CA1088607A - Isolated terminal electric heating element and method of making same - Google Patents
Isolated terminal electric heating element and method of making sameInfo
- Publication number
- CA1088607A CA1088607A CA274,932A CA274932A CA1088607A CA 1088607 A CA1088607 A CA 1088607A CA 274932 A CA274932 A CA 274932A CA 1088607 A CA1088607 A CA 1088607A
- Authority
- CA
- Canada
- Prior art keywords
- cold pin
- heating element
- moisture
- outer tubing
- hollow outer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Abstract
ABSTRACT OF THE DISCLOSURE
An electric heating element and a method of constructing such an element utilizing a moisture-impermeable insulating barrier to minimize the leakage current between a cold pin terminal connector and the hollow outer tubing surrounding an electric resistance heating wire. The cold pin has a protuberance formed thereon intermediate the ends.
A moisture-impermeable, electrically insulating layer is provided on 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 bushing defining an aperture through which the cold pin passes, with the bushing abutting the protuberance at the outer end of the aperture. The bushing also has a radially outwardly directed flange, a portion of the bushing being positioned within the hollow outer tubing and the flange engaging the end of the tubing to form a seal.
An electric heating element and a method of constructing such an element utilizing a moisture-impermeable insulating barrier to minimize the leakage current between a cold pin terminal connector and the hollow outer tubing surrounding an electric resistance heating wire. The cold pin has a protuberance formed thereon intermediate the ends.
A moisture-impermeable, electrically insulating layer is provided on 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 bushing defining an aperture through which the cold pin passes, with the bushing abutting the protuberance at the outer end of the aperture. The bushing also has a radially outwardly directed flange, a portion of the bushing being positioned within the hollow outer tubing and the flange engaging the end of the tubing to form a seal.
Description
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 .-~C~886~
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 1~88~07 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 B mb/J~ - 4 -: -' . : '' - 1~886~)7 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-B mb/~o - 4a -1~886~7 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 .
.
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-1~8~6~7 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 10886~'7 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.
1~886()~
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) 1~886~7 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 11~886~7 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.
~(~8t36~3~7 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 -1~886~7 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~
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 .-~C~886~
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 1~88~07 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 B mb/J~ - 4 -: -' . : '' - 1~886~)7 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-B mb/~o - 4a -1~886~7 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 .
.
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-1~8~6~7 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 10886~'7 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.
1~886()~
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) 1~886~7 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 11~886~7 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.
~(~8t36~3~7 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 -1~886~7 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~
Claims (11)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In an electric heating element comprising a hollow, outer tubing, a resistance heating element passing through the bore of said tubing in spaced relation thereto, cold pin terminating elements secured to the ends of said resistance heating element within said hollow outer tubing and extending beyond the ends thereof, a moisture-permeable, electrically insulative material having high heat transfer characteristics, positioned between said resistance heating element and the hollow outer tubing, and end seal means positioned about the ends of said hollow outer tubing in a manner to permit said cold pins to pass therethrough; the improvement wherein said cold pin defines a protuberance formed thereon intermediate the ends thereof and further comprising, in combination, a moisture-impermeable, electrically insulating barrier positioned along said cold pin element from said end seal means inwardly toward the junction of said cold pin and the resistance heating element, to form an insulation barrier for leakage current between said cold pin element and the hollow, outer tubing in the region adjacent said end seal means, said seal means including a first bushing member defining an aperture through which said cold pin passes, said bushing member abutting the protuberance at the outer end of said aperture, said bushing also defining a radially outwardly directed flange, a portion of said bushing being positioned within said hollow outer tubing at an end thereof, said flange engaging the end of said hollow outer tubing to form a seal.
2. The heating element of Claim 1 in which said end seal means is proportioned to permit the intake and expulsion of gases therethrough upon, respectively, cooling and heating of the heating element.
3. The heating element of Claim 2 in which said moisture-impermeable insulating barrier is a tube of heat-shrunk material having good thermal stability, tightly encasing each cold pin terminating element.
4. The heating element of Claim 2 in which said moisture-impermeable insulating member is a sleeve positioned against the inner wall of said hollow outer tubing.
5. The heating element of Claim 2 in which said moisture-impermeable insulating barrier is a sleeve positioned between said cold pin terminating element and said hollow outer tubing, being spaced intermediately between both said elements.
6. The heating element of Claim 2 in which said moisture-impermeable insulating barrier is a tube of heat-shrunk material having good thermal stability, tightly encasing each cold pin terminating element, plus a moisture-impermeable insulating sleeve positioned against the inner wall of said hollow outer tubing.
7. The heating element of Claim 2 in which said moisture-impermeable barrier member is made of a material selected from the group consisting of Teflon? and silicone rubber.
8. The heating element of Claim 1 including a resilient bushing defining an aperture through which said cold pin is positioned, said resilient bushing being positioned within said hollow outer tubing and abutting the inner end of said first bushing member, said moisture-impermeable insulating barrier being defined by a sleeve tightly positioned about said cold pin terminating element and extending from said resilient bushing inwardly beyond said resilient bushing for a predetermined distance which is greater than half way from the inner end of said resilient bushing to the junction of the cold pin and the resistance heating element.
9. The heating element of Claim 2 in which said moisture-impermeable insulating barrier is bonded to each cold pin terminating element to prevent the passage of moisture therebetween.
10. The method of sealing the terminating end of an electric heating element utilizing a cold pin terminating connector, comprising the steps of:
applying a layer of moisture-impermeable insulation to a central portion of said cold pin, placing an end cap element over said layer of insulation, securing a resistance heating element to the cold pin, inserting said 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 said hollow outer tubing, filling the space between the inner wall of said hollow outer tubing and said cold pin and resistance heating element with a particulate insulation material, and sealing the ends of said heating element with a seal that extends to the outer ends of the moisture-impermeable insulating barriers, whereby said moisture-impermeable insulating barriers prevent the passage of electric current from said cold pin to the hollow outer tubing in regions adjacent the end thereof.
applying a layer of moisture-impermeable insulation to a central portion of said cold pin, placing an end cap element over said layer of insulation, securing a resistance heating element to the cold pin, inserting said 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 said hollow outer tubing, filling the space between the inner wall of said hollow outer tubing and said cold pin and resistance heating element with a particulate insulation material, and sealing the ends of said heating element with a seal that extends to the outer ends of the moisture-impermeable insulating barriers, whereby said moisture-impermeable insulating barriers prevent the passage of electric current from said cold pin to the hollow outer tubing in regions adjacent the end thereof.
11. The method of Claim 10 in which said moisture-impermeable insulating member is made of heat-shrinkable tubing which is placed over the cold pin, and then heated to shrink down into a tight, sealing relation on the cold pin terminating means.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US75584776A | 1976-12-30 | 1976-12-30 | |
| US755,847 | 1976-12-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1088607A true CA1088607A (en) | 1980-10-28 |
Family
ID=25040901
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA274,932A Expired CA1088607A (en) | 1976-12-30 | 1977-03-28 | Isolated terminal electric heating element and method of making same |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1088607A (en) |
-
1977
- 1977-03-28 CA CA274,932A patent/CA1088607A/en not_active Expired
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