Abstrict
An electric heater assembly suitable for heating molten metal,
the electric heater assembly having a sleeve comprised of a closed
end suitable for immersing in the molten metal. The sleeve is fabricated
from a composite material comprised of 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; and an electric
heater located in the sleeve in heat transfer relationship therewith.
Claims
What is claimed is:
1. An electric heater assembly suitable for heating molten metal,
the electric heater assembly comprised of:
(a) a sleeve having a closed end suitable for immersing in said
molten metal, the sleeve fabricated from a composite material comprised
of titanium alloy and having an outside surface coated with a refractory
which is resistant from attack by said molten metal, when said sleeve
is exposed to said molten metal; and
(b) an electric heating element located in said sleeve in heat
transfer relationship therewith for adding heat to said molten metal.
2. The electric heater assembly in accordance with claim 1 wherein
the titanium alloy has a thermal expansion coefficient of less than
15.times.10.sup.-6 in/in/.degree.F.
3. The electric heater assembly in accordance with claim 1 wherein
the titanium alloy has a thermal expansion coefficient of less than
10.times.10.sup.-6 in/in/.degree.F. and a chilling power of less
than 5000 BTU.sup.2 /ft.sup.4 hr.degree.F.
4. The electric heater assembly in accordance with claim 1 wherein
the titanium alloy is selected from the group consisting of alpha,
beta, near alpha, and alpha-beta titanium alloys having a chilling
power of less than 500 BTU.sup.2 /ft.sup.4 hr.degree.F.
5. The electric heater assembly in accordance with claim 1 wherein
the titanium alloy is selected from the group consisting of 6242,
1100 titanium alloy and commercial purity grade titanium.
6. The electric heater assembly in accordance with claim 1 wherein
a bond coating is provided between the titanium alloy sleeve's outside
surface and the refractory.
7. The electric heater assembly in accordance with claim 1 wherein
the refractory is selected from the group consisting of one of Al.sub.2
O.sub.3, ZrO.sub.2, Y.sub.2 O.sub.3 stabilized ZrO.sub.2, and Al.sub.2
O.sub.3 --TiO.sub.2.
8. The electric heater assembly in accordance with claim 1 wherein
a bond coating having a thickness in the range of 0.1 to 5 mils
is provided between said titanium alloy and said refractory.
9. The electric heater assembly in accordance with claim 1 wherein
said refractory has a thickness in the range of 0.3 to 42 mils.
10. The electric heater assembly in accordance with claim 1 wherein
a bond coating is provided between said titanium alloy and said
refractory and said bond coating comprises an alloy selected from
the group consisting of a Cr--Ni--Al alloy and a Cr--Ni alloy.
11. The electric heater assembly in accordance with claim 1 wherein
the refractory comprises alumina.
12. The electric heater assembly in accordance with claim 1 wherein
the refractory comprises zirconia.
13. The electric heater assembly in accordance with claim 1 wherein
the refractory comprises yittria stabilized zirconia.
14. The electric heater assembly in accordance with claim 1 wherein
the refractory comprises 5 to 20 wt. % titania and a balance of
alumina.
15. The electric heater assembly in accordance with claim 1 wherein
the electric heating element has an outside surface in contact with
an inside surface of said sleeve.
16. An electric heater assembly suitable for heating molten metal,
the electric heater assembly comprised of a sleeve having a closed
end suitable for immersing in said molten metal, the sleeve fabricated
from a composite material comprised of:
(a) a base metal layer of a titanium alloy;
(b) a bond coat bonded to an outside surface of said base layer
to coat said surface to coat said surface to be exposed to said
molten metal;
(c) a refractory layer bonded to said bond coat, the refractory
layer resistant to attack by said molten metal; and
(d) an electric heating element located in said sleeve in heat
transfer relationship therewith for adding heat to said molten metal.
17. An electric heater assembly suitable for heating molten metal,
the electric heater assembly comprised of a sleeve having a closed
end suitable for immersing in said molten metal, the sleeve fabricated
from a composite material comprised of:
(a) a base metal layer of a titanium alloy selected from alpha,
beta, near alpha, and alpha-beta titanium alloys;
(b) a bond coat bonded to an outside surface of said base layer
to coat said surface to coat said surface to be exposed to said
molten metal;
(c) a refractory layer bonded to said bond coat, the refractory
layer resistant to attack by said molten metal; and
(d) an electric heater located in said sleeve in heat transfer
relationship therewith for adding heat to said molten metal.
18. The electric heater assembly in accordance with claim 16 wherein
said titanium alloy is selected from 6242, 1100 titanium alloys
and commercial purity grade titanium.
19. The electric heater assembly in accordance with claim 16 wherein
said base metal layer has a coefficient of thermal expansion of
less than 5.times.10-6 in/in/.degree.F.
20. The electric heater assembly in accordance with claim 16 wherein
said bond coat has a thickness in the range of 0.1 to 5 mils and
said refractory layer has a thickness in the range of 0.3 to 42
mils.
21. The electric heater assembly in accordance with claim 16 wherein
said refractory layer is selected from the group consisting of one
of Al.sub.2 O.sub.3, ZrO.sub.2, Y.sub.2 O.sub.3 stabilized ZrO.sub.2,
and Al.sub.2 O.sub.3 --TiO.sub.2.
22. The electric heater assembly in accordance with claim 16 wherein
said bond coat comprises an alloy selected from the group consisting
of Cr--Ni--Al alloy and Cr--Ni alloy.
23. The electric heater assembly in accordance with claim 16 wherein
the ratio of coefficient of expansion of the refractory layer to
the base metal layer is in the range of 5:1 to 1:5.
24. An electric heater assembly suitable for heating molten metal,
the electric heater assembly comprised of a sleeve having a closed
end suitable for immersing in said molten metal, the sleeve fabricated
from a composite material comprised of:
(a) a base layer of a titanium alloy;
(b) a bond coat bonded to an outside surface of said sleeve to
coat said surface to coat said surface to be exposed to said molten
metal;
(c) a refractory layer selected from a material comprising Al.sub.2
O.sub.3, ZrO.sub.2, Y.sub.2 O.sub.3 stabilized ZrO.sub.2, and Al.sub.2
O.sub.3 --TiO.sub.2 bonded to said bond coat, the refractory layer
resistant to attack by said molten metal; and
(d) a heating element located in said sleeve, said heating element
having an outside surface in contact with an inside surface of said
sleeve.
25. The electric heater assembly in accordance with claim 24 wherein
the refractory layer is Al.sub.2 O.sub.3 and said titanium alloy
is selected from 6242, 1100 titanium alloy and commercial purity
grade titanium.
26. The electric heater assembly in accordance with claim 24 wherein
said base layer has a chilling power in the range of 100 to 700
BTU.sup.2 /ft.sup.4 hr.degree.F.
Description BACKGROUND OF THE INVENTION
This invention relates to electric heaters, and more particularly,
it relates to electric heaters suitable for use in molten metals
such as molten aluminum.
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
It is an object of the invention to provide an improved electric
heater assembly.
It is another object of the invention to provide an improved electric
heater assembly for use in molten metal such as molten aluminum.
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 element, thereby substantially eliminating the
air gap between the heater and sleeve.
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.4 hr.degree.F.
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.
These and other objects will become apparent from the specification,
drawings and claims appended hereto.
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. An electric
heating element is located in the sleeve in heat transfer relationship
therewith for adding heat to the molten metal.
BRIEF DESCRIPTION OF THE FIGURE
The sole figure is a cross-sectional view of an electric heater
assembly in accordance with the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the Figure, 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. A lead 18 extends from electric heating element
14 and terminates in a plug 20 suitable for plugging into a power
source. A suitable element 14 is available from International Heat
Exchanger, Inc., Yorba Linda, Calif. 92687 under the designation
P/N HTR2252.
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. Electric heating element 14 is seated or secured in tube 30
by any convenient means. For example, swaglock nuts and ferrules
may be employed or the end of the tube may be crimped or swaged
shut to provide a secure fit between the electric heating element
and tube 30. In the invention, any of these methods of holding the
electric heating element in tube 30 may be employed. It should be
understood that tube 30 does not always have to be sealed. In a
preferred embodiment, electric heating element 14 is inserted into
tube 30 to provide an interference or friction fit. That is, it
is preferred that electric heating element 14 has its outside surface
in contact with the inside surface of tube 30 to promote heat transfer
through tube 30 into the molten metal. Thus, air gaps between the
surface of electric heating element 14 and inside surface of tube
30 should be minimized.
If electric heating element 14 is inserted in tube 30 with a friction
fit, the fit gets tighter with heat because electric heating element
14 expands more than tube 30, particularly when tube 30 is formed
from titanium.
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 NiF (364 NiFe) and NiTiC (40 Ni 60TiC),
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.
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.4 hr.degree.F.,
preferably less than 2000 BTU.sup.2 /ft.sup.4 hr.degree.F., and
typically in the range of 100 to 750 BTU.sup.2 /ft.sup.4 hr.degree.F.
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.
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.
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.
Specific alpha and near-alpha titanium alloys contain, by wt. %,
about:
(a) 5 Al, 2.5 Sn, the remainder Ti and impurities.
(b) 8 Al, 1 Mo, 1 V, the remainder Ti and impurities.
(c) 6 Al, 2 Sn, 4 Zr, 2 Mo, the remainder Ti and impurities.
(d) 6 Al, 2 Nb, 1 Ta, 0.8 Mo, the remainder Ti and impurities.
(e) 2.25 Al, 11 Sn, 5 Zr, 1 Mo, the remainder Ti and impurities.
(f) 5 Al, 5 Sn, 2 Zr, 2 Mo, the remainder Ti and impurities.
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.
Specific alpha-beta alloys contain, by wt. %, about:
(a) 6 A, 4 V, the remainder Ti and impurities.
(b) 6 Al, 6 V, 2 Sn, the remainder Ti and impurities.
(c) 8 Mn, the remainder Ti and impurities.
(d) 7 Al, 4 Mo, the remainder Ti and impurities.
(e) 6 Al, 2 Sn, 4 Zr, 6 Mo, the remainder Ti and impurities.
(f) 5 Al, 2 Sn, 2 Zr, 4 Mo, 4 Cr, the remainder Ti and impurities.
(g) 6 Al, 2 Sn, 2 Zn, 2 Mo, 2 Cr, the remainder Ti and impurities.
(h) 10 V, 2 Fe, 3 Al, the remainder Ti and impurities.
(i) 3 Al, 2.5 V, the remainder Ti and impurities.
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.
Specific beta titanium alloys contain, by wt. %, about:
(a) 13 V, 11 Cr, 3 Al, the remainder Ti and impurities.
(b) 8 Mo, 8 V, 2 Fe, 3 Al, the remainder Ti and impurities.
(c) 3 Al, 8 V, 6 Cr, 4 Mo, 4 Zr, the remainder Ti and impurities.
(d) 11.5 Mo, 6 Zr, 4.5 Sn, the remainder Ti and impurities.
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.
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, yittria
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.
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.
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.
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.
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.
The heater assembly of the invention can operate at watt densities
of 40 to 120 watts/in.sup.2.
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 back 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.
In conventional heaters, the heating element is not in intimate
contact with the protection tube resulting in an annular air gas
or space therebetween. 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.
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.
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.
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.
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