t
COMPUTER AIDED MODELLING OF HEAT REJECTION SYSTEM OF A THERMAL
POWER PLANT
by
YASHVIR SINGH GOEL
Thesis submitted
in fulfilment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
saw.%
Department of Mechanical Engineering
INDIAN INSTITUTE OF TECHNOLOGY, DELHI
Hauz Khas, New Delhi-110016 JULY, 1993
DEDICATED TO MY PARENTS
CERTIFICATE
This is to certify that the thesis entitled "Computer Aided Modelling of Heat Rejection System of a Thermal Power Plant" being submitted by Mr. YASHVIR SINGH GOEL to the Indian Institute of Technology, Delhi, for the award of the degree of "Doctor of Philosophy" is a bonafide record of candidate's own research work carried out by him under our guidance and supervision. In our opinion, the thesis has reached the standards fulfilling the requirements of all the regulations relating to the degree.
The results contained in this thesis have not been submitted, in part or in full, to this Institute or any other University or Institute for the award of any degree or diploma.
CJx. .55- 6/7 /3
Dr. V.R. Srivastava Assistant Professor
Chemical Engg. Department Indian Instt. of Technology New Delhi-110016
Professor P.B. Sharma Professor of Mechanical Engg.(On lien)
I.I.T. Delhi Principal
Delhi College of Engineering Delhi
ACKNOWLEDGEMENTS
The author expresses his deep sense of gratitude and sincere thanks to Professor P.B. Sharma and Dr. V.K.
Srivastava for their highly encouraging and inspiring guidance throughout this work. The interest shown by them together with their unfailing patience and indefatigability have been a source of inspiration to me and has enabled me to bring this work to a successful completion which may not otherwise have been possible.
I am also indebted to Shri M.W. Goklany, Chairman of M/s DESEIN Pvt. Ltd., New Delhi, and Shri Balakrishna, Executive Director of M/s Development Consultants Pvt. Ltd., New Delhi, for their guidance and encouragement.
I wish to express my gratitude to Dr. M.N. Gupta of Computer Centre of I.I.T. Delhi for his kind help and guidance in carrying out the work on main frame computer ICL 3980 installed in Computer Centre at I.I.T. Delhi.
I also wish to express my sincere gratitude to Prof. N.
C. Nigam, Director, I.I.T. Delhi, who has provided all help and encouragements to complete the present study. My sincere thanks to the authorities of super thermal power plants of National Thermal Power Corporation at Vindhyachal and Singrauli and of Uttar Pradesh State Electricity Board at Annapara for their kind support and cooperation.
Finally, I must mention, how much I owe to all the members of my family, particularly to my wife Anjli, for bearing with me all the hardships during the entire period of this work at I.I.T. Delhi. Last but not the least, the support of Mr. Bhalla for typing the thesis and Mr.
Chowdhury for the graphical assistance is gratefully acknowledged.
Dated: 5th July, 1993 (YASHVIR SINGH GOEL) New Delhi
ABSTRACT
Electrical energy is the key requirement for the economic progress of any country and particularly for a developing country like ours which has now entered an era of industrial advancement in the rapidly developing global 'economy. The installed capacity for power generation has risen to 66,000 MW in 1991 from about 1,700 MW in 1950(67).
Today, in India, thermal power contributes about 71% of the .total power generation. An additional capacity of 38,000 MW has been proposed in the Eighth Five Year Plan to sustain the economic growth of the country.
Consequently, there is a growing awareness in the power industry to optimise the turbine heat rate (THR), which is the prime efficiency function and is defined as the heat input to the working substance to produce one unit of electricity. Modelling and optimization of the constituent sub-systems is a logical and necessary step towards maximizing the overall performance of the thermal power plant. In the present work, computer aided modelling of heat rejection system of a thermal power plant has been carried out.
The main factors affecting the turbine heat rate are:
main steam pressure, hot reheat steam temperature, condenser vacuum and reheater pressure drop. Among these factors, the condenser vacuum is the single most important factor which contributes to the deviation in turbine heat rate from the
design value. The condenser vacuum depends upon the heat removal system being used for the power plant.
Approximately, 60% of the total heat added to the working fluid in the Rankine cycle is finally rejected to the atmosphere. This indicates the importance and dimension of the heat rejection system which should be simulated and analyzed for the efficient functioning of the thermal power plant. Modern thermal power plants require huge quantities of cold water which makes it imperative to use large size .cooling towers to optimize the cost and layout constraints.
Cold water temperature from the cooling tower directly affects the datum temperature of heat removal in a condenser in a Rankine cycle. This, in turn, affects the condenser back pressure which has a direct bearing on the turbine heat rate. This, two way coupling between the condenser and cooling tower, has been studied in the present work. An analysis of thermal performance of both counter flow and cross flow cooling towers is presented in this work. The cold water temperature is an output data from the cooling tower system and is affected by the ambient conditions (wet bulb temperature, dry bulb temperature, relative humidity), thermal load, liquid and air flow rate and the tower characteristic. Mathematical models have also been developed for the thermal performance of condenser which have been integrated with the cooling tower models to incorporate the above two-way coupling effect.
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Turbine heat rate correction factor has also been computed and a comprehensive model of turbine heat rate deviation calculation for different condenser back pressure and different conditions of cooling tower has also been developed. The models presented in the present work are considered as powerful tools for use by the designers, consultants and operating personnel of the steam power plant for analyzing and optimizing the heat rejection system.
The analytical work has been divided into four main 'sections:
(a) Cooling tower modelling (b) Condenser modelling
(c) Two-way coupling between condenser and cooling tower (d) Turbine heat rate deviation and consequent revenue loss
computation.
Following base mathematical models have been developed for the simulation of thermal performance of cooling tower:
(i) Analysis of counter flow cooling tower using Merkel's approach.
(ii) Analysis of cross flow cooling tower using Merkel's approach and unit volume method.
(iii) Rigorous analysis of counter flow cooling towers considering evaporation loss and inter-facial film resistance.
(iv) Analysis of cross flow cooling tower using Finite difference method.
(v) Rigorous analysis of cross flow cooling towers considering evaporation loss and inter-facial film resistance.
Under group (b), the following base mathematical models have been developed for the simulation of thermal performance of steam condensers:
(i) Analysis of condenser . using various codes being practised in Industry, viz. HEI, BEAMA etc.
(ii) Analysis of condenser using thermal resistance method.
(iii) Analysis of condenser using improved thermal resistance method.
(iv) Analysis of one pass steam condenser using discreet lumped cells model.
(v) Analysis of two pass steam condenser using discreet lumped cells model.
(vi) Transient analysis of steam condenser to model the effect of transient in working fluid temperature.
Under group (c) and (d), the following base mathematical models have been developed:
(i) Integrated models to describe the two way coupling between cooling tower and condenser.
(ii) An overall comprehensive model for describing the coupling among parameters of cooling tower, and condenser and their impact on turbine heat rate deviation and consequent revenue loss.
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The above mentioned mathematical models have been validated by the site data collected at a few super thermal power plants in India. Site data/manufacturer's curves collected from site have been used for validation of these models.
It has been observed that due to inherent assumption of Merkel's approach, there is a significant under-estimation of cooling tower characteristic by about 5 to 7%.
Further, it is observed that ambient conditions which Affect the cooling tower thermal performance also affect the turbine heat rate through a two-way coupling between condenser and cooling tower. A lower wet bulb temperature results in a lower cold water temperature which yields better condenser performance and lower turbine heat rate.
However, heat rate deviation due to ambient conditions is found to be more pronounced at part loads. Computer programmes have been made for all the mathematical models as stated above in FORTRAN 77 language. The computer work has been done on main frame computer ICL 3980, with VME operating system, installed in I.I.T. Computer Centre, Delhi.
CONTENTS
CERTIFICATE
ACKNOWLEDGEMENTS ABSTRACT
CONTENTS vi
LIST OF FIGURES
LIST OF TABLES xxi
LIST OF APPENDICES xxi
.NOMENCLATURE xxii
CHAPTER 1 INTRODUCTION
1.1 General 1
1.2 Objectives of the present work 5
1.3 Scope 9
1.4 Presentation of thesis 13 CHAPTER 2 REVIEW OF LITERATURE
2.1 General 16
2.2 Types of cooling tower 17 2.3 Selection of cooling towers 20 2.4 Development of theories of
cooling tower 21
2.5 Theories of steam condensers 47
2.5.1 General 47
2.5.2 Current methods used in industry 56 2.5.2.1 Use of various codes 56 2.5.2.2 Russian method 60
vi
2.5.2.3 Thermal resistance method 64 2.5.2.4 Improved thermal resistance method 73 CHAPTER 3 COOLING TOWER SIMULATION AND MODELLING
3.1 Introduction 87
3.1.1 General 87
3.1.2 Need for computer programming 89 3.2 Analysis of counter flow cooling
tower using Merkel's approach 91 3.3 Rigorous analysis of counter
flow cooling tower 94
3.4 Analysis of cross flow cooling
tower using unit volume method 103 3.5 Alternative model for analysis
of cross flow cooling tower
using finite difference method 107 3.6 Rigorous analysis of cross flow
cooling tower 110
3.7 Computer programme for cooling
tower performance calculation 111 3.8 Algorithm for computer programme
for cooling tower analysis 118 3.9 Subroutine programme for calculation
of cooling tower thermal performance 146
3.10 Validation 153
CHAPTER 4 CONDENSER SIMULATION AND MODELLING
4.1 Introduction 155
4.2 Analysis of condensers using
various codes 157
4.3 Analysis of condensers using
thermal resistance method 161 4.4 Analysis of condensers using
improved thermal resistance method 163
vi
i4.5 Rigorous analysis of steady state
two pass steam condensers 166 4.6 Rigorous analysis of steady state
one pass steam condensers 179 4.7 Transient analysis of steam
condensers 185
4.8 Computer programme for analysis
of steam condensers 190
4.9 Algorithm for computer programme
for analysis of steam condensers 195 4.10 Subroutine programme for analysis
of steam condensers 221
4.11 Validation 226
CHAPTER 5 TWO WAY COUPLING BETWEEN CONDENSER AND COOLING TOWER AND ITS IMPACT ON TURBINE HEAT RATE DEVIATION
5.1 General 227
5.2 Two way coupling between
condenser and cooling tower 229 5.3 Two way coupling between
condenser and turbine heat rate 232 5.4 Computation of turbine heat rate 233 5.5 Computation of condenser correction
factor and turbine heat rate deviation 234 5.6 Subroutine programmes for calculation
of turbine heat rate deviation 236 5.7 Algorithm for computer programmes 238
5.8 Validation 245
CHAPTER 6 DISCUSSION OF RESULTS
6.1 Thermal performance of counter flow
cooling tower using Merkel's approach 247 6.2 Thermal performance of counter flow
cooling tower using rigorous analysis 251
viii
6.3 Thermal performance of cross flow
cooling tower using unit volume method 259 6.4 Thermal performance of cross flow cooling
tower using finite difference method 262 6.5 Thermal performance of cross flow cooling
tower using rigorous analysis 266 6.6 Thermal performance of steam
condensers using various codes 272 6.7 Thermal performance of steam
condensers using conventional
thermal resistance method 273 6.8 Thermal performance of steam
condensers using improved thermal
resistance method 274
6.9 Thermal performance of two pass steam
condensers using rigorous analysis 275 6.10 Thermal performance of one pass steam
condenser using rigorous analysis 281 6.11 Transient analysis of steam condensers 285 6.12 Two way coupling between condenser
and cooling tower 289
6.13 Integrated model for cooling tower, condenser and turbine heat rate
deviation computations. 299 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS
7.1 Main conclusions 308
7.2 Suggestions for future work 314
REFERENCES 315
FIGURES APPENDICES
