Industrial electric heating furnaces – Induction furnace device
Reexamination Certificate
2000-04-14
2002-05-21
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Induction furnace device
C373S152000, C373S156000, C373S163000
Reexamination Certificate
active
06393044
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to induction melting systems that use magnetic induction to heat a crucible in which metal can be melted and held in the molten state by heat transfer from the crucible.
BACKGROUND OF THE INVENTION
Induction melting systems gain popularity as the most environmentally clean and reasonably efficient method of melting metal. In the induction melting furnace
1
shown in
FIG. 1
, the electromagnetic field produced by AC current in coil
2
surrounding a crucible
3
couples with conductive materials
4
inside the crucible and induces eddy currents
5
, which in turn heat the metal. As indicated in
FIG. 1
, the arrows associated with coil
2
generally represent the direction of current flow in the coil, whereas the arrows associated with eddy currents
5
generally indicate the opposing direction of induced current flow in the conductive materials. Variable high frequency AC (typically 100 to 10,000 Hz) current is generated in a power supply or in a power converter
6
and supplied to coil
2
. The converter
6
, typically but not necessarily, consists of an AC-to-DC rectifier
7
, a DC-to-AC inverter
8
, and a set of capacitors
9
, which, together with the induction coil, form a resonance loop. Other forms of power supplies, including motors-generators, pulse-width modulated (PWM) inverters, etc., can be used.
As shown in
FIG. 2
, the magnetic field causes load current
10
to flow on the outside cylindrical surface of the conductive material, and coil current
11
to flow on the inner surface of the coil conductor as shown in FIG.
2
. The crucible
3
in a typical furnace is made from ceramic material and usually is not electrically conductive. The efficiency of the furnace is computed by the formula:
η
=
1
1
+
D
1
D
2
·
ρ
1
ρ
2
·
Δ
2
Δ
1
Equation
(
1
)
where
&eegr;=furnace efficiency
D
1
=coil inner diameter
D
2
=load outer diameter
&rgr;
1
=resistivity of coil winding material (copper)
&rgr;
2
=resistivity of load (melt)
&Dgr;
1
=current depth of penetration in copper winding; and
&Dgr;
2
=current depth of penetration in load (melt).
The depth of current penetration (&Dgr;) is a function of a material's properties as determined by the formula:
Δ
=
k
·
ρ
f
·
μ
Equation
(
2
)
where:
&rgr;=resistivity in ohm·meters;
f=frequency in Hertz;
&mgr;=magnetic permeability (dimensionless relative value);
&Dgr;=depth of penetration in meters.
The constant,
503
, in Equation (2) is dimensionless.
Because current does not penetrate deep into the low resistivity copper material of the coil, the typical coil efficiency is about 80 percent when the molten material is iron. Furnaces melting low resistivity materials such as aluminum, (with a typical resistivity value of 2.6×10
−8
ohm·meters), magnesium or copper alloys have an even lower efficiency of about 65 percent. Because of significant heating due to electrical losses, the induction coil is water-cooled—that is, the coil is made of copper tubes
12
and a water-based coolant is passed through these tubes. The presence of water represents an additional danger when melting aluminum and magnesium and their alloys. In case of crucible rupture, water may get into molten aluminum and a violent chemical reaction may take place in which the aluminum combines with oxygen in the water (H
2
O), releasing free hydrogen which may cause an explosion. Contact between water and magnesium may similarly result in an explosion and fire. Extreme caution is taken when aluminum or magnesium is melted in conventional water-cooled furnaces.
Often, aluminum scrap is melted in gas-fired furnaces of a sort that are referred to as “stack furnaces.” As shown in
FIG. 3
, a stack furnace
19
consists of two chambers, a dry chamber
20
and a wet chamber
21
. The scrap
18
is loaded using a charge transfer bucket
22
that dumps the scrap into the dry chamber
20
as indicated by the arrows in FIG.
3
. The scrap is melted by the flame from a gas burner
23
. Molten metal runs from a bottom spout
24
of the dry chamber
20
into a bath
25
in the wet chamber
21
where additional heating is provided by a second gas burner
26
.
An object of the present invention is to improve the efficiency of an induction furnace by increasing the resistance of the load by using as the load a crucible made of a high temperature electrically conductive material or a high temperature material with high magnetic permeability. It is another object of the present invention to improve the efficiency of an induction furnace by reducing the resistance of the induction coil by using as the coil a cable wound of multiple copper conductors that are isolated from each other. It is still another object of the invention to properly select operating frequencies to yield optimum efficiency of an induction furnace.
It is a further object of the present invention to provide a high efficiency induction melting system with a furnace and power supply that do not use water-cooling and can be efficiently air-cooled. A further objective of the present invention is to use the high efficiency induction melting system of the present invention to melt metal from scrap, cast molds, and provide a continuous source of molten metal for processing, in a manner that is integrated with the induction melting system.
SUMMARY OF THE INVENTION
In its broad aspects, the present invention is an induction furnace that is used for melting a metal charge. The furnace has a crucible formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a plurality of conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic.
Copper is especially preferred for the conductors, because of its combination of reasonably high electrical conductivity and reasonably high melting point. An especially preferred form of the cable is Litz wire or litzendraht, in which the individual isolated conductors are woven together in such a way that each conductor successively takes all possible positions in the cross section of the cable, so as to minimize skin effect and high-frequency resistance and distribute the electrical power evenly among the conductors.
In another aspect, the present invention is an induction melting system that is used for melting a metal charge. The system has at least one power supply. The crucible that holds the metal charge is formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a large number of copper conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic. Preferably, the induction melting system is air-cooled from a single source of air that sequentially cools components of the power supply and the coil. The metal charge is placed in the crucible. Current is supplied from the at least one power supply to the at least one coil to heat the crucible inductively. Heat is transferred by conduction and/or radiation from the crucible to the metal charge, and melts the charge.
In another aspect, the present invention is an induction melting system for separating metal from scrap metal that contains heavy metal inclusions. The system includes at least
Belsh Joseph T.
Fishman Oleg S.
Mavrodin Aurelian
Mortimer John H.
Ranlof Richard A.
Hoang Tu Ba
Inductotherm Corp.
Post Philip O.
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