DE102007035080B3 - Production process for tubular heating element involves removal of unwanted material and making good tears, grooves or other recesses after compression - Google Patents

Abstract

The tubular heating element production process is for a tubular heating element (1) with a metal jacket (7) oxidized by heating. The process involves the removal of unwanted material and making good tears, grooves or other recesses caused during production after compression. The machining part of the process is followed by grinding.

Description

The The invention relates to a method for producing a compacted, at least simply curved tubular heater with a Metal jacket in which a heating coil embedded in a metal oxide which is responsible for high surface loads at high Heating cycles, especially for water heating in washing machines and Dishwashers is used.

Out DE 197 43 450 An electric tubular heater is already known in which the existing of a copper pipe protective jacket with a ceramic coating or with a metal coating of a chemically deposited Ni-P alloy is provided to prevent the formation of surface defects during compression.

The end DE 24 39 739 A1 Known method for improving the corrosion resistance of electric pipe heaters provides to cohesively connect a galvanically, chemically or mechanically nisch applied metal coating under high pressure with the surface of the metal shell.

The DD 7 119 A describes an electric tubular heater in which the surface of the heat conductor enclosing tube jacket is provided with two cover layers, of which the cover layer serves to produce the corrosion and temperature resistance, while the second cover layer, the radiation constant is to increase the performance.

These known methods or methods are not only procedural consuming and expensive, but also unsatisfactory insofar, As coatings can be easily damaged mechanically.

tans The type in question are usually in the way manufactured that a jacket pipe with a wall thickness of about 0.5 to 0.6 mm with a heating coil and filled with MgO granules becomes. After filling, the jacket tube is filled with the MgO granules and the heating coil embedded therein compacted. This compacting can be done by rolling, pressing or hammering respectively. Here, the diameter of the jacket tube is about 15% reduced. Subsequently, the tubular heaters soft annealed, then easier to shape by bending to bring in the required installation form. After bending, a so-called look-up usually takes place in a die for the densification of the bows.

While conventional tubular heaters used to heat water, usually have a maximum surface load of about 25 W / cm ² , recently tubular heaters, which are used for example in dishwashers or washing machines and are constantly flowed around by water, at least twice as high surface load required. The surface load is the most important parameter for the dimensioning of electrical heating elements. It is a measure of the stress and service life of a heating element. Especially with high-performance heating elements, the surface load is of central importance.

For The life of a tubular heater is also the Number of heating cycles is essential. In investigations the lifetime of such tubular heaters has been found that with high surface loads and high work cycles (Wechselbelastun conditions) of the metal jacket an extraordinary Vulnerability represents where these high loads are relatively fast cause leaks due to cracking, leaving water penetrate into the interior of the radiator and this unusable can make.

Further it was found that in most cases the causes the cracking in the material distortions and notches or scoring too are seen, either at the original production of the jacket tube or at the latest during the compression process be generated.

It is therefore the object of the present invention, the cause of to avoid or minimize this cracking or leakage formation.

Of the Invention is based on the object, a production process for compacted high performance tubular heaters indicate, by which the emergence of leaks, in particular Cracks in the metal jacket at least largely prevented and a long life is achieved.

Solved This object is achieved according to the invention in that the metal shell after compaction by mechanical, cutting Ablating a surface layer of the manufacture and / or material distortions and notches created during the compaction process, Scoring or similar depressions is released.

By the inventively provided by machining a surface layer in which the material distortions (also called Materialaufwerfungen), notches, grooves od. Like. Wells are included, either originate from the original pipe production or from the compression process or caused by the compression, can, as experiments have shown, the mechanical causes of Rissbil at least to the extent that the service life is increased many times over, even under extreme conditions.

While the removal of material distortions such as notches, grooves od. Like. Wells containing surface layer of the metal shell in principle can be done in any way, the machining has machining by grinding as particularly advantageous and relatively easy to carry out proved.

Further advantageous method steps are the subject of the claims 3 to 5.

Reference to the accompanying drawings, the invention will now be explained in more detail below. The drawing shows a pre-finished tubular heating element 1 ,

This tubular heater 1 has a ring-shaped curved power section 2 on, to the two straight, parallel to each other, obliquely upward and outward connecting sections 3 and 4 are bent. At the ends of these sections 3 and 4 There are electrical connection lugs 5 and 6 over which the power supply takes place.

The original tubular heater 1 consists of a metal jacket 7 , in which, embedded in a MgO filling, a heating coil is located, which is not shown in the drawing and at least over the circular curved portion of the Leistungsab section 2 extends and their ends with the terminal lugs 5 and 6 are connected.

The production of such a tubular heater takes place in such a way that in the existing of a straight pipe metal jacket 7 First, the heating coil and the MgO granules are filled. Thereafter, a radial compression, z. B. by rolling or radial compression, which has a diameter reduction of the metal shell by about 15% result.

at This compression process arise, as shown in studies have material dislocations or material dislocations, notches and Scoring or similar depressions in the outer surface of the Metal mantle, the exit for a later cracking while using the tubular heater can.

To eliminate these causes of cracking, the metal shell 7 depressions are removed after compaction by mechanical removal of a surface layer from the material buildup and notches, grooves or the like produced during manufacture and / or during the compacting process, so that the recognized causes of crack formation are at least largely eliminated.

Conveniently, This removal is done by machining by grinding.

there It may be beneficial if the surface of the metal shell even before the compaction process by machining Unevenness, in particular depressions, freed, so that the accumulations of material, notches and Riefen lower fail.

there It may be sufficient if the removal of the Materialaufwerfungen or Materialverwerfungen, notches, grooves od. Like. Wells containing Surface layer only in the areas of the highest Temperature load, d. H. in the areas that the heating coil include, takes place.

To this removal of said surface layer of the metal shell usually an annealing and then the bending of the tubular heater, for example in the in the drawing shown installation form. After this form bending it is expedient, at least the curved sections to recompress by looking up in the die again and this densified areas or sections, if necessary by repeated Grind the newly created material scooping, score formation and the like to eliminate.

By This manufacturing process can reduce the risk of cracking Metal jacket significantly reduced or even eliminated. Also has been proven to be a by this method a much higher lifetime even when used with Maximum load and high cycles of thermal cycling achievable.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 19743450 [0002]
• - DE 2439739 A1 [0003]
• - DE 7119 A [0004]

Claims (5)

Method for producing a compacted and at least simply bent tubular heating element ( 1 ) with a metal jacket ( 7 ), in which a heating coil is arranged embedded in a metal oxide, which is used for high surface loads during high heating cycles, in particular for heating water in washing machines and dishwashers, characterized in that the metal casing ( 7 ) is freed after compaction by mechanical, removing a surface layer of the material produced during manufacture and / or during the compression process material distortions and notches, grooves or the like depressions. Method according to claim 1, characterized in that that the removal by machining by grinding he follows. A method according to claim 1 or 2, characterized in that the surface of the metal shell ( 7 ) is already freed before the compaction process by machining unevenness, in particular depressions. Method according to one of claims 1 to 3, characterized in that taking place after compression Removal only in the highest temperature exposure exposed Surface regions of the metal shell takes place. Method according to one of claims 1 to 4, wherein the tubular heater ( 1 ) after compression and removal of the material distortions and notches, grooves or the like recesses containing surface layer is brought by form bending in its operative outer shape, characterized in that after the bending of the form and optionally after additional recompressing a further material removal, in particular in the Bends done.