U.S. patent number 3,624,335 [Application Number 05/049,646] was granted by the patent office on 1971-11-30 for microwave oven.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Edward C. Dench.
United States Patent |
3,624,335 |
Dench |
November 30, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
MICROWAVE OVEN
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.
Inventors: |
Dench; Edward C. (Annisquam,
MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
|
Family
ID: |
21960921 |
Appl.
No.: |
05/049,646 |
Filed: |
June 25, 1970 |
Current U.S.
Class: |
219/699; 219/742;
219/744 |
Current CPC
Class: |
H05B
6/782 (20130101); H05B 6/76 (20130101) |
Current International
Class: |
H05B
6/78 (20060101); H05B 6/76 (20060101); H05b
009/06 () |
Field of
Search: |
;219/10.55 ;333/73W |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IEEE Transactions On Microwave Theory & Techniques Vol. MTT-17,
No. 12 Dec. 1969 pages 1097-1101 Title-Slot Line Application to
Miniature Ferrite Devices Robinson & Allen.
|
Primary Examiner: Truhe; J. V.
Assistant Examiner: Jaeger; Hugh D.
Claims
I claim:
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.
Description
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" utilized 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. U.S. 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 collodial 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 U.S. 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, U.S. Pat. No. 3,365,562, issued Jan. 23, 1968 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 guages 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
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 conveyorized microwave oven apparatus
10. A substantially rectangular parallelepiped conductive enclosure
12 provides the main oven chamber. The entrance and exit access
ports of this chamber are enclosed by tunnel 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 disposed movable conveyor belt 18 carried by
drums 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 reducing gear mechanism 26 to drive the belt 18.
Microwave energy is introduced within the main oven chamber in
enclosure 12 through an opening in topwall 28 by means of waveguide
feed 30. The microwave energy source together with all electrical
controls is indicated generally by box 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 14 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 as slots 34 in the upper
walls 36 and 38 of access tunnels 14 and 16, respectively. In the
illustrative embodiment, shown in greater detail in FIGS. 2 and 3,
a portion of rectangular tunnel 14 is disclosed. In this embodiment
the narrow 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 of slots 34 will be disposed in at least one wall structure,
such as topwall 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
between slots 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. In the exemplary embodiment block members 42 and 44
completely enclose the slots and are fabricated in dense monolithic
sheets of a suitable energy absorbent material. Illustratively, a
material having a nonconductive composition with high temperature
resistant material dispersed throughout, such as asbestos, is
available under the tradename "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 in block
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. It is intended,
therefore, that the embodiment shown and described herein be
considered as exemplary only and not in a limiting sense.
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