Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to improvements in electric band or strip
heaters, and is more particularly concerned with such heaters of relatively
thin configuration and which are fabricated from a novel assembly of organic
bound ceramic particle strips sandwiching a resistance wire wound organic-
ceramic heater core element. The organic bound ceramic particle strips and
organic-ceramic core element consist of flexible sheets of organic material
densely packed with ceramic particles wherein the organic material may be
dissipated when subjected to heat.
In conventional band heaters of standard mica configuration, a
wire wound mica heating element is assembled between mica ins~lator strips.
The resultant mica sandwich is then encased in a sheet metal enclosure and
formed into a desired shape. The electrical mica insulators used are of
relatively low thermal conductivity and thus limit the heat transfer
efficiency. Also, these insulator strips undergo physical and chemical
changes upon exposure to temperature in excess of 1200F., which consist of
dehydration or baking out of the water of hydration. These changes further
decrease thermal conductivity and also reduce electrical insulating proper-
ties.
The presence of air voids and undesirable expansion under elevated
temperature inherent in conventional mica heaters reduce heat transfer
capability and result in loss of heater efficiency. These factors cause a
-~ conventional heater to operate at relatively higher than most efficient
internal temperatures, resulting in premature heater failure. Additionally,
where clamp force must be applied to maintain the heater in a given position,
e.g. around a nozzle of a tube having contents which must be heated as they
pass therethrough, expansion of the heater under elevated temperatures
causes loss of clamping force, resulting in heater inefficiency because the
heater must be hotter to achieve a given surface temperature, and the higher
temperature of the heater induces further expansion as the temperature is
elevated.
In a second type of conventional band heater, coils of element
wire are strung through preformed ceramic insulator blocks which are shielded
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by a light sheet metal cover. Such an assembly is then strapped around an
object to be heated. The resulting assembly can be likened to an oven
assembly wherein heat transfer to the heated object is principally by con-
vection rather than conduction. Such a heating system is not capable of
high wattage because the inefficient convection heat transfer will not remove
heat from the element fast enough, and thus would lead to over-temperaturing
of the wire and premature element failure. This limitation of wattage thus
increases heat-up time of any object to be heated. Due to the open design
of the casing for such conventional ceramic heaters, carbon forming materials
can enter the heater, causing grounding type failures, which also may con-
; stitute a safety hazard. ~lso inherent bulk and thickness requirements for
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conventional ceramic heaters prevent their use where space is critical.
In the present invention, during fabrication of the heater, instead
of a formed mica core and mica insulations strips as in conventional mica
~i heaters, and instead of preformed blocks strung with wire as in conventional
ceramic heaters, a resistance wire element is wound on a core strip of
~ organically bound ceramic particles, and sandwiched between similar organical-
;l~ ly bound ceramic particle strips; and the assembly is placed in a metal
housing and rolled or pressed to eliminate air voids between the elements,
whereupon the entire assembly is heated to bake out the binders and
agglomerate the ceramic particles into a densely packed unitary mass embedding
therein the heater wire element and the ceramic particles surround all exposed
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surfaces of the heater wire element.
These novel organically bound ceramic particle strips each com-
prise a thin pliable "green" sheet of ceramic particles, pressed and rolled
to a high density, and bonded together with binder materials, usually organic
in nature, to an overall thickness upwards of .018 inch. The ceramic
particles in the sheets are typical powdered ceramic materials, such as
particles of aluminum oxide, magnesium oxide, boron nitride, or silicone
dioxide. The binders for the ceramic particle sheets are typically silicone,
rubber, varnish, glyptal nr the like. These bonded "green" or unbaked
ceramic particle sheets conventionally are used in the fabrication of
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ceramic underlayment for printed circuits, the end product when baked out
being referred to as "ceramic substrate" but in their "green" state before
baking they are pliable and bendable.
In fabricating a heater according to the present invention, a
lower organic - ceramic particle strip is laid over the bottom wall of a -
metal housing, ~nd the core organic-ceramic particle strip which has been
formed with a Nichrome or other resistance wire element, which is placed ~.
over the lower strip. A second or upper organic-ceramic particle insulator
strip is placed over the resistance wire element core strip, and a metal
pressure plate is installed over the upper strip. The edges of the housing
are bent over the pressure plate, and the assembly is then rolled and
flattened, thereby eliminating air voids between the elements and
amalgamating and unifying the structure.
The assembly may then be shaped, for example, bent into a curved
band heater. When the heater assembly is in its final finished shape, it
is fired at an elevated temperature above the vaporization point of the
binder materials in the strips and below the melting point of the sheath
covering, preferably in an oxygen atmosphere, to vaporize and carbonize the
binders and oxidize the carbon, which is vented from the heater assembly
in the form of carbon dioxide. The assembly may be rolled again to further
compact the body. As a result of this process, the ceramic particles
agglomerate and are densely packed into a thin integral heat conducting and
electrically insulating mass. Leads may then be connected to the heater
element terminals and any desired heater mounting members may then be
attached.
It is therefore an object of the present invention to provide a
novel electric heater assembly of the character referred to.
According to one aspect of the invention there is provided a thin
integral ceramic band heater assembly comprising a closed metal housing, a
heater core body secured tightly within the housing without air voids, said
heater core body comprising a compacted agglomerated mass of unbound ceramic
particles electrically insulating and surrounding an electrical resistance
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heater element embedded therein, and leads for connecting the heater element
and a source of electric power insulated from and through said housing.
According to another aspect of the invention there is provided
a core body for a ceramic band heater assembly comprising a sandwich of
aligned sheets of ceramic particles bonded together by vaporizable binders
and an electrical resistance heater element arranged between said sheets,
said body being adapted to amalgamate into a unitary mass of agglomerated
ceramic particles embedding said heater element therein upon application of
sufficient heat to vaporize said binders.
According to a further aspect of the invention there is provided
in a method for fabricating a unitary electric ceramic band heater comprising
the steps of assembling resistance wire on an uncured sheet of ceramic
particles impregnated to a high density and bound together in heat dissipat-
able binder material, arranging said assembled wire and uncured sheet between
uncured sheets of insulator particles bound together in heat dissipatable ~ -binder material, compressing said arranged assembled wire and uncured sheet
and said uncured insulator sheets together to substantially eliminate air
voids between said sheets, and heating said compressed arranged assembly at
a temperature sufficient to substantially dissipate said binder materials and
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cure said ceramic and insulator particles into an integral mass.
Embodiments of the invention will now be described by way of
example, with reference to the accompanying drawings, in which
Figure 1 is a perspective view of a curved ceramic band heater
assembly embodying the invention;
Figure 2 is a perspective exploded view of the separated parts of
a heater assembly embodying the invention;
Figure 3 is a perspective view of a strip heater assembly embodying
the invention;
Figure 4 is a sectional view of assembled parts of the heater
before closing the housing and compressing and heating the assembly; and
Figure 5 is a sectional view of the heater assembly before com-
pression.
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With reference to the drawings, a curved band heater 10 (as shown
in Figure 1) or a strip heater 11 (as shown in Figure 3) may be fabricated
from a sheet metal channel 12 having a flat base 19 and upstanding sides 14,
and into the channel is placed, successively, a thin flat pliable insulator
sheet 15 of bound ceramic particles, a resistance wire element core 16, a
second or upper insulator sheet 17 of bound ceramic particles, and a metal
pressure plate 18 may be set thereover, all of which may be held together
and centered during initial assembly by means of suitable tape or adhesive.
The margins 29 of the upstanding sides 14 on channel 12 may be bent over
the pressure plate 18 to close the assembly and bind the pressure plate
thereover. The closed assembly is then rolled flat or is formed into a
curved finished shape to compress the parts together and eliminate air voids
between the elements, as shown in Figure 5.
Ceramic particle core element strip 16 is formed with Nichrome
; (Trademark) or other resistance wire 20 wound around a sheet of bound
ceramic particles similar to the sheets 15 and 17, and the ends of the wire
may be bound with terminal pads 21. The pre~sure plate 18 and insulator
strips 15 and 17 are of about the same length and width as the base 19, to
fit snugly within channel 12, but the core element 16, while about the same
length as the base 19, is narrower than the strips 15 and 17, to provide a
gap 24 for electrical clearance between the core 16 and its wire winding 20,
and the channel sides 14.
The bound ceramic particle insulator sheets 15 and 17 and the
ceramic particle core 16 each comprise high density ceramic particles bound
together by a binder, usually organic material, which has been fabricated by
pressing and rolling the material together. While the strips are green, i.e.
before heating to the vapor point of the binder material and agglomerating
and packing of the ceramic particles into a dense mass, the strips are
pliable and bendable, but after heating to a temperature above the vapor
point of the organic or inorganic binder material and after agglomerating
and packing of the ceramic particles, the strips amalgamate into a packed
dense mass of ceramic particles to insulate the resistance wire 20 embedded
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therein, while providing efficient heat transfer and low expansion character-
istics when a current is applied to the resistance wire element.
Before heating, the assembly is bendable and formable without
damaging the core 16 and insulator strips 15 and 17, so the assembly may be
shaped, for example into the configuration of a curved band heater 10, shown
in Figure 1, or left in its extended form to be completed as a strip heater
11, shown in Figure 3. After the forming step, the assembly is fired at an
elevated temperature, preferably in an oxygen atmosphere, sufficient to
, vaporize and bake out the binder materials of the strips 15 and 17 and the
binder of core 16 and to agglomerate the ceramic particles, joining them
! together into a single mass. The applied temperature for vaporization
;~ should be less than the melting point of the metal members, so as not to
weaken those parts.
Electric leads 25 and 26, respectively, may be connected to each
of the terminal pads 21, connecting the heater wires 20 to a power source.
A slight extension 13 may be provided on each edge of the channel to support
the lead wires, and the channel edges may be potted with suitable electrical
cement 35 to close and finish the connection to the heater assembly.
Means for mounting or clamping the heater assembly to or about a
surface to be heated may also be connected to the finished heater assembly.
Such means may comprise a band 27, which may be spot welded to the pressure
plate 18, having turned and apertured ends 30, through which apertures a
bolt 30 may be inæerted, and clamping may be accomplished by tightening a
nut 31 on the bolt.
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Supplementary Disclosure
In the drawings accompanying this Supplementary Disclosure:-
Figure 6 is a sectional view of the completed heater assembly;
Figure 7 is a perspective exploded view of the separated parts of
a heater assembly similar to Figure 2, except the wire wound heater element
consists of a metallic ribbon,
Figure 8 is a sectional view of a completed heater assembly having
the ribbon heater element shown in Figure 7;
Figure 9 is a detailed schematic view of the ceramic particles
before compression;
Figure 10 is a perspective view of a modified seamless housing
for the heater assembly.
`~ With reference to the embodiment of the invention shown in Figures
-~ 7 and 8, instead of the wire wound core element 16, a flat wire ribbon 36
assembled between the insulator strips 15 and 17 defines the heater core of
the assembly. The ends of this wire ribbon 36 may be connected to leads in
the manner previously described. 1`he assembly of the heater elements is sub-
stantially as described with reference to Figures 2 and 4-6, except, in the
case of this embodiment, the insulator strips 15 and 17 surround and embed
the ribbon 36 after the members are assembled and compressed and formed, and
the binders for the strips 15 and 17 are baked out in the manner previously
described.
The ceramic particles 40, which were bound together in the green
strip form, are packed into a dense mass during the compression step
described, the burning out of the binders leaving these ceramic particles
agglomerated. Additional pressing of the mass of ceramic particles 40 makes
the agglomeration tighter and even more dense, with a consistency similar to
very tightly compressed talc.
Where it is desired that the completed heater assembly have a
seamless housing, instead of utilizing a channel 12, the core 16 and insulator
strips 15 and 17 are assembled together, with or without a pressure plate 18,
and the assembly may be inserted into a flattened tube 42, and then formed,
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compressed and heated in the manner described to complete the ceramic band
heater.
Because the ceramic particles 40 are very small, the strips 15 and
17 and the core 16 may be relatively thin and only thick enough to provide
adequate electrical insulation without concern for the mechanical limitations
of conventional preformed ceramic blocks. The result is a relatively thin
completed heater assembly which has superior heat transfer characteristics
within minimum space requirements. Also, the structure permits the wire
element 20 or 36 to be formed in a wide variety of heater coils engineered
primarily for electrical requirements only and without consideration for the
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~ mechanical strength of the wire or coil.
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