US3027442A - High-frequency furnaces - Google Patents

Description

March 27, 1962 J. VERSTRATEN HIGH-FREQUENCY FURNACES Filed Feb. 13, 1961 INVENTOR JAN VERSTRATEN.

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AGENT umat:

United States altent 3,027,442 HIGH-FREQUENCY FURNACES JamVerst rateu, EmmasingehEindhoven, Netherlands, as-,

The invention relates tohigh frequency heating furnaces and more particularly to high frequency furnaces operating inthe microwave region.

This application is a continuation-in-part of my copending application, Serial No. 2-6,235'filed May 2, 1960, and entitled High Frequency Heating Furnaces Operating With Very High Frequencies. The heat furnace described therein: comprises a heating unit having a high frequency generator and compatible Waveguide system. The waveguide systemcomprises a parabolic planar reflecting surface bounded by a pair of parallel monoplanar conducting surfaces which are normal to the focal line o f'the' parabolic plane of the reflecting surface. A conveyor belt system isprovid'ed to place workpieces to be heated in juxtaposition with an opening of the waveguide system formed by the mutual intersections of each of the conducting surfaces with the mouth of the parabolic plane of' the reflecting surface. A linear radiator coupled tothegenerator by suitable load matchingmeans, is placed in substantial coincidence along its longitudihalaxeswith the focal line which is located, because of thepresence of'the' workpieces and/ or the conveyor helt'at the opening, within the parabolic plane between the verte'x line and mouththereof. In order to interceptthe direct radiations from the focal line, i.e., the linear radiator, and provide a uniform electro-magnetic field of constant intensity at the opening, reflector means are disposed between the opening and the focal line causing substantially the whoe radiation from the linear radiatorto' be reflected from the parabolic planar reflectingsurface in the direction of the opening. To provide radiations from" emanating from the sides of the opening', the linear radiator irradiates a linearly polarized field having an electrical field vector parallel to the di rection of the linear radiator and in a directioncoincident with" the" direction of movement of the belt of the conveyor' belt system past the opening. The energy vector (Poynting vector) is at right angles to the electric field vector and hence has no' component in the direction of movement of the conveyorbelt, and, consequently, only stray. rays can" emanate from the sides of the opening,

Coupled to each side of: the opening and at right angles I to the p'ara'l'le'l boundary surfaces are two aligned passages which further-attenuate these stray. rays and also provide an entrance and exit guide" passages to the opening of the waveguide system for the belt and the workpieces supported thereon. Beneath the opening the parabolic shaped surface is extended at themouth thereof pastthe boundary con'ducting; surfaces to form a rectangular trough below the. conveyor: belt. The trough contains an absorbing material to absorb the radiations which pass throughthe belt, so that" the generator and waveguide system. are adequately loaded even. in the absence of workpieces tolb e heated; In operationg'the workpieces whenfplaced in juxtaposition-to the opening, are heated by the dielectric displacement currents produced in the workpiece when subjected to the electro magnetic field.

In the heat furnaces of'the co-pending applicatiomsubpredetermined distances from each other.

stantially described herein above, while generally providing for satisfactory uniform heating, it was found that 3,927,442 Patented Mar. 27, 1962 uniform heat distribution because, inter alia, of the nonuniform. energy. absorption of the heterogeneous mixture. Inheatingapplicatious where the aforedescribed heat fur naces are utilized to heat frozen foodstuffs, itv was found that because of the small; penetration depth and, energy absorption of the food in thefrozen state, as well as the runaway effect of melted parts at the, surface, the nonuniform heat distribution. was even more noticeable, and, more especially, if the frozen foods were, also, of a heterogeneous nature. In addition it was often found, that, the. vapors, steam, etc., released. by the workpieces during the heating process, besides contaminating the surfaces andparts. of the system, also caused variations in the dielectric medium of. the'waveguide system resulting in. undesirable changesin the. operating characteristics ofthe system, such. asloss of power, etc., as well as, contributing; to the heat distribution problems mentioned above. Moreover, if these workpieces happened to be foodstuifs,.it was found,.that. they often released malingering odors, vapors and the like, in the system that would contaminateand/or, obnoxiously permeate similar or. different foodstufi's subsequently prepared and render them inedible or undesirable.

An object of this invention is to provide a high frequency heat furnacewith an improved uniform heat distribution for the workpieces heated thereby.

Another object of' this invention is to provide a heat furnace that alternately subjects the workpieces to zones ofinjected microwave energy by which heat is generated withiuthe workpiece, andzones of microwave energy in-. activity wherein the heat, previously generated becomes uniformly dispersed throughout the. workpiece.

Among other objects are included the stabilization, of the variations in the dielectric medium caused by vapors and the like released from workpieces heated by the heating furnace of this invention, and the, mitigation and/or izemova'liof such vapors and. their effects.

. Accordingly, this invention features a high frequency furnace for the heat treatment of workpieces by high frequency oscillations which comprises a plurality of heating units, and means to disperse the heat generated in. the workpiecesby each unit. Each heating. unit and waveguide means has a first parabolic planar reflecting surface and second and third monoplanar conducting surfaces, such that the second and third surfaces are disposed in a substantially parallel relationship with each other and are. at right: angles to the first. surface to provide an enclosure having an opening substantially normal to the principal axis of'the parabolic planar surface. Radiating means are disposed at the focal line of the parabolic planarsu-rface; with the focal linebcing disposed between the opening andvertex line of the parabolic planar surface. Wave energy reflecting means are disposed be:

tween the radiating means and the opening, as well as, means for coupling the generator to the radiating means. A conveyor'belt'member having a movable supporting surface is provided for conveyingthe workpieces normal to 1 each-of the openings to have heat energy generated inLthe workpieces bymicrowave energy emanating therefrom; the conveyor belt member being driven by suitable driven means; The heat dispersing means comprises a conductive. hollow" transport member disposed about the supportingsurface'of the belt member and the workpieces supported thereon, and the transport member also has a plurality, corresponding to the number of openings of the waveguide systems, of apertures spatially disposed at Each of the openings of the waveguide means are associated with one of the apertures, so that the. workpieces may be subjected to, the aforementioned, microwave energy at the sites thereof and during the conveyance of the workpieces to 7 ceeding waveguide means is therein distributed in a more uniform manner.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the following drawing, in which:

The single FIGURE is a perspective drawing of the heating furnace of this invention.

The high-frequency furnace according to the invention, shown'in the drawing, is mounted on frame 1 and is suitable for high-frequency heating of workpieces, as for example, prepared frozen foodstuffs carried on conveyor belt 2. having a supporting surface 2a and associated driving rollers 3, 4, 5, of which the roller 3 is directly driven by a driving motor 6. In the illustration, the objects to be heated, consist of frozen meals having a temperature of, for example, -30 C., arranged on plates 7, and are taken off the end of the conveyor belt system for direct consumption after being heated in the high-fre quency furnace to a temperature of 90 C. for 2 to 4 minutes. 7

' The high-frequency furnace comprises, for example, five separate high-frequency heating units, each having a high frequency generator, as for example, magnetron genlinear radiator 22. The relative distance between the rods and their distance from the linear radiator are, for example, 0.4 and 0.2, respectively, of the wavelength.

Substantially the complete radiation from the linear radiator 22 emanated in accordance with the radiation pattern described above is reflected from the parabolic cylinder surface 19 in the direction of the parabola axis 19a towards the openings of the waveguide system 18 to 18"". At the outlet openings, in the directions extending parallel to the boundary 'surfaces'20, 21, a uniphase, electromagnetic field of substantially constant intensity is produced, so that workpieces to be heated on the conveyor belt 2 passing the opening are heated substantially uniform.

The linear radiator 22 produces a linearly polarized field, of which the direction of the electric field vector E extends parallel, to the direction of the linear radiator 22, so that at the openings of the waveguide system 18 to 18" it extends in a direction indicated by the broken arrows 24 in the broken-off part of the waveguide 18. As a consequence, the energy vector (Poynting vector), which is at right angles to the electric field vector E,

- has no component in the transport direction of the conerator 8 of which only two are illustrated for the sake of clarity in the drawing. These generators may be, for example, rated at 2 kw., each, and produce oscillations having a wavelength of 12 centimeters. Associated with each generator is a separate voltage supply 9. A common supervisory and control-panel 10 is provided with a plurality of anode current meters 11, anode-current adjusting resistors 12, on-off switches 13, and safety fuses 14, each of which is associated with one of the generators.

Each of the output circuits of the magnetron generators 8 is coupled by means of a coaxial conductor '15, via adjustable load matching means 16, in a manner well known to those skilled in the art, to its associated waveguide system 18, 18, 18", 18", 18"", respectively, which' propagate the oscillations generated by the magnetron generators 8 to the outlet openings of the waveguide systems-18 to 18". The outlet opening is formed by the surfaces 19, 20 and 21, and is located in a plane normal to the principal'axis, illustrated in the drawing as extended broken line 19a of the parabolic planar reflecting surface 19. Workpieces to be heated are transported on the conveyor belt 2 past these openings. In order to illustrate in greater detail the interior of the waveguide systems 18 to 18"", which are identical for each heating unit, the front wall of the second waveguide system 18 is partly broken away.

To obtain, as described fully in the aforementioned copending application, a uniform heating throughout the width of the conveyor belt 2, which may, for example, be 70 cms, each of the waveguide systems 18 to 18" veyor belt 2, and, therefore, stray rays leave the waveguide system 18' to 18"" with considerable attenuation, for example by 25 db, with respect to the field in the waveguide system 18 to 18"". 7

While using the aforedescribed properties of the heating unit of a high-frequency furnace in accordance with the teachings of my aforementioned co-pending application, the present invention provides a considerable further improvement which consists in the use of a plurality of high-frequency heating units, with the further provision of a transport channel 25, surrounding the supporting surface 2a of the conveyor belt 2 and the workpieces supported thereon. The channel 25 is made from conductivematerial and is illustrated, by way of example, as having an upper and lower boundary surfaces 26, 27, respectively which extend parallel to the conveyor belt 2, and extend in the direction of movement of the conveyor belt 2. The transport member is provided with five apertures which are associated with the openings of the waveguide systems 18 to 18"" and as illustrated in the drawing at system 18', the aperture of the channel 25 thereat is coincident with the opening 18a. The parallel boundary surfaces 20, 21 of these openings, such as opening 18'a, are substantially at right angles to the common transport channel 25. At the area of the openings of the waveguide systems 18 to 18"" each boundary surface of the common transport channel 25 opposite the waveguide system is bent over in a direction leading away from the conveyor belt in the form of a rectangular trough 28, which acts as a support means for absorbing is formed by a parabolic planar reflecting surface 19 and planar surface, a linear radiator 22 is arranged. Bea tween the linear radiator 22 and the outlet opening 18'a of the waveguide system and remote from the vertex line, illustrated as extended broken line 190, of the parabolic planar surface 19, reflector means 23 is provided to reflect the directradiation of the linear radiator 22 towards the parabolic planar surface 19, so that by suitable arrangement of the linear radiator 22 and the reflector means 23, a radiation pattern is obtained at the opening 18'a-in planes at right angles to and along the linear radiator 22, while alongside the openings of the waveguide systems 18 to 18"" substantially no radiation occurs. In the embodiment shown, the reflector means 23 consists of two reflector rods, arranged parallel to the material which is placed therein to absorb the radiation from the linear radiator 22 which passes through the conveyor belt 2. Thus, the magnetron generators 8 are adequately loaded, even in the absence of workpieces to be heated on the belt 2.

The. variouswaveguide systems 18 to 18"" with their associated magnetron generators 8 are arranged, at distances which are most favorable for the heating process,

by properselection of the spacing between adjacent apertures of the channel 25 due to the low emanating stray radiation of each of the waveguide systems 18 to 18"", the magnetron generators 8 are relatively substantially decoupled, so that a relative destructive influence between adjacent waveguide systems is not effected, even under excessive, varying operating conditions. The high-frequency furnace described above pro,- vides a material increasein the output power, which results in a corresponding acceleration of the heating process and a material reduction of the overall stray radiation. The stray radiation existing outside of the multiheating unit furnace herein described is substantially the same as that existing in the single heating unit described eoams in my aforementioned co-pending'application because ofthe necessity of requiring damping in the transport channel: for only one equivalent unit; viz., the damping of the stray radiation from the inlet to the firstv waveguide system- 18-and the outlet of the last waveguide system 18 respectively. For. example, inthe high-frequency furnace described herein, thewmaximum. power. of. the stray" radiation. occurring beyond the. high; frequency furnace at the: area of the attending. staff. personnel is only onemilliwatt' per square centimeter. Also, a-material' improvement. in the: heating process andv in the efficiency thereof, is provided by the heat furnace of this invention, as. will be explained more fully hereinafter.

When the workpieces to be heated, illustrated in the drawing. inv the form of frozen. meals, are arrangedat the inlet of the transport channel 25, they are heated by high-frequency oscillations on their journey through the high-frequency furnace when passing by the outlet opening ofeach Waveguide system 18 to 18"". During the time required by the object to reach the outlet opens ing of the next following waveguide no high-frequency heating ocurs, so that alternate time periods with and without high-frequency heating are provided. In the time period without heating the heat developed during the preceeding heating period is allowed to disperse in the objectto be heated, which is advantageousfor a: 11111- form heating of the workpieces, especially those comprised of a heterogeneous mixture. Due to local variations in the dielectric constant of the workpieces to be heated, local discontinuities may occur in the highfrequency heating; in the present embodiment this occurs, for example, at the instants when the frozen meals thaw. In order to attain optimum conditions, the distances between the successive waveguide systems, which distances determine the time periods without heating, are to be judiciously selected since in the case of a selection of too small a distance the heat may not have the possibility of dispersing uniformly in the object, whereas in the case of too great a distance the object may become chilled.

With the high-frequency furnace described a uniform heating of the objects is obtained, which is further improved in the embodiment shown by connecting to the transport channel 25 at least one of the heating units, as for example, the waveguide system 18" of the third heating unit, in a direction opposite to that of the other waveguide systems 18, 18, 18", 18", which allows workpieces having a small penetration depth to be exposed to bilateral radiation.

In the high-frequency furnace described the material increase in efiiciency is important. The objects to be heated pass, during various heating stages, by the outletv openings of the waveguide systems 18 to 18" of the various high-frequency heating units, so that these objects constitute different loads for these high-frequency heating units. By adapting each of the magnetrons 8 by means of its adjustable load matching means 16 to the workpieces the efiiciency is raised to a maximum; for example, increases in efliciency of 30% are obtainable.

Consequently, by using the principles according to the'teachings of this invention, a high-frequency furnace is obtained which is distinguished not only by a considerable increase in the output power, for example, 10 kw.

and still maintaining a minimum stray radiation, but

produced during the high-frcquency heating, a blower system having acompressed air duct 30 is used in conjunction with the high-frequency furnace. The duct 30 communicates through gaps 31 extending parallel to the linear radiators 22 in the parabolic cylinders 19 in the proximity of the vertex line 19c directlywith the waveguides 18, 18, 18 -"',;18". The gaps 31', which extend parallel to the current paths in the parabolic cylinder, do not. produce a disturbance of the satisfactory operation of the high-frequency furnace, so that in the high-frequency furnace according to this invention, the vapors developed during high-frequency heating can be removed in a simple manner from the high-frequency furnace without theneed' for particular measures,- and, thereby; mitigate their aforementionedetfects.

It is to be understood that the heating units described hereinare selected by way' of example to teach the principles of my invention and that, therefore, other types of microwave heating units may be utilized, as well as, different arrangements and/or quantities of component parts of the units may also be utilized, as for example, the utilization" of a single high-frequency generator means associated commonly with two or more of the heatingunits: Thus',while I have described my invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in the art without departing from. the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A high-frequency furnace for the heat treatment of workpieces by high-frequency oscillations comprising high-frequency generator means, a plurality of heating units, each of said units having waveguide means having a first parabolic planar reflecting surface and second and third mono-planar conductive surfaces, said second and third surfaces being disposed in a substantially parallel relationship and at right angles to said first surface to provide an enclosure having an opening substantially normal to the principal axis of said parabolic planar surf-ace, radiating means disposed substantially at the focal line of said parabolic planar surface, said focal line being disposed between said opening and the vertex line of said parabolic planar surface, and means for coupling said generator means to said radiating means, a conveyor belt member having a movable supporting surface for con- .veying said workpieces past each of said openings to generate heat energy in said workpieces by microwave energy supporting surface and the workpieces supported thereby, said hollow member means having a plurality of apertures spatially disposed at predetermined distances from each other, each of said openings of said waveguide means, being associated with one of said apertures.

2. A high-frequency furnace for the heat treatment of workpieces by high-frequency oscillations comprising a plurality of heating units, each of said units having a high-frequency generator, waveguide means having a first parabolic planar reflecting surface and second and third mono-planar conductive surfaces, said second and third surfaces being disposed in a substantially parallel relation ship and at right angles to said first surface to provide an enclosure having an opening substantially normal to the principal axis of said parabolic planar surface, radiating means disposed at the focal line of said parabolic planar surface, said focal line being disposed between said opening and the vertex line of said parabolic planar surface, wave energy reflecting means disposed between said radiating means and said opening and means for coupling said generator to said radiating means, a conveyor belt member having a movable supporting surface for conveying said workpieces past each of said openings to generate heat energy in said workpieces by microwave energy emanating therefrom, means to drive said conapertures spatially disposed at predetermined distances from each other, each of said openings of said waveguide means, being associated with one of said apertures.

3. A high-frequency furnace according to claim 2, further comprising means to adjust the coupling of each generator of said heating units to the associated radiating means thereof.

mono-planar conductive surfaces, said second and third surfaces being disposed in a substantially parallel relationship and at right angles to said first. surface to provide an enclosure having an opening substantially normal to the principal axis of said parabolic planar surface, radiat ing means disposed at the focal line of said parabolic. planar surface, said focal line being disposed between said opening and the vertex line of said parabolic planar surface, wave energy reflecting means disposed between said radiating means and said opening and means for coupling said generator to said radiating means, a con veyor belt'member having a movable supporting surface for conveying said workpieces past each of said openings to generate heat energy in said workpieces by microwave energy emanating therefrom, meanstto drive said con-t veyor belt member, and means to disperse said heat energy comprising a conductive hollow member .having at least first and second boundary surfaces disposed in parallel relationship witlrand about said supporting surface of saidbelt member and the workpieces supported thereby, said hollow member having further a plurality of apertures spatially disposed at predetermined distances from each other located on at least one of said boundary surfaces each of said openings of said Waveguide means being associated with one of said apertures.

6. A high-frequency furnace. according to claim 5,

wherein at least one of said apertures is located on said first boundary surface and the remaining said apertures. are located on said second boundary surface.

7 7. A heat frequency furnace according to claim 5, further comprising means to support absorbing m'aterial'on the boundary surface of said channel opposite each of said apertures located on the other of said boundarysurfaces. r

Noreferences cited.

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