Note: Descriptions are shown in the official language in which they were submitted.
1~886~
BACKGROUND OF THE INVENTION
This invention relates generally to electric heating
elements, and more particularly to an improved method of
manufacturing the terminating ends of the heating element
to eliminate and/or substantially reduce leakage current
between the internal electrical element and the external
outer tubing.
Heretofore, the construction of electric heating
elements of the type commonly referred to as Cal-Rod heaters
has lent itself to the problem of electrical leakage of
substantial amounts between the electrically active cold
pins and the isolated sheath at ground potential. While
this current leakage occurs along the entire length of the
heater, it appears to be most severe at the heater ends,
where the moisture content is cyclic and not fully driven out
of the magnesium oxide insulation by the heating.
Some success has been experienced in reducing this
current leakage in high quality heaters by removing as much
ambient moisture as possible from the units, followed im-
mediately by sealing the heater ends. However, this is a
costly process. Seals can fracture, and experience has in-
dicated that heater life is substantially increased when an
element is permitted to "breathe", i.e. vent, with changes
in temperature.
It is consequently most desirable to construct a
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heater that can freely breathe, yet minimize the current
leakage problem, As the leakage current in the heater in-
creases, the electric potential sensed at the outer sheath
becomes of concern, as it is potentially hazardous to per-
sonnel who may inadvertently touch the unit or objects in
contact with the unit,
SU~MARY OF THE INVENTION
This invention provides a substan-tially new design
of electrical heating elements which minimizes the leakage
currents between the cold pins and the outer sheathing, and
as a direct result of this permits the construction of a
safer heating element, combined with the longer life and
reliability of conventional constructions.
This invention also provides an improved method of
manufacture of Cal-Rod units of the type disclosed herein,
which substantially reduces the cost of, while increasing
the reliability and efficiency of~ units intended for por~
tions of the market now being supplied by premium-cost sealed
units. It further permits substantial improvements of units
intended for non-premium markets at competitive costs.
Breifly, the electric heating element of this in-
vention is constructed substantially in the conventional
manner, using a hollow outer tubing with a resistance heating
element centrally passing therethrough. Cold pin terminal
connectors are secured to the resistance heating element and
extend from the ends of the hollow outer tubing in the con-
ventional manner. The cold pins are embedded with the usual
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porous insulation such as magnesium oxide.
In accordance with this invention, the cold
pin defines a protuberance formed thereon intermediate
the ends thereof. A moisture-impermeable, electrically
insulating barrier is positioned along the cold pin
element from the end seal means inwardly toward the
junction of the cold pin and the resistance heating
element, to form an insulation barrier for leakage
current between the cold pin element and the hollow,
outer tubing in the region adjacent the end seal
means. The seal means includes a first bushing
member defining an aperture through which the cold
pin passes, the bushing member abutting the protuberance
at the outer end of the aperture. The bushing also
defines a radially outwardly directed flange, a
portion of the bushing being positioned within the
hollow outer tubing at an end thereof with the flange
engaging the end of the hollow outer tubing to form
a seal.
This construction will not preclude the use
of buttons, grommets or bushings, nor other end
constructions, nor those constructions where these
hardwares are omitted. Furthermore, this construction
will permit normal heater aspiration or "breathing",
thus avoiding the problems associated with attempts
to reduce current leakage by hermetic sealing of
heater ends.
In its method aspect, the invention relates
to the method of sealing the terminating end of an
e-lectric heating element utilizing a cold pin
terminating connector, comprising the steps of:
applying a layer of moisture-impermeable insulation
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to a central portion of the cold pin, placing an
end cap element over the layer of insulation, securing
a resistance heating element to the cold pin, inserting
the resistance heating element and the cold pin
into a hollow outer tubing in such a manner that the
cold pin and resistance heating element are spaced .
from the inner surface of the hollow outer tubing, ~ :
filling the space between the inner wall of the
hollow outer tubing and the cold pin and resistance
heating element with a particulate insulation
material, and sealing the ends of the heating element
with a seal that extends to the outer ends of the
moisture-impermeable insulating barriers, whereby
the moisture-impermeable insulating barriers prevent
the passage of electric current from the cold pin
to the hollow outer tubing in regions adjacent the
end thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of one of
many types of electric heating elements wherein the
isolated end construction of this invention can be
advantageously employed.
Figure 2 is a longitudinal sectional view,typical
of one of the many commonly employed prior art end construc-
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tions of the seal formed at the end of electric heating
elements.
Figures 3A to 3F are lonaitudinal sectional views
showing a series of construction steps to manufacture an
electrical heating element employing one of the isolation
methods embodied by this invention.
Figures 4 through 7 are fragmentary, longitudinal
sectional views of different embodiments of heater ends made
in accordance with this invention.
_TAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
Referring now to Figure 1, there is seen an elec-
tric heating element designated generally by reference
numeral 10. The electric heating element 10 is of the type
commonly referred to as a Cal-Rod unit.
An electric resistance wire is positioned within
a hollow outer metal tubing 16 (Figure 2) and is insulated
therefrom by a material such as magnesium oxide in a manner
well known in the art. The electric resistance wire is con-
nected at its ends to terminating pins 11 and 12 which are
commonly referred to as cold pins. Seal means 13 and 14 are
positioned about the cold pins 11 and 12, respectively and
inserted into the terminating ends of the hollow outer tubing
to form a seal and improve centering between the tubing and
the cold pin.
Figure 2 illustrates one typical form of prior art
construction for providing a seal about the hollow outer
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1(1i886~D7
tubing and the cold pin. Here, the hollow outer metal tubing
16 carries a cold pin 17 extending therein. The cold pin 17
has a substantial length thereof inserted into the hollow
outer tubing and embedded in the magnesium oxide insulation
material 18, or similar moisture-permeable insulation. This
is conventional in configuration.
An annular end end cap element 19 is placed over
the cold pin 17 and inserted partially into the terminating
end of the hollow outer metal tubing 16. This arrangement
provides an annular seam or abutment 20 between the end cap
element 19 and the interior surface of the hollow outer
tubing 16, through which "breathing" or venting takes place.
There is also provided an annular seam or abutment
21 between the cold pin 17 and the end cap element 19. Gen-
erally, an outer heat-shrink tubing member 22 is provided
over the end of the tubing and over the annular seal 19.
The annular seal 19 may be secured in position by epoxy ce-
ment or other suitable means.
Upon evaluating this conventional configuration it
can be seen that a moisture path exists between the inner
face 24 into the annular chamber 26 and therefrom across the
inner face 27 into the annular chamber 28, and from there
through the inner face 20 into the insulating material 18.
Also, moisture can enter chamber 28 through annular joint 29,
and also can pass from chamber 26 to the insulating material
18 through joint 21 along the cold pin. The amount of mois-
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ture absorbed within the insulating material 18 is greatest
near the terminating end l9a of the annular seal cap 19.
It should be noted that all end constructions in
common usage do not utilize the same elements shown in
S Figure 2. Figure 2 only serves to indicate a typical con-
struction and the means by which a heater may aspirate and
absorb moisture, which is a key element in the current leak-
aye phenomenon.
Magnesium oxide is not a perfect insulator, but is
desirable for use because of its excellent heat transfer
characteristics. However, trace moisture common in the cold
end area of this type of heater can reduce the electrical
resistance of magnesium oxide to provide a direct, low con-
ductivity path between the electrically charged cold pin 17
and the outer sheath 16.
It is a purpose of this invention not necessarily
to eliminate the trace moisture nor to prevent the normal
aspiration of high quality tubular heaters, but to isolate
the pin from the sheath by providing a solid, moisture-
impermeable electrical barrier between the pin and the sheathover that length of the heater where the problem is most
severe, including the end segments of the magnesium oxide
insulator.
This barrier may be on the outer surface of the pin,
or on the inner surface of the wall, or-on both, or located
somewhere in between. If the direct electrical path radiating
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outwardly from the cold pin is interrupted by an insulative
barrier that extends beyond the length of highest trace
moisture in the magnesium oxide, then any leakage must
occur beyond the end of the barrier, where trace moisture
in the magnesium oxide and the resulting electrical conduc-
tivity are significantly lowered. Accordingly, little or no
electrical current passes across the magnesium oxide.
In accordance with this invention, Figure 3
illustrates the various steps of constructing one type of
cold pin-terminating end which substantially reduces the
effects of moisture. In Figure 3A, a bump or protuberance
33 is formed on a conventional cold pin 30. The next step
is to secure an electric resistance heating wire 32 to the
tapered end 31, by suitable means such as welding or the
like t as in Figure 3B.
In Figure 3C, the cold pin 30 is then provided with
a sleeve layer of insulation 34 to extend along a substantial
central portion thereof~ although this may be accomplished
prior to the attachment of the heating wire 32. Sleeve 34
may be a separate piece, or may be an attached layer, sup-
ported by cold pin 30.
In the instance illustrated in Figure 4 the cold
. pin is sleeved with one of any number of types of high tem-
perature heat~shrinkable tubing 34, such as Teflon or silicone,
if desired~ preceded by a pretreatment to improve bonding.
The pin and tubing are then uniformly raised in -temperature
to effect "recovery" of the tubing 34 to the pin diameter.
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Other insulating coatings may also be employed as
a substitute for tubing 34, including ceramics, fiber compo-
sites, dip and spray coatings, and certain enamels. The
choice of coatings will usually be determined by the subse-
~uent manufacturing steps and the intended end use.
The bond at interface 37 between cold pin 30 and
insulating member 34 has not proved to be especially critl-
cal in most applications due to its length, but a good bond
will reduce "wicking" or capillary travel of moisture along
this interface 37.
Continuing again with the assembly steps of Figure
3, in Figure 3D, a retaining element or bushing 39 is then
placed over the cold pin and insulating material to abu~
against the protuberance 33. The bushing 39, made of gen-
erally rigid material, is provided with a radially outwardly-
directed annular flange 40, to abut against the terminating
edge of the hollow, outer tubing as to be described hereinbelow.
As shown in Figure 3E, optional resilient bushing 42
is then placed over the insulating sleeve 34 to abut against the
bushing 39 along their interface 43. The bushing 42 can be
formed of high temperature silicone rubber, or the like.
The entire assembly is then inserted into an outer
sleeve 16, as in Figure 3F, which is then filled with insu-
lating material such as magnesium oxide 46. It will be
noted that the layer of insulation 34 extends a distance (d)
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beyond the termlnating end face 47 of the bushing 42, typi-
cally terminatlng as near to the end of the cold pln as the
heat stability of the insulator 34 permits, but generally
not beyond the cold pin. Therefore, trace moisture which-
may migrate through the inner face between the bushing 42
and the hollow outer tublng 14, or between the bushing 42
and the sleeving 34, will be more concentrated near inter-
face 47 and less concentrated at the terminus of the sleeve
34, where electrical resistance across the magnesium oxide
insulation 46 will accordingly be higher.
In the embodiment of Figure 4, briefly described
above, the cold pin element is designated by the reference
numeral 30. This pin is centered within the sheath 16,
which is filled with magnesium oxide or similar filler 46.
The filler is contained by a bushing 53, and in some con-
structions an outer cover 56. The principal path of moisture
transfer during "breathing" is along the interfaces 59, 55,
and 60, 61, into the filler 46.
Accordingly, moisture can be sucked into the porous
magnesium oxide lnsulation through those vent paths as the
heater cools, creating a reduced pressure inside of its outer
tubular housing 16. However, solid insulating sleeve 34,
made of silicone rubber or any other insulating material as
desired, serves as a moisture barrier to prevent the passage
of electric current from cold pin 30 to the outer sheet 16
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along the length of sleeve 34, even if the magnesium oxide
46 is damp in that area.
Upon use of the heater, -trace moisture is generally
expelled out of the ends thereof or diffused by the heat and
expansion of any gases in the coil.
Referring to Figure 5, a construction similar to
Figure 4 is shown, but where a solid, tubular moisture bar-
rier insulator 70 is positioned inside of outer sleeve 16,
rather than about tubular cold pin 30. The remaining parts
function in a manner similar to the embodiment of Figure 4.
Once again, moisture which seeps into the heater through any
of the various interfaces to increase the conductivity of
the magnesium oxide 46 at the ends of the heater will not
result in a significant increase of current passing from
the cold pin 30 to the outer sheath 16, because of the
presence of insulating barrier 70.
Referring to Figure 6, another embodiment of
similar structure is shown, but including a pair of tubular
insulating barriers 74, 76, one being positioned about cold
pin 30, and the other positioned on the inner diameter of
sheath 16. The remaining parts shown in Figure 6 are simi-
lar to their analogous parts in Figures 4 and 5. Accordingly,
the presence of moisture in the ends of magnesium oxide layer
46 will not result in a significant increase of current
passage between cold pin 30 and outer sheath 16 in that area.
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According to Figure 7, outer sheath 16 and cold
pin 30 are shown with this particular embodiment carrying
the optlonal flattened member 33 as illustrated in Figure
3. A tubular insulating structure 78 is provided, being sur-
rounded inside and out with insulating layers 80, 82 of
magnesium oxide. Once again, insulating sleeve 78 is made
of a water-lmpermeable insula-ting material, and terminates
short of the end of -the cold pin but preferably as near to
the end as the temperature stability of the material of in-
sulator 78 will permit. Thus, a current barrier from cold
pin 30 to outer sleeve 16 of the heater is provided near
the ends of the magnesium oxide layers.
Also, tubular insulating barrier 78 may include an
integrally-attached, enlarged bushing portion 84 to fit the
end of casing 16 and to receive cold pin 30 in a sealing
manner.
A11 of the embodiments of this invention illustrated
above exhibit the characteristic of preventing, by the use
of a water-impermeable insulating layer in addition to the
magnesium oxide insulator or similar porous, water-wettable
material, the excessive passage of current from the cold pin
to the outer sheath in the vicinity of the cold ends of the
heater. Accordingly, a heater which breathes by venting and
taking ambient gases at its ends is provided, for preserva-
tion of the life of the heater, coupled with a safer heater
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which is substantially free from the electrical current
passing across the ends of the magnesium oxide insulating
layer, as is often the case in heating coils which are not
hermetically sealed.
The above has been offered for illustrative pur-
poses only, and is not for the purpose of limiting the
invention of this application, which is as defined in the
claims below~
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