CA1078166A - Refractory granular embedding composition for electric heating coils - Google Patents
Refractory granular embedding composition for electric heating coilsInfo
- Publication number
- CA1078166A CA1078166A CA253,704A CA253704A CA1078166A CA 1078166 A CA1078166 A CA 1078166A CA 253704 A CA253704 A CA 253704A CA 1078166 A CA1078166 A CA 1078166A
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- ceramic material
- composition
- oxide
- ceramic
- granulated
- Prior art date
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Abstract
ABSTRACT OF THE DISCLOSURE
A refractory, granular electrically insulative composition useful as an embedding composition for electric heating coils comprises fused and granulated, possibly ground, difficultly fusible, ceramic and possibly oxide-ceramic material having polyalkylsiloxane, e.g., polymethylsiloxane, or polyaryl-siloxane admixed in a proportion of from 0.5 to 5.0% added weight. Use of the composition is suitable with tubular heaters which have been produced with it and which, after manufacture and before being brought into use, have been sub-jected to a heat treatment at a temperature of at least 65°C, for a period of at least 10 minutes. An electrical heating element comprises a housing and an electric heating coil which is embedded in the composition wherein the ceramic and possibly oxide-ceramic material has been ground.
A refractory, granular electrically insulative composition useful as an embedding composition for electric heating coils comprises fused and granulated, possibly ground, difficultly fusible, ceramic and possibly oxide-ceramic material having polyalkylsiloxane, e.g., polymethylsiloxane, or polyaryl-siloxane admixed in a proportion of from 0.5 to 5.0% added weight. Use of the composition is suitable with tubular heaters which have been produced with it and which, after manufacture and before being brought into use, have been sub-jected to a heat treatment at a temperature of at least 65°C, for a period of at least 10 minutes. An electrical heating element comprises a housing and an electric heating coil which is embedded in the composition wherein the ceramic and possibly oxide-ceramic material has been ground.
Description
:~7l~
The present inyention relates to ~ re~actor~, gr~nular electrically insulative compos-;~t~on su~ta~le ~or use as an embedding composition for electric heating coils~ advantageously for use with metal-covered electrical heater elements, more especially electrical tubular heaters~ a method o~ use of such a composition; and an assembly of such a composition and an electrical heating element embedded -therein.
Metal-covered electrical heating elements are used for heating li~uids in baths, for contact heating in cast or poured metals and for heating gaseous media. Conventional tubular heaters have an external, metallic tubular wall with a diameter which is greater than 6 mm, with a maximum of 8.5 mm, a heating coil of metallic resistance material being disposed in centred relation on the mean axis of the tubular wall. The interstice is filled with a ceramic or oxide-ceramic, small-grain insulating composition, advantageously fusea magnesium oxide.
The quality thereof as regards transfer of heat and electrical properties is mainly dependent on the electric, insulating embedding composition which has been introduced between the heating coils and the tubular wail. The fused magnesium oxide which is usually employed for this purpose has an excellent thermal conductivity and at the same time high electrical resistivity. The optimal values for the thermal conductivity and the electrical resistivity are obtained with a final consolidation vf the magnesium oxide of 3.1 to 3.2 g/ml.
Insulating compositions have been previously proposed -:- wherewith the electrically fused and thereafter ground magnesium ; oxide has additives admixed therewith in order to improve the - electrical properties at high operating temperatures. Such ~; 30 compositions are very efficient, but they only completely ;-~ ;
' ~ .'''' ~L~7~
satis~y the purpose ~or wh~Gh the~ a,re intended i~ the ends of the tubular heaters a~e sealed in such a ~ay that no moisture is able to penetrate into the magnesium oxide. Several construc-tional forms have been prevlously proposed for the water-tight sealing of tubular heaters. For example, constructions have been previously proposed wherein elastic sockets or nozzles are introduced into the open tubular heaters and are so compressed by means of atool that moisture is no longer able to penetrate into the end of the tubular heater. It is also known to intro-duce a plastics or a liquid composition into an open end of a`tube and then to provide a seal in the form of a tubular slaeve of silicone rubber, ceramics or PTFE (Teflon - Registered Trade Mark), which then is likewise subsequently fixed in position in such a way that any emergence thereof from the end of the tubular heater is prevented. With these insulating compositions and such tube seals, additional working steps are necessary in the manufacture of an electrical tubular heater, which involve a considerable additional expense for matQrial as well as the time which is involved. Despite this additional expense in the manufacture of such known heater elements, the possibility of moisture being able to penetrate into the open ends ~f the tubes immediately after an annealing operation is not reliably -~
excluded. Since the magnesium oxide has the property of being quite hygroscopic, a danger constantly exists of moisture being absorbed, and then electrical values, mora especially the resis-tance of the electrical-`insulation, can decrease strongly and a discharge current can rise to impermissible orders of magni-tude~
- It is an object of the present invention to provide a refractory, granular insulating composition suitable for use as =3-~7~
an embeddin~ co~position ~or electric heating coils, wherein the above-indicated disadyanta~es are eliminated or suhstantially reduced.
According to the present invention, therefore, we provide a refractory, granular electrically insulative composition suitable for use as an embedding composition for electric heating coils, which composition comprises fused and granulated, difficultly fusible, ceramic matexial having polyalkylsiloxane or polyarylsiloxane admixed therewith in a proportion of from 0.5 to 5.0~ by weight, expressed as an added weight, based on the total weight of the ceramic material and fused and granulated difficultly fusible, oxide-ceramic material.
The present invention, in another aspect, resides in an electrical heating element comprising a housing and an electric. :
heating coil which lS embedded in a refractory, granular elec-trically insulative composition suitable for use as an embedding composition for electric heating coils, which composition comprises fused, ground, and granulated, difficultly fusible, ceramic material and/or fused, ground and granulated, difficultly ::.
20 fusible oxide-ceramic material having polyalkylsilox~ne or polyarylsiloxane admixed therewith in a proportion of from 0.5 to 5.0% by weig~t, expressed as an added weight, based on the ;
total weight of the ceramic material and/or oxide ceramic - material. -- We have established by tests that the addition of poly-alkylsiloxanes or polyarylsiloxanes to the remainder of the - present composition results in a considerable improvement :~
thereof as regards the a~sorption of moisture and therefore also has a favourable influence on the electrical properties there- : :
30 of. ~ ::
.
;6 In a pre~exxed embodI~ent ~f ~h~ present co~osition, it comprises fused and granulated d~ficultly fus~le, oxide-ceramic material.
The polyalkylsiloxane preferably comprises polymethyl-siloxane.
The polyalkylsiloxanes or polyarylsiloxanes, have prefera-bly been admixed in a grain size from 20 to 200 j~ with the remainder of the present composition.
Particularly preferred results are obtained, when the presen~ composition is in use, when the ceramic material and, where present, the oxide-ceramic material, comprise~ a high-melting metal oxide, preferably magnesium oxide, beryllium oxide, titanium dio~ide, or silicon dioxide, or a mixture thereof.
In place thereof, it is, however, also preferable that the ceramic material comprise a ceramic silicate material, for example of clay.
According to a further feature of the present invention, we provide a method of use of a composition as defined above, in which method tubular heaters which have been produced with the composition, after manufacture and before being brought into use, have been subjected to a heat treatment at a temperature o~lat least 65C for a period of at least 10 minutes. This heat treatment can essentially be omitted if the manufactured tubular heater, as is usually the case, has to be suhjected in any case to an annealing treatment at relatively high tempera-ture for other reasons, for example, for eliminating material ~- -stresses, but the present me hod has proved to be necessary to maintain the resistance to moisture of the tubular heaters in the method.
-~ 30 According to a still further feature of the present ~7~
inVention~ we provid~ an eIectxic~l heatin~ element comprisin~
a hous~ng and an electric fieatin~ co~l w~ich is e~bedded in a composition as de~ined above wherein the ceramic material or the oxide-ceramic material has ~een ground.
The following Example illustrates the present composition.
EXA~LE
Several heating elements were produced with which poly-methylsiloxane had been admixed, in each case in a proportion between 0.5 and 5.0~, expressed as an added weight, based on the total weight of a composition comprising fused and granulated, difficultly fusible, ceramic and possibly oxide-ceramic mater-ial. The heating elements consisted of a tubular wall made of steel (St. 3402~, the embedding composition comprising essen-tially magnesium oxide as a basic composition and modified as indicated above and an electrical resistance element of a nickel-chrome alloy in the ratio of 4.1 (by weight). The embedding composition when poured in was compressed to a density of approximately 3.2 g/ml hy reduction of the cross-sectional area of the tubular wall. - -The specific surface loading for the heating element was
The present inyention relates to ~ re~actor~, gr~nular electrically insulative compos-;~t~on su~ta~le ~or use as an embedding composition for electric heating coils~ advantageously for use with metal-covered electrical heater elements, more especially electrical tubular heaters~ a method o~ use of such a composition; and an assembly of such a composition and an electrical heating element embedded -therein.
Metal-covered electrical heating elements are used for heating li~uids in baths, for contact heating in cast or poured metals and for heating gaseous media. Conventional tubular heaters have an external, metallic tubular wall with a diameter which is greater than 6 mm, with a maximum of 8.5 mm, a heating coil of metallic resistance material being disposed in centred relation on the mean axis of the tubular wall. The interstice is filled with a ceramic or oxide-ceramic, small-grain insulating composition, advantageously fusea magnesium oxide.
The quality thereof as regards transfer of heat and electrical properties is mainly dependent on the electric, insulating embedding composition which has been introduced between the heating coils and the tubular wail. The fused magnesium oxide which is usually employed for this purpose has an excellent thermal conductivity and at the same time high electrical resistivity. The optimal values for the thermal conductivity and the electrical resistivity are obtained with a final consolidation vf the magnesium oxide of 3.1 to 3.2 g/ml.
Insulating compositions have been previously proposed -:- wherewith the electrically fused and thereafter ground magnesium ; oxide has additives admixed therewith in order to improve the - electrical properties at high operating temperatures. Such ~; 30 compositions are very efficient, but they only completely ;-~ ;
' ~ .'''' ~L~7~
satis~y the purpose ~or wh~Gh the~ a,re intended i~ the ends of the tubular heaters a~e sealed in such a ~ay that no moisture is able to penetrate into the magnesium oxide. Several construc-tional forms have been prevlously proposed for the water-tight sealing of tubular heaters. For example, constructions have been previously proposed wherein elastic sockets or nozzles are introduced into the open tubular heaters and are so compressed by means of atool that moisture is no longer able to penetrate into the end of the tubular heater. It is also known to intro-duce a plastics or a liquid composition into an open end of a`tube and then to provide a seal in the form of a tubular slaeve of silicone rubber, ceramics or PTFE (Teflon - Registered Trade Mark), which then is likewise subsequently fixed in position in such a way that any emergence thereof from the end of the tubular heater is prevented. With these insulating compositions and such tube seals, additional working steps are necessary in the manufacture of an electrical tubular heater, which involve a considerable additional expense for matQrial as well as the time which is involved. Despite this additional expense in the manufacture of such known heater elements, the possibility of moisture being able to penetrate into the open ends ~f the tubes immediately after an annealing operation is not reliably -~
excluded. Since the magnesium oxide has the property of being quite hygroscopic, a danger constantly exists of moisture being absorbed, and then electrical values, mora especially the resis-tance of the electrical-`insulation, can decrease strongly and a discharge current can rise to impermissible orders of magni-tude~
- It is an object of the present invention to provide a refractory, granular insulating composition suitable for use as =3-~7~
an embeddin~ co~position ~or electric heating coils, wherein the above-indicated disadyanta~es are eliminated or suhstantially reduced.
According to the present invention, therefore, we provide a refractory, granular electrically insulative composition suitable for use as an embedding composition for electric heating coils, which composition comprises fused and granulated, difficultly fusible, ceramic matexial having polyalkylsiloxane or polyarylsiloxane admixed therewith in a proportion of from 0.5 to 5.0~ by weight, expressed as an added weight, based on the total weight of the ceramic material and fused and granulated difficultly fusible, oxide-ceramic material.
The present invention, in another aspect, resides in an electrical heating element comprising a housing and an electric. :
heating coil which lS embedded in a refractory, granular elec-trically insulative composition suitable for use as an embedding composition for electric heating coils, which composition comprises fused, ground, and granulated, difficultly fusible, ceramic material and/or fused, ground and granulated, difficultly ::.
20 fusible oxide-ceramic material having polyalkylsilox~ne or polyarylsiloxane admixed therewith in a proportion of from 0.5 to 5.0% by weig~t, expressed as an added weight, based on the ;
total weight of the ceramic material and/or oxide ceramic - material. -- We have established by tests that the addition of poly-alkylsiloxanes or polyarylsiloxanes to the remainder of the - present composition results in a considerable improvement :~
thereof as regards the a~sorption of moisture and therefore also has a favourable influence on the electrical properties there- : :
30 of. ~ ::
.
;6 In a pre~exxed embodI~ent ~f ~h~ present co~osition, it comprises fused and granulated d~ficultly fus~le, oxide-ceramic material.
The polyalkylsiloxane preferably comprises polymethyl-siloxane.
The polyalkylsiloxanes or polyarylsiloxanes, have prefera-bly been admixed in a grain size from 20 to 200 j~ with the remainder of the present composition.
Particularly preferred results are obtained, when the presen~ composition is in use, when the ceramic material and, where present, the oxide-ceramic material, comprise~ a high-melting metal oxide, preferably magnesium oxide, beryllium oxide, titanium dio~ide, or silicon dioxide, or a mixture thereof.
In place thereof, it is, however, also preferable that the ceramic material comprise a ceramic silicate material, for example of clay.
According to a further feature of the present invention, we provide a method of use of a composition as defined above, in which method tubular heaters which have been produced with the composition, after manufacture and before being brought into use, have been subjected to a heat treatment at a temperature o~lat least 65C for a period of at least 10 minutes. This heat treatment can essentially be omitted if the manufactured tubular heater, as is usually the case, has to be suhjected in any case to an annealing treatment at relatively high tempera-ture for other reasons, for example, for eliminating material ~- -stresses, but the present me hod has proved to be necessary to maintain the resistance to moisture of the tubular heaters in the method.
-~ 30 According to a still further feature of the present ~7~
inVention~ we provid~ an eIectxic~l heatin~ element comprisin~
a hous~ng and an electric fieatin~ co~l w~ich is e~bedded in a composition as de~ined above wherein the ceramic material or the oxide-ceramic material has ~een ground.
The following Example illustrates the present composition.
EXA~LE
Several heating elements were produced with which poly-methylsiloxane had been admixed, in each case in a proportion between 0.5 and 5.0~, expressed as an added weight, based on the total weight of a composition comprising fused and granulated, difficultly fusible, ceramic and possibly oxide-ceramic mater-ial. The heating elements consisted of a tubular wall made of steel (St. 3402~, the embedding composition comprising essen-tially magnesium oxide as a basic composition and modified as indicated above and an electrical resistance element of a nickel-chrome alloy in the ratio of 4.1 (by weight). The embedding composition when poured in was compressed to a density of approximately 3.2 g/ml hy reduction of the cross-sectional area of the tubular wall. - -The specific surface loading for the heating element was
2.1 watt/ml.
: -. ,After the cross-sectional area of the tubular wall had -~
been reduced,l the elements were annealed at a temperature of 800C. in a reducing atmosphere. The ends of the tubular heaters were not closed. The measurèment of the electrical resistance with a direct current of 500 volt show a value greater than 104 M-~. Heating elements produced for comparison and without any additlon of polymethylsiloxane to the embedding composition has the same electrical resistances immediately after manufacture.
~78~
After stora~e ,f,~r 72 hours ~n ,a,~oist cha~ber at ~0%
rel~tiye humidity and at 3~C, an electric~l resistance of greater than 10 ~n_was- measurea, show-ing no change, with the tubular heaters having the present embedding composition. The comparison heaters without any addition to the basic composition had fallen in value to less than 0.3 M~after being kept for 8 hours in the moist chamber. AEter being stored ~or 144 hours and after 262 hours in the said chamber, the measured electrical :
resistance of the tubular heaters, with polymethylsiloxane -added to the embedding composition, was always still greater than 10 M_f~.
: -. ,After the cross-sectional area of the tubular wall had -~
been reduced,l the elements were annealed at a temperature of 800C. in a reducing atmosphere. The ends of the tubular heaters were not closed. The measurèment of the electrical resistance with a direct current of 500 volt show a value greater than 104 M-~. Heating elements produced for comparison and without any additlon of polymethylsiloxane to the embedding composition has the same electrical resistances immediately after manufacture.
~78~
After stora~e ,f,~r 72 hours ~n ,a,~oist cha~ber at ~0%
rel~tiye humidity and at 3~C, an electric~l resistance of greater than 10 ~n_was- measurea, show-ing no change, with the tubular heaters having the present embedding composition. The comparison heaters without any addition to the basic composition had fallen in value to less than 0.3 M~after being kept for 8 hours in the moist chamber. AEter being stored ~or 144 hours and after 262 hours in the said chamber, the measured electrical :
resistance of the tubular heaters, with polymethylsiloxane -added to the embedding composition, was always still greater than 10 M_f~.
Claims (9)
1. A refractory, granular electrically insulative composition suitable for use as an embedding composition for electric heating coils, which composition comprises fused and granulated, difficultly fusible, ceramic material and/or fused and granulated, difficultly fusible, oxide-ceramic material having polyalkylsiloxane or polyarylsiloxane admixed therewith in a proportion of from 0.5 to 5.0% by weight, expressed as an added weight, based on the total weight of the ceramic and/or oxide-ceramic material.
2. A composition according to claim 1, wherein the ceramic material and, where present, the oxide-ceramic material, have been ground.
3. A composition according to claim 1, wherein the poly-alkylsiloxane comprises polymethylsiloxane.
4. A composition according to claim 3, wherein the poly-methylsiloxane has been added with a grain size of from 20 to 200 u.
5. A composition according to claim 4, wherein the ceramic material, and, where present, the oxide-ceramic material, comprise a high-melting metal oxide.
6. A composition according to claim 5, wherein the ceramic and/or oxide-ceramic material comprises magnesium oxide, beryllium oxide, titanium dioxide, or silicon dioxide, or a mixture thereof.
7. A composition according to claim 4, wherein the ceramic material comprises a silicate ceramic material.
8. A method of use of a refractory, granular electrically insulative composition suitable for use as an embedding composition for electric heating coils, which composition comprises fused and granulated, difficultly fusible, ceramic material and/or fused and granulated difficultly fusible oxide-ceramic material, having polyalkylsiloxane or polyarylsiloxane admixed therewith in a proportion of from 0.5 to 5.0% by weight, expressed as an added weight, based on the total weight of the ceramic and/or oxide-ceramic material, in which method tubular heaters which have been produced with the composition, after manufacture and before being brought into use, have been subjected to a heat treatment at a temperature of at least 65°C, for a period of at least 10 minutes.
9. An electrical heating element comprising a housing and an electric heating coil which is embedded in a refractory, granular electrically insulative composition suitable for use as an embedding composition for electric heating coils, which composition comprises fused, ground, and granulated, difficultly fusible, ceramic material and/or fused, ground and granulated, difficultly fusible oxide-ceramic material having polyalkyl-siloxane or polyarylsiloxane admixed therewith in a proportion of from 0.5 to 5.0% by weight, expressed as an added weight, based on the total weight of the ceramic material and/or oxide-ceramic material.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA253,704A CA1078166A (en) | 1976-05-31 | 1976-05-31 | Refractory granular embedding composition for electric heating coils |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA253,704A CA1078166A (en) | 1976-05-31 | 1976-05-31 | Refractory granular embedding composition for electric heating coils |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1078166A true CA1078166A (en) | 1980-05-27 |
Family
ID=4106080
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA253,704A Expired CA1078166A (en) | 1976-05-31 | 1976-05-31 | Refractory granular embedding composition for electric heating coils |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1078166A (en) |
-
1976
- 1976-05-31 CA CA253,704A patent/CA1078166A/en not_active Expired
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| MKEX | Expiry |