Note: Descriptions are shown in the official language in which they were submitted.
1. BACKGROUND OF THE INVENTION
This invention relates to electric heating units and
more particularly to such units in which coiled resistance
heating elements are embedded in insulating bodies.
In U.S. Patent 3,500,444 to W.K. Hesse, et al., a
lightweight ceramic fiber insulation is disclosed. Due to
energy saving advantages, such ceramic fiber insulation is
often used in place of more dense fire brick for furnace
linings. Most of these applications are for oil or gas fired
10. furnaces; however, there is increasing interest in using
electric heating elements in which heating coils are embedded .
in situ in ceramic fiber insulating bodies as taught in the
Hesse et al patent. In that patent, a portion of the heating
coil is exposed above the surface of the insulating body in ;
order to permit effective transmission and direction of heat.
The requirement that a portion of the coil be exposed `'
beyond the surface of the insulating body leads to great manu-
facturing difficulties. As taught in ~esse et al patent, the
insulating body is formed by a vacuum activated filtering
20. process in which the coil is positioned against a screen and
liquid from a molding slurry is drawn from the slurry past the
heating coil through the screen. Ceramic fiber from the slurry
is retained by the screen and accumulates around the coil. It
is difficult to arrange for each individual convolution of the
coil to be partially exposed from the insulating panel.
An object of this invention is to provide a highly
efficient heating unit having a heating coil embedded in an
insulating body, yet which is not subject to the difficulties
of manufacture resulting from exposting each convolution of the
30. coil beyond the surface of the insulating body.
It is among the advantages of this inventi ~ at a
flattened heating coil may be totally embedded in the insulating
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1. body and be maintained at a substantially lower temperature
for a given oven temperature than would otherwise be possible
for a totally embedded round coil.
SUMMARY OF THE INVENTION
According to the invention in one of its aspects, an
electrical heating unit comprises a resistance heating coil
embedded in an insulating body adjacent to and extending along -
beneath the surface of the insulating body, the coil being
flattened longitudinally such that each turn of the coil has a
10. substantially linear segment lying generally in the plane of
the surface of the insulating body.
According to other aspects of the invention, the
coil is also flattened longitudinally in a plane set in from
the surface of the insulating body and the coil is embedded in
situ in a ceramic fiber.
In accordance with the invention in another of its
aspects, an electrical heating unit is made by winding a `
circular heating coil, flattening the heating coil, placing
the heating coil on the bottom surface of a mold, introducing
20. a slurry including a suspension of ceramic fiber into the
mold, and drawing the liquid from the slurry to deposit a
body of ceramic fibers in the mold and thereby embed the heat-
ing coil within the body.
According to yet other aspects of the invention,
the method of making the electrical heating unit includes the
step of flattening the convolutions of the coil while they
are closely spaced in side-by-side relationship and then
stretching the heating coil after it is flattened.
BRIEF DESCRIPTION OF THE DRAWINGS ~ -
30. The foregoing and other objects, features, and
advantages of the invention will be apparent from the follow-
ing more particular description of a preferred embodiment of
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1. the invention, as illustrated in the accompanying drawings in
which like reference characters refer to the same parts
throughout the different views. The drawings are not necessarily
to scale, emphasis instead being place upon illustrating the
principles of the invèntion.
FIG. 1 is a perspective view of an electrical heating
panel unit having flattened coils embedded in a ceramic fiber
insulating body in accordance with the invention;
FIG. 2 is a front edge view of the electrical heating
10. unit of FIG. 1 with a portion thereof broken away to show the
generally oval shape of the flattened heating coils; -
FIG. 3 is a side edge view of FIG. 1, partially
broken away to show a flattened heating coil and terminal
connection pin.
FIG. 4 is a graph demonstrating the results of tests
run to compare the internal temperatures of two insulating 11
bodies respectively having flattened and round coils totally ` ~-
embedded therein; -
FIG. 5 is a sectional view of a furnace having
20. electrical heating retrofit or replacement units embodying the
invention mounted on the interior walls thereof.
DESCRIPTION OF PREFERRED EMBODIMENTS
An electric heating unit 12 embodying the present
invention is shown in FIG. 1. The unit includes a number of
flattened oval electric heating coils 14, 16, 20, 22, 24, 26,
28 and 30 embedded in a ceramic fiber insulating body 31. The
heating coils are positioned adjacent to the surface 32 of the
insulating body and that surface will be referred to as the hot
face of the heating unit. The opposite surface 33 of the heat-
30. ing unit shown in FIG. 2 will be referred as the cold face,although the temperature at that face is somewhat above ambient
during operation of the heating unit. The insulating body is
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1. preferably of the ceramic fiber type disclosed in U.S. Patent ~ -~
3,500,444 and may be molded to have a step 34 along three sides
thereof and an overhang 36 along one side. The step and over-
hang are provided for convenient interfitting of a number of
panel units in a furnace.
The electric heating coils are electrically connected
in series by connecting wires 38. These connecting wires are
welded to respective coil pairs prior to deposition of the
insulating material forming the insulating body 31. Alter- ~ ~
10. natively, the several coils may be formed from a single wire -
strand to avoid the need for intercoil welding. The nine
heating coils in series are electrically connected across a
low voltage, high current source by means of terminal pins
40 and 42 which extend through the insulating body 31 and
protrude from the cold face 33. These terminal pins are
electrically connected to respective coils 14 and 30 by lead
wires 44 and 46. Each lead wire extends through a hole drilled
transversely through the respective terminal pin and is welded `~ ~-
thereto. `~`
20. Anchoring ribbons 48 and 50 are also welded to the
terminal pins. Each of these ribbons is preferably about .060
inches thick, 1 inch wide, and 2 inches long and is positioned
within the insulating body with the ribbon faces perpendicular
to the hot face 32 of the insulating body. Each ribbon is
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bend about a line perpendicular to the hot face to form an `
angle of about 90 degrees to increase the anchoring effect
of the ribbons. With the anchoring ribbons, the terminal pins
are less likely to be pushed through the lightweight insulating
material if subjected to a sudden, accidental axial impact
30. from the cold face side of the unit.
Each coil is flattened longitudinally such that, as
best shown in FIG. 2, each turn of each coil has a substantially
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1. linear segment 52 lying generally in the plane of the surface
of the insulating body. Thus, a substantial portion of each
turn is at the hot face surface the insulation body contiguous
with the heated atmosphere while all coils are fully embedded
in the insulating material. Each coil is also flattened
longitudinally in a plane set in from the surface of the
insulating body in order that each turn of each coil has a
second substantially linear segment 54 lying generally in a
plane parallel to the surface of the insulating body. With
10. this flattening of the coils in the second plane, the interior
segments of the embedded coil are positioned closer to the hot
surface 32 and further from the cold face 33 (as seen in FIG. 4)
than would be the case with a nonflattened round coil. Thus,
the advantageous effects of the two flattened segments of each
turn are: (1) a reduced average distance of each turn from the
hot face, (2) an increased average distance from the cold face,
thereby reducing the insulation between the heat source and
heated environment and increasing the insulation between the
heat source and ambient environment, and (3) a greatly increased
20. projected area of the flattened coil which is facing (and
directed) through the insulating medium toward the interior of
the furnace to be heated.
In a preferred example of this embodiment, each of
the nine coil strips was formed of 14.44 feet of eight gage
Kanthan A-l wire, having a diameter of 0.128 of an inch. The `
h wire was wound in closely spaced turns around a mandrel of
.075 inch diameter circular cross section. The thus wound
;` coil was then removed from the mandrel and, while the turns
were still closely spaced in side-by-side relationship, the
30. turns were flattened to a generally oval configuration as seen
- in FIG. 4 ha~ing a major outer diameter of 1.244 inches and a
minor outer diameter of .715 inches as shown in this end view
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1. 56 of a flattened ovalized coil. Six of the flattened coils
were than stretched to 28 inches and three of the coils were
stretched to 33 inches. The coils were welded to the connect-
ing wires and ter~.inal pins and were placed on the bottom
surface of a mold in the final arrangement shown in FIGS. 1
through 3 with the terminal pins 40 and 42 extending upwardly
from the bottom surface of the mold. A slurry includlng a `
suspension of ceramic fiber was then introduced into the mold
and the liquid was vacuum drawn from the slurry to deposit a
10. body of ceramic fibers totally embedding the heating coils
therein. It is noted that the major axis of the flattened
ovalized coil is parallel with the hot face of the insulation
panel. Consequently, the projected area of the resistance
heating wire facing through the surrounding insulation medium
directly toward the interior of the furnace is much greater
than is the projected area of a round coil. In effect this
projected area of the flattened coil 56 as seen in FIG. 4 is at
a ratio of 1.244 to .961 as compared to the round coil 60. ` -;~
This ratio represents an increase of about 30 percent in the
20. area of the heater wire facing through the insulation medium
toward the furnace chamber.
From the method of manufacturing the electric heating
unit, the significance of the anchor ribbon structure can be
understood. Because the ceramic fibers settle down toward the
bottom of the mold as the liquid is drawn through the mold
; bottom, to totally embed any anchoring means in the insulating
unit, those anchoring means should advantageously be relatively
thin in any horizontal plane. In this preferred embodiment
the anchoring ribbons are only .06 inches thick. Although the
30. ribbon is thin, it resists axial movement of the terminal pin
because it is 2 inches long and bent at about 90 degrees; thus,
the axial forces are resisted as the bent anchoring ribbon is
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1. pressed against a planar internal surface of the insulating
body 31. The 1 inch width of the ribbon gives structural
stability to the anchor itself. ~ ;
The resistance heating unit described above was
tested with respect to a panel containing a round coil. The
round comparison coil was wound from identical eight gage
Kanthal A-l, .128 diameter wire; however, this coil was not
flattened and had an outer diameter of .961 inches. The
flattened and round coils were each totally embedded in separate
10. five inch thick panels of lightweight ceramic fiber insulating
material. The thus formed heating units were operated in two
test runs in separate ovens to provide hot face temperatures of ~
2,000 and 2,200 degrees Fahrenheit (F) in respective test `
runs. With the hot face temperatures set in each test run, the
cold face temperature and internal temperatures at 1 inch,
2 1/2 inches, and 3 inches from the cold face and at the back
of the coil were measured for each test coil. For each hot
face temperature of the test, it was determined that the back
of the round coil was at a higher temperature than the back of
20. the flattened coil. The results are shown graphically in
FIG. 4.
The test results for the 2,000 hot face tempera-
ture test run are illustrated by lines 60 and 62 in FIG. 4.
Line 60 shows the internal temperatures within the insulation
panel for the round coil heating unit at the selected points
and line 62 shows the internal temperatures for the flat coil ~ ;~
heating unit. In that test, the temperature at the back of
the round coil 58 was 2,295F, but the temperature at the back
of the flattened coil 56 was only 2,214F, a difference of
30. 81F. Thus, for the given hot face temperature of 2,000F,
the temperature to which the flat coil was heated was sub-
stantially less than that to which the round coil was heated--
1. an indication of substantial energy savings. Similarly, forthe 2,200F hot face temperature, illustrated by lines 64 and
66 for the round and flat coils, respectively, the back surface
of the round coil was 2,520F, and the back surface of the
flat coil was only 2,426F, a difference of 94F.
The above test results indicate that for heating at
a given temperature level, the temperature of the heating coil
and thus the energy input can be reduced by using a flat,
totally embedded coil rather than a round totally embedded coil.
10. And the increased temperature difference between thw two coils ~`
with increased oven temperature suggests that the advantage of
using a flat coil is particularly significant for high oven
temperatures. Research has proven that operating temperatures
in excess of 2,200F can be achieved using flattened, fully
embedded heating elements while still providing normal life
expectancy.
The 5 inch thick electric heating panel unit shown in
FIGS. 1 through 3 is ideal for use as the internal lining of a
new furnace. The unit provides both efficient electric heating
; 20. and high hest insulation from the ambient. However, embodi-
ments of this invention are not limited to heating units of
the dimensions set forth above. Not only may the coil dimen-
sions be varied, but also the depth of the insulating body is
readily varied by changing the depth of the ceramic fiber
slurry during the molding process. The dimensions o~ both the
coils and the insulating body will be dictated by power require-
ments, operating temperatures, physical limits of furnace ~-
chambers and so on.
By embedding the flattened coils described above in
30. a thin insulating body, such as one having a thickness no
greater than about twice the depth of the flattened resistance
heating coil within the insulating body, a heating unit which is
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1. ideal for retrofit refurbishing a old furnaces or for convert-
ing old furnaces to electric heating is provided. As shown in
FIG. 5, a furnace 68 includes an insulated top 70, insulated
bottom 72, and insulated side walls 74 and 76. These walls
may have electric heating elements embedded therein or the
furnace may simply be an oil or gas fired furnace or the like. -~
If the walls have electric heating elements embedded therein
which have deteriorated with age, they are electrically dis-
connected and left in place. Replacement heating units 78 and
10. 80 are then placed over the old heating elements and secured to
the interior of the furnace walls. Because the furnace already
has sufficient insulation the insulating bodies 82 and 84 of
the respective heating units 78 and 80 are relatively thin so
as not to reduce the furnace volume more than necessary.
As can readily be seen by comparing the depths of
the flattened coil 56 and round coil 58 from the hot face 32
in FIG. 4, a flattened coil can be totally embedded in a much
thinner insulating body than can the round coil. This favorable
result is over and above the advantage of high heating -
20. efficiency with ease of manufacture.
While the invention has been particularly shown and
desctibed with reference to preferred embodiments thereof, it
will be understood by those skilled in the art that various
changes in form and details may be made therein without depart- -
ing from the spirit and scope of the invention as defined
by the appended claims.
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