Design and Construction of an Induction Furnace (Cooling Sys
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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
YANGON TECHNOLOGICAL UNIVERSITY DEPARTMENT OF MECHANICAL ENGINEERING
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
YANGON TECHNOLOGICAL UNIVERSITY DEPARTMENT OF MECHANICAL ENGINEERING
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 78ºC. 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
CHAPTER 1
2
ix
TITLE INTRODUCTION
1
1.1 Objective
2
1.2 Outline of Thesis
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
3
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
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 3.2 Pump Selection
4
5
38 39
3.2.1 Essential Parameters Required in Selection
40
3.2.2 Selection Procedures
40
3.2.3 Calculations for Pump Selection
44
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
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
6
7
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
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
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, ∆T v / ∆To 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.
107
Comparison of Different Temperature Ranges at Constant Water Quantity
6.4.
Enthalpy-Temperature Diagram of Air and Water
108
by Changing L/G Ratio 6.5.
Enthalpy-Temperature Diagram of Air and Water
109
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 2.1. Induction Furnace Categories
Page 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
f (W1 ), f (W2 )
wind speed function for analytical, and empirical, respectively
Fo′
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/mºC
kr
water retention rate, m/min
kT
thermal rate, min-1
K
heat exchange coefficient, W/m2ºC
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, %
R1 , R DC1
resistance at temperature t1, and at temperature 20ºC, respectively, Ω
R2 , R DC2
resistance at temperature t2, and at temperature 60ºC, respectively, Ω
S
heat transfer surface, m2
S
monthly average of the sunshine hours per day at the location, hr
So
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, ºC
tc
coil thickness, cm
T
temperature, ºC
∆Tv
average temperature difference between the surface and bottom of the pond, ºC
∆To
temperature difference between the surface and the bottom of the pond, ºC
Ti ∗
normalized intake temperature, ºC
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
α1
temperature coefficient of resistance
αE
kinetic energy coefficient
β
coefficient of thermal expansion
β∗
proportional factor
cˆ
specific heat of water, J/kgºC
∈
roughness, mm
υ
kinematic viscosity, m2/min
ρ
water density, kg/m3
ρ1
resistivity, µΩcm
φ
latitude of the location, degree
φn
net solar heat flux, W/m2
φ sn
net solar (short-wave) radiation, W/m2
φ an
net atmospheric (long-wave) radiation, W/m2
φbr
back (long-wave) radiation, W/m2
φe
evaporative heat flux, W/m2
φc
conductive heat flux, W/m2
φs
solar radiation at water surface, W/m2
φ sr
reflected solar radiation, W/m2
φa
atmospheric (long-wave) radiation, W/m2
φ ar
reflected atmospheric radiation, W/m2
φ sc
extraterrestrial solar radiation, kJ/m2. day
φ sc
clear sky solar radiation, kJ/m2.day
ωs
sunset or sunrise angle, degree
δ
declination angle, degree
o
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 production efficiency is high. They are also capable of melting a higher fraction of alloy scraps. There are two main types of arc furnace, the direct arc and the indirect arc, as shown in Figure 2.1.
Power lead Carbon electrodes
Door Spout
Slag Metal Rammed hearth
(a) Direct Arc Furnace
Water-cooled roof Upper electrode (cathode)
Water-cooled panel
Eccentric bottom taphole
Bottom electrode (anode) Tilting 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 bottomtapping 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 semidrum 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 lid Cover plate
Upper case lining Back-up castable Insulating brick
Pouring spout Furnace platform Upper case assembly Upper case hearth Throat Blasch inductor lining Transformer Back-up castable Coil core Hydraulic cylinder Bushing Inductor assembly
Figure 2.3. Pictorial Diagram of Channel Induction Furnace
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 electricresistance 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
Molten metal layer Molten slag
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.
Metallic Charge Water-Cooled Induction Coils Molten Metal Heel
Heel Melter
Refractory Lined Steel Shell
Batch 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 batchmelting 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 Joule’s 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 1,538ºC. 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
Electric Arc
Electric-Resistance
[106 Btu/tona]
[106 Btu/tona]
Furnace [106 Btu/tona]
520 – 800b [5.0 – 7.6]
500 – 600 [4.3 – 5.2]
600 – 825 [5.2 – 7.9]
500 – 550c [4.3 – 4.8] 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 100% Transformer (1)
Input 100% Water-cooled Transformer cable (1) (1.5)
Inverter (4)
Coil (16)
Bus bar
Slag, etc. (1.5)
condenser
(2) Total efficiency 69%
(a) Distribution of losses in high-frequency of furnace. Heat loss (%) is given in ( ).
Heat conduction (3) Heat radiation (2)
Water-cooled cable (1.5)
Bus bar condenser (2)
Coil (17)
Total efficiency 67%
Heat conduction (7) Heat radiation (4.5)
(b) Distribution of losses in low-frequency furnace. Heat loss (%) is given in ( ).
Figure 2.7. Heat Balance Diagram of Crucible Type Induction Furnace Source: Energy Conservation in Iron Casting Industry (1998)
In above figure, 100 percent of input energy is used fully in both of these furnaces; high-frequency and low-frequency crucible type furnace which have electrical and heat losses. Electrical losses consist of transformer, frequency converter, water-cooled condenser, bus bar, wiring, cable and coil. Loss in coil is an essential factor, on which the furnace capacity depends. Heat losses in induction 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 user’s 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 highfrequency. 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 highfrequency 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 1500ºC 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 25ºC, to prevent condensation on the cooled components. The upper limit of water temperature leaving the coil should be no more than 70ºC, 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 deionizers 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 highlevel 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 coolingtower 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 Molten metal
Shell Pouring spout
Refractory cement Crucible Copper induction coils Rammed refractory
Tilting bail
Water cooling hoses Power leads 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 1,600ºC 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 78ºC 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 Material
Electrical Parameters
Copper
Input power
95 kW
2.0828 cm
Frequency
880 Hz
1.7018 cm
AC current
1,500 A
Outer surface area
3.4071 cm2
Rated voltage
650 V
Inner surface area
2
2.2741 cm
Water inlet temperature
28ºC (82.4ºF)
Number of turns
16
Water outlet temperature
Coil height
46.228 cm
Water pressure
2 to 4 MPa
Total length
20.96 m
Estimated melting time
1.56 hr
Coil outer diameter Coil inner diameter
54ºC (129.2ºF)
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 crosssectional area. This relation proposed by Loew [12] can be expressed as follow: RDC =
ρ1l A1
Equation 3.1
where, l = length of conductor in direction of current, cm A1 = area of conductor normal to direction of current, cm2
ρ1 = resistivity, µΩcm The resistivity is also called specific resistance of conductor material which depends upon the chemical and physical properties and measured in micro ohmcentimeters and micro ohm-millimeters. Resistivity is always expressed as at the standard temperature 20ºC (68ºF). 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 -234.5ºC.
Resistance
R2 R1 Tc
0
-234.5
t1
t2
Temperature, ºC
Figure 3.2. Variation of Resistance with the Temperature Since the usual range of interest runs from perhaps 20ºC 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,
R2 Tc + t 2 = R1 Tc + t1 R2 =
R1(Tc + t 2 ) Tc + t1
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: R2 = R1 + m(t 2 − t1 )
where m =
R1 and therefore Tc + t1 R2 = R1 +
⎡ ⎤ R1 1 (t 2 − t1 ) = R1 ⎢1 + (t 2 − t1 )⎥ Tc + t1 ⎣ Tc + t1 ⎦
34 The fraction 1/ (Tc+t1) is usually considered as α1 and is called the temperature coefficient of resistance. Ultimately, the relationship of resistance variation in a copper metal with temperature is shown as follow: R2 = R1 [ 1 + α1(t 2 − t1 )]
Equation 3.3
Because the temperature Tc for copper is 234.5ºC the temperature coefficient of resistance can be described as
α1 =
1 234.5 + t1
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 π(Do2 − Di2 ) π( 2.0828 2 − 1.7018 2 ) = = 1.1325 cm 2 4 4
A1 =
The resistivity of copper at 20ºC from Marks [14], ρ1 = 1.7 µΩcm By using the Equation 3.1, the DC resistance inside the coil is computed as R DC1 =
ρ1l 1.72 × 2096 = = 0.003183 Ω A1 1.1325
From the Equation 3.4, the temperature coefficient of resistance is calculated at t1 = 20ºC, 1 234.5 + 20 = 0.00393
α1 =
35 Another electrical resistance with the Equation 3.3 is calculated by using the induction coil temperature, t2 = 60ºC which is maintained by the effect of cooling water. This temperature measures from the operating condition of induction furnace. R DC2 = R DC1 [ 1 + α1(t 2 − t1 )] = 0.003183[ 1 + 0.00393( 60 − 20 )] = 0.00368
AC resistance of induction coil RAC is taken from R AC /R DC ratio that is read from the curve for skin effect and proximity effect. The skin effect is the phenomenon where the apparent resistance of copper tubing increases as the frequency increases. For round copper tubing, the Figure B.1 of the Appendix taken from Dwight [15] plotting the root of the ratio, frequency: DC resistance (ohm per 1,000 ft), versus RAC/RDC can be easily given to determine the AC resistance RAC. To compute AC resistance of copper tubing, the ratio of coil thickness divided by outer diameter tc/d must be known: t c 0.1905 = = 0.09146 d 2.0828
Based on DC resistance (ohm per 1,000 ft), RDC2 Ω
0.00368 Ω × 1000 ft × 0.3048 m 1000 ft 20.96 m = 0.0535 Ω =
f ⎛ RDC2 ⎞ ⎜ ⎟ ⎜ 1000 ⎟ ⎝ ⎠
=
880 0.0535
= 128
From Figure B.1 of the Appendix, R AC = 1.04 RDC R Ac = 1.04 × (RDC2 ) = 1.04(0.00368) ∴ R AC = 0.0038272 Ω Finally, power losses or heat generation rate of induction coil due to the effect of electrical resistance can be expressed as follows: Pg = I 2 R AC = ( 1500 2 ) × 0.0038272 = 8.61123 kW
36 3.1.4. Calculation of Heat Transfer Rate in Composite Refractory Shells In this calculation, a system with cylindrical symmetry and having heat conduction only in the radial direction is considered as this situation occurs in refractory shell in electric furnace. From the point of view of steady state heat conduction, the heat transfer rate through the refractory lining from the molten metal can be solved by using the prescribed physical conditions of induction coil. The schematic diagram of temperature distribution for a composite refractory cylindrical shell is shown in Figure 3.3.
ln( r2 / r1 ) ln( r3 / r2 ) ln( r4 / r3 ) 2πk A L 2πk B L 2πk C L
Figure 3.3. Temperature Distribution for a Composite Refractory Cylindrical Shell Thermal insulations in refractory shell of furnace comprises low thermal conductivity materials such as silica lining, asbestos sheet and asbestos cloth combined to achieve an even lower system thermal conductivity. The radii at various interfaces should be defined as - r1, r2, r3, the temperatures at these interfaces - Ts,1, Ts,2, Ts,3, Ts,4 and the conductivities of these materials - kA, kB, kC, respectively. The heat transfer rate of the surface of induction coil is considered at maximum melting point. In solving these heat transfer rates, all pertinent simplifying assumptions are carefully listed as follows:
37 1. Steady state conduction. 2. One-dimensional heat transfer by conduction across the cylindrical walls. 3. Using pure metals with the maximum point (1,600ºC). 4. Supplying the induction coil with the electrical current 1,500 A, AC voltage 650 V, power 95 kW and frequency 880 Hz. 5. Constant properties. 6. Neglecting the interfacial contact resistance. 7. Neglecting the radiation heat transfer. As the heat flow through the composite refractory shells is considered to be under steady state, whatever heat enters into a layer at one end must also leave it at the other. In the calculation of heat transfer, the values for the thermal conductivity of asbestos sheet, asbestos cloth and silica lining have been read from Phelps [16], Marks [14], and Kern [17]. From Figure 3.3 together with the thermal equivalent circuit, the temperature at one side of induction coil is available at 74ºC measuring the practical result and the other side is potentially maintained 60ºC from the practical measurement owing to the cooling water. Fourier's law expressed in Incropera and DeWitt [18] is used for the heat transfer rate as follow:
qr =
Ts,1 − Ts,2 Ts,2 − Ts,3 Ts,3 − Ts,4 = = ln (r2 /r1 ) ln (r3 /r2 ) ln (r4 /r3 ) 2πk A L1 2πk B L1 2πk C L1
Equation 3.5
By substituting the desired values: three different types of thermal conductivity and length and various radii in Equation 3.5, the heat transfer rate will be given by
qr =
1600 − 74 ln(0.193336/0.12) ln(0.194836/0.193336) ln(0.1973336/0.194836) + + 2π (7.2663)(0.46228) 2π (5.5189)(0.46228) 2π (1.5116)(0.46228)
According to the calculation, q r = 58.7354 kW
Finally, the heat transfer rate passing through the refractory lining and insulation materials has been obtained as the amount of heat flowing. Using the result of heat transfer rate, the flow velocity inside the induction coil will be determined in the next section.
38 3.1.5. Flow Velocity Designation In the previous subsections, the heat transfer rates have been computed by applying the effect of electrical resistance in induction coil and steady state heat conduction equation for composite refractory cylindrical shell. Now, the total heat transfer rate will be considered for the designation of flow velocity circulated in induction coil or power coil. The total heat transfer rate can be expressed as follows: Total Heat Transfer Rate = Heat Generation Rate + Heat Transfer Rate and substituting the required value from the previous subsections, it has been obtained as follows: Qt = (8.6112 + 58.7357) kW = 67.3469 kW
In case the analytical form of the total heat transfer rate, Qt is for 0.16 ton induction furnace system, heat flow and generating heat are not uniform throughout the operating condition. Both of these are dependent upon the frequency, supplied power and current, rated voltage given by operator, and thermal conductivity of insulation materials changing from the temperature. Assuming the maximum possible condition in this furnace, it will be continued to solve the flow velocity. According to the practical measuring result, the inlet temperature or cool water temperature into the tubing is about 28ºC and the outlet temperature or hot water temperature from the tubing is about 54ºC at the highest melting conduction in induction furnace. In order to take into account of the flow velocity of induction coil, Newton's law of cooling will be used for the heat exchange. Accordingly, Newton's law of cooling for incompressible fluid presented in Incropera and Dewitt [18] may be expressed as
Qt = m&C p (Tm,o − Tm,i )
Equation 3.6
where, Qt = total heat transfer rate or quantity of heat or heat exchange rate, kW m& = the mass flow rate, kg/s
Cp = specific heat at constant pressure, 4.179 kJ/kgºK Tm,o = the leaving temperature of water from the coil, ºC and Tm,i = the entering temperature of water into the coil, ºC Hence, using Equation 3.6 and putting in the desired values, the mass flow rate is
39
m&=
Qt 67.3469 kW = C p (Tm,o − Tm,i ) 4197 J/kg οK (54 − 28) ο C
= 0.6172 kg/s From Mott [19], the mass flow rate, m& is related to the volume flow rate, Qv by
m&= ρQv
Equation 3.7
where ρ = the density of the fluid, 986.4 kg/m3 at 54ºC
The volume flow rate, Qv can be driven as follows:
Qv = Ai vi
Equation 3.8
where, Ai = the inner area of the tubing, m2
vi = the average velocity of flow, m/s The inner area of the induction coil is calculated as:
Ai =
πDi2 π × 0.017018 2 = = 0.00022746 m 2 4 4
Using Equation 3.7 and Equation 3.8, the average velocity of flow can be solved as follows: vi =
m& 0.6172 kg/s = ρA i 986.4 kg/m 3 × 0.00022746 m 2
= 2.7507 m/s
Ultimately, the designation of flow velocity inside the induction coil can be defined satisfactorily as 2.7507 m/s.
3.2. Pump Selection
When selecting a pump for the application of cooling system in induction furnace, the possible factors must be considered to be economical and successful throughout the melting cycle. These factors involve the nature of liquid to be pumped, the required capacity to be sufficient, the total head on the pump and others. Especially, there are two pumps used in induction furnaces. For the induction coil to be circulated with cooled-water, one pump is operated continuously during the melting process. If the emergency cases such as power failure and unexpected error occur in the running condition, another pump or stand-by pump will be used not to be lack the cooled-water for the furnace ancillary and equipment. Hence the pumps are one of the equipments essential in cooling system.
40 During the pump selection process, the alternative pumping station layouts should be developed in sufficient detail so that the cost of pumps over the life of the project can be determined. The cost of pumps should include the capital cost and the operating costs which include cost of energy, maintenance, and replacement costs. It is usually best to consider all types of pumps when developing the pumping station layout unless it is obvious that certain ones are not applicable. The pump selection corresponding to the cooling system with the pipe line arrangements should be done in sufficient detail to follow the process without reference to additional catalogs or other such sources. 3.2.1. Essential Parameters Required in Selection The essential parameters required in selection of pumps are summarized as follows: 1. Number of units required. 2. The nature of the liquid to be pumped. 3. The required capacity (volume flow rate) as well as the minimum and maximum amount of liquid. 4. The conditions on the suction (inlet) side of the pump. 5. The conditions on the discharge (outlet) side of the pump. 6. The total head on the pump. 7. The continuous or intermittent service. 8. The type of system to which the pump is delivering the fluid. 9. The type of power source (electric motor, diesel engine and stream turbine) 10. Space, weight, and transportation limitations. 11. Location of installation. 12. Environmental conditions. 13. Special requirements of marked preferences with respect to the design, construction, or performance of the pump. 14. Cost of pump operation and pump purchase. 3.2.2. Selection Procedures There are various types of selection procedure to use a pump in pumping system. Chopey [20] gave a step-by-step procedure for choosing the class, type, capacity, drive and materials for a pump that will be used in an industrial pumping
41 system. Later, this procedure will be used for the pump selection in cooling system of induction furnace. Solution procedures for any pumping system are expressed as follows:
1. Sketch the proposed piping layout In the first procedure, both single-path and multiple-path diagram of the piping system should be sketched on the actual job conditions. Showing all the piping, fittings, valves, equipment, and other units in the system and marking the actual and equivalent pipe length on the sketch are involved in this procedure.
2. Determine the required capacity of the pump The required capacity is the flow rate that must be handled in gal/min, m3/min, or some similar measure. It has been obtained from the process conditions such as boiler feed rate, cooling water flow rate and chemical feed rate. The required flow rate for any process unit is usually given by the manufacturer. Once the required flow rate is determined, a suitable factor of safety is applied. Typical safety factors are in the 10 percent range.
3. Compute the total head on the pump The most common way of expressing the total head on a pump is the result in meter or feet of water. To compute the total head on the pump, the total static head; in meter or feet, must be considered from the pump piping arrangements with static suction lift and static discharge head. When both the suction and discharge surfaces are open to the atmosphere, the total static head equals the vertical difference in elevation. When the supply source is below the pump centerline, the vertical distance is called the static suction lift and otherwise, above the pump centerline, is called static suction head. The total static head, as described above, refers to the head on the pump without liquid flow. The friction losses in the piping system during liquid flow must also be considered to determine the total head on the pump. Thus, the actual total head on the pump is the sum of total static head and total friction-head loss.
4. Analyze the liquid condition The liquid conditions on a pump-selection should include the name and chemical formula of the liquid, maximum and minimum pumping temperature,
42 corresponding vapor pressure at these temperatures, specific gravity, viscosity at the pumping temperature, pH, flash point, ignition temperature, unusual characteristics (such as tendency to foam, curd, crystallize, become gelatinous or tacky), solids content, type of solids and their size, and variation in the chemical analysis of the liquid. Such these data are available from many pump manufacturers or can be prepared to meet special job conditions.
5. Select the class and type of pump Three classes of pumps: centrifugal, rotary and reciprocation are generally used today. Each class of pump is further subdivided into a number of types as shown in Table 3.2. Table 3.2. Pump Classes and Types Class
Type • Volute
Centrifugal
Rotary
Reciprocating
(or) Radial-flow Diffuser or turbine pump • Regenerative - turbine • Vertical - turbine • Mixed - flow • Axial - flow (propeller) • Gear • Vane • Cam and piston • Screw • Lobe • Shuttle - block • Direct - acting • Power (including crank and flywheel) • Diaphragm • Rotary - piston •
Note that these terms apply only to the mechanics of moving the liquid – not to the service for which the pump was designed. A general guide to the characteristics of various classes and types of pumps to be used in industrial process is shown in Table C.5 of the Appendix. This table describes that a centrifugal pump would probably be best when a large capacity at moderate pressure is required. It is also needed to be considered all the operating factors related to the particular pump. These factors include the type of service (continuous or intermittent), operating-speed preferences, future load expected and its effect on pump head and capacity, maintenance facilities available, possibility of parallel or series hookup, and other conditions peculiar to a given job. Once the class and type of pump are selected, pump selector should be consulted and adjusted with a rating chart, as illustrated in Figure B.2. of the
43 Appendix, or a rating table to determine if a suitable pump is available from the manufacturer whose unit will be used. When the hydraulic requirements fall between two standard pump models, it is usual practice to choose the next larger size of pump, unless there is some reason why an exact head and capacity are required for the unit. Some pumps are constructed for custom-built of a given job when precise head and capacity requirements must be met. In the engineering information of manufacturer; the characteristics curves for various diameter impellers in the same casing, variablespeed head-capacity curves for an impeller of given diameter, and other supporting data are included to be satisfied for the customers. More completed selection procedures can be found in Church [21], Peerless [22], Dickinson [23], Walker [24], Stepanoff [25], and Hicks and Edwards [26].
6. Evaluate the pump chosen for the installation In evaluating the pump chosen for the installation, the specific speed of a centrifugal pump is important to classify the impellers on the basis of their performance. It should be checked by using one of the suitable methods. Once the specific speed is known, the impeller type and approximate operating efficiency can be found from Figure 3.4
Figure 3.4. Approximate Relative Impeller Shapes and Efficiency Variations for Various Specific Speeds of Centrifugal Pumps (Worthington Corp.) Source: Daily (1950)
44 .
Then, it must be considered to see if the available net positive suction head
(NPSHA) is equal, or greater than, the required net positive suction head (NPSHR) of the pump. In addition to the checking of net positive suction head (NPSH), horizontal or vertical design of pump may be determined. From the stand point of floor space occupied, required NPSH, priming, and flexibility in changing the pump use, vertical pumps may be preferable to horizontal design in some installations. But where headroom, corrosion, abrasion, and ease of maintenance are important factors, horizontal pumps may be preferable. Lastly, before making a final purchase decision, all above presented steps must be checked and determined to be successful for the pumping system. In the next section, the necessitated pump for cooling pond system used in 0.16 ton coreless induction furnace will be chosen correctly according to the selection procedures. 3.2.3. Calculations for Pump Selection In this section, the candidate pump in cooling pond system is calculated by using the selection procedures for the given application. Calculation mainly involves the determination of pumping capacity, total head, specific speed and net positive suction head. Many manufacturers and system engineers currently use computerized procedures to select a pump that is most suitable for each given application. Such procedures are simply automated versions of the traditional selection method. Nevertheless, the calculations for the selected pump must be carried out accurately and correctly to meet the satisfactory system.
(i) Sketching the Piping Layout Before selecting the pump, the piping layout diagram is sketched on the actual job condition. The functional layout diagram of 0.16 ton coreless induction furnace has been shown in Figure 3.5. The different diameters of pipe are used in the piping system. The representation of bend or curve on the piping arrangements is illustrated as fittings such as elbow, valve, sudden enlargement and sudden contraction.
(ii) Determining the Pumping Capacity Based on the designation of flow velocity inside the induction coil, the required flow rate for pumping is considered for 0.16 ton coreless induction furnace. 0.16 ton coreless induction furnace in Figure 3.5 consists of two induction furnaces.
45
Furnace No.2 Furnace No.1
Control Panel
Capacitor Bank
Pump Discharge Pipeline
Suction Pipeline Cooling Pond
Figure 3.5. Functional Layout Diagram of 0.16 ton Coreless Induction Furnace
B
Q2 QT
A
Q4 Induction Coil
Q5 Q6
Water Cooled Cable
C
Q7
D
Capacitor Bank
Q1 Q8
Induction Coil
Q3 Control Panel
E
Q9 Q10
Water Cooled Cable
Q11
E
Figure 3.6. Sketch of Flow Branches in Pipes
46 According to the calculation of flow velocity designation in previous section, the velocity 2.75 m/s is essentially needed for both furnace 1 and furnace 2 because the high velocity rate causes the effective cooling. Omitting the frequency-conversion equipment, induction furnaces, cooling pond, emergency pump and all return pipes, the connections of pipe lines are schematically shown in Figure 3.6. To determine the required pumping capacity, the pipe joint A, B, C, D, E and F are defined as node points. First of all, the pipe from the outlet of pump holding the total flow rate, QT is in contact with the pipe joint A and it separates into two branches, Q1 and Q2. Similarly another pipe joints are also to go out in different directions as shown in Figure 3.6. Cooling system of coreless induction furnace has been constructed by using the multiple-path system in the pipe line arrangement. As the multiple-path system with pipe branches, the pipe flow regime is determined by the starting point of joint E. Joint E is defined as the junction of the flow rate Q9 and Q10. Pipe network for joint A is shown schematically in the following Figure 3.7.
(a) Representation of Pipe System with Three Branches
(b) Illustration of Flow Direction
Figure 3.7. Pipe Network for Joint E The general energy equation to each branch recommended by Fox and McDonald [28] is applied. This equation for the representation of the mechanical energy per unit mass at a cross section expresses as follows: 0
0
0
0
p v2 v2 pE + α E E + gz E = 9 + α 9 9 + gz 9 + hL ρ ρ 2 2
47 The following assumptions are used in the above equation. 1. v E2
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