GB1561913A

GB1561913A - Electrical resistance heating element with heat tgransferring characteristics

Info

Publication number: GB1561913A
Application number: GB45685/76A
Authority: GB
Original assignee: EGO Elektro Gerate Blanc und Fischer GmbH
Current assignee: EGO Elektro Geratebau GmbH
Priority date: 1975-11-03
Filing date: 1976-11-03
Publication date: 1980-03-05
Also published as: YU266576A; NO140321C; ES452703A1; FR2330241A1; NO763523L; DE2549006B2; IT1089852B; YU37430B; DE2549006A1; NO140321B; ATA746176A; AT351118B; DE2549006C3; FR2330241B1; SE7610508L

Description

(54) ELECTRICAL RESISTANCE HEATING ELEMENT WITH IMPROVED HEAT TRANSFERRING CHARACTERISTICS (71) We, E.G.O. El .EKTRo-GERAETE BLANC UND FISCHER, a German Company, of 7519 Oberderdingen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment:-- The present invention relates to a heating element comprising a heat transfer component with a groove heated by a tubular sheathed electric resistance heater arranged in the groove, and a method of manufacturing such a heater element.

One such heating element is known as a baking oven heater. It consists of a metal sheet having a groove impressed therein, a tubular sheathed electric resistance heater in repeatedly curved form being located in and following said groove. The heater is fixed in the groove by straps arranged at a distance from one another. The metal sheet is thin, and the heater is arranged in the groove so that it is exposed on one side.

It thus assumes a fairly high temperature and transmits the heat to the surroundings, this being desirable in the operation of a baking oven.

Furthermore, a roasting oven heater is known, where a helical heating wire is enclosed in an insulating mass directly between two metal sheets which are welded to one another. These heating elements have been used for some time by the applicant as roasting oven heaters for domestic cookers.

It is very difficult to combine heat transfer surfaces with a tubular electric resistance heater, especially if the latter has its own metal casing. Under variable conditions of heating or cooling the heat transfer bodies may become loose on the metal casing of the heater, and thus not only disturb the heat transfer, but also cause partial overheating of the heaters and produce unpleasant clicking noises during heating or cooling.

It is an aim of the present invention to solve these problems and in particular to provide a heating element which is of simple construction and allows good transfer and distribution of heat.

According to the present invention there is provided a heating element comprising two components which are welded together, at least one of which components is a cast metal heat transfer component, and a tubular heater comprising a metal sheath containing an electrical resistance wire located in an embedding mass, the tubular heater being located in a groove provided in the or each heat transfer component, and sandwiched under pressure between the components, the two components, apart from the welds, being spaced apart from each other by the tubular heater.

Such a heating element eliminates the problems of the quality of the heat transferring joint between the tubublar heater and the heat transfer components. The cast metal provides very good equalization of tempertaure and sufficient rigidity to ensure permanent compressive stress between the components and the tubular heater.

As a result the heat transfer is so good that differences in expansion hardly occur and under no circumstances can lead to a deterioration of the heat transfer or clicking noises. Furthermore, the cast metal can easily and inexpensively be brought into the desired shape, and surprisingly it is also possible to weld together the heat transfer components consisting of cast iron without special measures, by means of electric arc welding without filler. The welding takes place preferably along the outer edges running parallel with the tubular heater.It is preferred that the metal casing of the tubular sheathed electric resistance heater does not engage the wall of the groove at all points, i.e., the groove has a different profile to the cross section of the heater casing, cavities being thus defined by the wall of the groove and the heater casing, which cavities extend parallel to the casing. The heater can thus expand into these cavities when the element is compressed. This provision of cavities is preferably done by varying the transverse cross section of the groove or grooves frorn a semicircular form which would be complementary with that of the heater casing. For example with triangular shaped grooves a channel of a square section will result when two half shells which are substantially of the same cross-section are compressed together.

During this compression the tubular heater adapts itself by expanding into the corners, i.e., cavities, of the channel.

The heating element in accordance with the present invention may be constructed in a particularly preferred embodiment with two lateral sections comprising parallel tubular heater sections and with ribs running transversely to these. Such a heating element can be used in a very versatile manner, especially for space heating purposes. By virtue of its large surface and good thermal equalization within the tubular heater it makes possible a low surface temperature which is of benefit to the room climate. If in a preferred embodiment the heating element has openings between the lateral sections or fins an oblique or flat arrangement is also possible, beside the upright arrangement.

The individual heat transfer components may be constructed of optional lengths.

In a particularly preferred embodiment, however, the two heat transfer components are manufactured from a number of individual sections which are assembled with displaced butt joints. It is then only necessary to cast one length of a longitudinal section. Combined with two initial and end sections of different lengths, staggered lengths on a large scale can then be produced.

It is also possible to use the heating element in accordance with the invention for other purposes, for example as grill heaters.

In this case, where it is desirable to apply effective heating only to one side, namely the top side, it may be advantageous to provide only one of the components with a groove adapted to the cross-section of the tubular sheathed electric resistance heater and to design the other component without a groove. In this application too, full use is made of the advantage of the invention, namelv the eoualization of temperature and the reliable heat transfer under all conditions of use.

The present invention will now be further described, by way of example, with refer- ence to the accompanying drawings, in which: Fig. 1 shows a cross-section through a heating element in accordance with the invention serving as a convection heater including an enlarged detail, Fig. 2 shows a plan view of a section of a heating element according to Fig. 1, Fig. 3 shows a side elevation of the heating element, Fig. 4 shows a reduced schematic plan view of a complete heating element according to Fig. 1 to 3, Fig. 5 shows a partial cross-section through an embodiment in accordance with the invention in the form of a grill heater, Fig. 6 shows an elevation, partially in section, of a corner of such a grill heater seen from the left of Fig. 5, Fig. 7 is a variant of Fig. 3, and Fig. 8 is a variant of Fig. 1.

The heating element shown in Figs. 1 to 4 consists of a tubular metal sheathed electric resistance heater 13 and two heat transfer components 12 enclosing the heater 13 between them.

The heater 13 is constructed in the usual manner, i.e., it comprises a metal casing 14 and a helical resistance heating wire 16 located in an embedding mass 15.

The two heat transfer components 12 are identical castings made preferably of cast iron, i.e., a special grey casting of the type used for electric hotplates. The heat transfer components 12 each comprise two lateral sections 17 which are joined to one another by transversely extending ribs 18.

Thus a ladder-like construction with openings 19 between the lateral sections and the ribs is achieved. The ribs 18 project on the outsides over the lateral sections. By contrast the inside faces 20 of the heat transfer components 12 are substantially planar. In a thickened region of the lateral sections, grooves 21 are formed which are of triangular cross-section and which extend parallel to the longitudinal extension of the lateral sections 17.

In Fig. 3 is shown an end section 22 of a heat transfer component which at its right end in Fig. 3 has the same cross-section as that shown in Fig. 1, but which is then adapted so that the groove 21 forms a closed arc which joins the upper and lower grooves 21 in the two lateral parts together.

The assembly of the heating element 11 is carried out as follows: A tubular heater 13, bent according to the U-shape of the groove 21 evident from Fig. 3, is inserted between heat transfer components 12. The two heat transfer components 12 which are identical and svmmetrical in their cross-section are placed, with their flat inside surfaces 20 facing one another against the heating element in such a way that the latter is accommodated in the grooves 21 on both sides. The grooves are dimensioned so that when the casing 14 of the tubular heaters 13 is in contact with the surfaces of the grooves, a gap 23 is formed between the inside surfaces 20 of the heat transfer components.

The heat transfer components are manufactured in sections of a specified length, as can be seen for example from Fig. 4. They are arranged with staggered butt joints, so that a heating element of a length which is a multiple of the length of the length of the individual sections of the heat transfer components is formed. For this purpose, in addition to the normal straight heat transfer component 12 of standard length an initial section 12' of a different, preferably smaller, e.g., half length, and beside the normal end section 22 an end section 22' of a different, preferably half length, is required. From these three castings, which can be used practically without any machining, heaters of a great variety of different lengths can therefore be manufactured.

The two heat transfer components 12 with the tubular sheathed electric resistance heater 13 inserted between them are compressed on assembly to a heating element under such a high pressure that the tubular heater readily becomes deformed in the manner shown in Fig. 1, in that it adapts itself to the square shape defined by the two grooves facing one another. The adaptation is not complete, it merely creates certain flattenings in the region of the substantially flat groove surfaces and thus enlarges the area of contact. Owing to the corners of the deformed metal casing of the heater being rounded, an expanslon space remains at the regions of the inner corners of the grooves which prevents the metal tube of the heater from collapsing in the event of a change of cross-section caused in the fabrication or roughness in the grooves.

In these regions the tube can readily expand into the corners of the grooves. Hence in the region of the grooves too, unmachined castings can be used.

In the raw state the heat transfer components 12 have on their longitudinal outer edges projecting fins 24 which are indicated in Fig. 1 by broken lines. These fins serve as welding material for the subsequent welding. This is thus a welding without filler material which can take place under normal atmospheric conditions, i.e., without protective atmosphere. The welding and the cooling of the weld take place whilst the heat transfer components, with the heaters 13 inserted between them, are pressed together under a high pressure. A certain gap 23 should remain after welding, even if the gaD should become very narrow.

After welding, a relatively high elastic compression between the heat transfer components and the heater is maintained.

Thus, with the heaters situated in a region which is at a certain distance from the weld 25, and in spite of the very rigid construction of the heat transfer components resulting from their material and design, a certain resilience may nevertheless be produced which maintains the pressure acting on heaters. The heating elements 11 are intended primarily for use in the upright position, as shown in Fig. 1, that is in a position appropriate to the direction of air flow. By virtue of the openings 19 it is also possible, however, to have an air flow in a transverse direction or to install them in an inclined position without diminishing their effectiveness.

It would also be possible to provide openings in the region of the lateral sections outside the heaters. This might mean interrupting the weld. As a seal welding is here not necessary, this embodiment would be possible, but the continuous weld is much more favourable from a point of view of strength. Normally the sections of the heat transfer components 12 cast in one piece will be made longer than can be recognized from the schematic representation of Fig.

4.

In a particularly advantageous embodiment it is possible to omit a specific end section 22 by providing a standard heat transfer component 12 which has at least one end a groove as shown in dash-dotted lines in Fig. 3, in addition to the arcuate portion 26 of the groove. This groove extends in the dash-dotted region 27 to the short end face of the heat transfer component. Thus this heat transfer section may be used at the same time as an initial, intermediate and end section. It could then be made so that it ends in the centre of the opening 19' (Fig. 3).

In Figs. 5 and 6 a different form of, and application of, a heating element in accordance with the invention is shown. The heating element 30 is adapted for use as a contact grill. For this purpose it has an upper heat transfer component 31 which on its top has parallel, triangular cross sectioned ribs 32, and which has in the border region on the underside, a projecting rim 33.

On the underside 34 of the upper heat transfer component 31 grooves 36 are provided in the region of thickened regions 35, which grooves have a semi-circular base which corresponds to the transverse cross section of a tubular casing of a heater 13 inserted into the grooves 36. The grooves 36 are somewhat less deep than the diameter of the tubular casing of the heaters, so that the latter slightly protrude above the grooves (see Fig. 5).

The heaters 13 are pressed into the grooves 36 under high pressure by a plate 37 which has stiffening ribs 38 on its underside and which is inserted in the space between the edges 33. After the insertion of the heaters this plate is pressed under high pressure against them and is then welded at its edges by means of the weld 39 to the rim 33.

In this embodiment, where mainly the upper heat transfer component 30 is to be heated so that the matter to be grilled should be heated on the ribs 32, it is ensured by the design of the grooves adapted to the outer shape of the tubular sheathed electric resistance heater, and by the fact that a groove is only provided in the component 30, that the heat will be transferred substantially to the component 30, the plate 37 being only in contact with the tubular heater over a very small area and essentially ensuring that the tubular heater is pressed into the grooves and is adapted as much as possible to the shape of the groove. By means of a gap 40 between the underside of the thickened regions 35 and the plate 37 and by the only semi-circular shape of the base of the grooves a facility for expansion of the tubular heaters is provided.

However, in case the pressure is greater at some points than at others the recesses formed between the thickened regions 35 diminish the transfer of heat from the component 30 to the plate 37. The heating element described hereabove has the advantage that due to the relatively large area of contact between the tubular sheathed electric resistance heater and the heat transfer component or components, in comparison with the known arrangements, the thermately highly stressed surface, normally at a high temperature, of a heater is in effect enlarged many times in a simple manner and without practical disadvantages. A tubular heater of a relatively small diameter can be used, which will give a high output per unit of length. Nevertheless, a lower surface temperature is produced on the heating element.By the forcible compression of the tubular heater into the groove of the heat transfer component a good heat transfer is created and clicking noises are avoided. Depending on the design of the heat transfer components made of cast material the desired heat storage can be achieved. This is an advantage in space heating, because it improves in co-operation with a control element, the optimum adapt tation to the energy requirement of the room. Owing to the electrically as well as mechanically safe construction, utilization in many fields of application in the household and industry is possible.

The embodiment shown in Fig. 7 has two heat transfer components 12' of which only one is shown in plan view of the inside. It is largely similar to that according to Fig. 1 to 3. Similar parts have been given the same reference numerals.

In the heat transfer component 12', however, in the centre between the two grooves 21 a further groove 41 is provided, which accommodates a return tube 42 in the same way as the grooves 21 accommodate the tubular heater.

The embodiment represents an end piece where, however, the tubular heaters are not arranged in an arc, but terminate in the end region of the heat transfer components.

Such an embodiment is desirable in all cases where the heating elements are just as long as the tubular sheathed electric resistance heaters used. The connecting pins 43 at the ends of the heaters attached to the helical heating wires 16, Fig. 1, project into an insulating part 44 which is located in a recess 45 of the heat transfer components 12. In a recess in this insulating part 44 extends a contact bridge 46 to which are connected the connecting pins of the two tubular sheathed electric resistance heaters 13 and the return wire 47 of the return tube 42. In the example shown, the return tube is unheated, i.e., it is of a similar construction to a tubular sheathed electric resistance heater with a casing and an embedding mass, but it contains a return wire whose crosssection and conductivity are so great that it does not contribute to the generation of heat.This embodiment makes it possible, also in heating elements of great length, to make do with customary tubular sheathed electric resistance heaters which have customary heating wire thicknesses, and yet retain the advantageous connection of the heating element on one side only, i.e., the heater is connected to an electricity supply only at one end. Because of the return tube the two tubular sheathed electric resistance heaters are connected in parallel, which allows the heaters to be used individually or collectively. It is also possible, however, to operate without the return tube in which case the two tubular sheathed electric resistance heaters are connected in series. Instead of the return tube a normal tubular electric resistance heater may also be used which also serves for heat generation.

In such a case, however, it is advantageous to conduct the return tubular sheathed electric resistance heater over its entire length in a cIosed groove, and not only in the region of the ribs 18 as shown in Fig. 7.

The variant represented in Fig. 8 is similar in its basic structure to that of Fig. 1.

Here too similar parts have been given the same reference numerals. By contrast to Fig. 1, however, a large number of tubular electric resistance heaters running parallel with one another are provided which are each accommodated in grooves 21. Four are placed between respective lateral sections 17' and one in a central section 48 extending over the whole length of the heating element which also has a groove.

The ribs extend over all three sections and the welding takes place at the longitudinal edges as in the embodiment according to Figs. 1 to 3. In this embodiment with altogether five tubular electric resistance heaters, they may be connected in the most varied manner as forward and return conductors and numerous control possibilities are available.

A control by the switching off of individual tubular electric resistance heaters of a heating element or changepver between parallel and serial connection is possible, but the heating element in accordance with the invention is also particularly well suited for an intermittent control, since it has a heat capacity which by selection of material thicknesses etc. may be adapted to particular conditions. This heat capacity ensures that the intervals between switching on and off become so long that switching frequency and therefore switching noises and system faults remain within permissible limits, i.e., do not cause disturbances.

It will be understood that many further possible variations exist. Thus, for example, the shape of the ribs of the heating elements may be adapted to a particular use. A semi-circular shape of the ribs is for example, possible, so that the whole heating element becomes circular and may be introduced into a pipe to be heated (for example of an oil filled radiator).

WHAT WE CLAIM IS: 1. A heating element comprising two components which are welded together, at least one of which components is a cast metal heat transfer component, and a tubular heater comprising a metal sheath containing an electrical resistance wire located in an embedding mass, the tubular heater being located in a groove provided in the or each heat transfer component, and sandwiched under pressure between the components, the two components, apart from the welds, being spaced apart from each other by the tubular heater.

2. A heating element as claimed in claim 1, in which the metal sheath of the tubular heater directly engages the wall of the groove or grooves.

3. A heating element as claimed in claim 2, in which the metal sheath of the tubular heater does not engage the wall of the groove at all points, cavities being defined by the wall and the metal sheath, which cavities extend parallel to the metal sheath.

4. A heating element as claimed in any one of claims 1 to 3, in which the or each groove is not semi-circular in transverse cross section.

5. A heating element as claimed in claim 4, in which the or each groove has a triangular cross section.

6. A heating element as claimed in any one of the preceding claims, in which the said two components are cast metal heat transfer components of identical design.

7. A heating element as claimed in any one of the preceding claims in which the or each heat transfer component has ribs which extend between lateral sections of the component, the ribs running transversely to the tubular heater which extends through said lateral sections.

8. A heating element as claimed in claim 7, in which openings are provided which are defined by the said lateral sections and the ribs.

9. A heating element as claimed in any one of the preceding claims, in which each said component consists of several individual sections which are interengaged by displaced butt joints.

10. A heating element as claimed in any one of the preceding claims, in which an end section is provided with a curved groove to accommodate a curved portion of a tubular heater.

11. A heating element as claimed in any one of claims 1 to 9, in which a heat transfer component has two parallel grooves extending straight over the entire length of the heat transfer component, and in the region of one end has a curved groove to accommodate a curved portion of the tubular heater.

12. A heating element as claimed in any one of the preceding claims, in which a heat transfer component has a groove having a transverse cross section having the same dimensions as the cross section of the tubular heater and the other component is designed without a groove.

13. A heating element as claimed in claim 12, in which the heat transfer component having the groove has a rim to which the other, largely flat, component is welded.

14. A heating element as claimed in any one of the preceding claims, in which the welding takes place on the outer edges of the heat transfer components extending parallel with the tubular heater.

15. A heating element as claimed in any one of the preceding claims, in which the heat transfer components are constructed of cast iron.

16. A heating element as claimed in any one of the preceding claims, in which it has its electrical connections at one end only.

17. A heating element as claimed in claim 15, in which the tubular heaters end

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. each accommodated in grooves 21. Four are placed between respective lateral sections 17' and one in a central section 48 extending over the whole length of the heating element which also has a groove. The ribs extend over all three sections and the welding takes place at the longitudinal edges as in the embodiment according to Figs. 1 to 3. In this embodiment with altogether five tubular electric resistance heaters, they may be connected in the most varied manner as forward and return conductors and numerous control possibilities are available. A control by the switching off of individual tubular electric resistance heaters of a heating element or changepver between parallel and serial connection is possible, but the heating element in accordance with the invention is also particularly well suited for an intermittent control, since it has a heat capacity which by selection of material thicknesses etc. may be adapted to particular conditions. This heat capacity ensures that the intervals between switching on and off become so long that switching frequency and therefore switching noises and system faults remain within permissible limits, i.e., do not cause disturbances. It will be understood that many further possible variations exist. Thus, for example, the shape of the ribs of the heating elements may be adapted to a particular use. A semi-circular shape of the ribs is for example, possible, so that the whole heating element becomes circular and may be introduced into a pipe to be heated (for example of an oil filled radiator). WHAT WE CLAIM IS:

1. A heating element comprising two components which are welded together, at least one of which components is a cast metal heat transfer component, and a tubular heater comprising a metal sheath containing an electrical resistance wire located in an embedding mass, the tubular heater being located in a groove provided in the or each heat transfer component, and sandwiched under pressure between the components, the two components, apart from the welds, being spaced apart from each other by the tubular heater.

17. A heating element as claimed in claim 15, in which the tubular heaters end

in the region of one end of the components-- and are connected to one another and to a return conductor located within the components by way of a contact bridge.

18. A heating element as claimed in claim 17, in which the return conductor is placed inside a return tube similar to the tubular heater.

19. A method of manufacturing a heating element as claimed in claim 1, comprising the steps of forming said two components, at least one of which is a cast metal heat transfer component with a groove provided therein for receiving a tubular heater, inserting the tubular heater in the groove, compressing the two components together to sandwich the tubular heater therebetween, and welding the components together at the outer extremities of the heating element, whilst thy are still being pressed together.

20. A method as claimed in claim 19, in which the components are pressed together with sufficient force to deform the tubular heater.

21. A method of manufacturing a heating element as claimed in claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

22. A heating element constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. ~~