US2500885A - Electric heating plate - Google Patents

Description

March 14, 1950 s sz 2,500,885

ELECTRIC HEATING PLATE Filed July 25, 1947 Fig :1

Patented Mar. 14, 1950 ELECTRIC HEATING PLATE Stanislas Teszner, Paris, France Application July 23, 1947, Serial No. 763,061 In France March 6, 1941 Section-1, Public Law 690, August 8, 1946 Patent expires March .6, 1961 3 Claims. 1

The major drawback of the heating plates usually employed in electric cooking'is their poor efliciency which is due first of all to the necessity of insulating the heat source from the bottom of the heated vessel by meansof an-electric insulatorwhich at the same time is more or less av heat insulator. Said insulator may consist either of (a) air only (visible fire plates) and in this case itsv thickness is comparatively con siderable in order to exclude every possibility of contact between the vessel and the wire of the heating resistance; consequently, the efficiency is decidedly bad; or (b) a solid agglomerated insulator contained between the resistance wire and the outer envelope of the plate, and in this case in addition a more or less considerable air gap exists between said outer envelope and the bottom of the vessel to be heated since the former does not exactly conform the latter; after all, the efficiency is better than in the preceding case although it still remains insufiicient'.

Another inconvenience to be pointed out at least concerning invisible-fire plates defined under (1)), which are the only ones worth to be 1 considered, is the obligation of using a special set of thick and fiat bottomed cooking utensils. Such a set is practically indispensable with plates provided with a rigid envelope, but it is also quite useful where said envelope is comparatively flexible since such flexibility is never such that conformation to the bottom of the vessel is obtained. Nevertheless, since the vessel bottom cannot indefinitely remain even. the efliciency-will sink rapidly unless the set of cocking utensils is quite carefully maintained and rather frequently replaced. Moreover, every foreign body that may shim the bottom apart from the plate will most unfavourably influence the efficiency.

A last inconvenience to be pointed outiis the inertia of such plates, which is particularly troublesome in uses of short duration; The cause of the said inertia resides mainly in the presence of one or several layers of heat-insulati-ng material around the heat source and corn tingentlyalso of anouterenvelope whose heat capacity is by no means negligible.

That a fundamental solution has been found to these inconveniences by feeding the said heat ingplates with a very-lowvoltage ofthe-orderof about ten volts is well known. In this manner the. following advantages are, obtained:

(a) It, is no longer necessary to electrically nsula e be hea i g: elemen rom vessel o be heated, the former itself can then constitute the plate on which the latter rest. Moreover, since the resistance value to be obtained is comparatively low, no difliculties' are encountered in designing such aplate.

(b) A special set of cooking utensils is. no longer necessary.

(0) The inertia of the plate is reducedto a minimum.

(d) The danger of the user being shocked byv contact with the resistance wire or with the leads that feed the same is eliminated.

However. in prior applications of this principle, the plate didnot at least satisfactorily conform to the bottom of the vessel. The result was that a good efliciency was only obtained with utensils in perfect condition and in the absence of 1 all foreign bodies liable to shim the bottom apart.

Moreover, the efficiency was also affected by appreciable heat losses from the periphery of the plate which were the more considerable as a higher temperature prevailed at said periphery.

The purpose of this invertion is to obviate these inconveniences and with this endin view to improve electric heating plates fed with a very low voltage so that:

(a) The plates shall effectively conform the bottom of the vessel to be heated even if the latter happens to be considerably out of shape;

(1)) The losses from the periphery of the plate shall be minimized.

According to the invention, the result aimed at is attained:

(a) By having the heating plate made up of a plurality of flexible independently deformable pieces, with the result that the points of contact are multiplied and consequently-heat-transmission by conduction from the plate to the bottom of the vessel is increased; moreover, saidpieces are arranged to effectively conform the shape of the latter;

(2)) By stepping down the heatingof the plate surface towards the. periphery of the same so that the heat losses from the said periphery are practically negligible.

It should be emphasized that a condition essential to satisfactory performance of such a plate is the maintenance of the resiliency of the pieces of which the plate is made. This result can be secured either by limiting the possible deformation so that the elastic limit is not exceeded, and this, with the aid of one or several suitably arranged stops, or else by the provision of one or several springs acting on said pieces although being heatinsulated therefrom.

The following description, which is given merely by way of example, of two embodiments of the invention will aid to understand the features and advantages of the same, it being understood that the shapes and materials used may vary without thereby departing from the scope of the invention.

Reference will be had successively to the following figures:

Figures 1 and 2 which are a plan View and a vertical section respectively of a heating plate according to the invention;

Figure 3 which is a vertical partly sectional view of a modification comprising on auxiliary spring.

The plate whose top is shown in Fig. 1 comprises a metal sheet (e. g. a nickel-chromium or a stainless steel sheet) of suitable resistivity cut into eight separate pieces movable independently from one another and interconnected merely at their peripheries. They are separated by slits 2 and each of them is shaped owing to the intermediate slits 3 as a V having its point aiming at the centre of the plate, whereby its resiliency is further increased.

Fig. 2, which is a sectional view taken in the plane (AA) shows that the metal sheet is stamped in such manner that in unstressed condition it assumes the shape of an inverted dish. It is formed with a pair of lugs l and 5 for the inflow and outflow of very low voltage electric current.

It rests at its periphery on a ring support 6 (e. g. of steel or aluminum) with the interposition of a layer 7 of both thermally and electrically insulating material (e. g. asbestos); it may be wrapped over said support acting as a frame as shown in the figure or else reliably secured thereon by means of screws or rivets thermally and electrically insulated therefrom at least at one side. The support l is complemented by a dish 8 (e. g. of blank aluminium) fitted preferably in air-tight manner thereon and providing a reflector for the heat radiated downwards by the metal sheet. The dish 8 is secured (e. g. steatite) adapted to limit the possible deformation of the sections 1 by the effect of the weight of the vessel. Said prop rests on a flat portion provided at the centre of the dish and is cut with a thread on which a nut l l is screwed by which it is rigidly connected with the dish.

The latter is provided with preferably very small apertures l2 arranged in the free portion as close as possible to the axis and serving to let off liquids that might be spilled on the plate. The latter as a whole is secured on the frame of a range or stove or the like with the aid of lugs l3.

The modification illustrated in Fig. 3 differs from the one just described by the presence of a leaf spring l4 having a limited travel as shown in the drawing and by which the prop I0 is pressed against the lower face of the pieces 1 to maintain their initial deformation. The nut H is then modified to serve as a rest for the spring. The latter is rivetted to the dish 8 yet heat insulated therefrom by washers l5, e. g. made of asbestos. A shield l6 fitted on the dish and contingently provided with a central aperture H complements the structure.

The operation of the plate can be understood easily. In view of the convexity of the plate when not in use it will conform even comparatively concave vessel bottoms. Now, such a deformation of the bottoms is particularly frequent and was highly inconvenient up to the present from th electric heating viewpoint whereas in the case of the plate according to the invention the pieces of the heating metal sheet will conform to the shape of such bottoms as perfectly as possible. However, in the event the shape of the bottom should be different, e. g. convex, the plate would also conform it; effectively, as a result of the weight of the vessel, the pieces will yield and as far as possible conform to all the swellings and irregularities of the bottom surface. Even if on or more foreign bodies should be present on the bottom of the vessel and shim the latter from contact with the heating plate, which is much improbable in view of their high capacity of deformation, further pieces would still lay themselves flat thereagainst, thus providing for sufiicient heat transmission. In order to preclude any excessive deformation of the pieces beyond th elastic limit of the metal used therefor as a result of the presence of a particularly convex bottom and a comparatively heavy or loaded vessel the stop provided by the prop it? will limit such a deformation to a reasonable amount.

In the case of the modification illustrated in Fig. 3 the operation is the same except that on account of the auxiliary action of spring M a more considerable deformation of the pieces in either direction is admissible since the spring then will supply any deficiency or loss in the resiliency of the latter.

As to the distribution of the heat over the surface of the plate it will be appreciated immediately by considering Figs. 1 and 2 that (1) the electric resistance of the pieces is not uniformly distributed; (2) the lineal resistance is a maximum at the end of the slits 3 (inner chain-line circle); (3) the lineal resistance is a minimum at the periphery (outside the outer chainline circle). Nor will the temperature of the heating plate be uniform. In particular, it will decrease towards the outside from one half or two thirds of the diameter. The drop will be par ticularly evident in the vicinity of the periphery since the lineal resistance in that region is com-- paratively very low. Consequently, the heat losses (by convection and by radiation) from the periphery will be reduced considerably.

In certain conditions the shape of the pieces in the embodiment described hereinbefore may suffer from two inconveniences: on one hand the fragility of the ends of the pieces, even when rounded, and on the other hand, the current is not properly distributed therein. Effectively, the current density is low unless the slit is protracted, which is detrimental to mechanical strength, and such decrease in density is against the gradation aimed at in the heating effect.

For these reasons one may be led to adopt for the plate pieces a trapezoidal outline (preferably also with rounded angles) which while retaining the flexibility of the pieces will eliminate the inconveniences mentioned. Of course, the pieces remain slitted in the initially provided manner.

On the other hand, in order to prevent the pieces from being caught by the utensil to be heated it may be useful to provide in the general shape of the stamped plate an annular plane surface in the central region the inner contour of the annulus being substantially coincident with the end of the pieces and the outer contour being in the vicinity of the end of the slits. The said plane surface moreover provides a stable bearing surface for the utensil even if the diameter of the latter is decidedly smaller than that of the plate.

Finally, in spite of the very low operating voltage and of the very low resistance of the heating element providing the plate, it is necessary to cover the face of the latter in contact with the utensil to be heated with an insulating or semiconductive film in order to prevent the formation of small arcs that will result in a rapid destruction of the element. The purpose of the said fllm is to decrease to a practically negligible value the current shunted through the utensil irrespective of the material same is made of. The film must retain its insulating or semi-insulating properties at high temperature and possess a mechanical stiffness sufficient to resist any friction against the utensil.

A method of obtaining an insulating coating that will fulfil the aforesaid requirements and that is particularly valuable in the case of the plate according to the invention consists in coatme one of the faces of the resistance with a thin hard layer of enamel treated at high temperature and consequently capable of standing heat without damage. The insulating film thus formed provides for an insulation which is quite suflicient for the plate with respect to the utensil to be heated which does not appreciably interfere with heat transmission, which will resist all friction, which has a pleasant appearance and which is quite easy to maintain. Finally, this being a remarkable and important feature, it will also contribute to lay the pieces of the plate into contact with the utensil. Effectively, it will be appreciated that where the upper face of the pieces is enamelled the said pieces as they become heated have a tendency to warp upwards due to the fact that the enamel layer will not expand so much as the metal providing the plate. Consequently, they lay themselves flat against the utensil and thus establish an intimate contact with the latter to improve the conditions of heat transmission by conduction.

What I claim is:

1. An electric heater comprising a heating plate having a central opening and formed of a plurality of flexible blades adapted to be traversed by an electric current and distributed about said central opening, main current inlet and outlet means secured respectively on two adjacent blades, means connecting mechanically and electrically each two adjacent blades other than said two blades carrying said main current inlet and outlet means, said connecting means being provided only at the external periphery of the plate between the adjacent edges of said blades, whereby an electric current may traverse successively all the flexible blades, and supporting means provided with a bearing surface on which the flexible blades are secured only by their outer edges remote from the central opening, whereby the portions of said blades situated near said central opening are adapted to bend yieldingly independently of each other.

2. An electric heater comprising a heating plate having a central opening and formed of a plurality of flexible blades adapted to be traversed by an electric current and distributed about said central opening, main current inlet and outlet means secured respectively on two adjacent blades, and means connecting mechanically and electrically each two adjacent blades other than said two blades carrying said main current inlet and outlet means, said connecting means being provided only at the external periphery of the plate between the adjacent edges of said blades, whereby the portions of said blades situated near said central opening are adapted to bend yieldingly independently of each other, each flexible blade being provided with a substantially radial slit extending from the periphery of the plate to a point situated at a distance from the central opening, whereby the current flows in two opposite directions in each blade on either side of said slit and the resistance and the heat produced are greater in the central part of the plate than at its periphery.

3. An electric heater according to claim 1, in which said supporting means is a dish shaped support the upper face of which is a reflecting surface.

STANISLAS TESZNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 891,657 Berry June 23, 1908 1,108,329 Callahan Aug. 25, 1914 1,432,491 Phelps Oct. 1'7, 1922 FOREIGN PATENTS Number Country Date 290,514 Great Britain May 17, 1928 435,033 Germany Oct. 6, 1926

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