Inductive Melting and Holding Fundamentals Plants and Furnaces Process Engineering

Transcription

1 Erwin Dötsch Inductive Melting and Holding Fundamentals Plants and Furnaces Process Engineering

2 Inductive Melting and Holding

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4 III Erwin Dötsch Inductive Melting and Holding Fundamentals Plants and Furnaces Process Engineering VULKAN VERLAG

5 IV Bibliographic information published by Deutsche Nationalbibliothek Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available on the Internet at ISBN Vulkan-Verlag GmbH Huyssenallee 52-56, D Essen, Fed. Rep. of Germany Tel.: +49 (0) , Internet: All rights reserved. No part of this book covered by the copyrights hereon may be translated, reproduced or copied in any form or by any means graphic, electronic, or mechanical, including photocopying, taping, or information storage and retrieval systems without written permission of the publisher. The listing of utility names, trade names, descriptions of goods etc. without special marking or designation in this book does not allow the presumption that such names are considered free in the sense of the trademark and trademark protection laws and can, consequently, be used by any person. This book was prepared with much care and devotion. Nonetheless, authors and publishing company do not assume responsibility for the correctness of data, comments, suggestions and possible printing errors. Managing Editor: Dipl.-Ing. Stephan Schalm s.schalm@vulkan-verlag.de

6 FOREWORD V FOREWORD In the foreword to his book "Inductive melting", published in 1966 by Verlag W. Girardet, Essen, which was to become the standard work of its day, K.-H. Brokmeier (Dr.-Ing.) wrote "Inductive melting is an application in a state of rapid development". He was right: Much progress has been made in the field of induction engineering since that was written more than 40 years ago. The time has therefore come to present - in summary form - the current state-of-the-art of this technology, still young and open to new developments, as it is deployed in manufacturing and processing metal. This is what this book sets out to do. The work focuses mainly on the metallurgical processes involved in melting, holding and pouring using induction systems. The fundamentals of inductive power transmission and the design of induction plants are described to the extent necessary to understand the production processes. The same applies to the sections on melting metallurgy, the targeted deployment of which is made possible by inductive melting. Based on these deliberations, the chapters on operating the systems themselves should be understandable and useful both for interested newcomers and for "old hands" in this field. In conclusion, a few personal notes: My thanks go to my first boss in Dortmund, Mr Karl-Heinz Brokmeier, to whom I primarily owe my knowledge of the interplays described in this book and who made my first steps in industry easy after many years at university, to Mr Franz Neumann, for all he taught me about cast iron metallurgy and to our technical head of the furnace business in Dortmund for many years, Mr Fritz Hegewaldt, who encouraged my positive attitude towards the engineering profession and to whom I owe most of my technical knowledge. I would furthermore like to thank the General Manager of ABP Induction Systems GmbH, Mr Wolfgang Andree, who made it possible for me to write this book and, above all, who has given me the opportunity to remain in daily contact with the interesting discipline of inductive melting and holding. In this context, I would also like to thank all my colleagues at ABP for all the pleasure I have gained in working together with you over the years. Last but not least, thanks go to my wife, Dorothee, whose mental support and technical writing skills have been invaluable in composing this book. May the reader find it useful. Dortmund 2009 Dr.-Ing. Erwin Dötsch

7 VI PREFACE PREFACE Inductive melting and holding is a major area of application in electro-thermal processes and one of the best-earning, high-performance sectors for inductive applications. After the rapid growth of inductive melting based on line frequency engineering in the 1960s, the technology enjoyed renewed growth in the 1980s in the wake of the development of suitable medium-frequency energy supplies. The importance of induction melting in iron foundries, in steel works and in metal works for non-ferrous applications continues to rise rapidly throughout the world. Yet the development of heavy-duty induction melting furnaces, executed both as induction crucible furnaces and as induction channel furnaces, also increases the risks in the works if the furnaces are not constructed and operated properly. Modern plants must furthermore fulfil the latest requirements of the iron, steel and non-ferrous industries, these needing to be considered in the design and construction of furnaces. The "energy efficiency" of process heating plants nowadays presents a new challenge for induction furnaces, for which new ideas have to be developed. The specialist literature available on these topics is extremely limited or has already become rather outdated. The expert knowledge required for designing and constructing induction furnaces is today preserved in the heads of a handful of specialists. The majority of these, however, will be retiring from professional life shortly, if they have not done so already. Although the theories of inductive melting are still taught at some universities or are even being further researched there, the applied practice of design and construction in consideration of the different metallurgical applications continues to be rather uncharted territory. I therefore particularly welcome the author's initiative to fill this gap. In this work, he presents the expert knowledge, in detail and oriented to practice, that he has gained through his many years of experience in the field of induction melting, both in plant construction and in the various areas of application. The book will provide excellent support not only for numerous specialists in the trade, but also for many students in their daily studies, in research and development and in practical applications. I would like to thank Vulkan Verlag, the publishers who agreed to print and distribute this important, ground-breaking work, and also ABP Induction Systems GmbH, which made its compilation possible. I hope that all established and up-and-coming engineers and students working in this field will profit greatly from this book. Prof. Dr.-Ing. Bernard Nacke Managing Director of the Institute for Electroprocess Engineering at Leibniz University in Hannover

8 CONTENTS VII CONTENTS FOREWORD... V PREFACE... VI 1. INTRODUCTION FUNDAMENTALS Inductive Power Transmission Induction Furnace Structural Shapes Induction Crucible Furnaces Electrical Efficiency Field Line Bath Agitation Bath Meniscus Hight Bath Flow Induction Channel Furnace Efficiency Factor Electromagnetic Field and Melt Flow COMPONENTS OF A CRUCIBLE FURNACE PLANT Furnace Body Refractory Lining Quartzite Dry Masses Spinel-Forming Dry Masses Refractory Technology Coil and Transformer Yokes Tilting Frame and Furnace Cover Power Supply Line and Medium Frequency Connection Parallel and Series Resonant Converters Circuit Feedback Optimizing Power Utilization by Tandem 34 Arrangement Frequency Selection Electromagnetic Leakage Fields Peripherals Recooling Devices Furnace Cooling Circuit... 38

9 VIII CONTENTS Electrical System Cooling Circuit Frost Protection and Water Quality Waste heat utilization Charging Devices Skimming Devices Process Control Systems Overall Layout COMPONENTS OF CHANNEL FURNACES Furnace Vessel Design for Holding Design for dosed pouring Design for Melting Refractory Lining Inductors Structural shapes Refractory Lining Power Supply Line Frequency Power Supply Converter Power Supply Cooling Devices ELECTROMAGNETIC STIRRERS AND PUMPS FURNACE DESIGN AND ENERGY REQUIREMENTS Characteristic Data for Melting in Crucible Furnaces Furnace Power Consumption Specific Enthalpy Electrical Losses Thermal Losses Furnace Efficiency Design of Furnace Power Specific Power and Furnace Limit-Rating Frequency of the Coil Current Energy Requirements Electricity Price and Load Management Characteristic Data for Channel Furnaces Melting Non-Ferrous Metals Holding and Superheating of Iron Melts... 86

10 CONTENTS IX 7. MELTING METALLURGY OF IRON AND NON-IRON MATERIALS Cast Iron Iron-Carbon Diagram Cast Iron Types Oxygen Content of the Melt and C/Si Isotherms Inoculation and Nucleation Magnesium Treatment Sandwich Process Tundish Cover Process Converter Process (Georg-Fischer System) Wire Flushing Process Inmold Process Other Processes of Mg Treatment Desulfurization Monitoring the Melt Wedge Test Piece Thermal Analysis Cast Steel Aluminium Primary and Secondary Aluminium Cast and Wrought Alloys Quality of the Aluminium Melt Treatment of the Melt Copper Materials Copper Production Copper Types and Alloys Oxidizing and Reductive Melting OPERATION OF INDUCTION PLANTS IN IRON FOUNDRIES Melting in Induction Crucible Furnaces Process Engineering Features Induction Furnace Versus Cupola Furnace Line Frequency (LF) or Medium Frequency (MF) Crucible Furnaces Charge Materials Steel Scrap Pig Iron Return Material

11 X CONTENTS Cast Iron Scrap Chips Additives Producing the Melt Making up the Charge and Charging Adjustment of the Target Composition of the Melt Heating to Tapping Temperature Processor-Controlled Melting Run Deposit and Bridge Formation Extraction of Fumes Melting Zinc-Coated Scrap Melting Chips Charging Loose Chips Melting Chip Briquettes Melting Sponge Iron Production Characteristic Properties Melting Process Profitability Downtime Procedure Refractory Behavior Chemical Erosion Infiltration Via the Liquid or Vapor Phase Crucible Monitoring Duplicating, Holding and Combined Holding/Melting in the Crucible Furnace Holding in the Channel Furnace Integrating the Storage Furnace into the Production Cycle Characteristic Features of the Channel Furnace Method of Furnace Operation Commissioning Operational Procedures Automatic Process Monitoring Inductor Change Dealing with Disturbances Inductor Monitoring Wear Behavior of the Refractory Lining Erosion Infiltration Crack Formation Clogging Energy Consumption and Profitability

12 CONTENTS XI 8.4 Pouring with Pressure-Actuated Pouring Furnaces Variants and Features of the Teapot Pouring Device Use of Heated and Unheated Pouring Devices Pouring Mg Treated Melts Deposite Formation Caused by Suspended Oxides Magnesium Vaporization Magnesium Reactions with Metal Oxides and Sulfur Practical Experience Pouring with Intermediate Ladles Stopper for Automatic Pouring Pouring in a Regulated Closed-Circuit Integrated Inoculations Process Monitoring Continuous Supply of Molten Iron Melting Cycle in a Tandem Plant Plant Concepts for a Continuous Supply of Molten Iron Channel Furnace as Buffer Tandem Plant as Melting and Buffer Unit Pouring Furnace as Buffer Optimised Design of the Melting and Pouring Plant MELTING CAST STEEL IN INDUCTION CRUCIBLE FURNACES Frequency and Gas Absorption Adjusting the Target Composition INDUCTION CRUCIBLE FURNACES IN MINI STEEL WORKS Induction Furnaces Versus Electric Arc Furnaces Requrements on the Melting Operations Producing Different Types of Steel Structural Steel Alloyed Steels Example of an Induction Melting Plant to Produce Structual Steel Billets Production Figures Layout of the Induction Plant

13 XII CONTENTS 11. INDUCTION SYSTEMS IN THE ALUMINIUM INDUSTRY Crucible Furnaces in Foundries Metallurgical Process Engineering Benefits Duplicating in a Crucible Furnace Refractory Behavior Melting Aluminium Chips Melting in Works for Semi-Finished Product Shops Examples of Establihed Production Plants Behavior of the Inductor Refractory Lining INDUCTION PLANTS FOR COPPER MATERIALS Induction Furnaces in Copper Fondries Melting Pouring Copper Materials with Pressure-Actuated Pouring Furnaces Inductive Melting in Semi-Finished Product Shops Channel Furnaces with High-Performance Inductors Application of Crucible Furnaces Melting Brass Chips Recycling Cable Wastes Flexible Melting of Copper Alloys INDUCTION PLANT FOR MELTING ZINC Re-Melting Zinc Cathodes Producing Zinc Alloys Galvanized Strip LITERATURE INDEX

14 All you ever wanted to know on Refractory Materials REFRACTORY MATERIALS Design, Properties, Testing Gerald Routschka, Hartmut Wuthnow (Editors) The book aims to provide in a compact format basic knowledge and practically oriented information on specific properties of refractory materials, on their testing and inspection, and on interpretation of test results. Tables and illustrations are used to clarify fundamental concepts on a comparative basis. This pocket-format manual provides an overview of the diverse range of modern refractories and their application-relevant properties. Its main feature is a series of practice-derived articles by well-known authors in the field on the various material groups and their characteristic property data. The content has deliberately been kept concise and instructive, abstracting and more detailed works are referenced. ORDERFORM FAX: +49 (0) 201 / Ex. Pocket Manual Refractory Materials Design-Properties-Testing Editor: Gerald Routschka und Hartmut Wuthnow 3. edition 2008, 625 pages, plastic, (EUR) 60,00 ISBN Date of Publication: Company Name Street address/p.o.b. Postcode/Zip, Town/City Phone Fax Preferred payment method Standing order Bill I will receive a 10,- credit note after using the secure and safe payment method by standing order. Vulkan-Verlag GmbH Leserservice Postfach Essen Bank name, Town/City Bank account number Giro account number Date Signature

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16 1. INTRODUCTION 1 1. INTRODUCTION Inductive melting and holding has found wide acceptance in metal producing and processing industries. Two main types of induction furnace are deployed for this purpose: the induction channel furnace and the induction crucible furnace. These are depicted in diagram form in Fig. 1. The first technically feasible induction furnaces were based on the principle of the channel furnace [1]. Milestones were the first patent obtained by de Ferranti in 1887 and the construction of the first induction furnace in 1899 by the Swede Kjellin. In Kjellin's furnace, the melt is guided around the coil in a horizontal, open channel, hence the name "channel furnace". The open channel underwent further development, by the American Wyatt in particular, to become an enclosed refractory channel in a vertically standing U-form. Induction crucible furnaces became significant to industry at the start of the 1930s with the development of high frequency furnaces, fed by motor generators, so-called rotary converters. A large number of such furnaces were installed over a period of just a few years in the USA, Russia and Europe with capacities of up to 8-t for melting iron and non-ferrous metal materials. After World War II, there was firstly the development of high frequency (HF) to medium frequency (MF) crucible furnaces with the power being supplied Figure 1: Diagram of the induction crucible furnace and induction channel furnace

17 2 1. INTRODUCTION through rotary converters or static frequency multipliers (triple or quintuple multipliers). Then came the era of the line frequency (LF) crucible furnace. With the advantage of being able to be connected directly to the 50 or 60 Hz mains electricity grid, the LF crucible furnace experienced a rapid development as a melting unit in the foundry industry. This progress reached its provisional peak in 1970 when 60-t furnaces were installed with a power consumption of 21MW at an operating frequency of 60 Hz [2,3]. Inductive melting thus came to be regarded in iron foundries as an economically viable technical alternative to cupola furnaces, while it was able to supplant the majority of gas and oil-fired plants for melting non-ferrous metals. A drawback was the heel operation required for economical power consumption. On the one hand, this allows only small tapping amounts, and on the other hand, it requires upstream drying of scrap metal for reasons of operational reliability. Inductive melting was given a new impetus at the start of the 1980s with the development of the static frequency converter which, after the introduction of thyristor technology, was successful from a number of aspects: The efficiency of frequency conversion was raised from the 60 to 80 % at that time up to around 97 to 98 %. At the same time, high operational reliability and availability was achieved and procurement costs were cut by half. Compared to a line frequency furnace, the higher and more adaptable coil current frequency enabled a converter-fed crucible furnace with the same capacity to be operated at approximately three times the furnace power consumption and without heel, without lowering melting performance [4,5]. In the case of channel furnaces, the furnace power levels able to be installed were restricted for a long time, mainly because the inductor refractories had inadequate service lives. Ground-breaking developments in the 1980s led to a significant increase in performance while extending the life expectancy of refractory linings, particularly for melting non-ferrous metals. Compared to crucible furnaces, induction channel furnaces nevertheless have a much lower specific power based on capacity, due to their construction principle. Their particularity lies in the fact that large volume furnace units can be constructed in siphon execution with an enclosed gas chamber. This type of furnace is accordingly well suited for storing larger volumes of molten iron. Moreover, channel furnaces can also be advantageously deployed for melting non-ferrous metals because their high electrical efficiency, in the form of low energy consumption, pays off well here.

18 2. FUNDAMENTALS 3 2. FUNDAMENTALS 2.1 Inductive Power Transmission Power is transmitted in induction furnaces in that an alternating current is passed through a multi-winding coil to heat the charged material, usually an electrically conducting metal (Fig. 2) [6]. The alternating current running through the coil creates a magnetic field. Voltages are induced in the feed material when it is introduced into this magnetic field, whereby eddy currents are created due to the conductivity of the metal. The inducted current heats the charged material in accordance with Joule's law and it becomes molten after a certain heating time. Figure 2: Principle of inductive energy transmission The eddy currents flowing through the feed material generate a secondary magnetic field which is opposed to the primary magnetic field. This causes the resulting magnetic field in the middle of the charge to weaken and thus the induction effect in the inside is reduced. The overlap of the magnetic fields displaces the current in the charge towards the outside. The skin effect causes the current density to decrease from the outside towards the inside (Fig. 3). For an induction crucible furnace, the equation below gives an approximation of the current density S x at the distance r from the surface (1) The variable δ represents the penetration depth of the electromagnetic field

19 4 2. FUNDAMENTALS Figure 3: Course of current density during inductive heating [6] and designates the distance from the surface, in that the current density decays to 1/e of the surface value S o. The penetration depth results from (2) In this equation, κ is the electrical conductivity of the feed material, µ its permeability and f the frequency of the coil current. According to Fig. 2, the penetration depth can be interpreted as an equivalent plane conductor thickness, through which a current flows that has the surface current density So over the complete thickness. The feed material can thus also be understood as a cylindrical tube with the wallthickness δ, which has the same effect as the short-circuited secondary winding of a transformer. 2.2 Induction Furnace Structural Shapes Induction furnaces can be differentiated into crucible and channel furnaces; their construction principles are illustrated in Fig. 4 [7]. In a channel furnace, the inductor flanged onto the furnace vessel can be regarded as a transformer, consisting of one or more primary coils with multiple turns around a closed, ferromagnetic core and a short-circuited secondary winding. The current flowing through the inductor's primary coil generates a varying electromagnetic field that primarily flows through the iron core. The single-winding secondary circuit represents the refractory lined inductor channel filled with liquid metal, into which a high short-circuit current is induced. The thermal energy generated (Joule's heat) is transmitted from the inductor channel to the melt in the furnace vessel via the turbulent flows

20 2. FUNDAMENTALS 5 Figure 4: Principles of channel and crucible furnaces [7] formed by the electromagnetic forces and thermal forces of lift. This principle of functionality delivers greater electrical efficiency as the main advantage of channel furnaces over induction crucible furnaces. The latter work without an iron core (which is why they are also known as coreless induction furnaces) under the electrical principle of an air-core transformer. The cylindrical induction coil runs around the refractory crucible for approx. 80 % of the crucible's height. As almost the whole furnace space is subject to the electromagnetic field, these furnaces can be operated with high specific power. A further benefit is that the melt experiences a strong inductive stirring in a crucible furnace; the drawback is their lower electrical efficiency. 2.3 Induction Crucible Furnace Electrical Efficiency The following equation applies to the electrical efficiency η el of crucible furnaces η el = P i / (P i + P v ) (3)

21 6 2. FUNDAMENTALS Figure 5: Efficiency of an induction crucible furnace as a function of the ratio between feed stock diameter d to skin depth δ for different materials [7] The greatest part of the electrical losses P v occur in the furnace coil. The variable for the inducted power P i is decisively dependent upon the ratio of charge diameter d to penetration depth δ. For scrap feed materials, charge diameter d is the diameter of the scrap pieces directly after charging, for melted metal it is the diameter of the melt. The skin depth δ is very small in comparison to the dimensions of the furnace. It changes greatly during the heating process due to the temperature dependence of the electrical conductivity and the permeability. Thus an iron or steel melt has a skin depth of approx. 80 mm at 50 Hz and of approx. 25 mm at 500 Hz, while cold-charged feed material has a skin depth lower by a factor of 10. Fig. 5 shows the electrical efficiency factor for different metals and temperatures in dependence on the ratio d/δ [7]. For the induction crucible furnaces used in industry, d/δ is always greater than 6, thus in the high electrical efficiency range. The highest values of over 90 % are obtained when heating up cold ferromagnetic iron or steel. Once the Curie point has been passed at around 770 C, or where austenitic steel is concerned, the values fall back to approx. 80%. In the case of non-ferrous metals, the maximum electrical efficiency is far below as per Fig. 5.

22 2. FUNDAMENTALS Field Line The eddy currents in the material to be heated can only be approximately calculated by analytic processes. Thanks to the computer technology now available, numerical calculation methods are widely applied today. These obtain much more accurate results by simulating the crucible furnace in a twodimensional, axial symmetrical model. The numerical methods used have progressed with regard to user-friendliness and speed to the extent that they have become the standard procedures for designing induction crucible furnaces [8]. Such calculation programs enable both the exterior electrical data of an induction furnace and also the field line inside the furnace to be exactly determined. As an example, Fig. 6 a) shows the field line in a medium frequency crucible furnace, consisting of the coil, magnetic yokes and melt. The magnetic flux generated by the coil current partly runs through the melt, although the greatest part flows through the crucible wall between the coil and the melt. The exterior transformer yokes form a magnetic return path and thereby guide the outer magnetic leakage field. They thereby prevent any heating of the furnace structure and an inadmissibly high leakage field outside the furnace. It can be recognized from Fig. 6 a that the magnetic yokes protrude quite far out over the coil ends in order to catch the field leaking upwards and downwards. This is an important factor in the construction of a) b) Figure 6: a) Field pattern of a medium frequency crucible furnace; b) Power distribution and course of flow (source ABP)

23 8 2. FUNDAMENTALS industrial crucible furnaces, and is discussed in Chapter "Coil and magnetic yokes". The illustration also shows the high field density in the refractory wall, which leads to inductive heating of metal which has penetrated into the wall. This is described in more detail in Chapter "Refractory Lining" Bath Agitation During inductive power transmission, electromagnetic forces are created from the interplay of the eddy currents induced into the melt and the magnetic induction. These forces are responsible for the characteristic phenomenon of the bath meniscus and melt flow in induction crucible furnaces (Fig. 7). Figure 7: Bath meniscus and bath agitation in an induction crucible furnace [7] Bath Meniscus Hight The electromagnetic forces basically run in a radial direction to the crucible axis and press the melt away from the crucible wall towards the center. Gravity works against these forces, so that a meniscus is formed on the bath surface. The meniscus hight h Ü in the center axis has the following dependencies: (4)

24 2. FUNDAMENTALS 9 Figure 8: Calculated bath Meniscus hight h ü and melt flow in a 2-t crucible furnace at 2000 kw power and 50 Hz frequency with a) partly filled furnace, b) full furnace [9] a) b) P S is the power referred to the mass of the melt, f the frequency of the coil current and K a proportionality constant which depends upon the feed material, the furnace geometry and, in particular, upon the filling level. Fig. 8 a shows the form of the bath meniscus in a 2-t crucible furnace with a power input of 2000 kw/570 Hz [9]. The meniscus with the hight h ü is clearly recognizable with the coil only part filled. It is reduced at a higher bath level and tends towards zero when the furnace is filled above the level of the active coil (Fig. 8 b) Bath Flow Alongside the characteristic bath meniscus, agitating the bath by means of an intensive, turbulent melt flow is a further significant process engineering phenomenon for melting and duplication in the crucible furnace [10]. Flow velocity. The distribution of the average flow velocities is characterized by the formation of twin toroids whose directions of flow are opposed (Fig. 7). The maximum flow velocity in typical medium frequency high performance furnaces can reach up to 1.5 m/s and occurs near the crucible wall at the height of the upper center of the flow vortex [10]. As per Equation (5), the average velocity v is proportional to the root of the specific furnace power P S and, vice-versa, in larger furnaces from 5-t or so is proportional to the frequency to the power of minus 0,4: Turbulence. Apart from the average flow velocity v, turbulent fluctuation movements are particularly important for transporting heat and material in the (5)

25 10 2. FUNDAMENTALS Figure 9: Measured distribution of the average flow speed and turbulent, kinetic energy in a trial crucible furnace [9] melt. They can be described quantitatively as the distribution of the turbulent, kinetic energy of the bath flow. The relevant results of measurements for a trial crucible furnace are depicted in Fig. 9 together with the average flow velocity [9]. It can be seen that the characteristic distribution of the turbulent, kinetic energy in the melt reaches the highest values in the area between the toroid flow vortices, whereby the maximum occurs directly on the crucible wall. These high turbulences occurring between the two flow vortices are the cause of the good temperature and material homogenization between the upper and lower parts of the melt on the one hand, yet on the other hand, they help to separate out suspended oxides and thus encourage deposits to form on the crucible wall (see Chapter ). The turbulent, kinetic energy k of the melt flow and the associated transport of heat and material in the melt are proportional to the specific furnace pow- Figure 10: Bath meniscus and melt flow in the top part of an induction crucible furnace (source ABP)

26 2. FUNDAMENTALS 11 er and are reduced as the operating frequency rises in the following dependency: k ~ P s / f. (6) Bath agitation is a characteristic property of an induction crucible furnace. It is desirable because it brings about a homogenization of the melt composition and temperature, while allowing light substances to be stirred in. In particular, the interplay of the lower level of the bath at the crucible wall with the high flow velocity and turbulence prevailing there enables light substances and alloying agents to be stirred in an optimal manner (Fig. 10). However, formation of a bath meniscus and the flow also inhibit performance if furnace power consumption is too high (see Chapter 6.1.2). 2.4 Induction Channel Furnace Efficiency factor As with the induction crucible furnace, Equation (3) applies to the electrical efficiency η el. In the case of channel furnaces, the electrical losses P v are more or less equally attributable to the inductor coil, the casing and the cooling cylinder. The losses in the iron core are generally minor. The transformer principle with an iron core in the induction channel furnace provides a much better magnetic coupling between the inductor coil and the melt, and therefore leads to considerably higher degrees of electrical efficiency compared to induction crucible furnaces. The typical electrical efficiency factors lie between 95 % and 98 % for iron melts, and between 80 % and 90 % for nonferrous metals, whereby smaller inductors generally have a higher electrical efficiency than larger ones. In holding mode, overall electrical efficiency factors of between 90 % and 95 % are attained [11]. Although the electrical losses in the induction channel furnace are comparably low, set against this are greater thermal losses, particularly in the furnace vessel. The furnace heat losses thus consist of the thermal losses from the vessel, including the cover, from the inductor and from the water-cooled flange. Electromagnetic penetration depth and frequency in an induction channel furnace behave completely differently than in an induction crucible furnace. As the input current in the channel furnace basically flows along the channel, the skin effect in the channel has hardly any effect on the electrical behavior. Skin effects in the surrounding casing and other structural elements cause the unwanted eddy current losses occurring there to rise considerably at higher frequencies. Investigations into the influence of frequency on flow behavior have also shown that similarly high frequencies, as are deployed beneficially

27 12 2. FUNDAMENTALS in induction crucible furnaces, bring no advantage in an induction channel furnace. For this reason, induction channel furnaces are typically operated with line frequency or are supplied by a converter at a frequency slightly above line frequency [12,13] Electromagnetic Field and Melt Flow The design and arrangement of induction channel furnaces requires precise knowledge of the behavior of the electromagnetic field and the associated electromagnetic forces, and of the flow relationships. Contrary to the rotationally-symmetric field of an induction crucible furnace, the electromagnetic fields in the channel furnace need to be determined in three dimensions due to its complex geometry. There are meanwhile numerical methods which allow three-dimensional electromagnetic fields and three-dimensional fields of flow to be calculated. Field of flow calculations aim to optimize the melt flow through power and channel geometry in such a way that erosion or clogging in the channel are impeded local flow velocities are reduced intermixing of the melt in the furnace vessel is optimized pinch phenomena are prevented Fig. 11 shows the flow distribution in a trial arrangement, for which the flow relationships were calculated and confirmed by technical measurement using Wood's metal (alloy consisting of 50 % Bi, 25 % Pb, 13 % Sn, 12 % Cd with a melting point of 71 C). Contrary to popular belief, this found that the transport of heat and material in the inductor channel is not determined by a directed through flow of the channel. The results of the investigations instead show that the integral through flow velocity is very low, although intensive transversal vortices arise in the inductor channel. Furthermore, self-contained longitudinal vortices with high flow velocities can be created around the channel ends in the inductor throat. It is therefore impermissible to determine the directed through flow velocity from the maximum temperature difference setting in between the inductor channel and the furnace bath. Neither can any directed through-flows establish themselves on the basis of the distribution of electromagnetic fields and forces in the inductor channel. The melt flow occasionally observable on the surface of the melt in the furnace vessel at low bath levels is caused by the flow vortices created in the mouth at the transition between the inductor throat and the furnace vessel. These flow vortices are influenced by the form of the channel ends and the inductor throat. The transport of heat and substances in the inductor channel itself is mainly determined by the turbulent

28 2. FUNDAMENTALS 13 Figure 11: Measured distribution of velocity in an induction channel furnace [11] transversal vortices (dual vortex, butterfly vortex structures), which result in intensive, turbulent mixing and thus effectively transport heat and material [12,13]. Regardless of these flow phenomena in the inductor channel, so-called pinching results from the integral electromagnetic forces. Analogous to the effect in the crucible furnace, these try to push the melt away from the refractory wall facing the coil, while gravity works against them. If the electromagnetic pressure forces prevail, the self-escalating run of the melt in the inductor channel is constricted to the extent that the inducted electrical current (and thus the effect of the electromagnetic force) is interrupted. This enables the displaced melt to flow back in the channel and initiate pinching again with greater or lesser abruptness. The consequences are heavy vibrations in the furnace, which make it necessary to switch the furnace off and restart it at a low level of power. The optimum design of the inductor power output and the channel geometry aims to prevent pinch effects, even when the bath level in the furnace vessel is at a minimum. Nevertheless, changes in the channel dimensions in practice (perhaps due to clogging effects) are often the reason why the pinch effect still occurs, whereby the power input of the inductor is then restricted for a time.

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