EP0300548B1 - Radiant heater for cooking devices - Google Patents

Abstract

In known radiant heaters, the direct radiation emerging from halogen lamps and the radiation reflected via a reflector lead to an inhomogeneous radiation intensity on the cooking plate. In order to achieve an essentially homogeneous radiation intensity on the cooking plate (2) a reflector (14) is assigned to each halogen lamp (8), the reflector (14) having two regions (15, 16) which are designed essentially in the form of cylindrical sections similar to parabolas. The radiant heater permits energy-saving cooking with a reduced temperature stress. <IMAGE>

Description

The invention relates to a radiant heater for cooking appliances with a hotplate designed in particular as a glass ceramic plate, a concave reflector arranged at a distance from the hotplate and at least one halogen lamp arranged between the hotplate and the reflector, as for example from EP-A1-0 176 063 or known from EP-A1-0 176 027.

Such a radiant heater is e.g. through GB 2 154 405 A become known. This known design shows a radiant heater for cooking appliances, one or more rod-shaped halogen lamps being assigned to the concave reflector in order to achieve a uniform distribution of the radiation intensity, the filaments of which may have a gradient which varies over their length.

From DE-OS 1 565 798 an electric radiant heater has become known, to which two concave reflectors are assigned in such a way that they abut one another below the radiant heater to form a folded edge drawn up in the direction of this radiant heater. The rays coming from the heating element are thus reflected back past the heating element. The embodiment described in this document is an electric radiant heater for heating the washing drum of a washing machine. The raised crease edge of the reflector ensures that the rays emanating from the side of the radiator facing away from the drum are reflected in such a way that they hit the drum surface.

GB 2 137 060 A has disclosed a radiant heating cooking device in which the hotplate is opaque to light wavelengths (0.3 to 0.7 µm) and transparent to wavelengths of approximately 1 to 3 µm. This known design shows a parabolic mirror shape with a lamp arranged in the focal point.

From DE-AS 2 205 132 an electric cooking device with an upper plate made of highly heat-resistant glass-like material has become known, the plate having a plurality of heated hot plates which are formed by heating units attached below the plate. The glass ceramic plate forms the actual hot plate of the electric cooking device. The heating units contain tubular heating elements that are pressed onto the plate with a flattened side. The heating unit has a carrier shell, which consists of a heat-conducting, relatively thick-walled material, such as cast aluminum. The inner surface of the carrier shell is designed to be reflective, for example, by a corresponding surface treatment, while the outer surface is roughened and darkened, for example by coloring or anodizing. The outer surface should be as large as possible and have the largest possible radiation absorption ratio or a small reflection ratio. The portion of the total heat that flows away via the relatively large flattening and the glass ceramic plate as a result of heat conduction is very large. The jacket of the heating elements helps to conduct the heat flowing from the heating wire to the underside upwards, ie into the area of the flattening. The heat then radiated from the tubular heaters is then radiated back from the reflecting inner surface of the carrier shell. In this known design, the carrier shell thus essentially serves as Thermal insulation system, but not as an energy control system. The heat transfer from the heating elements to the heating plate takes place primarily through heat conduction between the tubular heating elements and the heating plate. Only residual heat radiation is radiated back through the carrier shell.

The object of the invention is to simplify and improve a radiant heater mentioned at the outset with regard to the homogeneity of the radiation distribution, the heat distribution and the energy yield.

• a) jeder Halogenlampe zwei im wesentlichen in Form parabelähnlicher Zylinderabschnitte ausgebildete Bereiche des Reflektors zugeordnet sind, die unter Bildung einer in Richtung auf die Halogenlampe hochgezogenen Knickkante aneinanderstoßen,
• b) die Kochplatte für eine Strahlung im Wellenlängenbereich von sichtbarem Licht bis mindestens 4 µm im wesentlichen durchlässig ist,
• c) der Reflektor aus Aluminium besteht.

This object is achieved according to the invention in that

• a) each halogen lamp is assigned two regions of the reflector which are essentially in the form of parabolic-like cylinder sections and which abut one another to form a bent edge which is drawn up in the direction of the halogen lamp,
• b) the hotplate is essentially transparent to radiation in the wavelength range from visible light to at least 4 μm,
• c) the reflector is made of aluminum.

The design of the reflector according to the invention enables angles of incidence which are favorable for the reflection, by means of which the radiation emanating from the halogen lamp and incident on the reflector is essentially reflected in the regions of the hotplate which are further away from the halogen lamp. The parabolic-like cylinder sections can have parabolic sections or sections formed from one or more additional elements of a higher order.

The reflected radiation thus affects those parts of the hotplate that are increasing due to the Distance from the halogen lamp can only be acted upon by a small proportion of the light emitted directly from the halogen lamp. This results in an essentially homogeneous radiation distribution over the hotplate and local overheating of the hotplate is avoided. The elimination of overheated areas of the hotplate also means that excessive heating of the space underneath the hotplate, including the reflector, is avoided. As a result, the use of complex components that are also resistant to very high temperatures is no longer necessary. This means that a simple, highly reflective aluminum reflector can be used, which could not withstand the heat load in the known radiant heaters.

The shape of the mirror with the raised crease edge results in angles of incidence such that the proportion of the radiation reflected on the halogen lamp, which can lead to overheating and thus impairment of the lamp life, is greatly reduced.

The fact that the hotplate is essentially transparent to radiation in the wavelength range from visible light to at least 4 microns results in a high proportion of the rays directly incident on the bottom of the vessel, and at the same time the undesired high heating of the hotplate is reduced. The food to be cooked is therefore mainly due to absorption of the radiation from the halogen lamp by the vessel or, in the case of transparent vessels, directly by absorption by the food itself. Since the heat transfer is essentially not caused by heat conduction between the hotplate and the bottom of the vessel or over an air gap between the hotplate and the bottom of the vessel, there are no special requirements for the flatness of the bottom of the vessel. Due to the essentially direct heating of the vessel or the food to be cooked by radiation emitted by the halogen lamp, the start of cooking takes place almost without inertia. Furthermore, due to the low radiation absorption by the hotplate, the residual heat that can be emitted by the hotplate after switching off the radiant heater is low, so that there is a comparatively small increase in the temperature of the cooked product after switching off the radiant heater.

The above-mentioned only moderate heating of the lamp room enables the use of an aluminum reflector with a limit temperature of approx. 420 °. This has the great advantage that the exceptionally high reflectivity of aluminum can now also be used in a radiant heater.

In the event that for some reason an undesirable temperature increase occurs in the lamp room, e.g. when using highly reflective pots, in one embodiment of the invention, a temperature sensor is arranged on the back of the reflector. When a predeterminable temperature is reached, the output of the halogen lamp can thus be reduced or switched off completely. This results in effective protection, not only for the reflector, but for the entire radiant heater, since the heating of the reflector results from the amount of heat radiated by the halogen lamp and the hotplate.

In a preferred embodiment, two halogen lamps are arranged on a hotplate, and the reflector is symmetrical to a central plane lying between the two halogen lamps. This results in a very homogeneous radiation distribution on the hotplate. The symmetrical design also results in a homogeneity in the middle area between the two halogen lamps. The regions arranged in one half of the reflector can have different cross-sectional shapes. The cross-sectional shapes of the areas adjoining each other on the plane of symmetry can be determined depending on the distance between the halogen lamps so that the most homogeneous radiation distribution results in the central area.

In an embodiment of the invention, the aluminum reflector has a coating of higher emissivity on its rear side. The resulting increased radiation prevents the reflector from overheating, so that the limit temperature of aluminum, which is approximately 420 °, is not exceeded.

In a further embodiment of the invention, the aluminum reflector is formed in one piece. Such reflectors are thus easy to manufacture and can be easily installed or removed in the radiant heater.

In a further embodiment of the invention, a blower device is assigned to the rear of the reflector, which can also be controllable via a temperature sensor. The reflector can thus be cooled in a simple manner and thus protected against overheating.

The maximum distance between the hotplate and reflector is 20 mm, for example. This leads to a low overall height of the radiant heater and enables its installation in Low-height parts, such as worktops of kitchen equipment or appliances.

In a further embodiment of the invention, the hotplate is essentially opaque to radiation in the range of visible light, so that the proportion of the disturbing visible light emission is reduced with somewhat reduced permeability for the total radiation of the halogen lamp.

A further embodiment of the invention is that one or more halogen lamps of a radiant heater have a filament with an incline that varies over its length. As a result, the homogeneity of the radiation directed onto the hotplate can be increased over the length of the halogen lamp and thus of the radiant heater.

An embodiment of a radiant heater according to the invention is explained with further details with reference to the drawing.

• Fig. 1 einen teilweise geschnittenen, in ein Herdoberteil eingesetzten Strahlheizkörper in perspektivischer Darstellung;
• Fig. 2 einen Querschnitt in schematischer Darstellung durch eine Hälfte des Strahlheizkörpers nach Fig. 1.
• Fig. 3 die Richtung direkter und reflektierter Strahlen bei einem Reflektor nach Fig. 1 und 2 sowie die sich ergebende qualitative Verteilung der Strahlungsintensität auf einer Kochplatte.

Show it:

• Figure 1 is a partially sectioned, in a stove top radiant heater used in perspective.
• FIG. 2 shows a cross section in a schematic representation through half of the radiant heater according to FIG. 1.
• 3 shows the direction of direct and reflected rays in the case of a reflector according to FIGS. 1 and 2 as well as the resulting qualitative distribution of the radiation intensity on a hotplate.

The radiant heater shown in FIG. 1, designated as a whole by 1, is arranged below a hotplate 2 in a housing part 3, for example a cooker (not shown). The radiant heater 1 has a housing 4, not shown, which can be connected to the housing part 3 and has a base plate 5, from which an annular wall 6 extends in the direction of the hotplate 2, which at its end opposite the base plate 5 into an annular end wall 7 passes. The end wall 7 lies essentially in a plane parallel to the one containing the base plate 5. The housing 4 consisting of the base plate 5, the annular wall 6 and the annular end wall 7 can be produced in one piece, for example as a sheet metal part.

To accommodate halogen lamps 8 are in opposite areas of the wall 6 to the annular End wall 7 open slot-shaped recesses 9 formed. In the illustrated embodiment, two halogen lamps 8 are arranged parallel to one another at a distance. However, it is also possible in the same way to construct a radiant heater according to the invention with only one halogen lamp or with a larger number of halogen lamps, which can be designed as linear halogen lamps or those with a curved shape. The halogen spotlights used are preferably optimized for maximum energy emission by radiation in the visible light range down to a wavelength of about 4 µm. This is due to the use of anhydrous quartz crystals, suitable inert gas fillings to reduce gas heat conduction losses and to increase the service life at high burning temperatures and Halogen filling to prevent blackening. Around 90% of the electrical power consumed can be converted into direct radiation.

For the mechanical fastening of the halogen lamps 8 and their electrical connection, 9 connection blocks 10 are connected to the annular wall 6 in the region of the elongated recesses, of which only one connection block 10 is shown in FIG. 1 for the sake of simplicity. The connection block 10, which is connected to the annular wall 6 in a manner not shown, has a slot-shaped recess 11 which is open towards the annular end wall 7 and into which a contact part 12 of the halogen lamp 8 can be inserted. An electrical connection to an electrical connection line 13 can be produced via the contact part 12 and the connection block 10 in a manner not shown.

Below the halogen lamps 8, a reflector 14 can be connected to the housing 4 and is assigned to each halogen lamp 8 has two regions 15, 16, which essentially have the shape of parabolic cylinder sections. At the edge regions of its long sides, the reflector 14 has flange-like projections 17 and one or more support ribs 18 extend from the reflector for support on the base plate 5. The annular space between the annular end wall 7 and the underlying annular surface of the base plate 5 is included an insulation 19 filled, which has a recess adjacent to the annular end wall 7. Between the recess and the overlying part of the annular end wall 7, a receptacle 20 is formed for one of the flange-like projections 17.

gebildet. Die konstanten Werte für die Bereiche 15, 16 sind dabei yo = -17,3 mm, $\text{A = 1,544 x 10⁻²mm⁻¹}$

, xo = 31,0 mm bzw. yo = -17,5 mm, $\text{A = 7,848 x 10⁻³mm⁻¹}$

und xo = 39,5 mm. Die angegebenen beispielhaften Werte haben günstige Reflektionseigenschaften für eine Halogenlampe mit einem Durchmesser von etwa 12 mm ergeben, deren nicht dargestellte Heizwendel in dem in Fig. 2 eingezeichneten Koordinatensystem die Lage xw = 36 mm, yw = -10 mm, aufweist. Abhängig von dem Durchmesser der eingesetzten Halogenlampen kann es zur Erzielung einer besonders günstigen Reflektionswirkung vorteilhaft sein, daß die parabelähnlichen Zylinderabschnitte gemäß einer, um mindestens ein Glied höherer Ordnung erweiterten Funktion gebildet sind.In the part of the reflector 14 shown schematically and enlarged in FIG. 2 and assigned to a halogen lamp 8, the two parabolic cross-sectional areas 15, 16 are parabolic areas according to the relationship $\text{y = yo + A (x - xo) ²}$

educated. The constant values for the areas 15, 16 are yo = -17.3 mm, $\text{A = 1.544 x 10⁻²mm⁻¹}$

, xo = 31.0 mm or yo = -17.5 mm, $\text{A = 7.848 x 10⁻³mm⁻¹}$

and xo = 39.5 mm. The exemplary values given have given favorable reflection properties for a halogen lamp with a diameter of approximately 12 mm, the heating filament of which is not shown has the position xw = 36 mm, yw = -10 mm in the coordinate system shown in FIG. 2. Depending on the diameter of the halogen lamps used, it may be advantageous to achieve a particularly favorable reflection effect that the parabolic-like cylinder sections are formed according to a function which is expanded by at least one higher-order element.

The adjoining ends 21 ', 21˝ of both areas 15, 16 have the smallest distance to the halo gene lamp 8 and run substantially parallel to the central axis. The transition between the ends 21 ', 21˝ can be formed in a manner not shown, for example. By a narrow area of convex curvature. With integrally formed reflectors, the ends 21 ', 21' can also merge into one another in the form of a folded edge.

For the reflector 14, the beam paths 22, 23 shown in FIG. 3 result for direct radiation directed from the halogen lamp 8 onto the hotplate 2 or the radiation reflected via the reflector 14 onto the hotplate 2. The shape of the reflector 14 results in the somewhat simplified, substantially homogeneous intensity 24 of the radiation directed onto the hotplate 2, which is plotted somewhat simplified in FIG. 3. The essentially homogeneous radiation intensity 24 is achieved because the rays 23 reflected by the reflector 14 are predominantly reflected in areas of the hotplate 2 which are distant from the halogen lamp 8 and are therefore only slightly exposed to direct radiation 22. Characterized in that the ends 21 ', 21˝ of both areas 15, 16 adjoin each other so that the distance of the reflector 14 from the halogen lamp 8 at the ends 21', 21˝ is smallest, a reflection of rays in the direction continues Avoided on the halogen lamp 8 and thus an inadmissible heating reducing its lifespan.

In order to increase the homogeneity of the radiation 22, 23 incident on the hotplate 2 over the entire length of the radiant heater 1, the filament pitch of the halogen lamps 8 can be varied over their length in a manner not shown. It has proven advantageous that the slope in the middle third of the Halogen lamps 8 is about 20% larger than in the end thirds that follow on both sides. Areas of high thermal load on the hotplate 2 are avoided by the essentially homogeneous radiation distribution 24.

The glass ceramic material for the hotplate 2 is such that it is transparent to radiation in the wavelength range from approximately visible light to 4 μm. It is thus avoided that a substantial amount of heat absorbed radiation energy is stored in the hotplate 2. On the one hand, this results in an essentially inertia-free start of cooking and, on the other hand, it avoids the fact that after switching off the radiant heater 1, heat can still be transferred to the food to a significant extent. Cooking material is thus heated predominantly by absorption of the radiation emitted by the halogen lamps 8 in the vessels containing the cooking material or, if these vessels are transparent, directly by absorption in the cooking material. In contrast to heating the vessel essentially through heat transfer, requirements regarding the flatness of the vessel bottoms therefore do not have to be made.

Since the amount of heat absorbed by the hotplate 2 is small, the emission from the hotplate 2 in the direction of the reflector 14 is likewise low. Because of the reduced heating of the reflector 14 compared to conventional radiant heaters, pure aluminum or specially anodized aluminum can be used, for example, to achieve a highly reflective surface. In order to further ensure that a permissible maximum temperature of the reflector 14, for example 450 ° C. in the case of aluminum, is not exceeded, the side of the reflector 14 facing away from the halogen lamp 8 can be covered with a coating Emission levels are provided, and this side of the reflector can also be cooled by a blower device, not shown. In a manner also not shown, that side of the reflector 14 which faces away from the halogen lamp 8 can be assigned a temperature sensor which, when a predeterminable temperature is reached, throttles the power of the halogen lamp 8 or switches it off entirely. This results in an effective overload protection, not only for the reflector 14 but for the entire radiant heater 1 and the hotplate 2, since the heat absorption by the reflector 14 is determined by the power of the halogen lamps 8 and the amount of heat absorbed by the hotplate 2.

The radiant heater can be designed with a small distance 25 between the hotplate 2 and the reflector 14 of approximately 20 mm and can be used in parts of low overall height such as worktops of kitchen equipment or appliances due to the small space requirement.

Claims (10)

1. Radiant heating element for cooking appliances with a cooking plate (2) designed especially as a glass ceramic plate, a concave reflector (14) arranged at a distance from the cooking plate (2) and at least one halogen lamp (8) arranged between the cooking plate (2) and the reflector,
   characterised in that,

   a) two areas (15,16) of the reflector (14) adjoin each halogen lamp, designed essentially in the shape of parabola-like cylindrical sections, which abut by forming a buckling edge (21', 21'') drawn up in the direction of the halogen lamp,

   b) that the cooking plate (2) is essentially transparent for radiation in the wavelength range from visible light to at least 4 µm,

   c) that the reflector is made of aluminium.
2. Radiant heating element according to claim 1, characterised in that two halogen lamps (8) are arranged on a cooking plate (2) and that the reflector (14) is designed symmetrically.
3. Radiant heating element according to claim 1 or 2, characterised in that the reflector (14) has on the side facing the associated halogen lamp a highly reflective surface and on the opposite side a coating of a higher degree of emission.
4. Radiant heating element according to one of claims 1 to 3 characterised in that the reflector (14) is designed in one piece.
5. Radiant heating element according to one of claims 1 to 4, characterised in that a temperature sensor is arranged on the side of the reflector (14) away from the halogen lamp (8).
6. Radiant heating element according to one of claims 1 to 5, characterised in that a blowing device adjoins the side of the reflector (14) away from the halogen lamp (8).
7. Radiant heating element according to one of claims 1 to 6, characterised in that the cooking plate (2) is essentially non transparent for radiation in the area of the visible light.
8. Radiant heating element according to one of claims 1 to 7, characterised in that one or more halogen lamps (8) have a coil with varied pitch over its length.
9. Radiant heating element according to one of claims 1 to 8 characterised in that the reflector (14) is made of extra-pure aluminium.
10. Radiant heating element according to one of claims 1 to 8 characterised in that the reflector (14) is made of specially anodised aluminium.

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