Abstrict
An improved electric heater assembly suitable for heating molten
metal, the electric heater assembly having a tube comprised of a
closed end suitable for immersing in the molten metal. The tube
is fabricated from a composite material comprised of a metal having
a coefficient of thermal expansion of less than 10.times.10.sup.-6
in/in/.degree. F. 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 tube 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 tube having a closed end suitable for immersing in said molten
metal, the tube fabricated from a composite material comprised of
a metal case having a thermal coefficient of expansion of less than
10.times.10.sup.-6 in/in/.degree. F.,
(i) the metal case fabricated from a metal selected from the group
consisting of titanium or titanium alloy stainless steel nickel
based alloys and iron based alloys; and
(ii) the tube having an outside surface to be exposed to said molten
metal coated with a refractory coating having a coefficient of expansion
less than 10.times.10.sup.-6 in/in/.degree. F. and being resistant
to attack by said molten metal;
(b) an electric heating element located in said tube in heat transfer
relationship therewith for adding heat to said molten metal; and
(c) a powdered contact medium provided in said tube between said
heating element and said tube, the contact medium having the ability
to conduct heat from said heating element to said tube to improve
heat transfer.
2. The electric heater assembly in accordance with claim 1 wherein
the metal case has a thermal expansion coefficient of less than
5.times.10.sup.-6 in/in/.degree. F.
3. The electric heater assembly in accordance with claim 1 wherein
the metal case is comprised of a titanium alloy selected from the
group consisting of alpha, beta, near alpha, and alpha-beta titanium
alloys having a thermal coefficient of expansion of 5.times.10.sup.-6
in/in/.degree. F.
4. The electric heater assembly in accordance with claim 1 wherein
the metal case is formed from a titanium based alloy selected from
the group consisting of 6242, 1100 and CP grade.
5. The electric heater assembly in accordance with claim 1 wherein
a bond coating is provided between the outside surface of the metal
case and the refractory.
6. The electric heater assembly in accordance with claim 1 wherein
the refractory coating 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,
SiAlON and Al.sub.2 O.sub.3 --TiO.sub.2.
7. The electric heater assembly in accordance with claim 1 wherein
a bond coating having a thickness in the range of 0.1 to 8 mils
is provided on said outside surface between said metal case and
said refractory.
8. The electric heater assembly in accordance with claim 1 wherein
said refractory has a thickness in the range of 0.3 to 42 mils.
9. The electric heater assembly in accordance with claim 1 wherein
a bond coating is provided between said outside surface and said
refractory coating and said bond coating comprises an alloy selected
from the group consisting of a Cr--Ni--Al alloy and a Cr--Ni alloy.
10. The electric heater assembly in accordance with claim 1 wherein
the refractory comprises alumina.
11. The electric heater assembly in accordance with claim 1 wherein
the refractory coating comprises zirconia.
12. The electric heater assembly in accordance with claim 1 wherein
the refractory coating comprises yittria stabilized zirconia.
13. The electric heater assembly in accordance with claim 1 wherein
the refractory coating comprises 5 to 20 wt. % titania and the balance
alumina.
14. The electric heater assembly in accordance with claim 1 wherein
the electric heating element is provided in said metal case which
is deformed by one of rolling and swaging, said rolling and swaging
performed prior to applying said refractory coating.
15. An electric heater assembly suitable for heating molten metal,
the electric heater assembly comprised of a tube having a closed
end suitable for immersing in said molten metal, the tube fabricated
from a composite material comprised of:
(a) a base metal layer of a titanium alloy having a coefficient
of expansion less than 10.times.10.sup.-6 in/in/.degree. F.;
(b) a bond coat bonded to an outside surface of said base layer
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, the refractory layer
having a coefficient of expansion less than 10.times.10.sup.-6 in/in/.degree.
F.;
(d) an electric heating element positioned in said tube in heat
transfer relationship; and
(e) a contact medium provided in said tube between said heating
element and said tube to fill air gaps between said element and
said tube, the contact medium having the ability to conduct heat
from said element to said tube and improve heat transfer.
16. The heater assembly in accordance with claim 15 wherein said
contact medium is one of a powdered material.
17. The heater assembly in accordance with claim 15 wherein the
powdered material has a median particle size in the range of 0.03
to 0.3 mm.
18. A method of forming an electric heater assembly for heating
molten metal, the electric heater assembly comprised of a tube having
a closed end suitable for immersing in said molten metal, the tube
fabricated from a composite material comprising the steps of:
(a) providing a tube of metal selected from the group consisting
of a titanium based alloy, nickel based alloy, iron based alloy
and stainless steels, said metal having a coefficient of thermal
expansion of less than 8.times.10.sup.-6 in/in/.degree. F.;
(b) providing a contact medium in said tube;
(c) locating an electric heater in said tube;
(d) forming said tube about said heating element thereby compressing
said contact medium;
(e) applying a bond coat bonded to an outside surface of said metal
to coat said surface to be exposed to said molten metal; and
(f) applying a refractory layer to said bond coat, the refractory
layer resistant to attack by said molten metal, the refractory layer
having a coefficient of expansion less than 10.times.10.sup.-6 in/in/.degree.
F.
19. The method in accordance with claim 18 wherein said forming
includes rolling or swaging.
20. An electric heater assembly suitable for heating molten metal,
the electric heater assembly comprised of:
(a) a tube having a closed end suitable for immersing in said molten
metal, the tube fabricated from a composite material comprised of
metal case having a thermal coefficient of expansion of 10.times.10.sup.-6
in/in/.degree. F. and having an outside surface to be exposed to
said molten metal coated with a refractory resistant to attack by
said molten metal;
(b) an electric heating element located in said tube in heat transfer
relationship therewith for adding heat to said molten metal; and
(c) a thermocouple positioned in said tube for purposes of monitoring
the heat output of said heating element and preventing said heating
element from overheating.
21. A composite material for use with molten metal, the composite
material having a tensile strength of greater than 30 ksi and being
resistant to attack by the molten metal, the composite material
comprising:
(a) a base layer of metal having an expansion coefficient of less
than 10.times.10.sup.-6 in/in/.degree. F.;
(b) a bond coating applied to a surface of said base layer, the
bond coating having a thickness in the range of 0.1 to 8 mils and
a thermal coefficient of expansion of less than 10.times.10.sup.-6
in/in/.degree. F.;
(c) a protective refractory coating applied to said bond coating,
the refractory coating having a coefficient of expansion of less
than 10.times.10.sup.-6 in/in/.degree. F., said refractory coating
bonded to said bond coating; and
(d) a molten metal substantially non-wetting coating applied to
said refractory coating, said non-wetting coating selected from
one of the group consisting of silicon carbide, boron nitride, silicon
aluminum oxynitride and silicon nitride.
22. The composite material in accordance with claim 21 wherein
said base layer of metal is selected from one of the group consisting
of a titanium based alloy, a nickel based alloy and stainless steel,
said base layer of metal having a coefficient of thermal expansion
less than 8.times.10.sup.-6 in/in/.degree. F.
23. The composite material in accordance with claim 21 wherein
said nonwetting coating is boron nitride.
24. The composite material in accordance with claim 21 wherein
said refractory coating has a porosity of 3 to 22%.
25. The composite material in accordance with claim 21 wherein
said refractory coating is selected from a material consisting of
Al.sub.2 O.sub.3, ZnO.sub.2, Y.sub.2 O.sub.3 stabilized ZnO.sub.2,
Al.sub.2 O.sub.3, SiAlON and TiO.sub.2.
26. A composite material for use with molten aluminum, the composite
material having a tensile strength of greater than 30 ksi and being
resistant to attack by said molten aluminum, the composite material
comprising:
(a) a base layer of titanium base alloy having an expansion coefficient
of less than 5.times.10.sup.-6 in/in/.degree. F.;
(b) a bond coating applied to a surface of said base layer, the
bond coating having a thickness in the range of 0.1 to 5 mils and
a thermal coefficient of expansion of less than 10.times.10.sup.-6
in/in/.degree. F.;
(c) a protective refractory coating having a coefficient of expansion
of less than 10.times.10.sup.-6 in/in/.degree. F. and resistant
to attack by said molten aluminum, said refractory coating bonded
to said bond coating and having a thickness in the range of 4 to
22 mils, the refractory coating having a porosity of 3 to 22%; and
(d) a boron nitride coating applied to said refractory coating,
the boron nitride being substantially non-wettable by said molten
aluminum.
27. The composite material in accordance with claim 26 wherein
said refractory coating is selected from a material consisting of
Al.sub.2 O.sub.3, ZnO.sub.2, Y.sub.2 O.sub.3 stabilized ZnO.sub.2,
Al.sub.2 O.sub.3, SiAlON and TiO.sub.2.
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
tubes are 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 or tube 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 or tube having a thermal expansion
coefficient of less than 15.times.10.sup.-6 in/in/.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 covering 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 or container having
a closed end suitable for heating molten metal, the sleeve or container
fabricated from a composite material comprised of a metal or nonmetal
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 or container 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 or tube 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. Also, heating elements are available from Watlow AOU,
Anaheim, Calif.
Preferably, protective sleeve or tube 12 is comprised of metal
tube 30 having a closed end 32. While the protective sleeve is illustrated
as a tube, it will be appreciated that any configuration or container
that protects or envelops electric heating element 14 from the molten
metal may be employed. By the use of sleeve or tube as used herein
is meant to include any kind of means, such as a metal case, container,
envelope, casing or covering used to protect the heating element
from the molten metal, and the heating element may be inserted into
the protective tube and/or the metal case may be formed around the
heating element, e.g., by swaging or rolling, and a protective layer
applied after forming. 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 metal 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. That is, often the electric heating element is surrounded
or protected with a metal tube such as a steel tube. The electric
element is separated from the metal tube with an insulating material
such as a metal oxide, e.g., magnesium oxide. It is the outside
of the metal tube which is provided with a friction fit with the
inside of the tube 30. Thus, air gaps between the surface of the
steel tube 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 a metal such as titanium having a low coefficient of expansion.
While it is preferred to fabricate tube or metal case 30 out of
a titanium based alloy, tube 30 may be fabricated from any metal,
or combination of metal and non-metallic or metalloid material with
suitable surface protection suitable for contacting molten metal
and which material is resistant to dissolution or erosion by the
molten metal. Other base materials that may be used to fabricate
tube 30 include silicon, niobium, chromium, molybdenum, cobalt,
iron, nickel based alloys including combinations of NiFe (364 NiFe)
and NiTiC (40 Ni 60TiC), IN783.RTM., INCONEL.RTM., LAPALLOY.RTM.,
INVAR.RTM. or KOVAR.RTM., particularly when such materials have
low thermal expansion, e.g., less than 10.times.10.sup.-6 in/in/.degree.
F., 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.
One of the important features 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 8.times.10.sup.-6 in/in/.degree. F. and typically less
than 5.times.10.sup.-6 in/in/.degree. F. All ranges herein include
all the numbers within the range as if specifically set forth.
As noted, the preferred material for fabricating into tubes 30
is a titanium base material or alloy 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 material or metal out of which tube 30 is fabricated preferably
has an interfacial shear stress with refractory coating 34 of 2
to 175 KSI and preferably 15 to 45 KSI and typically less than 35
KSI at a surface temperature of 1080.degree. F. of the tube and
a surface temperature of 1300.degree. F. refractory surface.
When the electric heater assembly is being used in molten metal
such as lead, for example, the titanium based 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
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 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 1 IV, 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, mullite, a combination
of alumina and titania or a material such as SiAlON (silicon aluminum
oxynitride). 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
if the interfacial shear stress is too high. Preferably, the refractory
coating has a porosity of about 3 to 22% and median pore diameter
of 0.01 to 0.15 mm. The refractory coating may be fully dense but
it is more subject to thermal shock.
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 to the refractory coating 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 8 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, silicon carbide, boron nitride,
silicon nitride, and other metal oxides, and combinations of carbides,
nitrides and oxides, may be applied as a thin coating on top of
the refractory coating. The thin coating should be non-wetting or
metallaphobic, that is, have a contact angle of greater than 90.degree.
with liquid or molten material in which the heater is immersed.
Thus, any non-wetting coating which has these characteristics may
be used. The preferred material is boron nitride. The non-wetting
coating may be applied mechanically, vacuum impregnated, sprayed,
or co-plasma sprayed with 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
non-wetting coating is not normally more than about 2 or 3 mils
in thickness, and typically it is less than 2 mils.
When boron nitride or other non-wetting refractory material is
applied dry or in a water dispersion, the particle size should be
sufficiently small, e.g., less than 75 .mu.m and typically less
than 30 .mu.m, to permit intrusion of the boron nitride particles
into the pores of the refractory coating.
The heater assembly of the invention can operate at watt densities
of 15 and preferably 40 to 375 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 intimate contact with the internal
element. Intimate contact between heating element and sheath I.D.
provides 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 another aspect of the invention, it has been found that intimate
contact or fit can be obtained by swaging metal tube 30 about or
onto heating element 14. It will be appreciated that element 14
is circular in cross section and, therefore, tube 30 can be swaged
tightly onto element 14, thereby substantially eliminating air gaps.
Swaging includes the operation of working and partially reshaping
metal tube 30, particularly the inside diameter, placing in compression,
the tube contents, and more exactly fitting the outside diameter
of element 14 to eliminate air gaps between element 14 and tube
30. It will be appreciated that intermediate tubes may be placed
between the heating element of the heater assembly and tube 30.
Further, the invention contemplates a heating element wire or rod
surrounded by an electrical insulating material such as a powder
which has good heat conduction, e.g., magnesium oxide, contained
by tube 30 without any intermediate tubes such as steel tubes.
When tube 30 is swaged on heater element 14, the refractory coating
is applied after swaging. Whether the heater assembly is made by
inserting heating element 14 into tube 30 or by swaging, as noted,
it can be beneficial to use a contact medium for better heat conduction
between heating element 14 and tube 30. The contact medium can be
a powdered material located between the heating element and the
tube. The powdered material can be selected from silicon carbide,
magnesium oxide and carbon or graphite if the heating element is
contained in an intermediate tube. If no intermediate tube is used,
the contact medium must provide electrical insulation as well as
good heat conduction. The powdered material should have a median
particle size ranging from about 0.03 to 0.3 mm. The powdered material
has the effect of filling any voids between the heating element
and the tube. The range of size for the powdered material improves
heat conduction by minimizing void fraction. Swaging is very beneficial
with the powdered material because the swaging effectively packs
the powder tighter for improved heat conduction.
The inside of tube 30 may be treated to provide a roughening effect
or controlled RMS for improved packing of powder against the inside
wall of tube 30. That is, having a range of particle size and a
roughened inside wall provides a higher level of contact by said
powdered contact medium and therefore a greater level of heat conduction
to the wall. In addition, providing the element with a roughened
surface improves heat conduction to the powdered contact medium.
If an intermediate metal tube, e.g., a steel tube, is used, then
it is also important to provide it with a roughened surface for
heat transfer.
Another contact medium that may be used includes high temperature
pastes such as anti-seize compounds having a nickel or copper base.
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 low coefficient
of expansion of the composite sheath, which is lower than the heating
element, maintains intimate contact of the heating element with
the composite sheath.
In another feature of the invention, a thermocouple 40 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 by an
analog method. 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.
In a preferred embodiment, thermocouple 40 is positioned such that
tip 42 of thermocouple 40 is located adjacent end 16 of the heating
element. Having tip 42 positioned adjacent or near end 16 ensures
that the heater assembly is immersed in the liquid metal. That is,
because of the high level of heat generated by the heater assembly,
it is important that the heating element be submerged in order to
remove heat efficiently. If part of the heating element extends
above the metal line, the element can overheat causing damage to
the assembly.
In the present invention, it is important to use a heater control.
That is, for efficiency purposes, it is important to operate heaters
at the highest watt density while not exceeding the maximum allowable
element temperature. The thermocouple placed or positioned in the
heater assembly senses the temperature of the heater element. The
thermocouple can be connected to a controller such as a cascade
logic controller to integrate the heater element temperature into
the control loop. Such cascade logic controllers are available from
Watlow Controls, Winona, Minn., designated Series 988.
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. |