Abstrict
A vacuum furnace has a heating chamber heated by metal strip electric
heater elements. The elements have insulated mechanical support
between their ends to hold them in position in the furnace. Support
is provided by a ceramic post having an internal thread in an outer
end threaded onto a rod secured to a portion of the furnace. A transverse
member at an inner end of the post includes a straight surface normal
to the length of the post for engaging the heater element. The heater
element is compliantly connected to the straight surface. One embodiment
of heater element support is in the form of a T-shaped ceramic post
having an internal thread in the leg of the T and a ceramic crossbar
having a straight surface normal to the length of the post for receiving
the heater element. Another embodiment has a ceramic post having
an internal thread in the outer end of the post and a metallic crossbar
having a straight surface normal to the length of the post for receiving
a heater element.
Claims
What is claimed is:
1. A mechanical support for an intermediate portion of an electric
heater element in a furnace comprising:
a ceramic post having a length and an internal thread in an outer
end;
a threaded rod secured to a portion of the furnace and engaging
the internal thread of the post;
a transverse member at an inner end of the post including a straight
surface normal to the length of the post; and
means adjacent to the straight surface for securing a heater element
to the straight surface.
2. A support as recited in claim 1 wherein the transverse member
is a ceramic member integral with the inner end of the post and
forming a T-shaped ceramic support.
3. A support as recited in claim 2 wherein:
the post is generally cylindrical and has an axial passage therethrough,
the passage having an inner end and an outer end, the inner end
of the passage being enlarged for low heat transfer, the outer end
of the passage comprising the internal thread; and
the transverse member comprises a flat inner surface, a flat outer
surface, and diagonal reinforcing wings connecting the flat outer
surface and the post.
4. A support as recited in claim 3 wherein the transverse member
also includes a pair of spaced apart holes adjacent each end of
the member for receiving a tie wire.
5. A support as recited in claim 1 wherein the post comprises a
hollow portion in at least the inner end of the post.
6. A support as recited in claim 1 wherein the transverse member
comprises is a metal member secured to the post and forming a T-shaped
support.
7. A support as recited in claim 6 wherein the post comprises a
transverse slot in the inner end of the post and the metal member
comprises an L-shaped sheet having a first leg of the L-shaped sheet
in the slot and a second leg of the L-shaped sheet forming the straight
surface.
8. A support as recited in claim 7 further comprising a pair of
holes through the post transverse to the slot and a pair of holes
through the first leg, each hole in the first leg being aligned
with a hole through the post, and a wire extending through each
set of holes in the first leg and post, respectively, for securing
the L-shaped sheet in the slot.
9. A support as recited in claim 1 further comprising an axial
passage extending through the post, the thread being in the outer
end of the passage.
10. A support as recited in claim 1 wherein the post comprises
an enlarged inner end, a transverse slot in the enlarged inner end,
a pair of holes extending through the enlarged inner end transverse
to the slot, and an axial passage extending through the post, the
passage having an inner end and an outer end, the thread being in
the outer end of the passage.
11. A support as recited in claim 1 further comprising a second
ceramic post identical to the first mentioned post and spaced apart
therefrom, and wherein the transverse member comprises a member
extending between the first and second posts and forming a U-shaped
support.
12. A support as recited in claim 11 wherein the transverse member
is a metal sheet.
13. A support as recited in claim 11 wherein each post comprises
an external thread at the inner end of each post and further comprising
a ceramic nut on each external thread clamping the transverse member
to the post.
14. A support as recited in claim 11 further comprising an axial
passage extending through the post, the passage having an inner
end and an outer end, the internal thread being in the outer end
of the passage.
15. A support as recited in claim 1 wherein the post comprises
an external thread at the inner end of the post and further comprising
a ceramic nut on the external thread.
16. A mechanical support for an intermediate portion of an electric
heater element in a furnace comprising:
a T-shaped ceramic post having a leg, an internal thread in the
leg of the post and a ceramic crossbar having a straight surface
normal to the length of the leg for receiving a heater element.
17. A support as recited in claim 16 wherein the T-shaped ceramic
post has an inner end and comprises a hollow portion in at least
the inner end of the post.
18. A support as recited in claim 16 wherein:
the leg is generally cylindrical and has an axial passage therethrough,
the passage having an inner end and an outer end, the inner end
of the passage being enlarged for low heat transfer, the outer end
of the passage comprising the internal thread.
19. A support as recited in claim 16 wherein the crossbar comprises
a straight inner surface, an outer surface, and diagonal reinforcing
wings connecting the outer surface and the post.
20. A mechanical support for an intermediate portion of an electric
heater element in a furnace comprising:
a ceramic post having a length and an internal thread; and
a metallic crossbar connected to the post and having a straight
surface normal to the length of the post for receiving a heater
element.
21. A support as recited in claim 16 wherein the post has an inner
end and comprises a transverse slot in the inner end and further
comprising a L-shaped metal member having one leg of the L-shaped
member in the slot and the other leg of the L-shaped member forming
the straight surface.
22. A support as recited in claim 21 further comprising a pair
of holes through the post transverse to the slot and a pair of holes
through the leg of the L-shaped member in the slot, each hole in
the leg being aligned with a hole through the post, and a wire extending
through each set of holes in the leg and post, respectively, for
securing the leg of the L-shaped member in the slot.
23. A support as recited in claim 20 further comprising an axial
passage extending through the post, the passage having an outer
end, and the thread being in the outer end of the passage.
24. A support as recited in claim 20 wherein the post comprises
an enlarged inner end, a transverse slot in the enlarged inner end,
a pair of holes extending through the enlarged inner end transverse
to the slot, and an axial passage extending through the post, the
thread being in the outer end of the passage.
25. A vacuum furnace comprising:
a furnace shell;
a heating chamber in the furnace shell;
at least one metal strip heater element in the heating chamber
of the furnace;
means for making electrical contact with each end of the heater
element for passing electric current therethrough; and
at least one mechanical support for an intermediate portion of
the electric heater element comprising:
a T-shaped ceramic support for an intermediate portion of the heater
element having the leg of the T-shaped ceramic support connected
to the heating chamber; and
means for compliantly securing the heater element to the crossbar
of the T-shaped ceramic support without holes in the heater element.
26. A vacuum furnace as recited in claim 25 wherein an outer end
of the ceramic support is spaced apart from the heating chamber.
27. A vacuum furnace as recited in claim 25 wherein an outer end
of the T-shaped ceramic support is threaded and comprising a threaded
connection between the T-shaped ceramic support and the heating
chamber.
28. A vacuum furnace as recited in claim 27 wherein an outer end
of the T-shaped ceramic support comprises an internal thread and
further comprising an external thread connected to the heating chamber,
the external thread and internal thread being the only connection
between the leg of the T-shaped ceramic support and the heating
chamber.
29. A vacuum furnace as recited in claim 25 wherein the T-shaped
support comprises a ceramic post having an outer end connected to
the heating chamber and a metal member connected to an inner end
of the post forming a portion of the crossbar of the T-shaped support.
30. A support as recited in claim 29 wherein the ceramic post comprises
a transverse slot in the inner end of the post and the metal member
comprises an L-shaped sheet having a first leg of the L-shaped sheet
in the slot and a second leg of the L-shaped sheet forming a straight
surface.
31. A vacuum furnace comprising:
a furnace shell;
a heating chamber in the furnace shell;
at least one metal strip heater element in the heating chamber
of the furnace;
means for making electrical contact with each end of the heater
element for passing electric current therethrough; and
at least one mechanical support for an intermediate portion of
the electric heater element comprising:
a ceramic post having a length and an outer end connected to the
heating chamber by a thread on the ceramic;
a metal crossbar on an inner end of the ceramic post for supporting
an intermediate portion of the heater element; and
means for compliantly securing the heater element to the metal
crossbar without holes in the heater element.
32. A vacuum furnace as recited in claim 31 wherein the thread
on the ceramic post is an internal thread and the post is connected
to the heating chamber solely by a threaded rod.
33. A vacuum furnace as recited in claim 31 wherein the ceramic
post comprises a slot in the inner end of the post transverse to
the length of the post and the metal crossbar comprises an L-shaped
sheet metal piece, one leg of the L-shaped piece being in the slot
and the other leg of the L-shaped piece supporting the heater element.
34. A vacuum furnace comprising:
a furnace shell;
a heating chamber in the furnace shell;
at least one metal strip heater element in the heating chamber
of the furnace;
means for making electrical contact with each end of the heater
element for passing electric current therethrough; and
at least one mechanical support for an intermediate portion of
the electric heater element comprising:
a first ceramic post having an outer end connected to the heating
chamber by a thread on the ceramic;
a second ceramic post identical to the first mentioned post and
spaced apart therefrom;
a metal bridge between the first and second ceramic posts forming
a U-shaped support; and
means for compliantly securing the heater element to the metal
bridge without holes in the heater element.
35. A support as recited in claim 34 further comprising an axial
passage extending through each post, the passage having an inner
end and an outer end, the thread being an internal thread in the
outer end of the passage.
36. A support as recited in claim 35 wherein each post comprises
an external thread at the inner end of the post and further comprising
a ceramic nut on the external thread.
Description BACKGROUND OF THE INVENTION
This invention relates to mounting of electric strip heating elements
in high temperature vacuum or protective atmosphere furnaces.
Electric heating elements in high temperature vacuum furnaces are
often made of strips of sheet molybdenum or the like. Electrical
contact may be made to the ends of a long metal strip which wraps
around the hearth of the furnace. Such a long heating element requires
mechanical support intermittently along its length to hold it in
proper position in the furnace and for preventing shorting to other
parts of the furnace or the load being heated in the furnace.
Various techniques for providing insulated mechanical support for
such a heater element have been employed, but few are completely
satisfactory. Less than satisfactory heating element supports are
described and illustrated in U.S. Pat. Nos. 3,737,553 by Kreider,
3,812,276 by Cyrway and 4,056,678 by Beall, for example.
Some heater supports have relied on an insulated post, or the like,
extending through a hole in the strip heater. Although good mechanical
support can be provided with such an arrangement, the hole is quite
undesirable. The hole necessarily results in the heater having a
narrower effective width at the location of the hole. Since there
is less metal cross-section to carry the heating current, there
is excess heating around the hole. This excess heating can be severe
enough to burn out a heater under some circumstances and, if nothing
else, it shortens the heater lifetime in that region. Heater elements
most commonly fail at the end electrical contacts or in proximity
to such holes.
Another type of heater support that does not require holes through
the heater element is in a general form of a T. A sheet metal "post"
forms the leg of the T, and the heater element lies on top of the
top crossbar of the T. The heater element is secured to the crossbar
by a rod lying on top of the heater element with twisted wires securing
the bar to the crossbar of the T. Ceramic sleeves insulate the crossbar
from the leg of the T to provide a compliant connection. Supports
shown in the Kreider and Cyrway patents are of this general type.
Such heater supports have been plagued with deformation problems.
There is insufficient rigidity in the T-shaped mounting to support
the heater element as it tries to move under the forces of thermal
expansion, cooling gas flow, mechanical vibrations, and the like.
The crossbar of the T tends to tilt relative to the leg, which may
result in shorting of the heater element to other parts of the furnace
structure or the load in the furnace. Breakage is also a problem
when attempting to remove or replace the heater element supports.
In high-temperature furnaces, heater elements, heat-shields and
supports for the heater elements are often made of molybdenum. This
material becomes quite brittle after heating to elevated temperatures.
It is also important to provide good electrical insulation between
the heater element and other portions of the furnace. This electrical
insulation must not only isolate the heater element when the furnace
is first put into service, but must also maintain such isolation
after heating. A problem encountered in high-temperature vacuum
furnaces is "metallizing." Components of the furnace and
articles being heated in the furnace may evolve metal vapors that
deposit on electrical insulators and provide an electrically conductive
path which shorts such a heater element to other parts of the furnace.
The electrical insulation should resist such shorting when metallizing
occurs.
The heater support must also accommodate dimensional changes in
the heater and the furnace. Typically, one part of the support is
at relatively low temperature, while another part is at relatively
high temperature. The heater element itself undergoes thermal expansion
as it is heated. The consequent dimensional changes must be accommodated
by the support without applying large mechanical loads on the brittle
heating element, which could result in breakage.
U.S. Pat. No. 4,771,166 has provided an excellent support for an
electric heating element in a vacuum furnace. The heater support
has two similar support assemblies spaced apart with a rigid bridge
compliantly mounted therebetween. The heater strip is compliantly
connected to the bridge. Each of the mounting assemblies has a metal
post connected to the heating chamber of the furnace and surrounded
by a ceramic sleeve. The ceramic sleeve is surrounded by two ceramic
tubes which hold the end of the bridge.
Additional improvements could, however, be made in such a heater
support for reducing heat conduction though the metal post that
secures the tubes in place in the furnace chamber. This mounting
support also has a number of separate pieces which must be separately
fabricated and it takes appreciable time to assemble. It is desirable
to provide a heater support which does not deform upon heating,
supports a heater element compliantly to accommodate thermal expansion,
does not penetrate the heater element and lead to localized heating,
does not short out to the furnace structure due to metallizing and
is made with a minimum number of parts.
SUMMARY OF THE INVENTION
To address such problems, there is provided in practice of this
invention according to a presently preferred embodiment, a mechanical
support for an electric heating element in a vacuum furnace or the
like which comprises a ceramic post having an internal thread in
its outer end. A threaded rod secured to a portion of the furnace
engages the internal thread of the post for mounting the support.
A transverse member at the inner end of the post includes a straight
surface normal to the length of the post for engaging a flat heater
element. The heater element is secured to the straight surface by,
for example, a rod over the heater element which is secured to the
transverse member by twisted wires.
Preferably, the post and transverse member are integral and form
a T-shaped ceramic support. Alternatively, the post has a transverse
slot in its inner end of the post and an L-shaped metal sheet has
one leg of the L in the slot and the other leg of the L forming
the flat surface. In still another embodiment, a second ceramic
post identical to the first post is spaced apart from the first
post. A transverse metal member extends between the first and second
posts to form the flat surface, forming a U-shaped support for the
heater element.
DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the present invention
will be apparent from the following detailed description when considered
in connection with the accompanying drawings wherein:
FIG. 1 is an end perspective view into a vacuum furnace; and
FIG. 2 is a side elevation view of a T-shaped heater element support
mounted in the furnace shell;
FIG. 3 is a plan view of the T-shaped heater element support;
FIG. 4 is an end elevation view of the T-shaped heater element
support;
FIG. 5 is a side elevation view of another embodiment of T-shaped
heater element support;
FIG. 6 is a plan view of the heater element support of FIG. 5;
FIG. 7 is a side elevation view of the heater element support of
FIG. 5; and
FIG. 8 is a side elevation view of a U-shaped embodiment of heater
element support.
DETAILED DESCRIPTION
An exemplary vacuum furnace comprises a horizontal cylindrical
shell 10 having a sealing flange 11 at the end, against which a
door (not shown) is sealed to close the furnace. The furnace illustrated
in FIG. 1 is schematic and does not include a variety of conventional
features such as support legs, doors, vacuum pumps, water cooling
jackets, gauges, power supplies, etc., which are not required for
an understanding of this invention. The drawing simply shows the
general location of the heater elements and their supports employed
in practice of this invention.
In the exemplary furnace, there is a horizontal cylindrical hot
zone 12 suspended away from the furnace shell by upper support brackets
14 a short space above lower brackets 13. Minimal cross-section
supports are employed for minimizing heat transfer from the heating
chamber to the water-cooled furnace shell. In a typical embodiment,
the hot zone comprises a double walled sheet metal plenum 16 into
which cooling gas can be circulated for rapid cooling of the hot
zone and its contents.
Inwardly from the plenum is a layer (or layers) of thermal insulation
17 which may be in the form of a plurality of parallel metal radiation
shields, fibrous ceramic insulating "wool", graphite "wool",
or ceramic or graphite insulating sheets. Regardless of the insulation
employed in the hot zone, the innermost face is typically formed
of sheet metal or flexible graphite sheet (Grafoil) which may be
bonded to other materials. Such thermal insulation is conventional
and need not be further described for an understanding of this invention.
A plurality of electrical heating elements 18 extend circumferentially
around the interior of the hot zone. In the embodiment illustrated,
there are a plurality of lower heating elements 18a, each of which
extends around approximately half of the circumference of the hot
zone. Similar upper heating elements 18b extend around the upper
half of the hot zone. Each heating element has conventional bolted
electrical contacts 15 at each end for passing electric current
through the heating element. Each heating element is mechanically
supported between its ends by a plurality of supporting assemblies
illustrated in greater detail in the other drawings.
A plurality of furnace load supporting structures 20 extend from
the furnace shell between the heater elements into the hot zone.
Baskets of parts (not shown) or other objects to be heated are placed
on such supporting structures when the furnace is in use.
To give an order of magnitude, a furnace such as illustrated in
FIG. 1 may have a hot zone with a 1.5 meter diameter. Supports for
the heating elements are spaced in the order of 30 cm. apart around
the hot zone. Typical molybdenum heating elements are from 6 to
15 cm. wide. Four or more such heating elements are spaced along
the length of the hot zone, depending on its total length. Such
heating elements are generally not a continuous semicircle, but
instead are formed as a plurality of straight sections between adjacent
support assemblies, with a small straight section adjacent to each
support assembly. Small angle bends are sufficient to form such
a heating element for an exemplary 1.5 meter diameter furnace. Such
bends are readily made on a break and avoid the need for rolls for
rolling a continuously curved heating element.
An exemplary T-shaped heater support is illustrated in FIGS. 2
to 4. The side elevation view of FIG. 2 looks at such a support
assembly circumferentially around the hot zone, that is, along the
length of the heating element 18.
In the description of the heater supports, the portion of the heater
support that connects to the heating chamber of the furnace is referred
to as the outer portion since it is radially outward in a circular
furnace as illustrated herein. Similarly, the opposite end that
extends toward the center of the heating chamber is referred to
as the inner portion.
The plenum 16 around the heating chamber is formed by an outer
sheet metal wall 23 and an inner sheet metal wall 24. Typically,
these walls are steel rolled into a cylinder. A steel tube 26 extends
through the plenum and is welded to the inner and outer plenum walls
at the location of each mounting assembly. Additional spacers between
the sheets may be employed, but other details of the plenum are
not required for an understanding of this invention.
A stainless steel nut 27 is welded into the outer end of the tube
26. A molybdenum rod 28 with roll formed threads is threaded into
the nut and extends inwardly toward the center of the furnace. Molybdenum
is used for this and other structural elements which may be exposed
to elevated temperatures because of its ability to withstand the
temperatures encountered in the vacuum furnace. Depending on the
temperature requirements for the furnace, the various metal and
ceramic parts may be fabricated of lower cost materials than the
molybdenum, stainless steel and alumina mentioned herein.
One type of thermal insulation commonly employed in vacuum furnaces
comprises a plurality of sheet metal radiation shields 29. In a
vacuum radiation is the principal mechanism of heat transfer. A
plurality of reflective radiation shields can be quite effective
in providing a temperature gradient between the inner hot zone of
the furnace and the surrounding shell. In an exemplary embodiment
as illustrated in FIG. 2, five such radiation shields are employed
inwardly of the plenum walls, which themselves act as radiation
shields. At the location of the mounting assembly, U-shaped sheet
metal spacers 31 keep the radiation shields spaced apart from each
other. Three or four innermost radiation shields and spacers may
be fabricated of molybdenum while the outer ones are safely fabricated
of less expensive stainless steel.
The radiation shields and spacers each have a hole for providing
ample clearance around a ceramic post 32 of the heater element support
to permit shifting of the shields due to thermal expansion without
applying loads on the heater support. A high-temperature ceramic
such as alumina or alumina-based composition is preferred for the
heater support. An exemplary ceramic composition which is commercially
available from Coors Ceramics Co. of Golden, Colo., is 97.3% alumina.
It is a feature of the ceramic used for the heater support that
it can be molded and/or machined to complex shapes, including threads.
Lower alumina ceramics may be used for lower cost, it being recognized
that 100 or more heater supports may be used in an exemplary furnace.
Cost, however, is not a great concern since the improved heater
element supports provide a prolong lifetime for the heater elements
in the furnace and the time before it is necessary to rebuild the
heater system of the furnace can be significantly prolonged.
The post 32 of the heater support has an axial passage 33 through
the full length of the post. The outer end of the passage includes
an internal thread 34 so that the ceramic post can be threaded onto
the molybdenum rod 28. The passage has a large diameter counterbore
36 near its inner end, thereby reducing the wall thickness of the
ceramic for minimizing heat transfer.
A flat transverse member 37 is integral with the post so that the
heater support is T-shaped. The transverse member is further connected
to the post by integral diagonal wings 38. In this embodiment, the
top surface of the T is flat. If desired, the transverse member
could have a shallow V-shape in traverse cross-section (viewed as
in FIG. 4) as long as there is a straight central line for supporting
a heater element.
The heater element 18, which is, for example, a sheet of molybdenum,
is bent in a shallow V-shape adjacent the heater support so that
the heater element extends straight from support to support around
the inside of the furnace. The heating element rests on top of the
crossbar of the T-shaped heater support. It is compliantly secured
to the crossbar by a molybdenum retaining rod 39 which has a length
greater than the width of the heating element. Each end of the retaining
rod has an L-shaped bend, and the rod is tied to the bridge by twisted
loops of molybdenum wire 42 extending through holes 41 near each
end of the crossbar. There are no holes in the heating element.
By loosely tying the retaining rod to the crossbar, the heater element
is compliantly secured so that it can shift as required by thermal
expansion.
If desired, the heating element may not be completely flat but
may have stiffening ridges bent into the sheet metal extending along
its length. The retaining rod in such an embodiment lies atop the
stiffening ridges.
The ceramic post is threaded onto the molybdenum rod about 2.5
cm. When the ceramic post and molybdenum rod are threaded together,
the connection is stopped before the outer end of the post engages
the inner wall 24 of the plenum or the end of the tube 26 through
the plenum. This means that there is a small amount of compliance
of the T-shape heating element support relative to the plenum. The
support can tilt or rotate slightly as required to accommodate thermal
expansion variations that would otherwise apply undue loads on the
heating elements.
FIGS. 5 to 7 illustrate another embodiment of T-shaped heating
element support having a ceramic post connected to the cooling gas
plenum by a threaded connection. In this embodiment, a round ceramic
post has an axial passage 47 through the full length of the post
for reducing heat transfer and aiding in forming an internal thread
48 at the outer end of the post. The connection of the post to the
furnace by a threaded rod is not illustrated in these drawings since
the connection is essentially identical to the connection hereinabove
described and illustrated in FIG. 2.
The inner end of the post has an enlarged somewhat T-shaped head
49 which is generally rectangular in transverse cross-section (FIG.
6). A transverse slot 51 extends the full length of the rectangular
head. An L-shaped sheet 52 of molybdenum has one leg of the L in
the slot. The sheet of molybdenum is loosely held in place by a
pair of molybdenum wires 53 passed through aligned holes in the
head of the post and sheet respectively. The ends of the wire are
bent over to keep them in place. The other leg of the L-shaped sheet
of molybdenum forms the flat surface of the crossbar of the T to
which the heater element is attached. The heater element 18 is compliantly
secured to the molybdenum sheet by a retaining rod 54 compliantly
tied to the sheet by retaining wires 56.
Similar principles are employed for constructing a U-shaped heater
element support which can be used for retrofitting furnaces originally
constructed with heater element supports as described and illustrated
in U.S. Pat. No. 4,771,166. In such an embodiment, as illustrated
in FIG. 8, there are a pair of substantially identical ceramic posts
61. Each of the posts includes an axial passage 62, the outer end
of which includes an internal thread 63. The ceramic posts are each
mounted to the gas plenum of the furnace by a threaded rod (not
illustrated in FIG. 8) passing through a nut welded to the gas plenum
as hereinabove described and also as employed in U.S. Pat. No. 4,771,166.
The inner end of each of the ceramic posts has an external thread
64. A ceramic nut 66 on this external thread holds a sheet molybdenum
bridge 67 between the two posts. The heater element 18 is compliantly
secured to the bridge by a molybdenum retaining rod 68 tied to the
bridge by twisted loops of molybdenum wire 69. The nut is not threaded
tightly against the molybdenum bridge so as to accommodate thermal
expansion. If desired, a molybdenum key wire 65 passed through a
hole transverse to the post may be used for retaining the nut on
the external thread of the post.
A ceramic post threaded onto a rod attached to the gas plenum provides
good electrical insulation between the heating element and adjacent
metallic parts of the furnace. Metallizing of the ceramic during
operation of the furnace is not a problem. The outer end of the
ceramic rod passes through holes in the radiation shields or other
thermal insulation employed in the furnace. The insulation shadows
the outer end of the rod and thereby, prevents any appreciable deposition
of metal. Furthermore, the outer end of the ceramic post is not
in contact with the gas plenum. The only contact with electrically
conductive structure is to the threaded rod in the outer end of
the post and the face of the post surrounding the rod is amply protected
from metallizing.
Although limited embodiments of heater element supporting structure
have been described and illustrated herein, many modifications and
variations will be apparent to one skilled in the art.
It will be recognized that the exemplary furnace is just one of
many possible embodiments. Such a furnace may have a vertical cylindrical
shell or be rectangular or have any desired shape or size. It may
be a bottom loading or top loading furnace instead of the end loading
furnace as illustrated. On a smaller diameter furnace, the electrical
heating elements may extend substantially completely around the
circumference of the hot zone. On larger furnaces, heating elements
may extend less than half way around the hot zone. Continuous strip
heating elements may be used which make repeated paths around the
furnace or which are connected for three phase power. Such heater
supports may also be used for supporting intermediate portions of
sinusoidal heating elements which traverse longitudinally through
the furnace shell. They may also be used for supporting parts of
flat heating elements in rectangular furnaces.
Many other variations and modifications of electric vacuum furnaces
or the like in which this invention may be employed will be apparent.
The invention is also described in connection with a vacuum furnace,
however it will be apparent that it is equally applicable in protective
atmosphere furnaces, or in furnaces operated in air when oxidation
resistant materials are employed. Molybdenum is not the only material
for fabrication of the parts of the heater elements and their supports.
Tantalum and tungsten are other exemplary high temperature materials.
Analogous supports may be used for graphite or alloy heating elements
as well. Although high alumina ceramics are preferred for elevated
temperature resistance, other structural ceramics may be used for
lower temperature applications.
For such reasons, it is to be understood that within the scope
of the appended claims, this invention may be practiced otherwise
than as specifically described. |