US2958121A

US2958121A - Method of forming integral finned sheathed heaters

Info

Publication number: US2958121A
Application number: US535427A
Authority: US
Inventors: Toscher Sol
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1955-09-20
Filing date: 1955-09-20
Publication date: 1960-11-01
Anticipated expiration: 1977-11-01
Also published as: FR1161125A

Description

S. TOSCHER Nov. 1, 1960 METHOD OF FORMING INTEGRAL FINNED SHEATHED HEATERS Filed Sept. 20, 1955 lnvenior Sol Toscher y. 6 M

His AHorney United States Patent OF FORMING INTEGRAL FINNED SHEATHED HEATERS Filed Sept. 20, 1955, Ser. No. 535,427 5 Claims. (Cl. 29-15564) METHOD This invention relates to radiator type electric heating units, and more particularly to units of the sheathed type having integral fins thereon.

A heating unit of the sheathed type is presently formed by placing a resistance element through a length of metal tube and, keeping the resistance element centered, compacting an inorganic substance about the resistance element by means of vibration and tamping. The tube is then compressed by rolling or swaging in order to compact the inorganic insulating material into a solid mass. If it is desired to provide a finned heater, helical fins are then brazed on the outside of the tube.

In general, fins made integrally with the sheath provide better heat transfer and greater resistance to vibration than do fins that are made separately and joined to the sheath by brazing or other similar means. It is known that integral fins may be formed on tubing by inserting a mandrel into the tube and applying radially inward pressure at spaced intervals along a helical path to extrude material outwardly to form the fins. However, this process is not well adapted to form fins on a sheath type of heating unit in which the tubing contains a resistance element surrounded by an insulating powder. Accordingly, a primary object of the present invention is to provide a method of forming integral fins on a sheath type of heating unit when the sheath contains a resistance element surrounded by an insulating powder. It is an object of this invention to produce an integral finned sheathed heater in an inexpensive, simplified manner in one or more steps.

Another object of this invention is to utilize the finning process to compact the inorganic insulator, thus eliminating one step in the fonnation of the heating unit.

Still another object is to form integral fins on a tube placed over the sheath of a sheathed heating element while bonding the tube to the sheath.

A further object of this invention is to provide a method for forming an integral finned sheathed heater that is low in cost and has high heat transfer and vibration resistance.

In carrying my invention into effect in one form thereof, a heater wire is axially centered in an outer metallic sheath and surrounded by insulating material. Pressure is then exerted on the exterior of the sheath in order both to extrude fins thereon and to compact the insulating material. In another form of my invention, the sheath is compacted by swaging or rolling prior to the finning operation. In still another form, a preformed sheathed heater is encased in a separate tube on which the fins are extruded while bonding the tube to the sheath of the heater.

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawing, in which Fig. 1 is a longitudinal elevation, partly in section, of one form of apparatus for accomplishing the extruding and compacting process;

Patented Nov. 1 1960 'ice Fig. 2 is a vertical sectional view of the apparatus shown in Fig. 1;

Fig. 3 is a sectional view of the final product formed by the method illustrated in Figs. 1 and 2;

Fig. 4 is a longitudinal elevation, partly in section, showing apparatus employed in an alternative method, wherein the tube is loaded with a preformed sheathed heater before extruding the fins; and

Fig. 5 is a sectional view of the product formed by the method illustrated in Fig. 4.

Referring to the drawings, Figs. 1 and 2 illustrate apparatus for practising one form of the method of my invention. A spirally coiled heater wire 11 is axially centered in an outer metallic tube or sheath 12, and inorganic insulating material 13 is placed about the resistance wire 11. The resulting structure is then fed through a tubular guide 14 to a plurality of roll dies 15 shown in Fig. 2, only one of which is shown in Fig. l, which exert a radial inward pressure at spaced intervals along a helical path to extrude material outwardly to form the generally radially outwardly and circumferentiallyextending fins 16 shown in the drawing. The die 15 is'formed of sections of increasing diameter containing grooves 17 of increasing depth, the material of the sheath 12 being successively forced down into the grooves 17 of increasing depth. in such a manner that the fins are gradually extruded outwardly as shown at 18. During this process, the radial inward pressure from the extruding tool 15 serves to efiect a reduction of the internal dimensions of the tube 12 so as to compact the insulating material 13 to a rock-like consistency, thus forming a sheathed heating element having integral fins thereon. As will be apparent to those skilled in the art, the insulating material 13 is required to be confined within the tube or sheath 12 during the combination finning and reducing operation, just as in the case of the conventional methods of manufacturing heaters of this general type.

It is readily apparent that the method of my invention provides means for producing finned sheathed heaters that embodies several advantages over methods heretofore known. First, the fins 16 are integral with the sheath 12 and thus provide excellent heat conduction and resistance to vibration. Secondly, a manufacturing step has been eliminated in that the insulating material 13 is compacted by the extrusion process which forms the fins '16, rather than having to be compacted before the fins are provided. Thus, the manufacturing cost is low.

The invention also contemplates the production of parallel instead of helical fins, if desired, although the provision of such parallel fins may require an intermittent rather than a continuous process.

In another form of my invention the tube or sheath 12 may be compressed by rolling or swaging prior to the finning operation. This step should be followed by annealing in order to place the metallic sheath in condition for finning. The annealed structure is then passed through the tubular guide 14 and finned by the process shown in Figs. 1 and 2. The finning operation may here be used to further compress the insulating material 13. Further elongation due to additional compression may add an important design factor.

Fig. 3 is an end section of the finished product of the method of Fig. 1 showing a radial fin 16 on the metallic tube or sheath 12 which contains compressed insulating material 13 surrounding the resistance wire 11.

Apparatus for carrying out a modified form of my invention is shown in Fig. 4. Here a metallic tube 19 is placed over a preformed sheathed heating unit comprising an outer metallic tube or sheath 20 filled with the inorganic insulating material 13 and containing the coiled heater wire 11. The combination is then fed through the tubular guide 14 and fins 21 are extruded on the tube 19 by the dies 15, only one of which is shown in Fig. 4, in a manner similar to that of the method illustrated in Figs. 1 and 2. The pressure applied in the extr-uding process also forces the metallic tube 19 into engagement with the sheath 21 of the heating element, bonding them together in such a manner that heat is readily transferrable therebetween. As is apparent from Fig. 4 of the drawing, the preformed heater alone serves as a supporting mandrel for the tube 19 during the extrusion of the fins; and where further compacting of the insulating material is desired to be provided simultaneous with the fin forming, the preformed heater forms a yieldable mandrel permitting a reduction of the inner dimensions of the tube 19 simultaneous with the fin-forming operation. This essentially forms an integral sheath from the tube 19 and the sheath 2d and produces an integral finned sheathed heater. This method can also utilize a standard sheathed heater, the sheath 20 of which is preferably annealed before the extruding process. By proper selection of the thickness of the metallic tube 19, it is possible to regulate the diameter of the fins 21 by providing enough material for any desired fin diameter.

Fig. 5 shows an end section of the finished product made by the method of Fig. 4. The resistance wire 11 surrounded by the inorganic insulating material 13 is contained within the metallic sheath 20, which is surrounded by the outer metallic tube 19 having fins 21 formed thereon.

Thus it can be seen that by any of the modifications of the method of the invention, a finned sheathed heating unit having the fins integral with the outer sheath may be formed. Such a unit has the advantages mentioned above, namely, that it has greater heat transfer characteristics and higher resistance to vibration than units constructed by methods heretofore known. The unit may be inexpensively constructed either by using the single step of extruding and compressing outlined in the description of the method of Figs. 1 and 2 or by using a standard sheathed heater and selecting a suitable thickness of outer tubing in order to obtain a desired fin diameter as discussed in the method of Fig. 4.

While I have shown particular embodiments of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A method of forming an integrally finned sheathed heating element comprising providing a tubular metal member surrounding a resistance element embedded in granular heat-conducting electrically insulating material which is confined within the tubular member and which is adapted to be compacted into a substantially solid mass, and utilizing the granular insulating material as a mandrel, subjecting the tubular member to external pressure directed generally radially inwardly thereof from a plurality of roll dies arranged equidistantly about the circumference of the tubular member to form radially outwardly and generally circumferentially extending heat radiating fins integral with the tubular member, the pressure from said roll dies being sufiicient to reduce the inner dimensions of the tubular member simultaneously with the forming of the fins so as to effect compacting of the insulating material.

2. A method of forming'a finned sheathed heating element comprising providing a tubular member surrounding a resistance element embedded in heat-conducting electrically insulating material confined within the tubular member, and utilizing the insulating material as a mandrel, simultaneously subjecting the tubular member to external pressure directed generally radially inwardly thereof from a plurality of'roll dies arranged equidistantly about the circumference of the tubular member and effecting relative movement between the tubular member and the roll dies to form radially outwardly and generally circumferentially extending heat radiating fins integral with the tubular member, the pressure from said roll dies being suflicient to reduce the inner dimensions of the tubular member simultaneously with the forming of the fins to compact the insulating material.

3. A method of forming a finned sheathed heating element comprising stretching a resistance element through a metal tube, centering the resistance element, vibrating and tamping a compactible inorganic heat-conducting electrically insulating substance about the resistance element to at least partially compact the insulating substance, and by utilizing the partially compacted insulatsubstance as a mandrel and subjecting the tube to pressure directed generally radially inwardly from a plurality of roll dies arranged equidistantly about the circumference of the tube, simultaneously extruding generally circumferentially extending integral fins on said tube and reducing the inner diameter of the tube whereby the insulating substance is compacted to a substantially solid mass.

4. A method of forming a finned sheathed heating element comprising providing a preformed heating element including a tubular member in which is received a resistance element surrounded by at least partially compacted heat conducting electrically insulating material; telescopically assembling a second tubular member over the first tubular member; and, utilizing the preformed heating element alone as a mandrel, simultaneously subjecting the second tubular member to external pressure from an extruding tool to form generally radially outwardly and circumferentially extending heat radiating fins integral with the second tubular member, said pressure being of a value suflicient to reduce the inner dimensions of the second tubular member to bond said tubular members in intimate thermal contact.

5. A method of forming a finned sheathed heating element comprising providing a preformed heating element including a tubular member in which is received a resistance element surrounded by at least partially compacted heat conducting electrically insulating material confined in said tubular member; telescopically assembling a second tubular member over the first tubular member; and utilizing the preformed heating element alone as a yieldable mandrel, subjecting the second tubular member to external pressure from an extruding tool to form generally radially outwardly and circumferentially extending heat radiating fins integral with the second tubular member, with said pressure being of a value sufiicient to reduce the inner dimensions of the second tubular member to bond said tubular members in intimate thermal contact and to effect a reduction in the inner dimensions of the first tubular member simultaneously with the forming of the fins and bonding of said tubular members so as to further compact the material surrounding the resistance element.

References Cited in the file of this patent UNITED STATES PATENTS 1,359,400 Lightfoot Nov. 16, 1920 1,909,005 Paugh May 16, 1933 1,997,844 Weigand Apr. 16, 1935 2,432,169 Morgan Dec. 9, 1947 2,586,653 Hill Feb. 19, 1952 2,591,442 Lacy-Hulbert et a1 Apr. 1, 1952 2,677,172 Oakley May 4, 1954 2,735,162 Huck Feb. 21, 1956 2,779,223 Schuster Jan. 29, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2358, 121 November l 1960 Sol Toscher It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line l8 for "insulat read insulating Signed and sealed this 25th clay of April 1961.,

(SEAL) Attest:

DAVID L LADD ERNEST Wa SWIDER Attesting Oflicer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Nos 2 958 l2l November l 1960 S01 Toscher It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 18 for insulat read KW insulating Signed and sealed this, 25th day of April 1961 (SEAL) Atteet: ERNEST W, SWIDER DAVID Lo LADD v Commissioner of Patents Attesting Oflicer