US3582598A

US3582598A - Methods and apparatus for heating a body conveyor system for heating bodies by a microwave cavity

Info

Publication number: US3582598A
Application number: US782389A
Authority: US
Inventors: Peter Astley Wincott
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1967-12-09
Filing date: 1968-12-09
Publication date: 1971-06-01
Anticipated expiration: 1988-06-01
Also published as: NL6817623A; ES379218A1; GB1243009A; BE725142A; DE1813554A1; FR1598340A; DE1813554B2; ES361198A1; AT295680B

Abstract

Method and apparatus for heating a body, especially a long continuous body such as rubber belting, by microwave heating in an enclosure which is moved relative to the body being heated. In the examples described this is done by a reciprocating device, the belting being held stationary during heating. The support structure on which the enclosure is movably mounted is preferably pivoted at one end on a fixed axis and pivoted at the other end to the moving platen of a vulcanization press.

Description

Inventor Peter Astley Wincott Luton, England Appl. No. 782,389 Filed Dec. 9,1968 Patented June 1, 1971 Assignee The Eglish Electric Company Limited London, England Priority Dec. 9, 1967 Great Britain 56105/67 METHODS AND APPARATUS FOR HEATING A BODY CONVEYOR SYSTEM FOR HEATING BODIES BY A MICROWAVE CAVITY {56] References Cited UNITED STATES PATENTS 3,277,580 10/1966 Tooby 219/10.55 2,603,741 7/1952 Seifried et a1. 219/10.55X 2,722,589 11/1955 Marguardt.... 219/10.61 3,177,333 4/1965 Lamb 219/10.55 3,436,506 4/1969 Smith 2 l 9/1055 FOREIGN PATENTS 944,99 3 12/1963 Great Britain 219/ 10.55

Primary ExaminerJ.V. ruhe Asristan! Examiner-LB. Bender AttorneysMisegades & Douglas, Keith Misegades and George R. Douglas Jr.

ABSTRACT: Method and apparatus for heating a body, especially a long continuous body such as rubber belting, by

2 Claims 3 Drawing Figs microwave heating in an enclosure which is moved relative to US. Cl 20/101159, the body being heated. 1n the examples described this is done 219/1055, 219/ 1() 7 3 by a reciprocating device, the belting being held stationary Int. Cl H05b 5/00, during heating. The support structure on which the enclosure H05b 9/06 is movably mounted is preferably pivoted at one end on a fixed Field of Search 219/ 10.73, axis and pivoted at the other end to the moving platen of a vul- 10.55, 10.61 canization press.

MICROWAVE SOURCE 2o\fl MUmDOw ATENTED JUN 1 m1 SHEET 2 OF 3 PATENTED JUN Han SHEET 3 0F 3 FIG.3

METHODS AND APPARATUS FOR HEATING A BODY CONVEYOR SYSTEM FOR HEATING BODIES BY A MICROWAVE CAVITY This invention relates to methods and apparatus for heating a body by microwave energy and to bodies so heated.

Microwave energy, that is to say the energy in radio frequency radiation, has for some time been known to induce heating in materials which are not transparent or reflective to such radiation. It is usual to provide a microwave enclosure, (sometimes called a microwave cavity or microwave oven) which consists essentially of an enclosed chamber having an inlet for the radiation. The body to be heated is placed in the enclosure and subjected to the radiation which is supplied through the inlet from a suitable source such as a magnetron. Perhaps the best known example of an application of this method up tonow is in the cooking of food: microwave cooking ovens are available commercially.

Microwave heating has the advantages over more conventional heating methods which involve heating the atmosphere around the body to be heated (so that heat is transferred to the body from said atmosphere), that:

a. the atmosphere is not itself heated by the microwaves,

and

b. the enclosure can be arranged so that virtually the only substances exposed to the radiation, which are not transparent and reflective thereto, are those of the body to be heated. Thus substantially all of the energy produced can be converted into heat in the body, so that there can be negligible waste heat. This, combined with the fact that radiation of considerable-intensity can be produced quite easily, means that the body can be heated very much more quickly than by more conventional methods, and also in most cases more economically.

One problem inherent in microwave heating is that, because the heat is produced within the body being heated by virtue of radiation directed at the body in a straight beam or beams (direct from the microwave source and/or by reflection from suitable surfaces), standing patterns of radiation are set up which make it difficult to obtain a substantially even temperature distribution over the surface of the body. ln practice, hot spots," i.e. points at which the temperature is considerably higher than in the surrounding parts of the surface, tend to occur on the surface of the body by virtue of these standing patterns.

An object of the present invention is to reduce the effect of hot spots and thus tend to promote a more even temperature distribution on the surface of the body to be heated than has hitherto been possible in microwave heating techniques.

The body to be heated may be (for example) a single separate article or an elongate body such as a sheet or strip. lt has been proposed to heat sheet or strip material in a microwave enclosure in which the material to be heated is held stationary or is passed through the enclosure with a steady unidirectional movement. In the latter case, the continuous movement of the material through the enclosure will tend to eliminate the effect of hot spots; but such continuous movement is not always possible or desirable for various reasons. For example, if the heating process is part of a process of which the next stage requires that part of the sheet or strip shall be stationary, then the whole sheet or strip must be stationary.

It is therefore a further object of the invention to provide a means of reducing the effect of hot spots where the body to be heated is stationary while being heated by microwave energy.

The term sheet material" as used herein includes materials in the form of discrete sheets and elongate sheets or strips for example conveyor and transmission belting, including V-belting, and is limited to materials having dielectric properties suitable for allowing heating of the material by absorption of microwave energy, for example natural and synthetic rubbers and plastics materials. A body heated or to be heated by methods and in apparatus according to the present invention may comprise sheet materials as defined above.

The term microwave enerby" as used herein means electromagnetic energy within the approximate frequency range 300 to 30,000 megacycles/second.

The usual method of reducing the effect of hot spots is to employ a device known as a mode stirrer, which is a rotatable bladed device in which the blades are inclined in opposite directions to each other so as not to act as a fan but to stir the microwave beam. This does not eliminate the hot spots however; it merely reduces the temperature of each hot spot. The provision of at least one mode stirrer in association with the source of microwave energy (or with each said source if there is more than one) is to be assumed in interpreting the claims and the remainder of this specification, since the use of mode stirrers is essential in practice.

According to the invention in one aspect, in a method of heating a body by microwave energy, the body is heated in a microwave enclosure by a microwave source which is subjected to movement relative to the body while heating the body, the body being stationary.

According to the invention in another aspect, in a method of heating a body by microwave energy, the body is heated in a microwave enclosure having means for admitting said energy into the enclosure from a microwave source wherein the body is stationary and the enclosure is subjected to movement relative to the body while heating the body, the body being stationary.

According to the invention in another aspect, in a method of heating a body by microwave energy, the body is heated in a microwave enclosure having means for admitting said energy into-the enclosure from'a microwave source wherein the body is stationary and the enclosure is subjected to movement relative to the body while the body is being heated.

Preferably, said movement is reciprocating movement. The movement of the enclosure relative to tlgestationary body ensures that the hot spots do not remain stationary for more than an instant, if at all.

Where said body is a continuous elongate body longer than the enclosure, successive portions of the body are preferably moved into the enclosure, stopped, heated and then moved out of the enclosure.

According to the invention in a further aspect, a method of preparing rubber sheet material for vulcanization in a press includes introducing a portion of the sheet material into a microwave enclosure; heating said portion in the enclosure by microwave energy while the sheet material is stationary and while the enclosure is subjected to reciprocating motion relative to the sheet material; moving the sheet material so that the heated portion thereof enters a vulcanizing press adjacent the enclosure and so that the next adjacent portion of the sheet material is then in the enclosure; and pressing the heated portion in the press.

According to the invention in yet another aspect, apparatus for heating a body by microwave energy includes a microwave enclosure having means for admitting microwave energy to the body, support means for holding the body stationary independently of the enclosure, and means for moving the enclosure relative to the body.

According to a preferred feature of the invention, where the enclosure has an inlet opening for admission of the body, said inlet opening is provided with a choke for reducing escape of microwave energy from the enclosure.

According to another preferred feature of the invention, the enclosure has said inlet openingat one end and an exit opening at the other end, said exit opening also being provided with a said choke.

Said support means preferably includes a conveyor belt mounted independently of the enclosure but passing therethrough through said openings.

The apparatus preferably includes a bed fixed in the enclosure so as to support the conveyor belt.

Where the means for moving the enclosure is a reciprocating device for applying motion to the enclosure, the reciprocating device and support means are preferably mounted on a support structure on which the enclosure is mounted for linear motion relative to the support structure in response to said reciprocating motion. ln such an apparatus, the said support structure is pivoted adjacent one end of the enclosure on a fixed axis and is pivoted adjacent the other end of the enclosure to a first part of a press which is movable relative to a cooperating second part of the press for pressing a portion of an elongate body previously heated in the enclosure.

In one preferred form of apparatus according to the invention, the enclosure includes a plurality of intercommunicating cells each having a source of microwave energy and mode stirring means.

A body heated by a method, and/or in apparatus, according to this invention, is included in the scope of the invention.

Heating apparatus in two forms according to the invention will now be described, by way of example and with reference to the accompanying diagrammatic drawings, ofwhich:

FlG. 1 is a diagrammatic part sectional elevation showing a microwave enclosure in one form, in combination with a steam-heated flat molding press;

FIG. 2 is a part sectional elevation showing a microwave en closure in another form; and

FIG. 3 is a cut away general view of a typical choke for a microwave enclosure.

With reference to FIG. 1, a rectangular microwave enclosure (or cavity) 1 has two inputs 2 connected to a source of microwave energy 3 the frequency of which is 2,450 megacycles/second. Each input 2 is associated with one of two mode stirrers 4, located within the enclosure 1 and each driven by an electric motor 5.

The enclosure 1 is supported by flanged wheels 6 which engage and are arranged to run along rails 7. A reciprocating unit 8, incorporating an electric motor and crank, is con nected to the enclosure 1 by a connecting rod 9 and pivot 10 for reciprocating the enclosure 1 along the rails 7. The frequency and stroke of the reciprocating motion of the unit 8 are respectively about 6 cycles/minute and about l5 cm.

The opposite ends 11 of the enclosure 1 have horizontal slots 12 arranged for the passage of rubber conveyor belting (Which is to be heated in the enclosure) into, through and out of the enclosure 1.

The slots 12 are defined by chokes 13 arranged to prevent emission of microwave energy from the enclosure 1 through the slots 12.

In the arrangement shown in FIG. 1, the enclosure 1, rails 7 and reciprocating unit 8 are supported on vertical hydraulic rams 14. The rams 14 are capable of moving the enclosure 1, rails 7 and unit 8 vertically.

Closely adjacent the end of the enclosure 1, remote from the unit 8, is a flat molding press 15. The press 15 has a fixed body 16 and a movable platen 17. The body 16 has a fixed platen 18. The movable platen l7 and the fixed platen 18 having cooperating metal surfaces, heated by steam, between which rubber conveyor belting to be consolidated can be placed for molding,

The movable platen 17 is supported by three hydraulic rams 19. The rains 19 are capable of moving the movable platen vertically, and, in cooperation with the fixed platen 18, of subjecting rubber conveyor belting placed between the

platens

17 and 18 to a pressure of at least 175,000 kg./m. (250 pounds per square inch).

The

platens

17 and 18 are heated by steam introduced to the press 15 by way of an inlet 20 and exhausted from the press 15 by way of an outlet 21. The slots 12 and

platens

17 and 18 are aligned so that rubber conveyor belting can pass straight through the enclosure 1 and between the platens l7 and 18.

Adjacent the end of the enclosure 1 remote from the press 15 is a hydraulically actuated gripper shown diagrammatically at 22, which when operated grips the rubber conveyor belting. Adjacent the end of the press 15 furthest from the enclosure 1, and attached to the movable platen 17, is a hydraulically operated stretcher shown diagrammatically at 23, which, when operated after the gripper 22 has gripped the rubber conveyor belting, grips and stretches the rubber conveyor belting suspended between the gripper 22 and the stretcher 23 by a predetermined amount. The gripper 22 and stretcher 23 are arranged to grip the rubber conveyor belting at a position in which the belting suspended therebetween passes straight through the slots 12 and between the platens l7 and 18.

The

rams

14 and 19 are supplied with hydraulic fluid by way of a pipe 24 from a source'not shown. All rams 14 and 19 are arranged to operate in synchronism with each other.

In an example of operation of the apparatus according to FIG. 2, a section of rubber conveyor belting 24, comprising several rubber-impregnated canvas plies bonded between rubber outer covers, is to be vulcanized in the press 15. The section of belting 24 is positioned in the enclosure 1, and the gripper 22 and stretcher 23 are operated to engage and stretch the belting. The enclosure 1 is supplied with microwave energy from the source 3, the mode stirrers 4 being at the same time rotated by the electric motors 5, and the reciprocating unit 8 being operated to reciprocate the enclosure 1 inch relative to the conveyor belting.

The said section of belting is heated by the microwave energy in the enclosure to a temperature of between C. and C. When the required temperature is reached, the gripper 22 and stretcher 23 are released. The rubber conveyor belting is then moved through the apparatus (by means not shown) until the heated section of belting is between the

platens

17 and 18 of the press 15. The gripper 22 and stretcher 23 are then operated again to engage and stretch the belting. The

rams

14 and 19 are now supplied with hydraulic fluid so as to raise the movable platen 17, the enclosure 1, reciprocating unit 8, gripper 22 and stretcher 23 until the heated section of belting 24 is clamped firmly between the platens l7 and 18. During this operation the belting still extends in a straight line through the apparatus. The platens l7 and 18, supplied with steam by way of the inlet 20, raise the temperature of the heated belting still further to the vulcanizing temperature of between C. and C, and exert a pressure in the approximate range 175,000-315,000 kg./m. (250-450 pounds/square inch) on it.

The combination of this temperature and pressure vulcanizes the rubber conveyor belting.

The apparatus can accommodate a range of different thicknesses and widths of rubber conveyor belting and the time taken to heat these to between 80 C. and 100 C. can be maintained substantially constant by adjustment of the power supplied by the source 3.

It will be appreciated that a section of rubber conveyor belting is heated in the enclosure 1 at the same time as a section already heated in the enclosure 1 is vulcanized in the press 15.

It will also be appreciated that the length of the section accommodated in the enclosure 1 is the same as that of the section vulcanized by the press 15.

The enclosure 1 is also suitable for use in combination with a continuous press. in this case belting is passed continuously through the enclosure 1 and the continuous press.

It will be appreciated that means may be provided for supporting sheet material in and/or for conveying sheet material through the enclosure.

It has been found that belting, having discrete metal inserts extending laterally of the belting, can be heated in apparatus and by methods according to the invention.

The apparatus shown in FlG. 2 is similar in principle but constructed differently from that in FIG. 1. in H0. 2, the jacks 14 are omitted and the microwave unit, comprising a microwave enclosure, shown at 30, is supported on a rigid support frame 31 which is mounted through a pivot 32 at one end to a fixed pedestal 33, and through a pivot 34 at the other end to a part of the press 15 which carries the lower platen 17 and which is movable up and down relative to the upper platen 18 by means of the jacks l9.

The microwave unit is arranged as follows. The enclosure 30 consists of a number of open-ended rectangular cells 35 (two of which are shown in section), mounted in a framework 36 comprising longitudinal l-beams 37 which carry upstanding portal frames 38 between which the cells 35 are supported. The framework 36 is mounted for limited longitudinal movement on the support frame 31 by means of rollers 39 carried by the beams 37. The rollers 39 can roll in guides 40 fixed to the support frame 31. A motor 41, mounted in the support frame 3], drives an oscillating mechanism 42 which is coupled to the framework 36 so as to oscillate the enclosure 30 backwards and forwards.

Chokes 43 are provided at each end of the enclosure 30, to prevent escape of microwave energy from the latter through the holes in the ends thereof through which the rubber belting 24 to be heated passes. The belting 24 is carried through the enclosure 30 by a continuous conveyor belt 44, of polypropylene or other material substantially transparent to microwave energy. The belt 44 extends through the enclosure 30 and passes over rollers 45 to 49 mounted on the support frame 31. The roller, 45 is motor driven so that the conveyor belt 44 and rubber belt 24 are moved through the enclosure in the direction indicated in the Figure.

A bed, shown diagrammatically at 50, extends longitudinally through the entire length of the enclosure 30. The bed 50, which is of heavy polypropylene or other material substantially transparent to microwave energy, is rigidly supported in the enclosure 30 so as to prevent sagging of the belt 44.

Each cell 35 has its own microwave source, shown diagrammatically at 51, at the top, and a mode stirrer 52 associated therewith.

In operation, the roller 45 is driven so as to bring into the enclosure 30 a section of the belt 24 equal in length to the enclosure. The roller 45 is then stopped. The motor 41 is started, and at the same time the microwave sources 51 and mode stirrers 52 are energized. The belt 24 is thus subjected to microwave energy which heats it to the required temperature. The incidence of hot spots is reduced by the mode stirrers, the hot spots being moved continuously back and forth along the belt by the oscillating action of the unit 42.

It should be noted that since microwave heating effects tend to be reduced close to metal surfaces, the bed 50 should be supported in such a way as to support the conveyor belt 44 and belt 24 as far as possible away from any metal below it.

For this purpose the bed may itself be supported in the enclosure 30 by a support structure made entirely or partly of a material transparent to microwave energy. In this connection it will be appreciated that the bed preferably oscillates with the enclosure, as it does in the example described.

The chokes to be used at the open ends of microwave enclosures in apparatus according to the invention can be of any suitable design for preventing harmful microwave energy from escaping. Their precise design is dependent on the dimensions and character of the material to be processed by the apparatus. Such a choke typically comprises several hollow microwave elements, i.e. spaces, which may be interconnected, the dimensions of these elements being such as to align and alternate the microwave power in such a way that any radiation from the open outer end of the choke is below the permitted maximum power level.

One example ofa choke of this kind is shown in FIG. 3, and consists of a single hollow fabricated aluminum member 60, having a rectangular opening 61 at the outer end and another, 62, at the other end, at which the choke is fixed to the outside of the end wall 63 of a microwave enclosure. The member 60 has three pairs of transverse channel section enlarged portions 64-66, each constituting a microwave element such that the enlarged portions serve to align and alternate, as discussed above, microwave power escaping through the opening 62.

It will be realized, however, that the microwave elements of the choke need not be transverse or of channel section.

lclaim: 1. Apparatus for heating an elongate body by microwave energy, including a microwave enclosure having means for admitting microwave energy to the body, support means for holding the body stationary independently of the enclosure, and a reciprocating device for subjecting the enclosure to a reciprocating motion in a direction parallel to the plane of the body, said enclosure having an inlet opening at one end for admission of the body and an exit opening at the other end, a choke being provided at each of said inlet and exit openings for reducing escape of microwave energy from the enclosure, and said support means including a conveyor belt mounted independently of the enclosure but passing therethrough through said openings.

2. Apparatus according to claim 1, including a bed fixed in the enclosure so as to support the conveyor belt.

Claims

1. Apparatus for heating an elongate body by microwave energy, including a microwave enclosure having means for admitting microwave energy to the body, support means for holding the body stationary independently of the enclosure, and a reciprocating device for subjecting the enclosure to a reciprocating motion in a direction parallel to the plane of the body, said enclosure having an inlet opening at one end for admission of the body and an exit opening at the other end, a choke being provided at each of said inlet and exit openings for reducing escape of microwave energy from the enclosure, and said support means including a conveyor belt mounted independently of the enclosure but passing therethrough through said openings.