A HEATING DEVICE

Abstract: A heating device for food preparing purposes, primarly cooking pots and grill tables in large kitchens and the like includes electrical resistor elements adapted to supply thermal energy to a surface to be heated, such as the outer surface of a cooking pot and the underside of a grill surface or table. According to the invention the resistor elements (2-5) comprise resistor filaments in the form of an electrically conductive foil, which is vulcanized firmly to the aforementioned surface (12; 13) with the aid of a thin layer of a vulcanizable rubber material. In accordance with one embodiment, when the heating device is a cooking pot (1) the foil is applied to the peripheral surface (12) of the cooking pot (1) in the form of strip-like elements (23, 24, 25) which extend around the pot in superposed relationship from the bottom (13) thereof. An element (35) is also provided on the bottom (13) of the pot, and covers substantially the whole of the pot bottom.

Description: A heating device The present invention relates to a heating device preparing food, particularly in large kitchens and the li The vessels used to prepare food in large kitchens are themselves large in size. Thus, the cooking pots norm ly used have a volumetric capacity of several hundred lit Steam is the conventional mediumused to heat such large cooki pots. The pot has two walls which present therebetween e_sp into which steam is introduced, so as to be brought into contact with substantially the whole of the inner wall de fining the chamber in which food is prepared. Steam-heate cooking pots are encumbered with a number of disadvantage however. One serious problem is that the pot is not heated an even temperature. The highest temperature is obtained the upper part of the pot and the lowest temperature in t bottom part thereof. As a result, condensation collects a the bottom of the pot. This uneven temperature distributi is highly disadvantageous. An undesirably high temperatur is obtained particularly at that part of the pot located above the level of liquid therein. When the liquid in the pot boils, for example soup, the liquid will always splash or sputter to some extent. Because of the high temperature prevailing in the upper part of the pot, the liquid carrie in these splashes will bake onto the bare wall surfaces of the pot. This makes subsequent cleaning of the pot extreme difficult. The lower temperature at the bottom of the cooking pot creates in the liquid contained therein a flow pattern in which there is formed an upwardly moving stream immedia tely adjacent the vertical inner wall of the pot and a dow wardly moving stream immediately adjacent the upwardly moving stream radially inwards thereof. As a result hereof a central volume in the pot rema relatively stationary and is consequently heated to a lowe temperature than the remaining liquid. In addition, because of the temperature distribu¬ tion prevailing in such a cooking pot, it is difficult to keep small amounts of liquid hot. Another serious disadvantage encountered with steam- heated cooking pots is that it is difficult to control the temperature thereof. This difficulty is particularly mani¬ fested when a quick change in temperature is desired. A further problem is that the maximum level of power availabl is not always as high as would be desired in certain stages of food preparing processes, such as boiling-up large quan- ties of cold liquid, due to the fact that for different reasons a steam-heated cooking pot has not been designed to enable the energy input to be controlled over a sufficientl wide range. In order to avoid these problems endeavours ha been made to provide electrically heated cooking pots, in which the heat used is obtained through electrical resista ce elements in contact with the pot. These endeavours, how ever, have not been particularly successful, since it has not been possible to conduct the desired maximum power through the pot material and into the liquid with sufficie efficiency. This has caused the resistance element to burn out when applying desired maximum power. The problem is basically due to the fact that the ratio between the wall- surface area of the pot and i * ts volume is too low. As a result the heat-transfer surface available is too small in relation to the level of power desired to be introduced to the volume of liquid in the pot. It can be mentioned by wa of example that the desired power input of pots having a volumetric capacity of 300 liters is as high as 50 kW. The present invention fully solves the aforesaid problems and provides a heating device to which a high lev of power can be applied and with which the power input can be regulated unlimitedly throughout the whole power range. Thus, the present invention relates to a heating device for preparing food, primarily cooking pots and grilling plates or tables in large kitchens and the like, said device comprising electrical resistance elements adap to supply thermal energy to a surface to be heated, such a the outer surface of a cooking pot and the underside of a grill surface, and being characterized in that the resis¬ tance elements comprise resistance elements in the form of electrically conductive foil which is vulcanized firmly to the aforesaid surface with the aid of a thin layer of vulc nizable rubber material. The invention will now be described in more detail with reference to an embodiment thereof illustrated in the accompanying drawing, in which Figure 1 schematically illustrates a cooking pot; Figure 2 is a detail view of part of a heating element; Figure 3 is a sectional view in larger scale taken o the line A-A in Figure 2; Figure 4 is a block schematic illustrating electroni control apparatus; Figure 5 is a side view of the cooking pot illustra¬ ted in Figure 1; Figure 6 schematically illustrates a grill plate or table. The present invention is well suited for application with cooking pots of the aforedescribed kind. The invention however, can also be equally as well applied to grill plate or tables and other food-preparing devices where a high maximum power input is desired. Figure 1 illustrates a heating device in the form of a schematically illustrated cooking pot 1 , while Figure 6 illustrates a further example of a heating device in the form of a grill surface or table 2, seen from above. The heating device comprises electrical resistance elements 2-5; 3-11 adapted to supply thermal energy to the surface to be heated. The cooking pot 1 is a single-wall vessel and the elements 2-5 lie against its peripheral sur- face 12 and against the bottom 13 respectively. With regard to the grill table, the elements 6-11 are placed on the undersurface of the plates 14 of the table. In accordance with the invention the resistance elements 2-5; 6-11 comprise resistance filaments 15 in the form of an electrically conductive foil, which is vulca¬ nized firmly to the aforesaid surface with the aid of a thin layer 16 of a vulcanizable rubber material. The foil used is preferably a so-called Kanthal- foil produced by Bulten-Kanthal AB in Sweden. It will be understood that other makes of foil can also be used, however. The electrically conductive foil 17 comprises a slender electrical conductor 15 which extends backwards and forwards in a pattern such that the width of the foil excee the width of the conductor 15, cf Figure 2. Figure 3 illustrates in larger scale a section of a foil 15 taken on a line corresponding to the line A-A in Figure 2. As will be seen from Figure 3, the foil 17 is also provided with a vulcanized rubber layer 18 on the sid remote from said surface. The thin layer 16 of rubber material located betwee the foil 17 and said surface 12,13 has a thickness beneath 1 mm, preferably beneath 0.5 mm. The lower thickness limit is set with a view to the fact that the layer 16 shall for an electric insulation between the conductor 15 and the wa 12,13. The walls of the cooking pot are made of stainless steel. According to a preferred embodiment the layer 16 comprises vulcanizable silicone rubber. The aforementioned problems relating to the diffi¬ culty of supplying electrically a sufficiently high level of energy to the cooking device are solved fully by vulca¬ nizing the foil 17 firmly to the aforesaid wall 12,13 in accordance with the invention. It has been found that the thermal conductivity of vulcanized silicone rubber is sufficient to achieve power inputs of up to 10 W/c when transferring heat from the conductor 15 to the aforesaid wall. Full scale experiments have shown that when the wall 12,13 is cooled, for example by a liquid being heated the temperature of the outer sur¬ face 19 of the foil is only slightly increased when the 2 power input to the foil is about 5 W/cm . When a voltage is applied to the conductor 15 it is initially heated rapidly. During the steady state the cond tor will, of course, be hotter than the wall 12,13 of the pot. The temperature of the conductor 15 may reach to abou 300°C while the wall temperature of the pot is about 100°C or therebelow. The differences in linear expansion between the pot wall on the one hand and the conductor on the othe is partly taken-up in the vulcanized rubber layer 16. This ensures that complete abutment of the foil 17 with the wal 12,13 via the layer 16, is obtained irrespective of the differences in temperature. This is of the utmost importan It has been found that if this abutment is not complete, t conductor will burn-out immediately when subjected to the aforementioned high surface energies. By arranging resista elements in the aforedescribed manner in accordance with t invention, extremely high power inputs can be applied elec trically to a surface to be heated. The foil 17 is preferably applied to said surface i strip-like elements, as illustrated in Figure 1. The width of the strip-like elements is suitably from 5 to 20 cm. In the case of cooking pots, the foil is preferably mounted on the peripheral surface 12 of the pot in strip¬ like elements 23,24,25 extending in superimposed, relation¬ ship around the pot from the bottom 13 thereof. An element 35 is also mounted on the bottom 13 of the pot. The element 35 covers substantially the whole of the pot bottom. Electrical conductors 20,21,22,26 connect respective elements to a current source. In accordance with one preferred embodiment the heating device includes control means 27 arranged to delive separately the desired power to each of the strip-like ele- ments 23,24,25 or groups of elements, and the element 35 located on the pot bottom. This enables the power input, and therewith the temperature, to be varied at different parts of the pot wit o regard to the food-preparing process and the volume of liquid in the pot. When preparing soop, for example, the pot is filled to a level L2, cf. Figure 5, and all the elements, includi the element on the pot bottom, are energized. When a volume of liquid represented by the level L1 is to be boiled-up, only the element 25 and the bottom element 35 are energized. In this case those parts of the pot wall lying above the level L1 will be relatively cold, and hence splashing or spluttering liquid will not burn on the wall surface of the pot. When the liquid is simply to be kept warm, only the bottom element 35 is energized. The bottom element 35 is effective in achieving good agitation of the liquid in the pot and a uniform temperature through out said liquid, as opposed to steam-heated cooking pots. The aforedmentioned disadvantages encountered with steam-heated cooking pots are thus fully eliminated with a cooking pot according to the invention. Temperature sensing device, such as thermistors 28,29,30,31, are suitably placed adjacent each element 23,24,25,35 for sensing prevailing temperatures. In this respect, the thermistors 28-31 are connected to the afore¬ said control means 27. The control means 27 may be provide with control devices of a more or less sophisticated character. Preferably, there is connected to the control means 27 a microprocessor 32, containing a program by means of which it can calculate a suitable power and suitable set- point values for the temperatures of the various elements 24,25,35 in dependence on the desired mean temperature of the liquid, the volume of liquid in the pot etc.. The thermistor will then serve as real-value sensors. The microprocessor 32 includes a keyboard 33, on which the desired temperature, volume cooking time, etc. can be set, and a display 34 showing the information in¬ serted into the microprocessor. With regard to a grill surface or table, the foil 17 is vulcanized to the underside of the plates 14, in different regions 6-11. This enables an extremely high lev of power to be applied to one or more regions to achieve desired heating of theplates, and the power supplied to various parts of the plate can be regulated individually in use, in a manner corresponding to that described with reference to a cooking pot. 2 In the aforegoing a power of 10 W/cm has been ment ned with respect to the powers which can be supplied. In the case of a cooking pot having a volumetric capacity of 300 liters the maximum power input is suitably about 20-30 kW with respect to the bottom element 35 of th pot and about 10-15 kW in respect of each element 23,24,25 The elements 35 located on the bottom of the pot wi at maximum, withstand a slightly higher surface charge effect than the elements 23,24,25 on the peripheral surfac of the pot, due to the fact that heat is transferred more effectively to the liquid at the bottom of the pot than at the peripheral surfaces thereof. Various embodiments have been described in the afor going. It will be understood by those skilled in this art that modifications can be carried out without departing fr the concept of theinvention. Thus, the present invention shall not be considered to be restricted to the aforedescribed embodiments, but that modifications can be made within the scope of the following claims.

Claims: CLAIMS 1. A heating device for preparing food, primarily cooking pots and grill surfaces in large kitchens and the like, comprising electrical resistance elements adapted to supply thermal energy to a surface to be heated, such as the outer surface of a cooking pot and the underside of a grill table, characterized in that the resistor elements (2-5; 6-11) comprise a resistance filament (15) in the form of an electrically conductive foil (17), which has been vulcanized firmly to said surface (12; 13) with the aid of a thin layer (16) of a vulcanizable rubber material 2. A heating device according to Claim 1 , charac¬ terized in that the electrically conductive foil (17) comp ses a thin electrical conductor (15) which extends back¬ wards and forwards in a pattern such that the width of the foil exceeds the- width of the conductor, said foil (17) also being provided with a vulcanized rubber layer (18) on the side remote from said surface (12; 13). 3. A heating device according to Claim 1 or 2, characterized in that said thin layer (16) of rubber mater between the foil (17) and said surface (12;13) has a hick of beneath 1 mm, preferably beneath 0.5 mm. 4. A heating device according to Claim 1,2 or 3, characterized in that the rubber material between the foil (17) and said surface (12; 13) comprises a vulcanizable rubber layer. 5. A heating device according to Claim 1,2,3 or 4, characterized in that said foil (17) is mounted to said surface (12; 13) as a strip-like element having a width of about 5 cm to 20 cm. 6. A heating device according to Claim 1,2,3,4 or 5 when said device has the form of a cooking pot (1) charac¬ terized inthat said foil (17) is applied to the peripheral surface (12) of the cooking pot (1) in the form of strip-l elements (23,24,25) which extend in superposed relationshi around the pot from the bottom thereof; and in that an element (35) is mounted on the bottom (13) of the pot so as to substantially cover the whole of said bottom. 7. A heating device according to Claim 5 or 6, characterized by control means adapted to control individu ly each of the said strip-like elements (23,24,25) or grou of strip-like elements and an element (35) mounted on the bottom (13) of the pot (1).

Inventors: HERMANSSON JAN, JANSSON HAOKAN, HERMANSSON, JAN, JANSSON, HAOKAN
Applicants: KOPAL FOERSAELJNINGS AB
Application year: 1985
Main classification: H05B3
IPC codes: A47J27, H05B3