JPS6127869B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a sheathed heater that is widely used for household and industrial purposes, and an object of the present invention is to simplify the structure and improve safety, especially at the end of its life. A conventional sheathed heater has a structure in which a coiled heating wire is housed inside a metal pipe with magnesium oxide interposed therebetween. This heater has many advantages such as excellent insulation, long life, and easy processing, and is widely used.
As the sheathed heaters are increasingly used at high temperatures such as red-hot conditions, it has become clear that there is a safety issue at the end of the life of the sheathed heater. That is,
After long-term use, the heating wire will eventually break, but when this happens, the broken part will become white hot.
Metal pipes may melt. Also, depending on the diameter of the heating wire and the pitch conditions of the coil, the temperature of the incandescent part can reach as high as 2000℃, and the temperature remains high for several minutes as it moves along the metal pipe, resulting in several melted parts of the pipe. It can be as large as a centimeter.
This phenomenon is a major drawback of sheathed heaters from the standpoint of safety, and a sheathed heater that does not cause incandescence when the heating wire breaks and does not cause the metal pipe to become molten has been desired. The inventors clarified the cause of this phenomenon in order to develop a sheathed heater that does not cause the above-mentioned incandescence or melting of the metal pipe. When a heating wire is used for a long period of time, its resistance value becomes uneven due to factors such as the diameter of the wire becoming thinner in some parts, and when electricity is applied, the temperature becomes uneven. As this non-uniformity expands, the heating wire melts in the high temperature portion and eventually breaks.
When the wire breaks, arc discharge occurs and the magnesium oxide near the break becomes molten. Magnesium oxide, which is an insulator in its solid state, also has remarkable ionic conductivity in its molten state, making it a good electrical conductor of 10 ^-1 to ^{10 -2} Ωcm. Therefore, even after the heating wire is disconnected, the current continues to flow, and the metal pipe melts and becomes incandescent due to the Joule heat in the molten state of the magnesium oxide. Due to the causes mentioned above, even if the heat generated when the wire breaks becomes high, if it is filled with a substance that is not a good conductor of electricity, the metal pipe will melt or glow when the wire breaks, as in the case of magnesium oxide. It is thought that this will not happen. On the other hand, fillers for sheathed heaters are required to have high thermal conductivity and high insulation properties. The inventors investigated various inorganic insulators and discovered silicon nitride as a material that satisfies the above-mentioned conditions. Embodiments of the present invention will be described below with reference to the drawings. [Embodiment 1] Fig. 1 shows a sheathed heater which is an embodiment of the present invention, in which a coiled heating wire 2 disposed at the center of a metal pipe 1 is filled with silicon nitride powder. An inorganic insulating layer 3 is formed. In addition, in the figure, 4 is a terminal bar connected to both ends of the heating wire, and 5 is a sealing member that seals both ends of the metal pipe 1. A sheathed heater with such a configuration first attaches a heating wire 2 to a metal pipe with an appropriate diameter.
is inserted and filled with silicon dioxide powder, both ends of the metal pipe are sealed with a sealing member 5, and then the diameter of the metal pipe is reduced by rolling. The silicon titanide powder used was
Contains 97% by weight of Si ₃ N ₄ , with the remainder containing SiO ₂ , Al ₂ O ₃ ,
Contains impurities such as Fe ₂ O ₃ . Heating wire is JIS standard NCH
-1 wire rod with a diameter of 0.5 mm, the winding diameter is 2 mm,
Coiled bodies with pitches of 1, 5, 3, 5, and 7 mm were used. Metal pipe is SUS3
21 (JIS standard), the outer diameter after rolling is 6.6mm, and the inner diameter is
It is 5.8mm. Next, the appearance and internal state of the sheathed heater using magnesium oxide as in the past and the sheathed heater using silicon nitride as in the present invention for forming the inorganic insulator layer after the wire is disconnected are as follows: coil pitch 1, Table 1 shows the comparison results for 5, 3, 5, and 7 mm.

[Table] As is clear from Table 1, in a conventional sheathed heater filled with magnesium oxide, the pipe will melt if the pitch is less than 5 mm, and if the pitch is less than 3 mm, the melted part of the pipe will expand and reach up to 10 cm. . Magnesium oxide is melted inside. On the other hand, in the sheathed heater filled with silicon dioxide as in the present invention, the metal pipe does not melt at any coil pitch. Moreover, the internal state is only that the heating wire is melted, and no change is observed in the silicon nitride. In this way, if silicon nitride is used as the inorganic insulator used to insulate a sheathed heater, it will be safer at the end of its life, that is, when the wire breaks, compared to the conventional insulator that uses magnesium oxide. becomes possible. The results described above were shown for the case where silicon titanide with a purity of 97% was used, but highly pure silicon titanide is expensive and difficult to obtain. Therefore, when producing sheathed heaters on an industrial scale and at low cost, it is desirable to use silicon nitride with low purity. As mentioned above, many of the impurities are oxides such as silicon oxide, aluminum oxide, and iron oxide. On the other hand, since the pipe fusing phenomenon at the time of wire breakage is caused by melting of oxides, the amount of impurities, especially the total amount of oxides, greatly affects the safety of the sheathed heater. Therefore, in order to find out how much oxide is allowed to exist as an impurity, we decided to
A sheathed heater is created by filling silicon oxide, aluminum oxide, iron oxide, and magnesium oxide in a proportion of 5% to 50% by weight as an insulating layer. , we investigated the state of the disconnection. At this time, the heating wire
Winding diameter of 0.5mm wire made of NCH-1 material is 2.
A coiled body with a pitch of 1.5 mm and a pitch of 1.5 mm was used. The metal pipe is made of SUS321 and has an outer diameter of 6.6 mm and an inner diameter of 5.8 mm after rolling. The experimental results will be explained with reference to Table 2.

[Example 2]

Example 1 was carried out by using silicon titanide powder alone or by mixing each of magnesium oxide, aluminum oxide, and silicon oxide powder with silicon titanide powder in an amount of 20% by weight or less. A sheathed heater was made in the same manner as above, and a life test was conducted under accelerated conditions to determine the time it took for the heating wire of the sheathed heater to break. Table 3 shows the results.

[Table] As is clear from the above table, the lifespan is significantly extended when magnesium oxide, aluminum oxide, and silicon oxide powders are mixed with silicon titanide powder. In this case, as shown in Example 1, it is necessary that the mixing ratio does not exceed 20% by weight. The reason why the service life is extended when magnesium oxide, aluminum oxide, or silicon oxide powder is mixed with silicon nitride powder is considered to be as follows. In other words, silicon titanide powder generally has a relatively high porosity, so when silicon titanide powder is filled alone, the packing density is low and the thermal conductivity is somewhat low. Powders of the above-mentioned oxides, including magnesium oxide, have high thermal conductivity. Therefore, by mixing such powders, thermal conductivity increases, and as a result, the temperature of the heating wire can be lowered. This is thought to extend the life of the heating wire as described above. As described above, according to the present invention, in a sheathed heater in which a heating wire is housed in a tubular body made of a good thermal conductor through an insulating layer, the insulating layer contains silicon nitride as a main component, and the silicon nitride Add at least one selected from the group consisting of magnesium oxide, aluminum oxide and silicon oxide to 20% of the total amount of silicon oxide.
Since we use a mixture containing up to % by weight, when the heating wire breaks, the broken part will not become incandescent and the tubular body will melt, and as a result, a safe sheathed heater can be obtained. At the same time, the increased thermal conductivity keeps the heating wire at a lower temperature, resulting in a longer service life.

[Brief explanation of the drawing]

The figure is a sectional view of a sheathed heater that is an embodiment of the present invention. 1... Metal pipe, 2... Heating wire, 3... Insulating layer.

Claims (1)

[Claims] 1 In a sheathed heater in which a heating wire is housed in a tubular body made of a good thermal conductor through an insulating layer, the insulating layer contains silicon nitride as a main component, and the silicon nitride contains magnesium oxide, aluminum oxide, and silicon oxide. A sheathed heater characterized by using a mixture of at least one selected from the group consisting of up to 20% by weight with respect to the total amount of silicon titanide.