INDUCTION
FURNACE
ABSTRACT
A
new generation of industrial induction melting furnaces has been developed
during the last 25 years. Present practices followed in Induction Furnaces are
discussed in this paper. Through a literature review account of various
practices presently being followed in steel industries using Induction Furnaces
has been carried out with a view to gather principal of working. Apart from
this a pilot study has also been carried out in few industries in India. We
provide some recommendations for the productivity improvement .Due to non
availability of the proper instrumentations the effect of the ill practices can
not be precisely judged. If this is properly measured, the percentage of
productivity improvement in steel melting Induction Furnace can be calculated. The
review is carried out from the literature in the various journals and manuals.
Keywords Induction Furnace, molten metal , productivity, Melt rate.
INTRODUCTION.
An
Induction Furnace uses induction to heat a metal to its melting point which is
based on the theory of Electro Magnetic Induction. Depending on their frequency
(50 Hz 250 kHz) these can be divided to
three types:
1.
High Frequency
2.
Medium Frequency
3.
Low Frequency
Their
capacities range from less than 1kg to 100MT, which are used for remelting of
iron and steel (steel scrap), copper, aluminium, precious metals and alloys.
Even most modern foundries use this type of furnaces and now more iron
foundries are replacing Cupolas with Induction Furnace to melt cast iron as the
former emit lots of dust and other pollutants. The Steel making via Induction
Furnace route has certain advantages and disadvantages:
ADVANTAGES OF INDUCTION FURNACE
1. It has no electrodes and electric
arcs which allow productions of steel and alloys low in
carbon and occluded gases without
any quality problem.
2.
Low melting losses and alloying elements.
3.
High power efficiency, therefore, cost effective.
4.
Precise control of the operating parameters.
DISADVANTAGES OF
INDUCTION FURNACE
1.
Refining in Induction Furnace is not as intensive or effective as in Electric
Arc Furnace (EAF).
2.
Life of Refractory lining is low as compared to EAF.
3.
Removal of S and P is limited, so selection of charges with less impurity is
required.
CONSTRUCTION AND WORKING
Combustion furnaces and induction
furnaces produce heat in two entirely different ways. In a combustion furnace,
heat is created by burning a fuel such as coke, oil or natural gas. The burning
fuel brings the interior temperature of the furnace above the melting point of
the charge material placed inside. This heats the surface of the charge
material, causing it to melt. Induction furnaces produce their heat cleanly,
without combustion. Alternating electric current from an induction power unit
flows into a furnace and through a coil made of hollow copper tubing. This
creates an electromagnetic field that passes through the refractory material
and couples with conductive metal charge inside the furnace. This induces
electric current to flow inside the metal charge itself, producing heat that
rapidly causes the metal to melt. Although some furnace surfaces may become hot
enough to present a burn hazard, with induction, you heat the charge directly,
not the furnace.
Fig. 2: Current flowing in
one direction in the induction coil induces a current flow in the opposite
direction in the metal charge. This current heats the metal and causes it to
melt
A. Induction Electrical
System Configurations:
Induction
furnaces require two separate electrical systems: one for the cooling system,
furnace tilting and instrumentation, and the other for the induction coil
power. A line to the plant’s power distribution panel typically furnishes power
for the pumps in the induction coil cooling system, the hydraulic furnace
tilting mechanism, and instrumentation and control systems. Electricity for the induction coils is
furnished from a threephase, high voltage, high amperage utility line. The complexity
of the power supply connected to the induction coils varies with the type of
furnace and its use. A channel furnace
that holds and pours liquefied metal can operate efficiently using mains
frequency provided by the local utility. By contrast, most coreless furnaces
for melting require a medium to high frequency power supply. Raising the
frequency of the alternating current flowing through the induction coils
increases the amount of power that can be applied to a given size furnace.
This, in turn, means faster melting. A 10 ton coreless furnace operating at 60
Hz can melt its capacity in two hours. At 275 Hz, the same furnace can melt the
full 10 ton charge in 26 minutes, or four times faster. An added advantage of
higher frequency operation is that furnaces can be started using less bulky
scrap and can be emptied completely between heats. The transformers, inverters
and capacitors needed to “tune” the frequency required for high efficiency
induction furnaces can pose a serious electrical hazard. For this reason,
furnace power supplies are housed in key locked steel enclosures, equipped with
safety interlocks.
B. Safety Implications:
Typically,
the induction coil power supply and the other furnace systems are energized
from multiple electric services. This means that foundry workers cannot assume
that the power to the furnace coil has stopped because service has been
interrupted to the furnace’s cooling system or hydraulic pumps. Review the lock
out/tag out section provided in this safety guide
C. Input And Output Parameters
Of The Induction Furnaces:
In order to study the prevailing practices in
steel plants using Induction Furnaces, the following parameters have been
identified as
1) Raw Material: Induction Furnaces are using Steel
melting scrap, Sponge Iron & Pig Iron/Cast Irons. On an average the ratio
of these items is 40% sponge Iron + 10% Cast Irons or Pig Iron. The technology
of melting these input materials varies according to the availability of raw
materials and location of the plant and inputs of sponge iron consumed is as
high as 85 % as charge mix on bigger furnaces.
2) Power Supply: An A.C. current from the transformer
is fed to the rectifier of the furnaces electronic circuit. This converts A.C.
to D.C, voltage is smoothed out by a D.C. choke, and then fed to the inverted
section of the furnace. Here the D.C is converted to a high frequency A.C.
current and this is fed to the coil.
3) Refractory Lining: The material used for lining is
crushed quarts. This is a high purity silica material. The linings are of two
types, acidic lining and basic lining.
4) Water: The cooling system is a through one way flow system
with the tubular copper coils connected to water source through flexible rubber
hoses. The inlet is from the top while the outlet is at the bottom. The cooling
process is important because the circuit of the furnace appears resistive, and
the real power is not only consumed in the charged material but also in the
resistance of the coil. This coil loss as well as the loss of heat conducted
from the charge through the refractory crucible requires the coil to be cooled
with water as the cooling medium to prevent undue temperature rise of the
copper coils.
5) Molten Metal : The molten metal is the desired
output of the Induction furnace. The quantity depends upon the capacity of the
furnace, and the quality depends upon the raw material and alloy
composition. The tapping temperature
depends upon the type of steel, as well as the distance of end use of the molten metal.
6) Waste Heat: The surface of the molten metal bath
is exposed to atmosphere. This results in the major thermal energy loss
through radiation. The Coils of furnace are water cooled this also
results in heat loss.
7) Slag : During the operation of electric induction
melting furnaces, non-metallic are produced from the various sources described
earlier. Depending on the specific process being used and the type of iron or
steel being melted, the composition of the slag will vary.
8) Slag Composition: The composition of furnace and ladle
slags is often very complex. The slags that form in electric furnace melting
are the results of complex reactions between silica (adhering sand on casting
returns or dirt), iron oxide from steel scrap, other oxidation by products from
melting, and reactions with refractory linings. The resulting slag will thus
consist of a complex liquid phase of oxides of iron, manganese, magnesium and
silicon, silicates and sulfides plus a host of other compounds, which may
include alumina, calcium oxides and sulfides, rare earth oxides and sulfides
and spinels and fosterites.
TYPES OF INDUCTION FURNACES
A. Coreless Induction
Furnaces:
The
coreless induction furnace is a refractory lined vessel with electrical current
carrying coils surrounding the refractory crucible. A metallic charge
consisting of scrap, pig iron and ferroalloys are typically melted in this
vessel.
B. Channel Furnaces :
In
a channel furnace, induction heating takes place in the “channel,” a relatively
small and narrow area at the bottom of the main bath. The channel passes
through a laminated steel core and around the coil assembly.
C. Pressure Pour
Furnace:
A
pressure pour is, in essence, a channel furnace, as described above, that is
carefully sealed so that the metal can be moved out of the furnace by way of
pressurizing the chamber above the molten metal bath in the furnace.
SAFETY IMPLICATIONS:
Accident
investigation reports indicate that most foundry accidents happen due to one of
the following reasons:
•
The introduction of wet or damp metal into the melt, causing a water/molten
metal explosion
•
Lack of operator skill during temperature taking, sampling or the addition of
alloying compounds, causing metal splash.
•Dropping
large pieces of charge material into a molten bath, causing metal splash
•
Improper attention to charging, causing a
bridging conditions
•
Failure to stand behind safety lines, causing a
tapping situation
•
Coming into contact with electrical conductors, overriding safety interlock
switches or coming into contact with incompletely discharged capacitors,
causing electric shock or electrocution
•
Lack of operator training
ELECTRONBEAM FURNACE
An electron
beam furnace (EB furnace) is a type of vacuum furnace
employing high energy electron beam
in vacuum as
the means for delivery of heat to the material being melted. It is one of the electronbeam
technologies.
Electron
beam furnaces are used for production and refining of
high purity metals (especially titanium, vanadium, tantalum, niobium, hafnium,
etc.) and some exotic alloys.[1]
The EB furnaces use a hot cathode for production of electrons and high voltage
for accelerating them towards the target to be melted.
ELECTRIC ARC FURNACE
INTRODUCTION
The
use of electric arc furnaces
(EAF) for steelmaking has grown
dramatically in the last decade
in the United States. In
1975 electric furnaces
accounted for 20% of the
steel produced in the U.S.; by
1985 this figure
had grown to 34%. Electric
furnaces range in capacity from
a few tons
to as many as 400, and a
steelmaking shop can have
a single furnace
or up to
three or four. In brief,
these furnaces melt steel
by applying an AC current
to a steel scrap
charge by means of graphite electrodes. It
requires about 500 kwh of
electricity to produce
a ton of steel; consequently, these
furnaces use a tremendous
quantity of electricity.
Trans former loads may reach 120 MVA. The melting
process involves the
use of large quantities of energy in a
short time (1-2 hr) and in
some instances the process
has caused disturbances
in power grids. These
disturbances have usually been
characterized as “flicker”
brief irregularities in
voltage a fraction
of the 60 Hz cycle
in length, and “harmonics” irregularities that
tend to occur in a pattern
repetitive to the 60 Hz cycle.
The features of electric
arc furnaces were described in a CMP Tech Commentary on Electric Arc
Furnaces (Vol. 1, No. 3, 1985).
The purpose of the present
Technology commentary, is to give
utilities and steel
mills a better understanding of
electric furnaces from an
electrical viewpoint.
ENERGY NEEDS
Furnaces
are often classified
by power requirement levels.
A scale indicating power classification ranging
from ultra high power (UHP), with over 700 kVA per ton,
down to low power, with less than 200 kVA per ton. It
is important to
consider the energy balance for
a typical modem EAF because; 70% of the total
energy is electrical, the
remainder being chemical energy arising
from the oxidation of elements such
as carbon, iron,
and silicon and the
burning of natural
gas with oxy-fuel burners.
About 53% of the total
energy leaves the
furnace in the liquid steel, while
the remainder is
lost to slag, waste
gas, or cooling.
STEEL MAKING CYCLE
To
achieve meltdown as
quickly as possible, electrodes
are initially lowered to a
point above the
material, the current is
initiated, and the
electrodes bore through the
scrap to form
a pool of liquid
metal. The scrap itself protects
the furnace lining from
the high intensity arc.
Subsequently, the arc is lengthened by increasing the
voltage to maximum power.
Most modem furnaces are
equipped with water cooled panels in the upper
half of the sidewall, rather than
refractories, which allows for
longer arcs and
higher energy input into
the furnace. In the final
stage, when there is
a nearly complete
metal pool, the arc is
shortened to reduce radiation heat
losses and to
avoid refractory damage
and hot spots. After
melt dawn, oxygen usually
is injected to oxidize
the carbon in the steel or
the charged carbon.
This process is an important
source of energy;
the car bon monoxide
that evolves helps
minimize the absorption
of nitrogen and flushes
hydrogen out of
the metal. It also foams
the slag, which
helps minimize heat loss.
DETAILED ELECTRICAL
OPERATION
After an
electric furnace is charged
with scrap and the
roof is in place,
the operator lowers the
electrodes, each of which
has its own regulator and mechanical drive.
The electrodes are connected
to the furnace
transformer‘s secondary delta winding,
which may be rated
from ;bout 600 to 850 volts. No current
flows when the
first electrode contacts
scrap, but a line to line
path through the
scrap and an
arc are established when
the second electrode completes the
circuit. The regulators for each
of these two electrodes then signal
the drives to
raise the electrodes
until the selected
current voltage ratio for the
arc is achieved. Initiation
of the third arc
depends on the
scrap’s location, which is
unpredictable, hence the duration
of the unbalance is short but random.
While the scrap
is still un-melted, the
arc may easily be extinguished by
a minor overshoot in
an electrode regulator or
by physical movement
of the scrap. As the
scrap melts, it can
often shift and
fall away from an
electrode extinguishing the
arc, or against the electrode
possibly breaking it.
Because of the physical
movement and settling of the scrap,
wide excursions can
take place on
a random basis in
the secondary circuit.
The abrupt initiation and
interruption of current flow
provides a source
of harmonic currents and
causes considerable disturbance
to high impedance circuits. (About 75% of the
total impedance is in the secondary circuit.)
Voltage and current waves
deviate considerably from
symmetrical sinusoidal patterns,
but they do not
attain full rectangular
shape, according to findings in the CMP
report, “Arc Furnace Power
Delivery Scoping Study.”’ Disturbances
are worst during early
meltdown, and they
occur at varying
frequencies. Many attempts have
been made to establish the
human eye’s reaction
to the flicker of a lamp.
Generation
of harmonics may result
in further flicker problems,
and equipment on
the power system
may also be damaged. If static capacitors
are to be used
to improve the
power factor, an analysis to ensure that
resonance does not exist
at any of the harmonic frequencies should
be made. Harmonics
contribute to wave
distortion and to the increase in effective inductive reactance. This
increase is often
in the 10 to 15% range and
has been re- ported
as high as 25%. Current into the
furnace is therefore less
than what would be
expected from calculations based on
sinusoidal wave shapes,
and losses in frequency-sensitive equip- ment
such as transformers
are higher than the
sinusoidal wave shape
would produce.
IMPORTANCE OF SCRAP
Scrap
is available in a wide variety of sizes,
densities, and chemical compositions,
and a mixture is usually used.
If only the lightest,
least dense material is
charged, several buckets
of scrap must be placed in the
furnace to make a full heat. This is generally uneconomical
due to oxidation losses and
the need to open the furnace
for several separate charges,
which results in loss of both time
and heat. Nor is the
use of large heavy scrap
alone optimum. A large piece
might protrude and
interfere with roof closure
or require placement by
magnet, a process which
takes time. The furnace
operator therefore tries to blend several
types of available
scrap to suit his needs. It is
beneficial to arrange the heavier
pieces near the bottom
of the charge.
After about 20 minutes of operation,
depending on available power and
other practices, the electrodes
will have opened
some voids, and cave-ins
can occur. If
large pieces of scrap
are on top
of the pile, they
can possibly slide
into and break an electrode. It
is generally believed
that light, uniform
scrap produces a smoother meltdown
than does large heavy
scrap. However, this
is not al- ways the
case. If heavy scrap is charged,
full power can be applied. If all the
scrap is light, on full power
the electrodes may bore
through, damaging the furnace bottom
before a sufficient pool of
liquid metal has
formed. Generally, the initial
period of melting causes the
most electrical disturbances. As the scrap temperature begins to rise, a liquid pool
forms, and disturbances begin to diminish. This
is generally about 10 minutes or
so after power-on
and can vary depending
on power levels and
shop practices.
MELT DOWN
Heating
steel scrap to approximately 3000"
F requires large quantities of energy rapidly
applied. Therefore, full power
is called for
during meltdown. The arc
during meltdown can be long because the electrode
and arc are
boring a hole down through
the scrap, and the
roof and sidewalls are
not exposed to arc radiation. If
the arc is extiquished, the regulator
will lower the electrode to re-establish it.
This can take several
seconds if the
scrap has moved out
from under the
electrode.
MAIN MELTING PERIOD
After about 20 minutes, most
electric furnaces will have
begun converting scrap to
liquid metal. Hence,
wide swings in disturbances will
diminish considerably. When sufficient
molten metal exists (in some high-powered furnaces only 8-10 minutes is
required), the arc is
shortened by an
adjustment to the electrode regulators.
The cur- rent will
rise since overall
resistance is reduced, and
the power factor
and arc power will
decline. Arc length
is changed so that the
shorter arc will
deliver a higher portion of
its heat to the metal below
the electrode than
will the longer arc,
which radiates more
heat to furnace sidewalls. Many
studies have been conducted
which confirm the
ad- vantages of the long
arc for meltdown of
heavy scrap and the
short arc for operation
after sufficient liquid
metal has been formed.
The short arc is
much more stable than
the long arc,
and operation during the
refining period follows sinusoidal
concepts much more closely.
REFERENCES
1.
Presidential address on the occasion of xx annual general meeting &
interactive seminar on 16th November 2005 at New Delhi overview of iron and steel industry steel markets Asia
conference 2005.
2.
Ikbal Nathani ( November 15, 2005) importance of recycling and current ferrous
scrap scenario. Ambassador, Indian sub-continent bureau of international
recycling Brussels.
4.
Advanced Melting Technologies: (November 2005 Energy Saving Concepts and
Opportunities for the Metal Casting Industry BCS, Incorporated 5550 Sterrett
Place, Suite 306 Columbia, MD 21044 www.bcs-hq.com.
5.
D. Sager, S. Tambe, R. Stadler, R. Pai, Steel Grips 6 (1) (2008) 8–15.
6.
T. Handrich, M. Huck, M. Abel, P. Petrov, in: AISTech 2013 Proceedings, 2013,
pp. 791–794.
7.
H.R. Manouchehri, Looking at shredding plant configuration and its performance
for developing shredding product stream (an overview): Report JK 88011
2007-09-03, 2007).
8.
J. Madias, Acero Latinoamericano 527 (2011) 48–56 (in Spanish).
No comments:
Post a Comment
You can make your comment here