Design and Construction of an Induction Furnace (Cooling Sys

YANGON TECHNOLOGICAL UNIVERSITY

DEPARTMENT OF MECHANICAL ENGINEERING

DESIGN AND CONSTRUCTION OF AN INDUCTION FURNACE:

COOLING SYSTEM

BY

MAUNG THANT ZIN WIN

Ph.D. THESIS

NOVEMBER, 2005

YANGON



DESIGN AND CONSTRUCTION OF AN INDUCTION FURNACE:

COOLING SYSTEM

BY

MAUNG THANT ZIN WIN

A THESIS SUBMITTED TO

THE DEPARTMENT OF MECHANICAL ENGINEERING

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR

THE DEGREE OF DOCTOR OF PHILOSOPHY

(MECHANICAL ENGINEERING)

NOVEMBER, 2005

YANGON



We certify that we have examined, and recommend to the University Steering

Committee for Post Graduate Studies for acceptance of the Ph.D. thesis entitled:

"DESIGN AND CONSTRUCTION OF AN INDUCTION FURNACE:

COOLING SYSTEM" submitted by Maung Thant Zin Win, Roll No. Ph.D. M.7

(October, 2003) to the Department of Mechanical Engineering in partial fulfilment of

the requirements for the degree of Ph.D. (Mechanical Engineering).

Board of Examiners:

1. Daw Yin Yin Tun

Associate Professor and Head .

Department of Mechanical Engineering, Y.T.U. (Chairman)

2. Dr. Mi Sandar Mon

Associate Professor .

Department of Mechanical Engineering, Y.T.U. (Supervisor)

3. Daw Khin War Oo

Lecturer .

Department of Mechanical Engineering, Y.T.U. (Co-supervisor)

4. Dr. Sandar Aung

Associate Professor .

Department of Mechanical Engineering, Y.T.U. (Member)

5. Dr. Kyaw Sein

Professor and Advisor .

Ministry of Science and Technology (External Examiner)

i

ACKNOWLEDGEMENTS

First and foremost, the author sincerely wishes to express my deep gratitude to

His Excellency Minister U Thaung, Ministry of Science and Technology, for opening

special intensive courses leading to Ph.D Degree in Yangon Technological

University.

Special thanks are extended to Minister Dr. Chan Nyein, Ministry of

Education, for his guidance and kind help, and deep thanks are due to Deputy

Minister U Kyaw Soe, Ministry of Science and Technology, for his advice and keen

interest to produce the cooling system of induction furnace.

The author also wishes to thank Daw Yin Yin Tun, Associate Professor and

Head of Department of Mechanical Engineering, for her invaluable guidance and

helpful suggestions throughout the study.

Associate Professor Dr. Mi Sandar Mon, my thesis supervisor, provided me

with expert guidance throughout the study and the author is deeply grateful for it. She

was very helpful. Also, Daw Khin War Oo, my thesis co-supervisor, supported me

with the helpful suggestions in improving the thesis.

Sincere thanks are then extended to Associate Professor Dr. Sandar Aung, for

her critical review and inspiring guidance. Special thanks are extended to Professor

Dr. Kyaw Sein for his participation in the Board of Examiners of my thesis. His help

and advice are gratefully acknowledged. The author shall not forget Ko Cho Min Han,

who skillfully drew the necessary figures for my thesis.

Furthermore, the author would like to express my heart felt gratitude to my

parents and to all my teachers who taught me everything from childhood till now.

Finally, thanks are to the persons who contributed directly or indirectly towards the

success of this thesis.

ii

ABSTRACT

In coreless induction furnaces, water cooling system is the heart of the

induction coil which consists of a hollow section of heavy duty and high conductivity

copper tubing, and the coil must be water-cooled because of its high temperature

about 78C. The purpose of this thesis is to prevent overheating and damage to the

induction coil due to heat generated by the passage of alternating current to induce the

charge around the coil and heat transferred through the refractory lining from the

molten metal. For this reason, cooling pond system is theoretically designed and

practically constructed for 0.16 ton coreless induction furnace. It is used in two

induction furnaces for the alternative melting in foundry shop.

The calculations of required pond area and volume are carried out according to

the temperature difference between the hot water and cold water. The mass flow rate

passing through the inside of induction coil is mainly calculated according to the

increasing temperature. For 0.16 ton melting capacity of electric induction furnace,

the centrifugal pump, the size which is of 11 kW and pumping capacity 0.69 m3/min

is used to suck the amount of water sufficiently. To be a free flow of water, the size of

2.5 inches diameter galvanized iron pipes for inlet and outlet section of water from

cooling pond, and 1.5 and 2 inches diameter polyvinyl chloride plastic (PVC) pipes

have been used for the connection of pipelines to induction coil, capacity bank and

control panel.

Moreover, cooling tower system with induced draft counter flow type has been

designed for the continuous operating time and mass production in the melting

process. In addition, cooling tower is more efficient rather than cooling pond in that

the duration of operating time is limited with its volume. As a result, cooling pond

surface area 1,000 ft2 and volume 6,000 ft3 are obtained for 0.16 ton melting capacity

of two induction furnaces. Finally, their influences and operating capacity on cooling

system of induction furnace have been discussed with the recommendations.

iii

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS i

ABSTRACT ii

TABLE OF CONTENTS iii

LIST OF FIGURES vi

LIST OF TABLES viii

NOMENCLATURE ix

CHAPTER TITLE

1 INTRODUCTION 1

1.1 Objective 2

1.2 Outline of Thesis 2

2 LITERATURE REVIEW 3

2.1 Electric Melting Furnaces 3

2.1.1 Arc Furnace 4

2.1.2 Induction Furnace 6

2.1.3 Resistance Furnace 9

2.2 Operating Principle of Coreless Induction Furnace 10

2.3 Features of Induction Melting Furnace 12

2.4 Energy Requirements and Coil Cooling Energy Losses 13

2.5 Heat Balance of Induction Furnace 15

2.6 Water Cooling System 17

2.6.1 Water Requirements 19

2.6.2 Effects of Water Quality 20

2.6.3 Water Purification/ Maintenance 20

2.6.4 Filtration 21

2.6.5 Effects of Impurities 21

2.6.6 Energy Water Supply and Cooling System 22

2.7 Types of Cooling Water System for Electric Induction Furnace 23

2.7.1 Cooling Pond System 23

2.7.2 Spray Pond System 24

iv

2.7.3 Evaporative Cooling Tower-Open Circuit System 25

2.7.4 Fan-Radiator Closed-Circuit System 26

2.7.5 Water/Water Heat Exchanger Dual System 27

2.7.6 Dual System with Closed-Circuit Cooling Tower 28

2.8 Selection of Cooling System 28

3 FLOW CALCULATION AND PUMP SELECTION 30

3.1 Consideration of Flow Velocity 30

3.1.1 Specifications of Induction Coil 31

3.1.2 Effect of Electrical Resistance in Induction Coil 32

3.1.3 Heat Generation Rate Calculation 34

3.1.4 Calculation of Heat Transfer Rate in Composite 36

Refractory Shells

3.1.5 Flow Velocity Designation 38

3.2 Pump Selection 39

3.2.1 Essential Parameters Required in Selection 40

3.2.2 Selection Procedures 40

3.2.3 Calculations for Pump Selection 44

4 COOLING POND DESIGN 57

4.1 Pond Design Parameters 57

4.2 Conceptual Study for Steady-State Cooling Pond Design 58

4.2.1 Classification of Ponds 59

4.2.2 Equilibrium Temperature and Surface Heat Flux 61

4.2.3 Traditional Model 67

4.3 Design Model Consideration 69

4.4 Design Calculation 72

5 EVAPORATIVE COOLING TOWER SYSTEM 80

5.1 Cooling Tower Fundamentals 80

5.1.1 Principal Criteria 81

5.1.2 Classification of Cooling Towers 81

5.1.3 Main Components and Tower Operation 84

5.1.4 Cooling Tower Fill 87

5.2 Conceptual Study for Induced Draft Cooling Tower System 89

5.2.1 Cooling Tower Theory 89

5.2.2 Heat-Balanced Process 91

v

5.2.3 Tower Coefficients 92

5.2.4 Factors Affecting on Cooling Tower Performance 93

5.3 Design Calculations 94

5.4 Operation Considerations 99

6 RESULTS AND DISCUSSIONS 101

6.1 Flow Velocity Calculation Results 101

6.2 Cooling Pond Performance 102

6.3 Cooling Tower Performance 106

6.4 Process Influence on Tower 107

7 CONCLUSION, RECOMMENDATION AND 110

FURTHER SUGGESTIONS

7.1 Conclusion 110

7.2 Recommendation 111

7.3 Further Suggestions 112

REFERENCES 113

APPENDICES 117

APPENDIX A PROGRAM 117

APPENDIX B GRAPHS 121

APPENDIX C TABLES 125

vi

LIST OF FIGURES

Figure Page

2.1. Electric Arc Furnaces 4

2.2. Pictorial Diagram of Coreless Induction Furnace 7

2.3. Pictorial Diagram of Channel Induction Furnace 8

2.4. Pictorial Diagram of Electric Resistance Furnace 10

2.5. Simplified Cross Section of Coreless Induction Furnace 10

2.6. Melting Design Difference between Heel Method and Batch 11

2.7. Heat Balance Diagram of Crucible Type Induction Furnace 15

2.8. A Sample Induction Coil with Cooling Water 18

2.9. Sample of Damaging Induction Coil 18

2.10. Typical Sketch of Cooling Pond System 23

2.11. Sample Spray Pond System 25

2.12. Open-Circuit System with Evaporative Cooling Tower 25

2.13. Fan-Radiator Closed-Circuit System 26

2.14. Dual System with Water/Water Heat Exchanger 27

2.15. Dual System with Closed-Circuit Cooling Tower 28

3.1. Internal View of 0.16 ton Coreless Induction Furnace 30

3.2. Variation of Resistance with the Temperature 33

3.3. Temperature Distribution for a Composite Refractory 36

Cylindrical Shell

3.4. Approximate Relative Impeller Shapes and Efficiency Variations 43

for Various Specific Speeds of Centrifugal Pumps

3.5. Functional Layout Diagram of 0.16 ton Cooling Pond System 45

3.6. Sketch of Flow Branches in Pipes 45

3.7. Pipe Network for Joint E 46

3.8. Sketch of Suction and Discharge Line in Pumping System 49

4.1. Correlation between Pond Number, IP and Normalized 60

vii

Temperature Gradient, ov TT / 4.2. Components of Surface Heat Transfer 63

4.3. Example of Plug-Flow Pond 67

4.4. Schematic Elevation View of Completely Mixed Pond 67

4.5. Illustrative Example of Cooling Pond Model 69

4.6. Illustration for the Equilibrium Condition 70

4.7. Heat Transfer Mechanism in Cooling Pond and 72

the Symbolic Notations

5.1. Mechanical Draft Cooling Towers 82

5.2. Natural Circulation Cooling Towers 83

5.3. Cutaway View of Induced Draft Counterflow Cooling Tower 85

5.4. Drift Eliminator used in Induced Draft Counterflow Cooling Tower 86

5.5. Water Distribution System 86

5.6. Illustration of Typical Splash Fill 87

5.7. Illustration of Typical Film Fill 87

5.8. Typical Film Fill Shape and Texture 88

5.9. Process Heat Balance Diagram of Counterflow Cooling Tower 91

5.10. Enthalpy-Temperature Diagram of Air and Water 96

5.11. Toolkit Software Dialog Box 98

5.12. Output Results Comparison 98

6.1. Cooling Pond Performance Curve 102

6.2. Effect of Cooling Pond Configurations 104

6.3. Comparison of Different Temperature Ranges at 107

Constant Water Quantity


by Changing L/G Ratio


at the Close Approach Condition

B.1. Skin Effect in Isolated Rounded Copper Tubings 121

B.2. Composite Rating Chart for a Typical Centrifugal Pump 122

B.3. Moody's Diagram 123

B.4. Nomograph of Cooling Tower Characteristics 124

viii

LIST OF TABLES

Table Page

2.1. Induction Furnace Categories 8

2.2. Electricity Use in Electric Melting Furnaces 14

3.1. Specifications of Induction Coil 32

3.2. Pumps Classes and Types 42

3.3. Total Losses for Pipe Sections 50

3.4. Operating Speed versus Required Specific Speed 52

3.5. Pump Types Listed by Specific Speed 53

3.6. Atmospheric Pressures at Various Altitudes 55

4.1. Iterative Solutions of Equilibrium Temperature 74

4.2. Resulting Values of the Water Temperature and the Operating Time 78

5.1. Enthalpy Difference by Using the Numerical Integration Method 95

5.2. Enthalpy Difference by Using the Chebyshev Method 97

6.1. Comparison of Process Variables in Tower Design 108

C.1. Pipe Roughness - Design Values 125

C.2. Resistance in Valves and Fitting expressed as Equivalent Length 125

in Pipe Diameters

C.3. Properties of Water at Various Temperatures 126

C.4. Comparison of Different Roofing Materials 126

C.5. Characteristics of Modern Pumps 127

ix

NOMENCLATURE

A area of pipe line, m2

Ap pond surface area, m2

A1, Ai copper conductor area in general, and for inner area,

respectively, cm2

a, b regression coefficient

C cloud cover of the sky

Cp specific heat of constant pressure, kJ/kg K

D diameter of a pipe, m

Do outer diameter of induction coil, cm

Di inner diameter of induction coil, cm

Dv vertical dilution

ea vapour pressure, mmHg

esat saturation vapour pressure, mmHg

E thermal energy, W

f rated frequency, Hz

f fraction factor

f internal fraction factor )( ),( 21 WfWf wind speed function for analytical, and empirical, respectively

oF densimetric Froude number g gravitational constant

G air loading, kg/(hr m2)

h loss, m

hl energy losses from the system, m

H enthalpy of air-water vapor mixture at the

wet bulb temperature, J/kg

H' enthalpy of air-water vapor mixture at the

bulb water temperature, J/kg

Ha atmospheric pressure, m

x

Hf total friction-head loss, m

Hn net heat exchange rate, W

Hp pond depth, m

Hs total suction head or lift, m

Ht actual total head on the pump, m

Hts total static head, m

HDU height of a diffusion unit, m

I rated alternating current, A

IP pond number

Isc solar constant

kA, kB, kC thermal conductivity for silica lining, for asbestos sheet, and for

asbestos cloth, respectively, W/mC

kr water retention rate, m/min

kT thermal rate, min-1

K heat exchange coefficient, W/m2C

Kx overall enthalpy transfer coefficient, kg/(hr m2)

KxaV/L tower coefficient

l length of copper conductor, m

L liquid loading, kg/(hr m2)

L1 height of crucible, m

L length of flow path, m m slope of the straight-line portion of the curve

m& water mass flow rate, kg/s n Julian day number

nd number of diffusion unit

N pump rotative speed, rpm

Ns pump specific speed, rpm

p pressure, Pa

ps possible sunshine hour, hr

Pg power loss of induction coil, kW

qr heat transfer rate, kW

Q water outflow rate, m3/min

Qt total heat transfer rate, W

Qv volume flow rate, m3/min

xi

QT total volume flow rate, m3/min

r pond cooling capacity

r1, r2, r3, r4 radii at various interfaces, m

R water inflow rate, m3/min

Re Renold number

RH relative humidity, %

1 ,1 DCRR resistance at temperature t1, and at temperature 20C,

respectively, 2

,2 DCRR resistance at temperature t2, and at temperature 60C,

respectively, S heat transfer surface, m2

S monthly average of the sunshine hours per day at the location,

hr

oS monthly average of the maximum possible sunshine hour per

day at the same location, hr

t operating time, hr

t1, t2 temperature of the copper tubing related to the resistance R1,

and R2, respectively, C

tc coil thickness, cm

T temperature, C

vT average temperature difference between the surface and bottom of the pond, C

oT temperature difference between the surface and the bottom of the pond, C

iT normalized intake temperature, C

v, vE, vi flow velocity, for joint E and for the inside of induction coil,

respectively, m/sec

V volume, m3

w pond width, m

Wc circulating water flow rate, m3/min

Wd drift loss, m3/min

We water evaporative loss, m3/min

xii

Wm make-up water, m3/min

W2 wind speed at two meters above the water surface, mph

z, zE elevation in general, and for joint E, respectively, m

Z height of cooling tower, m

Greek Letters

1 temperature coefficient of resistance E kinetic energy coefficient

coefficient of thermal expansion proportional factor

c specific heat of water, J/kgC

roughness, mm kinematic viscosity, m2/min water density, kg/m3

1 resistivity, cm latitude of the location, degree

n net solar heat flux, W/m2

sn net solar (short-wave) radiation, W/m2

an net atmospheric (long-wave) radiation, W/m2

br back (long-wave) radiation, W/m2

e evaporative heat flux, W/m2

c conductive heat flux, W/m2

s solar radiation at water surface, W/m2

sr reflected solar radiation, W/m2

a atmospheric (long-wave) radiation, W/m2

ar reflected atmospheric radiation, W/m2

osc extraterrestrial solar radiation, kJ/m2. day

sc clear sky solar radiation, kJ/m2.day s sunset or sunrise angle, degree

declination angle, degree

xiii

Subscripts

a ambient air

atm atmosphere

AC alternating current

b pond number

c copper conductor material

d dew point

DC direct current

E equilibrium

i inlet into the pond

m major

m,i entering water into the coil

m,o leaving water from the coil

n minor

o outlet from the pond

p pond

s surface

sd static discharge head

sl static suction lift

s,1 molten metal

s,2 silica lining

s,3 asbestos sheet

s,4 asbestos cloth

t tower

w wet bulb

1 hot water

2 cold water

1

CHAPTER 1

INTRODUCTION

The basic metal melting processes require application of heat to raise the

metals to their respective melting points. The major melting processes available for

foundry industries include electric induction furnace, arc furnace, resistance furnace,

gas furnace and cupola furnace. Among them, the electric induction furnace is

suitable for not only ferrous and non-ferrous applications but also high temperature

melting because of its energy concentration, and installation space is reduced as

compared with other types of melting furnace. Especially, coreless induction furnaces

are used for the various types of metal.

An induction furnace consists of a refractory structure surrounded by high

conductivity copper tubing with the cooled water in which the alternating current is

passed. This current generates a magnetic field that induces a current on the surface of

the metal. The heat generated by this current is conducted into the metal, causing

melting. Heat carried away through the refractory lining due to the molten metal

inside the crucible, and heat generated by the magnetic field (frequency of the power)

and its intensity (power input) inside the induction coil itself, are simultaneously

conducted and reach the water-cooled coil which is wound into a helical coil. Its heat

causes the melting effect to the water-cooled coil. Not to be damaged and not to melt

the induction coil, it is essential for the water cooling system to feed the cooling water

to the coil. There are different varieties of cooling system used in induction furnaces.

Most of the newer coreless induction melting system uses a recirculating system for

getting a great quality of cooling water. To be more efficient and effective, some

foundry industries are using the cooling ponds, cooling towers, fan radiators, and heat

exchangers for operating continuous batch method during the day.

Nowadays, industrial zones are rapidly growing and the demand of coreless

induction furnace for foundry industries is also increasing. In Myanmar, it has the

promising regions for installing and setting up the induction furnaces to produce the

good quality products more efficiently. If the induction furnaces can be built in

foundry industries locally and commercially, it will save cost, and improve the

2

productivity towards the industrialized nation. Thus, the design and construction of an

induction furnace essentially requires careful selection, installation, and maintenance

of the water cooling system. Here, the further investigations of mostly used cooling

system such as cooling pond and cooling tower system are of broad interest to design

more compact and efficient in coreless induction furnace.

1.1. Objective The objectives of the present study are:

(a) To design and construct the cooling pond system for 0.16 ton melting capacity.

(b) To design the evaporative cooling tower (induced draft counterflow type) system

for the continuous operating time and mass production in melting process.

(c) To support the foundry industries in melting with coreless induction furnace

where the cooling system is an essential part of furnace.

1.2. Outline of Thesis This research is directed to the understanding of the design and construction of

an induction furnace with water cooling system. The objectives and outline of the

thesis are expressed in chapter one. In chapter two, the relevant literature on cooling

system of coreless induction furnace is reviewed. There are significant differences

among cooling systems. Flow calculation and pump selection of cooling pond system

are described in chapter three. In chapter four, design and calculation of cooling pond

system is presented by using the concepts of equilibrium temperature and surface heat

flux. Theoretically, it describes design processes of the evaporative cooling tower

system (induced draft counterflow type) in chapter five. The results and discussions

on the study with all the problems are presented in chapter six. Finally, conclusion,

recommendation and further suggestions are expressed in chapter seven.

3

CHAPTER 2

LITERATURE REVIEW

This chapter covers the literature review of electric melting furnace essentially

required in foundry sector without any calculation for design, and energy

requirements and cooling coil energy losses. Various types of water cooling system

mostly used in induction furnaces are described with the necessary diagrams. Water

related problems and effects of impurities for induction melting system are presented

in this chapter.

2.1. Electric Melting Furnaces

In electric melting furnaces, energy is introduced by radiation, convection, or

induction directly to the metal to be melted. Raw ferrous materials consist mostly of

scrap and some cold pig iron. For this reason, the electric furnace plays an important

role in the recovery and recycling of waste iron resources. In area where an abundant

supply of scrap and electric power are available, the properties of steelmaking via the

electric furnace route is relatively high, because both energy consumption and

equipment investment are substantially smaller than via the integrated route using a

blast furnace and blast oxygen furnace to produce steel from ore.

They are being increasingly used for melting metal and many new and

improved types of furnace have been produced in year by year and installed at

foundries. Electric melting methods are flexible in terms of the metal charged and can

have very high melting rates. Their relative importance and the various types can be

seen in the order of their industrial significance. Electric melting furnaces are usually

divided into three main classes according to the method of pouring the metal from the

crucible, the heating method, and several configurations. They are:

1. Arc furnace

2. Induction furnace

3. Resistance furnace

4

2.1.1. Arc Furnace

Electric arc furnaces are refractory-lined melting furnaces that obtain heat

generated from an electric arc within the furnaces. They are used more extensively for

steelmaking and the other majority of applications, including the melting of gray iron,

brass, bronze and gunmetal, as well as many nickel alloys, because its capacity is

large and p iency is high. They are also capable of melting a higher

fraction of a

indirect arc,

C

Spo

roduction efficlloy scraps. There are two main types of arc furnace, the direct arc and the

as shown in Figure 2.1.

a

u

B

TPower lead

rbon electrodes Door

t Slag Metal Rammed hearth

(a) Direct Arc Furnace

Water-cooled roof

Upper electrode (cathode)

Water-cooled panel

Eccentric bottom taphole ottom electrode (anode)

ilting device

(b) Indirect Arc Furnace

Figure 2.1. Electric Arc Furnaces

5

In Figure 2.1. (a), direct arc furnace is so called because an arc is struck

directly between the electrode and the metal to be melted. The electrodes are of

graphite or amorphous carbon, and the furnaces are either single-phase unit for very

small furnaces or more generally, three-phase unit with three overhead, vertically

disposed electrodes suspended over what is normally a bowl-shaped refractory hearth.

Practically all modern arc furnaces are circular in plan, the kettle-shaped

structure with a removable lid, with refractory sidewalls and a domo-shaped roof

provided with holes for inserting the electrodes. The carbon electrodes provide the

current for the process. They are totally removable in an upward direction to allow the

top of the furnace to be removed. The tapping spout is used at the end of the process

to allow the molten steel to be poured from the furnace. During the process it is sealed

to keep the heat in. The operating door on a top-charged furnace is used for making

alloy of slag additions, for rabbling the molten metal and for removing the slag if

necessary. The furnace can usually be tilted backwards to assist this operation.

Direct arc furnaces are either acid or basic-lined, depending on the melting

operation to be carried out. Basic linings are used for steelmaking when sulphur and

phosphorus removal are required and are generally recommended for high-alloy steels,

such as stainless and manganese steels. Acid linings consist entirely of siliceous

materials and are restricted to the melting of cast iron and the production of steel

castings from scrap requiring no removal of sulphur and phosphorus.

The changing process to the furnace is in itself damaging the refractory lining

by both impart and the chilling effect of the cold scrap. The aggressivity to the

refractory lining is further increased by rapid temperature increase during melting,

combined with the attack by slag fluidizers such as fluorspar. Preferential attack of the

refractory lining occurs in the hot spot areas (opposite the electrodes) caused by flare,

and at the slag line, owing to low basicity slags, and high FeO slags, often employed

to aid phosphorous removal.

Indirect arc furnaces are so called because the arc is struck between two

carbon electrodes and is therefore independent of the charge, which is heated

indirectly by radiation. A typical indirect electric arc furnace is shown

diagrammatically in Figure 2.1. (b). The efficiency of heating, melting, and

decarburization in the indirect arc furnace has been substantially increased by

adopting an ultra high-power transformer and an oxy-fuel burner, as well as by

supplying coal power and pure oxygen gas.

6

Cooling the furnace walls and ceiling with water-cooled panels have also been

enhanced, enabling an increase in production efficiency from 80 to 120 ton/h. The

indirect arc furnace offers lower unit consumption of power, electrodes, and

refractories, and both noise and flicker are also lower. The preheating and continuous

charging equipment for scrap decrease the energy consumption because preheating is

carried out by the high temperature exhaust gas, and heat loss by opening the furnace

lid during conventional scrap charging can be prevented. The eccentric bottom-

tapping allows efficient tapping without tilting the vessel, and is desirable for

maintaining the cleanliness of the molten steel, because the carry over of oxidizing

slag into the ladle during tapping can be prevented.

2.1.2. Induction Furnace

Electric induction furnace is used in both ferrous and nonferrous melting

applications. It is also an AC electric furnace in which the primary conductor

generates, by electromagnetic induction, a secondary current that develops heat within

the metal charge. Many small furnaces are being used by the foundry can be operated

in several configurations, including single furnace system, tandem operation, melter

and holder configuration, and power sharing.

In the conventional single furnace system, each furnace body is supplied from

its own power supply. In tandem operation, two furnace bodies (usually identical) are

fed from a single power supply that is switched from one furnace to the other.

In melter/holder systems, an additional small power supply is used for holding

requirements. The power sharing configuration is similar to melting/ holding except

that a single power supply simultaneously provides melting power to one furnace and

holding power to the second. In both these configurations, the two furnaces alternate

in their melting and pouring roles. Metal production can be increased by up to 20

percent with this type of operation presented by Mortimer [1].

The advantages and disadvantages of induction melting systems are:

Advantages

- The system permits but does not require the use of a slag.

- The system exhibits good melt agitation, relatively easy fume control and

rapid heat-up.

- It is not as inherently dusty as electric arc melting, producing only 20 percent

as much effluent dust.

7

Disadvantages

- There is an increased risk of cross-contamination between melts due to

reactions between refractory lining and the metal and also the slag.

- Molten slag is removed by skimming for which the furnace may be opened

releasing fumes and dust.

There are two main types of induction furnace. They are coreless type

induction furnace and core or channel type induction furnace.

(i) Coreless Induction Furnace

In a coreless induction furnace, a water-cooled helical copper coil surrounds a

refractory-lined cavity containing the charge material, as shown in Figure 2.2. An

induced current is produced in the charge material by an alternating current in the coil.

Once the charge is molten, stirring action occurs as a result of the interaction of

currents in the melt with the magnetic field.

Steel shell

Cooling coil

Magnetic yoke

Power coil

Refractory lining

Cooling coil

Figure 2.2. Pictorial Diagram of Typical Coreless Induction Furnace

Stirring velocity increases at high powers and lower frequencies. The amount

of stirring is characterized by the velocity of the molten metal circulation as well as

the resulting height of the molten metal meniscus. Horwath et al. [2] classified three

categories of induction furnace depending on the capacity and melting rate required,

and the frequency of the current supplied as shown in Table 2.1.

8

Table 2.1. Induction Furnace Categories

Frequency Designation Frequency (Hz)

Mains (or line) 50-60

Low 150-500

Medium or high 500-10,000

For melting high melting point alloys, all grades of steels and irons as well as

many non-ferrous alloys, the coreless induction furnace has been widely used in

foundry as the crucible furnace. This furnace can be used for remelting and alloying

because of the high degree of control over temperature and chemistry while the

induction current provides good circulation of the melt.

(ii) Core or Channel Induction Furnace

Another type of induction melting furnace is the channel furnace or core type

induction furnace. The configurations may be horizontal drum type furnace or semi-

drum or low-profile furnace with removable cover or vertical type furnaces. In a

coreless induction furnace, the power coil completely surrounds the crucible. In a

channel furnace, a separate loop inductor is attached to the upper-body, which

contains the major portion of the molten metal bath.

Movable lidUpper case lining

Back-up castableCover plate Insulating brick

Pouring spout

Furnace platform

Upper case assembly Upper case hearth Throat

Blasch inductor lining Transformer Back-up castable

Figure 2.3. Pictorial Diagram of Channel Induction Furnace

Hydraulic cylinder

Inductor assemblyBushing

Coilcore

9

Attached to the steel shell and connected by a throat is an induction unit which

forms the melting component of the furnace. The induction unit consists of an iron

core in the form of a ring around which a primary induction coil is wound. This

assembly forms a simple transformer in which the molten metal loops comprise the

secondary component. The heat generated within the loop causes the metal to

circulate into the mail well of the furnace. The circulation of the molten metal effects

a useful stirring action in the melt. A vertical channel furnace may be considered a

large bull ladle or crucible with an inductor attached to the bottom. In Figure 2.3, it is

illustrated that the furnace has insoluble components, such as slag, accumulate over

time in the induction loop or throat area. Buildup on the sidewalls of channel furnaces

is also a common occurrence.

Channel induction furnaces are commonly used for melting low melting point

alloys and or as a holding and superheating unit for higher melting point alloys such

as cast iron. They can be used as holders for metal melted off peak in coreless

induction units, thereby reducing total melting costs by avoiding peak demand

charges. Channel induction melting furnaces have been built with capacities

exceeding 100,000 pounds. Overall required efficiency should be around 75 percent.

Channel induction furnaces have capacities in the range of 1 ton to 150 tons.

2.1.3 Resistance Furnace

The electrical- and heat-resistance reverberatory melting furnace is used for

zinc and aluminum melting. This furnace is constructed with an aluminum-resistant

refractory lining and a structural steel shell. The furnace is heated by silicon carbide

or carbon electrode or other resistance elements mounted horizontally above the both.

Heat is transferred through direct radiation from the refractory roof and sides. The

details are seen in ACMA et al. [3].

Another type of electric resistance furnace uses electric immersion-type

elements. The elements are inserted into silicon carbide tubes that are immersed in the

molten aluminum. Through radiation, the element passes its heat to the silicon carbide

tube. Through conduction, the tube releases its heat into the bath.

To clarify the structure of electric resistance furnace, the example of electric-

resistance ash melting furnace is shown in Figure 2.4 and it uses carbon electrodes

and performs the reduction melting treatment of ash in a fully closed structure. Molten

10

slag and molten metal are separated by the difference in specific gravity and each has

a separate discharge port. Molten slag is discharged utilizing the head pressure. Power supply

Incineration ash + Fly ash Exhaust gas

Ash layer Radiated heat transfer

Molten slag layer Heat convection

Molten metal layer Molten slagMolten metal

Figure 2.4. Pictorial Diagram of Electric Resistance Furnace

2.2. Operating Principle of Coreless Induction Furnace

The principle of operation of the coreless induction furnace is the phenomena

of electromagnetic induction. Many induction furnaces are widely constructed by

using the phenomena of electromagnetic induction. All electrically conductive

materials can be heated quickly and cleanly with pollution free induction heating. A

simplified cross section of a coreless induction furnace with the molten charge and the

crucible lining is shown in Figure 2.5.

It is composed of a refractory-lined container with electrical current carrying

coil that surrounds the refractory crucible. Holding the molten container which is

surrounded by a water cooled helical coil is connected to a source of alternating

current. A metallic charge consisting of scrap, pig iron and ferroalloys are typically

melted in such a container. Electrical current in the coil forms a magnetic field, which

in turn creates thermal energy, melting the charge.

Figure 2.5. Simplified Cross Section of Coreless Induction Furnace

11

Otherwise, the induction (generation) of the electrical current in a conductive

metal (charge) placed within a coil of conductor carrying electrical current is known

as electromagnetic induction of secondary current. The magnetic currents in the

molten metal cause an intense stirring action, thus ensuring a homogenous liquid.

During the melting process, slag is generated from oxidation, dirt, sand and

other impurities. Slag can also be generated from the scrap, erosion and wear of the

refractory lining, oxidized ferroalloys and other sources. It normally deposits along

the upper portion of the lining or crucible walls and above the induction coils. The

hottest area of high frequency coreless induction furnaces is at the mid-point of the

power coil, where insufficient metal turbulence from magnetic stirring occurs.

Two methods or melter are used for operating a coreless induction furnace. In

the heel method (also called tap and charge), a portion of the liquid charge is

retained in the furnace and solid charge material is added. The batch method requires

the furnace to be completely emptied between melts. Batch melting on a large has

become more common for the development of reliable high-power components for

variable frequency equipment and technology that allows utilization of full power

input during the entire melting cycle.

The energy losses associated with holding iron between melts, as well as the

larger overall furnace sizes resulted in high overall energy consumption rates. The

basic design differences between heel melt and batch melt induction furnaces are

shown in Figure 2.6.

MetallicCharge

Water-CooledInduction

Coils

Molten MetalHeel

Batch Melter Refractory

Lined Steel Shell Heel Melter

Figure 2.6. Melting Design Difference between Heel Method and Batch Method

12

The older power supplies were also very inefficient, with losses approaching

40 percent. The heel was used primarily to help reduce stirring associated with line

frequency melting, and it also required that charges be preheated to ensure that no wet

charges were put into the molten iron in the furnace heel.

As more sophisticated solid-state power supplies with increasingly higher

power ratings become available, the batch furnace increases in numbers. A batch-

melting furnace empties the furnace after each melting cycle, reducing the holding

power requirements. Over time, methods were developed to increase the frequency of

the power supplies, allowing for increased power densities and smaller furnace sizes.

Another inherent advantage of the batch induction melter is that when a

magnetic charge such as solid scrap iron and cold pig iron are melted, the coil

efficiency can be as high as 95 percent, compared to 80 percent when heating the

molten bath in a heel melter. Hysteresis losses associated with induction heating of a

solid ferrous material are responsible for this increased coil efficiency during the first

part of the melting cycle.

2.3. Features of Induction Melting Furnace

In metallic material placed in magnetic field generated by the current in

induction coil of the furnace, electromotive force is induced by the action of

electromagnetic induction, and induced current flows to heat up the material by its

Joules heat. Compared to other types of melting furnace, induction furnace has the

following features:

1. Its heat efficiency is high because the material is directly heated by

electromagnetic induction.

2. No carbon dioxide is produced and little smoke and soot is emitted because

cokes are not used as fuel.

3. Metal loss by oxidation is little, thus little contamination of metal because of

heating without air.

4. Temperature control is simple, uniform composition of metal product is

attained by agitation effect and alloyed cast iron is easily produced.

5. Induction melting is suitable for high temperature melting because of its

energy concentration, and installing space is reduced as compared with other

types of melting furnace.

13

6. It is possible to melt not only steels very low in carbon but also ferrous and

non-ferrous metals because there are no electrodes in arc furnace and

resistance furnace.

7. As the electricity causes heat in an induction furnace, and the molten metal/air

interface is relatively small, off-gas volumes are smaller for induction furnaces

than for electric arc furnace given by A.D. Little [4].

2.4. Energy Requirements and Coil Cooling Energy Losses

The overall efficiency of coreless induction furnaces depends on furnace

operating parameters and factors related to the charge. Energy consumption in

coreless induction furnaces is affected by the contaminants (e.g. rust, sand, oil, water,

coatings) on the charge since these materials contribute to slag formation. Removing

the slag requires additional time during the melt cycle, thereby lowering the efficiency.

About 20 percent more energy is required to melt virgin gray iron in coreless

induction furnaces than using scrap metal. Researchers theorize that it takes a higher

temperature and longer melting time to melt the virgin material to produce carbon.

These differences between virgin materials and scrap have not been shown, however,

for carbon and low-alloy steel. Further details can be found in Horwath et al. [2].

Other variables affecting energy use during coreless induction melting include

the melting method (heel versus batch); power application (step power versus full

power); use of covers; and furnace condition (e.g. hot, medium, or cold). For ferrous

materials, heel melting typically requires less energy than batch melting (in the order

of 5 percent less for stainless steel), as does the use of a hot furnace (about 2 percent

to 4 percent less for gray iron and low-alloy steel compared to cold conditions).

Coreless induction melting furnaces have electrical efficiencies in the range of

76 percent to 81 percent although the efficiency of an inductor is around 95 percent.

Induction furnaces operated in tandem can achieve a maximum electric power

utilization exceeding 80 percent (excluding power plant losses).

About 75 percent of the energy delivered to the furnace is used for increasing

the temperature of the metal. The main source of energy loss is via the coil water

cooling system, typically a 20 percent to 30 percent loss. The above energy percents

are given by ACMA et al. [3], and Smith and Bullard [5].

Other energy loses in a coreless induction furnace come from

- conductive losses through the lining,

14

- heat losses associated with the slag, and

- radiation losses when the furnace lid is open.

Heat losses associated with slag are a function of the temperature and

composition of the slag produced. The heat content of a typical slag in furnace is

about 410 kWh/ton at 1,538C. Unless large quantities of slag are produced, the heat

loss due to slag does not detract substantially from the overall performance of the

furnace [6].

Radiation heat loss from an uncovered molten bath and the bottom of an

opened cover can reach 130 kW for a 10-ton furnace. However, radiant heat loss

caused by iron melting is less than that by aluminum melting. Table 2.2 summarizes

the energy requirements for various types of electric melting furnaces.

Table 2.2. Electricity Use in Electric Melting Furnaces

Electricity Use in Electric Melting Furnaces

(kWh/metric ton of metal)

Induction

[106 Btu/tona]

Electric Arc

[106 Btu/tona]

Electric-Resistance

Furnace

[106 Btu/tona]

520 800b [5.0 7.6]

500 550c [4.3 4.8]

500 600 [4.3 5.2] 600 825 [5.2 7.9]

Sources: Smith and Bullard (1995), Booth (1996) and Process Metallurgy International (1998)

a Using electricity conversion factor of 10,500 Btu/kWh.

b Ferrous melting. Medium frequency coreless. When an ancillary equipment

energy use is included, the tool ranges from 550 to 650 kWh/metric ton of

metal.

c Molten, efficient furnaces.

Energy consumption for medium-frequency induction melting is generally in

the range of 520 to 800 kWh/metric ton. The use of furnace covers can reduce

melting-rated energy consumption to as low as 500 kWh/metric ton. Allowing for

holding power requirements and ancillary equipment, overall energy consumption is

reported to be in the range of 550 to 650 kWh/metric ton.

15

With modern, efficient, solid state power electronics, the energy required in

many induction furnaces can be as low as 500 kWh/metric ton for aluminum or iron at

high utilization rates. Energy consumption for electric arc furnaces ranges from 450 to

550 kWh/ton of charge, depending on the scrap type and length of time heat is applied.

For the electric resistance furnace, the only heat loss is through the shell and from

exposed radiant metal surfaces.

2.5. Heat Balance of Induction Furnace

As the induction furnace is operated with the large amount of temperature,

heat balance of the furnace must be understood fully to make the proper decision

about cooling effects inside the induction coil to resist the overheating condition and

power source side such as frequency conversion equipment and power-factor

improving capacitor. Efficiency of induction furnace is expressed as a total, deducting

electrical and heat transfer losses. Heat balance diagram of crucible type induction

furnace is shown is Figure 2.7. Input Input 100%100%

Water-cooled Transformer Transformer

(1) cable (1) (1.5)

Bus bar Coil Inverter condenser (16)(4)

(2) Bus bar condenser

(2)

Slag, etc.(1.5)

Heat conduction

Total Total (3) efficiency efficiency Heat

radiation 67% 69%

(2)

(b) Distribution of losses in (a) Distribution of losses in low-frequency furnace. high-frequency of furnace. Heat loss (%) is given in ( ). Heat loss (%) is given in ( ).

Figure 2.7. Heat Balance Diagram of Crucible Type Induction

Source: Energy Conservation in Iron Casting Industry (1998)

In above figure, 100 percent of input energy is used fully

furnaces; high-frequency and low-frequency crucible type furna

electrical and heat losses. Electrical losses consist of transformer, freq

water-cooled condenser, bus bar, wiring, cable and coil. Loss in co

factor, on which the furnace capacity depends. Heat losses in inWater-cooled cable

(1.5)

Coil (17)

Heat conduction (7)

Heat radiation (4.5)

Furnace

in both of these

ce which have

uency converter,

il is an essential

duction furnace

16

consist of conduction loss of heat escaping from furnace wall to coil side, radiation

loss of heat released from melt surface, absorption loss in ring hood and slag melting

loss. Heat efficiency of high-frequency furnace (69%) is slightly larger than that of

low-frequency furnace (67%). Low-frequency furnace is larger in heat loss

(conduction and radiation) due to long melting time, while high-frequency furnace is

larger in electrical loss (transformer, inverter and bus bar) due to short melting time.

To improve heat efficiency of furnace, the proper decision about the kind of

material, size and shape of charging materials to be melted, melting amount,

connection with pouring line and layout of the melting shop should be made and

adjusted carefully by users side. Induction furnace equipment should be melted with

minimum distance between each of equipment to reduce wiring losses. To reduce the

wiring losses remarkably, it is essential to shorten the distance between furnace body

and power-factor improving capacitor as very large current flows between them.

Moreover, skin effect and effect of agitation are considered to improve the

heat efficiency and induction current flows concentratedly in the surface of material to

be melted. This concentration of current becomes more remarkable as the frequency

become higher, resulting in better heating efficiency. Diameter or thickness of

material to be melted in the furnace may be decreased accordingly as the frequency

becomes higher when cast iron is melted in high-frequency induction furnace, there is

practically no limitation in its size, but in low-frequency furnace when starting with

cold metal, melting has to be started only by the use of starting block. Continuous

melting is to be preformed with residual molten metal.

In the effect of agitation, molten metal is agitated to raise its surface in the

center because molten metal is excited by current opposite to current flowing in

induction coil. Surface of molten metal is raised higher as frequency becomes lower.

So, agitation of molten metal occurs stronger in low-frequency furnace than in high-

frequency. This effect of agitation makes it possible to ensure uniform temperature of

molten metal and its uniform quality as well as to promote entrapment of material

charged and fusion of chemical composition adjusting agents, specially carbon

addition. In this respect, as compared with low-frequency furnace, high-frequency

furnace can be charged with larger electric power at the same agitation degree, which

will speed up the melting and improve the furnace heat efficiency because high-

frequency furnace can be operated with power density about three times larger than

low-frequency furnace.

17

To improve the heat efficiency in operating condition, the following should be

carried out as:

(a) Lower tapping temperature

To keep the tapping temperature lower, it is necessary to take care

throughout measurement such as ladle traveling distance and preheating and

covering of ladle.

(b) Close furnace cover

In practice of furnace operation, especially in case of small-sized

furnace, furnace cover sometimes remains open carelessly. It is important to

train personnel and make necessary preparation so as to charge materials and

adjusting agent regulator as quick as possible.

(c) Required temperature and duration for melting metal

Molten metal should be held, when required, at low temperature, or

turn off power supply. Preparatory operations should certainly be performed

so that there is no unmatching with mold assembly or waiting for crane.

(d) Dust collecting hood

Dust collecting degree and time should be controlled according to

furnace running conditions.

(e) Clean of sand, rust and other dirts

Sand or rust adhered to cast iron or steel scrap may react with furnace

refractory to form slags. Power loss at 1500C is about 10 kWh/ton if slags are

formed about 1 percent in melting of 3 tons iron.

2.6. Water Cooling System

In coreless induction melting systems, water is vital to the success of a

complete operating system. It needs the high quality water to maximize system

reliability and component longevity for the cooling of power supplies and furnaces. In

a coreless induction furnace much of the heat loss by the metal passes through the

furnace lining. Heat is also generated in the power coil or induction coil itself by the

passage of current. To prevent damage and overheat to the coil it must be water

cooled. A sample of the cooling water passing through inside the thick-walled copper

tubing is shown in Figure 2.8.

18

Figure 2.8. A Sample Induction Coil with Cooling Water

Figure 2.9. Sample of the Damaging Induction Coil

Flow velocity and monitoring of all water circuit should be considered for the

cooling of induction coil. Bailey [10] recommended that all cooling-passages should

be designed so that the flow velocity is not less than 1 meter per second, to prevent

any suspended solids settling-out in the system. All complete water circuits should be

designed so that the flow can be monitored, either by open-ended pipes or by

instrument indication. Monitoring with instrument indication may be expensive, but

accuracy is good and reliable for the whole system. Temperature should also be

monitored at each outlet. Flow switches should be provided at each outlet to trip out

the furnace power supply in the event of a failure. The over flow-bucket types are

preferred in an open system.

If the cooling water cannot be sufficiently provided to the induction coil and

the necessary components in some installations such as the frequency-conversion

equipment, the power cables, the control panel and the capacitors, the coil may be

damaged and exploded to the surrounding where the employees will be working

19

inside the foundry shop. Simultaneously, it will affect the productivity, the mental and

physical power of workers and all works of industry. A sample of the damage of

induction coil is shown in Figure 2.9.

Sometimes, it may also be necessary in some installations to cool the water in

the frequency-conversion equipment, the capacitors and power cables. In channel

furnaces the coil and the inductor casing are usually water-cooled. The cooling water

supply temperature should not be below 25C, to prevent condensation on the cooled

components. The upper limit of water temperature leaving the coil should be no more

than 70C, and that from the capacitors and frequency-conversion equipment should

not exceed the value specified by the manufacturers. If too cold water is allowed to

return to the system (cold temperature is defined as water temperature lower than the

ambient air temperature), condensation will then form on the electrical parts and the

coil. The life expectancy of these components is related to their operating temperature

and maintenance.

There are various types of cooling system to support the induction coil,

frequency- conversion equipment, the capacitors and the control panel. They are

installed and constructed in many foundry shops according to the requirements of

installation space, the annual operating costs, the furnace sizes and capacities, and the

environmental conditions, and the area of the industry. The types of water cooling

system used in most of the application for coreless induction melting systems will be

described in section 2.7.

2.6.1. Water Requirements

The quality and quantity of water required to cool a coreless induction melting

system should be specified in the equipment manufacturers literature or quotation. If

a new coreless induction melting system is proposed to be installed in an existing

facility with established plumbing in place, several design factors relating to water

flow and pressure must be considered. Additional water supply must exist within the

plant. Then, there is adequate flow and pressure to satisfy the equipment

manufacturers specifications. The present water quality characteristics do meet the

specifications of the induction furnace manufacturer. The addition of the new system

will affect flow and pressure to the existing and new system may be required. If a new

line is required, it should be designed to eliminate friction losses along with assuring

that there is an adequate supply of emergency water.

20

2.6.2. Effects of Water Quality

There are three detrimental effects of poor water quality in melting equipment

cooling paths are:

(a) The reduction in the ability to transfer heat that leads to subsequent damage to

the components from overheating.

(b) Electrochemical corrosion of tubing.

(c) Degradation of the electrical performance of the melting equipment due to the

water having too high an electrical conductivity.

All of these effects are directly related to impurities in the water scale

formation, fouling due to products of corrosion or fouling due to biological growth.

When this fouling does occur, in order to maintain the same heat transfer, the

temperature difference between the water and the component will increase. As the

fouling continues to build up, the temperature increases and the components fails.

This process is further aggravated by the reduction of water flow caused by the

reduction in the cross sectional area of the path.

Electrochemical corrosion is the deterioration of solids by liquid electrolytes.

In this case, the electrolyte is the contaminated cooling water, which attacks metal

components in the system. Under severe corrosion conditions the components can

corrode or rust in less than a year time. High electrical conductivity is directly related

to the amount of dissolved solids in the water. The resulting problems are the

distortion of the electrical control signals to solid-state devices and the desensitizing

of the ground detector circuits.

2.6.3. Water Purification/Maintenance

The highly de-ionized water has very corrosive properties and it can cause

damage to the induction coils. Corrosion of iron in the piping can add enough iron in

suspension to affect conductivity. Therefore, the newer water systems will usually

include a de-ionizer to main the conductivity of the water at acceptable levels. The de-

ionizers are used to maintain a water conducting level of 50 micromhos/cm or lower.

It is generally accepted that an operational water conductivity range of 100 to 300

micromhos/cm is adequate for operation for the water system.

In a closed water system if the water is not changed periodically a microscopic

organism will develop. This organism will attack the copper surfaces of the water

system and if not addressed will eventually lead to water leaks throughout the system.

21

By removing a hose on the furnace coil and inspecting the inside diameter of the

copper tubing it can be determined if there are microorganisms present. The inside of

the copper tubing will show a shiny black surface and will be very slipping.

Treatment for microscopic organisms can be done by draining the system of all water,

then acid wash the entire system with water. Then refill the system, making sure to

remove all of the entrapped air.

2.6.4. Filtration

Many filtration units have been used with high maintenance requirements. The

centrifugal separator, one of the filtration units, is used in water systems to remove

solids from liquids. Many advantages of using these devices are as follows:

1. No moving parts to wear out

2. No screens, cartridges, cones or filter elements to replace

3. No backwashing

4. No routine maintenance or downtime requirements

5. No standby requirement needs

6. Low and steady pressure loss

7. Easily automated

By removing the solids from the water, the life of the pumps can be extended,

fouling of cooling towers and heat exchangers can be virtually eliminated and allow

for optimum efficiencies.

2.6.5. Effects of Impurities

It is important that there are the effects of impurities in circulating water

system. Typical water impurities affect water quality. High water conductivity can

result in distortion of control signals and it can lead to corrosion of pipe nipples. If the

water is over saturated with calcium bicarbonate, calcium carbonate will form on the

piping interior. This deposited scale will restrict water flow and decrease heat transfer.

The suspended solids can also accumulate in equipment, particularly at low points,

causing clogging and reducing heat transfer. Suspended solids in makeup and

circulating water can be removed by either filtration or centrifugal separation.

Water that contains a high amount of free mineral acid is required. Acidity is

evidenced by effervescence when in contact with carbonate. This makes the water

very corrosive. The measure of pH of a solution is a measure of acidity of the solution.

22

Acid solutions have a pH of less than 7. Other effects of impurities are alkalinity,

slime and algae biological fouling, and dissolving oxygen and corrosion. If the

alkalinity is determined to be in excess, treatment of water with acid may be

necessary. Slime and algae biological fouling can offer and occur in once through and

open circulating systems. It is formed by the excessive growth or accumulation of

lower forms of plant life. Chemical treatment, usually chlorine, may be used for

control of these growths to avoid loss in heat transfer and to minimize biological

fouling on metal surfaces.

Dissolving oxygen and corrosion is accelerated by dissolved gases such as

oxygen, ammonia, carbon dioxide or sulfur dioxide, dissolved solids and high

temperature. The gases mentioned cannot be removed by mechanical means because

they tend to ionize in the water. The life of electrical conducting components in

induction systems relies heavily on the quality of the water supplied by the water

system. Nevertheless, the selection of a high quality cooling system for coreless

induction melting systems is of prime importance.

2.6.6. Emergency Water Supply and Cooling System

In all coreless induction furnace systems, a source for emergency water must

be used to supply cooling water to the furnace during times when the water system

loses power or has a pump failure. Many water systems are provided with a standby

pump in case of primary pump failure; but in a case where there is a power outage and

the recirculating pumps cannot be run, an emergency water system is the only

alternate source for cooling water. This is due to the fact that both the molten metal in

the furnace and the refractory system have significant amount of stored energy that

must be removed through the recirculating water at all times. Energy transfer to

unrecirculated water in the coil will cause the temperature of the water contained

within it to rise. The temperature will continue to elevate until the water turns to

steam where it will expand in volume.

Since the water is closed, the pressure in the coil will increase until hoses blow

off of the coil and all of the water contained within will be expelled. At this point

there is nothing to remove the stored energy in the furnace and it will transfer to the

coil and raise its temperature to that exceeding the ratings of materials in contact with

it. This will result in a significant expense to the foundry as regards to equipment

damage as well as loss of production due to loss of service of the equipment. In this

23

situation, if possible, there should be a procedure to empty the furnace immediately of

molten metal, thereby eliminating the largest amount of the stored energy that needs

to be removed.

The emergency cooling system should be provided to cool the furnace coil in

the event of power failure. The emergency water should be gravity-fed from a high-

level storage tank, supplied from the mains, and connected directly to the furnace coil

via a check valve that should be opened automatically when the pressure in the

normal, pumped supply falls. The emergency water will flow through the coil to the

buffer tank, and then to the drain through an overflow pipe.

2.7. Types of Cooling Water System for Electric Induction Furnace

Various types of cooling water system for electric induction furnace are as

follows:

1. Cooling pond system

2. Spray pond system

3. Evaporative cooling tower-open circuit system

4. Fan-radiator closed-circuit system

5. Water/water heat-exchanger system

6. Dual system with closed-circuit cooling tower

2.7.1. Cooling Pond System

Cooling pond system is one of the cooling systems of induction furnace

melting. When large ground areas are available, cooling ponds offer a satisfactory

method of removing heat from water. A pond may be constructed at a relatively small

investment by pushing up on earth dike 1.8 to 3.1 m (6 to 10 ft) high. For a successful

pond installation, the soil must be reasonably impervious, and location in a flat area is

desirable. Typical sketch of cooling pond is shown in Figure 2.10.

Hot water inlet Cool water outlet

Water surface

Pond

Figure 2.10. Typical Sketch of Cooling Pond System

24

In many cases, the pond water must be treated with chlorine, thus it is more

economical to use an open loop for the treated water. Acceptable circulation rates

vary from hour by hour for a complete change of water. They should be considered to

resist the corrosive effects of the chlorine in the pond water and scaling or corrosion.

Four principal heat-transfer processes are involved in obtaining cooling from

an open pond. Heat is lost through evaporation, convection, and radiation and is

gained through solar radiation. The required pond area depends on the number of

degrees of cooling required and the net heat loss from each square foot of pond

surface.

2.7.2. Spray Pond System

The hot water from the induction coil needs to be cooled to the desirable

temperature before pumping it. The cooling process is carried out in spray ponds after

which the water is pumped back to the induction coils.

In spray ponds, the exchange of heat between the hot water and ambient air is

performed by conduction process between the fine droplets of water and the

surrounding air. The efficiency of the system is mainly dependent on the relative

humidity of the air. Due to loss of water from the pond, fresh water makes up system

operating on pond level is required.

Spray ponds provide an arrangement for lowering the temperature of water by

evaporative cooling and, in so doing, greatly reduce the cooling area required in

comparison with a cooling pond. A spray pond uses a number of nozzles which spray

water into contact with the surrounding air. A well-designed spray nozzle should

provide fine water drops but should not produce a mist which would be carried off as

excessive drift loss.

The pond should be placed with its long axis at right angles to the prevailing

summer wind. A long, narrow pond is more effective than a square one, so that

decreasing pond width and increasing pond length will improve performance.

Performance can also be improved by decreasing the amount of water sprayed per

unit of pond area, increasing the height and fineness of spray drops, and increasing

nozzle height above the basin sides.

A typical spray pond system with evaporative cooling, which is by far the

most effective factor, is shown is Figure 2.11.

25

Figure 2.11. Sample Spray Pond System

2.7.3. Evaporative Cooling Tower-Open Circuit System

An induction furnace requires a great quality of cooling water, so a

recirculating system should be used to conserve water and save cost. In this system,

water from the furnace coil and, if necessary, the other ancillaries cascades through

the splash matrix of an evaporative cooling tower are cooled by a counter-current of

air supplied by a fan. The water gravitates to a sump, from which it is pumped

through the coil and other circuits before being returned to the tower via a buffer tank.

Simplified schematic arrangement of this system is shown in Figure 2.12.

Figure 2.12. Open-Circuit System with Evaporative Cooling Tower

This type of system has advantages and disadvantages as follow:

Advantages

- Simplicity.

- Low capital cost.

26

- Cooling water with the ambient wet-bulb temperature.

Disadvantages

- Water is lost by evaporation, so that solids dissolved in the system concentrate

and cause electrical conductivity problems.

- Airborne dust and impurities are drawn into the tower and cause corrosion and

fouling problems.

- If the make-up water is hard, scaling can result, reducing heat transfer and

even causing total blockage.

- Cooling towers are temperature and humidity dependent; in conditions of high

temperature and high humidity their efficiency will be decreased.

2.7.4. Fan-Radiator Closed-Circuit System

This system provides an essentially closed-circuit system which prevents

entrainment of dust particles and other atmospheric pollutants. It consists of a heat

exchanger in the form of a fan-blown radiator, a circulating pump, and a buffer tank

to allow for expansion. Schematic diagram of fan-radiator (closed-circuit) system is

shown in Figure 2.13.

Figure 2.13. Fan-Radiator Closed-Circuit System

Advantages and disadvantages in this system are as follow:

Advantages

- Water circuit can be made completely enclosed.

- Loss of water is slight, so expense for water is lower than in evaporative

towers.

27

Disadvantages

- Radiators are large for a given thermal duty.

- Radiator fins are subject to blockage by atmospheric dust, and may be difficult

to clean.

- Radiators are ambient temperature dependent and are less effective in warm

ambient conditions.

2.7.5. Water/Water Heat Exchanger Dual System

This system is shown in Figure 2.14. It consists of two circuits: primary open

circuit and secondary closed-circuit.

1. Primary open circuit _ with cooling tower, circulating-pump and heat

exchanger.

2. Secondary closed circuit _ with furnace coil and other circuits, buffer tank and

circulating-pump.

Figure 2.14. Dual System with Water/Water Heat Exchanger

The primary system supplied cooled water at near ambient temperature to the

heat exchanger, where heat is removed from the secondary circuit and returns to the

cooling tower. The secondary circuit carries heat away from all furnace circuits to a

buffer tank, from which the water is pumped back through the heat exchanger. Its

advantages and disadvantages are as follows:

Advantages

- The water/water heat exchanger is more compact and easier to clean and

maintain than the fan-radiator system.

28

Disadvantages

- A primary source of cooling-water is required.

2.7.6. Dual System with Closed-Circuit Cooling Tower

In this arrangement, the splash system of the normal evaporative cooler is

replaced by a tube bundle, through which the furnace cooling-water is circulated. The

primary water trickles over the bundle against the flow of air provided by a fan, and

so it is cooled at the same time as heat is transferred from the secondary water to the

primary water. Schematic arrangement of this system is shown in Figure 2.15. Its

advantages and disadvantages are as follows:

Advantages

- Water/water heat exchanger is eliminated.

- Piping and pumping costs are lower than in conventional tower with heat

exchanger.

Disadvantages

- Slightly more expensive than conventional tower with heat-exchanger.

Figure 2.15. Dual System with Closed-Circuit Cooling Tower

2.8. Selection of Cooling System

It depends upon:

1. Furnace size

2. Furnace environment

3. Local water board regulations

4. Nature of water supply available

29

5. Local noise-control requirement, particularly at night

6. Cost

To eliminate noise level in a furnace environment, cooling pond system gives

a satisfactory solution. This system reduces the maintenance costs compared with

other types of cooling system. Although it is suitable for small furnaces, the space

available in foundry for pond surface area becomes the major factor for the larger

furnaces. For small furnaces, it is often more economical to use a sample, open

recirculating system with a cooling-tower. For larger furnaces, a fan-radiator system

or dual system with a water/water heat exchanger is preferable. Fan radiators should

not be used in a dusty environment, or where noise is likely to be nuisance,

particularly at night. Noise can be reduced by installing fans at ground level, wherever

possible, and by using foundry buildings to screen the noise. A closed-circuit cooling-

tower may be useful for larger furnace, where it could be smaller than the normal

tower in a dual system.

30

CHAPTER 3

FLOW CALCULATION AND PUMP SELECTION

As the flow velocity of induction coil (power coil) and the feasible pump of

pumping the water sufficiently are the important factors, the considerations and

calculations based on these factors are solved analytically by using the solution

procedures. To obtain the prefect flow rates, pump selection should be carried out for

the cooling system. The required flow rate and pump for 0.16 ton coreless induction

furnace are focused in this chapter by using the equation of heat transfer and fluid

mechanics.

3.1. Consideration of Flow Velocity

To consider the flow velocity inside the induction coil, there are two portions:

heat transfer due to the effect of heat generated by the alternating current and

transferred through the refractory lining from molten metal and heat carrying from

fluid flow due to the pumping device. Before considering the flow velocity of the

induction coil, the internal structure of 0.16 ton coreless induction furnace is shown in

Figure 3.1.

Trunion Shell Molten metal Pouring spout

Refractory cement

Crucible

Copper induction coils

Rammed refractory

Tilting bail

Power leads Water cooling hoses

Stand

Figure 3.1. Internal View of 0.16 ton Coreless Induction Furnace

31

Firstly, the temperature of molten metal in the crucible is approximately about

1,600C according to the melting points of various types of metal. This crucible is

made up of silica lining, which is surrounded by an asbestos sheet, which is again

surrounded by an asbestos cloth. Heat from molten metal passes through the silica

lining, asbestos sheet and asbestos cloth, and then it conducts to induction coil. The

temperature of coil will be maintained at about 78C because of the effect of cooling

water and the high flow velocity.

In accordance with the temperature of molten metal in the crucible, the flow

velocity of induction coil is considered for the cooling system. It should be selected

for the suitable pump corresponding to the designative flow velocity. Flow velocity

may affect not only the service life of high conductivity copper coil but also overall

system of furnace. It is also the main point among the most important design

parameters. Nevertheless, the flow velocity for all cooling passages, especially the

induction coil, should be designed more than 1 meter per second that had been met as

described in the aforementioned chapter.

3.1.1. Specifications of Induction Coil

The design of induction coil is typically manufactured with a copper tube

wound with a carefully selected tubing profile and number of turns on the coil to

match the melting process into the power supply used. It may be either flattened,

round, or elongated vertically [11]. The round section allows the large water passages

within the coil and assures maximum water circulation together with efficient cooling,

but the flatted section permits a higher input per unit of coil height.

The use of heavy copper tubing prevents coil distortion when the coil is

positioned and clamped immovably inside the casing. The power for the coil is carried

in flexible water cooled leads which can be connected either left hand or right hand

side of the coil.

One of the induction coils recommended by low power transmission resistance

is produced from copper material for 0.16 ton induction furnace made in Russia. The

specifications of induction coil concerning with the physical and electrical parameters

are described in Table 3.1. The electrical parameters such as input power, rated

voltage and frequency may be varied throughout the melting and pouring time. The

maximum possible ratings for the specifications of induction coil are also described in

Table 3.1.

32

Table 3.1. Specifications of Induction Coil

Physical Parameters Electrical Parameters

Material Copper Input power 95 kW

Coil outer

diameter 2.0828 cm Frequency 880 Hz

Coil inner

diameter 1.7018 cm AC current 1,500 A

Outer surface area 3.4071 cm2 Rated voltage 650 V

Inner surface area 2.2741 cm2 Water inlet temperature 28C (82.4F)

Number of turns 16 Water outlet temperature 54C

(129.2F)

Coil height 46.228 cm Water pressure 2 to 4 MPa

Total length 20.96 m Estimated melting time 1.56 hr

3.1.2. Effect of Electrical Resistance in Induction Coil

The electrical resistance due to the heat generating rate is formed inside the

induction coil itself while passing through the alternating current. It is a measure of

the degree to which a body or tubing opposes the passage of an electric current. The

electrical resistance of high conductivity copper tubing is similar to the hydraulic

resistance of a pipe and it varies directly with the length and inversely with the cross-

sectional area. This relation proposed by Loew [12] can be expressed as follow:

1

1

AlRDC = Equation 3.1

where, l = length of conductor in direction of current, cm

A1 = area of conductor normal to direction of current, cm2

1 = resistivity, cm The resistivity is also called specific resistance of conductor material which

depends upon the chemical and physical properties and measured in micro ohm-

centimeters and micro ohm-millimeters. Resistivity is always expressed as at the

standard temperature 20C (68F). When the resistivity of copper tubing is known, the

total resistance of its material may readily be computed from its dimension. The

electrical resistance of a pure metal is directly varied with the temperature, as

33

illustrated in Figure 3.2 for the case of copper, and its resistance would be reduced to

zero when the temperature reached -234.5C.

R2

Res

ista

nce

R1

Tc

0 t1 t2-234.5

Temperature, C

Figure 3.2. Variation of Resistance with the Temperature

Since the usual range of interest runs from perhaps 20C to a few hundred

degrees above zero, a straight-line law of variation may be assumed for the usual

condition. The resistance-temperature relationship is apparent from Figure 3.2 that the

rule of similar triangles may be applied to find the resistance R2 of copper tubing at

any temperature t2, if the resistance R1 at some other temperature t1, and the

temperature intercept Tc of the conductor material of copper are known.

From similar triangle, 1

2

1

2

tTtT

RR

c

c

++=

1

212 tT

)t(TRR

c

c

++= Equation 3.2

In another consideration of this relationship, if the slope of the straight-line

portion of the curve in Figure 3.2 is designed as m, the equation from analytic

geometry may be written as follows:

)tm(tRR 1212 +=

where 1

1

tTRm

c += and therefore

++=++= )t(ttTR)t(ttTRRR

cc12

1112

1

112

11

34

The fraction 1/ (Tc+t1) is usually considered as and is called the

temperature coefficient of resistance. Ultimately, the relationship of resistance

variation in a copper metal with temperature is shown as follow:

1

)]t(t[RR 12112 1 += Equation 3.3 Because the temperature Tc for copper is 234.5C the temperature coefficient

of resistance can be described as

1

1 52341

t. += Equation 3.4

On the other hand, the calculation formulas of the electrical resistance for

various conductor materials can be seen in electrical handbooks.

3.1.3. Heat Generation Rate Calculation

In the flow velocity consideration, the generation of heat in induction coil with

respect to the electrical resistance is one of the important factors. Paschkis and

Persson [13] studied the common feature in induction heating in which heat

generation is always localized, whereas in dielectric heating the generation of heat

may be uniform. The locality of temperature in the induction coil can be approached

by heat generation rate according to the supplied power, rated voltage and the usage

of frequency. Table 3.1 given by the specifications of induction coil for 0.16 ton

melting capacity will be used for the calculation of heat generation rate.

The area of copper tubing is

22222

1 cm 1325147018108282

4.)..()D(DA io ===

The resistivity of copper at 20C from Marks [14], cm 711 . = By using the Equation 3.1, the DC resistance inside the coil is computed as

003

Documents

Design and Construction of an Induction Furnace (Cooling Sys