US3624335A - Microwave oven - Google Patents

Abstract

The entrance and exit access tunnels of a conveyorized microwave heating oven are provided with means for permitting the flow of the material to be processed without any significant escape of radiated energy through the open ports. Means for prevention of such radiation includes slotted tunnel wall structures with energy absorbing material disposed in such a manner as to prevent any contamination of, particularly, food products processed through the oven. The orientation and dimensions of the slotted structures is determined by dimensions of the access tunnels and frequency of operation of the energy source.

Description

United States Patent [72] Inventor Edward C. Dench Annisquam,Mass'. [21] AppLNo. 49,646 [22] Filed June25, 1970 [45] Patented Nov.30, 1971 [73] Assignee Raytheon Company Lexington, Mass.

[54] MICROWAVEOVEN 8 Claims, 3 Drawing Figs.

52 U.S.C1 219 1055, 333/73W s11 lnt.Cl. HOSb9/06 [50] FieldolSearch 219/1065; v 333/73W [56] ReferencesCited UNITED STATES PATENTS 2,868.939 1/1959 Pound 219/1055 3,048,686 219/1055 3.365.562 219/1055 3.353.123 11/1967 Met 333/73W 3,451,014 6/1969 Brosnahan et a].

OTHER REFERENCES IEEE Transactions On Microwave Theory & Techniques Vol. MTT-l7, No. 12 Dec. 1969 pages 1097- l 101 Title-Slot Line Application to Miniature Ferrite Devices Robinson & Allen Primary Examiner-J. V. Truhe Assistant ExaminerHugh D. Jaeger I Attorneys-Harold A. Murphy and Joseph D. Pannone MICROWAVE ENERGY GENERATOR BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the process and apparatus for treating materials with microwave energy.

2. Description of the Prior Art The processing of materials, particularly, food products by means of microwave energy radiated within an enclosure has become widespread in the home and industry. The attractive advantages of microwave heating are the absence of any thermal lag to result in exceedingly fast processing times, as well as cleanliness, relatively low cost and absence of radiated heat from the oven enclosure during operation. Since most products are lossy dielectric materials essentially all of the microwave energy is absorbed by the products to result in more uniform heating.

In order to assist in the understanding of the invention the term microwave in the present description shall be construed to mean very short electromagnetic energy waves having wavelengths in the order of between 1 meter and 1 millimeter and frequencies in excess of 300 megacycles per second. Suitable energy generators include the crossed field magnetron oscillator of World War II radar fame, klystrons and high power vacuum triodes. The common frequencies of operation allocated by the Federal Communications Commission for microwave heating are 915 and 2450 megacycles.

A serious drawback in the utilization of microwave heating arises in continuous flow process systems where the access openings can leak considerable microwave energy into the surrounding environment. Such leakage may have deleterious results biologically to persons in the area, as well as electrically in interfering with adjacent radio or television communication systems and apparatus. In closed oven embodiments efficient door seals with microwave energy chokes substantially reduce all stray radiation. A need, however, is evident in the art for efficient means of reducing microwave energy leakage in high volume processing apparatus having means for continually feeding the material to be treated through open end access ports.

Numerous solutions have been proposed in the prior art to reduce the aforementioned stray energy leakage. US. Pat. No. 2,868,939, issued to R. V. Pound, Jan. 13, 1959 suggests the use of lossy dielectric material energy absorbing panels in one or more planes of maximum electric field excitation in a vestibule adjacent to the main oven enclosure. The panels are suitably coated with a resistive material such as a colloidal graphite suspension or other finely divided conductive material bonded to the panel to form a resistive film. Such a structure is utilized in processing of large size building materials such as wallboard panels, as well as other items including sand core molds, foam rubber cushions, mattresses and the like. Due to the deposition of a foreign material such as particulated graphite on the lossy panels, this structure is wholly unsuited for conforming to rigid health standards in the processing of any food products.

An alternate solution is disclosed in US. Pat. No. 3,048,686, issued Aug. 7, 1962, to W. Schmidt. This patent discloses the use of plural quarter-wave blocking filters with adjusting members in each of the entrance and exit waveguide tunnels. In addition, slots are disclosed in the wall of the waveguide ducts, however, the reference makes note of the fact that such slotted structure is not essential, in principle, for the operation of the device. Apparently, the slots are provided for reflection of one desired type of wave within the main oven cavity. The structure is rather cumbersome and costly but,

'most importantly, the inner sides are again desirably coated with a dissipative substance such as graphite to absorb nonreflected residual microwave energy. The processing of food products with such foreign substances on the interior walls is again questioned.

Finally, US. Pat. No. 3,365,562, issued Jan. 23, I968 to Morris R. Jeppson discloses the use of a rather elaborate lossy dielectric liquid circulating system within chambers or compartments adjacent to the entrance and exit ports to absorb the stray microwave energy radiation. This structure is effective, as well as sanitary for food product processing. The additional cost involved, however, of conduits, circulating pumps, housing, valves and gauges may be prohibitive so as to prevent wide acceptance.

A continuing need for an efficient, as well as inexpensive stray microwave energy radiation absorbing means in continuous flow microwave oven apparatus is still prevalent in the art.

SUMMARY OF THE PRESENT INVENTION In conformance with the teachings of the invention a continuous flow conveyorized microwave oven is provided with entrance and exit waveguide tunnels having open ports. At least one wall structure of each of said waveguide tunnels is provided with means of for disrupting the flow of electrical currents and reflecting substantially all the microwave energy. Such means includes a plurality of slots having a frequency determined dimension in spacing so as to be nonradiating at the frequency of operation of the microwave energy generator. In an embodiment disclosed herein the waveguide member of the tunnel arrangement has a dimension in the order of one-half of the operating frequency wavelength. A series of slots spaced at intervals of approximately one-half of a wavelength at this frequency and oriented so as to provide a discontinuity in current flow will effectively reflect substantially all the microwave energy attempting toescape from the main oven chamber. Additionally, a suitable energy absorbing material is exteriorly disposed in communication with each of the discontinuities to attenuate any escaping energy. A suggested lossy material is disclosed having a highly absorbing characteristic such as Portland cement together with an asbestos fiber and is commercially available under the trade name Transite. The oven apparatus of stainless steel construction with the disclosed stray radiation attenuating means will satisfactorily meet the requirements for handling and processing food products while being equally adaptable to any other products.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, as well as the details for the provision of an illustrative embodiment, will be readily understood after consideration of the following detailed description and reference to the accompanying drawings wherein:

FIG. 1 is a perspective view of the illustrative embodiment of a conveyorized microwave oven;

FIG. 2 is a detailed cross-sectional view taken along the line 2-2 in FIG. 1; and

FIG. 3 is a partial perspective view of the slotted microwave tunnel arrangement of the illustrative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 illustrates the embodiment of the invention incorporated in the conveyorizedmicrowave oven apparatus 10. A substantially rectangular parallelepipedconductive enclosure 12 provides the main oven chamber. The entrance and exit access ports of this chamber are enclosed bytunnel structures 14 and 16 which are open at their respective ends. The product to be continuously processed traverses the microwave oven apparatus by means of a coaxially disposedmovable conveyor belt 18 carried bydrums 20 and 22. The belt is formed of a flexible nonconductive material such as plastic or leather and is nonabsorbent to microwave energy.Drum 22 is actuated by motor means 24 coupled through a reducinggear mechanism 26 to drive thebelt 18.

Microwave energy is introduced within the main oven chamber inenclosure 12 through an opening intopwall 28 by means ofwaveguide feed 30. The microwave energy source together with all electrical controls is indicated generally bybox 32. Any of the conventional devices such as the magnetron oscillator hereinbefore referred to may be employed in the oven apparatus.

In accordance with the teachings of the present invention the waveguide access tunnels l4 and 16 are each provided with electrical current flow interruption means for substantially reflecting all escaping microwave energy from the main oven chamber. Structure for reflecting such escaping energy comprises a number of spaced wall discontinuities such asslots 34 in theupper walls 36 and 38 of access tunnels l4 and I6, respectively. In the illustrative embodiment, shown in greater detail in FIGS. 2 and 3, a portion ofrectangular tunnel 14 is disclosed. In this embodiment thenarrow sidewalls 40 of the tunnel have a height dimension of one-half or less of the operating frequency of the microwave energy generator. In order to reflect any escaping energy a nonradiating array ofslots 34 will be disposed in at least one wall structure, such astopwall 36, to disrupt the flow of electrical currents in the conductive walls. A dimension of one-half of the wavelength of the operating frequency is, therefore, selected for the spacing betweenslots 34. The wall structure discontinuities, then, have a spacing dimension which is frequency dependent to provide a nonradiating energy reflective structure at each end of the oven apparatus.

In order to further assure attenuation of substantially all the escaping energy a member of a lossy absorbent material encloses all of the slots. ln the exemplaryembodiment block members 42 and 44 completely enclose the slots and are fabricated in dense monolithic sheets of a suitable energy ab sorbent material. lllustratively, a material having a nonconductive composition with high temperature resistant material dispersed throughout, such as asbestos, is available under the trade name Transite. For the purposes of the practice of the present invention any other suitable material having high energy absorbing characteristics can be coupled to the slots. By reason of the disposition of such material exteriorly of the inner walls of the waveguide access tunnels the possibility of contamination by foreign substances in the food products being processed is substantially reduced. The overall dimensions inblock members 42 and 44 are empirically selected to provide the high heat characteristics, as well as long life throughout the operation of the microwave oven apparatus.

While an array of energy reflective discontinuities have been shown in one wall structure of the waveguide access tunnel, numerous applications may exist where it is desirable to provide such discontinuities in more than one wall structure. There is thus disclosed an efficient and relatively low cost means for the attenuation of escaping microwave energy in continuous flow-type microwave oven apparatus having open ports at each end. Various modifications and alterations will readily occur to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims. lt is intended, therefore, that the embodiment shown and described herein be considered as exemplary only and not in a limiting sense.

lclaim:

1. Microwave oven apparatus comprising:

a conductive enclosure; a microwave energy generator for radiating electromagnetic waves within said enclosure; hollow conductive means for providing access to the interior of said enclosure;

means defined by said access means for reflecting substantially all such energy escaping from said enclosure;

said reflecting means including an array of nonradiating energy discontinuities in at least one wall of said access structure;

said discontinuities being uniformly spaced apart a predetermined fraction of a wavelength at the operating frequency of said generator sufficient to interrupt the flow of electrical currents in said wall; and a layer of dense energy absorbing high temperature resistant material covering all said discontinuities.

2. Microwave oven apparatus comprising:

a conductive enclosure; a microwave energy generator for radiating electromagnetic waves within said enclosure: and electrically conductive wall means defining an open ended access passageway to the interior of said enclosure:

at least one of said wall means having an array of nonradiating slots to reflect substantially all such energy escaping from said enclosure:

slots being arranged a frequency dependent distance determined by the operating frequency of said generator: and a block member of a dense energy absorbing high temperature resistant material covering all said slots.

3. Microwave oven apparatus comprising:

a conductive enclosure; a microwave energy generator for radiating electromagnetic waves within said enclosure;

metallic wall means defining an open ended access passageway to the interior of said enclosure;

at least one of said wall means having an array of spaced nonradiating slots to reflect substantially all such energy escaping from said enclosure;

said slots being disposed a frequency dependent distance determined by the operating frequency of said generator;

and a block member of an energy absorbing material covering all said slots.

4. Microwave oven apparatus according to claim 3 wherein said slots are spaced apart approximately one-half a wavelength of the operating frequency of said generator.

5. Microwave oven apparatus according to claim 3 wherein said energy absorbing means are disposed exteriorly of said slotted wall means.

6. Microwave oven apparatus according to claim 3 wherein said energy absorbing means comprise a dense high temperature resistant material of principally cement with an asbestos fiber disposed therein.

7. A conveyorized microwave oven apparatus comprising:

A conductive enclosure defining a main heating chamber; access tunnel means coupled to at least one wall of said enclosure and means providing a continuous flow of material to be processed in said chamber;

a microwave energy generator coupled to said enclosure; means provided by at least one wall of said tunnel means to reflect substantially all such energy escaping from said enclosure;

said reflecting means including an array of nonradiating discontinuities in said wall spaced apart a frequency dependent dimension;

and dense energy absorbing high temperature resistant means covering all said discontinuities.

8. A conveyorized microwave oven apparatus comprising:

a conductive enclosure defining a main heating chamber and having an access tunnel means in axial alignment joined to opposing walls of said chamber:

A coaxially disposed nonconductive conveyor belt adapted to continuously move materials through said enclosure:

a microwave energy generator for radiating electromagnetic waves within said chamber;

at least one wall of each of said tunnel means having an array of nonradiating slots spaced apart a frequency dependent dimension to reflect substantially all such energy escaping from said chamber;

and a member of a dense energy absorbing high temperature resistant material disposed exteriorly of said tunnel wall and covering said slots.

* i I! l t UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,624 335 Dated November 30, 1971 ln e fl Edward Ci Dench It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1,line 20, after "microwave" insert utilized Column 2, line 6, change "housing" to housings Column 2,line 18, after "means" delete "of" Column 4, line 13, before"s1ots" insert said Column 4, line 67, claim 8, after "wall" insert means Signed and sealed this 13th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTISCHALK Attesting Officer Commissioner of Patents

Claims (8)

1. Microwave oven apparatus comprising: a conductive enclosure; a microwave energy generator for radiating electromagnetic waves within said enclosure; hollow conductive means for providing access to the interior of said enclosure; means defined by said access means for reflecting substantially all such energy escaping from said enclosure; said reflecting means including an array of nonradiating energy discontinuities in at least one wall of said access structure; said discontinuities being uniformly spaced apart a predetermined fraction of a wavelength at the operating frequency of said generator sufficient to interrupt the flow of electrical currents in said wall; and a layer of dense energy absorbing high temperature resistant material covering all said discontinuities.

2. Microwave oven apparatus comprising: a conductive enclosure; a microwave energy generator for radiating electromagnetic waves within said enclosure; and electrically conductive wall means defining an open ended access passageway to the interior of said enclosure; at least one of said wall means having an array of nonradiating slots to reflect substantially all such energy escaping from said enclsoure; said slots being arranged a frequency dependent distance determined by the operating frequency of said generator; and a block member of a dense energy absorbing high temperature resistant material covering all said slots.

3. Microwave oven apparatus comprising: a conductive enclosure; a microwave energy generator for radiating electromagnetic waves within said enclosure; metallic wall means defining an open ended access passageway to the interior of said enclosure; at least one of said wall means having an array of spaced nonradiating slots to reflect substantially all such energy escaping from said enclosure; said slots being disposed a frequency dependent distance determined by the operating frequency of said generator; and a block member of an energy absorbing material covering all said slots.

4. Microwave oven apparatus according to claim 3 wherein said slots are spaced apart approximately one-half a wavelength of the operating frequency of said generator.

5. Microwave oven apparatus according to claim 3 wherein said energy absorbing means are disposed exteriorly of said slotted wall means.

6. Microwave oven apparatus according to claim 3 wherein said energy absorbing means comprise a dense high temperature resistant material of principally cement with an asbestos fiber disposed therein.

7. A conveyorized microwave oven apparatus comprising: a conductive enclosure defining a main heating chamber; access tunnel means coupled to at least one wall of said enclosure and means providing a continuous flow of material to be processed in said chamber; a microwave energy generator coupled to said enclosure; means provided by at least one wall of said tunnel means to reflect substantially all such energy escaping from said enclosure; said reflecting means including an array of nonradiating discontinuities in said wall spaced apart a frequency dependent dimension; and dense energy absorbing high temperature resistant means covering all said discontinuities.

8. A conveyorized microwave oven apparatus comprising: a conductive enclosure defining a main heating chamber and having An access tunnel means in axial alignment joined to opposing walls of said chamber; a coaxially disposed nonconductive conveyor belt adapted to continuously move materials through said enclosure; a microwave energy generator for radiating electromagnetic waves within said chamber; at least one wall of each of said tunnel means having an array of nonradiating slots spaced apart a frequency dependent dimension to reflect substantially all such energy escaping from said chamber; and a member of a dense energy absorbing high temperature resistant material disposed exteriorly of said tunnel wall and covering all said slots.

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