US3440383A

US3440383A - Heating of articles

Info

Publication number: US3440383A
Application number: US592108A
Authority: US
Inventors: Peter Harold Smith
Original assignee: Microtherm Ltd
Current assignee: Microtherm Ltd
Priority date: 1965-11-04
Filing date: 1966-11-04
Publication date: 1969-04-22
Anticipated expiration: 1986-04-22
Also published as: DE1565731A1; SE329451B; GB1164278A; FR1510898A

Description

A-pril 22, 1969 Filed NOV. 4;. 1966 P. H. sMlrH 3,440,383

HEATING 4OF ARTI CLES lNvENToR PETER HAROL 'SM/TH 5 .PE/vomm /vEz/MA/v .SF/BOLD 8 W/LL/MS ATTORNEY sheet @f5v April 22, 1969 Filed No?. 4. 1966 Sheet 3 isssf HG. 5

' INVENTOR PETER HAROLD SMITH 5y PENDLETON, NEI/MAN SE/BOLD a W/LL/AMS ATTORNEY April 22, 1969 P. H. SMITH y HEATING oF ARTICLES med Nov. 4, 1966 sheet 3 @f3 77\ A I 76 73` C 72 INVENTOR PETER HAROLD .9M/TH By PE/VL E70/V, /VEUMAN .STE/BOLD 8 WML/AMS` ATTQRNEY' U nited States Patent U.S. Cl. 219-10.55 8 Claims ABSTRACT OF THE DISCLOSURE A microwave heating device for heating food articles has a microwave source and a waveguide connected to the source and with a normally open end adjacent the food article. The food articles are contained in individual disposable containers including a conductive sheet surrounding all but one side of each container, the latter being permeable to the microwave energy. The containers are positioned with the permeable side adjacent the open end of the waveguide, to extend the waveguide and enclose it around the food article, which serves as a load for the microwave energy. In one embodiment, a pressure sensitive device is connected to the waveguide to furnish a signal indicating when an air-tight coupling exists between the waveguide and the container.

This invention relates to the heating of articles by microwave or similar electromagnetic energy. The invention is particularly, but not exclusively, applicable to the heating of food products, especially at a point of sale. However, the invention can be used in other circumstances where relatively small articles are to be heated in this way.

In the past, suggestions have been made for the heating of articles by microwave energy. In these suggestions, the article to be heated, usually in a suitable container, is placed in an enclosure, which in the case of foodstuffs may bear some resemblence to a conventional oven, and microwave energy is then fed to the enclosure, for example by means of a waveguide feed system.

The present invention has for its object to provide a simpler arrangement than those hitherto suggested, by in effect locating the article to be heated directly within the waveguide or similar feed device. In accordance with a feature of the invention, a package including the article to be heated enclosed within a container is removably secured in juxtaposition to a feed device, such as a waveguide, the package providing a termination for the -feed device, so that energy can be fed through the feed device to the load presented by the article with a minimum of mismatch and with a minimum of radiation leakage to the surroundings.

Other features and advantages of the invention will y appear from the following description of various embodiments thereof, given by way of example, in conjunction with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic vertical cross-sectional view through a heating device and a package including an article to be heated;

FIGURE 2 is a diagrammatic plan view of the package shown in FIGURE 1;

FIGURE 3 is a diagrammatic perspective view of the package shown in FIGURE 2;

FIGURE 4 is a diagrammatic vertical cross-sectional View through a modified heating arrangement and a moditied form of package;

FIGURE 5 is a diagrammatic sectional View of a detail of a heating apparatus, showing ay connection flange;

FIGURE 6 is a diagrammatic side elevation view of 3,440,383 Patented Apr. 22, 1969 means for clamping in position on the end of a heating apparatus an article to be heated;

FIGURE 7 is an end view of the arrangement shown in FIGURE 6;

FIGURE 8 is a diagrammatic cross-sectional view of a modiiied form of the apparatus shown in FIGURE I;

FIGURE 9 is a diagrammatic cross-sectional view showing a modified form of feed device and a different form of package including an article to be heated; and

FIGURE 10 is a detail of FIGURE 9, showing the edge of the package.

The apparatus shown in FIGURE 1 comprises a microwave generator consisting of a magnetron 10, coupled to a rectangular waveguide feed tube assembly 11. Energy is coupled from the magnetron into the waveguide in the basic TEm mode by means of a stub antenna 12; the adjacent end of the waveguide is closed by a shortcircuit plate 13.

Microwave energy is propagated along the waveguide,

f and is directed into a package 14 which is removably secured to the waveguide 11, the package including the article to be heated, in this case shown by way of example as a hamburger |15, and its container 16. The package is arranged so that it forms a termination for the waveguide 11, with the article 15 forming a resistive load for the radiation. The container has an electrically conductive wall portion 19 which is substantially impermeable to the radiation, and

conductive anges

17 and 18 respectively on the waveguide 11 and the wall portion 19 of the package co-operate to seal the joint against radiation. In this embodiment the wall portion 19 deiines a cavity in which the article 15 is located, the cavity having an open end at 20, which is permeable to radiation, The article 15 is spaced from the closed end wall and the shorter side walls of the cavity since the heating energy falls away towards these walls, but may be engaged by the wider side walls of the cavity.

It is desirable to reduce as far as possible reflection of power down the waveguide 11 to the magnetron 10, and to this end the package 14 is mounted to the waveguide 11. In particular, the cross-section of the package at the end 20 is arranged to be the same as that of the waveguide 11, so as to present an effectively continuous surface at the joint to the radiation, and the food package is arranged to absorb rnost of the power reected 'f from the end wall of the container.

The wall portion 19 of the container may be made of metallic or metallised foil, or metallised plastics material, or metallised or metallic coated board. Metallic foil is presently preferred, as being simpler to form in the required shapes than metallised plastics or board material. The shape of the container is related to the article 15 which is contained within it. In particular, the cavity formed by the wall portion 19 is shaped to ensure the optimum distribution of heating energy in the article 15, and this may be contrasted with the usual kind of microwave oven cavity which is adapted to receive a variety of articles, and whose shape is according a compromise, not especially suited for any particular article. In the case of a hamburger sandwich, or similarly round article, the end of the container is rounded as at 21 in FIGURE 2 to conform to the shape of the article.

It is also desirable, for reasons of hygiene and the like, that the container should provide a complete enclosure for the article .15. Accordingly, the end 20 of the container may be closed by a film or the like 22 of a material permeable to microwave energy, such as a transparent plastic. It is not essential that the closure provided by the film 22 should be wholly nonconducting, provided that it is adequately transmissive to microwave energy.

In use, it is intended that the microwave heating apparatus should form a static installation and that the package, including the article to be heated, and its container, forms a separate unit which is secured to the end of the waveguide to receive heating energy, the package being subsequently removed from the waveguide, and extracted from the container for consumption. Accordingly, it is convenient if the container 16 has means of readily opening it to give access to the heated contents, and for this purpose the wall of the container is perforated or otherwise weakened, for instance to provide a tear strip 23, FIGURE 3, which surrounds the container. The container can then be readily separated into two parts, and one of these two parts may serve as a holder for the heated article whilst it is being consumed.

In some circumstances, it is desirable to ventilate the package 14 during heating. This can be done by passing a current of air through perforations in the container, with either excess or reduced pressure in the container. In copending Smith application No. 586,429 filed Oct. 13, 1966 it is described how this invention can be combined with air cooling of the magnetron, the heat which is dissipated in use by the magnetron being used to supplement the heating of the article by microwave energy. A similar arrangement adapted to the present apparatus is shown in FIGURE 4. In this gure parts corresponding to those shown in FIGURE 1 bear like reference numerals, but in FIGURE 4 the magnetron generator 10 is provided with a cooling motor-driven fan 25 and by means of a suitable duct or cowling 26 coolant air is passed over the magnetron The waveguide has in its openings 27 which are of a size such as to be substantially impermeable to microwave energy, leading into the waveguide, so that the now warmed air driven by the blower 25 will pass into the waveguide, as indicated by arrows 28.

The article to be heated has its cotnainer 16, as before, coupled to the waveguide, but the container in this case is made permeable to air, so that the warm air can pass through the container. The container can be of material which is inherently permeable to air or of perforated but otherwise impermeable material. The passage of the heated dry air over the article 15 has the efr'ect of reducing the degree to which it may become damp or soggy due to any steam that is generated as it is heated.

FIGURE S illustrates a means of further reducing leakage of microwave energy from between the

anges

17 and 18. As shown, the iiange 17 has a portion 30 which denes a recess, in which is disposed a dielectric material having a relatively large loss factor. By making the dimensions of the recess appropriate to the wavelength of the microwave energy a lossy double quarter-wave choke is formed. This much reduces leakage of energy.

Preferably means are provided whereby the package 14 consisting of the article to be heated and its container can be rapidly and securely clamped to the end of the waveguide during the heating process, to reduce leakage of the energy transmitted to the article. It is also desirable that there should be means whereby the openings in the container and the waveguide should be located accurately in register during the heating operation. FIGURES 6 and 7 show a suitable means by which this can be accomplished. A plate or sheet 31, which may form part 0f a shell for the heating apparatus, is fastened to ilange 17. Plate 31 has in it an opening corresponding to the open end of the waveguide. Locating means are provided for the flange 18 of the container, which can take various forms but are shown as upper pegs or pins 32 which engage in corresponding apertures in the flange 18, and lower pegs 33 which engage the lower edge of the ilange 18. In this way the two ilanges are brought accurately into register. The abutting tlanges are adapted to be held by a U-shaped clamping plate 34 which is hinged along its lower edge at 35 to the edge or a part of plate 31. The plate 34 can be swung away from the

pins

32 and 33, to enable the flange 18 to be slipped between the plate 34 and the ange 17. The plate 34 may have

lateral projections

36 and 37 which serve to locate the flange 18 when it is initially inserted. The plate 34 is pulled into clamping engagement with the flange 18 by means of a solenoid 39, the plunger 40 of which' is pivoted at 41 to

arms

42, 43, which are attached to the extremities of the arms of plate 34. The operation of solenoid 39 is linked with the control apparatus which is associated with the operation of the magnetron, so that clamping pressure will be automatically applied by the solenoid when the cycle of the heating operation is started.

It is usually desirable to ensure that there is an adequate seal between the abutting

flanges

17 and 18 and a means of monitoring this general condition is indicated in FIG- URE. `8. In this arrangement, a differential pressure is established between the interior and exterior of the waveguide; if the seal between the

flanges

17 and 18 is not properly made, the differential pressure will be reduced and can be detected by pressure sensitive means. As shown in FIGURE 8, the article to be heated is of the kind the end wall of which is not permeable to air, so that when the

flanges

17 and 18 are in contact the waveguide should be hermetically sealed. A pump 44, such as a small vacuum pump, is in communication with the interior of the waveguide through a pipe 45, and a pressure sensitive switch 46, with a diaphragm 47 and switch 48, is also in communication with the interior of the waveguide through pipe 49. If the waveguide is properly sealed, the switch 48 will be operated in one way when the pump 44 is in operation, but if there should be a leak in the system then the switch will not operaate in this manner and suitable supervisory alarm or control means can be operated. Conveniently, it can be arranged that the pump 44 is started at the beginning of the cycle, and that the switching on of the magnetron is controlled by the closure of switch 48 when adequate vacuum pressure has been established with the waveguide. In some circumstances, depending upon the nature of the package to be heated, it is possible to use this arrangement to ensure that the package 14 is not opened, or cracked or, if initially sealed, has had the seal fractured in any way. If the diaphragm 22 is permeable to air, a partial vacuum should be maintained inside the container 15, and the pressure diiferential will again be greater for a perfect container, properly positioned, than for an imperfect container or imperfect joint with the waveguide 11. If the wall portion 19 is permeable to air, the arrangement can be used to draw oit any steam or cooking vapours which may be generated by the article 15 when it is being heated.

In the embodiments of the invention described so far, the cross-sectional dimensions of the waveguide are usually chosen to be sufficiently small to support only one propagation mode, and if this arrangement is adopted it clearly imposes a practical limitation on the size of the article that can be heated conveniently. In FIGURES 9 and 10 is shown an arrangement in which substantially larger articles appropriately packaged, can be heated by direct inclusion in a waveguide system, while avoiding problems which can be caused by reection of power back to the magnetron by avoiding mismatches in the system.

In FIGURE 9, the microwave generator 10 has the coupling aerial 12 as before, and this extends into a short waveguide launching section 50, which can be of either circular or rectangular cross-section as may be convenient. This waveguide section is terminated at its one end by the short-circuit plate 13. The other end of the waveguide 50 is coupled to a` tapered section 52, of gradually increasing cross-sectional area from its junction with section 50 to an outermost section 53, which is shaped to receive the package 14, presented by the article 54 to be heated, and its container. In this case, the container is composed of two parts, a dished portion 55 which is made of a low loss material permeable to microwave energy and an outer cover 56 which is impermeable to microwave radiation, and can be, for example, of metal foil. In the manner which is shown more clearly in FIG- URE 10, the outer edge 57 of the tray-like portion of the container is embraced by the edge of the foil cover 56, which is turned over, as at 57, so that it is in good contact with a flange 58 formed around the end of section 53.

Once again, the package can be positioned on the open end of the waveguide system, the cover portion 56 of the container forming a termination for the waveguide. A flange 58 is formed around the edge of the cover 56, and co-operates with a corresponding flange on the portion 53 of the waveguide system to form a radiation seal. The cover 56 is shaped to ensure an optimum distribution of heating energy in the contents of the package, and is spaced from the contents.

The package may be circular or rectangular in crosssection and the flange 58 formed around the edge of section 53 has a corresponding cross-section. It may be convenient for the tapered section 52 to have a cross-sectional shape which resembles that of the article, in order to secure a more uniform distribution of energy throughout the article. It is also arranged that the tapering of the section gives a good impedance match between the article and the launching section 50. In order to prevent the leakage of energy, it is desirable to use a clamping means, such as a ring 60 which firmly clamps the foil-covered edge of the container to the liange 58. In this case, the foil cover 56 may be perforated if desired, to allow the escape of any steam produced in the heating process. The perforations should be of a size to prevent any substantial leakage of microwave energy.

What I claim is:

1. The combination of a package, a source of radio frequency electromagnetic radiant energy, and a feed device connected to said source for feeding the energy from said source to said package, wherein said package comprises an article to be heated and a container in which said article is received, said container including a first wall portion which is substantially impermeable to said radiant energy and a second wall portion which is permeable to said radiant energy, an end of said feed device being `adapted to receive said package whereby said article is positioned to receive said radiant energy, said first wall portion and said end of the feed dev-ice being adapted to couple together when said package is received by the end of the feed device, whereby said package forms a termination for said feed) device, said package being adapted to form a substantially hermetic seal to said end of the feed device when received correctly thereby, and means for detecting incorrect positioning of the package comprising means for generating a pressure differential between the interior of said feed device and its surroundings, and means responsive to the magnitude of said pressure differential.

2. In heating apparatus for heating a food article contained within a disposable container by exposing said food article to radiant energy while in said container, the combination comprising a source of radiant energy, a waveguide having a first flange on a normally open end and its other end connected to said source, said container having a conductive wall portion and a second flange adapted to be connected with said first iiange to extend said waveguide and close said open end when said container is juxtaposed with said open end so that said extended waveguide includes the space within said package, whereby said food article may be heated by said radiant energy.

3. Apparatus according to claim 2, wherein said container includes a rectangular wall substantially permeable to said radiant energy adapted to be juxtaposed with the CIIA open end of said waveguide, the remainder of the walls of said container including conduct-ive material impermeable to said radiant energy.

4. Apparatus according to claim 2, including a plurality of containers containing different food articles, each of said conta-iners including a conductive wall portion adapted to extend said waveguide and close said open end when said container is juxtaposed with said open end, said conductive wall portion of each container surrounding a food article, at least some of said food articles being of different size or shape from the others, each of said containers having a size and shape related to the characterstics of the food articles within it, whereby the optimum distribution of heating energy in the food article is obtained.

5. The combination according to claim 1 wherein said feed device includes a first hollow waveguide section, connecting means for connecting said first waveguide section to said source, the cross-section of said first waveguide section being substantially uniform along its length and being 'chosen so that said first waveguide section can maintain only one mode of radiation from said connecting means, and a hollow waveguide taper section which extends betwen said 'first waveguide section and said end of the feed device, the cross-section of said taper section being substant-ially identical to that of said rst waveguide section where they meet; and increasing progressively towards said end of the feed device.

`6. The combination according to claim 2 and including clamp means for releasably urging together said flanges, whereby to secure said package releasably in position at said end of the feed device.

7. The combination according toclaim 1 wherein said container includes a readily frangible portion for opening said package.

8. The -combination of a package, a source of radio frequency electromagnetic radiant energy, and a feed device connected to said source for feeding the energy from said source to said package, wherein said package comprises an article to be heated and a container in which said article is received, said container including a first wall portioni which is substantially impermeable to said radiant energy, and a second wall portion which is permeable to said radiant energy, an end of said feed device being adapted to receive said package whereby said article is positioned to receive said radiant energy, said first wall portion and said end of the feed device being adapted to couple together when said package is received by the end of the feed device, whereby said package forms a termination for said feed device, said package being adapted to form a substantially hermetic seal to said end of the feed device when received correctly thereby, and means for detecting incorrect positioning of the package comprising means for generating a pressure differential between the interior of said feed device and its surroundings, and means responsive to the magnitude of said pressure differential.

References Cited UNITED STATES PATENTS 2,586,754 2/1952 Wild 219-10.55 X 2,599,033 6/1952 Wild 219--10.55 2,820,127 1/1958` Argento et al 219-10.55 2,827,537 3/1958 Haagensen 219-1055 2,993,973 7/1961 Johnson et al 219-10.55

RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner.