US3597567A - Microwave applicator for heating continuous web - Google Patents

Microwave applicator for heating continuous web Download PDF

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Publication number
US3597567A
US3597567A US860657A US3597567DA US3597567A US 3597567 A US3597567 A US 3597567A US 860657 A US860657 A US 860657A US 3597567D A US3597567D A US 3597567DA US 3597567 A US3597567 A US 3597567A
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Prior art keywords
cavity
web
sidewall
source
electric field
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Expired - Lifetime
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US860657A
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English (en)
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Ray M Johnson
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MICRODRY Inc A CORP OF KENTUCKY
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Individual
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Assigned to MICRODRY, INC., A CORP. OF KENTUCKY reassignment MICRODRY, INC., A CORP. OF KENTUCKY ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: MICRODRY 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

Definitions

  • Lungmus ABSTRACT A resonant cavity in the form of a right circular cylinder is excited in the TM mode by a source of microwave energy to generate an electric field with lines extending generally parallel to the axis of the cylindrical sidewall of the cavity.
  • a pair of aligned slots are formed at diametrically opposite locations in the cavity sidewall for passing the web through the applicator in a plane which passes through the axis of the sidewall.
  • the intensity of the electric field within the applicator increases in a radial direction of the cavity from a minimum at the sidewall to a maximum along the axis.
  • the electric field at its greatest intensity, extends parallel to the plane of the web for maximum coupling of energy to the web.
  • Air is circulated through the applicator for carrying away moisture and for cooling the sidewalls of the cavi ty.
  • An adjustable stub mounted for radial movement in the cavity, tunes the resonant frequency of the cavity to match the frequency of the source.
  • the present invention relates to a system for applying microwave energy to a web to heat it; more particularly, it pertains to a microwave applicator adapted to heating a web which is continuously fed through the applicator so that the heating may be carried on uninterrupted.
  • Waveguides have been folded into a serpentine configuration to permit the product to pass through aligned slots in opposing broadwalls of a number of wave guide sections folded so that all of the slots are aligned.
  • Energy is coupled into one end of the waveguide to excite a traveling wave which makes a number of passes through the material transverse of the direction of movement of the material finally terminating in a water load.
  • a resonant cavity in the form of a right circular cylinder is excited in the TM mode by a source of microwave energy.
  • the magnetic field vectors extend in concentric circles, and each circular magnetic. field line extends in a plane perpendicular to the axis of the applicator and has its center located on that axis.
  • the word cavity refers to the conductive wall members as well as the volume defined thereby, which volume is sometimes referred to as the heating chamber.
  • the resonant cavity provides a heating chamber for the web being treated.
  • Slots for passing the web transversely through the axis of the cavity are formed on diametrically opposite sides of the cylindrical sidewall.
  • the slots may communicate with each other by a third slot formed in one of the transverse end plates thus forming two cantilevered half-cylinders out of the cavity.
  • This latter arrangement facilitates loadingof .a weblike material into the cavity and passing the same in a direction transverse of and through the axis of the cavity.
  • the material being treated passes through the zone of maximum electric field intensity and is oriented such that the electric field lines are parallel with the web surface for maximum coupling of microwave coupling of microwave energy into the web.
  • An adjustable screw is located in the cylindrical sidewall to adjust the resonant frequency of the cavity; and tuning of the cavity to the source frequency under load is facilitated by means of a coupling probe located on one of the end plates.
  • the tuning screw' is adjusted to tune the resonant frequency of the cavity to the operating frequency of the excitation source.
  • the axial length of the cavity is preferably maintained less than one-half wavelength of the excitation frequency to confine the electric field vector to the TE mode.
  • a number of cavities could be arranged in tandem with the web being transported sequentially through all of the cavities, if needed.
  • the cavities may be excited by a loop, a probe, a coupling aperture, or a feed waveguide.
  • the present invention provides a microwave applicator which is simple in construction and useful in applying highly concentrated microwave energy in an orientation which permits maximum coupling of microwave energy into a web being treated in a reasonably small space and with high efficiency, safety and reliability.
  • FIG. 1 is a perspective view of an applicator adapted to heat a web according to the present invention
  • FIG. 2 is a transverse cross section view of the applicator of FIG. I;
  • FIG. 3 is a perspective view of an alternate embodiment of an applicator, partially cut away to show the interior
  • FIG. 4 is a transverse cross section view of the applicator of FIG. 3.
  • FIGS. 1 and 2 there is shown an applicator generally designated 10 which is adapted to apply microwave energy to a web denoted W.
  • the web W is fed to the applicator from a source diagrammatically illustrated by the box 11; and after passing through the applicator, it may be collected by any suitable means (such as a reeler diagrammatically shown by the block 12) or transported through a second, independent applicator similar to the one shown.
  • the web material may be a woven or nonwoven, natural or synthetic fiber, or any material that is nonconductive. However, better results in heating the web if it has a relatively high dielectric loss factor. Further, the web shown may be a conveyor belt of low-loss material for supporting thin articles being heated, as long as the articles pass through the applicator without substantial loss of the microwave energy by radiation through the passage slot, as will be clear from subsequent description.
  • the heating applicator 10 takes the form of a right circular cylinder including a cylindrical sidewall 13 and first and second transverse end plates 14 and 15 (FIG. 2) which define the heating chamber.
  • the cylindrical sidewall 13 defines an axis which extends transverse of the direction of motion of the web W.
  • the transverse end plates 14 and 15 together with the cylindrical sidewall 13 provide the resonant electromagnetic cavity which has the form of a right circular cylinder. These boundary walls are sometimes collectively referred to herein as the. housing of the cavity.
  • a slot 16 is formed in the housing of the cavity.
  • the slot includes three communicating sections: one segment is designated 17 in FIG. 2 and extends along the sidewall 13 parallel to its axis; a second segment 18 is formed in the end plate 14; and the third segment 19 is formed in the sidewall 13 at a location diametrically opposite and parallel to the slot 17.
  • the slot 16 is formed in a plane which passes through the axis of the cavity; and it divides the housing of the cavity into cantilevered half sections.
  • an operator may first connect the web to the transport mechanism before loading it into the applicator. Loading is simply and safely accomplished by flexing the web laterally, aligning it with the segment 17, placing it in its operating position. If loading or unloading the web does not present a problem, the segment 18 in the end plate is not necessary.
  • a threaded aperture 21 is formed in the sidewall 13 for receiving a flange member 22.
  • the flange 22 is adapted to be connected to a coaxial cable or waveguide for coupling a source of microwave energy (not shown) to the applicator to excite the resonant cavity.
  • a feed center conductor 23 extends through the aperture 21 and is provided with an end coupling loop 24 for exciting the resonant cavity.
  • An externally threaded screw member 25 is also received in a threaded aperture located in the cylindrical sidewall 13.
  • the screw 25 is adjustable in a radial direction in the sidewall for tuning the resonant frequency of the cavity to that of the source.
  • a locknut 26 secures the tuning screw 25 in a fixed position once the adjustment is made.
  • a coupling probe generally designated 28 in FIG. 2 is threadably received in the end plate and it includes a center conductor 29 which partially extends within the cavity and is supported by means of a connector (which may be a Type N coaxial connector). The center connector 29 is then connected by means of coaxial cable to a power meter to monitoring the power coupled from the cavity. It will be observed from FIG. 2 that the location of the coupling probe 28 is toward the outer peripheral edges of the plate 15. This minimizes the amount of power coupled from the cavity (preferably, of the order of milliwatts).
  • a second function of this probe is to permit matching the input impedance of the cavity to the output impedance of the source by the coupling loop by merely observing power levels.
  • the air will be exhausted through the slot 16 described above; and this feature is important for applications in which the web (or an article it supports) contains water desired to be evaporated because the combined effect of the heated air and applied microwave energy is advantageous in reducing the transportation time and power requirements.
  • the TE mode may be suppressed by maintaining the axial length of the cavity less than one-half of the cavity wavelength for the TE. mode.
  • the applicator heats a web designated W.
  • the applicator generally denoted 35, includes a cylindrical sidewall 36 and first and second transverse end plates 37 and 38.
  • the opening for admitting passage of the web takes the form of two separate slots 39 and 40 formed respectively in diametrically opposing locations of the sidewall 36.
  • the cavity provides a heating chamber 46; and a tuning screw 51 similar to the previously identified tuning screw 25 is threadably received in the cylindrical sidewall 36 at about its longitudinal center.
  • Air input conduit for coupling a source of pressurized air to the interior of the cavity is designated 53.
  • a power-monitoring probe 54 may be secured to the end plate 38. Such a probe may be similar to the probe 28 of the embodiment of FIGS. 1 and 2.
  • FIG. 3 is excited by means of an input wave guide which couples output energy from a microwave source, not shown.
  • An adapter for connecting the waveguide to the cavity is designated generally by reference numeral 55, and it includes a mounting flange 56 and a waveguide tapered section 57 integral at its widest portion to the flange 56.
  • An input coupling aperture 58 is formed in the cylindrical sidewall 36 of the cavity 45; and the narrow end of the tapered waveguide section 57 is secured to the exterior surface of the cylindrical sidewall 36 about the input coupling aperture 58.
  • the side edges of the aperture 58 as at 59 and 60 are designed as matching iriss to match the characteristic impedance of the input feed waveguide to the input impedance of the cavity.
  • the resonant cavity may also be excited by means of an input probe coupled to a coaxial cable or, alternatively, the cavity may be symmetrically excited with coupling apertures properly phased so that the TE mode is not excited.
  • the cavity of the microwave applicators in each of the illustrated embodiments is excited in the TM mode.
  • the magnetic field lines extend substantially circumferentially about the axis of the cavity in a series of concentric circles. That is, the magnetic field lines, in any one plane transverse of the axis, are arranged in concentric circles, all centered on the axis of the cavity.
  • the intensity of the mag- I netic field is at a maximum near the interior cylindrical wall of the cavity; and it diminishes proceeding toward the axis of the cavity.
  • the electric field lines are orthogonal to the magnetic field lines; hence, the electric field lines extend perpendicularly between the transverse end plates in substantially parallel arrangement, all of the electric field lines being parallel to the axis of the cavity.
  • the intensity of the electric field lines is uniform in the axial direction and varies along a radius as the Bessel function of the first kind, order zero. That is, in a transverse plane, the electric field intensity is at a minimum along the cylindrical sidewalls of the cavity and increases to a maximum along the axis of the cavity; and the profile of the electric field intensity is substantially the same for any such transverse plane taken along the axis.
  • the intensity also increases with frequency. Heating of the web, of course, increases with electric field intensity; but the intensity must be maintained at a safe level to avoid breakdown.
  • the radius of the cavity is selected to be slightly lower than the wavelength of the lowest expected resonant frequency of the particular power oscillator used to excite the cavity.
  • an additional frequency shift may be caused by the heating of the cavity due to PR losses which will cause an expansion of the walls of the cavity.
  • the compensation afforded by adjustment of the tuning screws in each embodiment offsets such frequency shifts.
  • the condition of resonance permits an electric field to be generated intensity than would otherwise be possible (as for example, with multimode cavities), and these increased intensities may be attained with moderate input power.
  • High electric field intensities can be directly related to rapid heating rates in a lossy dielectric material being treated.
  • the current flow exhibits the skin effect phenomenon along lines substantially parallel with the slots formed in the two illustrated embodiments for transportation of the web. Since the slots do not interrupt current, there is little or no escape of microwave energy through radiation.
  • the intensity requirement for the electric field will, of course, vary from application to application depending upon the axial length of the applicator and the load presented to the source. It will, however, be observed that the web passes transversely through the location of maximum field intensity (i.e. along the axis of the cylindrical sidewall) and that the orientation of the electric field lines is parallel with the surface of the material being treated at the location of maximum field intensity. This effects a maximum coupling of microwave energy into the material being treated for heating it, or in the case in which the web is a conveyor, for heating articles supported by it.
  • Impedance matching between the source and the resonant cavity in the device of FIGS. 1-2 is achieved by orientation of the end loop 24 of the center conductor 23 projecting into the cavity to couple the maximum amount of power into the cavity.
  • impedance matching is provided by a properly designed iris as part of the coupling aperture 58. Matching of the frequency at which the cavity resonates to the source frequency in order to minimize the amount of power reflected back to the source is accomplished by means of the tuning screws. With the frequency of the cavity tuned to the frequency of the source and the impedance of the cavity matched to the impedance of the source and feeding waveguide, a maximum field intensity within the cavity and a higher efficiency are achieved.
  • the monopole coupling probes 28 and 54 located in the end plates of the cavities couple a small amount of power (of the order of milliwatts) from their respective cavities.
  • the cavity is tuned by means of the tuning screw until the power coupled out through the probe is maximized.
  • This peaking of output power for a constant input power indicates the resonant condition of the cavity-that is, the frequency is adjusted until resonance is achieved.
  • the tuning screw permits the exact matching of the cavity resonant frequency with the oscillation frequency of the magnetron (or other source of microwave energy).
  • the input power coupler places this loop in the side of the cylinder at a position of high magnetic field intensity but low electric field intensity.
  • the provision of the means for blowing air into the cavity insures that the air within the resonator does not become saturated with moisture; and it serves to cool the interior surface of the structure since the power dissipated in the cavity walls results in a pronounced heating ofthe metal.
  • the efficiency of the resonator is strongly influenced by the resistivity of the metal used for fabrication (of particular importance because of the skin effect is the metal forming the interior boundary of the cavity).
  • the lower the resistivity of this metal the smaller are the losses to the resonator walls for a given electric field intensity along the centerline of the cavity.
  • the power coupled to the material along the centerline is directly proportional to the square of the electric field intensity-thus, the resultant increase in efficiency is apparent.
  • Silver has a very low resistivity and therefore is a desirable material for use in interior surface of a cavity.
  • the plating thickness of 0.2 mil is sufficient to exceed the skin depth of the currents at 2.45GHz. and thereby to provide the necessary shielding from the bulk portion of the metal comprising the resonator.
  • a system for applying heat to a web or to articles supported by a web comprising: a source of microwave energy, a resonant cavity having a cylindrical conductive sidewall and an axial length less than one-half wavelength of the resonant frequency thereof to suppress the TE mode, excitation means receiving microwave energy from said source to excite said cavity in substantially the TM mode, said cavity further including means for admitting transportation of said web transverse of the axis of said sidewall wherein the surface of said web extends substantially parallel to the electric field lines within said cavity.
  • conduit means for coupling a source of pressurized air into said cavity means to circulate air therethrough, whereby moist air is purged from the interior of said cavity and the sidewalls of said cavity are maintained in equilibrium temperature.
  • the system of claim 3 further comprising probe means extending partially within said cavity to couple power therefrom to monitor the power within said cavity and assist in adjusting the resonant frequency thereof to the frequency of said source.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Constitution Of High-Frequency Heating (AREA)
US860657A 1969-09-24 1969-09-24 Microwave applicator for heating continuous web Expired - Lifetime US3597567A (en)

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4186044A (en) * 1977-12-27 1980-01-29 Boeing Commercial Airplane Company Apparatus and method for forming laminated composite structures
US4626640A (en) * 1984-10-02 1986-12-02 U.S. Philips Corporation Microwave arrangement for heating material
US4877938A (en) * 1986-09-26 1989-10-31 U.S. Philips Corporation Plasma activated deposition of an insulating material on the interior of a tube
US4992133A (en) * 1988-09-30 1991-02-12 Pda Engineering Apparatus for processing composite materials
US5217656A (en) * 1990-07-12 1993-06-08 The C. A. Lawton Company Method for making structural reinforcement preforms including energetic basting of reinforcement members
US5866060A (en) * 1989-12-06 1999-02-02 C. A. Lawton Company Method for making preforms
US6901683B2 (en) 2002-02-15 2005-06-07 International Business Machines Corporation Method and apparatus for electromagnetic drying of printed media
US20120091127A1 (en) * 2009-05-02 2012-04-19 Electrolux Home Products Corporation N.V. Microwave sealing device of an opening for a rotating shaft
US9282594B2 (en) 2010-12-23 2016-03-08 Eastman Chemical Company Wood heater with enhanced microwave launching system
US20220402058A1 (en) * 2019-11-15 2022-12-22 National Institute Of Advanced Industrial Science And Technology Mounting wiring board, electronic device mounting board, method of mounting electronic device, microwave heating method, and microwave heating apparatus

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SE419494B (sv) * 1979-12-21 1981-08-03 Husqvarna Ab Genomstromningsvermare av mikrovagstyp innehallande en cylindrisk mikrovagsapplikator
FR2683420B1 (fr) * 1991-11-05 1996-07-12 Bordeaux 1 Universite Dispositif pour l'application de micro-ondes en vue du traitement d'un materiau.

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2560903A (en) * 1949-08-27 1951-07-17 Raytheon Mfg Co Wave guide dielectric heating apparatus
US3277580A (en) * 1963-07-05 1966-10-11 Hammtronics Systems Inc Method and apparatus for drying
US3321605A (en) * 1964-08-06 1967-05-23 Gen Electric Electronic oven
US3412227A (en) * 1965-11-18 1968-11-19 Tappan Co Electronic oven protection circuit
US3449836A (en) * 1967-10-25 1969-06-17 Bechtel Int Corp Air suspension system in microwave drying
US3475827A (en) * 1967-12-06 1969-11-04 Bechtel Int Corp R.f. seal in microwave drier

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2560903A (en) * 1949-08-27 1951-07-17 Raytheon Mfg Co Wave guide dielectric heating apparatus
US3277580A (en) * 1963-07-05 1966-10-11 Hammtronics Systems Inc Method and apparatus for drying
US3321605A (en) * 1964-08-06 1967-05-23 Gen Electric Electronic oven
US3412227A (en) * 1965-11-18 1968-11-19 Tappan Co Electronic oven protection circuit
US3449836A (en) * 1967-10-25 1969-06-17 Bechtel Int Corp Air suspension system in microwave drying
US3475827A (en) * 1967-12-06 1969-11-04 Bechtel Int Corp R.f. seal in microwave drier

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
Radar Electronics Fundamentals, Navships 900.016 Washington D.C. June 1944 page 373 & 372 *

Cited By (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4186044A (en) * 1977-12-27 1980-01-29 Boeing Commercial Airplane Company Apparatus and method for forming laminated composite structures
US4626640A (en) * 1984-10-02 1986-12-02 U.S. Philips Corporation Microwave arrangement for heating material
AU581449B2 (en) * 1984-10-02 1989-02-23 Philips Electronics N.V. Microwave arrangement for heating material
US4877938A (en) * 1986-09-26 1989-10-31 U.S. Philips Corporation Plasma activated deposition of an insulating material on the interior of a tube
US4992133A (en) * 1988-09-30 1991-02-12 Pda Engineering Apparatus for processing composite materials
US6001300A (en) * 1989-12-06 1999-12-14 C.A. Lawton Company Method for making rigid three-dimensional preforms using directed electromagnetic energy
US5866060A (en) * 1989-12-06 1999-02-02 C. A. Lawton Company Method for making preforms
US6004123A (en) * 1989-12-06 1999-12-21 C.A. Lawton Company Apparatus for making preforms
US5827392A (en) * 1990-07-12 1998-10-27 C.A. Lawton Company Method for making structural reinforcement preforms including energetic basting of reinforcement members
US5217656A (en) * 1990-07-12 1993-06-08 The C. A. Lawton Company Method for making structural reinforcement preforms including energetic basting of reinforcement members
US6901683B2 (en) 2002-02-15 2005-06-07 International Business Machines Corporation Method and apparatus for electromagnetic drying of printed media
US6938358B2 (en) 2002-02-15 2005-09-06 International Business Machines Corporation Method and apparatus for electromagnetic drying of printed media
US20120091127A1 (en) * 2009-05-02 2012-04-19 Electrolux Home Products Corporation N.V. Microwave sealing device of an opening for a rotating shaft
US9907123B2 (en) * 2009-05-02 2018-02-27 Electrolux Home Products Corporation N.V. Microwave sealing device of an opening for a rotating shaft
US9282594B2 (en) 2010-12-23 2016-03-08 Eastman Chemical Company Wood heater with enhanced microwave launching system
US9456473B2 (en) 2010-12-23 2016-09-27 Eastman Chemical Company Dual vessel chemical modification and heating of wood with optional vapor
US20220402058A1 (en) * 2019-11-15 2022-12-22 National Institute Of Advanced Industrial Science And Technology Mounting wiring board, electronic device mounting board, method of mounting electronic device, microwave heating method, and microwave heating apparatus

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Publication number Publication date
SE341907B (https=) 1972-01-17
DE2047119A1 (de) 1971-04-01

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AS Assignment

Owner name: MICRODRY, INC., A CORP. OF KENTUCKY, KENTUCKY

Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:MICRODRY CORP.;REEL/FRAME:005278/0185

Effective date: 19880629