Basics and Applications of Induction Furnaces

Technical basics and applications of  induction furnaces

Prof. Dr.-Ing. Egbert Baake Institute of Electrotechnology Leibniz University of Hanover (Germany)

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Outline  

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Introduction Fundamentals and application of the induction crucible furnace Fundamentals and application of the induction channel furnace Other industrial induction melting applications

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Industrial process requirements for  melting in induction furnaces 

Mixing and homogenisation of the entire melt

Homogenisation of the temperature, avoiding of  local local over overhea heatin ting g , but but reali realizin zing g of  sufficient superheating of the entire melt 

Intensive stirring at the melt surface (melting of small-sized scrap, carburization process) 

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Optimisation of the heat and mass exchange in the melt

Avoiding of erosion and clogging of the ceramic lining

Avoiding of melt instabilities, splashing or pinching 

Intensive stirring for cleaning of  the melt (zinc removing) 

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Physical correlations in induction furnaces magnetic field - distribution of power  - electromagn. forces

velocity field homogenisation of  melt

meniscus shape geometry of melt

temperature field - overheating - heat flow

skull formation

alloy composition

liquid-solid-interface

melt components

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Induction furnaces for melting Induction crucible furnace

Induction channel furnace

Used mainly for melting

Used mainly for holding and pouring

Medium high efficiency

High efficiency

Operating frequency: 50 ... 1000 Hz

Operating frequency: 50 Hz, 60 Hz 5

Construction of induction crucible furnace

meniscus melt melt flow steelconstruction

crucible induction coil

concrete-ring

magnetic yoke

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Meniscus shape and melt flow of the crucible induction furnace

Inductor curr ent J1 :

Electromagnetic force density: F = J 2  x B

Induced current density in the melt:J 2 

Melt flow pattern Magnetic field:

B

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Example: Induction crucible furnace

meniscus melt melt flow steelconstruction

Velocity v of the melt is proportional to the inductor current I: v ! I Height of meniscus is proportional to:

crucible induction coil

concrete-ring

magnetic yoke

Velocity v of the melt is proportional to:

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Characteristics of turbulent flow in induction furnaces local melt flow velocity in dependence on time Vmax

≈

20 cm/s

Shared in: 1. Time averaged flow velocity  convective heat and mass transfer  2. Instationary fluctuations and oscillations  turbulent heat and mass transfer 

Spec. kinetic energy of turbulence:

k = ½ (v´x12 + v´x22 + v´x32) 9

3D hydrodynamic model of an industrial induction crucible furnace

P = 4540 KW Hind = 1.33 m Rcr  = 0.49 m Filling level 90 %

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Calculation of the melt flow velocity in the ICF: (3D transient LES)

symmetric state

unsymmetric state 11

Calculation of the melt flow velocity in the ICF: Melt surface (3D transient LES)

calculated time: 5 sec

calculated time: 40 sec 12

Example of medium frequency induction crucible furnace: 12 t/9,3 MW/250 Hz

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MF-ICF-melting installation with two furnaces: 12 t, 9,3 MW/250 Hz

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Induction crucible furnace installation for  melting of grey cast iron

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Induction craucible furnace during pouring Medium frequency-ICF Capacity: - 6 t grey cast iron Power: - 3300 kW/250 Hz

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Induction crucible furnace: capacity 8 t grey cast iron with charging vehicle

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Back tilt position of an ICF for 8 t grey cast iron during slag cleaning process

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Induction crucible furnace (ICF): Advantages in comparison with competitive processes Homogenisation of the melt due to stirring of the melt Less oxidation losses (dross) because no local overheating High accurate alloying process Well defined melt temperature Simple automatic process control High quality of the melt even with cheap charge material (scrap) High throughput because fast melting and fast heating up of the melt High furnace efficiency Simple handling of the furnace and the process Well defined controlling of the power input Good working conditions for the workers at the furnace Environmental friendly (small dust emission, no exhaust gas) 19

Induction crucible furnaces melting material, capacities, power rates, frequencies Material

Frequencies

[t]

Power rates [MW]

1,3 ... 100

0,5 ... 21

50 ... 60

Light metal

0,5 ... 15

0,2 ... 4

50 ... 60

Heavy metal

1,5 ... 40

0,5 ... 7

50 ... 60

0,25 ... 30

0,3 ... 16

150 ... 1000

Light metal

0,1 ... 8

0,2 ... 4

90 ... 1000

Heavy metal

0,3 ... 70

0,3 ... 16

65 ... 1000

LFCast iron, steel furnaces:

MFCast iron, steel furnaces:

Capacities

[Hz]

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Charging of line-frequency induction crucible furnace

heal

starting blocks

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Energy supply of induction crucible furnaces

a) LF-Energy supply via switching transformer

b) MF-Energy supply via converter 

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Energy flow diagramm of a MF induction crucible

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Parallel operation of two ICF

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Design of a melt processor control system

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Melt processor  with operator 

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Melt processor  main menu Operation modes:

- sintern - starting with cold crucible - melting - overheating - holding - crucible wear  detection

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Induction channel furnace (one loop design)

28 Prof. Dr.-Ing. E. Baake / Institute of Electrotechnology, Leibniz University of Hannover 

Design of a single loop induction channel furnace Channel inductor (ABP): - single loop - P = 250 kW - symmetrical channel

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Dopple loop induction channel furnace

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Melting in the induction channel furnace

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Homogenisation of the entire melt

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Sufficient overheating of  the entire melt

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Avoiding of overheating of  the melt in the channel

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Efficient heat and mass exchange between the channel and the furnace vessel is the key point

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Melt flow in a model single loop induction channel furnace (measurements)

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3D-Simulation model for electromagnetic calculation of the channel inductor  (power density and electromagnetic force density)

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Channel inductor: distribution of power density in the melt

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Channel inductor: distribution of electromagnetic force density

Parameters: P = 60 kW I = 850 A Melt: Wood-metal 35

Channel inductor: distribution of elektromagnetic force density

Measured Melt flow distribution

Parameters: P = 60 kW I = 850 A 36

Line frequency induction channel furnace used for storing, holding and pouring of cast iron

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CIF used in non-ferrous metal industry (aluminium foundry)

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CIF used in non-ferrous metal industry (aluminium foundry)

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CIF installation used in non-ferrous metal industry (aluminium foundry)

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Channel inductor for melting of copper and brass

• power: 2400 kW • frequency 50...70 Hz • mass:15 t

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Induction channel furnace melting material, capacities, power rates, frequencies

Melting materials

Capacities

Power rates

Frequencies

[t]

[MW]

[Hz]

10 ... 135

0,1 ... 3

50 ... 60

 Aluminium, Al-alloys

5 ... 70

0,1 ... 6

50 ... 60

Copper, Co-alloys

5 ... 160

0,5 ... 10

50 ... 60

10 ... 100

0,2 ... 10

50 ... 60

Cast iron

Zinc, Zinc-alloys

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Energy flow diagram of an induction channel furnace

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Comparison: induction crucible vs. induction channel furnace Crucible furnace

Channel furnace

melting, holding,

Holding, melting,

all metal materials

all metal materials

Melting rate (power)

high (MF)

medium

Meltíng process

heal, scrap pieces

Heal

Furnace volume

medium

big

Lining live time

good

vessel very good,

application

Channel sufficient Efficiency

sufficient

high

Operating frequencies

50 ... 1000 Hz

50 ... 60 Hz (60 ... 120 Hz) 44

Design of a pressure controlled pouring installation with induction heating

1 pressure vessel with melt 2 channel inductor 3 input spout 4 output spout

5 output valve 6 melt level controlling 7 main frame with rolls 8 casting mould 45

Induction pouring installation 10 t, 500 kW

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Induction pouring installation Presspour®, 2,5 t, 130 kW

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Holding processor for channel furnaces

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Practical problems: induction channel furnace Clogging, erosion and infiltration of the ceramic lining of the channel inductor  influenced by e.g.: 

Heat transfer and temperature distribution in the channel

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Mass transfer in the channel and in the vessel

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Type of alloy (purity)

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Type of ceramic lining

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and many others …

Clogging and erosion of a channel inductor  used for holding of grey cast iron

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Holding processor for channel furnaces Inductor-diagram

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Crucible inductor furnace

Industrial furnace for casting of grey cast iron Industrial applications: • holding and casting of grey cast iron • melting and holding of non-ferrous metal • reduction process in steel industry

Experimental furnace

Comparison with induction channel furnace: + flexible operation,energy saving, easier to clean due to complete emptying – but: lower efficiency 51

Features of the Induction Furnace with Cold Crucible melt flow

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slitted crucible to realize efficient electromagnetic transparency

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free melt surface and intensive melt stirring, based on electromagnetic forces

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water cooled bottom and crucible segments leads to solid layer (skull)

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heat losses by radiation and conduction depending on the meniscus shape

radiation

slit crucible segment

(water cooled)

inductor 

(water cooled)

current

melt with meniscus shape bottom

(water cooled)

EM-forces

skull

heat conduction

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Melting in the Induction Furnace with Cold Crucible 

high reactive and high purity materials, e.g. TiAl

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melting, alloying, overheating and casting in one process

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no pre-alloys necessary, using of scrap material

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good homogenisation of the melt due to intensive electromagnetic stirring

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