Electrical heating means for cookers or hot plates

An electrical heating device for cookers and hot plates is disclosed, having a heat-resistant support member disposed below a heat-radiation transmitting plate, in particular, a glass-ceramic plate. The support member is adapted to receive at least one electric heater filament, wherein the support member is disposed within a hollow space between the heat-radiation transmitting plate and a heat-insulating layer or body. The support member is provided with holes or cut-out portions enforcing convection of the air enclosed within the hollow space, and furthermore, the heat-insulating layer being covered by a reflecting sheet or foil, so that a substantial portion of the heat radiated downwards by the heater filament is directed partially by reflection at the sheet or foil and partially by the air current passing over the sheet or foil onto the heat-radiation transmitting plate and onto a cooking or frying vessel standing thereupon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an electrical heating means for cookers and hot 
plates having a heat-resistant support disposed below a heat-radiation 
transmitting plate, in particular a glass-ceramic plate, said support 
member being adapted to receive at least one electric heater filament. 
2. Description of the Prior Art 
Electric cookers equipped with so-called "radiation hot plates" are being 
used to an increasing extent. Their cooker surfaces are formed, for 
example, by a glass plate. Circular, square or rectangular electrical 
heating means, usually of different sizes, are disposed below the glass 
plate. The heat produced by the electrical heating means is transmitted 
mainly by radiation onto cooking or frying vessels placed onto the glass 
plate. 
Examples of embodiment are known, wherein the heater filament producing the 
heat is placed at a definite distance from the lower side of the glass 
plate. A shell-shaped structure made of a heat-resistant material is 
secured to the lower side of the glass plate. The heater filament is 
placed into the bottom of the shell-shaped structure. These embodiments 
provide the advantage that the glass plate is not subjected directly to 
the relatively high temperature of the heater filament and that each 
heater filament irradiates a large section of the glass plate, because of 
the widening of the radiation cone. In this way the radiation passing 
through the glass plate is thus distributed. It is a disadvantage of these 
embodiments, however, that a relatively high proportion of the heat energy 
produced by the heater filament is conducted away in downward direction 
and therefore becomes lost. These losses adversely affect the energy 
requirements and, furthermore, the heater filament or spiral must be 
heated to a high temperature in order that the required heating power may 
be transferred to the cooking or frying vessel positioned on the hot 
plate. This high temperature appreciably shortens the life of the heater 
filament. 
BRIEF SUMMARY OF THE INVENTION 
It is the object of the invention to propose heating means of the kind 
described above, wherein the downward heat-flow from the heater filament, 
i.e. away from the glass plate, is substantially reduced. Furthermore, the 
filament support member is to consist of a material having poor 
electrical, but very good thermal conductivity, for example pressed MgO 
profiles, ceramics and the like. In comparison with known embodiments 
having the filament directly placed into the insulating material, a 
lowering of the temperature of the filament, having the same dimensions, 
by about 50.degree. to 100.degree. C. may thus be achieved, leading to a 
doubling or quadrupling of the operational life. 
The present invention provides an electrical heating means for cookers and 
hot plates having a support member disposed below a heat-radiation 
transmitting plate, in particular a glass-ceramic plate, said support 
member being adapted to receive at least one electric heater filament, 
wherein said support member is disposed within a hollow space between said 
heat-radiation transmitting plate and a heat-insulating layer or body, 
said support member being provided with holes or cut-out portions 
enforcing convection of the air enclosed within said hollow space, and 
furthermore, said heat-insulating layer being covered by a reflecting 
sheet or foil, so that a substantial portion of the heat radiated downward 
by said heater filament is directed partially by reflection at said sheet 
or foil and partially by the air current passing over said sheet or foil 
onto said heat-radiation transmitting plate and onto a cooking or frying 
vessel standing thereupon. 
In a preferred embodiment of the electrical heating means of the present 
invention said support member consists of a body of refractory brick, 
preferably of magnesite, having an H-shaped cross-section, the flanges of 
said body being supported at the lower side of said heat-radiation 
transmitting plate and at said heat-insulating layer or said reflecting 
sheet or foil, and the plate-shaped central web portion of said H-shaped 
body, which bridges said flanges, receiving said heater filament. 
According to a further preferred embodiment of the electrical heating means 
of the present invention, the helically wound heater filament is arranged 
in known manner in a groove of said support member and the bottom of said 
groove is provided with through holes or cut-out portions through which a 
convective air current is drawn and directed upwards against said 
heat-radiation transmitting plate though said heater filament when in 
heated state, and furthermore, between the windings of said heater 
filament additional holes or cut-out portions are provided, through which 
the convective air current streams downwards against the sheet or foil.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
A support member 1 for a helically wound heater filament 2 consists of a 
refractory brick, for example, of pressed magnesite, having a H-shaped 
cross-section, i.e. a round or square or even rectangular plate 3 is 
integrally formed with flanges 4. Grooves 5 are pressed into the surface 
of the plate 3. Acccording to the required power stages of the plates 
used, the grooves may be provided with a single heater filament 2 or a 
plurality of heater filaments 2 to be operated in series or parallel or as 
a single unit. 
The bottom of the groove 5 is provided with spaced through holes or 
slot-shaped cut-out portions 6, through which a convective air current is 
drawn by the heater filament 2 in heated state during operation, as is 
indicated by the arrows in FIG. 1. This convective air current cools the 
heater filament so that its temperature is about 30.degree. to 50.degree. 
l C. lower in comparison with that of an embodiment using the same power 
but not provided with the holes or cut-out portions 6. Between the 
windings of the heater filament 2 further holes or cut-out portions 7 are 
provided, through which the convective air current may pass downwards 
after having given off heat to the glass-ceramic plate 8. 
Furthermore, a bracket or a shell 9 is provided to support a 
heat-insulating layer 10 or a heat-insulating body. A radiation-reflecting 
sheet or foil 11, in particular an aluminium foil, is placed onto the 
heat-insulating body 10. The flanges 4 of the support member 1 are 
supported on one side by the lower side of the glass plate 8 and on the 
other side by the sheet or foil 11 or at the heat-insulating layer 10. 
In this manner a hollow space is created, which is divided by the plate 3 
into two approximately equally sized halves 12a and 12b. To achieve 
suitable temperature control the temperature within the hollow space 12a, 
for example, may be detected by means of a thermostat device 13 passing 
diagonally through the arrangement. 
As in this new arrangement the heater filament 2 is disposed at a distance 
from plate 8, the area of intersection of the radiation cone 14 with the 
plate 8 is correspondingly large, so that the heat is transmitted 
uniformly to the cooking vessel or a pan (not shown) placed onto the plate 
8 and undesired local overheating is avoided. Furthermore, this limits the 
thermal load on the plate 8. 
The major portion of the heat radiated away in downward direction from the 
lower side of the plate 3 is reflected by the sheet or foil 11. Herein the 
hollow space 12b has two very important functions: 
If the sheet or foil 11, for example an aluminium foil, were to be disposed 
directly at the lower side of the plate 3, then it would melt, because 
temperatures of up to 900.degree. C. may exist at this location. The 
hollow space 12b may be of such dimensions, that the heat reflecting sheet 
or foil 11 attains no temperature which might be higher than its softening 
temperature or even its melting temperature. Now convection of air 
accordingly takes place on the other side, because of the cut-out portions 
6, 7, thus providing a suitable balance. The convection currents may form 
in very differing manner, according to the temperature on the upper side 
of the plate. In general, the temperature at the plate 8 will be lower 
than the temperature at the surface of the sheet or foil 11, because of 
the contents of the cooking or frying vessels placed onto the hot plate. 
Air which has become cooler will accordingly pass downwards through the 
cut-out portions 7 and will cool the space 12b, whereafter it then again 
rises upwards through the other cut-out portions 6. This effect is 
particularly marked when the thermostat device 13 has switched off strands 
of the heater filament 2. Suitable comparison tests have shown that with 
the new embodiment the energy transfer to the contents of the cooking 
vessels is improved by more than 10% when compared with known embodiments. 
As the temperature of a heater filament having the same, usual dimensions 
may be lowered by about 50.degree. C. the operational life of the heater 
filament 2 is approximately doubled when compared with the heater 
filaments of other embodiments.