Abstract:
An electric hotplate, particularly intended for continuous operation in commercial use, is provided on the bottom surface with at least two superimposed reflectors (24, 28), which in each case form the lower boundary of an insulating zone (29, 30) and while incorporating an outer flange edge (4) of hotplate (2) seal the bottom surface thereof. This leads to a very low no-load power of the electric hotplate with a very rapid response to a higher power setting.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electric hotplate, particularly for continuous operation in industrial or commercial use, with a hotplate body constructed as a solid body from iron material and particularly as a casting, which forms a cooking surface on its top and within a downwardly projecting outer flange edge is provided on the bottom with at least one electrical heating element distributed over an annular heating zone, as well as a lower seal below the heating element and comprising at least two superimposed sealing parts, whilst incorporating the outer flange edge. 
     In certain fields electric hotplates are subject to completely different operating conditions to those normally encountered when used in domestic kitchens and in normal operation. In the latter, the electric hotplate is generally heated from the cold state for so-called parboiling initially to a relatively high operating temperature, followed by finishing boiling for a period generally less than one hour at a reduced operating temperature. However, there are also electric hotplates intended to remain in continuous operation for several hours, e.g. from morning to evening and which are therefore not switched off during a normal working day. Such electric hotplates are used in the form of large hotplates in commercial kitchens, e.g. works canteens and the like. These electric hotplates, where there is a continuous operation, must be operable both at a low no-load temperature and under a favorable finishing boiling temperature in order to obtain a low power consumption, but must still be suitable to temporarily rapidly supply the necessary increased power in a heating phase interrupting the no-load or finishing boiling phase. The no-load power is e.g. required if no cooking utensils are located on the electric hotplate for a certain amount of time, but the hotplate cannot be switched off in order to maintain the necessary permanent operational readiness. The no-load power can also be used for finishing foods in the case of a relatively low power requirement. 
     Hitherto, the aforementioned requirements could not be satisfied in an optimum, simultaneous manner. Thus, in general an improvement to the no-load operation, both with regards to a reduction in the no-load temperature and as regards to the no-load power requirement has led to a detectable deterioration to the heating operation with respect to speed and vice-versa and this is particularly noticed when different sizes of cooking utensil are used. 
     An electric hotplate of the aforementioned type is e.g. known from EP-B No. 0 024 621. This electric hotplate, which is particularly intended for normal operation and is correspondingly designed, has as the lower seal incorporating the outer flange edge, a substantially closed sheet metal cover plate engaging on said flange edge and above the same a further sealing part in the form of insulation, which contains an aluminium crinkled foil secured between the cover plate and the outer flange edge. As this design is mainly provided for shielding the electric hotplate in the downward direction by insulation, a corresponding insulation increase would be necessary to increase the shielding effect, if this electric hotplate was to be made suitable for continuous operation. 
     However, those electric hotplates which, instead of a solid iron material body, have a hotplate body of glass ceramic or the like are less suitable for continuous operation due to their specific characteristics. With respect to severe operating conditions, electric hotplates made from an iron material have a much longer life and due to their thermal mass can better absorb briefly occurring temperature fluctuations. In addition, the thermal stresses occurring in the case of glass ceramic plates are of a completely different type to those occurring with hotplate bodies constructed in solid form from an iron material and which have an outer flange edge, whereby said hotplate bodies can be made either from steel or as a casting, e.g. from grey cast iron. Electric hotplates with glass ceramic plates, such as are e.g. known from U.S. Pat. Nos. 3,909,592, 4,032,750, 3,733,462 and 3,987,275 are admittedly provided with reflectors below the heating elements, but for the reasons indicated hereinbefore, quite different effects occur as compared with those with an electric hotplate of the initially mentioned type. 
     SUMMARY OF THE INVENTION 
     The problem of the present invention is to provide an electric hotplate of the aforementioned type, where on the one hand it is possible in a simple manner to reduce the no-load power with a stable no-load temperature and on the other hand it can supply a greatly increased heating with a low time delay and on a temporary basis. 
     In the case of an electric hotplate of the aforementioned type, this problem is solved in that at least one sealing part located closer to the heating element is constructed as a reflector substantially located within the outer flange edge, at the most extending up to its bottom surface and directed against the bottom of the hotplate body and that below the same is provided an insulating zone extending at least approximately from the outer flange edge and over the heating zone so as to shield the bottom of the reflector and which is bounded by the lower sealing part. Thus, the reflector action is so limited to the area within the outer flange edge in one part with the hotplate body, that substantially all the heat irradiated downwards from the heating elements is reflected back in at least one reflector stage against the bottom of the hotplate body and the inside of the outer flange edge. A heat loss through heat conduction to the underside of the electric hotplate is simultaneously counteracted in that the insulating zone provides a shielding action against this. Thus, in superimposed arrangement are provided at least one reflector and an insulating zone within an area, which is particularly hermetically sealed on the bottom of the hotplate body and which is bounded on the outer circumference by the outer flange edge. It has been found that this leads to a very uniform and surprisingly low no-load power in the case of a relatively high temperature on the cooking surface and that the stepped combination of heat reflection and thermal insulation in the case of a brief power increase leads to an extremely low delay response of the temperature on the cooking surface. 
     This is particularly the case if there are provided in superimposed manner at least two successively mounted reflectors, whereof preferably the lowermost collector is arranged in an insulating zone provided below the overlaying reflector. The inventive construction has the further important advantage that it is suitable both for electric hotplates or hotplate bodies which are circular in plan view and for those which differ from the circular shape, namely e. g. for at least roughly rectangular hotplates, even if they have an edge length of 300 mm or more than 400 mm. 
     A further improvement to the downward thermal insulation both with respect to the heat radiation and with respect to the heat conduction or convection can be achieved if between the heating element and the reflector adjacent thereto an insulating zone is provided, whose height is smaller than that of the outer flange edge and/or is roughly the same as the height of a portion of the centre stud of the hotplate body projecting over the bottom of the heating element. Thus, the reflected heat radiation can be distributed in a simple, uniform manner over the bottom of the hotplate body. 
     To obtain an undelayed direct reflection, it can be advantageous if the insulating zone is at least partly and e. g. completely constructed as a cavity. Instead of this or in addition thereto, the insulating zone can be at least partly filled with a reflecting and/or thermally insulating material. A particularly advantageous construction is obtained if the insulating zone provided below the reflector adjacent to the heating element is provided connected to the underside of the seal with thermally insulating and/or reflecting material, the area located above said reflector then being constructed as an empty area or merely as an air-filled area, which is appropriately sealed in such a way that there is no air flow. 
     In the case of a very simple, low-weight construction, at least one thermally insulating and reflecting material layer, which in particular upwardly defines a cavity, is formed by a metallically bright crinkled foil or the like, which substantially prevents any absorption of moisture. 
     In order to be able to seal the bottom of the electric hotplate in an effective and stable manner, particularly against the penetration of moisture, the reflector and/or bottom of the seal is formed by a cup-shaped sheet metal cover, whose edge preferably engages on the outer flange edge. 
     To obtain a precisely defined engagement of the top reflector on the bottom of the hotplate body, whose surface structure is generally uneven or irregular, the edge of said reflector is appropriately at least partly provided in ring flange-like outwardly directed manner in the vicinity of the bottom of the heating element and preferably a reflector plate covering the associated cover in closed manner on the top surface is provided, which is then supported with its ring disk-like outer edge and/or via its remaining radial extension on the bottom of the heating element or the hotplate body. The edge can engage directly or indirectly on the bottom of the hotplate body or heating elements. It is possible to interpose, as a dimensional tolerance compensating plate, a thin spacing plate made from an at least partly compressible elastic material, particularly an insulating material and which is appropriately constructed in such a way that it is either self-reflecting or it allows through, in relatively low resistance manner, the thermal radiation to the reflector or reflector plate. 
     The edge of a lower and particularly the bottom cover plate can also be directed outwards in ring flange-like manner and preferably engages frontally on the bottom of the outer edge flange of the hotplate body. If a packing or seal is placed between the edge of the lower cover plate and the outer edge flange, which can be flat, angular or Z-shaped in cross-section, a particularly good hermetic seal is obtained, especially against the penetration of moisture, which is very advantageous for maintaining the insulating and reflecting characteristics of the different reflector and insulation arrangements in the closed space between the bottom of the hotplate body and the lower seal. The packing is appropriately made from a heat-resistant sealing material, silicone rubber, asbestos-containing sealing materials and the like being suitable. The sealing material can also be made e.g., from a hardening paste, which on pressing the cover plate against the outer flange edge adapts to the cross-sections thereof between them. Thus, the cover part can at least approximately be fixed without metal contact to the hotplate body. 
     It is conceivable to construct, in inwardly directed ring flange-like manner, the edge of particularly the bottom cover plate and it preferably engages on the underside of the hotplate body outside the heating element, so that the latter can also form a subassembly with the cover plate and can be positioned at a limited distance from the hotplate body to ensure direct radiation action thereon. The inwardly directed, ring flange-like edge also ensures a very precise alignment of the cover plate, including the parts provided thereon with respect to the hotplate body. 
     In the case of a further, constructionally very simple embodiment, which still provides a reliable seal, the edge of in particular the lower cover plate has a substantially constant width up to its upper end, so that said edge is formed by an upper portion of the circumferential wall of the cover plate which in cross-section is substantially at right angles to the cooking surface. The edge engages preferably at least approximately up to the bottom of the heating element in the inner circumference of the outer flange edge and consequently forms a double-walled circumferential seal therewith. 
     As a result of the construction according to the invention, the temperatures of the hotplate body in the vicinity of its exposed parts on its bottom surface can be so low, e.g. below 300° C., that the hotplate can at least partly be provided, particularly on the inner and outer circumferential surface of the outer flange edge, with a water-repelling surface coating, such as e.g. a silicone coating. This possibility particularly exists if the electric hotplate is controlled with a regulator, which limits the entire power to a maximum temperature. This control can e.g. be constructed in the manner described in German patent application No. P 34 43 529.8 and reference should be made thereto for further details. 
     Both the insulating and reflecting action can be considerably improved in that the filling of at least one insulating zone comprises at least two superimposed layers of approximately identical and/or different thickness, as well as in particular different materials, whereof the top one is preferably in the form of a reflector layer. The insulating material can e.g. be introduced in the form of a loose bulk material in layers corresponding to the subsequently desired layer arrangement. It can e.g. have pyrogenic silicic acid as the base material. The reinforcing fibres can be ceramic fibres, e.g. aluminosilicate fibres. It is also possible to add a hardener, e.g. high-melting glass frits, metal oxides, etc, which permit a hardening of the surface boundary layer on heating in such a way that reflector characteristics are obtained. The thermal insulating material for at least one insulating layer, particularly that engaging directly with the heating element, is appropriately formed by ceramic fibres, preferably of aluminosilicate, which are compressed to a type of plate to ensure adequate strength, but for dimensional tolerance compensation appropriately also has elastic deformability. Such a material is marketed under the trade name Fiberfrax. 
     The advantages according to the invention can be achieved in the case of electric hotplates with different heating systems. For example, at least one heating element can be formed by a resistance wire heating coil embedded in a ceramic embedding material in a rib-defined spiral slot on the bottom of the hotplate body and on whose bottom surface is provided preferably one insulating cavity defined by the associated reflector. The reflector which is appropriately in the form of a radiation cover leads to a marked improvement to the efficiency in the case of a very simple construction, the efficiency being further improvable by placing insulation between said cover and the lower cover plate. 
     In addition to or instead of said heating system, at least one heating element can be formed by a spirally positioned tubular heater pressed against the bottom of the hotplate body with a resistance heating wire enclosed in an insulating embedding material within an outer tubular jacket, said heating element being preferably pressed by means of a through, flat insulating plate by a reflector against the bottom of the hotplate body, on which the heating element engages either in a rib-defined spiral slot without embedding, or on a continuously smooth and in particular planar surface. Due to the insulating plate, the heating element is kept in good thermally coupled engagement on the hotplate body and for this purpose the insulating plate has a slight elastic deformability. 
     In addition to or instead of the above heating system, a heating element can also be formed by a more particularly spirally positioned radiant heater arranged in spaced manner below the bottom of the hotplate body on the top surface of an insulator constructed as a reflector and in conjunction with the solid hotplate body made from an iron material and having an outer flange edge leads to a particularly favourable thermal behaviour of the electric hotplate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings, wherein show: 
     FIG. 1, an electric hotplate according to the invention in part sectional view. 
     FIG. 2, the electric hotplate according to FIG. 1 in a view from below. 
     FIG. 3, a detail of FIG. 1 in a larger scale axial section. 
     FIGS. 4 to 16, other embodiments of the electric hotplates in representations corresponding to FIG. 3. 
     FIG. 17, another embodiment in a view corresponding to a detail of FIG. 3, but on a larger scale. 
     FIGS. 18 and 19, two further embodiments in representations corresponding to FIG. 17. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A shown in FIGS. 1 to 3, an electric hotplate 1 according to the invention has a cast material, solid plate body 2, which is provided on its surface with an also centrally continuously planar cooking surface 3 in the form of a standing surface or base for cooking utensils. On its circumference, the cooking surface passes via a sloping bevelled surface into an upright circumferential surface 5, which is formed by a ring disk-like edge, which forms that part of the hotplate body projecting furthest over the outer circumference of body 2. An outer flange edge 4 projects downwards over the bottom of this ring disk-like edge and is constructed in one piece with the hotplate body 2 and has a roughly constant distance from the circumferential surface 5 over its entire circumference, so that in axial view it has a shape corresponding to the outer contour of the electric hotplate. This outer flange edge 4, whose wall thickness is smaller than the smallest thickness of the hotplate body in the vicinity of cooking surface 3, is cross-sectionally downwardly tapered at an acute angle and has a much greater height, by e.g. 3 to 5 times, than the ring disk-like part. The bottom 8 of the outer flange 4 forms an uninterrupted parallel end face to the cooking surface 3. From said bottom 8, the outer circumferential surface of outer flange edge 4 cross-sectionally slopes outwards at an acute angle, whilst its inner circumferential surface slopes inwards at an acute angle. In the upper area located in the vicinity of the bottom of the ring disk-like edge, its outer circumferential surface is outwardly displaced and is cross-sectionally approximately at right angles to cooking surface 3, so that in this area a spillage rim 6 can be fixed to the outer flange edge 4. This cross-sectionally, approximately U-shaped spillage rim 6 made from thin sheet metal or the like can e.g. be pressed onto the outer flange edge 4 with its inner ring part forming U-shaped legs and engages with the outside of its U-shaped cross web on the lower ring shoulder 9 of the ring disk-like edge. The spillage rim 6 projecting outwards over circumferential surface 5, when the electric hotplate 1 is incorporated into a hob 7, engages over a raised cross-sectionally stepped edge defining the associated assembly opening. The area of ring shoulder 9 adjacent to the outer circumferential surface of the outer flange edge 4 is offset with respect to said surface by a slot-like depression, so that the ring disk-like edge according to FIG. 3 in cross-section freely projects downwards in ring-like manner. Directly following onto the inner circumferential surface of outer flange edge 4 is provided on the underside of the hotplate body 2 at least one spiral slot 11 curved round the central axis 10 of hotplate body 2, which extends from the inner circumferential surface of edge 4 over only part of the distance up to central axis 10, so that body 2 is provided in the centre with a zone 12 free from heating elements and surrounding central axis 10. Spiral slot 11, which cross-sectionally downwardly widened and defined in trapezoidal manner, is defined between its turns by a spiral web 13 projecting downwards from the underside of hotplate body 2 and whose cross-sectional thickness is much smaller than the cross-sectional width of spiral slot 11. The distance of the base surface of spiral slot 11 running in a parallel plane to the cooking surface 3 is slightly larger than the cross-sectional width of spiral slot 11, said base surface being roughly level or slightly lower than the ring shoulder 9. The cross-sectional height of spiral slot 11 is slightly greater than its cross-sectional width. It is possible to provide two or more interengaging spiral slots with corresponding interengaging spiral webs, depending on the number of independently switchable heating elements used. The heating element 14 is formed by a resistance wire heating coil placed spirally and with a wall clearance on all sides in the particular spiral slot 11 and is embedded in electrically insulating manner in an embedding material 15 filling spiral slot 11. Its connection ends lead (not shown) from the bottom 16 of hotplate body 3 formed by the lower end face of spiral web 13, so that they can be connected to leads of the cooker. In the central axis 10 of hotplate body 2 is provided a centre stud 17 projecting downwards from the plane of base surface of spiral slot 11 free from the underside of hotplate body 3. The lower end face of the stud 17 parallel to cooking surface 3 is located below bottom surface 16 and above bottom surface 8 of outer flange edge 4. The outer circumferential surface of the downwardly acute-angled, conically tapered centre stud 17 is radially spaced from the inner spiral turn of spiral web 13, which therefore form an inner flange edge 18 spacedly surrounding the centre stud 17 and whose distance from the outer flange edge 4 is much larger than that from the centre stud 17. 
     The underside of the hotplate body 2 is closed by two superimposed sealing parts 19, 20, which are spaced from one another in contact-free manner and are e.g. fixed in the vicinity of a thread bolt 21 screwed into an inner thread of centre stud 17 and which is located in central axis 10 and projects over the underside of the electric hotplate 1 for securing the latter in the hob 7. Both the sealing parts 19, 20 are formed by deep-drawn sheet metal covers, the upper, cup-shaped sealing part 19 having a greater height than the lower cup-shaped sealing part 20. By means of its closed base wall extending approximately up to the outer flange edge 4 and traversed by thread bolt 21, the upper sealing part 19 engages on the lower end face of centre stud 17 against which it can be fixed by a nut (not shown) or the like. Its outer circumferential wall is outwardly deformed in ring flange-like manner, at least in parts, so that shoulder members 22 are formed, whose radial extension is at least as large or slightly larger than the width of spiral slot 11. The upper sealing part 19 by means of the shoulder members 22 is directly supported on the underside 16 of hotplate body 2 following on to the inner circumferential surface of the outer flange edge 4. Between the shoulder members 22, (the circumferential wall of the sealing part 19 ends in upwardly directed manner, so that it engages with its upper edge face on the bottom of the embedding material 15. Base wall 23 of sealing part 19 has an outer, relatively narrow ring disk-like area adjacent to the circumferential wall, with respect to which the remaining central, planar part of the base wall is downwardly displaced by means of a sloping ring step, the top of said part being located in the plane of the lower end face of the centre stud 17 and has a relatively narrow through-bore for the passage of the thread bolt 21 or a similar fixing member. The top surface of at least base wall 23, but particularly also the circumferential wall of sealing part 19, is constructed in a continuous manner over its entire extension in the form of a reflector 24, which is directed against the bottom 16 of hotplate body 2 over an area extending from the circumferential surface of centre stud 17 approximately up to the inner circumferential surface of outer flange edge 14. With respect to the outer flange edge 4, reflector 24 provides a heat shielding action and in particular the circumferential wall of the sealing member 19 reflecting on the inside reflects back the thermal radiation before it reaches the outer flange edge 4. If the sealing part 19 is not from the outset held with a limited spacing and in completely contact-free manner with respect to the outer flange edge 4, the contacting faces between sealing part 19 and edge 4 are kept extremely small as a result of the aforementioned construction, being formed by edge faces. Between the circumferential wall of sealing part 19 and the inner circumferential surface of outer flange edge 4 is provided a small spacing, at least approximately over the entire height of sealing part 19. 
     The flat dish-shaped sealing part 20, which is much lower than sealing part 19 also has a ring flange-like, cross-sectionally outwardly directed edge 25, which with its upper end face engages uninterruptedly on the bottom 8 of outer flange edge 4 and appropriately extends roughly up to its outer circumferential surface. Edge 25 is only slightly wider than bottom 8 of outer flange edge 4. In cross-section, edge 25 passes via a downwardly directed bend directly into the lower circumferential wall of sealing part 20, which in turn passes via a bend into the planar base wall 26 which is parallel to the bottom 8 of outer flange edge 4 or at right angles to central axis 10. In the centre of base wall 26 is provided a relatively narrow through opening for thread bolt 21 or the like. A nut 27 on thread bolt 21 is so fixed against the bottom of base wall 26 that, accompanied by slight elastic deformation of the latter, edge 25 is resiliently pressed against the bottom 8 of outer flange edge 4. Between the base wall 26 and base wall 23 can be provided at least one spacing member, e.g. a spacing sleeve surrounding thread bolt 21, so that a single nut is sufficient for fixing both sealing parts 19, 20 with respect to centre stud 17. Such a spacing sleeve is e.g. shown in FIG. 16. The upper sealing part 20, at least on the top surface of base wall 26 and in particular on its entire inside, is continuously constructed as a reflector 28, which is directed against the bottom of the upper sealing part 19. Any heat radiation passing downwards from the upper sealing part 19 is consequently reflected back upwards. 
     The area terminated both by the upper sealing part 19 and also between the two sealing parts 19, 29 is provided as an insulating zone 29, 30, both of which are formed by substantially hermetically sealed, air-filled cavities. The upper insulating zone 29 is positioned above the bottom 8 of outer flange edge 4 and the lower insulating zone 30 extends downwards and only slightly over bottom 8 of slange edge 4. The two sealing parts 19, 20 together form the lower seal 31 of the hotplate body, the outer flange edge being incorporated into said seal as a casing wall, whose inner circumference 32 bounds the outer circumference of the insulating zone 30. 
     In FIGS. 4 to 19, corresponding parts are given the same reference numerals as in FIGS. 1 to 3, but different letter are used. 
     Electric hotplate 1a according to FIG. 4 differs from that according to FIGS. 1 to 3 essentially only in that in the lower insulating zone 30a is provided an additional insulating and/or reflecting part in the form of an aluminium crinkled foil 33, which only takes up part, e.g. roughly half the smallest height of insulating zone 30a and is arranged in contact-free manner with respect to the upper sealing part 19a, in the same way as the lower sealing part 20a, so that the single metallic connection between the two sealing parts 19a, 20a is formed by the outer flange edge 4a and the thread bolt 21a. The aluminium crinkled foil 33 is arranged directly on base wall 26a of the lower sealing part 20a at a height which is roughly the same as the tray height of the sealing part 20a. It extends in uninterrupted manner approximately from the inner circumference 32a of the outer edge flange 4a over the entire ground plane of the lower sealing part 20 a and is centrally traversed by the fixing bolt 21a. The outer edge of the aluminium crinkled foil 33 can be secured between edge 25a of sealing part 20a and the bottom 8a of the outer flange edge 4a. Here again, the inside of the lower sealing part 20a can be constructed as a reflector 28a, so that there are three successive reflectors. Between crinkled foil 33 and the base wall 26a of sealing part 20a are formed several cavities which are reciprocally closed and/or closed with respect to the remaining insulating cavity 30a and whose shape can e.g. be determined by a wavy laying of the aluminium crinkled foil 33. 
     In the embodiment according to FIG. 5, most of the volume of insulating zone 30b is filled with insulation 34, which is appropriately located on base wall 26b or reflector 28b and extends in the manner of a plate of constant thickness only up to the underside of the central lowered region of base wall 23b of sealing part 19b. Thus, the insulation 34 is contact-free with respect to the outer, ring disk-like part of base wall 23 and with respect to the circumferential wall of sealing part 19b. The insulation 34 also forms the spacing member between the two sealing parts 19b, 20b, so that the two together can be fixed against centre stud 17b by nut 27b exposed on the bottom of seal 31b or some similar tightening member. Insulation 34 extends uninterruptedly from the outer circumference of thread bolt 21b to the inner circumference 32b of outer flange edge 4b. As shown in FIG. 6, at least one sealing part, particularly the upper sealing part 19c, can severe as a support body for a connecting member 35 for the electrical connection of heating element or elements 14c. Appropriately connecting member 35 is fixed to the bottom of base wall 23c, so that the reflector or reflectors 24c only has to be traversed by relatively narrow passage openings for the connecting leads 36, which connect the connecting member 35 to heating element 14c and are placed in the insulating zone 29c. The connecting member 35, which is e.g. fixed by screws in detachable manner against the bottom of base wall 23c and which has a casing made from insulating material traverses by its casing a through opening, having a closely adapted cross-section, in the base wall 26c of the lower sealing part 20c, so that its electric connections 37 on the underside of the casing can be freely exposed e.g. in the form of terminals on the bottom of the seal 31c. Connecting member 35 is appropriately nearer to centre stud 17c than to outer flange edge 4c and is appropriately located in the vicinity of inner flange edge 18c, which projects slightly further downwards than the remaining spiral web 13c. As is also shown in FIG. 6, the spillage from rim 6c can be directly formed by the hotplate body 2c, so that the ring shoulder 9c connected to circumferential surface 5c is constructed for resting on the raised edge of the hob. 
     In the embodiment according to FIG. 7, edge 25d of the lower sealing part 20d does not have an angular or bent cross-section and instead forms a continuous, upwardly directed extension of the circumferential wall of the sealing part 20d, which extends over the level of bottom 8d of outer flange edge 4d or the bottom of the upper sealing part 19d. In the represented embodiment, edge 25d extends level with the top of the upper sealing part 19d, in such a way that the end face of edge 25d engages on the bottom 16d of hotplate body 2d. Edge 25d surrounds the outer circumference of sealing part 19d with a small clearance, its upper end face engaging either on the bottom of the shoulder member 22d and/or passes round the outer circumference of the sealing part 19d in the vicinity of its top surface and in a centering manner. In the first case, sealing part 19d is also secured in the vicinity of its outer circumference by means of sealing part 20d against the bottom 16d of hotplate body 2d. The outer circumference of the lower sealing part 20d engages exclusively in the vicinity of its top surface, i.e. in the region of the outer edge line of its upper edge face on the inner circumference 32d of the outer flange edge 4d, so that also in the vicinity of the circumferential wall of sealing part 20d, approximately over the entire associated height of edge 4d is provided a substantially uninterrupted clearance over its circumference between the circumferential wall and flange 4d, which insulates the latter with respect to the circumferential wall. In the embodiment according to FIG. 7, the height of insulating zone 30d is greater and in particular at least twice as great as the height of insulating zone 29d, whereas in the embodiment of FIGS. 1 to 3 it is only roughly half as large. 
     According to FIG. 8, the construction of the lower sealing part described relative to FIG. 7 can also be provided in the hotplate constructed according to FIG. 6. Insulating zone 30d is filled over more than half its height by insulation 34e, whose top surface is arranged in spaced manner below the flange plate of the connecting member 35e fixed to the sealing part 19e or the fixing screws thereof. Unlike in the case of FIG. 6, the flange plate is directly arranged in whole-area manner on the bottom surface of sealing part 19e and not by means of spacing parts engaging over small areas. The lower end of the casing of connecting member 35e which is at right angles to cooking surface 3e and is provided on its top surface with the outlet for connecting leads 36e is arranged roughly level with the base wall 26e of sealing part 20e, so that it only slightly projects downwards over base wall 26e and the leads 37e provided on its bottom surface are located directly below sealing part 20e. The casing of connecting member 35e is embedded in insulation 34e over the entire thickness thereof. For the close connection of insulation 34e to the thread bolt 21e is provided a sleeve 38 which surrounds the latter and which is e.g. mounted on thread bolt 21e and passes through the entire thickness of insulation 34e, so that the insulation engages on the outer circumference of sleeve 38. Sealing parts 19e, 20e are made from varying thick sheet metal, the sheet metal of sealing part 19e being thicker and in particular twice as thick as that of sealing part 20e. 
     In the embodiment according to FIG. 9, there are three superimposed sealing parts 19f, 20f, 39 in the form of cover plates, the two lower parts 20f, 39 surrounding a further insulating zone and which is at least partly and in particular completely filled with an insulation 40 of at least one of the described materials. Sealing parts 19f, 20f are constructed in accordance with sealing parts 19, 20 of FIG. 3, whilst sealing part 39 has a reduced height corresponding to sealing part 20d according to FIG. 7. The upper edge of sealing part 39 is in engagement with the bottom and/or outer circumference of the ring flange-like edge 25f of the sealing part 20f and therefore has its upper edge face on the bottom 8f of the outer edge flange 4f. The circumferential wall of the sealing part 20f engaging from above approximately over its entire height in sealing part 39 is spaced from the circumferential wall of sealing part 39, which cross-sectionally over its entire height passes through in an approximately linear manner and on its lower end connects the through, planar base wall 41 of sealing part 39 which is at right angles to central axis 10f. At least the top surface of base wall 41 and in particular the entire inner surface of sealing part 39 is constructed as a reflector 42. As described relative to FIG. 4, in sealing part 20f is provided a crinkled foil 33f or the like, e.g. made from aluminium as a reflector and insulating layer. Thus, in superimposed manner, there are three or four reflectors and two insulation layers separated from one another by sealing part 20f and two insulating cavities separated by the sealing part 19f. Whilst optionally incorporating an insulating 40, sealing parts 20f, 39 can be joined together to form one component, e.g. welded in the vicinity of there engaging edges. 
     In the embodiment according to FIG. 10, the upper sealing part 19g is formed by a planar plate, whose top surface engages on the bottom surface 16g of hotplate body 2g and which extends with its circumference side edge face approximately to the inner circumference of outer flange edge 4g. Between sealing part 16g and the lower sealing part 20g constructed in accordance with FIG. 7 is provided a multilayer insulation 34g substantially completely filling the associated insulating zone and which is only centrally provided with a through opening for the passage of the center stud 17g and the thread bolt 21g. In the represented embodiment, there are three separate layers 43, 44, 45 made from different insulating materials and/or different compression densities. The bottom, thickest layer 45, which extends above the bottom 8g of the outer flange edge 4g bounds the thinnest, appropriately hardest, compressed layer 44, on whose top surface rests the medium thickness layer 43, whose bottom surface bounds the sealing part 19g. One or the central layer 44 can also be constructed on its top surface as a reflector and can e.g. be formed by a sheet metal plate. As in the embodiments according to FIGS. 7 to 9, here again the lower seal 31g projects relatively well over the bottom 8g of the outer flange edge 4g and at least by the height thereof. 
     In the embodiment according to FIG. 11, heating element 14h or the heating elements are formed by a tubular heater, which comprises a resistance heating wire 48 arranged within an outer tube casing 46 and is kept spaced therefrom by embedding in an insulating embedding material 47. The spirally positioned tubular heater is arranged in spiral slot 11h and can be embedded in the described manner in an embedding material 15h connecting onto the outer circumference of a tube casing 46 and which substantially completely fills the spiral slot 11h and provides a particularly good thermal coupling to the hotplate body 2h. The upper, plate-like sealing part 19h is located, as described relative to FIG. 10, in engagement on the bottom of hotplate body 2h, but its inner circumference only extends up to the outer circumference of the further downwardly projecting inner edge flange 18h, which also applies for the insulating layer 43 located immediately below it and for the layer 44h. However, layer 45h extends almost up to the outer circumference of center stud 17h. However, in this embodiment, edge 25h of sealing part 20h engages by its upper edge face on the bottom of layer 44h, so that said edge 25h for the entire circumferential wall of sealing member 20h can be kept spaced from the inner circumference of outer edge flange 4h. In this embodiment, a reflector or reflectors are only provided in the heated zone of hotplate body 2h, whilst the zone 12h free from heating elements which surrounds the central axis 10h only up to the outer circumference of the inner flange edge 18h is only covered on the bottom surface by an insulation, which passes through in one part in the height direction. 
     In the embodiment according to FIG. 12, the height of the spiral web 13i is smaller than the external diameter of the heating element 14i, e.g. formed by a tubular heater, so that it projects slightly over the bottom of spiral web 13i. The heating element 14i is not embedded in spiral slot 11i and is instead loosely placed in the cross-sectionally upwardly tapering spiral slot 11i in such a way that its outer circumference is in contact both with the base face and the two side faces and to this extent is thermally very well coupled. Heating element 14i is pressed upwards into the spiral slot 11i by the sealing member 19i supported in prestressed manner on its bottom and is consequently kept in good thermal conducting engagement, despite thermal stresses. for this purpose, sealing member 19i is provided on its top surface with a flat plate, which may be fixed thereto and which can be constructed in accordance with the sealing part 19g according to FIG. 10 and appropriately is constructed as a reflector plate 49, i.e. on its top surface as a reflector. On said plate is provided an insulating plate 50 which is much thinner than the height of the outer flange edge 4i and the sealing part 19i and which has a certain pressure elasticity, so that it transfers in the manner of a permanent elastic intermediate layer the contact pressure of sealing part 19i to heating element 14i. Insulating plate 50 is spaced below spiral web 13i and to the extent that it projects radially inwards into this area, is also spaced below the inner flange edge 18i. In the interior, sealing part 19i forms the completely hollow insulating zone 29i, whilst below it is provided the much lower insulating zone 30i. 
     In the embodiment according to FIG. 13, only the bottom sealing part 20j is formed by a sheet metal plate, whilst the upper sealing part is formed by insulating plate 50j, which engages in the upper edge 25j of sealing part 20j in such a way that it projects upwards slightly above its upper edge face. Below insulating plate 50j is provided a further layer 44j of roughly the same thickness, which can also be made from insulating material, but is appropriately less strongly compressed. The layer 45j filling the adjacent space below it corresponds to that according to FIG. 11. 
     For corresponding installation cases, the lower seal 31k of hotplate body 2k can, at least in the vicinity of its bottom surface, be free of any sheet metal part and can be formed on its bottom surface solely by an insulating material body 45k, which is pressed by clips or similar fixing means only partly covering its bottom surface against the bottom surface of hotplate body 2k, said fixing means simultaneously serving for fixing electric hotplate 1k into the hob. According to FIG. 14 the upper sealing part is only formed by insulating plate 50k, so that seal 31k has no sheet metal parts. The bottom of hotplate body 2k has no projecting spiral web and optionally also no inner flange edge, so that it is completely planar between the inner circumference of outer flange edge 4k and the outer circumference of centre stud 17k. Heating element 14k engages on this planar face and in particular in this case the heating element can have a cross-sectionally flattened portion, e.g. with a semicircular or triangular construction, with which it appropriately engages on the associated overlying bearing face of hotplate body 2k. 
     The embodiment according to FIG. 15 has a heating element 14m  arranged and constructed similarly to that of FIG. 14, but the hotplate body 2m and lower seal 31m are constructed much as in FIG. 11. Hotplate body 2m has an inner flange edge 18m, from which the innermost turn of the heating element 14m is spaced. Insulating plate 50m engages directly on heating element 14m and is supported on its underside by a reflector plate 44m, whose bottom surface is roughly located in the plane of the bottom surface of the inner flange edge 18m. As a result the insulating layer 45m is connected to the bottom of inner flange edge 18m, so that heating element 14m is located substantially freely in a cavity closed on both the inner and outer circumferences. 
     In the embodiment according to FIG. 16, heating element 14n is arranged in a closed cavity, but said heating element 14n is in the form of a radiant heater and is spaced below the planar bottom 16n of hotplate body 2n. Sealing part 20n, whose base wall 26n is roughly in the plane of the bottom 8n of the outer flange edge 4n extends with its circumferential wall up to the bottom 16n of the hotplate body 2n, the upper edge 25n of said circumferential wall engaging in ring flange-like manner on the bottom 16n and is inwardly direct. Sealing part 20n receives a lower insulating layer 45n and an overlying insulating material layer 50n which serves as a support for the heating element 14n, so that heating element 14n is at least partly fixed in position by embedding in said support layer. On the outer circumference of the heating zone having heating element 14n, layer 50n projects upwards over said heating element and extends up to edge 25n, so that the space receiving the heating element is defined on the outer circumference by said layer 50n approximately up to bottom surface 16n. On the inner circumference of the heating zone, layer 50n engages in the inner flange edge 18n in such a way that it partly surrounds the underside and outer circumferential surface thereof, layer 45n extending into the heating element-free zone 12n. Bottom wall 26n of sealing part 20n is fixed against the bottom of centre stud 17n, whilst interposing a spacing sleeve 51 surrounding thread bolt 21n. 
     As shown in FIG. 17, sealing part 20p can engage in the outer flange edge 4p, whilst interposing a ring packing 52, which is in cross-section approximately flat and rectangular and is fixed between the bottom 8p of the outer flange edge 4p and edge 25p of sealing part 20p. On both the inner and outer circumferential surfaces and on the bottom 8p, outer flange edge 4p is provided with an uninterrupted surface coating 53, which can e.g. be of silicone. The base wall 26q of sealing part 20q can, according to FIG. 18, be located above edge 25q thereof, so that the sealing part 20q engages from below in centered manner in the outer flange edge 4q and a cross-sectionally angular ring packing 52q or a ring packing located between the inner circumference of the outer edge flange 4q and the outer circumference of the circumferential wall of the sealing part 20q can be used. The ring packing 52r according to FIG. 19 is cross-sectionally approximately Z-shaped, so that it also engages on base wall 26r of sealing part 20r.