US3474209A - Dielectric heating - Google Patents

Dielectric heating Download PDF

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Publication number
US3474209A
US3474209A US629440A US3474209DA US3474209A US 3474209 A US3474209 A US 3474209A US 629440 A US629440 A US 629440A US 3474209D A US3474209D A US 3474209DA US 3474209 A US3474209 A US 3474209A
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Prior art keywords
waveguide
energy
product
guide
heating
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US629440A
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William N Parker
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RCA Corp
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RCA Corp
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/78—Arrangements for continuous movement of material
    • H05B6/782—Arrangements for continuous movement of material wherein the material moved is food
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S65/00—Glass manufacturing
    • Y10S65/04—Electric heat

Definitions

  • microwave heating in the processing of dielectric products is the possibility of reduced cost-per-unit of the product due to greatly reduced processing time. A shorter time permits more processing cycles per day and thus provides more total product output. Furthermore, the product load depth can be several times greater with microwave dielectric volumetric heating than for conventional heating methods. Economic studies indicate that the cost savings derived by utilizing microwave heating, for example, in a freeze drying process, can vary between fifty and eighty percent as compared with the more conventional freeze drying techniques. In addition, the capitalization costs for necessary equipment are often substantially reduced when dielectric microwave heating techniques are used to provide energy for a drying function.
  • United States Patent O ice isV sufficient to state at this point that the uniformity of microwave dielectric heating has heretofore been difficult to regulate and control in a large fixed processing enclosure.
  • the invention disclosed herein provides novel apparatus for use in a process requiring as a step the exposure of a lossy dielectric product to a source of heat in the form of a microwave generator.
  • the apparatus takes the form of a waveguide having a properly designed non-linear taper along one dimension thereof into which the product is placed. Microwave energy is launched into the waveguide and propagated along the direction of the taper.
  • the described waveguide permits the product Ibeing processed to ⁇ be exposed to a uniform amount of heat at every point within the waveguide wherein the product is situated.
  • the invention provides a novel method for the heat treatment of the products being processed utilizing the disclosed apparatus.
  • an object of the present invention to provide an improved method of heating a product using microwave energy as the heating source, wherein a high degree of uniformity of heating is maintained.
  • a further object is to provide an improved apparatus for use with a source of microwave energy to apply uniform heating to a product, particularly, in applications where a continuous iiow technique is used.
  • FIGURE 1 is a perspective view, partly in cross section, of heating apparatus in accordance with the present invention
  • FIGURE 2 is a vectorial depiction of the electric eld distribution for a TEM mode propagating within the apparatus shown in FIGURE 1 at any point therealong;
  • FIGURE 3 is a graph wherein waveguide characteristic admittance is plotted as a function of guide height for various materials having different relative dielectric constants.
  • FIGURES 4 and 5 are perspective views of additional embodiments in accordance with the concepts of the present invention.
  • Freeze drying is one of the newest techniques used in the field of food preservation. It is a sublimation process wherein moisture is removed from frozen products without changing their shapes, colors, flavors, or nutritional value, In the conventional freeze drying process, the frozen raw or cooked products are placed on trays in an atmospherically regulated chamber. A controlled amount of heat (heat of'sublimation) is then applied to the products within the chamber through liquid heated trays or platens.
  • the frozen moisture within the food evaporates (sublimes) and the dried food emerges in a solid, spongelike condition.
  • the products can be shipped or stored Without the need for refrigeration.
  • Prior to heating, cooking, or otherwise handling as fresh food the products require only re-hydration.
  • the outer layers of food tend to dry more readily and surround the undried frozen inner portions thereby acting as a heat-flow barrier and 'retarding the further transfer of heat from the hot platens to the interior of the product. Drying times of from eight to twenty-four hours are often required to sublime the ice completely.
  • Dielectric heating is volumetric in nature and tends to speed up the drying rate significantly thereby increasing the product output and reducing the cost per unit of output.
  • a variety of approaches toward alleviating the aforementioned problem were investigated.
  • One solution considered was to decrease the length of the waveguide, terminate it at the remote end with a matched load and utilize only a fraction of the propagating power in the guide to heat the food. This approach was found to be unsatisfactory in that it resulted in a large waste of electric power.
  • FIGURE 1 shows a waveguide 10 having a non-linear taper 11 along its length such that the height of the guide decreases from an initial height of HI at the input end 12 of the guide to a height of Hc at the output or cutoff end 14 of the guide.
  • a conveyor belt 16 for carrying food or other product 18 to -be processed passes down the length of the waveguide
  • the belt 16 is preferably made of a thin non-lossy material and is of a constant width, as is the waveguide 10.
  • a source of energy e.g., a microwave generator (not shown), supplies energy to the input end 12 of the waveguide 10 via a pair of antenna probes 20, coaxial leads 22, and, as shown in the ernbodiment of FIGURE 1, a power divider 24.
  • a microwave generator not shown
  • the waveguide 10 As the power enters the waveguide 10 via the antenna probes 20, a traveling wave is established along the length of the guide; the wave propagating down the length of the guide in the direction of cutoff 14. Where it is desirable that .the process be continuous fiow in nature, the direction of conveyor movement may be either toward the input end of the guide 12 or, as shown in FIGURE 1, toward the cutoff end 14 of the guide 10. Should it be desirable to utilize a batch-type process the product 18 to be heated can be positioned within the guide 10 and then permitted to remain stationary therein for the desired period of time. To obtain the maximum heating effect from the traveling wave to the product being processed, the waveguide 10 is designed to support a TEM mode such that the electric field strength is always greatest in a plane slightly above the plane which is defined by the belt or support 16.
  • FIGURE 2 represents the electric field distribution E at any point along the waveguide 10. It is important that the guide be designed to have a width 26 narrower than a half-wavelength such that the guide will be unable to support undesired TE or TM modes. As shown in FIG- URE 2 the belt 16 is positioned slightly beneath the center of the waveguide 10 so that the maximum electric field Em passes through the center of the processed product 18 which sits upon the belt 16.
  • a natural first approach for supplying microwave power as described is to attempt to utilize a wave traveling along a waveguide of constant cross section containing the lossy dielectric to be heated.
  • the heating effect of such an applied traveling wave is rapidly diminished as it progresses down the length of the waveguide due to the power within the traveling wave being attenuated as it passes through the lossy medium.
  • Equation 1 defines the amount of power dissipated as heat:
  • F is the frequency in cycles per second
  • Em is the peak electric field strength in volts/cm.
  • e is the loss factor of the processed material.
  • Equation 2 defines the following relationship between Pp the power propagating through the waveguide, Y the admittance of the waveguide at any point along its length, and V the voltage across the waveguide:
  • Equation l the only way to maintain E2 constant so that Pd can remain constant (Equation l) is to decrease the admittance Y of the waveguide along its length so that the ratio of Pp to Y remains constant. Recognizing that the admittance Y is a function of the cross section of the guide at any point therealong, the effect of the foregoing relationships maybe summarized by saying that to deliver to a lossy dielectric material within a waveguide a uniform heat density, it is necessary that the cross section of the guide be appropriately varied as a function of guide length.
  • the width 26 of the waveguide 10 which for the purpose of attaining maximum heating efficiency should be as close to the Width d of the product as possible, be incapable of supporting a mode other than a TEM mode.
  • a half wavelength Will be approximately 6.45 inches in free space. It is important therefore to make sure that the width 26 of the waveguide 10 be designed to be somewhat narrower than this figure.
  • the next step in the calculation is to determine the cutoff height HC of the waveguide at its narrow end 14.
  • Cutoff is defined for purposes of this application as that point along the length of the tapered waveguide where, for a constant frequency and guide width, the characteristic admittance of the waveguide 10 becomes zero and the waveguide is incapable of supporting a travelling wave thereby resulting in the power propagated becoming zero.
  • curves such as shown in FIGURE 3. These curves plot the waveguide admittance Y as a function of guide height H for materials having various dielectric constants. These curves may be calculated based on the work of P. H. Vartanian et al.
  • the next step is to determine the height HI of the guide at its input 12 and then the height at various intermediate points thereby determining lthe shape of the taper.
  • To determine the input height HI it is first necessary to calculate the value of a parameter referred to as the conductance G of the dielectric which is defined as follows:
  • the input height must be selected so as to be incapable of supporting a TE mode other than a TEM mode, i.e. a TEOZ mode, and since the input height HI is a function of the guide length as has been shown, the length chosen should be selected so as to satisfy this requirement.
  • the intermediate heights H1 and HC of the guide can be readily determined by interpolation.
  • the food to be dried is first placed into a frozen state in accordance with state of the art techniques. While in such condition it is then admitted into an atmosphere which is controlled as provided in the present state of the art such that the partial pressure of water therein is below the saturation vapor pressure of the frozen food. The presence of such an atmosphere is necessary to permit sublimation of the frozen water within the food during the subsequent heating step. Also contained within said atmosphere is one or more tapered waveguides, designed in accordance with the properties of the food as discussed supra.
  • the frozen food is then either held or passed through the waveguide and thereby exposed to the microwave energy propagated therein, resulting in the evaporation of the water directly to the gaseous state.
  • the guide can be provided with a series of strategically located narrow slots 28 (or holes) as shown in FIGURE 4.
  • the power propagates through the waveguide it Will be attenuated due to absorption by the food as a result of its lossy dielectric properties. A uniform electric field is maintained however as a result of the decreasing admittance characteristic of the waveguide.
  • the food to be processed is in this manner essentially exposed to a uniform heat density through the entire length of the waveguide.
  • the food emerges from the waveguide it may either be removed from the controlled atmosphere and then processed for pack-aging or, if desired, it may be passed through additional waveguides within the atmosphere for further drying before being removed and packaged.
  • FIGURE 4 shows a further embodiment of the invention wherein two tapered waveguides 10 are connected in tandem with their power input ends 12 coinciding. Propagating energy is simultaneously launched in both directions along the lengths of each guide via the Iantenna probes. This has the effect of decreasing the number of source inputs 22 per waveguide thereby further reducing the overall cost of the process.
  • a large continuous process installation might employ several such tandem sections in cascade.
  • FIGURE presents a further embodiment which will nd particular application in the continuous drying or curing of relatively thin products such as textiles, paper, plywoods, adhesives, plastics, etc.
  • the thin product 30 is passed through the width of the waveguide 10 rather than down its length while the direction of the traveling wave ⁇ and the electric field distribution remain the same as shown in FIGURE l.
  • Such a configuration will often be desirable where the width of the product to be dried is greater than the maximum Width to which the guide can be designed Iand still prevent the establishment of undesirable modes.
  • a number of waveguides may be placed one next to the other i.e., side by side, and a thin slot provided in their sides to permit the passage of the product therethrough.
  • FIGURE 5 further illustrates an alternative method of coupling microwave power to the waveguide by the use of loops 27.
  • Apparatus for processing a lossy dielectric material comprising -a tapered waveguide dimensioned to hold therein said material, means for applying electromagnetic energy at the largest end of said waveguide so that said energy propa- -gates along the tapered length of said waveguide,
  • the degree of said taper being determined to maintain a constant ratio between said energy propagated in said waveguide and the admittance of said waveguide with respect to said energy along said tapered length in the presence of said material within said waveguide.
  • An apparatus for heating a moisture laden article comprising,
  • the degree of said taper being determined to maintain a constant ratio between said energy propagated in said waveguide and the admittance of said waveguide with respect to said energy along said tapered surfaces in the presence of said article within said waveguide.
  • said last-mentioned means being designed and arranged to continuously convey said article into and out of said waveguide.
  • said first-mentioned means operating to establish said propagating energy in a TEM mode within said waveguide.
  • An apparatus for use with -a source of microwave energy to provide uniform density heating to a moisture laden article comprising, l
  • a hollow chamber arranged to allow the passage of said article therethrough and along one dimension of said chamber, said energy being propagated within said chamber along said one dimension with said chamber having a non-linear taper along said one dimension designed to provide a constant ratio between said propagating energy and the admittance of said chamber with respect to said energy at any point along said one dimension.
  • Apparatus for processing a lossy dielectric material comprising,
  • a first hollow chamber arranged to allow the passage of said material therethrough, said rst chamber having a non-liner taper along one dimension thereof,
  • a second hollow chamber arranged to allow the passage of :said material therethrough, said second chamber having a non-linear taper along one dimension thereof,
  • said first and second chambers being disposed in axial alignment along said one dimension with the larger ends of said tapered chambers coinciding
  • the degree of taper of said chambers being determined to maintain a constant ratio between said energy propagated in each of said chambers and the admittance of each of said chambers with respect to said energy along said tapered length in the presence of said material within said chamber.
  • Apparatus for processing a lossy dielectric material comprising,
  • each waveguide having a non-linear taper along one dimension thereof, said waveguides being disposed so that the tapered surfaces of each waveguide coincide to form a continuously tapered surface, said waveguides ybeing formed with openings to permit said material being processed to pass through each of said waveguides in a direction transverse to said tapered dimension,
  • each of said tapers being determined to maintain a constant ratio between said propagated energy and the admittance of said waveguide with respect to said energy along said tapered length in the presence of said material within each of said waveguides.
  • a method for heating an article having lossy dielectric properties which comprises the steps of,
  • a method for freeze drying a water laden lossy dielectric article wherein the water content within said article has been previously placed into a frozen state which comprises the steps of positioning said article within a waveguide having nonlinear tapered surfaces in one dimension thereof,

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Drying Of Solid Materials (AREA)
  • Constitution Of High-Frequency Heating (AREA)
US629440A 1967-04-10 1967-04-10 Dielectric heating Expired - Lifetime US3474209A (en)

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DE (1) DE1765150C3 (enrdf_load_stackoverflow)
FR (1) FR1567335A (enrdf_load_stackoverflow)
GB (1) GB1181208A (enrdf_load_stackoverflow)
SE (1) SE333028B (enrdf_load_stackoverflow)

Cited By (19)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4100386A (en) * 1975-08-27 1978-07-11 Automatisme & Technique Process for sintering ceramic products
US4401873A (en) * 1979-11-28 1983-08-30 Stiftelsen Institutet For Mikrovagsteknik Microwave heating device with tapered waveguide
US4874915A (en) * 1988-12-30 1989-10-17 Lifeblood Advanced Blood Bank Systems, Inc. Apparatus for the rapid microwave thawing of cryopreserved blood, blood components, and tissue
US5369250A (en) * 1991-09-27 1994-11-29 Apv Corporation Limited Method and apparatus for uniform microwave heating of an article using resonant slots
US5408074A (en) * 1991-11-05 1995-04-18 Oscar Gossler Kg (Gmbh & Co.) Apparatus for the selective control of heating and irradiation of materials in a conveying path
US5457303A (en) * 1993-05-05 1995-10-10 Apv Corporation Limited Microwave ovens having conductive conveyor band and applicator launch section to provide parallel plate electric field
US5958275A (en) * 1997-04-29 1999-09-28 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6246037B1 (en) 1999-08-11 2001-06-12 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6259077B1 (en) 1999-07-12 2001-07-10 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6753516B1 (en) 1999-12-07 2004-06-22 Industrial Microwave Systems, L.L.C. Method and apparatus for controlling an electric field intensity within a waveguide
US20050092741A1 (en) * 2003-10-24 2005-05-05 The Ferrite Company, Inc. Choke assembly for continuous conveyor microwave oven
US6965099B1 (en) * 2000-08-28 2005-11-15 Georgia Tech Research Corporation Geometry for web microwave heating or drying to a desired profile in a waveguide
US20070131678A1 (en) * 2005-12-14 2007-06-14 Industrial Microwave Systems, L.L.C. Waveguide exposure chamber for heating and drying material
US20070271811A1 (en) * 2004-04-12 2007-11-29 Takaharu Tsuruta Method And Apparatus For Drying Under Reduced Pressure Using Microwaves
EP2086285A1 (en) * 2008-02-01 2009-08-05 Anton Paar GmbH Applicator and Apparatus for heating samples by microwave radiation
US20100200573A1 (en) * 2007-08-06 2010-08-12 Industrial Microwave Systems, L.L.C. Wide waveguide applicator
WO2022030331A1 (ja) * 2020-08-07 2022-02-10 マイクロ波化学株式会社 マイクロ波照射装置、及びマイクロ波照射方法
US11369937B2 (en) 2019-02-10 2022-06-28 Dwight Eric Kinzer Electromagnetic reactor
US11576409B2 (en) * 2017-10-24 2023-02-14 Societe Des Produits Nestle S.A. Method for preparing a foodstuff with a food processing system

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2820127A (en) * 1953-03-30 1958-01-14 Raytheon Mfg Co Microwave cookers
US3027442A (en) * 1960-02-29 1962-03-27 Philips Corp High-frequency furnaces
GB978197A (en) * 1961-07-05 1964-12-16 Radyne Ltd Improvements in or relating to high frequency heating equipment
US3209113A (en) * 1961-07-17 1965-09-28 Philips Corp Furnace for high-frequency heating with the aid of oscillations of very high frequency
US3242304A (en) * 1963-07-22 1966-03-22 Philips Corp High frequency heating apparatus

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2820127A (en) * 1953-03-30 1958-01-14 Raytheon Mfg Co Microwave cookers
US3027442A (en) * 1960-02-29 1962-03-27 Philips Corp High-frequency furnaces
GB978197A (en) * 1961-07-05 1964-12-16 Radyne Ltd Improvements in or relating to high frequency heating equipment
US3209113A (en) * 1961-07-17 1965-09-28 Philips Corp Furnace for high-frequency heating with the aid of oscillations of very high frequency
US3242304A (en) * 1963-07-22 1966-03-22 Philips Corp High frequency heating apparatus

Cited By (29)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4100386A (en) * 1975-08-27 1978-07-11 Automatisme & Technique Process for sintering ceramic products
US4401873A (en) * 1979-11-28 1983-08-30 Stiftelsen Institutet For Mikrovagsteknik Microwave heating device with tapered waveguide
US4874915A (en) * 1988-12-30 1989-10-17 Lifeblood Advanced Blood Bank Systems, Inc. Apparatus for the rapid microwave thawing of cryopreserved blood, blood components, and tissue
US5369250A (en) * 1991-09-27 1994-11-29 Apv Corporation Limited Method and apparatus for uniform microwave heating of an article using resonant slots
US5408074A (en) * 1991-11-05 1995-04-18 Oscar Gossler Kg (Gmbh & Co.) Apparatus for the selective control of heating and irradiation of materials in a conveying path
US5457303A (en) * 1993-05-05 1995-10-10 Apv Corporation Limited Microwave ovens having conductive conveyor band and applicator launch section to provide parallel plate electric field
US5958275A (en) * 1997-04-29 1999-09-28 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6075232A (en) * 1997-04-29 2000-06-13 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6259077B1 (en) 1999-07-12 2001-07-10 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6590191B2 (en) 1999-07-12 2003-07-08 Industrial Microwaves Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6246037B1 (en) 1999-08-11 2001-06-12 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6396034B2 (en) 1999-08-11 2002-05-28 Industrial Microwave Systems, Inc. Method and apparatus for electromagnetic exposure of planar or other materials
US6753516B1 (en) 1999-12-07 2004-06-22 Industrial Microwave Systems, L.L.C. Method and apparatus for controlling an electric field intensity within a waveguide
US6965099B1 (en) * 2000-08-28 2005-11-15 Georgia Tech Research Corporation Geometry for web microwave heating or drying to a desired profile in a waveguide
WO2005043953A3 (en) * 2003-10-24 2005-11-10 Ferrite Company Inc Choke assembly for continuous conveyor microwave oven
US7002122B2 (en) * 2003-10-24 2006-02-21 The Ferrite Company, Inc. Choke assembly for continuous conveyor microwave oven
US20050092741A1 (en) * 2003-10-24 2005-05-05 The Ferrite Company, Inc. Choke assembly for continuous conveyor microwave oven
US7665226B2 (en) * 2004-04-12 2010-02-23 Kitakyushu Foundation For The Advancement Of Industry, Science & Technology Method for drying under reduced pressure using microwaves
US20070271811A1 (en) * 2004-04-12 2007-11-29 Takaharu Tsuruta Method And Apparatus For Drying Under Reduced Pressure Using Microwaves
US20070131678A1 (en) * 2005-12-14 2007-06-14 Industrial Microwave Systems, L.L.C. Waveguide exposure chamber for heating and drying material
US7470876B2 (en) 2005-12-14 2008-12-30 Industrial Microwave Systems, L.L.C. Waveguide exposure chamber for heating and drying material
US8324539B2 (en) 2007-08-06 2012-12-04 Industrial Microwave Systems, L.L.C. Wide waveguide applicator
US20100200573A1 (en) * 2007-08-06 2010-08-12 Industrial Microwave Systems, L.L.C. Wide waveguide applicator
US20090194528A1 (en) * 2008-02-01 2009-08-06 Anton Paar Gmbh Applicator and apparatus for heating samples by microwave radiation
EP2086285A1 (en) * 2008-02-01 2009-08-05 Anton Paar GmbH Applicator and Apparatus for heating samples by microwave radiation
US8969768B2 (en) * 2008-02-01 2015-03-03 Anton Paar Gmbh Applicator and apparatus for heating samples by microwave radiation
US11576409B2 (en) * 2017-10-24 2023-02-14 Societe Des Produits Nestle S.A. Method for preparing a foodstuff with a food processing system
US11369937B2 (en) 2019-02-10 2022-06-28 Dwight Eric Kinzer Electromagnetic reactor
WO2022030331A1 (ja) * 2020-08-07 2022-02-10 マイクロ波化学株式会社 マイクロ波照射装置、及びマイクロ波照射方法

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Publication number Publication date
DE1765150B2 (de) 1974-11-14
DE1765150A1 (de) 1972-01-13
SE333028B (enrdf_load_stackoverflow) 1971-03-01
FR1567335A (enrdf_load_stackoverflow) 1969-05-16
DE1765150C3 (de) 1975-07-10
GB1181208A (en) 1970-02-11

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