Sheathed resistance heater

Details Sheathed resistance heater Sheathed resistance heater

1983-01-18

1985-03-19

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

045062510

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a sheathed resistance heater of the type which comprises a heating wire received in a metal pipe and an electrically insulating powder packed in the metal pipe by which the resistance heater has a prolonged life and can be maintained at a high level of insulation resistance in a working condition after having been used over a long term.


BACKGROUND ART

Sheathed or shielded resistance heaters have widely been used in many fields as heating parts because of their very excellent performance, quality and convenience. The commercial range of the heaters has now increased including not only domestic electric articles, but also specific applications such as in various industries, space developments and atomic power services. Among various classes of the sheathed electric heaters, sheathed heaters for high temperature purposes will more and more increase in applications.
Upon reviewing the performance and quality of sheathed resistance heaters in a world-wise sense, it will be found that they have defects in that the insulation resistance in a working condition (hereinafter referred to as insulation resistance under self-heating conditions) lowers as a function of time, coupled with another disadvantage in that it takes only a short time before breakage of the heating wire.


DISCLOSURE OF THE INVENTION

The present invention contemplates to provide a sheathed resistance heater in which there is used an electrically isnulating powder which contains a specific type of a powder therein whereby the life before breakage of the heating wire is prolonged and the heater exhibits a high insulation resistance when measured under working or self-heating conditions after long-term use.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional sheathed resistance heater; and FIGS. 2, 5, 8 and 11 are, respectively, characteristic graphs showing the relation between the total test period and the insulation resistance under self-heating conditions, FIGS. 3, 6, 9 and 12 are characteristic graphs showing the relation between the amount of metallic powder and the insulation resistance under self-heating conditions; and FIGS. 4, 7, 10 and 13 are characteristic graphs showing the relation between the amount of metallic powder and the life.


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 heat 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 wire therein, an electrically insulating powder 4 such as electrofused magnesia, electrofused silica, electrofused alumina and the like filled up in the metal pipe 3, and optionally, a glass 5 and a heat-resistant resin 6 sealing opposite ends of the metal pipe 3 therewith.
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, to which were added different amounts of nickel powder and mixed together to obtained samples of electrically insulating powder 4.
The electrofused magnesia powder used had a composition indicated in Table 1.


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 in the form of a coil having a winding diameter of 2 mm. The heating wire was connected with terminal bars 1 at opposite ends thereof.
The metal pipe 3 was a NCF 2P pipe (commercial name Incoroi 800) having a length of 413 mm, an outer diameter of 8 mm and a thickness of 0.46 mm. Into the metal pipe 3 was inserted the heating wire 2 connecting the terminal bars 1 at opposite ends thereof.

REFERENCES:
patent: 2932577 (1960-04-01), Shaw et al.
patent: 3201738 (1965-08-01), Mitoff
patent: 3477058 (1969-11-01), Vedder et al.
patent: 3622755 (1971-11-01), Vedder
patent: 3623921 (1971-11-01), Behringer et al.
patent: 4087777 (1978-05-01), Classen et al.
patent: 4280932 (1981-07-01), Borom et al.
patent: 4376245 (1983-03-01), Lindskog et al.

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