US2595748A - Heating apparatus - Google Patents

Description

May 6, 1952 G. E. ANDREWS HEATING APPARATUS Filed Maroh14, 194'? T M 00 mm WC www. w

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Patented `May 6, 1,952

HEATING APPARATUS Glenn E. Andrews,

Andover, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application March 14, 1947, Serial'No. 734,568

1`Claim. l

This invention relates to heating apparatus, and more particularly to a heat-responsive controlling means for a microwave heating or cooking device.

An object of this invention is to provide a means, useful in conjunction with apparatus for heating dielectric bodies by alternating electrical energy, for automatically controlling the supply of energy to the body being heated.

Another object is to devise a means for automatically cutting off the supply of alternating electrical energy to a dielectric body being heated, when said body reaches a predetermined temperature.

A further object is to provide a device for microwave cookers which automatically shuts oi the microwave generator when the food being cooked reaches a predetermined temperature, this device being capable of manual adjustment to vary said predetermined temperature.

A still further object is to devise a controlling means whereby dielectric bodies being heated may be brought to a certain predetermined end or nal temperature, after which the energy is automatically decoupled from said body, this end temperature being attained regardless of the initial temperature of the body, the size of the body, or the rate of energy absorption of the body.

An additional object is to provide an automatic bodyheatresponsive, microwave energy controlling means for a microwave heating apparatus, which will not interfere with the absorption of microwave energy by the body being heated and will not be affected by said microwave energy.

The foregoing and other objects of the invention will be best understood from the following description of some exemplifcations thereof, reference being 'had to the accompanying drawing. wherein:

Fig. 1 is a schematic illustration of an arrangement whereby the invention may be carried out;

Fig. 2 isa schematic illustration of a modied arrangement; and

Fig. 3 is a partial View, on an enlarged scale, showing a detail.

Now referring to Fig. 1, a hollow rectangular prismoidal enclosure or. cavity I is made of a suitable metal and has rather thin walls as shown; enclosure I is adapted to serve as the oven of the heating or cooking apparatus. A dielectric body 2 to be heated, which may be a container of food for example, is positioned in enclosure I, said body resting on the bottom wall of the ovenwhile the body is being heated. In order to allow access to the interior of the oven for placing the body 2 therein and for removing said body therefrom, an opening 3 is provided in the front wall ofthe oven I, this opening being closable by means of a hinged metal door 4. When door 4 is closed, the enclosure I is entirely closed, except for the openings Il and I8 to be described hereinafter.

Numeral 5 generally designates an electrondischarge device of the magnetron type, which includes,` for example, an evacuated envelope 6, made of highly conductive material, such as copper, and provided with a plurality of inwardlydirected, radially-disposed anode vanes` 1. The arrangement is such `that each pair of adjacent anode vanes forms, together with that portion of the envelope lying therebetween, a cavity resonator whose natural resonant frequency is, as is well known to those skilled in the art, a function of the geometry of the physical elements making up the same. For an oven as above described, it is desirable that the dimensions of each such cavity resonator be such that the predetermined wavelength of the electrical oscillations adapted to be generated therein has a value which lies in .the microwave region of the frequency spectrum. However, this invention is equally applicable inheating apparatus utilizing alternating electrical energy of any frequency useful for heating dielectric bodies by electrical energy..

Centrally located in envelope 6 is a highly `electron-emissive cathode member 8, for example, of the well-known alkaline-earthmetaloxide type, said cathodemember being provided with .conventional means (not shown) for raising the temperature thereof to a level sufficient for thermionic emission.

The electron-discharge device or magnetron oscillator or radio-frequency oscillator 5 is completed by magnetic means (not shown) for establishing a magnetic field in a direction transversely of the electron path between the cathode and anode members thereof.

Anode or envelope 6 of the oscillator is connected, by a conductor 9, directly to the positive terminal of a suitable power source or direct current supply, while cathode 8 of the oscillator is connected, in series with the normally-closed contacts I0 of a relay II, by a conductor 39, to one contact 42 of a switch I2, the other contact 4I of said switch being connected by a conductor 40 to the negative terminal of said power source; switch I2 is normally closed to connect contacts 4I and 42, as will be later described. It will be seen that, normally, oscillator 5 is connected to be energized from the power source, and that by 'the opening o contacts IIJ or the opening of contacts 4I-42 the oscillator will be disconnected from the power source and thereby deenergized.

When energized, radio-frequency oscillator 5 delivers radio-:frequency energy to a hollow metallic waveguide I3 through a coaxial transmission line I4 which is coupled to oscillator 5 by a loop I5. The central conductor I6 of line I4 extends into the interior of waveguide I3 near one end thereof, through an opening provided in the wall thereof, to serve as an exciting rod or exciting probe for said guide.

The end of guide I3 nearest this exciting rod is closed, `while the opposite end of said guide is fastened to the rear wall of enclosure I and is open. The interior of guide I3 is placed in energy-transmitting relationship with the interior of the cavity I by means of an aperture I'I provided in the rear wall of said cavity, this aperture being of the same size and configuration as the interior of guide I3 and being aligned with said guide to place the interior of said guide in communication with the interior of cavity I.

When magnetron oscillator 5 is energized, the radio-frequency or microwave energy produced thereby is propagated down waveguide I3 and emanates from the open end thereof into the interior of enclosure I. This energy impinges on the body E! and is in eect absorbed thereby, resulting in heating or raising the temperature of said body.

Pursuant to this invention, an aperture I8 is provided in the top wall of enclosure or oven I. this aperture preferably being located in the center of said wall, being circular, for example, and having a diameter d. Extending outwardly or upwardly from the top wall of oven I, at right angles thereto, and aligned with aperture I8, is a hollow metallic tube I9 which has an internal diameter equal to d. The diameter d of aperture Iand oi tube I3 is made such that for the wave mode present in enclosure I, the cutoff wavelength of tube I9, considered as a hollow waveguide, is below the wavelength of the oscillations generated by oscillator 5. The cutoff wavelength for hollow waveguides depends lon the cross-sectional dimensions of such guides, as is well-known to those skilled in the art.

Mounted adjacent the outer end of tube I9 is a heat-responsive or heat-sensitive device 25, which may be, for example, a sensitive bank or series of thermocouples in a vacuum. Device 2l] is here represented as including a bimetallic hot junction 2| enclosed in an evacuated envelope 22, and a bimetallic cold junction 23. The arrangement of tube I9 and device 2B with respect to body 2 is such that junction 2I is exposed to the heat radiated from said body. In other words, the axis of tube I9 is pointed at the body and the inner end of the tube is exposed to the body, so that thermocouple device 20 in eifect looks at the body. Junction 23 is maintained at a so-called fixed or reference temperature, and when the temperature of junction 2l is raised above that of junction 23, a thermal electromotive force, direct in nature, is produced between the output terminals 24 and 25 of the device 20.

As used herein, the phrase thermal electromotive torce has the meaning commonly associated with it by those skilled in the science of physics. A current flows in a circuit composed of two metallic conductors of different materials if the temperatures of the two junctions are different; in such a circuit there is an electromotive force dependent on thermal conditions and called a thermal electromotive force.

The electromotive force thus produced between terminals 24 and 25 is connected as the input voltage of a direct current vacuum-tube amplifier 26, and to this end terminal 24 is connected by means of a lead 21 to one of the input terminals 28 of said amplier, and terminal 25 is connected by means of a lead 29 to the other input terminal 3S of said amplifier. A lead SI connects one end of the operating winding 35 of relay I I to one of the output terminals 32 of amplifier 2S, while a lead 33 connects the other end of said winding to the other output terminal 34 of said amplifier.

Amplifier 26 has a variable gain, and this gain is such that normally the output of said amplifier is insufficient to energize relay II; as a result said relay is unenergized and contacts I0 are closed.

Switch I2, as shown in Fig. 3, may be mounted on the rear face of the depending portion of the iront wall of enclosure I, adjacent to the hinge of door 4. The actuating rod of switch I2 is spring-biased to the position indicated in dotted lines in Fig. 3, in which position the contacts 4I and 42 of said switch are not connected to each other or are open. The switch actuating rod is arranged to be forced, by the movement of door 4 from its dotted-line open position to its solidline closed position, against the bias of the spring, to the position indicated in solid lines in Fig. 3, in which the contacts 4I and 42 of the switch are closed or connected to each other. When door 4 is opened, the pressure on the switch actuating rod is released, so that said actuating rod is moved to the right by the spring to open the contacts 4I and 42. Thus, when door 4 is closed, after body 2 is placed inside the oven I, the contacts ll-42 of switch I2 will be closed, and when door 4 is opened to remove body 2 said contacts will be opened.

Assume, now, that a dielectric body 2, which is at some initial temperature and which is desired to be heated to a predetermined final temperature, is placed in enclosure I and door 4 is closed. Switch I2 is closed by the closing of said door and, since junctions 2l and 23 are at the same temperature, device 25 produces no thermal electromotive force and conditions are normal, being such that relay I I is unenergized and contacts IB are closed. Therefore, the controllable circuit which connects oscillator 5 to the power' source is completed, and said oscillator, being energized, produces radio-frequency energy which is supplied to the interior of enclosure I, where it impinges on dielectric body 2 to heat the same. As body 2 is heated or as it rises in temperature, it radiates heat. This radiant energy in the form of heat falls on junction 21 (since junction 2I is exposed to this radiant heat energy of body 2), producing a rise in temperature at said junction and consequently a direct thermal electromotive force between terminals 24 and 25. This electromotive force is applied to the input of amplifier 26, in such a direction as to tend to increase the output of said amplifier. This thermal electromotive force is proportional to the temperature of junction 2l, and, when it reaches a denite predetermined value, will cause the output of amplier 26 to increase to a level suicient to energize relay II, opening its contacts IIJ, thus 5. deenergizing oscillator 5, thereby stopping Ithe supply of radio-frequency energy to enclosure I and body 2. The aforesaid denite value of thermal electromotive force necessary to produce energization of relay I I is Variable at will by-varying the gain of ampliiier 26, thereby varying the output level of the amplifier relative to the input level thereof. The heat energy radiated from body 2 is proportional to the temperature of said body, and the temperature of junction 2I is proportional to the radiant heat energy falling thereon, so that the thermal electromotive force produced between terminals 24 and 25 is proportional to the temperature of body 2. Thus, when body 2 reaches a predetermined temperature, oscillator is automatically deenergized and the supply of radio-frequency energy to said body thereby stopped.

When magnetron oscillator 5 has been so deenergized, the door 4 may be opened and the body 2 removed. Opening of said door causes the contacts of switch I2 to be opened, so that even though body 2, which is now a source of radiant heat, be removed from the oven, thereby also removing the thermal electromotive force from amplifier 2S and deenergizing relay II to reclose contacts I0, oscillator 5 will not be reenergized unnecessarily, because of the series connection of the contacts of switch I2 and of contacts I0. The contacts of switch I2 are reclosed by the reclosure of door 4 after another body 2 has been placed in the oven I, to energize the oscillator 5 for the next heating cycle, contacts II of relay II being closed at this time because of the absence of a thermal electromotive force from thermocouple 20.

As explained above, the denite value of thermal electromotive force (and therefore also the temperature of junction 2I and the temperature of body 2) at which relay II is energized to deenergize oscillator 5 is variable by varying the gain of amplifier 26. Therefore, when body 2 is food, the food can be heated to various selected temperatures by appropriately varying the amplifier gain. For instance, the food could be cooked rare, medium, or Well done.

The microwave energy of oscillator 5 is automatically cut off when and only When body 2 reaches a predetermined temperature. This means that, when body 2 is food, such food is by this invention heated until it reaches the temperature at which it is cooked, regardless of its initial temperature, its size, or its rate of energy absorption. If a preset timing device were used to control the application of radio-frequency energy to the dielectric body, rather than the device of this invention, various different initial temperatures of the bodies, different sizes of the bodies, or diiferent rates of energy absorption of the bodies might result in underheating or overheating of the bodies. However, with the system of the invention, such possible undesired end results are eliminated.

Since tube I9 is designed to make its cutoff wavelength below the wavelength of the radiofrequency oscillations of source 5, such oscillations will not propagate down said tube as a hollow waveguide. Therefore, radio-frequency energy will not be transmitted away from the interior of enclosure l to be lost for purposes of heating body 2, so that the means of this invention will not interfere with the absorption of energy by the body being heated. Also, since radio-frequency energy is not transmitted down tube I9, the heat-responsive device will not be 6 a actuated or affected by such energy, but will be actuated by or sensitive to only the` radiant heat energy of body 2.

In ovens or enclosures of the above-described type, the air within the enclosure is not heated appreciably by the radio-frequency energy, so that device 29 is not affected by the temperature of the-atmosphere surrounding the dielectric body, as would be the case with an ordinary oven which relies on heat transmission by conduction of heat energy through the atmosphere from the heat source to the body.

It is within the scope of this invention to make the aperture I8 of any convenient shape, element I9 having an opening therethrough of a corresponding configuration. For example, aperture I8 may be rectangular and element I9 may have a hollow rectangular cross-section, in which case the cutoff wavelength formulae for rectangular hollow waveguides Would-be used to calculate the internal dimensions of portion I9 so as to make the cutoff wavelength of waveguide portion I9 less than the wavelength of the oscillations 0f oscillator 5.

Now referring to Fig. 2, in which similar elements are denoted by the same reference numerals as in Fig. l, a modified structure for coupling the heat-responsive device or thermocouple 20 to the enclosure I is disclosed. In this modiflcation, a coaxial stub 3'I is provided, said stub being a quarter-wavelength long and having an open lower end and a short-circuited upper end. The open lower end of stub 3'I is aligned with enclosure aperture I8, with the axis of the stub pointed at body 2 and with the open end of the stub exposed to said body.

The shorted end of stub 31 has a relatively small aperture 38 therein, and the junction 2I of thermocouple 20 is placed directly above this aperture, so that the thermocouple junction 2I is exposed to the heat radiated from body 2, the thermocouple thereby in effect looking at the body 2.

Since junction 2 I is exposed to the heat radiated from body 2, a thermal electromotive force will be produced between terminals 24 and 25 of the thermocouple as body 2 heats, and this electromotive force may be utilized, as in Fig. l, to control the radio-frequency oscillator in such a way as to stop the supply of energy to body 2 when the same reaches a predetermined temperature.

Due to the utilization of the shorted quarterwavelength coaxial stub 31 in the above-described manner, no radio-frequency load additional to that of the body 2 is imposed on the radiofrequency source, so that no radio-frequency energy is abstracted away from the dielectric body 2 and, in addition, the thermocouple is effectively isolated from the intense radiofrequency radiations, so that the heat-responsive device or thermocouple 20 will not be actuated or affected by such radio-frequency energy.

Of course, it is to be understood that this invention is not limited to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. For example, it may in most cases be preferable to put the thermocouple tube or stub in the side wall of the oven, rather than in the top wall as shown, in order to prevent the entrance of cooking vapors, steam, etc. into the tube or line leading to the thermocouple. Various other variations will suggest themselves. It is accordingly desired that the appended claims be given a broad inter- 7 pretation commensurate with the scope of this invention Within the art.

What is claimed is:

In combination, a metallic enclosure adapted to serve as an oven, an oscillator for supplying electromagnetic wave energy to the interior of said enclosure to heat a dielectric body positioned in said enclosure, a controllable circuit for energizing said oscillator from a power source, means for generating a thermal electromotive force, said means being exposed to the heat radiated from said body and being cou-pled to said oven in such a way that said means is isolated from said wave energy, and means for utilizing said electromotive force to control said circuit to deenergize said oscillator when said body reaches a predetermined temperature.

GLENN E. ANDREWS.

REFERENCES CITED UNITED STATES PATENTS Name Date Wallis et al. Oct. 14, 1919 Number Number Name Date 1,981,631 Northrup Nov. 20, 1934 2,233,788 Lewin Mal. 4, 1941 2,275,265 Mead Mar. 3, 1942 2,279,525 Rogers Apr. 14, 1942 2,364,526 Hansell Dec. 5, 1944 2,370,161 Hansen Feb. 27, 1945 2,386,966 MacMllin Oct. 16, 1945 2,448,008 Baker Aug. 31, 1948 2,495,415 Marshall Jan. 24, 1950 2,495,435 Welch Jan. 24, 1950 2,498,720 Wild et al Feb. 2B, 1950 2,500,752 Hanson et al. Mar. 14, 1950 2,526,226 Gross Oct. 17, 1950 OTHER REFERENCES Stack: Vacuum Thermocouples of the Radiation Type, General Electric Review, vol. 42, No. 8, August 1939, pages 365 and 366.

Hutcheson: Electronic Torch, The Welding Engineer, December 1945, page 90.

Walsh: Homemaking, The Washington Daily News, October, 9, 1946, page 38.

Magnetron Adapted to Cooking Purposes, Electrical Engineering, December 1946, page 591.

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