JPS583352B2 - Manufacturing method of sheathed heater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a sheathed heater, and more particularly to a method for manufacturing a sheathed heater that improves the reduction in dielectric strength between a heating element and a main body.

Generally, sheathed heaters have a sealed structure in which the heating element is protected by a sheathed pipe, and are therefore widely used as immersion heaters, explosion-proof heaters, and the like.

This sheathed heater generally has a filler such as fused magnesia powder filled inside the main body to electrically insulate the main body and the heating element, but as is well known, magnesia is highly hygroscopic and In order to prevent poor insulation between the heating element and the main body, the terminal portion is hermetically sealed, and there are two types depending on the sealing method: a glass-sealed type and a resin-sealed type using silicone resin or the like.

The reliability of the airtightness of the sealed part is much higher in the former than in the latter, and sheathed heaters that have traditionally been required to have high reliability and are used at relatively high temperatures are generally sealed with glass. .

The conventional glass-sealed sheathed heater has a
Produced in steps ~f.

First, a heating element is inserted into the metal sheathed tube body (a), filler powder such as magnesia is filled in this state (b), and in order to further increase the filling degree of the filler, the body is subjected to swaging process (c), and an annealing process (d) After removing distortion from the main body, it is bent into a predetermined shape (e), and the terminal portion is sealed with glass in an electric furnace (f).

When manufacturing a sheathed heater using this process, cracks often occur in the filling material in the bent portion during the bending process, and glass sealing is performed in the furnace in this state.
After sealing, the sheathed heater inevitably becomes depressurized due to the filling of expanded air and cooling, and the voltage applied when the heater is used causes depressurized discharge in the cracks of the filler, resulting in poor insulation between the main body and the heating element. It was causing this.

Of course, such insulation defects due to reduced pressure discharge will not occur if cracks are prevented from forming in the magnesia filler, but in the process of bending the main body, the internal space of the main body will inevitably increase or decrease. It is inevitable that the filling material will crack in the enlarged space.

On the other hand, even if a crack occurs in the filler, as long as the inside of the main body is not in a reduced pressure state, the dielectric strength of the cracked portion will not drop significantly and discharge can be prevented in the practical voltage range.

The present invention aims to provide a method for manufacturing a sheathed heater that prevents the occurrence of insulation defects due to discharge in the cracks of the filler under reduced pressure, which occurs in the manufacturing process of the conventional glass-sealed sheathed heater as described above. This is the purpose.

That is, the present invention uses a mixture of at least one of barium carbonate and strontium carbonate, which are alkaline earth metal carbonates, as an insulating filler in a range of 3 to 5 wt%. After swaging, annealing, and bending the main body and sealing the glass in the same process as the heater, heat the bent part and its surroundings using a gas burner, etc. to a temperature above the decomposition temperature of the alkaline earth metal carbonate. The treatment is characterized by increasing the pressure inside the main body to above normal pressure.

By this heat treatment, the added alkaline earth metal carbonate, for example in the case of CaCO3, decomposes according to the following formula CaCO3→CaO+CO2, releases carbon dioxide gas, and acts to increase the pressure inside the sealed sheathed heater.

In addition, in the manufacturing process of a sheathed heater, there are heat treatment processes such as an annealing process and a glass sealing process before the glass sealing process. 930° C. and strontium carbonate at 1250° C., and a carbonate having a decomposition temperature sufficiently higher than the annealing temperature can be selected.

On the other hand, the amount of alkaline earth metal carbonate mixed with fillers such as magnesia is 3 to 5% by weight according to the inventors' study.
If added, it was sufficient to restore the reduced pressure inside the main body to around normal pressure, but if necessary, the internal pressure can be raised to above normal pressure, so even if a larger amount is added, there will be no problem. However, if it is less than 3% by weight, it will not be sufficient to recover the internal pressure, and no effect of addition can be expected.

EXAMPLES The present invention will be explained in more detail with reference to Examples below.

Example 1 4% by weight of barium carbonate was added to electrofused magnesia and dry mixed using a V-type mixing material.

Following the normal manufacturing process for sheathed heaters, a heating wire is inserted into a protective metal pipe made of stainless steel, etc., and then the above mixed powder of magnesia and barium carbonate is filled, followed by swaging and sealing of the terminals. It was bent into shape.

Next, the bent portion is heated by a heating means such as a burner.
Local heating was performed at 850-900°C for 5-10 minutes.

In the sheathed heater obtained as described above, the insulation resistance was improved from several MΩ after the bending process to several hundreds to several thousand MΩ or more.

Example 2 After adding and mixing 5% by weight of calcium carbonate to electrofused magnesia and manufacturing a sheathed heater in the same process as in Example 1, the bent portion and its surroundings were heated at 950 to 1000°C for 5 to 10 minutes.
Local heating was performed for a minute to release carbon dioxide gas and increase the pressure inside the tube.

The insulation resistance of the sheathed heater thus obtained was improved to the same extent as in Example 1.

As explained above, according to the present invention, it is possible to prevent insulation failure at the bent portion, improve the reliability of the sheathed heater, and expect a significant improvement in manufacturing yield.

[Brief explanation of drawings]

The figure is an explanatory diagram showing an overview of the manufacturing process of a conventional glass-sealed sheathed heater. In the figure, a to f indicate each step.

Claims (1)

[Claims] 1. A step of filling the main body with a powder mixed with at least one alkaline earth metal carbonate selected from barium carbonate, calcium carbonate, and strontium carbonate as a filler, and after sealing the terminal portion, the main body is A method for manufacturing a sheathed heater, comprising the step of heating at least the decomposition temperature of the alkali metal carbonate. 2. The method for manufacturing a sheathed heater according to claim 1, wherein at least 3% by weight of alkaline earth metal carbonate is used in the filler. 3. A method for manufacturing a sheathed heater according to claim 1 or 2, wherein fused magnesia is used as the filler. 4. A method for manufacturing a sheathed heater according to any one of claims 1 to 3, wherein glass sealing is used to seal the terminal portion.