AN ENERGY, EXERGY AND CFD ANALYSIS ON THE PERFORMANCE OF THE INDUCED DRAFT WET COOLING TOWER OF WARRI REFINING AND PETROCHEMICAL COMPANY LIMITED

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ABSTRACT

The aim of this work was to evaluate the performance of WRPC cooling tower using energy, exergy and CFD analysis methods. To achieve this, the evaluation of performance parameters of the cooling tower using energy method with the aid of Numerical integration via Excel spread sheet, the exergy analysis of the cooling tower was carried out with the aid of engineering equation solver, and additionally, the CFD analysis of air flow using Comsol Multi-physics. The evaluation of  performance characteristics of  WRPC the cooling tower reveals that all the parameters(range, effectiveness, evaporation rate, cooling capacity and performance coefficient) subjected to the comparative analysis between the operational data and design/standard data of WRPC cooling tower  showed that the cooling tower setup is grossly underperforming far below its design/standard operational capability; the CFD analysis of air revealed that the velocity, temperature, and relative humidity  profiles within the cooling tower are in sync with postulated and calculated characteristic of air within the cooling tower. Furthermore, the exergy destruction increases with the increases in mass flow rates of the cooling water, makeup water and air and verse versa, also, the change in exergy of water increases quadratically with temperature of water; the exergy of air remains constant when varied with relative humidity of air, while exergy efficiency and exergy destruction decrease with increase in relative humidity. Maximization of efficiency and Exergy Destruction with Varying Mass Flow Rates produces efficiency with a maximum value of  (0.9589[-]) while exergy destruction has a minimum value of 13.55 at minimum values (1.5kg/s, 0.15 (kg/s), 4.5 (kg/s)) of mass flows rates of Air, Makeup water and cooling water respectively; Minimization of Efficiency and Exergy Destruction with Varying Relative Humidity of air produces  optimal vales  of (0.8022[-]) for exergy efficiency while exergy destruction value of 6.012[W] at a maximum relative humidity value of 0.95[-]. The following recommendation are suggested; Proper evaluation of performance parameters of cooling towers should be done in a controlled environment for accurate results; CFD analysis should be carried out to test for new designs of cooling towers; Aside from low Re-K-e flow conditions other conditions of flow should be tested and results compared.





TABLE OF CONTENTS

Title Page i
Declaration ii
Dedication iii
Certification iv
Acknowledgements v
Table of Contents vi
List of Tables                                                                                  vii
List of Figures                                                                                viii

CHAPTER 1: INTRODUCTION
1.1. Background to the Study 1
1.2 Statement of Problem 3
1.3 Aim and objectives of Study 4
1.4 Scope of Study 4
1.5 Justification for the Study 5

CHAPTER 2: LITERATURE REVIEW
2.1 Cooling Towers 6
2.1.1 Theory of cooling towers 7
2.1.2 Classification of cooling towers 8
2.2 Conceptual Frame Work 9
2.2.1 Cooling tower performance analysis 9
2.2.2 Exergy Analysis 13
2.2.3 Computational flow dynamics of air through towers 15
2.3 Research Approach 16

CHAPTER 3: MATERIALS AND METHODS
3.1 Materials 18
3.1.1 Components of the cooling tower 18
3.1.2 Mode of operation 21
3.1.3 Description of study area 22
3.2 Methods                              22
3.3 Energy and Exergy Performance Evaluation Parameters     23
3.4 Performance Evaluation Parameters of Cooling Towers 23
3.5 Exergy Analysis of Cooling Tower Performance Parameters 27
3.6 Exergy optimization Modelling 32
3.7 Computational Flow Dynamics (CFD) of Air Flow through the Tower 33

CHAPTER 4: RESULTS AND DISCUSSION
4.1 Analysis of Cooling Tower Performance Characteristics 35
4.2       Exergy Analysis 42
4.3       Exergy Optimization 55
4.4       Cfd Simulation                                                           59
4.5 Validation of Results 70

CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion 72
5.2 Contributions to Knowledge 75
5.2 Recommendations 76
References                       78
Appendices             83






LIST OF TABLES

3.1 Material selection 18
4.1(a) Values of Range 35
4.2(b) Values of Approach 36
4.1(c) Values of Cooling Capacity 36
4.1(d) Values of Effectiveness 37
4.1(e) Values of Evapouration Rate 38
4.1(f) Values of Coefficient performance 38
4.1(g) Number of transfer unit (NTU) 39
4.2(a) Values of exergy destruction, exergy efficiency, exergy of water, air and mass flow  rates of air, cooling water and makeup water.    44
4.2(b)  Values of exergy destruction, exergy efficiency, exergy of water, air and temperatures of inlet air, return cooling water , makeup water    45
4.2(c)  Values of exergy destruction, exergy efficiency, exergy of water, air and relative humidity of air     45
4.3(a): Maximization of efficiency and exergy destruction with varying mass flow rates 55 
4.3(b): Minimization of efficiency and exergy destruction with varying Mass Flow Rates 56
4.3(c): Minimization of efficiency and exergy destruction with 
varying relative humidity 57
4.3(d): Maximization of efficiency and exergy destruction with relative humidity 58
4.3(e): Maximization of efficiency and exergy destruction with cooling water and makeup temperature    59







LIST OF FIGURES

1.1 Cooling tower 3
2.1 Schematic of cooling tower 7
2.2 Showing the variation in temperature of cooling tower 8
3.1 Geometry of the cooling tower 20
3.2 (a) Picture of the cooling tower 21
(b) Schematic diagram of the cell cooling tower 
3.3       Exergy flow sketch 28
4.1 Cooling tower performance characteristics graphs (Bar Charts) 42
4.2 (a) Exergy of air variation with mass flow rate of air 46
4.2 (b) Exergy of water variation with mass flow rate of water. 47
4.2 (c) Relationship between exergy efficiency and mass flow rate of air, Makeup water and cooling Water   48
4.2 (d) A Bar Graph of exergy destruction and mass flow rate of air, makeup water and cooling water     49
4.2 (e)  A Bar Graph of exergy destruction and temperature of air, makeup water and cooling water    50
4.2 (f)   Relationship between exergy efficiency and temperature of air, makeup water and cooling Water    51
4.2 (g) Exergy of Air Variation with Temperature of air 52
4.2 (h) Exergy of water variation with temperature of water. 53
4.2 (i) Relative humidity of air variation with exergy of air, exergy efficiency, exergy    destruction    54
4.4(a) Modeled geometry of the cooling tower 60
4.4(b-d) Categorization and selection of materials 62
4.4(e-h) Air Properties Plots Against Temperature 63
4.4(i-l) Water Properties Plots Against Temperature 64
4.4(m-p) Boundary/Domain Views 65
4.4(q-r) Generated mesh for the cooling tower 66
4.4(s-v) Pressure, temperature, relative humidity and velocity distribution`` 68




NOMENCLATURE

C_Pa Specific heat capacity of air
C_pv Specific heat capacity of air at constant volume
C_pw Specific heat capacity of water at constant pressure
〖CWT〗_1 Cooling water temperature
Ε_l Evapouration Loss
G Mass flow rate per unit area of air
h_w Enthalpy of water
h_a Enthalpy of air
K_ω a Coefficient of heat transfer
L Mass flow rate per unit area of water
NTU Number of transfer unit
mw(MUP) Mass flow rate of makeup water
m ̇_w Mass flow rate of water
P Atmospheric pressure
P_ο Pressure of ambient environment
P_v2 Vapour pressure of air at exit
P_v2(sat) Saturated vapour pressure of air at exit
PHRC Port Harcourt Petrochemical Company
Q_C Cooling capacity
R_a Gas constant at air
R_v Gas constant at specific volume
t_db2 Dry bulb temperature of air at exit
t_wb2 Wet bulb outlet temperature
t_r  or Tw, in Return temperature of cooling water
T_ο Ambient/sink temperature
t_wb1 Wet bulb inlet temperature
t_s  or  Tw,out Supply temperature of cooling water
T_A Approach
T_R Range
V Volume of air
WRPC Warri Refining and Petrochemical Company Ltd

GREEK SYMBOLS
ψ_ch Chemical exergy
ψ ̇_(a(conv)) Convective exergy of air
η Energy efficiency
ψ ̇_(a(evap)) Evapourative exergy of air
〖 ψ〗_(w,makeup) Exergy of makeup water
ψ ̇_D Exergy destruction
η_ψ Exergetic efficiency
ω_ο Humidity ration of ambient surrounding
ω_2 Humidity ratio of air at the exit
ψ_me Mechanical exergy
ψ ̇_w Net exergy of water
〖 ψ〗_(a.in) Net air inlet exergy
〖 ψ〗_(w,in) Net water inlet exergy
Фo Relative humidity
ψ_th Thermal exergy






CHAPTER 1
INTRODUCTION

1.1 BACKGROUND TO THE STUDY
As the country experiences technological and industrial advancement, one of the major bye-product of these activities is heat; heat if not well managed causes unquantifiable and irreparable damages to both mechanical equipment’s and the environment, consequently the cooling towers are needed to manage the challenge heat poses. The major focus of this work is to examine the performances of Warri refining and petrochemical company   (WRPC) cooling tower setup. Performance assessment is an important issue in the research of the cooling tower: CTI (cooling tower institute) is responsible for setting the standard specifications and methods used the evaluation of the performance of cooling Towers (Cooling Tower Institute, 1990). Performance assessment methods are categorised into effectiveness-NTU based method (Soylemez, 2004); exergy based analysis method (Muangnoi et al; 2007); software simulation based method (Ataei et al ;2008 and Reuter and Kroger, 2010); Poppe equation based analysis (Smrekar et al;2012); and fouling model development (Khan, 2004). 

According to (Reuter and Kroger, 2010 and Klimanek et al; 2010), mass/energy conservation laws and mass/heat transfer equations, CFD (computational fluid dynamics) models are utilized to develop and simulate the cooling tower operation. However, differences between the modelling world and the actual operations keep the simulation models from being utilized in the real-time operation. Smrekar et al (2011), posited that cooling towers are not usually a subject for annual examinations and overhauls, which gradually results in their degradation in the form of broken pickings, plugged sprayers, growing algae, etc. These eventually leads to a reduction in the heat and mass transfer rates which is reflected in the cooling tower local non-homogeneities. These have an impact on the cooling tower efficiency and hence on the efficiency of the energy system. Since the energy flows through natural draft cooling tower are usually huge, small improvements may contribute to large energy savings. Thus, a positive impact can be expected on the economic as well as environmental issues.

Oko and Ogoloma (2011), posited that at present, there is an intense production of thermal energy as a result of the recent industrial activities within the country (Nigeria); this has led to the increase in overall heat generated within process plants. Because of the relevance of  engineering in the society, the onus falls on them to design and develop systems that will economically and safely remove this heat from processing plants via the process of cooling (cooling towers). 

According to Wilbert and Jerold (1982), a cooling tower is a device that cools water by contacting it with air and evaporating some of the water. In most cooling towers serving refrigeration and air conditioning systems, one or more propeller or centrifugal fans move air vertically up or horizontally through the tower. A large surface area of water is provided by spraying through the nozzles or splashing the water down the tower from one baffle to another. These baffles or fill materials have traditionally been wood but may also be made of plastic or ceramic materials. A cooling tower configuration sometimes used for large capacity power plants applications has hyperbolic shape, which resembles a chimney 50 to 100(m) high in which the flow of air takes place by natural convection. 

Rajput (2008), asserted that the capacity of cooling towers and spray pond depends upon the amount of evaporation of water that takes place, which in turn depends upon the following factors: amount of surface water exposed to air,  length of exposure time, velocity of air passing over the water droplets formed in the cooling powers, wet bulb temperature of the atmospheric air. The cooling tower which exists in large scales would be considered to be down scaled in a modular set up. This mini cooling tower is expected to function as the conventional type and still retain all the components of the cooling tower. Fig 1.1, succinctly describe a cooling tower.
 
Fig. 1.1: Cooling tower(Wikepedia,2010)

1.2 STATEMENT OF PROBLEM
Waste heat in the form of hot water is a major bye product of industrial processes, if not properly disposed or recycle poses a great and significant danger to the entire production or generation chain. Beside the fact that water supply is limited, the discharge of this hot water causes thermal pollution to the environment. In lieu of this, it has become imperative to carry out a performance evaluation of one of the most potent, reliable and cost-effective mechanical devices (cooling towers) used to solve the challenges of heat disposal and evacuation. A performance analysis of WRPC Cooling Tower, Warri was carried out to ascertain how effective and reliable the cooling tower is in carrying out the design objectives of heat rejection and subsequent cooling of the return hot water. Also, an exegetic analysis was carried out using Equation solver, while computational flow analysis was perform using COMSOL to study the behavior of the parameter which affects air fluid in the cooling tower.

1.3 AIM AND OBJECTIVES OF STUDY
The aim of the work is to carry out a performance evaluation of WRPC cooling tower in Warri, Nigeria; while the specific objectives are to:
1. Analyze the cooling tower performance characteristics. 

2. Carry out an exegetic analysis of the system; and

3. Carry out a computational fluid dynamic of air through the tower.

1.4 SCOPE OF STUDY
 This research work is focused on the performance evaluation of WRPC cooling tower in Warri, Nigeria. To effectively perform this evaluation, energy, exergy and CFD approaches where adopted and used for analysis.  The energy approach involved the analysis of the performance parameters such as the range, efficiencies/effectiveness, cooling capacity, Number of transfer unit (NTU), evapouration rate, coefficient of performance of the Towers will be performed, while the exergy approach entails the analysis of exergy destruction, exergy efficiency, exergy of water and air. Also, the work will extend to a computational fluid dynamics (CFD) of air in the   cooling towers. 

1.5 JUSTIFICATION FOR THE STUDY
The work via its objectives, significantly added both to knowledge and literature. Also, the work presented a data driven, physically meaningful model that is clear to the operators and addressed the application of a model-based control to a real plant. The work effectively and efficiently suggested wats to make the plant work better, also, it highlighted the major parameters used in assessing the performance of the cooling towers. The behaviour of air (in terms of velocity, temperature, relative humidity profiles etc.) as it passes through the tower were thoroughly examined via Comsol model.


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