Electrical resistors – Incased – embedded – or housed – Element inpowdered insulation with outer metallic sheath
Patent
1983-01-17
1986-04-29
Mayewsky, Volodymyr Y.
Electrical resistors
Incased, embedded, or housed
Element inpowdered insulation with outer metallic sheath
174102P, 219544, 219552, 338243, 338274, 501108, H01C 103
Patent
active
045860203
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a sheathed resistance heater which comprises a metal pipe receiving therein a heating wire and an electrically insulating powder packed in the metal pipe whereby the heater has a prolonged life and exhibits a high level of insulation resistance in a working condition after having been used over a long time.
BACKGROUND ART
Sheathed resistance or electric heaters have been widely used in many fields because of their very excellent performance, quality and convenience, extending the commercial range including not only domestic electric articles, but also specific applications such as in various industries, space developments, atomic power services and the like. Among various classes of sheathed resistance heaters, sheathed heaters for high temperature purposes are considered to have more and more increasing applications in the future.
Upon reviewing the performance and quality of sheathed resistance heaters in a world-wide sense, they have drawbacks in that the insulation resistance of the heaters in a working condition (hereinafter referred to as insulation resistance under self-heating conditions) lowers as a function of time, coupled with another disadvantage that it takes only a short time before disconnection of the heating wire.
DISCLOSURE OF THE INVENTION
The present invention contemplates to provide a sheathed resistance heater and a method for fabricating the heater, in which an electrically insulating powder used is a powder admixed with the specific type of a powder whereby the heater has a prolonged life before breakage of the heating wire and exhibits high insulation resistance under self-heating conditions even after its long-term use.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an ordinary sheathed resistance heater; and
FIGS. 2-27 refer to embodiments of the present invention, in which FIGS. 2, 5, 8, 11, 14, 17, 20 and 23 are characteristic graphs of the insulation resistance under self-heating conditions in relation to variation in test period, FIGS. 3, 6, 9, 12, 15, 18, 21, 24, 26 and 26 are characteristic graphs of the insulation resistance under self-heating conditions in relation to variation in amounts of oxides, and FIGS. 4, 7, 10, 13, 16, 19, 22 and 25 are characteristic graphs of the life in relation to variation in amounts of oxides.
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention are described with reference to the accompanying drawings.
In general, a sheathed resistance heater comprises, as particularly shown in FIG. 1, a coil-like heating wire 2 provided with terminal bars 1 at opposite ends thereof, a metal pipe 3 receiving the heating wire therein, and an electrically insulating powder 4 such as of electrofused magnesia, electrofused silica, electrofused alumina or the like which is filled up in the metal pipe. Optionally, the metal pipe 3 may be sealed with a glass 5 and a heat-resistant resin 6 at opposite sides thereof.
We have paid particular attention to the electrically insulating powder 4 and made extensive studies on the powder.
[EXAMPLE 1]
An electrofused magnesia powder was used as a main component of the electrically insulating powder 4, which was admixed with NiO in different amounts to obtain samples of electrically insulating powder 4.
The electrofused magnesia powder had a composition indicated in Table 1 below.
TABLE 1 ______________________________________
MgO 96-97 wt %
CaO 0.2-0.3 wt %
SiO.sub.2 2-3 wt %
Al.sub.2 O.sub.3
0.4-0.5 wt %
Fe.sub.2 O.sub.3
0.14-0.16 wt %
______________________________________
The heating wire 2 used was a nichrome wire of the first kind having a diameter of 0.29 mm and wound in the form of a coil with a winding diameter of 2 mm. The wire was connected with terminal bars 1 at opposite ends thereof. The metal pipe 3 used was a pipe, NCF2P (commercial name Incoroi 800), having a length of 413 mm, an outer diameter of 8 mm and a thickness of 0.46 mm.
Into each of the metal pipes 3 was inserted the heat
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patent: 4506251 (1985-03-01), Narvo et al.
Kawanishi Hidekata
Naruo Noboru
Matsushita Electric Industrial Company Limited
Mayewsky Volodymyr Y.
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