JPS6231477B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for sealing a sheathed heater.

Magnesia, which is generally used for insulation between the metal pipe and heating coil of a sheathed heater, has the disadvantage that when left in the air, the insulation resistance decreases due to absorption of moisture in the air. To prevent this drawback, it is necessary to seal the ends of the metal pipes. The method involves placing a heating coil inside a metal pipe, filling the space with magnesia, and then passing a ring-shaped sintered body made by molding and sintering low-melting glass powder through the lead wire of the heating coil through the opening of the metal pipe. After putting the metal pipe into a heating furnace, it is heated to 300 to 400°C for a predetermined period of time to remove moisture absorbed by the magnesia powder, and then the inside of the heating furnace is heated to the melting temperature of the sintered body, for example, 480°C. It was raised and sealed.

When sealing is performed in such a method, when the temperature inside the heating furnace rises to the melting temperature of the sintered body, the sintered body melts and closes the openings at both ends of the metal pipe. As a result, the air remaining in the magnesia inside the metal pipe is sealed at almost the same temperature as the heating furnace.
Air circulation is stopped. Next, if the temperature inside the furnace is lowered, the molten sintered body will also harden.
Glass sealing is completed. At this time, the remaining air inside the metal pipe is sealed at 480°C and cooled to room temperature. Then, according to Boyleshall's law, the pressure inside the metal pipe becomes negative when it is cooled to room temperature.

For example, assuming that the furnace temperature is 480℃ and the furnace pressure is 1Kg/ ^cm2 , the internal pressure when cooled to room temperature (20℃) is the internal volume of the sheathed heater at 480℃, which is V,
If the same volume at room temperature is V', then P・V/T=P′・
From V'/T', if V≒V', then P'=PT'/T=1・20+273
/480+273≒0.4 Kg/cm ² and is sealed at a pressure lower than atmospheric pressure. In reality, V=V' is not true, so 0.4
Although it is slightly larger than Kg/cm ² , it is not enough to cause a large change considering the value of the thermal expansion coefficient, and a negative pressure is inevitable.

The sheathed heater completed as described above, which has a negative internal pressure, has particularly poor dielectric strength. According to Perthien's law, the discharge voltage (Vs) is a function of the product (Pd) of the distance (d) between the electrodes and the atmospheric pressure (P). That is, Vs=(Pd).

FIG. 4 shows the relationship between discharge voltage (Vs) and (Pd). For a sheathed heater with internal pressure at atmospheric pressure (Pd), the corresponding value (Pdc) of the minimum discharge voltage (Vsm)
Since it is sufficiently larger, it is sufficient to consider the range of (Pd)>(Pdc). Therefore, from this relationship, it is clear that if the internal pressure decreases, the dielectric strength characteristics deteriorate. Therefore, the completed sheathed heater may not be able to withstand the legally stipulated voltage and may become defective.

In order to improve this point, it is necessary to increase the diameter of the metal pipe of the sheathed heater to increase the insulation distance, which is extremely inconvenient.

In order to improve this inconvenience, the present invention removes the moisture absorbed by the magnesia powder at a high temperature when sealing, and then maintains it at a low temperature to the extent that the magnesia powder does not physically absorb moisture. By heating only the sealed portion after taking it out and melting the low melting point glass, the internal pressure after cooling is prevented from decreasing significantly, thereby preventing a decrease in dielectric strength characteristics.

The sealing method of the present invention will be described in detail below. Figure 1 is a cross-sectional view before sealing treatment, 1 is a heating coil,
2 is a metal pipe, 3 is magnesia powder, and 4 is a lead wire of a heating coil. The product in such a state is placed in a temperature-controlled heating furnace as shown in FIG. 3, and heated to 350° C. or higher to remove moisture absorbed by the magnesia powder 3. If the water adsorbed to magnesia powder is physically adsorbed,
It can be removed by heating to 100°C or higher, but chemically bonded substances cannot be removed unless heated to 350°C or higher. Therefore, it is not possible to determine whether the bond is physical or chemical from the appearance of magnesia powder 3, so if heated at 350°C or higher, magnesia powder 3
The moisture adsorbed on the substrate can be completely removed, and the electrical characteristics can be improved. In this way, the magnesia powder is heated in a heating furnace with a temperature of 350℃ or higher for a predetermined period of time to remove the moisture adsorbed on the magnesia powder, and then the inside of the heating furnace is heated to a temperature at which moisture is not physically adsorbed to the magnesia powder. Only lower the temperature to a lower temperature, for example 100℃. Then, it is heated at this temperature for a predetermined time and maintained until the entire sheathed heater reaches 100°C. Thereafter, the sheathed heater is taken out of the heating furnace, and the open end of the metal pipe 2 is sealed with low-melting glass. As a sealing method, a glass frit is put into the opening of the metal pipe 2 and heated and melted with a bart etc., and a sintered body 5 made of low melting glass formed into a ring shape as shown in Fig. 3 is used.
There is a method of inserting the glass into the opening of the metal pipe 2 and heating and melting it, but in either method, it is necessary to heat only the sealing part of the metal pipe 2 to melt the low melting point glass.

As described above, in the sealing method of the present invention, moisture adsorbed on magnesia powder is first completely removed in a heating furnace at a temperature of 350°C or higher, and then the water is heated to about 100°C, where the magnesia powder does not physically adsorb moisture. The entire sheathed heater is completely heated in a low temperature heating furnace of 100%
℃, then take out the sheathed heater from the heating furnace, immediately insert low melting point glass into the open end of the metal pipe, and heat only the vicinity of the sealing part of the metal pipe to melt the low melting point glass. , is to be sealed.

As described above, the present invention takes into consideration the reduction of internal pressure during slow cooling after sealing, heats the moisture in magnesia powder in a high-temperature heating furnace to completely remove it, and then lowers the temperature to a low temperature using a sheathed heater. After keeping the whole glass at the same temperature as inside the heating furnace, it is taken out of the heating furnace and only the sealing part is heated to melt the low melting point glass, so the internal pressure after sealing is
The pressure is cooled from 120℃ to room temperature of 20℃.
That is, P'=PT'/T=1・20+273/100
+273≒0.8Kg/cm ² , and the pressure in the pressure section can be greatly improved compared to the conventional one.
Since moisture is completely removed, a sheathed heater with good insulation resistance and extremely good dielectric strength can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of a sheathed heater before sealing treatment for explaining the sealing method of the present invention;
The figure is a cross-sectional view showing a sintered body of low-melting glass inserted into the opening of a metal pipe to be sealed, Figure 3 is a graph showing the temperature of the heating furnace, and Figure 4 is a graph showing Perthien's law. It is. In the figure, 1 is a heating coil, 2 is a metal pipe,
3 is magnesia powder, 4 is a lead wire, and 5 is a sintered body of low melting point glass.

Claims (1)

[Scope of Claims] 1. A heating coil with a lead wire connected is inserted into the center of a metal pipe, magnesia powder is filled between the heating coils, and the metal pipe is heated at a high temperature in a heating furnace. After removing the moisture adsorbed by the sheathed heater, lower the temperature inside the heating furnace to a low enough temperature that the magnesia powder does not physically adsorb moisture, and maintain that temperature until the entire temperature of the sheathed heater reaches the same temperature. A method for sealing a sheathed heater, comprising heating only the sealing portion of the metal pipe and heating and melting low-melting glass in the end opening. 2 The temperature in the above heating furnace is high temperature of 350℃ or higher,
A method for sealing a sheathed heater according to claim 1, characterized in that the low temperature is 100°C or higher.