Abstrict
An electric heater assembly using powdered media for generating
heat.
Claims
What is claimed is:
1-27. (Cancelled).
28. An electric heater assembly comprised of: (a) a metal tubular-shaped
container having an inside surface and having a layer of electrically
insulating material on said inside surface, said layer having a
thickness in the range of 10 to 500 mils, said tubular container
having an upper portion and a lower portion; (b) a compacted powdered
heating media having a controlled electrical resistivity contained
in said container extending between said upper portion and lower
portion; (c) a first electrical current conduction means contacting
said upper portion of powdered media; and (d) an electric current
feed conductor extending through said powdered media to a second
electric current conduction means contacting said lower portion
of said powdered media to permit electric current to flow from said
first means through said powdered media to said second means, the
electric current feed conductor having a layer of electrically insulating
material on the surface thereof to prevent electrical short circuiting,
the electrical resistivity of said media generating heat upon flow
of said electric current.
29. The electric heater assembly in accordance with claim 28 wherein
the resistivity of said powdered media ranges from 5 to 75 .OMEGA.-mm.
30. The electric heater assembly in accordance with claim 28 wherein
the resistivity of said powdered media ranges from 10 to 55 .OMEGA.-mm.
31. The electric heater assembly in accordance with claim 28 wherein
said powdered media comprises a powder selected from the group consisting
of SiC, C, Mo, W, TiO2, Si.sub.3N.sub.4, SiO2 and BN.
32. The electric heater assembly in accordance with claim 28 wherein
said powdered media comprises a mixture of an electrical conductive
powder and an electrical non-conductive powder.
33. The electric heater assembly in accordance with claim 28 wherein
said powdered media comprises a mixture of carbon powder and a powder
selected from the group consisting of SiC, TiO.sub.2, Si.sub.3N.sub.4,
SiO.sub.2 and BN.
34. The electric heater assembly in accordance with claim 28 wherein
said powdered media comprises a mixture of carbon and SiC powder.
35. The electrical heater in accordance with claim 28 wherein said
powdered media has a melting point in the range of 800.degree. to
4000.degree. C.
36. The electrical heater in accordance with claim 1 wherein said
powdered media has an average particle size in the range of 5 to
3000 microns.
37. The electric heater assembly in accordance with claim 28 wherein
said electrically insulating inside surface layer is comprised of
alumina, magnesia, mullite, silicon carbide, silicon nitride or
SiAlON.
Description BACKGROUND OF THE INVENTION
[0001] This invention relates to electric heaters, and more particularly,
it relates to electric heaters using novel heating means suitable
for use in molten metals such as molten aluminum.
[0002] In the prior art, electric heaters used for molten aluminum
are usually enclosed in ceramic tubes. Such electric heaters are
very expensive and are very inefficient in transferring heat to
the melt because of the air gap between the heater and the tube.
Also, such electric heaters have very low thermal conductivity values
that are characteristic of ceramic materials. In addition, the ceramic
tube is fragile and subject to cracking. Thus, there is a great
need for an improved electric heater suitable for use with molten
metal, e.g., molten aluminum, which is efficient in transferring
heat to the melt. The present invention provides such an electric
heater.
SUMMARY OF THE INVENTION
[0003] It is an object of the invention to provide an improved
electric heater assembly.
[0004] It is another object of the invention to provide an improved
electric heater assembly for use in molten metal such as molten
aluminum.
[0005] Yet, another object of this invention is to provide an improved
electric heater assembly for use in molten metal, the electric heater
assembly having a protective sleeve that has intimate physical contact
with the heating media.
[0006] And yet, another object of the invention is to provide an
improved electric heater assembly for use in molten metal, the electric
heater assembly having a protective sleeve having a thermal conductivity
of less than 30 BTU/ft hr.degree. F. and having a thermal expansion
coefficient of less than 15.times.10.sup.-6 in/in/.degree. F. and
having a chilling power of less than 5000 BTU.sup.2/ft.sup.4hr.degree.
F.
[0007] And yet, it is a further object of the invention to provide
an improved electric heater assembly for use in molten metal, the
electric heater assembly having a protective sleeve comprised of
a material resistant to erosion or dissolution by molten metal such
as molten aluminum.
[0008] These and other objects will become apparent from the specification,
drawings and claims appended hereto.
[0009] In accordance with these objects, there is disclosed an
improved electric heater assembly suitable for heating molten metal.
The electric heater assembly is comprised of a sleeve having a closed
end suitable for heating molten metal, the sleeve fabricated from
a composite material comprised of titanium or titanium alloy and
having an outside surface to be exposed to the molten metal coated
with a refractory resistant to attack by the molten metal. A novel
electric heating material is located in the sleeve in heat transfer
relationship therewith for adding heat to the molten metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a cross-sectional view of an electric heater assembly
in accordance with the invention.
[0011] FIG. 2 is a cross-sectional view of an electric heater assembly
employing compacted powdered heating media in accordance with the
invention.
[0012] FIG. 3 is a graph showing changes in electrical resistance
of powdered media with pressure.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0013] Referring to FIG. 1, there is shown a schematic of an electric
heater assembly 10 in accordance with the invention. The electric
heater assembly is comprised of a protective sleeve 12 and an electric
heating element 14 when the heater is used for heating molten metal.
A lead 18 extends from electric heating element 14 and terminates
in a plug 20 suitable for plugging into a power source.
[0014] Preferably, protective sleeve 12 is comprised of titanium
tube 30 having a closed end 32. While the protective sleeve is illustrated
as a tube, it will be appreciated that any configuration that protects
or envelops electric heating element 14 may be employed. Thus, reference
to tube herein is meant to include such configurations. A refractory
coating 34 is employed which is resistant to attack by the environment
in which the electric heater assembly is used. A bond coating may
be employed between the refractory coating 34 and titanium tube
30.
[0015] While it is preferred to fabricate tube 30 out of a titanium
base alloy, tube 10 may be fabricated from any metal or metalloid
material suitable for contacting molten metal and which material
is resistant to dissolution or erosion by the molten metal. Other
materials that may be used to fabricate tube 30 include silicon,
niobium, chromium, molybdenum, combinations of NiFe (364 NiFe) and
NiTiC (40 Ni 60 TiC), particularly when such materials have low
thermal expansion and low chilling power, all referred to herein
as metals. For protection purposes, it is preferred that the metal
or metalloid be coated with a material such as a refractory resistant
to attack by molten metal and suitable for use as a protective sleeve.
[0016] Further, the material of construction for tube 30 should
have a thermal conductivity of less than 30 BTU/ft hr.degree. F.,
and preferably less than 15 BTU/ft hr.degree. F., with a most preferred
material having a thermal conductivity of less than 10 BTU/ft hr.degree.
F. Another important feature of a desirable material for tube 30
is thermal expansion. Thus, a suitable material should have a thermal
expansion coefficient of less than 15.times.10.sup.-6 in/in/.degree.
F., with a preferred thermal expansion coefficient being less than
10.times.10.sup.-6 in/in/.degree. F., and the most preferred being
less than 5.times.10.sup.-6 in/in/.degree. F. Another important
feature of the material useful in the present invention is chilling
power. Chilling power is defined as the product of heat capacity,
thermal conductivity and density. Thus, preferably the material
in accordance with the invention has a chilling power of less than
5000 BTU.sup.2/ft.sup.4hr.deg- ree. F., preferably less than 2000
BTU.sup.2/ft.sup.4hr.degree. F., and typically in the range of 100
to 750 BTU.sup.2/ft.sup.4hr.degree. F.
[0017] As noted, the preferred material for fabricating into tubes
30 is a titanium base material or alloy having a thermal conductivity
of less than 30 BTU/ft hr.degree. F., preferably less than 15 BTU/ft
hr.degree. F., and typically less than 10 BTU/ft hr.degree. F.,
and having a thermal expansion coefficient less than 15.times.10.sup.-6
in/in/.degree. F., preferably less than 10.times.10.sup.-6 in/in/.degree.
F., and typically less than 5.times.10.sup.-6 in/in/.degree. F.
The titanium material or alloy should have chilling power as noted,
and for titanium, the chilling power can be less than 500, and preferably
less than 400, and typically in the range of 100 to 300 BTU/ft.sup.2
hr.degree. F.
[0018] When the electric heater assembly is being used in molten
metal such as lead, for example, the titanium base alloy need not
be coated to protect it from dissolution. For other metals, such
as aluminum, copper, steel, zinc and magnesium, refractory-type
coatings should be provided to protect against dissolution of the
metal or metalloid tube by the molten metal.
[0019] For most molten metals, the titanium alloy that should be
used is one that preferably meets the thermal conductivity requirements,
the chilling power and the thermal expansion coefficient noted herein.
Further, typically, the titanium alloy should have a yield strength
of 30 ksi or greater at room temperature, preferably 70 ksi, and
typical 100 ksi. The titanium alloys included herein and useful
in the present invention include CP (commercial purity) grade titanium,
or alpha and beta titanium alloys or near alpha titanium alloys,
or alpha-beta titanium alloys. The titanium-base alloy can be a
titanium selected from the group consisting of 6242, 1100 and commercial
purity (CP) grade. The alpha or near-alpha alloys can comprise,
by wt. %, 2 to 9 Al, 0 to 12 Sn, 0 to 4 Mo, 0 to 6 Zr, 0 to 2 V
and 0 to 2 Ta, and 2.5 max. each of Ni, Nb and Si, the remainder
titanium and incidental elements and impurities.
[0020] Specific alpha and near-alpha titanium alloys contain, by
wt. %, about:
[0021] (a) 5 Al, 2.5 Sn, the remainder Ti and impurities.
[0022] (b) 8 Al, 1 Mo, 1 V, the remainder Ti and impurities.
[0023] (c) 6 Al, 2 Sn, 4 Zr, 2 Mo, the remainder Ti and impurities.
[0024] (d) 6 Al, 2 Nb, 1 Ta, 0.8 Mo, the remainder Ti and impurities.
[0025] (e) 2.25 Al, 11 Sn, 5 Zr, 1 Mo, the remainder Ti and impurities.
[0026] (f) 5 Al, 5 Sn, 2 Zr, 2 Mo, the remainder Ti and impurities.
[0027] The alpha-beta titanium alloys comprise, by wt. %, 2 to
10 Al, 0 to 5 Mo, 0 to 5 Sn, 0 to 5 Zr, 0 to 11 V, 0 to 5 Cr, 0
to 3 Fe, with 1 Cu max., 9 Mn max., 1 Si max., the remainder titanium,
incidental elements and impurities.
[0028] Specific alpha-beta alloys contain, by wt. %, about:
[0029] (a) 6 A, 4 V, the remainder Ti and impurities.
[0030] (b) 6 Al, 6 V, 2 Sn, the remainder Ti and impurities.
[0031] (c) 8 Mn, the remainder Ti and impurities.
[0032] (d) 7 Al, 4 Mo, the remainder Ti and impurities.
[0033] (e) 6 Al, 2 Sn, 4 Zr, 6 Mo, the remainder Ti and impurities.
[0034] (f) 5 Al, 2 Sn, 2 Zr, 4 Mo, 4 Cr, the remainder Ti and impurities.
[0035] (g) 6 Al, 2 Sn, 2 Zn, 2 Mo, 2 Cr, the remainder Ti and impurities.
[0036] (h) 10 V, 2 Fe, 3 Al, the remainder Ti and impurities.
[0037] (i) 3 Al, 2.5 V, the remainder Ti and impurities.
[0038] The beta titanium alloys comprise, by wt. %, 0 to 14 V,
0 to 12 Cr, 0 to 4 Al, 0 to 12 Mo, 0 to 6 Zr and 0 to 3 Fe, the
remainder titanium and impurities.
[0039] Specific beta titanium alloys contain, by wt. %, about:
[0040] (a) 13 V, 11 Cr, 3 Al, the remainder Ti and impurities.
[0041] (b) 8 Mo, 8 V, 2 Fe, 3 Al, the remainder Ti and impurities.
[0042] (c) 3 Al, 8 V, 6 Cr, 4 Mo, 4 Zr, the remainder Ti and impurities.
[0043] (d) 11.5 Mo, 6 Zr, 4.5 Sn, the remainder Ti and impurities.
[0044] When it is necessary to provide a coating to protect tube
30 of metal or metalloid from dissolution or attack by molten metal,
a refractory coating 34 is applied to the outside surface of tube
30. The coating should be applied above the level to which the electric
heater assembly is immersed in the molten metal. The refractory
coating can be any refractory material, which provides the tube
with a molten metal resistant coating. The refractory coating can
vary, depending on the molten metal. Thus, a novel composite material
is provided permitting use of metals or metalloids having the required
thermal conductivity and thermal expansion for use with molten metal,
which heretofore was not deemed possible.
[0045] When the electric heater assembly is to be used for heating
molten metal such as aluminum, magnesium, zinc, or copper, etc.,
a refractory coating may comprise at least one of alumina, zirconia,
yttria stabilized zirconia, magnesia, magnesium titanite, or mullite
or a combination of alumina and titania. While the refractory coating
can be used on the metal or metalloid comprising the tube, a bond
coating can be applied between the base metal and the refractory
coating. The bond coating can provide for adjustments between the
thermal expansion coefficient of the base metal alloy, e.g., titanium,
and the refractory coating when necessary. The bond coating thus
aids in minimizing cracking or spalling of the refractory coat when
the tube is immersed in the molten metal or brought to operating
temperature. When the electric heater assembly is cycled between
molten metal temperature and room temperature, for example, the
bond coat can be advantageous in preventing cracking, particularly
if there is a considerable difference between the thermal expansion
of the metal or metalloid and the refractory.
[0046] Typical bond coatings comprise Cr--Ni--Al alloys and Cr--Ni
alloys, with or without precious metals. Bond coatings suitable
in the present invention are available from Metco Inc., Cleveland,
Ohio, under the designation 460 and 1465. In the present invention,
the refractory coating should have a thermal expansion that is plus
or minus five times that of the base material. Thus, the ratio of
the coefficient of expansion of the base material can range from
5:1 to 1:5, preferably 1:3 to 1:1.5. The bond coating aids in compensating
for differences between the base material and the refractory coating.
[0047] The bond coating has a thickness of 0.1 to 5 mils with a
typical thickness being about 0.5 mil. The bond coating can be applied
by sputtering, plasma or flame spraying, chemical vapor deposition,
spraying, dipping or mechanical bonding by rolling, for example.
[0048] After the bond coating has been applied, the refractory
coating is applied. The refractory coating may be applied by any
technique that provides a uniform coating over the bond coating.
The refractory coating can be applied by aerosol, sputtering, plasma
or flame spraying, for example. Preferably, the refractory coating
has a thickness in the range of 0.3 to 42 mils, preferably 5 to
15 mils, with a suitable thickness being about 10 mils. The refractory
coating may be used without a bond coating.
[0049] In another aspect of the invention, boron nitride may be
applied as a thin coating on top of the refractory coating. The
boron nitride may be applied as a dry coating, or a dispersion of
boron nitride and water may be formed and the dispersion applied
as a spray. The boron nitride coating is not normally more than
about 2 or 3 mils, and typically it is less than 2 mils.
[0050] The heater assembly of the invention can operate at watt
densities of 40 to 120 watts/in.sup.2.
[0051] The heater assembly in accordance with the invention has
the advantage of a metallic-composite sheath for strength and improved
thermal conductivity. The strength is important because it provides
resistance to mechanical abuse and permits an ultimate contact with
the internal element. Intimate contact between heating element and
sheath inside diameter provides for substantial elimination of an
annular air gap between heating element and sheath. In prior heaters,
the annular air gap resulted in radiation heat transfer and also
backs radiation to the element from inside the sheath wall which
limits maximum heat flux. By contrast, the heater of the invention
employs an interference fit that results in essentially only conduction.
[0052] In conventional heaters, the heating element is not in intimate
contact with the protection tube resulting in an annular air gas
or space there between. Thus, the element is operated at a temperature
independent of the tube. Heat from the element is not efficiently
removed or extracted by the tube, greatly limiting the efficiency
of the heaters. Thus, in conventional heaters, the element has to
be operated below a certain fixed temperature to avoid overheating
the element, greatly limiting the heat flux.
[0053] The heater assembly of the invention very efficiently extracts
heat from the heating element and is capable of operating close
to molten metal, e.g., aluminum temperature. The heater assembly
is capable of operating at watt densities of 40 to 120 watts/in.sup.2.
The low coefficient of expansion of the composite sheath, which
is lower than the heating element, provides for intimate contact
of the heating element with the composite sheath.
[0054] In another feature of the invention, a thermocouple (not
shown) may be inserted between sleeve 12 and heating element 14.
The thermocouple may be used for purposes of control of the heating
element to ensure against overheating of the element in the event
that heat is not transferred away sufficiently fast from the heating
assembly. Further, the thermocouple can be used for sensing the
temperature of the molten metal. That is, sleeve 12 may extend below
or beyond the end of the heating element to provide a space and
the sensing tip of the thermocouple can be located in the space.
[0055] Another embodiment of the heater is shown in FIG. 2. Heater
40 comprises a tube 42. In the embodiment shown in FIG. 2, tube
42 is comprised of an electrical insulating or dielectric inside
surface layer 44, a metal or metalloid layer 46 and a molten metal
protective layer 48. The molten metal protective layer is only necessary
when the heater is used for heating molten metal such as molten
aluminum, which would attack the metal layer 46.
[0056] Metal layer 46 can be comprised of any metal. However, when
a refractory or protective layer is applied, it is preferred to
use a metal or metalloid having a low coefficient of expansion such
as referred to herein. Also, molten metal protective layer or refractory
48 may be the same as referred to herein. Further, protective layer
48 may be applied as described herein.
[0057] When tube 42 is comprised of a metal layer 46, then electrical
insulating layer 44 is required on the inside surface to prevent
the heater from short circuiting. Any high thermal conductivity
electrical insulating layer, which electrically isolates the metal
from the heating element, may be used. Typical electrical insulating
layers are comprised of alumina, magnesia, mullite, silicon carbide,
silicon nitride and SiAlON. These layers may be applied by casting
in-place, spray deposition, or mechanical insertion of a pre-cast
or extruded form. Such layers usually have a total thickness in
the range of 10 to 500 mils.
[0058] In the embodiment shown in FIG. 2, an end cap 50 is used
to protect the end of the heater tube. End cap 50 may be comprised
of a refractory or carbon material.
[0059] The heater of the invention employs a powdered media 60
as the electrical resistive material for generating heat on the
passage of electrical current there through. In the embodiment shown
in FIG. 2, the powdered media extends from an upper portion of the
tube referred generally as 62 to lower portion of tube referred
to generally as 64. Any powdered material may be used that has the
characteristics to generate heat on the passage of electric current.
Preferably, the powdered media has an electrical resistivity in
the range of 3 to 95 .OMEGA.-mm and typically a resistivity in the
range of 8 to 30 .OMEGA.-mm. The powdered media may comprise a mixture
of an electrically conductive powder such as carbon and an electrically
non-conductive powder such as silicon carbide. Thus, the powdered
media may be comprised of a powder selected from the group consisting
of SiC, C, Mo, TiN, MoSi.sub.2, Al.sub.2O.sub.3, MgO, TiB.sub.2,
Si.sub.3N.sub.4, TiO.sub.2 and BN. One or more of these powders
may be mixed or combined to provide the desired resistivity. For
example, carbon powder may be combined with at least one of SiC,
Si.sub.3N.sub.4, SiO.sub.2 and BN powder. When carbon is used as
the electrical conductive media, it may be present in the range
of 8 to 30 vol. % and typically in the range of 12 to 24 vol. %,
depending on the resistivity desired. A typical media can comprise
22 wt. % carbon and 78 wt. % SiC. Because of the high temperatures
that can be obtained with the heaters of the invention, it is important
to have media with high melting points. Thus, the powdered media
can have a melting point up to 4000.degree. C., for example, and
the melting point can range from about 800.degree. to 4000.degree.
C., depending on the media.
[0060] In order to provide for packing of the powder in tube 42,
the powdered media should have good packing density. The powder
media can have an average particle size ranging from about 5 to
3000 .mu.m. When a mixture of carbon and SiC is used, the carbon
component of the powdered media has an average size range of about
0.5 to 60 .mu.m, and the SiC particles have an average size of about
3 to 200 .mu.m. Proper mixing of the components has been found to
be an important consideration, and the preferred method for accomplishing
this is to use a splitter.
[0061] Thus, it will be seen that in the present invention, the
powdered media comprises the electrical heating element or member
for the heater.
[0062] In the embodiment shown in FIG. 2, an electrical current
feed post 70 is shown extending from top 72 to bottom 74, which
terminates in a plate member 76. Post 70 is provided with an electrical
insulating layer 78 to prevent electrical short-circuiting. In the
embodiment shown, layer 78 extends from top 73 to plate member 76
and can comprise an alumina tube or layer. Other dielectric material
may be used, such as referred to for use with inside tube 62. It
will be noted that plate member 76 is not insulated and passes electric
current to powdered media 60. Electric current passes through powdered
media 60 and is removed from media 60 through member 80, which is
concentric with post 70. Post 70 and member 80 are connected to
an electrical power source via lines 84 and 86. To avoid overheating
at the interface between plate member 76 and member 80, a media
having lowered electrical resistivity is provided. In FIG. 2, the
resistivity is shown being increased in two steps going from a low
resistivity media 88 such as carbon to a higher resistivity media
such as a 50 wt. % carbon, 50 wt. % SiC media before contacting
resistive media 60.
[0063] It has been discovered that compaction stress is important
to permit the powdered media to function as an electrical resistive
heating element. That is, at a low compaction stress range, the
resistivity of a particular powdered media composition is low and
changes rapidly with increasing compaction stress (see FIG. 3).
It is particularly desirable for the resistivity values to be a
constant and repeatable as possible. This occurs at higher compaction
stress values where it is believed that intimacy between the individual
particles is high. Thus, for purposes of the invention, the stress
applied to the powdered media should be applied in the range of
35,000 to 60,000 psi. It has also been discovered that throughout
heater operation, the compaction stress values do not need to be
maintained as high as during the initial compaction. Relatively
low compaction stress can be applied and maintained as a pressure
within the heater itself.
[0064] In the invention as illustrated in FIG. 2, pressure may
be applied to the powdered media through spring 102. Pressure may
be applied to spring 102 using threaded fastener 106. It should
be noted that spring 102 compresses member 108 and is concentric
with section 109 of member 80. It should be noted that member 108
is also formed from a dielectric material such as used on post 70
or on the inside of tube 42 in order to prevent short circuiting
with metal tube 46.
[0065] A housing 120 is shown provided around spring 102 and top
portion of heater 40. The housing can be purged with gas such as
argon, which acts as a gettering agent for oxygen, or other gases
released from the powdered media. Also, a thermocouple 130 having
a probe 132 is shown located in housing 120. This may be used to
provide control of the heat output and avoid overheating. Leads
from the heater and thermocouple are removed through valve 140.
[0066] Rod 70 and member 80 may be comprised of titanium or other
electrical conductive material. Because of the compressive forces
applied to powdered media 60 by spring 102, it is preferred that
dielectric layers 44 and 78 either be intimately bonded to the metal
surface, for example, by plasma spraying or be flexible initially
to avoid cracking as the powdered media is compacted.
[0067] While the invention has been described in terms of preferred
embodiments, the claims appended hereto are intended to encompass
other embodiments, which fall within the spirit of the invention.
[0068] Having described the presently preferred embodiments, it
is to be understood that the invention may be otherwise embodied
within the scope of the appended claims.
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