal Machines Heat Transfer

The design of a Rankine cycle starts with the working fluid. One needs to understand thebasic principles of the characteristics of the working fluid as well as how it behaves in thecycle. The thermodynamics is the main design parameter and everything is based on thethermo dynamical behaviour of the working fluid. There are various characteristics of everyfluid and in some instances, a few assumptions have to be made that has to be very accurateand based on the actual cycle characteristics. In the Feed heating Rankine cycle, the workingfluid passes through feed heaters that preheat the fluid before the boiler, and thereforebackground knowledge is important for heat transfer.

1 .
http://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/TS_diagramhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isothermal_processhttp://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/Critical_point_(thermodynamics)http://en.wikipedia.org/wiki/Creep_(deformation)http://en.wikipedia.org/wiki/Carnot_efficiencyhttp://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/TS_diagramhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isothermal_processhttp://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/Critical_point_(thermodynamics)http://en.wikipedia.org/wiki/Creep_(deformation)http://en.wikipedia.org/wiki/Carnot_efficiency


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1.1.1MECHANICAL DESIGN


Material Science Strength of Materials Mechanical Design Structure analysis Engineering Graphic Design Computer methods of programming

To design and develop all the main parts for a Rankine cycle, it is important to understand allthe principles of design and the design considerations for various parts. There are a fewthings to take into account when it comes to design. There are various materials and with thatthere are lots of ways to implement them. One of the main factors of a machine is itsresistance to fail, and this can be prevented with the right designing methods and materials.

1.1.1CONTROL AND INSTUMENTATION


Control Systems Measure and control Machine dynamics

On the plant, there will be a control system that will adjust the system and all the cycleparameters to achieve the desired work load. There will also be various safety measurementinstruments that take measurements and can adjust the cycle or shut-down the cycle forsafety reasons. Also if there is a need for maintenance on the machines or parts of it.

1.1 PROBLEM STATEMENT

Formulate a Rankine cycle for calculating, optimising and graphic representation throughprogramming in EES. For the closed cycle the system consists of the following components:

The atmospheric conditions namely Tmin and Patm. There are two pumps with different work input and different pressures. One boiler with a temperature limit due to metallurgy namely Tmax. A High pressure turbine followed by a reheat to Tmax. After the reheat, an Intermediate pressure turbine followed by two low pressure

turbines.

From various point, steam is bled off for the feed water heaters. After there the turbines, the fluid enters the condenser where the heat gets rejected.

The feed water heater and boilers have heat transfer efficiencies as well as pressure losses.The two pumps have isentropic efficiencies. Depending on the power delivery, the drivemechanism has either a mechanical or isentropic efficiency.The maximum temperature that can be achieved is limited by the metallurgic limit of thematerial used.

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1.1 BOUNDARY VALUES AND ASSUMPTIONS FOR IDEALAND ACTUAL CYCLE

Boundary Value Value DescriptionPatm 90 kPa The atmospheric pressureTatm 25 oC The atmospheric temperatureTmax 535 oC The maximum temperature of the steam in the boiler

Pcondenser 5 kPa The vacuum pressure in the condenser Tcondenser 32.88

oC The temperature in the condenserTable 1: Boundary values for Ideal cycle

Boundary Value Value DescriptionPatm 90 kPa The atmospheric pressureTatm 25

oC The atmospheric temperatureTmax 535

oC The maximum temperature of the steam in the boilerPcondenser 8 kPa The vacuum pressure in the condenser

Tcondenser 41.49 oC The temperature in the condenser

Pressure lossesPboiler 5.61 MPa The head and pipe losses from the feedpump to the boilerPvalve 150 kPa The pressure loss over the turbine inlet valve

EfficienciesHPturbine 92% The efficiency of the HP turbineLPturbine 88% The efficiency of the LP and IP turbineFPturbine 85% The efficiency of the steam turbine driving the feedpumppump 90% The efficiency of the Feedpump

Table 2: Boundary values for Actual cycle

The assumptions that were made are as follow:

There is an isentropic efficiency for the two pumps. There is isentropic efficiency for the HP turbine. There is isentropic efficiency for the three turbines after the reheat. The three turbines can be seen as one turbine. There are pressure losses on various components of the cycle. The feed water heaters have an efficiency of 100%. There is no subcooling in the HP and LP heaters.

1.1 GOAL STATEMENT

In the design of the Rankine cycle, all four turbines are mounted on a common shaft.

The following is needed to achieve the optimum performance of the cycle:

The heat input for the cycle, qin [kJ/kg] The heat rejected for the cycle, quit [kJ/kg] Specific work output, wnett [kJ/kg] The efficiency of the Carnot and Rankine cycle, Rankine en Carnot [%] Specific Steam Consumption, SSC [kgs/kWh] Dryness Fraction, x [%]

The cycles will be represented graphically on a T-s diagram. In order to achieve theoptimum efficiency as well as the highest specific work output, graphs for various pressures

and different points for steam bleeding will be shown.

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1 TASK

1.1 TASK DESCRIPTION

1.1.1PRIMARY PROBLEM STATEMENT

The problem state that one has to formulate an advance Rankine cycle as encountered in theindustry and then optimise the cycle for maximum efficiency possible for a certain specificwork output. A report has to be compiled from the design as done in EES.

The design will be built up from the simple Carnot cycle to the Rankine cycle with feed waterheating. The advanced cycle will have pressure losses, efficiencies etc. and will be solved forvarious configurations to optimise the cycle. The effect on the cycle will be discussed for eachconfiguration.

There will also be a discussion about various configurations that influence the plant andRankine cycle such as a steam driven feed water pump, wet and dry cooling systems andcoal analysis.

1.1.2OUTPUTS

The following calculations will be done for the different cycles:

The heat input for the cycle, qin [kJ/kg] The heat rejected for the cycle, quit [kJ/kg] Specific work output, wnett [kJ/kg] The efficiency of the Carnot and Rankine cycle, Rankine en Carnot [%] Specific Steam Consumption, SSC [kgs/kWh]

Dryness Fraction, x [%]

The following graphical representations will be shown: The cycles T-s diagrams Efficiency against pressure for the feed pump Specific work output against pressure for the feed pump

1.1.1METHOD OF WORK

The method used for the process for reaching the advance Rankine cycle with feed heating

was to start with the simple Carnot cycle, then build one element at a time into the design inorder to reach the advance cycle. The steps are shown in the following points to come.

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1.1.1.1 CARNOT CYCLE

For a theoretical Carnot cycle, the boiler produces dry saturated steam at a pressure of 16MPa. It then expands through a turbine and exhausts into a condenser at 5 kPa.

-1.0 0.1 1.2 2.3 3.4 4.5 5.6 6.7 7.8 8.9 10.00

50

100

150

200

250

300

350

400

s [kJ/kg-K]

T[C]

Carnot Cycle

1

4 3

2

Figure 1: Carnot Cycle

For the Carnot cycle: Carnot= Tmax-TminTmaxFrom this equation, it can be seen that the greater the difference between that of the heatsource and that of the heat sink, the greater the efficiency of the cycle. For the limit to themaximum temperature, due to the strength of the material, only the minimum temperature canstill play a part and that is limited by the atmosphere and the surroundings. With a fewcalculations, it can be seen that the heat sink temperature is more important than themaximum temperature.

Even though the efficiency of the cycle is good, the Carnot cycle can never work in practice.One of the reasons for this is that the turbine will not last that long due to the dryness factor at

the last stage and therefore the blades will not last long. The other and main reason for this isthat from point 4, there is a need for a compressor to compress wet steam to saturated water.Even if this were possible, the size of the compressor will be over 50 % of that of the turbine.

By letting the steam condense fully from point 3 to saturated water, the water can then bepumped and the cycle evolves into a simple Rankine cycle.

Parameter

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Cycleqin

[kJ/kg]wnett

[kJ/kg]qout

[kJ/kg]

[%]SSC

[kg/kWh]

Dryness

Fraction

P[MW]

Carnot Cycle 930.7 471.7 45950.6

8

7.63 0.60 118

Table 3: Carnot Cycle Values

1.1.1.2 RANKINE CYCLE

For a simple Rankine cycle, a pump would now replace the compressor of the Carnot cycle.The pump increases the pressure of the saturated water to 16 MPa. The boiler needs to heatfrom point 5 to 1 as well as from point 1 to 2, same as in the Carnot cycle.

-1.0 0.1 1.2 2.3 3.4 4.5 5.6 6.7 7.8 8.9 10.00

50

100

150

200

250

300

350

400

450

s kJ/k -K

T[C]

Rankine Cycle

1

5

43

2

Figure 2: Rankine Cycle

In the figure shown, the working fluid condenses fully to saturated water. The pump takes thewater from 5 kPa and pressurise it to 16MPa. It also shows that the work done by the pump ismuch less than the compressor in the Carnot cycle.

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0.5 0.5 0.5 0.5 0.5 0.5

32.03

32.81

33.59

34.38

s kJ/k -K

T[C]

Rankine Cycle

4

5

Figure 3: Rankine Cycle Pump Work (zoom)

Even though the pump work is less, the heat input is greater and therefore the efficiency isless than for the Carnot cycle, but the specific work output is higher.

Parameter

Cycleqin

[kJ/kg]wnett

[kJ/kg]qout

[kJ/kg]

[%]SSC

[kg/kWh]

Dryness

Fraction

P[MW]

Carnot Cycle 930.7 471.7 45950.6

87.63 0.60 118

Rankine Cycle 2426 967 145939.8

53.72 0.60 242

Table 4: Rankine Cycle Comparison

Also, much more heat is rejected to the condenser which corresponds to the lower cycleefficiency. The smaller pump work, as well as the greater turbine work output, gives a muchsmaller SSC. The first step to improving this simple Rankine cycle is to superheat the steam.

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1.1.1.3 RANKINE CYCLE WITH SUPERHEATING

For the Rankine cycle with superheating, the steam is heated into the superheated region.The steam still gets pressurised through the feed pump into the boiler. The boiler superheatsthe steam from where it expands through the turbine into the condenser.

-1.0 0.1 1.2 2.3 3.4 4.5 5.6 6.7 7.8 8.9 10.0

0

50

100

150

200

250

300

350

400

450

500

550

s kJ/k -K

T[C]

Rankine Cycle with Superheating

1

6

54

3

2

Figure 4: Rankine Cycle with Superheating

It can be seen that the addition of superheating to the Rankine cycle, it greatly increases thework done for the cycle. This is because the mean temperature at which the heat is added isgreater than the cycles previously considered. Even though there is an increase in the workoutput and the efficiency, on the downside is that the boiler needs to be bigger to superheatthe steam as well as the condenser. However, the improvement of the dryness fraction is ofimportance both for the reduced erosion of the LP turbine blades as well as the wet stageefficiency. The next step to improve the cycle is to add a reheat cycle before the second stageturbines.

Parameter

Cycleqin

[kJ/kg]wnett

[kJ/kg]qout

[kJ/kg]

[%]SSC

[kg/kWh]

Dryness

Fraction

P[MW]

Carnot Cycle 930.7 471.7 45950.6

87.63 0.60 118

Rankine Cycle 2426 967 145939.8

53.72 0.60 242

Rankine Cycle

with

3243 1421 1822 43.8

2

2.53 0.75 355

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Superheating

Table 5: Rankine cycle with superheating comparison

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1.1.1.4 RANKINE CYCLE WITH REHEATING

From the superheated cycle, the reheat cycle improves the cycle even further. For the reheatcycle, the steam expands partially through a HP turbine to a lower pressure and lowertemperature. At this lower temperature, the steam gets heated once more to the maximumtemperature.

In this Rankine cycle, the feed pump pressurises the water to a pressure of 16 MPa into theboiler. The boiler superheats the steam to a maximum temperature of 535C and it partiallyexpands through the HP turbine. Then it enters the boiler for the second time to be reheatedto the maximum temperature of 535C and it then expands through the IP and two LP turbineinto the condenser. In the condenser it condenses to saturated water.

-1.0 0.1 1.2 2.3 3.4 4.5 5.6 6.7 7.8 8.9 10.00

50

100

150

200

250

300

350

400

450

500

550

s [kJ/kg-K]

T[C]

16 MPa

3.4 MPa

Rankine Cycle with Reheating

1

6

5

4

3

2

8

7

Figure 5: Rankine Cycle with Reheating

There is a relative slight gain in efficiency with the reheating element in the boiler. The main

advantage and real purpose of the reheating is that on the final stage of the turbine, thedryness fraction is greater and therefore there is less wet steam in the final stage.

The major gain in thermal efficiency due to reheating is achieved therefore as a result ofreduction in wetness of the steam passing through the latter stages, thereby improving theindividual stage efficiencies.

There is a particular pressure at which it is most economical to reheat the steam. That this isso may be seen by considering that, if the steam is reheated early in the expansion, theadditional quantity of the heat will be supplied will be small with a consequently small gain. Ifthe reheating is done at a fairly low pressure, then although a large amount of heat issupplied, the steam will still have a high degree of superheat at the entry to the condenserwith the result of a large amount of the heat supplied in the process will be thrown to waste.

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Parameter

Cycleqin

[kJ/kg]

wnett

[kJ/kg]

qout

[kJ/kg]

[%]

SSC

[kg/kWh]

Dryness

Fraction

P

[MW]

Carnot Cycle 930.7 471.7 45950.6

87.63 0.60 118

Rankine Cycle 2426 967 145939.8

53.72 0.60 242

Rankine Cyclewith

Superheating3243 1421 1822

43.82

2.53 0.75 355

Rankine Cyclewith

Reheating3813 1733 2080

45.46

2.08 0.86 433

Table 6: Rankine Cycle with feed heating comparison

1.1.1.5 RANKINE CYCLE WITH FEED HEATING

The Rankine cycle with feed heating is the same basic cycle as in the reheat cycle. The boilerproduces superheated steam with quality 16 MPa and 535C. The steam expands through aHP turbine to a pressure of 3.4 MPa from where the steam is reheated. The steam thenexpands through a IP turbine to 1 MPa. In the IP turbine, steam is bled off at 2 MPa to a HPheater to preheat feedwater before it enters the boiler. This HP heater is a closed heatexchanger.

The steam then expands through two LP turbines from 1 MPa to 5 kPa in the condenser. Inthe LP turbine, bled steam is taken at 600 kPa to a deaerator. The deaerator is a contact heatexchanger and the steam is fed directly into the feedwater. The distillate from the HP heateralso feeds into the deaerator. Also from a pressure of 150 kPa in the LP turbine, bled steam istaken to a LP feed heater, which is the same as the HP heaters, only at a lower pressure andanother stage. The distillate of the LP heater exhausts into the condenser.

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-1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.00

50

100

150

200

250

300

350

400

450

500

550

s [kJ/kg-K]

T[C]

Rankine Cycle with Feed Heating

m1m1

m2m2

m3m3

20

19

18

17

16

15

13

12

9,10

7,8

216

5

4

3

21

14

Figure 6: Rankine Cycle with feed heating

For this feedwater heating cycle, two feed pumping stages are now necessary because of theintersection of the deaerator. A condenser extraction pump (point 7 & 8) takes the saturatedwater from the condenser and pressurise it from 5 kPa to 600 kPa and discharges it into the

deaerator. The second and main feed pump takes suction in the deaerator and pressurise itfrom 600 kPa to 16 MPa for an ideal cycle, and discharge it into the boiler after passingthrough the HP heater.

For the calculation of the mass flows for the bleed steam, an energy balance is made forevery heater. The energy balances shown is with respect to the T-s diagram above.

Figure 7: HP feedwater heater

1 .

(m1. h12)+mt.h10= m1.h15+(mt. h14)h12

h10h

14h

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Figure 8: Deaerator contact feedwater heater

Figure 9: LP feedwater heater

It is important to note that the efficiency of the regenerative feed heating cycle may beconsiderably improved by increasing the number of stages of the feed heating, but it is limitedfor practical reasons in the industry. The main reason for this improvement is that less heat isthrown to the condenser without doing any work.

On the practical side, there are a few additional improvements, such as due to feed heating,the mass flow in the final stages of the turbine is lower and therefore a smaller turbine can beused. Also, due to less mass flow in the condenser, a smaller area is needed for heat transferso the condenser can be smaller.

Parameter

Cycleqin

[kJ/kg]wnett

[kJ/kg]qout

[kJ/kg]

[%]SSC

[kg/kWh]

Dryness

Fraction

P[MW]

Carnot Cycle 930.7 471.7 459

50.6

8 7.63 0.60 118

Rankine Cycle 2426 967 145939.8

53.72 0.60 242

Rankine Cyclewith

Superheating3243 1421 1822

43.82

2.53 0.75 355

Rankine Cyclewith

Reheating3813 1733 2080

45.46

2.08 0.86 433

Rankine Cyclewith FeedHeating

3058 1517 154149.6

22.37 0.86 379

1 .

(m2. h16)+m1.h8 + mt-m1-m2.h20=(mt. h9)h20

h9

h16

h15

h21

(m3. h18)+mt-m1-m2.h8= mt-m1-m2.h20+(m3. h21)h8h

20h

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Table 7: Rankine cycle with feed heating comparison

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2 PRACTICAL SITUATION

2.1 ACTUAL RANKINE CYCLEFor the ideal Rankine cycle, all the calculations have been done and compared to other

cycles with added elements. Now, the Rankine with feed heating cycle have to be done as itis in reality. For a practical cycle in the industry, every part of the cycle has losses andefficiencies. The following will be added to the ideal cycle to make it an actual cycle:

Pressure losses over the boiler, for pipe and head loss. Pressure losses over the valves at the inlet of the turbines. Isentropic efficiencies for the HP, IP and LP turbines. Isentropic efficiencies for the extraction and boiler feed pump.

The optimisation is done so that the balance between cycle efficiency and specific workoutput can be found. For optimisation, the following will be looked at:

The working pressure of the boiler, or delivery pressure of the main feed pump. The effect of the maximum temperature of the steam.

The reheat cycle pressure, or inlet pressure for the IP turbine. The bleed steam tapping point for the HP heater. The bleed steam tapping point for the deaerator which will also be the delivery

pressure of the extraction pump as well as the inlet pressure for the main feed pump. The bleed steam tapping point for the LP heater.

Even though there are a lot of configurations for the optimum cycle, there are a fewparameters that cannot be changed that are set to a value that is determined by other factors.These parameters are the following:

The maximum temperature of the boiler is limited to the metallurgic limit of thematerial used. Therefore, due to practical reasons, the maximum temperature is fixedat the temperature before the metal excursions takes place.

The minimum temperature in the condenser is also a set value, because of themaximum vacuum pressure in the condenser as well as the atmospheric temperatureand the cooling water temperature.

The maximum pressure of the boiler cannot exceed the critical pressure of water. The dryness fraction on the final stages cannot exceed a certain amount because of

erosion on the turbine blades.

In the figure below, the efficiency of the turbines were added as well as the pressure lossesover the valves. The boiler feed pump normally delivers a pressure of 22 MPa into the boilerwhere the pressure has dropped to 18 MPa due to the head and pipe loss. The pressuredrops even further through pipe and valve loss to 16 MPa for the HP turbine. The steamexpands through the turbine to a pressure of 3.5 MPa before the entry to the IP turbine wherethere is another valve loss to a pressure of 3.4 MPa. The steam expands through the IP and

LP turbines to 5 kPa in the condenser where the steam condenses to saturated water. Thecondenser extraction pump takes suction from 5 kPa and delivers 600 kPa which is thedeaerator pressure and the suction pressure of the boiler feed pump.

The points where the steam is bled from is normally 2 MPa for the HP heater, 600 kPa for thedeaerator and 150 kPa for the LP heater.

For this cycle, a steam drive feed pump is added to the cycle where steam is bled off from 2Mpa and expands into the condenser.

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-1 0 1 2 3 4 5 6 7 8 90

50

100

150

200

250

300

350

400

450

500

550

s [kJ/kg-K]

T[C]

Actual Rankine Cycle

m1m1

m2m2

m3m3

20

19

18

17

16

15

13

12

9,10

7,8

216

5

4

3

21

14

m4m4,22

,23

Figure 10: Actual Rankine Cycle

1.1 OPTIMISATION

1.1.1BOILER FEED PRESSURE

-1.0 0.1 1.2 2.3 3.4 4.5 5.6 6.7 7.8 8.9 10.00

50

100

150

200

250

300

350

400

450

500

s [kJ/kg-K]

T[C]

18 MPa21.5 MPa

Steam

Critical Pressure Point

Figure 11: T-s diagram for Steam

For water as working fluid, the critical point is the point where no change of state can takeplace when the pressure is sufficient or if heat is added. At the critical point the water andsteam can't be distinguished, and there is no point referring to water or steam. For statesabove the critical point the steam is supercritical. Supercritical is not the same as superheated- which is saturated steam at lower pressures and temperatures heated above the saturationtemperature.

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For a supercritical steam boiler, the operating pressure is so high (22.06 MPa) that actualboiling ceases to occur, the boiler has no liquid water - steam separation. There is nogeneration of steam bubbles within the water, because the pressure is above the criticalpressure at which steam bubbles can form. It passes below the critical point as it does work ina high pressure turbine and enters the condenser.

For this design, a normal boiler will be used with only superheated capability. The figurebelow, for the actual cycle, show that the greater the pressure of the feedpump, the greater isthe specific work output and the more improved the efficiency. However, the boiler pressure islimited by the critical pressure of steam and therefore cannot exceed that pressure. The boilerpressure is lower than the delivery pressure of the feedpump due to the pipe and head lossesby a 18/22 factor.

17 19 21 23 25 270.424

0.428

0.432

0.436

0.44

0.444

1258

1262

1267

1271

1276

1280

Pfeedpump [MPa]

RANKINE

RANKINERANKINE

wnetto

[kJ/kg]

wnettownetto

Feedpump Pressure

Figure 12: Efficiency and work output for feedpump pressure

Therefore the boiler pressure selected will be close to the critical pressure for the betterperformance of the cycle. However the dryness fraction decreases and this will lead to morewet steam in the final stages of the turbine. For the ideal cycle, the dryness fraction is toosmall for the final stage of the turbine to prevent erosion on the blades, but for the actualcycle, the steam is still dry enough in the final stages of the turbine. The next step is to find

the optimum outlet pressure of the HP turbine which will also be the reheat cycle pressure.

3 .
http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Critical_temperature_and_pressurehttp://en.wikipedia.org/wiki/Critical_temperature_and_pressurehttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Critical_temperature_and_pressurehttp://en.wikipedia.org/wiki/Critical_temperature_and_pressure


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1.1.2TEMPERATURE

For the maximum temperature that the boiler can deliver, the efficiency as well as the specific

work output improves as the temperature increases. However there is a limit to this maximumtemperature, because of the creep rupture strength of the tubes in the boiler. For atemperature of 540 C, the temperature in the boiler is around 1600 C and therefore the limitto the temperature is in the boiler and not in the HP turbine.

500 510 520 530 540 550 560 5700.428

0.432

0.436

0.44

0.444

0.448

1260

1280

1300

1320

1340

1360

1380

1400

Tmax [C]

RANKINE

RANKINERANKINE

wnettownetto

wnetto

[kJ/kg]

Temperature

Figure 13: Maximum temperature

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1.1.3REHEAT CYCLE PRESSURE

For the pressure at which the reheat should take place, there are a few parameters to

consider. One of the parameters that will change is the efficiency; because of the heat inputwill change as the pressure changes. Its the same with the specific work output that willchange with the pressure change. Another parameter that is controlled by the reheat pressureis the dryness fraction in the final stage of the turbine or inlet into the condenser; because theexpansion through the IP and LP turbines are depended of the reheat pressure.

2 3 4 5 6 7

0.43

0.432

0.434

0.436

0.438

0.44

1270

1290

1310

1330

1350

1370

Preheat [MPa]

RANKINE

RANKINERANKINE

wnettownetto

wnetto

[kJ/kg]

Reheat Cycle Pressure

Figure 14: Efficiency and work output for reheat cycle pressure

For this figure shown, the efficiency improves the greater the pressure, but it flattens out.However, the specific work output improves as the pressure lowers, and keeps improving.The reason for this is that the HP turbine delivers more work and the steam gets reheated tothe same temperature and then expands through the IP and LP turbines. The reason for theefficiency decreasing at lower pressure is that the temperature drops through the turbine, sothe further the pressure drops through expanding in the turbine, the more heat input is

needed and therefore the efficiency is less. Therefore a balance between the efficiency andthe specific work output is needed for the optimum performance of the cycle.

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Another factor that plays a roll on the reheat pressure is the dryness fraction. If the drynessfraction is too low, the steam is too wet in the final stages in the turbine and then erosionoccurs. If the dryness fraction is too high, more heat is rejected to the condenser andtherefore the efficiency drops. So the higher the pressure, the lower the dryness fraction andtherefore the efficiency are higher.

2 3 4 5 60.9

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.431

0.432

0.433

0.434

0.435

0.436

0.437

0.438

0.439

0.44

Preheat [MPa]

x6

x6x6

RANKINERANKINE

RANKINE

Figure 15: Dryness fraction for reheat cycle pressure

The optimum performance for the cycle will be decided with the balance in the pressure forthe efficiency and the specific work output. The next step of the optimisation is to decidewhere the tapping points for the bleed steam will be and what effect it will have on the cycle.

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1.1.4BLEED STEAM TAPPING POINTS

For the feed heating cycle, there are three points where the steam can be bled from. The first

is in the IP turbine cycle where the steam is bled for the HP heater. For the feed heatingcycle, the HP heater is there to preheat the water after the feedpump and before the boiler.

In the graph shown, the efficiency improves as the bleeding pressure increases, up to a pointand then it decreases again. The specific work output decreases as the pressure rises,because there is less mass flow to do work in the turbine as the bleeding pressure increases.If the bleeding pressure is less, it is possible for the turbines to do more work before thetapping point. Therefore it will be best to find the right balance for the tapping point dependingon the interest in efficiency or in specific work output. For this practical cycle, a value inbetween will be picked for the best efficiency and output.

1 1.5 2 2.5 3 3.50.434

0.435

0.436

0.437

0.438

0.439

0.44

1240

1260

1280

1300

1320

1340

1360

1380

Pm,dot,1 [MPa]

RANKINE

RANKINERANKINE

wn

etto

[kJ/kg]

wnettownetto

Pressure for HP heater tapping point

Figure 16: Pressure for bleed point for HP heater

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The deaerator pressure is the delivery pressure of the condensate extraction pump and theinlet pressure of the main feedpump. Therefore the deaerator pressure is the same pressurefrom which the steam for the contact heater is bled. The steam is bled from the LP turbinecycle for the contact heater.

In the graph shown, there are a specific point where the efficiency and specific work output isa maximum at a certain pressure. As the pressure increases, the efficiency as well as thework output is decreased. The reason for this is that the turbines do less work and theextraction pump must do more to deliver the sufficient pressure. Therefore the tapping pointpressure will be selected on the point for maximum efficiency and specific work output.

0.5 1 1.5 2 2.5 30.43

0.432

0.434

0.436

0.438

0.44

0.442

0.444

1295

1300

1305

1310

1315

1320

1325

1330

1335

Pm,dot,2 [MPa]

RANKINE

RANKINERANKINE

wnettownetto

wnetto

[kJ/kg]

Pressure for deaerator tapping point

Figure 17: Pressure bleed point for deaerator

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The LP heater takes steam from the LP turbine cycle and heats up the water from the outletof the extraction pump and heats it up to a certain point. For obvious reasons, if the pressureis too low, there can be no work done and therefore the efficiency also lowers. The graphshows that there is a certain point where the values are a maximum. Therefore the pressure

will be selected for the maximum efficiency and specific work output.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40.432

0.433

0.434

0.435

0.436

0.437

0.438

0.439

0.44

1310

1315

1320

1325

1330

1335

1340

Pm,dot,3 [MPa]

RAN

KINE

RANKINERANKINE

wnetto

[kJ/kg]

wnettownetto

Pressure for LP heater tapping point

Figure 18: Pressure for bleed point for LP heater

In the table below, the values for the optimisation changes are shown.

Parameter ValuePfeedpump 26 MPaPreheat 4 MPaPcondenser 8 kPaPm_dot_1 1.75 MPaPm_dot_2 0.66 MPaPm_dot_3 0.1 MPaPdeaerator 0.66 MPa

PSFP 1 MPaTable 8: Optimisation changes

Cycleqin

[kJ/kg]wnett

[kJ/kg]qout

[kJ/kg]

[%]SSC

[kg/kWh]

Dryness

Fraction

P[MW]

Rankine Cycle withFeed Heating

(Ideal)3058 1517 1541

49.62

2.37 0.86 379

Rankine Cycle withFeed Heating

3042 1336 1706 43.92

2.694 0.9303

334

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(Actual)

Table 9: Comparison for Ideal and Actual

2 ADDITIONAL

2.1 MATERIAL SELECTION

The material selection for a boiler is very important for the performance of the boiler in termsof the heat transfer to the working fluid, the resistance against corrosion and resistanceagainst abrasive wear. The typical challenges for the selections of materials for a boiler arethe high temperature and pressure at which the boiler operates as well as the aggressivemedia in the boiler.

The material used for the tubes in the boiler is mainly stainless steel. The difference betweennormal stainless steel and these used for the boiler tubes is that the material is more stable inthe austenite region. The reason for this stability is the increased content CrNi and CrNiMo in

the steel over the standard 18/8 CrNi and 18/8/2 CrNiMo steels, and more especially byadditions of nitrogen, which is particularly effective in promoting the austenite stability.Typical properties of austenitic heat resistance materials include:

High creep rupture strength above 550 C Resistance to high-temperature corrosion and oxidation Excellent processing characteristics

Creep rupture strength is one of the most important properties of materials used for tubesworking under pressure. The improvement in creep rupture strength can be attributed tometallurgical changes related to specific alloying. The stainless steels alloys with the highestcreep rupture strength are the supercritical grades. The high creep rupture strength in thehigh-nickel steels is due to precipitates in the matrix. Despite differences in the mechanicaland physical properties of the materials, due to their chemical composition, there are certainsimilar characteristics that are attributable to their metallurgical face-centred cubic latticestructure.

Besides creep rupture strength, ductility is an important factor governing the suitability of amaterial for a given application. Under long-term stress both stabilized and non-stabilizedsteels display lower values of reduction in area after fracture. This is due to the hardeningeffect of special carbide precipitates and metallic phases. However, high-nickel steels, as wellas nickel base alloys, also shows very good ductility values after long-term exposure toservice stresses across the range of high-temperature applications.

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1.1 STEAM VS ELECTRICALLY DRIVEN FEEDPUMP

For a practical Rankine cycle, there are two pumps providing pressure for the system. The

first pump is the condenser extraction pump and is much smaller than the main boilerfeedpump. The extraction pump takes suction from the condenser and delivers the water inthe deaerator with a pressure that will be the intake to the main feedpump. The extractionpump is electrically driven, which is very practical because it is quite small compared to themain feedpump.

For the main feedpump which takes suction from the deaerator and delivers pressure to theboiler, is quite large and takes a lot of power from the system to pump the condensate. Thedifference between the steam turbine driven and the electrically driven pumps is theiradvantages and disadvantages as well as the electrical units sent out. The table below showsa few characteristics of the two different drive systems: (the figures shown is for a 600 MWunit with 700 MW available before the HP turbine)

Steam turbine driven Electrically drivenAdvantage Disadvantage Advantage Disadvantage

Better efficiency forthe whole cycle

Expensive turbine Very easy on start-up Expensive motor

No auxiliary powerneeded

Difficult to use ondifferent loads

Easy to control overdifferent loads

Needs auxiliarypower (20 MW)

Efficiency 85% Efficiency 75%Generator reading 591.6 MW Generator reading 616 MWUnits Generated 573.9 MW Units Generated 597.5 MWUnits sent-out 571 MW Units sent-out 563 MW

Table 10: Pump drivers comparisonFor a steam driven feedpump, the tapping point for the bleed steam is not that important as iswas for the HP and LP heaters. The amount of work needed by the pump is equal to the

amount of work that the turbine will deliver. So for a higher pressure selected, the mass flowof the steam through the turbine will be less. In the figure shown, the efficiency and thespecific work output of the cycle increases as the tapping pressure decreases.

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.4391

0.4392

0.4392

0.4393

0.4393

0.4394

0.4394

1336

1336

1336

1336

1336

1336

1336

1336

1337

1337

PSFP [MPa]

RANKIN

E

RANKINERANKINE

wnettownetto

wnetto

[kJ/kg

]

Steam turbine driven feedpump tapping point pressure

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Figure 19: Pressure tapping point for turbine driven feedpump

1.2 WET VS DRY COOLING SYSTEMS

In the power generating industry, there are always factors that cannot be changed and forms

part of the location of the power station. These factors are the coal, water, ambienttemperature. One of the most important components of the power generating cycle is thecondenser, which condense the steam back to water to be pumped. For the condenser tocondense the steam, a cooling system is needed to cool down the water used to condensethe steam. There are three ways of doing so, the first is a wet cooling system, the second adry cooling system using fans and the third also a dry cooling system, but a dry cooling toweris used.

The wet cooling tower is full of tubes that circulate the water through the cooling tower andcondenser. The cooling water is cooled by sprinkler system in the tower, with a large dam atthe bottom of the tower. The disadvantage of this system is that the location of the powerstation needs to be close to water, because a lot of water is needed and it evaporates.

The two dry systems are basically the same. The first using fans for the convection heat

transfer between the tubes and air, whereas the second uses a tower that is full of tubes, butthe tower is shaped so that there is natural draft of air through the tower which provides theconvection heat transfer. The disadvantages of the first system are that it uses a lot ofauxiliary power and the second is that it relies on the ambient temperature.

In the table below, the three systems are compared.

Wet cooling system Dry cooling systemFan draft system Tower draft system

Efficiency of the cycle ishigher

Not very expensive Very expensive construction

Specific work output isgreater

Maintenance needs to bedone on the fans

Very little maintenance

The water used for thissystem is high

Uses a lot of auxiliary power No auxiliary power needed

Table 11: Cooling system comparison

2

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3 CONCLUSION

The generating of power starts with a simple principal of energy cannot be created ordestroyed. Therefore when it comes to a coal fired power station, the energy that is in the coal

as it comes from the mine, is converted into heat energy and used to heat water whichbecomes steam. The steam is then used in turbines that put out work and drives thegenerator and delivers electricity which is then fed into the grid.

The cycle used for delivering work is a Rankine cycle that has been optimised for betterefficiency and work output. It all starts with a simple Carnot cycle and from there it isdeveloped into a Rankine cycle. The normal Rankine cycle is then developed into a cycle thattakes the steam into the superheated region, with a reheat cycle and steam is bled off for thefeed water heating. These added components are there to improve the cycle and to make itmore feasible for practical cases.

After the basic cycle has been formed, the optimisation of the cycle starts. This is done to getthe best possible performance out of the turbines as well as what is needed and certaincompromises that have to be made. Components that can be optimised are the feedpumpdelivery pressure, feed heater tapping points and reheat pressure. The optimisation can belimited by certain parameters that cannot be changed and has to for part of the cycle.

The power station as a whole has a major impact on the environment and the location whereit is based. The pollution coming from a power station and the effect of the cooling water thatevaporates into the sky can also have an effect on the weather. Nevertheless, power isneeded everywhere and therefore it must be generated, but the challenge is to make it asclean and efficient as possible.

steam turbine design project 2011

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