SIMULATION AND OPTIMIZATION OF

COMPRESSION-ABSORPTION

REFRIGERATION SYSTEM

ANIL KUMAR PRATIHAR

Mechanical Engineering Department

Submitted

in fulfillment of the requirements of the degree of Doctor of Philosophy

to the

Indian Institute of Technology, Delhi

SEPTEMBER, 2006

DELHI.

(Decficatec17o )lCmighty God;

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CERTIFICATE

This is to certify that the thesis entitled "Simulation and Optimization of

Compression-Absorption Refrigeration System" being submitted by Mr. Anil

Kumar Pratihar for the award of the degree of Doctor of Philosophy is a record of

bonafide research work carried out by him in the Mechanical Engineering Department

of Indian Institute of Technology. Delhi.

Mr. Anil Kumar Pratihar worked under our guidance and supervision and has fulfilled

the requirements for the submission of this thesis, which to our knowledge, has

reached the requisite standard. The matter embodied in this thesis has not been

submitted in part or in full to any other university or institute for the award of any

degree.

ç.

(Prof. S. C. Kaushik)

(Prof. R. S. A arwal)

Centre of Energy Studies, Mech. Engg. Deptt.,

II T Delhi, Hauz Khas, II T Delhi, Hauz Khas,

New Delhi- 110 016 New Delhi- 110 016

ACKNOWLEDGEMENTS

I wish to express my deep sense of gratitude to Prof. R. S. Agarwal and Prof. S. C.

Kaushik for their kind blessings and invaluable guidance, which led me to carry

out this research work. Encouraging and generous words of Prof. Agarwal and

Prof. Kaushik have been inspiring forces for me throughout the span of this work.

I shall always be indebted to Prof. P. L. Dhar and Dr. Sanjeev Jain for their invaluable

suggestions and generous help provided in carrying out the simulation work.

I acknowledge Prof. S. C. Mullick, Dr Sanjeev Jain and Dr. P.M.V. Subbarao, the

SRC members, for their kind interest in this research work.

I can never forget to mention about Prof. N. K. Sharma, former Head, Mechanical

Engineering Department, College of Technology, Pantnagar, who always provided all

the support for carrying out this work.

My sincere thanks are due to my friends Prof. L. Varshney, Dr. R. S. Jadoun and

Mr. Raj Kumar Singh who always helped me at various stages of this work.

I cannot forget my fellow research scholars, Mr. Arun Asati, Mr. Ritunesh Maurya,

Mr. Rajeev Kukreja and Mr. Vishal Singh, for their kind co-operation.

The credit of successful completion of this research work goes to my sons, Mr. Shivin

& Aryan and my wife, Mrs. Gunjan as they had constantly encouraged me and co-

operated with me when I was busy with my work. My sons have sacrificed their

unforgettable golden days of infancy, which they had to spend waiting for me when I

was busy with my research work.

to

My father always encouraged me for hard work during my studies and blessed me for

an excellent career in my life. He is the main motivating force who provided me

moral and all round support to accomplish this research work. The love and blessings

of my parents have always been there with me to provide me strength in every walk of

my life.

My siblings need special mention for their invaluable moral support and help rendered

when I was busy with my research work.

I wish to express my thankfulness to my father in-law who always appreciated me and

encouraged me to carry out quality work. I can never forget the kind of support he

provided during the onset of my Ph.D. programme.

I wish to thank Ms. Anupam, especially for her sincere help in the preparation and

correction of this manuscript at the juncture when I was running short of time.

Last but not the least, I would like to express my thanks to all those who directly or

indirectly helped me during the course of this research work.

Dated: September 04, 2006 (Anil Kumar Pratihar)

Place: Delhi

iv

ABSTRACT

It is widely reported in the literature that the compression—absorption systems are

potential alternatives to vapour compression and vapour absorption systems due to

some of their distinguishing features. These systems make use of environment

friendly refrigerant-absorbent mixture like ammonia-water, which is ozone friendly

and has very low global warming potential. Further, these systems have better or

comparable COP than conventional vapour compression systems.

A literature review on compression-absorption systems reveals about various

theoretical and experimental studies performed on these hybrid systems. It is found

that most of the work has been carried out on heat pump applications and the

theoretical analysis of the system has been performed using UA value (overall heat

transfer coefficient-area product) in most of the cases without modelling the actual

processes occurring in the absorber, desorber and the solution heat exchanger of the

system.

An insight into the economics of the compression-absorption systems emphasizes the

need of system optimization. This is due to the fact that compression-absorption

systems are considered to be costlier than vapour compression systems. Further, it is

well known that two stage compression-absorption systems give higher COP

compared to single stage systems but at the cost of reduced capacity. Thus the capital

cost per unit capacity increases in the case of the two stage systems. Therefore, the

present work is aimed at carrying out thermodynamic analysis, simulation and

optimization of single stage compression-absorption refrigeration system of solution

recirculation type in order to carry out feasibility study for water chilling application

for summer air conditioning.

V

In the thermodynamic analysis, two studies have been performed; parametric studies

to know the effect of various operating parameters on the COP and a comparison with

the vapour compression cycle. Thermodynamic analysis of compression—absorption

cycle has been carried out using Engineering Equation Solver (EES) to study the

effect of temperature glide in the absorber, degree of subcooling of weak solution at

the inlet to the absorber and desorber pressure on the performance of the cycle. The

results show that for a given weak solution inlet temperature and concentration and a

given pressure ratio. an optimum absorber glide exists at which COP of the cycle

attains a maximum value. It has also been found that the COP of the cycle increases

as the degree of subcooling of weak solution entering the absorber decreases i.e. when

the solution is heated to nearly saturated condition in the solution heat exchanger. In

the comparison, the performance of the compression-absorption cycle operating on

ammonia-water mixture has been compared separately with three vapour compression

cycles operating on ammonia, R134a and R22 refrigerants. The COP of the

compression-absorption cycle has been found to be always higher than that of vapour

compression cycle, for the given range of operating temperatures.

The absorber and the desorber of the compression-absorption have been modelled as

vertical co-current shell and tube heat exchangers, with the solution as falling film on

inside tube surface, vapour in the core, co-current with the solution and water on

baffled shell side in counter-current direction with the solution. The solution heat

exchanger has been modeled as multi-tube hairpin type for 400 kW system and

double tube type counter flow heat exchanger for 100 kW plant. The simulation of the

absorber and the desorber has been carried out by solving mass material and energy

balance equations written in the form of ordinary differential equations and for

validation of the model, the simulation results have been compared with the published

vi

results. The results of the simulation agree reasonably well with the published data.

The other components, being the standard components, have been modelled using

standard simulation procedures.

The simulation of complete compression-absorption system has been carried out by

solving the governing equations of each component, in a sequential manner, using

Warner's technique. This technique proved to be very successful in the fast

convergence of the iterations. Simulation and optimization results for two systems,

one having large capacity of 400 kW and the other having small capacity of 100 kW,

have been presented. Simulations of 400 kW system has been performed for three

different configurations of system having three different relative areas of the solution

heat exchanger viz. 17 %, 23 % and 30%.

It is found from the results of simulation that the COP of the system can be increased

by increasing the solution heat exchanger area, maintaining low mass flow rate of

weak solution and low cooling water temperature in the absorber. The study of the

effect of relative area distribution, through system simulations, suggests that the

increase in the solution heat exchanger area beyond a certain limit does not help to

increase the COP; rather COP attains an optimum value at a definite relative area of

solution heat exchanger. The COP was found to be optimum at 39 % and 45 %

solution heat exchanger area in case of 100 kW and 400 kW systems, respectively, as

indicated by simulation results. This implies that in large systems, comparatively

more surface area can be provided to solution heat exchanger as a measure to increase

the COP. In case of 400 kW system, COP increased by about 16 % on increasing the

solution heat exchanger area from 10 to 30 %, keeping total area constant. However,

the capacity of the system decreases on account of increase in the solution heat

vii

exchanger area. Besides relative area distribution, the other parameters studied are the

mass flow rate of water in the absorber as well as the desorber, mass flow rate of

weak solution, pressure drops in the solution heat exchanger and tube side pressure

drops in the absorber and the desorber.

It was also observed that turbulent flow conditions could be obtained in the absorber

but not in the desorber where laminar flow prevailed due to lower operating

temperatures (in the case of refrigeration application). This necessitates more compact

and efficient designs of the desorber for refrigeration applications.

Different configurations of a system can be generated for accomplishing a desired

objective, but obviously at varying overall costs. This not only indicates the need for

system optimization but also makes it mandatory, in order to slash the extra costs

incurred on an incorrectly sized system, characterized by high capital as well as high

operating costs. Therefore, optimization has been carried out to configure a system to

minimize the life cycle cost using a nontraditional optimization technique known as

`Differential Evolution'. The effect of inflation in the cost of power has been taken

into account in the calculation of life cycle cost. Further, the optimization procedure

has also been extended to find an optimum relative area distribution among the

absorber, desorber and the solution heat exchanger with an objective to minimize the

capital cost of the system. Besides system optimization, optimization of the absorber

and the desorber has also been carried out to find their optimum designs and to study

the effect of inside tube diameter, baffle spacing and tube pitch on the optimum

designs of these components.

viii

TABLE OF CONTENTS

CERTIFICATE

ACKNOWLEDGEMENTS

ABSTRACT

LIST OF FIGURES

LIST OF TABLES

NOMENCLATURE

CHAPTER 1 Introduction 1

CHAPTER II Literature Review 5

2.1 Description of compression-absorption system 5

2.2 Compression-absorption cycles 8

2.2.1 Vapour compression cycle with single stage

solution circuit 9

2.2.2 Vapour compression cycle with two stage

solution circuit 10

2.2.3 Desorber/absorber heat exchange (DAHX) cycle 12

2.3 Theoretical/experimental investigations 13

2.3.1 Thermodynamic investigations 13

2.3.2 Simulation studies and experimental investigations 14

2.4 Working fluids for compression-absorption systems 20

2.5 Review on modelling of components 22

2.5.1 Compressor 22

2.5.2 Absorber and desorber 24

2.5.3 Solution heat exchanger (SHX) 24a

2.6 Scope of the research work 24b

ix

2.7 Objectives of the present work 27

CHAPTER III Thermodynamic Analysis of Compression-Absorption

Refrigeration System 29

3.1 Introduction 29

3.2 Thermodynamic analysis 30

3.2.1 Governing equations 33

3.2.2 Performance calculations 35

3.2.3 Results and discussions 37

3.3 Comparison of compression-absorption and vapour

compression cycle 41

3.4 Conclusions 42

CHAPTER IV Modelling and Simulation of Components 45

4.1 Introduction 45

4.2 Simulation of the absorber 45

4.3 Simulation of the desorber 53

4.4 Solution heat exchanger (SHX) 58

4.5 Expansion device 61

4.6 Mixer 62

4.7 Pump 64

4.8 Compressor 64

4.9 Thermodynamic and physical properties of ammonia water 66

4.10 Conclusions 66

CHAPTER V Simulation of the Compression-Absorption Refrigeration

System 67

5.1 Introduction 67

x

5.2 The system 69

5.3 Simulation procedure 70

5.4 Solution technique 76

5.5 Simulation of 400 kW CA system 77

5.6 Results of simulation of 400 kW system and discussions 79

5.6.1 Effect of mass flow rate of water (sink) in the

absorber 81

5.6.2 Effect of mass flow rate of water (source) in the

desorber 81

5.6.3 Effect of relative area distribution 83

5.6.4 Temperature profiles in the absorber and the desorber 86

5.6.5 Effect of the mass flow rate of weak solution and

SHX area 90

5.6.6 Tube side pressure drop 95

5.6.7 Influence of the cooling water temperature in the

absorber 95

5.6.8 Effect of the desorber pressure 96

5.7 Simulation of 100 kW compression-absorption system 97

5.7.1 Results of simulation 98

5.7.2 Effect of relative area distribution in 100 kW System 99

5.7.3 Temperature profiles in the absorber and the desorber 103

5.7.4 Effect of pressure drop in the solution heat exchanger 103

5.8 Validation of the simulation model 104

5.9 Problems encountered in the simulation and corrective

measures taken 104

5.10 Conclusions 105

CHAPTER VI Optimization of Compression-Absorption and System

Components 107

6.1 Introduction 107

6.2 Nontraditional optimization techniques 108

6.3 Optimization procedure 110

6.4 Validation of code 111

6.5 Optimization of compression-absorption system 112

6.5.1 Formulation of optimization problem 112

6.5.2 Design variables 115

6.5.3 Constraints 11

6.5.3.1 Boundary constraints 116

6.5.3.2 Inequality constraints 116

6.6 Results of optimization of 400 kW compression-absorption

system 117

6.6.1 Case-I: Optimum area distribution that minimizes

fixed cost 118

6.6.2 Case-2: Optimization of system for minimization of

life cycle cost 120

6.7 Results of optimization of 100 kW system 120

6.8 Component optimization 122

6.8.1 Optimization of absorber 122

6.8.1.1 Effect of inside tube diameter 124

6.8.1.2 Effect of baffle spacing 126

6.8.1.3 Effect of tube pitch 127

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6.8.2 Optimization of the desorber 127

6.8.2.1 Effect of inside tube diameter 129

6.8.2.2 Effect of baffle spacing 129

6.9 Discussions on results of optimization of system and

individual components 132

6.10 Conclusions 132

CHAPTER-VII Main Conclusions and Scope for Future Work 135

REFERENCES 139

APPENDICES

APPENDIX A Thermodynamic and physical properties of

Ammonia-water mixture 145

APPENDIX B 1 Warner's Technique 153

APPENDIX B2 Results of simulation of 400 kW

compression-absorption system 157

APPENDIX B3 Results of simulation of 100 kW

compression-absorption system 163

APPENDIX CI Differential Evolution Technique 165

APPENDIX C2 Results of optimization of 100 kW

compression-absorption system 173

APPENDIX C3 Results of optimization of the desorber 183

APPENDIX C4 Functions for calculation of cost of tubes 190

and outer diameter of tubes