A HIGH FREQUENCY HEATING APPARATUS WITH ELECTRIC HEATING DEVICE

Abstract: A high frequency heating apparatus (1) wherein oven heating and grill heating are performed by means of a flat heating element (14b), and no heater and projections such as an insulator for holding a heater are provided, whereby, when high frequency is supplied with a heating chamber with an oven plate placed in the heating chamber, abnormal heating such as local heating and sparks do not occur. Also, with the oven plate (10) placed in the heating chamber, heating can be performed by means of a high frequency and a heater which can be used alternately or in combination depending on situations to obtain the most preferable heating condition.

Description: The invention relates to a heating apparatus for heating with combined radiant heat and/or microwaves, comprising: a microwave-tight heating chamber for accomodating food material to be heated, and having oven plate holding means on sidewalls spaced downwardly from an upper wall; and one chamber sidewall having a microwave supply opening therein between said holding means and said upper wall to which microwaves of a frequency oscillation device are fed into said heating chamber. It is known to provide a heating apparatus with two methods for heating food material in a heating chamber. One method is to heat the food material by heat energy generated from an electric heating device provided near the ceiling and the bottom of the heating chamber, whereas the other method is to directly irradiate the food material in the heating chamber by high frequency energy. A selection between the two methods is made depending on the type of food material and the cooking menue. Constructions of such conventional heating apparatuses are shown in Figs. 1 to 5. Fig. 1 is a cross-sectional view of a conventional high frequency heating apparatus with an electric heater, comprising a main body 1 provided with a heating chamber 2 which accommodates food material. Provided within the heating chamber 2 are a food material placing plate 3, a turn table 4 for preventing non-uniform heating of food material, an oven heater 5a, and an oven and grill heater 5b, disposed at the bottom and the ceiling of heating chamber 2 for heating food material by raising the temperature of the heating chamber 2 and for performing oven and grill heating by means of radiant energy. Also provided in heating chamber 2 are high frequency oscillation devices, such as a magnetron 6 and a high voltage transformer 7. High frequency is introduced into the heating chamber 2 through an opening portion 9 provided at a portion in heating chamber 2, and conducted by a waveguide 8. The methods for heating food material by means of a high frequency heating apparatus with a heating device include a method using only high frequency, a method using high frequency and heaters 5a and 5b alternately, and a method using only heaters 5a and 5b. Thus, a preferable heating condition is obtained by selecting a heating method suitable for food material and a cooking menu. When the food material is heated only by heaters 5a and 5b, as shown in Fig. 2, the food material placed on an oven plate 10 is also heated with the oven plate 10 made of metal and held approximately at the middle portion of heating chamber 2 by rails 11 mounted the sidewalls of heating chamber 2. Several rails 11 for holding the oven plate are longitudinally positioned stepwisely at the sidewalls of heating chamber 2 to permit variation of the position of oven plate 10 so that food material can be heated most preferably depending on its size and heating method employed. The output from heaters 5a and 5b can be adjusted to obtain a preferable heating condition in this heating apparatus. In the case of oven and grill heating using heaters 5a and 5b, heating is performed after the temperature in heating chamber 2 has been raised. Therefore, preheating must be performed in heating chamber 2, and the bulk temperature of food material is raised by high frequency with the material placed on the plate 3 and, thereafter, the material is placed on the oven plate 10 to heat by heaters 5a and 5b in order to obtain a preferable condition by raising the temperature of heating chamber 2 and of the material. Thus, the heating operation according to this method, is very troublesome. Furthermore, long time period is required to heat the material using only heaters, thus wasting energy. Apparently, according to this method, the essential characteristics of a high frequency heating apparatus cannot be attained, i.e., the heating operation is not easy, and high speed heating and energy saving cannot be accomplished. Heating of food material can be performed with the material placed on the plate 3 after heating by high frequency, but the advantage obtained by this method only saves the labor of shifting the material from the plate 3 to the oven plate 10. Furthermore, the time required for heating the material by this method is not so different from the time required when material heating is performed using only heaters 5a and 5b, and the amount of the material which can be cooked by one heating in this method is about 50% less than the amount possible when heating is performed using only heaters. For example, heating gratin for four men in this method must be performed twice. More specifically, the heating chamber 2 is usually rectangular in plan view, and the oven plate 10 is almost as large as heating chamber 2 so that heating chamber 2 is effectively used, however, the plate 3 is round in plan view and must be rotated to prevent high frequency from non-uniformly heating the material, and, thus, the quantity of material which can absorb heat considerably decrease. As shown in Fig. 3, it is supposed that easy operation, quick heating, and energy-savings can be accomplished when heating is performed by using a method of irradiating high frequency to material with the oven plate 10 placed in the heating chamber 2 and thereafter, heating the material by heaters 5a and 5b. But, in practice, the heating chamber 2 is partitioned by the oven plate 10, so that the heating chamber 2 is small compared with the quantity of high frequency oscillated by the magnetron, and the electric field becomes very turbulent, which could cause, during high frequency material heating, the material to be non-uniformly heated, abnormal heating in the heater 5b and insulator 12, sparks in the heater 5b because the electric field converges on projections such as the heater 5b and insulator 12 which holds heater 5b, during high frequency heating, and increased electric wave leakage generated by the heater 5b in heating chamber 2. Thus, heating according to this method is very dangerous. Food material heating by the heaters is also disadvantageous because the material is non-uniformly heated when the heater 5b is positioned too close to the material, thus preventing fast heating. Therefore, safe, easy, fast, and energy efficient heating is very difficult to accomplish. This is particularly true for a grill heater. In a grill heater using radiant heat generated by heater 5b, there are two methods of heating food material depending on the material; one method is to heat the material by means of only heater 5b provided close to the ceiling, and the other one is where heater 5b is mainly auxiliary driven by heater 5a to raise the bulk temperature of the material so that a preferable heating condition can be obtained. But, in this method, of course, the material is mainly heated by heater 5b. In this method, when the quantity of food material and generated heat are constant, heating time periods and energy quantity required for heating the material are determined by the distance between the heater 5b and food material, the volume of the heating chamber partitioned by material placing plate 10 held by the sidewalls of heating chamber, the difference of radiant energy when a certain quantity of heat is generated by heater 5b, and a means for preventing heat energy from escaping from heating chamber 2. The time period required for heating is also important to obtain a preferable heating condition especially when grill heating is performed, so that the smaller a heating space is, and the higher heat temperature generated by heater 5b is, and the larger temperature retaining efficiency of the heating space is, the more effectively and uniformly food material is heated. Therefore, the position of rails 11 for holding material placing plate 10, the insulation material 13 provided at the outside of heating chamber ceiling, and the heat temperature raised by heater 5b are commonly designed to obtain a speedy, energy efficient, and preferable heating condition. A conventional heater 5b generally employs a sheathed heater whose configuration is shown in Fig. 4. As seen from the drawing, this configuration aims mainly at preventing various food materials from non-uniformly heating during oven heating. When heating is performed by placing food material adjacently to heater 5b to make fast, energy efficient heating, the material is scorched, as shown with scorch 14, in a short period of time at the portion close to the sheathed heater, and the material is not heated enough at the portion distant from the sheathed heater. Accordingly, when a sheathed heater 5b is placed close to the material, the material is very non-uniformly heated. When the material is placed distant from heater 5b to reduce the possibility of non-uniform heating of the material, radiant heat energy quantity per unit area is reduced, and a heating space is increased to a great extent. Consequently, fast, energy efficient heating cannot be accomplished, and in addition, heating is performed at low temperatures and the time period necessary for the heating is long. This is disadvantageous because grill heating must be performed after quickly raising the temperature of food material, resulting in that the surface of the material (food) becomes dry. When food material is heated by high frequency, as shown in Fig. 1, the heating is performed with turn table 4, on which the material is placed and rotated to reduce the possibility of non-uniform heating of the material. Likewise, when food material is heated by an electric heater, as shown in Fig. 2, heating is performed with the material placed on the oven plate 10 which is placed on rail 11 mounted on the sidewalls of heating chamber 2. At this time, heating is performed by two heaters 5a and 5b mounted on the bottom and the ceiling of heating chamber 2 to heat the material from the top and bottom thereof. Heaters 5a and 5b mounted on the bottom of heating chamber 2, are round, thereby solving the problem where heat generated by the heater 5a is collected under the turn table 4 and leads to the reduction of heat efficiency. But this method has a disadvantage because the heated degree of the center bottom portion of the material is much smaller than the other portions, hence, non-uniform heating because convection currents and radiant heat generated by heater 5a flow only around the periphery of turn table 4. From US-A-35 36 878 a microwave energy heating apparatus is known in which the high frequency energy is coupled to a cooking chamber. This publication mainly deals with the coupling structure to effectively feeding the microwave energy into the cooking chamber. The apparatus neither has an oven plate nor an oven plate holding means on the sidewalls of the heating chamber, but only provides a turn table at the bottom. A heating element is placed at the upper part of the heating chamber for additionally raising the temperature within the oven. From US-A-44 55 319 an apparatus is known which only uses radiant heat to evenly heat food placed in a heating chamber. The heating elements are in the form of heating tubes embedded either within a cast aluminium body forming the walls of the heating chamber, or in the form of a sandwich steel sheet construction also forming the walls of the chamber. There are no provisions for introducing microwave energy into the heating chamber, and accordingly, the heating chamber is not made microwave-tight. It is an object of the present invention to solve the problems described above and to provide a high frequency heating apparatus using microwave energy as well as radiant heat generated by heaters, and which is optimal in its construction, heating operation and efficient heating. The heating apparatus according to the invention is characterized by a) a flat radiant heat supplying heating element mounted against the outside of the metal upper wall of said heating chamber for supplying heat to the metal of said upper wall for raising the temperarture of the entire area of the upper wall for causing radiant heat to be emitted from the metal upper wall at temperatures up to food grilling temperatures, the upper part of the heating chamber above the oven plate holding means constituting a smooth area without any protruding parts disturbing microwave heating and b) the sidewall of said heating chamber opposite said microwave supply opening and above said oven plate holding means having an outwardly bulged portion for increasing the volume of the portion of the heating chamber above said oven plate holding means, whereby when an oven plate is placed on said holding means, it defines a smaller upper portion of the heating chamber in which the increased volume of the bulged portion better accomodates the microwaves. Preferable embodiments are defined in the dependent claims. These and other features of the present invention will become apparent from the following description taken in conjunction with a preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a conventional high frequency heating apparatus with a heater, as already referred above; Fig. 2 is a similar view of the apparatus during oven heating; Fig. 3 is a similar view of the apparatus in which oven heating is performed by high frequencies; Fig. 4, (a) and (b), are a perspective view showing a heating condition of food material and a heater in a conventional high frequency heating apparatus with a heater when grill heating is performed with food material placed adjacently to a heater; Fig. 5 is a perspective view showing a lower heater to be provided in a conventional high frequency heating apparatus having a turn table at the bottom of a heating chamber; Figs. 6 through 8 are cross-sectional views of a high frequency heating apparatus with electric heating device in accordance with an embodiment of the invention; Fig. 9 is an exploded view of a flat heating element to be employed in the apparatus of the invention; Fig. 10 is a perspective view of a turn table to be employed in the apparatus of the invention; and Fig. 11 is a schematic diagram showing the relationship between heating conditions and heating speed of a conventional apparatus and an apparatus of the invention. Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. The first embodiment of the invention will be described with reference to the accompanying drawings of Figs. 6 through 10. A main body 1 of a high frequency heating apparatus with a heating device is provided with a heating chamber 2 for accommodating food material therein. The heating chamber 2 is provided therein with a pair of flat heating elements 14a and 14b at the bottom and ceiling thereof. Main body 1 is provided with a magnetron 6 for generating electric wave of high frequency, and a high voltage transformer 7 for oscillating high frequency. The high frequency oscillated by these devices 6, 7 and conducted to waveguide 8 is introduced into heating chamber 2 a high frequency supply opening 9 provided at a portion on the sidewall of heating chamber 2, positioned between a flat heating element 14b mounted on the upper portion of heating chamber 2 and a rail 11 provided on the sidewall of heating chamber 2 for serving as a means to hold an oven plate 10. As shown in Fig. 6, above the oven plate 10 placed on the rail 11 in heating chamber 2, there is provided with a bulge portion 15 at the sidewall of the heating chamber 2 opposite to high frequency supply opening 9 in such a direction as to enlarge the heating chamber 2. The bulge portion 15 is provided to eliminate non-uniform heating of food material and to improve irregularities of high frequency caused by drastic changes of impedance depending on load imparted to magnetron 6. With this arrangement, the high frequency to be supplied into the heating chamber 2, with placing the oven plate 10 on the rail 11 of heating chamber 2, never causes heating abnormalities and sparks which may occur in a conventional high frequency supply oscillation apparatus because of the convergence of electric field at projections such as a heater and an insulation, and electric wave leakage generated by the heater 5b from the heating chamber 2 of Fig. 1. Further, effective high frequency heating can be accomplished within the heating chamber 2 because preferable distribution characteristics and matching high frequencies are performed by the provision of bulge portion 15, with oven plate 10 placed in heating chamber 2. In addition, as shown in Fig. 7, fast energy-efficient heating is easily obtained because grill heating is performed by a flat heating element 14b after raising the bulk temperature of food material by means of effective high frequency heating. At this time, the food material is uniformly heated because the flat heating element 14b generates heat over the entire surface of food material. Therefore, food material can be preferably placed adjacent to flat heating element 14b. Because of a reduced heating space 16, heating energy is not wasted and high temperatures can be very quickly obtained in heating space 16. According to the apparatus of the invention, the disadvantages of a conventional high frequency heating apparatus that include the lack of a preferable heating condition because of the long heating time period can be eliminated. The bulge portion 15, shown in Fig. 8, serves as a means for improving non-uniform heating of the food material and non-preferable impedance matching which occur in the heating chamber 2 when heating is performed only by high frequency with the oven plate 10 placed in heating chamber 2. But, as seen from Fig. 8, the bulge portion 15 does not influence the high frequency response characteristic in view of the cubic measurement of bulge portion 15 in comparison with that of heating chamber 2 when the oven plate 10 is not put in heating chamber 2. But when the oven plate 10 is placed in heating chamber 2, the bulge portion 15 greatly influences the high frequency response characteristic, i.e., the high frequency response characteristic of heating chamber 2 can be improved to a preferable extent by varying the depth (ℓ) of bulge portion 15. According to this construction, a similar advantage can be also obtained in a heating oven which is performed by raising up the temperature of heating chamber 2, and also in heating using a heater. The operation of a high frequency heating apparatus of the invention is easy, fast and energy-efficient, and a preferable heating condition can be obtained by selecting or combining heating methods based on the full of food material and a cooking menu. Preferably, the heating space 16 is closed to form a compartment by the oven plate 10 and walls of heating chamber 2 when the oven plate is placed in heating chamber 2 in order to stabilize high frequency performance for heating food material by high frequency. As shown in Fig. 9, the flat heating element 14a is composed of three insulation sheets 14e, 14f, and 14g each made of mica and heating wire 14h. Since the heating wire 14h is wound such that it concentrates at the center portion of insulation sheet 14f, the temperature of the center portion becomes higher than the other portions of insulation sheet 14f. As shown in Fig. 10, the turn table 4a has many small perforations 18, thereby permitting the heat of the above-described flat heating element 14a to smoothly escape upward. The above-described perforations 18 of the turn table must not be disposed near the rotation axis of electric motor 19 in order to prevent it from damaging. However, the perforations 18 with heating element 14a, which is heated at the center portion stronger than the other portions thereof, permits the food material to be uniformly and flatly heated. The heat capacity of turn table 4a can be reduced because the turn table 4a has many perforations 18, thus, the the material is effectively heated in cooperation with the above-described smooth upward movement of radiant heat. Further, since flat heating element 14a is disposed at the outside of heating chamber 2, the above-described turn table 4a can be disposed close to the bottom of heating chamber 2. Therefore, the heating space in heating chamber 2 is increased and a cleaning for the flat bottom of heating chamber 2 is easily performed by taking out the above-described turn table 4a. Various experiments indicated that the total area of perforations 18 of the above-described turn table 4a should be greater than the area of turn table 4a by 1/10 so that the above-described heat efficiency can be improved and the food material does not non-uniformly heated. As described above, according to the apparatus of the invention, the following advantages are obtained. (1) Since oven heating and grill heating are performed by means of a flat heating element, i.e., no heater and projections such as an insulator for holding a heater are provided, when high frequency is supplied with a heating chamber with an oven plate placed in the heating chamber, abnormal heating such as local heating and sparks do not occur, and electric waves do not leak from chalk mounted on an outlet portion of the heating chamber. Accordingly, with the oven plate placed in the heating chamber, heating can be performed by means of a high frequency and a heater which can be used alternately or in combination depending on situations to obtain the most preferable heating condition. Moreover, a fundamental and important heating method, i.e., heating of material by a heater after rapidly raising the bulk temperature of material by high frequency of heating is easily established. Because of this method, efficient heating, i.e., fast energy-efficient heating can be performed by a simple operation; namely, this method solves the problems where it takes a long time to heat when heating is performed by a heater and where a preferable heating condition cannot be obtained. (2) Heating using a flat heating element permits food material to be heated uniformly because heat is irradiated from the flat heating element, and, accordingly, non-uniform heating of food material does not occur. Further, food material may be placed close to the flat heating element, thereby greatly decreasing heating time periods, and achieving fast energy-efficient heating. (3) Since placing an oven plate in a heating chamber reduces heating space for the heater and food material and the heating space is effectively used for heating food material a rapid rising of the atmosphere temperature in the heating chamber can be easily accomplished when the food material is heated by a heater and energy is effectively used by a preferable heating condition. The relationship between heating speed and heating condition is shown in Fig. 11. (4) When high frequency heating is performed with the oven plate placed in the heating chamber, very non-uniform heating and impedance irregularities occur in a conventional apparatus. But, the provision of a bulge portion such as a diaphragm on the surfaces of the heating chamber walls prevents the lack of high frequency output caused by non-uniform heating, impedance irregularities, and abnormal heating of a magnetron and moding, i.e., abnormal oscillation, hence, greatly improving high frequency response characteristics. That is, a bulge portion, provided at a heating chamber wall, opposite to a high frequency supply opening has little influence on impedance when an oven plate is not placed in the heating chamber, however, the bulge portion greatly influences impedance when the oven plate is placed in the heating chamber. (5) Since the heating element on the heating chamber ceiling is flat, food material is uniformly heated when the material is placed close to it. When the distance between the heating element and the food material is small, heat irradiated by the heating element which strikes the material is very effective for heating the material, hence, fast energy-efficient heating can be easily performed. Further, temperature rising time of the heating space is greatly reduced because the heating space is very small, thus preferable heating suitable for a grill heating is obtained. (6) Since the heating chamber is provided with a heater at the bottom thereof and the metal turn table has many small diameter perforations, when food material is heated by a heater, non-uniform heating of food material does not occur and the heat generated by the heater disposed at the bottom of the heating chamber smoothly rises. Further, since heat capacity of the turn table can be reduced, preferable heat efficiency is obtained. Moreover, this construction prevents heat from being trapped in the bottom of the heating chamber. Accordingly, electric parts, such as an electric motor does not fail. Further, heat is conducted uniformly to the turn table. Also, since the turn table is disposed directly above the heater disposed at the bottom of the heating chamber, the turn table may be provided such that the turn table covers the entire bottom of the heating chamber. Therefore, heat is effectively used and the apparatus is easy to handle.

Claims: A heating apparatus for heating with combined radiant heat and/or microwaves, comprising: a microwave-tight heating chamber (2) for accomodating food material to be heated, and having oven plate holding means (11) on sidewalls spaced downwardly from an upper wall; and one chamber sidewall having a microwave supply opening (9) therein between said holding means (11) and said upper wall to which microwaves of a frequency oscillation device (6) are fed into said heating chamber (2); characterized by a) a flat radiant heat supplying heating element (14b) mounted against the outside of the metal upper wall of said heating chamber (2) for supplying heat to the metal of said upper wall for raising the temperature of the entire area of the upper wall for causing radiant heat to be emitted from the metal upper wall at temperatures up to food grilling temperatures, the upper part of the heating chamber (2) above the oven plate holding means (11) constituting a smooth area without any protruding parts disturbing microwave heating; and b) the sidewall of said heating chamber (2) opposite said microwave supply opening (9) and above said oven plate holding means (11) having an outwardly bulged portion (15) for increasing the volume of the portion of the heating chamber above said oven plate holding means (11), whereby when an oven plate (10) is placed on said holding means (11), it defines a smaller upper portion (16) of the heating chamber (2) in which the increased volume of the bulged portion (15) better accomodates the microwaves. A heating apparatus as claimed in claim 1 in which the lower wall of said heating chamber (2) is of metal exposed to the interior of said chamber, and a further flat radiant heat supplying element (14a) is mounted against the outside of the lower wall for supplying further radiant heat to the interior of said chamber (2). A heating apparatus as claimed in any of claims 1 or 2, characterized by a metal turntable (4a) in the bottom of said heating chamber (2) and rotatable for rotating food material positioned thereon. A heating apparatus as claimed in claim 3, characterized in that the further flat radiant heat supplying element (14a) is of ring-shaped form applying heat substantially only around the area of the metal turntable. A heating apparatus as claimed in claim 2, characterized by a metal turntable (4a) in the bottom of said heating chamber (2) and rotatable for rotating food material positioned thereon, said turntable having a plurality of perforations (18) therein each having a dimension such that the microwaves cannot pass therethrough but which permit passage therethrough of radiant heat; the flat radiant heater means (14a) at the lower wall of said heating chamber (2) covering substantially the entire outer surface of the lower wall of the heating chamber (2) and the radiant energy passing through the perforations (18) of the metal turntable (4a). A heating apparatus as claimed in claim 5 in which said perforations (18) cover greater than one-tenth the total surface area of said turntable (4a).

Inventors: AKIYOSHI, MITSUO, HIRAI, KAZUMI, HORI, ICHIROH, MITSUMOTO, YOSHIO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: F24C7
IPC codes: H05B6