MSc. Project Report - A. Hanan - 13084052

School of Engineering and Technology MSc. Project

Optimization of The Performance of Heat Exchanger Used in Power Plant Applications 1

MSc Project Report

Optimization of the Performance of Heat

Exchanger Used in Power Plant Applications

By: Ahmad Hanan

Supervisor: Dr. Sami H. Nasser

September 2014

SCHOOL OF ENGINEERING AND TECHNOLOGY


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications i

DECLARATION STATEMENT

I certify that the work submitted is my own and that any material derived or quoted from

the published or unpublished work of other persons has been duly acknowledged (ref. UPR

AS/C/6.1, Appendix I, Section 2 – Section on cheating and plagiarism)

Student Full Name: Ahmad Hanan

Student Registration Number: 13084052

Signed: …………………………………………………

Date: 08/09/2014


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications ii

ABSTRACT

The study explores the heat exchangers and their usage in power plants. The

purpose of this research is to investigate various types of power plants, heat

exchangers and usage of heat exchangers in power plants. Subsequently, selection

of a heat exchanger for design optimization which is widely used in power plants

was made. Star CCM+ was used for simulating the heat exchanger in order to

analyse heat transfer in different models made under design optimization. Heat

exchanger was optimized by altering geometrical components and design

parameters. In the end results were compared amongst each other and supported

from the facts obtained from literature review.


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications iii

ACKNOWLEDGEMENTS

Author has taken keen efforts in this project. However, it would not have been

possible without the blessings of Almighty Allah who gives the wisdom and strength

to act upon it. The author would like to express the gratitude towards his parents for

their moral support and encouragement which helped a lot in this project.

Author is highly indebted to the honourable project supervisor, Dr. Sami H. Nasser

who has given his full efforts in guiding the author in achieving the goal as well as

giving him encouragement to maintain the progress in track. Author would also like

to appreciate Dr. Nasser for providing necessary information regarding initiating the

project. His crucial role in progress of the project cannot be denied. Author also likes

to acknowledge the facilities provided by the institution, it would have been an

arduous job to conduct the research without the facilities available at University of

Hertfordshire.

Last but not the least author would also like to mention the names and extend

sincere thanks to the individuals who helped in various stages of the project. Talha

Usman, Ziv Bachar, Afrasiab Nadeem, Sarfraz Sheikh and specially Syed Mujtaba

Hussaini who has taken an immense interest in the research and helped the author

with his abilities.


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications iv

Table of Contents

DECLARATION STATEMENT .................................................................................. i

ABSTRACT ..............................................................................................................ii

ACKNOWLEDGEMENTS ........................................................................................ iii

LIST OF FIGURES .................................................................................................. vi

LIST OF TABLES .................................................................................................. viii

GLOSSARY ............................................................................................................. ix

NOMENCLATURE ................................................................................................... x

1. INTRODUCTION .............................................................................................. 1

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

1.2. AIMS AND OBJECTIVES .......................................................................... 1

1.3. METHODOLOGY ...................................................................................... 2

2. LITERATURE REVIEW .................................................................................... 4

2.1. TYPES OF POWER PLANTS .................................................................... 4

2.1.1. THERMAL POWER PLANT................................................................ 6

2.1.2. RENEWABLE ENERGY ................................................................... 10

2.2. TYPES OF HEAT EXCHANGERS ........................................................... 13

2.3. USAGE OF HEAT EXCHANGERS IN POWER PLANT APPLICATION .. 19

2.4. MODES OF HEAT TRANSFER ............................................................... 20

2.5. SELECTION OF HEAT EXCHANGER .................................................... 23

2.6. HEAT EXCHANGER TO BE OPTIMIZED ................................................ 24

2.6.1. COMPONENTS .................................................................................... 25

2.6.2. DESIGN DELIBERATIONS ................................................................... 26

2.6.3. PROBLEMS IN STHE ........................................................................... 29

2.7. STAR CCM+ ............................................................................................ 29

3. DESIGN, MODELING AND SIMULATION ...................................................... 30

3.1. COMPUTER AIDED DESIGN (CAD) MODEL ......................................... 30

3.2. DESIGN DATA AND ANALYTICAL CALCULATIONS ............................. 31

3.2.1. SHELL SIDE CALCULATIONS ............................................................. 31

3.2.2. TUBE SIDE CALCULATIONS ............................................................... 33

3.2.3. HEAT TRANSFER CALCULATIONS .................................................... 34

3.3. MESHING ................................................................................................ 36

4. RESULTS ....................................................................................................... 38

4.1. MODEL 1: DEFAULT DESIGN ................................................................ 38


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications v

4.2. VELOCITY .................................................................................................. 40

4.2.1. MODEL: 2 ............................................................................................. 40

4.2.2. MODEL: 3 ............................................................................................. 41

4.3. MATERIALS ................................................................................................ 43

4.3.1. MODEL: 4 ............................................................................................. 43

4.3.2. MODEL: 5 ............................................................................................. 45

4.4. MODEL 6: SUMMER ................................................................................... 46

4.5. MODEL 7: BAFFLES ................................................................................... 48

4.6. MODEL 8: TUBES LAYOUT ....................................................................... 50

DISCUSSION ........................................................................................................ 52

CONCLUSION AND RECOMENDATIONS ............................................................ 56

REFERENCES ...................................................................................................... 57

BIBLIOGRAPHY .................................................................................................... 61

APPENDIX A ......................................................................................................... 62


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications vi

LIST OF FIGURES

Figure 1: Power Cycle ............................................................................................. 4

Figure 2: Classification of Power Plants ................................................................... 5

Figure 3 (a): Schematic diagram of Rankine Cycle ................................................. 6

Figure 3 (b): T-s diagram of Rankine Cycle ............................................................. 6

Figure 4(a): Schematic diagram of Joule-Brayton Cycle ......................................... 7

Figure 4(b): T-S diagram of Joule-Brayton Cycle .................................................... 7

Figure 5: Pressurized water Reactor (PWR) ........................................................... 7

Figure 6: Geothermal Power Plant .......................................................................... 8

Figure 7: Impulse and Reaction Turbine ................................................................. 9

Figure 8(a): Schematic diagram of Combine cycle ................................................ 10

Figure 9: Hydroelectric power plant ....................................................................... 11

Figure 10: Horizontal and Vertical Axis wind Turbine ............................................ 12

Figure 11: Solar dish, parabolic solar trough and solar power tower ..................... 12

Figure 12: Classification of Heat exchanger ........................................................... 14

Figure 13: Plate heat exchanger ........................................................................... 15

Figure 14: Spiral heat exchanger .......................................................................... 16

Figure 15: Plate-fin heat exchanger ...................................................................... 16

Figure 16: schematic diagram of a gas turbine power plant having a recuperator .. 17

Figure 17: Steam generator .................................................................................. 17

Figure 18: Regenerator ......................................................................................... 18

Figure 19: HRSG .................................................................................................. 18

Figure 20: Conduction ........................................................................................... 21

Figure 21: Velocity and temperature profile ........................................................... 22

Figure 22: Shell and tube heat exchanger ............................................................. 24

Figure 23: Shell Nozzles ....................................................................................... 25

Figure 24: Baffles .................................................................................................. 26

Figure 25: Type of flow ......................................................................................... 26

Figure 26: Tube layout .......................................................................................... 26

Figure 27: Tube arrangement ................................................................................ 27

Figure 28: Baffle Spacing ...................................................................................... 27

Figure 29: Tube pitch ............................................................................................. 28

Figure 30: Types of Baffles .................................................................................... 28

Figure 31: CAD model ........................................................................................... 30

Figure 32: Meshed model ...................................................................................... 37

Figure 33: Default Designed Model ........................................................................ 38

Figure 34: Temperature Distribution for Model 1 .................................................... 38

Figure 35: Themperature Distribution for Model 3 .................................................. 42

Figure 36: Velocity Distribution for Model 3 ............................................................ 42

Figure 37: Heat Transfer plot for Model 3 .............................................................. 43


Figure 39: Velocity Distribution for Model 4…………………………………………….44



Optimization of The Performance of Heat Exchanger Used in Power Plant Applications vii



Figure 43: Heat transfer plot for Model 5 ................................................................ 46



Figure 46: Modified Geometery for Model 7 ........................................................... 49



Figure 49: Heat Transfer Plot for Model 7 .............................................................. 50

Figure 50: Figure 50: Modified Geometry for Model 8 (Hidden Outer Shell) ........... 50




Optimization of The Performance of Heat Exchanger Used in Power Plant Applications viii

LIST OF TABLES

Table 1: Commonly used heat exchangers in various power plants ....................... 20

Table 2: Working fluid properties ........................................................................... 36

Table 3: Calculated Properties for Model 2 ............................................................ 40

Table 4: Calculated Properties for Model 3 ............................................................ 42

Table 5: Material Properties for Model 4 ................................................................ 43

Table 6: Material Properties for Model 5 ................................................................ 45

Table 7: Tubular fluid properties for Model 6 .......................................................... 47

Table 8: Results of Model 1-8 ................................................................................ 52

Table 9: Heat Transfer Model 1-8 .......................................................................... 52

Table 10: Outlet temperate Model 1-8 .................................................................... 53


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications ix

GLOSSARY

PWR : Pressurized Water Reactor

BWR : Boiling Water Reactor

HVAC : Heaving Ventilation and Air Conditioning

HRSG : Heat Recovery Steam Generator

HP : High Pressure

LP : Low Pressure

STHE : Shell and Tube Heat Exchanger

TEMA : Tubular Exchanger Manufacturers Association

CAD : Computer Aided Design

UK : United Kingdom

LMTD : Log Mean Temperature Difference

UNS : Unified Numbering System

Al : Aluminium

Cu : Copper

T-s : Temperature-Entropy


Optimization of The Performance of Heat Exchanger Used in Power Plant Applications x

NOMENCLATURE

.

Q : Rate of Heat Transfer

k : Thermal Conductivity

T : Temperature

x : Wall Thickness

As : Surface Area

P : Pressure

h : Convective Heat Transfer Coefficient

ϵ : Emissivity

σ : Stephen-Boltzmann Constant

0C : Degree Celsius

0K : Degree Kelvin

d : Diameter

v : Velocity

ρ : Density

μ : Dynamic Viscosity

Re : Reynolds Number

Pr : Prandtl Number

Nu : Nusselt Number

I : Turbulence Intensity

K : Turbulence Kinetic Energy

ε : Turbulence Dissipation Rate

Cp : Specific Heat

R : Thermal Resistance

U : Overall Heat transfer Coefficient



1. INTRODUCTION

Heat Exchangers are the devices used for the exchange of heat from two fluids that

are at different temperatures, usually preventing them from mixing with each other.

The media which keeps them from mixing is a metallic wall of highly conductive

material which helps in an effective heat transfer between the fluids. There is an

observation that sometimes in a few cases, the fluids mixes with each other e.g.

open feedwater heater. The fluids can be at a same phase i.e. either liquid or gas

or they can be at different phases like heat exchange between a liquid and a gas

[1]. Heat transfer in heat exchangers involves conduction through the separating

walls and convection in each fluid. Heat exchangers are widely used in various

industrial and domestic applications such as, car radiator, refrigerator, air

conditioning, space heating, power plants, chemical plants, oil refineries, and

natural gas processing. The usage of heat exchangers in power plants will be

discussed in detail in this report

1.1. BACKGROUND

Heat exchangers have a vast usage in various types of power plants. It serves as a

necessary component of every power plant. The type of heat exchanger varies in

different types of power plants as discussed below. It is also seen that more than

one heat exchanger is used in a single power plant at different stages of the cycle.

And it is also noted that a power plant can accommodate different types of heat

exchangers. So in conclusion the choice of heat exchanger in a power plant varies

from design to design. Although the heat exchangers serves in various power plants

are designed according to the requirement, but still there is always a window for

improvement. Since heat exchangers are one of the major components upon which

the efficiency and net power output of the plant is dependent, it would be very useful

if the design of the heat exchanger is improved, whilst maintaining the basic

operation and function of the heat exchanger. The design can be modified or

improved by changing certain parameters. Alteration in different components,

increase or decrease in a number of certain parts or changing their arrangements

has a vast effect on the efficiency of a heat exchanger, thus affecting the power

plant’s efficiency.

1.2. AIMS AND OBJECTIVES

Aim: Optimization of the performance of heat exchanger used in power plants



Objectives:

The objectives of this thesis are listed below

• Study the types of heat exchangers and their significance in various power plants

• Select a particular heat exchanger to be further analysed and optimized

• To understand the parameters considered for the design of heat exchanger and

observe the results caused by the modifications made

• Analyse the relevant case studies for the optimization of the performance of heat

exchanger and understand the strategies and their results

• Construct a solid model of heat exchanger by assembling various components

• Flow and thermal analysis using computational packages such as Star CCM+ and

Phoenics.

• Select the best result to be used in a typical scenario.

1.3. METHODOLOGY

As the title of the project is “Optimization of the performance of heat exchanger

used in power plant applications”, the research begins with literature review that

includes the working of power plants and the purpose of heat exchangers used in it,

author’s research of various power plants and heat exchangers used in them. The

basics principles of heat transfer and the operation of heat exchanger was also

studied in order to consider the design parameters of heat exchanger to be

optimized

The next milestone is selection of a particular heat exchanger that used vastly in

power plants as compared to the others. Literature review will be continued in order

to study the design parameters that affect the heat transfer between the working

fluids and overall performance of that heat exchanger

In order to observe the effects of the variations, it is deemed appropriate to be

guided by the literature review in selection of the domain size, allowing simulations

and testing to be carried out within the timescales of the project. There will be an

investigation through experimenting controlled variances of the parameters such as

materials and fluid inlet conditions.

For investigating the flow pattern and the heat transfer in the heat exchanger, a

model will be modelled and simulated using software packages such as STAR-

CCM+ according to the design. This will allow the researcher to analyse the results

obtained from the simulation showing an effective heat transfer. The results will be

documented in this report.

In the last step the effect of heat transfer through the heat exchanger with the

variation of design parameters, such as the number of baffles, tube arrangement,



flow rate, materials and temperature differences will be investigated and discussed

briefly the effects caused by the variations

The report is divided in different sections as follows

• Section 1 – Introduction

• Section 2 – Literature Review

• Section 3 – Design, Modelling and Simulation

• Section 4 – Results

• Section 5 – Discussion

• Section 6 – Conclusion and Recommendations



2. LITERATURE REVIEW

2.1. TYPES OF POWER PLANTS

A power plant is a facility used for generation of electric power. Mostly power plant

is composed of power source, generator and transformer. Generator is the rotating

part which is driven by mechanical power and it converts mechanical energy into

electrical energy. Further the generated electrical energy is transferred to the

transformer for regulation of the voltage. In the end the electricity flows towards grid

station for transmission of electricity to consumers as shown in figure 1.

Figure 1: Power Cycle

Power Plants can be classified by the types of power sources. There are two main

categories Thermal and Renewable Energy as shown in figure 2.



Power Plant Classification

Thermal

Fuel

Fossil Fuel

Coal

Natural Gas

Oil Nuclear

Bio Mass

Geothermal

Dry Steam

Flash Steam

Binary Steam

Prime Mover

Gas Turbine

Steam Turbine

Impulse

Reaction

Combined Cycle

Reciprocating Engine

Spark Ignition (Otto)

Compression Ignition (Diesel)

Renewable Energy

Hydroelectric

Wind

Horizontal Axis

Vertical Axis

Solar

Geothermal

Marine

Marine Current

Osmotic

Ocean Thermal

Tidal

Wave

Biomass

Figure 2: Classification of Power Plants



2.1.1. THERMAL POWER PLANT

In thermal Power Plants, mostly Steam is generated by different methods which

drive the turbine. According to the second law of thermodynamics there is always a

heat loss; it means not all the thermal energy is transformed into mechanical

energy. Sometimes turbine is driven by combustion of fuel in calculated pressure

conditions, and the expansion of gasses produced by combustion causes the

driving of the moving parts. [2]

Thermal power plants can be classified by the type of prime mover installed and the

type of fuel used.

BY FUEL

FOSSIL FUEL

A fossil fuel power plant uses fossil fuel such as natural gas, coal or oil to produce

electricity. Chemical energy is been stored in fossil fuel and when they burn in the

presence of oxygen, the chemical energy is transformed into thermal energy which

drives the turbine (mechanical energy) with accordance to the principles of second

law of thermodynamics and finally electrical energy is obtained[3]. Fossil Fuel

power plants either operate on Rankine cycle, Joule-Brayton cycle or combined

cycle.

If the power plant is working on Rankine cycle, the heat source is combustion of

coal, natural gas or gasoline which heats up the water in boiler in order to generate

steam which in turn drives the turbine. Turbine exhaust cools down in condenser

and pumped again to the boiler [4]. Schematic and T-s diagrams of Rankine cycle

are shown in fig 3.

Figure 3(a): Schematic diagram of Rankine

Cycle [5]

Figure 3 (b): T-s diagram of Rankine Cycle [5]



In Joule-Brayton cycle, the working fluid is air, air is drawn from the intake and

compressed in the compressor, then the compressed air is injected into the

combustion chamber where it is mixed with fuel, and a spark is introduced to ignite

the fuel-air mixture. The turbine is situated right after the combustion chamber.

When the hot gasses enters the turbine, they make turbine blades move, hence the

turbine starts rotating. Power can be obtained from the rotary motion [6]. Schematic

and T-s diagrams of Gas turbine and Joule-

Brayton cycle are shown in fig 4

Figure 4 (a): Schematic diagram of Joule-Brayton Cycle

[5]

NUCLEAR

Nuclear power plant is a type of thermal power plant and as in the conventional

thermal power plants water is heated to generate steam which drives the turbine in

order to obtain power, nuclear power plants differ from fossil fuel power plants, as

the heat is provided from the controlled “chain reaction” inside the nuclear reactor.

The reactor uses uranium or some other highly radioactive material rods as fuel and

heat is generated by nuclear fission. Water is pumped through the reactor in

separate channels to remove the heat away. As shown in figure 5. This type of

arrangement is known as PWR (Pressurized water reactor). There is another

arrangement, in which water directly flows in reactor vessel and vaporizes due to

the heat generated in result of nuclear reaction. This type of arrangement is known

as BWR (Boiling water reactor). [7]

Figure 5: Pressurized water Reactor (PWR) [7]

Figure 4(b): T-S diagram of Joule-Brayton Cycle [5]



As the nuclear reaction is chained and it is highly dangerous, it must be controlled.

The reaction is controlled by boron control rods; they absorb neutrons and hence

slow down the reaction up to a desirable limit. [8]

BIOMASS

Bio-mass comprises of organic living or recently lived organisms. Usually they are

garbage, plants, agriculture and animals waste. Power experts take keen interest in

obtaining energy from biomass as it is carbon dioxide neutral and hence a

renewable energy source [10]. There are two common methods for making

biomass into use. Chemical process, i.e. obtain methane gas from biomass and use

it as fuel in fossil fuel power plants or processing plants such as “Jatropha” to get

bio-diesel and operate the thermal power plant with biodiesel [9]. The second and

most commonly practiced method is by direct combustion of bio gas in order to keep

heating up the water following a steam turbine [10]

GEOTHERMAL

Geothermal energy is one of the renewable energy sources. Earth’s inner core is at

a high temperature while the surface acts as an insulator. Thermal energy

generated by the hot masses can be utilized by circulation of a fluid. The common

method of utilizing the geothermal energy is to drill deep holes into the surface

(same as drilling of oil) until a suitable thermal spot is found. After finding a

significant geothermal spot the thermal energy can be utilized by three methods

depending on the quality of the hot masses. [11]

Dry Steam: Dry steam power plants directly use underground generated steam by

earth’s heat to drive the turbine [11]. As shown in figure 6

Flash Steam: Flash steam power plants use hot fluids (above 180 oC) to vaporize

water in the flash tank in order to generate steam which drives the turbine [11]. As

shown in figure 6

Binary cycle: In Binary cycle power plants, hot fluids (below 180 oC) are used to

vaporize the water through a heat exchanger. The steam generated in the heat

exchanger is used to drive the turbine and hence generator, while the extracted fluid

from geothermal source is returned back to the aground after the heat is utilized

[11].As shown in figure 6

Figure 6: Geothermal Power Plant [12]



BY PRIME MOVER

STEAM TURBINE

Steam Turbine is the one used in power plants operating on Rankine cycle. It is the

most commonly used turbine around the world for power generation [13]. Steam

turbines are usually classified on the basis of blade and stage design. There are two

common types, Impulse and Reaction turbine.

Impulse Turbine: An impulse turbine has fixed nozzles that ejaculate steam in the

form of high velocity jets towards a shaft having rotor blades on its edges. Kinetic

energy of the jets causes the rotation of shaft. Due to expansion at nozzle outlet,

steam leaves the nozzle with a very high velocity, and the shaft blades accumulate

a large portion of jet’s velocity. Pressure drop occurs at the nozzle outlet and the

pressure drops again when jet strikes the stationary blades. As shown in figure 7.

There are three types of impulse turbines commonly used, Pelton turbine, and

Turgo Turbine and Crossflow turbine [13]

Reaction Turbine: A reaction turbine has rotor blades arranged in the form of

nozzles on the main rotating shaft, there is a stator in the shaft which directs the

steam into the rotor (nozzles). Reaction turbine develops torque by the reaction of

ejaculating fluids. Increase in velocity takes place when the steam leaves the stator

and fills the circumference of the rotor, after a significant increase in the velocity; it

decreases in the rotor itself. Pressure drop occurs at both the stages, i.e. when the

steam leaves the stator and in rotor as well. As shown in figure 7. There are three

types of impulse turbines commonly used, Kaplan turbine, Francis turbine and

Propeller turbine. [13]

Figure 7: Impulse and Reaction Turbine [14]

COMBINED CYCLE

Thermal efficiencies of Joule-Brayton and Rankine cycles are usually low, this is

because of the rejected heat quantity is a considerable fraction of the fuel energy

consumed. Efficiency of the gas turbine or a steam turbine power plant can be



enhanced by combining the Joule-Brayton and Rankine cycle, in such a way that

the heat rejected from the gas turbine is used to heat up the vapour content present

in the steam turbine cycle. This arrangement is known as combined cycle. In this

arrangement both turbines supplies power to the system and utilization of the

rejected heat increases the overall plant efficiency [15]. Schematic and T-s

diagrams of combined cycle are shown in fig 8

RECIPROCATING ENGINE

Internal combustion engines can also be used to generate electric power. They are

usually of a small scale and are often used as a backup power source. Internal

combustion engines drive the shaft and a generator can be placed on the end of the

rotating shaft in order to obtain electrical energy. Commonly used internal

combustion engines in power generation are Diesel, Otto and Stirling engines. [16]

2.1.2. RENEWABLE ENERGY

Renewable energy is defined as energy that comes from naturally replenishes able

resources. By the evolution of technology the dependability on natural resources is

increasing significantly. As the fossil is eventually running out, utilization of

Figure 8(a): Schematic diagram of Combine cycle [5] Figure 8(b): T-s diagram of Combine cycle [5]



renewable energy resources in power generation is increasing day by day. There

are certain types of renewable energy power plants described below.

HYDROELECTRIC

Hydroelectricity means the power generated by hydropower. It is the most widely

used form of renewable energy. There are several methods to generate

hydroelectricity; the most common is by water storage dams. The hydroelectricity

extracted from water depends on the flow rate of water and the height difference

between the height of source and water outflow, known as head. Greater the height

difference and flow rate, greater will be the power produced. [17]

Water for hydroelectric generation is usually stored in a reservoir known as dam.

Water travelled from dam to the power house through special ducts called penstock.

Water carries potential energy due to its gradient and kinetic energy due to its flow,

when strikes the turbine transform into mechanical energy which is then converted

into electrical energy via generators. As shown in figure 9.

Figure 9: Hydroelectric power plant

WIND

Wind energy can be converted into useful electrical energy by using wing turbines in

the areas with healthy wind potential. Wind turbines can be constructed in-shore or

off-shore usually in the grid called wind farms. Cross winds make the blades rotate;

there is a gear box and generator connected with a shaft, which eventually starts

working with the movement of fan blades. [18] There are two types of wind turbines,

Horizontal axis and vertical axis as shown in figure 10.



Figure 10: Horizontal and Vertical Axis wind Turbine [18]

SOLAR

There are numerous ways to make infinite solar energy into use; the device used for

this purpose is called solar cells. Solar cells are made up of highly photovoltaic

material, they emit electron when sun light shines upon them. So the electrons

starts flowing and inverters convert direct current into alternating current for

domestic and industrial purposes. In small scale an individual plate of solar cells is

used to absorb solar radiations and conversion of electricity. While in larger scale

various techniques are used for converging radiations to a common point

(concentrating) solar radiations and hence get more power. For this purpose,

parabolic trough, dish or solar power tower is used (as shown in figure 11).

Sometimes a solar power is utilized for heating and vaporizing the water in tubes

which further drives the steam turbine, this system is called solar thermal power

plant [19][20]

Figure 11: Solar dish, parabolic solar trough and solar power tower [19][20]

MARINE

Marine energy refers to the ocean energy carried by its tides, waves, temperature

difference and salt concentration. This is not a very popular method of power

generation, still various types exists.

Ocean Thermal Energy: In ocean thermal energy conversion, temperature



difference between cold deep and relatively warmer surface temperature is sued to

run a heat engine and hence obtain electrical power. [21]

Tidal: Tidal power is the form of hydropower in which tidal energy is utilized for the

generation of electric power.[22]

Osmotic: Osmotic power is also known as salinity gradient power. It is the method

of obtaining energy from the difference in salt concentration between seawater and

river water. [23]

2.2. TYPES OF HEAT EXCHANGERS

Heat exchangers can be classified on flow arrangement, construction, process

function, heat transfer mechanism, transfer process and flow configuration. As

shown in figure 12. Based on these factors various types of heat exchangers have

been developed, some of them are discussed below. [24]



Figure 12: Classification of Heat exchanger

Classification of Heat Exchanger

Flow Arrangement

Single-Pass

Multi-Pass

Construction

Tubular

Plate Type

Regenerative

Process Function

Condenser

Boiler

Cooler

Heater

Heat Transfer Mechanism

Single Phase Convection

Two Phase convection

Combined (Convection and radiation)

Transfer Process

Direct Contact

Indirect Contact

Flow Configuration

Parallel Flow

Counter Flow

Cross Flow



PLATE HEAT EXCHANGER

The plate and frame heat exchanger consists of corrugated metal plates having

holes in them for the passage of fluids between which heat transfer will take place.

The plates are fixed together with a gasket in the corners; the gasket also directs

the hot and cold fluid into separate channels. Cold fluid enters from a hole and sub

sequential plates make a passage for the cold fluid to pass to the rear end. The

same happens with the hot fluid. Both fluids make exit from separate outlets. As

shown in figure 13. The heat is transferred through the plates which have both cold

and hot passages providing a large area for heat transfer. The corrugation of plates

provides support to the adjacent plate and the arrangement also makes channels

for both fluids respectively. [25]

Figure 13: Plate heat exchanger [25]

SPIRAL

In spiral heat exchangers there are two spiral channels, in which fluids at different

temperatures flow in separate channels. Flow arrangement is usually counter flow.

Either of the fluid enters from the centre and flows towards outer edge in spiral

pattern. Alternatively, the other fluid enters the heat exchanger from the outer edge

and flows towards the outlet which is at the centre, right behind or next to the inlet of

the first fluid. As shown in figure 14. Sometimes it is taken into account that the fluid

arrangement in spiral heat exchangers is parallel, i.e. both fluids enter from the

centre through separate inlets and travel to the outer edge or vice versa. [26]



Figure 14: Spiral heat exchanger [27]

AIR COIL

Air coil heat exchangers are also known as plate-fin heat exchanger. They have a

vast usage in HVAC (Heating Ventilation and Air Conditioning) systems and

domestic air conditioners. They have similitude with plate and frame heat

exchangers. It consists of tubes penetrating through the parallel arranged fins. As

shown in figure 15. A refrigerant flows through the tubes which reduces the

temperature of air flowing between the fins (cross flow). [28]

Figure 15: Plate-fin heat exchanger [29]

RECUPERATOR

Recuperator is a type of heat-recovery heat exchanger. It is also a special purpose

heat exchanger that cannot be used elsewhere. Recuperators positioned between

the inlet of combustion chamber and exhaust of a thermal system. They are usually

used in gas turbine power plants. The recuperator transfers some of the waste heat

from turbine exhaust towards compressor outlet where it heats the compressed air

(preheating) before it enters the combustion chamber and thus less fuel is needed

to heat up the gasses up to the turbine inlet temperature [30]. Figure16 shows the

schematic diagram of a gas turbine power plant having a recuperator.



Figure 16: schematic diagram of a gas turbine power plant having a recuperator [31]

The preheat efficiency or the temperature of the preheated fluid depends on the

temperature of exhaust fluid. Usage of recuperator generally saves 20% on fuel

consumption.

STEAM GENERATOR

Steam generator is a type of heat

exchanger used in nuclear power

plant. In nuclear power plant heat is

generated in a result of nuclear

reaction so it is not recommended to

generate steam by making direct

contact of water with radioactive

masses. For this purpose, another

heat carrying fluid is made in to use,

this additional fluid circulates in a

separate pathway, receives heat from

the reactor and transmit it in the

steam generator through numerous

small tubes. In steam generator, the

cooler fluid is water which turns into

steam under influence of heat

carrying fluid. Steam generator is

based on shell and tube heat

exchanger design [7]. figure17.

Figure 17: Steam generator [32]



REGENERATOR

Like recuperator, regenerator is a special

purpose heat-recovery heat exchanger. They

also positioned between the inlet of

combustion chamber and exhaust of a

thermal system. They are used in gas turbine

power plants with the same purpose as

recuperators have. They differ from

recuperators on the basis of their working and

operation. In recuperator hot and cold

streams passes simultaneously through

separate channels while in regenerator, heat

from the hot body is stored in an intermediate

medium before it is transferred to cold fluid in

order to recover some wasted energy. [33]

Figure 18: Regenerator [33]

HRSG

Heat Recovery Steam Generators (HRSG) is a type of heat exchanger used in

combined cycle power plants. HRSG serves as the interface link between air and

steam cycle. It utilizes heat from a hot gas stream coming from the exhaust of gas

turbine and produces steam that can be used in cogeneration or can be used in

driving the steam turbine. Main components of HRSG are Evaporator, superheater

and economizer. For providing better heat recovery in the HRSG operation is

carried out in high and low pressure levels. HRSG increases the power output of the

system by 30-40%. [34]

Figure 19: HRSG [34]



2.3. USAGE OF HEAT EXCHANGERS IN POWER PLANT

APPLICATION

Heat exchangers have vast usage in various types of power plants. They serve as a

necessary component of every power plant. The type of heat exchanger varies in

different types of power plants as discussed below. It is also seen that more than

one heat exchanger is used in a single power plant at different stages of the cycle.

And it is also noted that a power plant can accommodate different types of heat

exchangers. So in conclusion the choice of heat exchanger in a power plant varies

from design to design.

There are some typical heat exchangers used in power plants discussed below

In some power generation industries (usually in combined cycle power plants) there

is a steam producing heat exchanger called Heat Recovery Steam Generators

(HRSG) which utilizes heat from a hot gas stream and produces steam that can be

used in cogeneration or can be used in driving the steam turbine in combined cycle

[34]

Feedwater heaters are used in steam power plants; they are shell and tube heat

exchangers. A fraction of steam is bleed after driving the high pressure turbine thus

heating the feedwater. [35]

There is a special type of heat exchanger called Recuperator which is used in gas

turbine power plants. The recuperator transfers some of the waste heat from turbine

exhaust towards compressor outlet where it heats the compressed air (preheating)

before it enters the combustion chamber and thus less fuel is needed to heat up the

gasses up to the turbine inlet temperature. [31]

Sometimes geothermal power plants also need heat exchangers where the

geothermal steam is contaminated with excessive suspended solid particles or

corrosives. [36]

In Nuclear power plants there is a pressurized water reactor (PWR) in which water

under high pressure is heated due to nuclear reactions and the heated water flows

into a heat exchanger called steam generator where it transfers the thermal energy

to water in order to convert water into steam which further drives the turbine. Steam

generators are based on the shell and tube heat exchanger. [7]

For better efficiency and safer operation of gas turbines, a small plate or cross flow

heat exchanger can be used for steady cooling of rotor blades, bearings and lube

oils [37]

Surface Condensers are a vital part of a steam or thermal power plant. It is basically

a shell and tube heat exchanger. The exhaust steam from turbine enters the shell of

the heat exchanger where it is cooled to condensate (water), due to the cold water



flowing in the tubes. It is necessary to condensate the exhaust steam because the

pump works on liquid which drives the water to the boiler for further operation [38]

Some gas turbine power plants operating on modified Joule-Brayton cycle uses

Regenerator. Regenerator is a special type of Heat exchanger in which heat from

the hot body is stored in an intermediate medium before it is transferred to cold fluid

in order to recover some wasted energy. [33]

Table 1 shows some of the commonly used heat exchangers in various power

plants

Table 1: Commonly used heat exchangers in various power plants

Power Plant Heat

Exchanger

Type of Heat

Exchanger

Primarily Function

Combine cycle

Power Plant

HRSG

HRSG

Economize, Super

heat and

evaporation

Steam turbine

Power Plant

Condenser Shell and tube heat

exchanger

Condensing the

vapour

Steam turbine

Power Plant

Close

Feedwater

heater

Shell and tube heat

exchanger

heating the bleed

steam

Steam turbine

Power Plant

Open

Feedwater

heater

Direct contact/Open

heat exchanger

heating the bleed

steam

Gas Turbine

Power Plant

Recuperator Recuperator Preheating the

compressed air

Gas Turbine

Power Plant

Regenerator Regenerator heat storage and

recovery

Nuclear Power

Plant

PWR, BWR Shell and tube heat

exchanger

Vaporizing the

water

Geothermal

Power Plant

Geothermal

heat

exchanger

various

Vaporizing the

water

Various

-

Plate, cross flow or

Spiral heat

exchanger

Cooling rotor

blades, bearings,

lube oil

2.4. MODES OF HEAT TRANSFER

Heat is a form of energy and according to the law of conservation of energy it

cannot be created or destroyed. But it is interconvert-able and can be transmitted

from one body to another. The driving force which undergoes the heat transfer is



temperature difference. Heat always transfers from high temperature region to low

temperature region, and heat transfer stops when the two regions reach the same

temperature. [39]

Heat can be transferred by three modes as discussed below. There is only one

basic requirement for all of them, which is the existence of temperature difference.

CONDUCTION

Conduction is a mode of heat transfer as a result of interaction of high energy

particles with adjacent less energetic particles. Conduction is valid in solids, liquids

and gasses. Since the molecules of solids are more compact than liquids and

gasses, so heat transfer through conduction in solids is greater than conduction in

liquids and gasses experiences. In solids conduction is due to the movement of free

electrons and vibration of molecules in the lattice. While in liquids and gasses, it is

due to diffusion and collision of the molecules during their random motion. [39]

Figure 20: Conduction

The rate of heat transfer through conduction is directly proportional to the

temperature difference across the medium and the wall or area normal to the

direction of heat transfer, but it is inversely proportional to the thickness of the wall

or layer through which heat is being transferred (fig 20). “k” serves as a constant of

proportionality known as thermal conductivity, which is the ability of the material to

conduct heat. [39]

.

Q = kAs ΔT/Δx (equation 1)

In the above equation “.

Q ” is rate of heat transfer, while “k” is thermal conductivity,

“As” is surface area and “ΔT” is temperature difference and “Δx” is wall thickness.

The term “ΔT/Δx” is known as temperature gradient and the equation is called

“Fourier’s law of heat conduction”. [39]



CONVECTION

Convection is the transfer of heat energy between a solid surface and the adjacent

liquid or gas that is in motion. Convection also occurs when an immiscible liquid or

gas passes over another fluid. An illustration of convection is in fig 21, in which air is

flowing over a hot block, velocity is zero at the surface of block (no slip condition)

while the temperature is maximum at the layer adjacent to the hot surface, this heat

is than carried away by convection. [39]

Figure 21: Velocity and temperature profile [39]

Phase change heat transfer processes including boiling and condensation, are also

considered as convection because of the fluid motion induced during the process

such as liquid droplets falling during conduction and vapour rising during boiling.

Despite all the complexities involved in convection, the rate of heat transfer through

convection is simply proportional to the temperature difference “ΔT” and surface

area “As” through which conduction takes place where “h” serves as a convective

heat transfer coefficient. Rate of heat transfer through convection can be expressed

by Newton’s law of cooling. [39] Equation 2

.

Q = hAs ΔT (equation 2)

RADIATION

Every existing body in the universe having matter radiates energy in the form of

different waves, all bodies having temperature above absolute zero emits some

thermal radiations. By definition, radiation is the thermal energy emitted by the

matter in the form of electromagnetic waves. Radiation differs from conduction and

convection as heat transfer by radiation doesn’t require the presence of any

intermediate medium, in fact the process is even faster if there is no medium at all

(vacuum). [39]

All solids, liquids and gasses emits radiation to different degrees, it is a volumetric

phenomenon, but sometimes it is considered as a surface characteristic, because in

most of the solids and some liquids, the radiation emitted by the interior region can



never reach the surface and hence absorbed in between the surface and the core.

[39]

Rate of heat transfer through radiation can be expressed by the Stephen-

Boltzmann law. Equation 3

.

Q = ϵσAsT4 (equation 3)

Where “ϵ” is emissivity, it’s a surface characteristic and its value lies in the range of

0 ≤ ϵ≤ 1. Basically it is the measure of how close a body approximates black body,

which is a perfect emitter having emissivity = 1. “σ” is Stephen-Boltzmann constant,

having constant value σ= 5.67E-8 W/m2.K. “As” and “T” represents the area and

temperature of the surface emitting radiation. [39]

2.5. SELECTION OF HEAT EXCHANGER

One of the most important decisions taken by a designer is selection of appropriate

heat exchanger for a particular system or application. There are no general rules for

selection of heat exchanger; however there are certain factors which are related to

the specific application, which must be taken into account by the designer while

selecting the type of heat exchanger. [40]

a. Thermal and Hydraulic requirements: This is the most important of all the

factors, as it deals with the amount of heat to be exchanged, inlet and outlet

temperatures, pressure drop and other flow characteristics. [40]

b. Compatibility with fluids and operating conditions: The materials must be

compatible with fluids in resistance of corrosion. The fouling factor must be

carefully assessed and the selected exchanger must be able to operate for

the required period of time. The heat exchanger must be able to withstand

generated stresses due to fluid pressure and temperature differences

(thermal stresses) [40]

c. Maintenance: The characteristics of fluids and components should be

carefully assessed in order to meet the requirement s for cleaning and

periodic replacement of various units. In the design a room for possible

future modification should be left.[40]

d. Availability: There are some components which can be delivered rapidly by

the manufacturers, however some components of the heat exchanger has to

be fabricate specially on the requirement. These factors have to be taken in

consideration. [40]

e. Economic factors: If several heat exchangers can meet the requirement,

then the selection is based on economic aspects; being economic but



achieving the expected effectiveness and efficiency is one of the basic

requirements for any set-up. In this case, however, simple changes can

create huge differences, thus deliberate attention is needed in this step. Cost

of heat exchanger includes material cost, durability, and operating cost

including any coolant used. [40]

2.6. HEAT EXCHANGER TO BE OPTIMIZED

Traditionally in many industries, the choice of heat exchanger is shell and tube heat

exchanger, for which there are well established designs and operation standards

and long operational history which can be helpful in maintenance and operation.

[40] Also STHE is the most widely used heat exchanger in power plants (Refer to

literature review), which is the main topic of this research, so the researcher decided

to select STHE for further heat transfer analysis and optimization.

As discussed above there are several types of heat exchangers used in power

plants but because of the versatility of usage and diversity of applications in various

power plants author selected Shell and tube heat exchanger for further analysis and

optimization. Besides usage and applications, there are other major advantages of

STHE.

Figure 22: Shell and tube heat exchanger [42]

Shell and tube heat exchanger’s flexibility of design allows high pressures and

temperatures. As it name implies it comprises of a large shell, large number of tubes

(up to hundred) and baffles for guiding the flow. A fluid flows through the tubes and

the other one through the shell at different temperatures. Shell and tube heat

exchangers have a widespread of usage, they are used in power plants, boilers,

steam engine locomotives, oil refineries and more, but still they cannot be used in

aircrafts and automotives because of their large size and weight. [40] Some of the

main components and design consideration are described below.



2.6.1. COMPONENTS

Tubes: Tubes are one of the major components of shell and tube heat exchanger

which facilitates the heat transfer between the fluid flowing through the tubes and

the one flowing across the shell. Tube walls are main cause of heat transfer so the

tube material should be of good thermal conductivity and diameter and wall

thickness should be enough to tolerate thermal stresses due to pressure. [40]

Shell and Nozzles: Shell has a circular cross section and it serves as a container

and passage for one of the fluids. Generally it is made by any of the pipe

manufacturing technique, it can be made by rolling a metallic sheet into the shape of

the cylinder and then the joints are weld. Or it can be made by hot rolling and cold

drawing of a metallic billet so it doesn’t have a joint which is beneficial for bearing

high stresses. Inlet and outlet nozzles are mounted on the shell. The inlet nozzle

usually has an impingement plate to divert the incoming flow in the direction of

tubes and avoid the direct impact of high velocity flow on the tubes and other

components as it can result in vibration and corrosion. [41]

Baffles: The primarily function of the baffles is to guide the flow in shell chamber

across the tubes while making the flow turbulent and thus provide high heat transfer

rate. Besides that, the baffles help to support the tubes and maintain uniform

spacing between them during operation and thus minimize the vibrations due to

eddies. The diameter of baffle should be less than the inner diameter of the shell.

The baffles should be precisely machined to avoid leakage from the outer edges or

from the holes for supporting tubes because leakage can increase vibrations and

reduce thermal efficiency. [42]

Figure 23: Shell Nozzles [43]



Figure 24: Baffles [44]

2.6.2. DESIGN DELIBERATIONS

Type of flow: The flow in shell and tube heat exchangers can be classified as either

parallel flow or cross flow. In parallel flow, both fluids enter from the same end in

shell and tubes, travel in the same direction and leave from the other end. While in

counter flow the hot and cold fluid enters from the opposite ends and move in the

opposite directions. [39]

Figure 25: Type of flow [45]

Tube layout: Tube layout is the geometrical pattern of arrangement of tubes. It

affects the flow properties inside the shell and so does the heat transfer coefficient.

For example, in the comparison of triangular and square pitch, the heat transfer

coefficient for triangular layout is as higher than that of square pitch. [46]

Figure 26: Tube layout [46]



Tubes arrangement: Tubes arrangements can be classified as single pass and

multi-pass. It can be single pass shell and single tube pass or multi shell and multi

tube pass, or single shell and multi-tube pass. Most heat exchangers are one, two

or four tube pass design. [46]

Figure 27: Tube arrangement

Baffles spacing: It is the distance between two adjacent baffles. Increasing the

number of baffles also increases the turbulence; on the negative side it hinders the

shell flow. If the baffle spacing is large, it will result in vibrations due to large

unsupported tube span and less heat will be transferred. For avoiding both

possibilities of problems, optimum baffle spacing would be used. According to

TEMA (Tubular Exchanger Manufacturers association) the distance between the

adjacent baffles should be 1/5 of shell diameter. [44]

Figure 28: Baffle Spacing [47]

Material: Material properties of the heat exchanger should be considered while

designing, as the exchange of heat widely depends on the thermal conductivity of

the material used. Besides this, the material should be corrosion-resistant and have

high mechanical strength which makes it long lasting [44]

Tube Pitch: The distance between the centres of adjacent tubes is called tube

pitch. It is an important aspect and calculated carefully because the shell fluid

passes in between the tubes, tube pitch should not be too small to hinder the flow

on the other hand using large pitch restrict the user to accumulate large number of

tubes. Usually it is 1.25 times the diameter of tube. [46]



Figure 29: Tube pitch

Type of Baffles: Usually baffles are either segmented, helical, double segmented

or disc and doughnut type. It has been noted that flow properties change

significantly with the usage of different types of baffle. Fig 30 (a) shows conventional

segmented baffle which have a cut on either side which allows the shell flow. The

main problem with this type of baffle is that it creates dead regions at the base;

dead regions are those regions where the shell fluid cannot reach. Pressure drop is

another common problem in STHE. For overcoming these problems, different types

of baffles have been introduced such as double segmented and disc and doughnut

type. Fig 30 (b) (d). But the ultimate solution was helical baffles. Helical baffles

allows smooth flow in spiral pattern fig 30 (c), studies shows that helical baffles

gives 1.8 times more overall heat transfer than segmental baffles. Despite all the

benefits there are some limitations. Spiral surface machining is difficult, installation

requires special expertise because helix angle must remain same for all the baffles

and maximum spacing should be provided at shell’s inlet and outlet. Risk of leakage

is more than that in segmental baffles. Those were some of the reasons because of

which segmented baffles are still most popular. [44]

Figure 30: Types of Baffles



Along the above mentioned design considerations there are many other parameters

which should be kept in mind while designing a heat exchanger. The most important

of them are surface area, temperature and pressure difference, pressure drop, type

of fluid used, flow rate, tube diameter, fluid viscosity, fouling tendency etc. [40]

2.6.3. PROBLEMS IN STHE

FOULING

The performance of heat exchangers usually decline with the passage of time as a

result of accumulation of sediments on the heat exchanging surfaces. The layer

causes additional resistance to heat transfer and serves as insulator hence cause

the rate of heat exchange to decrease. Heat exchanger is designed on the base of

calculations which depicts the need of the operation. Calculations were based on

exchange of heat between two fluids around the metallic walls of tubes. If anything

else (fouling) came in between the thermal circuit, the amount of heat exchange

would be decreased from which was expected initially. The main cause of fouling is

excessive heat, low velocity fluids, and contaminated water causing algae. [39]

LEAKAGE

If the baffles are not tightly placed in the shell, leakage through baffle occurs.

Leakage is undesirable because it changes the flow pattern, disturb pressure

distribution and decreases overall heat transfer rate [48]

TUBE BURST

Although STHE is well known for safe operation, but there are incidents reporting

tube failure. There are several reasons investigated behind those failures. Such as

improper selection of material, design or manufacturing fault, blockage due to

fouling, stresses caused by high temperature and pressure, thermal fatigue, but the

main cause behind this failure is presence of excess of chloride ions in the coolant.

[49]

2.7. STAR CCM+

Star CCM+ is the software package used for tackle problems involving multi-physics

and complex geometries. It helps to model geometry along with entirely automating

the simulation workflow and performing iterative design studies. It is used for

designing, simulating and analysing of products and complex systems in a wide

range of applications. [50]



3. DESIGN, MODELING AND SIMULATION

3.1. COMPUTER AIDED DESIGN (CAD) MODEL

For the heat transfer analysis of Shell and tube heat exchanger, keeping in mind the

limitations and considerations of software package (STAR CCM+) a scaled model of

real life example of steam condenser is taken, because a full scale model would

require computing time which is beyond the scope of this project. Model geometry

is based on a STHE from Walchandnagar industries ltd (APPENDIX A).

The geometry of the model used for conducting simulations was kept to an optimum

size for various reasons but primarily due to restrictions in available computing

memory provided by the software package which effects the meshing operation and

simulations.

The model is made up of Aluminium, having density 2702 kg/m3, specific heat 903

J/Kg-K and Thermal conductivity 215 W/m-K. Shell and tubes of is 1.364 m were

made. Shell diameter is 304.8mm while tubes have diameter of 11mm. Tubes were

arranged in square arrangement and they are 121 in number. There is a distance of

16mm between the centres of two adjacent tubes, so there must be a gap of 5mm

for the shell liquid to flow in between the tubes. There are total 6 baffles having

thickness and baffle spacing of 4mm and 200mm. Nozzles on both ends on shell

have been made for inlet and outlet of steam having diameter of 73mm. Figure 31

shows the CAD model.

Figure 31: CAD model



3.2. DESIGN DATA AND ANALYTICAL CALCULATIONS

As design optimization is based on a real life scenario, in which a STHE is working

in a steam turbine power plant. After driving the turbine wet or saturated steam

enters the shell and condensed till it leaves the chamber, as a condensate (water).

For cooling purposes chilled water from nearby river is pumped into the tubes of

exchanger. The inlet temperature of steam is assumed to be 380 0K (107 0C) while

temperature of water flowing in tubes is 278 0K (5 0C), which is an average water

temperature of rivers in UK during winter season [51]. Geometry and material

properties have been described in CAD section

Initial velocity of steam is 10ms-1 at the inlet of shell while cooling water enters in the

tubes having velocity of 1.5ms-1. Shell side gauge pressure is estimated to 0.275265

bar while water enters the tubes at atmospheric pressure. Based on the

temperature, Pressure and velocity assumptions and dimensions of heat exchanger,

following calculation has been made.

Shell inlet/outlet diameter = dn = 73mm

Shell diameter = ds = 304.8mm

Tube diameter = dt = 11mm

Shell inlet velocity = vs = 10ms-1

Tube inlet velocity = vt = 1.5ms-1

3.2.1. SHELL SIDE CALCULATIONS

Reynolds number

nsdvRe

ρ = 0.74825 kg/m3

vs = 10ms-1

dn = 73mm

μ = 1.25037E-5 kg/m.s

Re = 0.74825 x 10 x 0.073 / 1.25037E-5

Re = 43685

Reynolds number is greater than 4000, so flow is turbulent

Turbulence Intensity

I = 0.16Re-1/8



I = 0.16(43685)-1/8

I = 0.0420835

Turbulence Kinetic Energy

K =

(vsI)

2

K =

(10 x 0.0420835)2

K = 0.63125 J/kg

Turbulence Dissipation Rate

ε = c0.75 x K1.5 / l

c = 0.09

l = 0.07 dn

l = 0.07 x 0.073

l = 5.11E-3

ε = (0.09)0.75 x (0.6312525)1.5 / 5.11E-3

ε = 16.12745 m2/s3

Prandtl Number

Pr = Cpμ/k

Cp = 0.05413 J/kg.K

k = 0.025875 W/m.K

Pr = 0.05413 x 1.25037E-5 / 0.025875

Pr = 0.99527

Nusselt number

Reynolds number is greater than 10000 so Nusselt number can be calculated by

Dittus-Boelter equation.

Nu = 0.023 x Re0.8 xPr0.3

Nu = 0.023 x (43685)0.8 x (0.99527)0.3

Nu = 118.40662

Heat transfer coefficient

hs = Nu x k / dn

hs = 118.40662 x 0.025875 / 0.073



hs = 42.1722 W/m2.K

3.2.2. TUBE SIDE CALCULATIONS

Reynolds number

ttdvRe

ρ = 999.96693kg/m3

vt = 1.5ms-1

dt = 11mm

μ = 1.51813E-3kg/m.s

Re = 999.96693 x 10.5 x 0.011 / 1.51813E-3

Re = 10868

Reynolds number is greater than 4000, so flow is turbulent

Turbulence Intensity

I = 0.16Re-1/8

I = 0.16(10868)-1/8

I = 0.050073

Turbulence Kinetic Energy

K =

(vtI)

2

K =

(1.5 x 0.050073)2

K = 8.46206E-3J/kg

Turbulence Dissipation Rate

ε = c0.75 x K1.5 / l

c = 0.09

l = 0.07 dn

l = 0.07 x 0.011

l = 7.7E-4

ε = (0.09)0.75 x (8.46206E-3)1.5 / 7.7E-4

ε = 0.166114m2/s3



Prandtl Number

Pr = Cpμ/k

Cp = 4.20495J/kg.K

k = 0.57057W/m.K

Pr = 4.20495 x 1.51813E-3 / 0.57057

Pr = 11.18822

Nusselt number

Reynolds number is greater than 10000 so Nusselt number can be calculated by

Dittus-Boelter equation.

Nu = 0.023 x Re0.8 xPr0.3

Nu = 0.023 x (10868)0.8 x (11.18822)0.3

Nu = 80.40366

Heat transfer coefficient

ht = Nu x k / dt

ht = 80.40366 x 0.57057 / 0.011

ht = 4170.538W/m2.K

3.2.3. HEAT TRANSFER CALCULATIONS

Heat Conduction through tubes

R =ln(do/di) / 2πktL

Where R = thermal resistance

k = Thermal conductivity of Aluminium

and L = Length of tube

R = ln(11/9) / 2π(215)(1.364)

R= 1.08906 E-4 W/K

Overall Heat transfer Coefficient

Overall Heat transfer Coefficient = U

1/U = 1/hs + 1/ht + R



1/U = 1/42.1722 + 1/4170.538 + 1.08906 E-4

U = 41.75W/m2.K

Surface Area

As = nπdtL

Where n = number of tubes

As= 121 x π x 0.011 x 1.364

As = 0.75451m2

Log Mean Temperature Difference (LMTD)

ΔTLM = [ΔT1 – ΔT2] / [ln(ΔT1 / ΔT2)]

Where

ΔT1 = Tsi - Tto

ΔT2 = Tso – Tti

Tsi, Tso, Tti, Tto can be assumed as follows

Tsi = Shell inlet temperature = 380 0K (107 0C)

Tso= Shell inlet temperature = 310 0K (37 0C)

Tti= Shell inlet temperature = 278 0K (5 0C)

Tto= Shell inlet temperature = 298 0K (25 0C)

ΔT1 = 380 – 298

ΔT1 = 82

ΔT2 = 310 – 278

ΔT2 = 32

ΔTLM = [82 – 32] / [ln(82/ 32)]

ΔTLM = 53.136 0K

Heat Transfer Rate

.

Q = UAsΔTLM

.

Q = 41.75 x 0.75451 x 53.136

.

Q = 1672.384 W

Table 2 shows data in the form of initial and computed values used for simulations

in order to observe the heat transfer in shell and tube heat exchanger.



Table 2: Working fluid properties

PROPERTIES SYMBOL SHELL-SIDE TUBE-SIDE UNIT

Inlet Temperature T 380(107) 278(5) 0K (0C)

Absolute Pressure P 128851 101325 Pa

Inlet Velocity v 10 1.5 m/s

Density ρ 0.74825 999.96693 Kg/m3

Dynamic Viscosity μ 1.25037E-5 1.51813E-3 kg/m.s

Hydraulic Diameter d 73 11 mm

Specific Heat Cp 0.05413 4.20495 J/kg.K

Thermal

Conductivity

k 0.025875 0.57057 W/m.K

Reynolds Number Re 43685 10868 -

Prandtl Number Pr 0.99527 11.18822 -

Nusselt Number Nu 118.40662 80.40366 -

Turbulence

Intensity

I 0.0420835 0.050073 -

Turbulence Kinetic

Energy

K 0.63125 8.46206E-3 J/kg

Turbulence

Dissipation Rate

ε 16.12745 0.166114 m2/s3

Heat Transfer

Coefficient

h 42.1722 4170.538 W/m2.K

3.3. MESHING

By definition, meshing is the process of dividing the geometry into small number of

cells for solving and studying the engineering phenomenon. For appropriate results

high number of cells or fine mesh is preferable. However, if the number of cells is

higher, higher would be the computational memory required for simulations. So it is

a tough decision for a designer to make, by using finest possible mesh supported by

system. [52]

In this project Polyhedral mesh models were used. Polyhedral mesh is a type of

volumetric mesh, it can accumulate larger number of cells as compare to other

meshing models because it generate mesh in polyhedral cells and it is less diffusive

and more stable. For betterment of surface, surface remesher model was used to

retriangulate the surface edges and preserve the original triangulation. The meshed



model is shown in figure 32. The selected mesh has 1mm base size with relative

minimum size of 25 % and relative target size of 50%. [52]

Figure 32: Meshed model



4. RESULTS

4.1. MODEL 1: DEFAULT DESIGN

The first model of heat exchanger has been made according to the calculations

shown in fig 33 and meshed model shown in fig 32. The results obtained for

Temperature and Velocity are shown in figure 34 and 35 respectively. Figure 36

shows the graph of heat transfer between the working fluids.

Figure 33: Default Designed Model

Figure 34: Temperature Distribution for Model 1



Figure 35: Velocity Distribution for Model 1

Figure 36: Heat Transfer Plot for Model 1

Design optimization has been carried out by varying different components and

parameters, as described below.



4.2. VELOCITY

4.2.1. MODEL: 2

The second model’s geometry is the same as the first model (fig 33), the only

difference is; it is simulated on high velocity (20ms-1). Shell inlet velocity has been

doubled to study the effects of heat transfer and temperature change under

modified conditions. Parameters related to velocity also changes by varying the inlet

velocity. Calculated properties for this case are mentioned in table 3.

Table 3: Calculated Properties for Model 2

Properties Symbol Value Unit

Reynolds Number Re 87369 -

Nusselt Number Nu 207.55387 -

Heat transfer coefficient h 73.5675 W/m2.K

Turbulence Intensity I 0.03858783 -

Turbulence Kinetic Energy K 0.8934122 J/kg

Turbulence Dissipation rate ε 27.154312 m2/s3

The results obtained for Temperature and Velocity are shown in figure 37 and 38

respectively. Figure 39 shows the graph of heat transfer between the working fluids.





Figure 39: Heat Transfer plot for Model 2

4.2.2. MODEL: 3

The third model’s geometry is the same as the first and second model (fig 33), the

only difference is; it is simulated on low velocity (5ms-1). Shell inlet velocity has been

reduced to half in order to study the effects of heat transfer and temperature change

under modified conditions. Parameters related to velocity also changes by varying

the inlet velocity. Calculated properties for this case are mentioned in table 4.



Table 4: Calculated Properties for Model 3


Reynolds Number Re 21842 -




Turbulence Kinetic Energy K 0.07896722 J/kg

Turbulence Dissipation

rate

ε 0.71358123 m2/s3


respectively. Figure 42 shows the graph of heat transfer between the working fluids.

Figure 35: Themperature Distribution for Model 3





4.3. MATERIALS

4.3.1. MODEL: 4

The fourth model’s geometry is the same as the first model (fig 33), the only

difference is; Aluminium tubes and baffles are replaced with Stainless steel

(UNSS30200) tubes and baffles. Material has been changed to study the effects of

heat transfer and temperature change under modified conditions. Material

properties for stainless steel (UNSS30200) are mentioned in table 5.

Table 5: Material Properties for Model 4


Density ρ 7944 kg/m3

Specific Heat Cp 490 J/kg.K

Thermal Conductivity k 16 W/m.K


respectively. Figure 45 shows the graph of the heat transfer between the working

fluids.








4.3.2. MODEL: 5

The fifth model’s geometry is the same as the first model (fig 33), the only difference

is; Aluminium tubes and baffles are replaced with Stainless Copper tubes and

baffles. Material has been changed to study the effects of heat transfer and

temperature change under modified conditions. The material properties of Copper

(Cu) are mentioned in table 6.

Table 6: Material Properties for Model 5


Density ρ 8940 kg/m3

Specific Heat Cp 390 J/kg.K

Thermal Conductivity k 400 W/m.K



fluids.





Figure 43: Heat transfer plot for Model 5

4.4. MODEL 6: SUMMER

As stated earlier, the cooling water was drawn from rivers. In summer the

temperature of rivers and open streams rises by an average of 12-13 degrees [51].

The research was carried out on the conditions of summer i.e. water temperature in

tubes are 2910K (180C) in order to determine the reduction in heat transfer and state

of condensate (shell outlet). While shell inlet conditions remain unchanged.

Properties of the coolant (water) are listed in table 7



Table 7: Tubular fluid properties for Model 6


Density ρ 998.5973 kg/m3

Specific Heat Cp 4.186317E-3 J/kg.K

Thermal Conductivity k 0.594865 W/m.K

Dynamic Viscosity µ 1.05267E-3 kg/m.s

Velocity v 1.5 m/s

Reynolds number Re 15652 -

Prandtl Number Pr 7.408 -




Turbulence Kinetic

Energy

K 7.7245161E-3 J/kg

Turbulence Dissipation

Rate

ε 0.021831 m2/s3



fluids






4.5. MODEL 7: BAFFLES

Baffle spacing plays an important part in heat transfer. In this model number of

baffles has been increased from 6 to 11. And baffle spacing is been reduced from

200mm to 100mm.as shown in figure 52. Rest all the initial conditions are same as

for the default model.



fluids



Figure 46: Modified Geometery for Model 7






4.6. MODEL 8: TUBES LAYOUT

Geometrical arrangement of tubes in the tube bundle also affects the flow properties

of the shell fluid and net heat transfer. For visualizing the heat transfer square pitch

tubes are replaced with triangular pitch tube bundle. Tubes were having 16mm

diameter, and their centres are 22 mm apart from each other so there is a gap of

6mm in between the tubular surfaces for the passage of shell fluid. Tubes are

arranged on 600 angle, making an equilateral triangle. . Rest all the initial conditions

are the same as for the default model. The geometry for this model is shown in

figure 56The results obtained for Temperature and Velocity are shown in figure 57

and 58 respectively. Figure 59 shows the graph of the heat transfer between the

working fluids

Figure 50: Figure 50: Modified Geometry for Model 8 (Hidden Outer Shell)








DISCUSSION

The results of heat transfer and temperature of condensate steam obtained at shell

outlet is given in table 8. It is observed that varying designs, materials and inlet

properties increases or decreases the heat transfer and the shell outlet temperature

significantly.

Table 8: Results of Model 1-8

Heat transfer was first calculated manually (section 3.2) and then compared with the

simulated model. In mathematical calculations the value of rate of heat transfer was

1.6724 kW while in the end of simulation in Star CCM+ the value of heat transfer

was 1.75 kW. The small difference of 77.6 W was because in the calculations,

turbulence properties such as Turbulence kinetic energy and Turbulence dissipation

rate were not taken into account and pressure drop across each stage of baffle was

not computed. Also by using LMTD method, the outlet temperature of Shell and

tubes were roughly assumed. While the simulations give real time accurate values

of heat transfer.

Table 9: Heat Transfer Model 1-8

0

0.5

1

1.5

2

2.5

3

Model1

Model2

Model3

Model4

Model5

Model6

Model7

Model8

He

at T

ran

sfe

r (k

W)

Heat Transfer (kW)

Model

Description

Outlet

temperature 0K(0C)

Heat Transfer in

kW

1 Default 320(47) 1.749976

2 v =20ms-1 340(67) 2.5844

3 v =5ms-1 305(32) 1.185894

4 Stainless Steel

(UNSS30200)

325(52) 1.527283

5 Copper (Cu) 310(37) 1.929292

6 Summer 320(47) 1.556012

7 Baffles 290(17) 1.420365

8 Tubes Layout 300(27) 2.0718



Heat transfer of different models was computed by simulating them in Star CCM+

and compared with the initial default model. Table 9 gives the comparison of heat

transfer in kilo-Watts for each model. In comparison of varying the velocities,

increasing velocity up to double of its initial value (Model 2) gives almost 48%

increase in heat transfer (2.5844 kW) while when velocity was reduced to its half i.e.

5ms-1, the heat transfer decreased by 32%. (1.185894). When the material of tubes

and baffles was changed from aluminium (Al) to Stainless steel (UNSS30200), it

was observed that the heat transfer was decreased by 12%, however an increment

of 11% is noticed when Copper (Cu) is used instead of Aluminium (Al). Results

shows that weather conditions do not affect heat transfer that much, as in the

summers less than 10% reduction in heat transfer is observed as compared to that

of the winter. Increasing number of baffles or reducing baffle space makes it difficult

for the shell fluid to flow in baffle chambers; hence a remarkable reduction of 19% is

noticed. As a triangular pitched tube bundle is arranged in hexagonal pattern inside

the shell, it occupies more space than a square pitched tube bundle, so the

experiment shows that by using triangular pitch tubes, the heat transfer is

considerably increases by 18%

Table 10: Outlet temperate Model 1-8

After heat is exchanged and operation of the heat exchanger is completed, outlet

temperature of the concerned fluid is also scrutinized. Table 10 shows the

comparisons of outlet temperatures of various models. In comparison of altering

velocities, if velocity is increased up to its double, then it was observed that the

decision was not favourable regarding to the temperature of the condensate, as fluid

temperature received at the outlet of shell was 670C, 20 degrees more than the one

received at initial default conditions. On the other hand, if velocity is reduced to its

half, outlet temperature decreases by 15 degrees as compared to the original.

When the material of tubes and baffles was changed from aluminium (Al) to

0

10

20

30

40

50

60

70

80

Model 1 Model 2 Model 3 Model 4 Model 5 Model 6 Model 7 Model 8

Tem

pe

retu

re 0 C

Outlet Temperature (0C)



Stainless steel (UNSS30200), condensate temperature rose by 5 degrees, however

a favourable decrement of 10 degrees is noticed when copper (Cu) is used instead

of aluminium (Al). Weather conditions do not affect that much, as the temperature of

condensate obtained in warm season is almost equal to that of obtained in chilly

season. Increasing the number of baffles and decreasing the baffle spacing causes

outstanding contrast with the initial value of outlet fluid. Condensate temperature

drops by 30 degrees by making a change in baffle arrangement. Results show that

triangular tube arrangement also prompts a remarkable depletion in condensate

temperature; it differs with that of the initial default model by 20 degrees.

It has been observed that changes in material cause changes in heat transfer and

condensate temperature; this is because of thermal conductivity of the material, as

copper has the highest value of “k” Aluminium holds smaller value, while Stainless

steel has the lowest. Their heat transfer chart shows a trend in the same order table

9. If the weight of heat exchanger is not an issue than copper is the recommended

material, but it has a disadvantage; because of higher density it is heavier than

Aluminium. Aluminium is a light material and it gives high strength against stresses,

it would be useful where mechanical properties are concerned. Stainless steel is

recommended where the main concern is with durability. It gives high level of

protections from rust and fouling.

Is it observed from the results that increasing velocity results in higher heat transfer

and also gives higher condensate temperature, and if velocity is reduced,

condensate temperature decreases as well as the heat transfer so if condensate

temperature is not concerned, high velocity stream of inlet jet is recommended

because it gives the highest value of heat transfer as compared to the other models.

On the other hand if the sole concern is a low condensate temperature, low inlet

velocity would be a favourable option.

Increment in the number of baffles and reduction in baffle spacing would be a good

option in those facilities where low condensate temperature is more than the

amount of heat transferred. Because this arrangement of baffles gives the lowest

condensate temperature, exchanging heat between steam and coolant in several

chambers makes it possible.

Besides model 5 (Copper) Model 8 is the only model in which higher heat transfer

rate and lower condensate temperature is achieved. Triangular tube layout helps

the steam to travel in zigzag path between the tube bundles and hence more heat is

exchanged.

It has been observed by the experimental results that velocity profile in Shell and

tube heat exchanger is more dependent on geometry than inlet conditions. However

the magnitude of velocity depends on the initial conditions. It is obvious that

changes in the environmental conditions and material properties can affect the

velocity imperceptibly. However if the inlet velocity is doubled to its original, the

velocity obtained at first two baffle cuts was 6-8 ms-1, which was 3-4 ms-1in default

settings. On the other hand if inlet velocity is reduced to its half as depicted in model

3, velocity obtained at the same position was 1.5-2ms-1. It would not be wrong if one



had to say that the velocity at first and second baffle cuts is 30-40% of inlet velocity.

As shown from the results, Placing baffles too close, in arranging tubes in triangular

pitch causes hindrance in the shell flow to pass, and hence reduces velocity

noticeably, that’s why velocity vectors nearly disappears while approaching the

other end.



CONCLUSION AND RECOMENDATIONS

The analysis of Shell and Tube heat exchanger was carried out in Star CCM+. The

results obtained by initial simulation at default input conditions were compared to

the seven modified models in order to observe the variation in heat exchange,

velocity profile and condensate temperature.

The heat transfer between the working fluids changed significantly by changing

input conditions, materials and geometry whilst maintaining the vapour temperature

constant. Although heat transfer is directly proportional to the temperature

difference but still temperature gradient is not the only thing on which heat transfer

is dependent, that is why in some models rate of heat transfer increases and

condensate temperature decreases or vice versa. It has been observed that if heat

transfer is the concern than Model 2,5 and 8 are recommended on the other hand if

condensate temperature is the sole concern regardless of the amount of the heat

transferred, Model 3,7 and 8 will be useful. Nevertheless overall best results have

been obtained by using copper tubes and baffles and triangular tube pitch, (Model 5

and 8) as it gives cooler condensate and high heat transfer.

Hence it is concluded that the aims and objectives of this research are successfully

accomplished. The effect of varying design parameters and geometrical

components were investigated and results were compared with each other.

However due to limitation of time scale, the research lacks some further

advancement, as there is always a window for improvement, further research can

be carried out by using double segmented, disc and doughnut or helical baffles,

analysing pressure drop, flow directions and increase in the number of tube passes.



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APPENDIX A

Documents

MSc. Project Report - A. Hanan - 13084052