US2515211A - Method and apparatus for dielectric heating - Google Patents

Description

July 18, 1950 ca. E. GARD METHOD AND APPARATUS FOR DIELECTRIC HEATING Filed Jan. 24, 1947 I N VEN TOR.

Patented July 18, 1950' UNITED STATES PATENT OFFlCE METHOD AND APPARATUS FOR DIELECTRIC HEATING Application January 24, 1947, Serial No. 724,186

This invention relates to dielectric heating and; in particular, to a method and'apparatus for maintaining substantially uniform distribution of the lines of force through the mass to be heated.

In heating certain masses'bydielectric effect the distribution of thelines of force through the mass is not uniform and the rate of generating heat differs in various portions of the mass. 'This is undesirable in many cases-where uniform heating throughout the mass'is necessary to obtain a uniform product. 'The-nonuniform distribution 'of lines 'of force is increased when the mass has a; dimension normal to the usual electrode plates which isof the same order as or greater than the transverse dimension. In such cases, the lines of force through the mass are spread farther apart-adjacent the median transverse plane than at the surfaces of theelectrodes. As a -result,'more heat is generated'adjacent the electrodes than in the intermediate 'portionof'the mass.

I have invented a novel method and apparatus for dielectric'heating whereby substantially uniform distribution-of the lines of force through the mass to be heated may be obtained even though thedimension of the mass normal to the electrodes 'is several times the'other dimension. In a preferred embodimentyl'provide a Voltage divider coil in the form of a helix surrounding the mass to be heated, and connect the ends of the coilto the electrodes, thus placing the inductance of the coil in parallelWith the capacity of the mass to be heated. Theinductance thus tends to balance the load capacity. It also reduces the non-uniformity of distribution of the lines. of force through the mass which would exist if the mass were outside of the coil. By properly spacing the successive turnsof the coil, a high degree of uniformity in the distribution of lines of force may be obtained. A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawing illustrating a preferred embodiment andcertain modifications. .Inthe drawings,

Figure 1 is a plan View;

Figure 2 is a section therethrough taken along the-plane of line II-II of-Figurel;

Figure 3 is a view simi1artoFigure2show- -inga modification; and

Figure-4- is'a view of the -cross-section of-the embodiment in "Figure -1, illustrating closer 14 Claims. (01. 219-47) a 2 spacing of the successive coil turns adjacent .the top and bottom portions of the mass.

Referring in detail to the drawings and, for the present, to Figures 1 and 2, a mass IDof dielectric material to be heated is disposed between spaced electrode plates II and I2. An

insulating layer I3 is disposed between the mass and each electrode plate. If the mass is composed of flowable material, e. g., cork particles coated with a heat-activatable'binder, adapted to be converted into a solid massofcork composition, itis preferably confined in a mold l4 0f Which the electrode. plates II and I2 serve as the top and bottom, respectively. The mold may be of any suitable'shape or dimensions. In the form illustrated, it is.a cylinder composed of laminated paper impregnated with ceresin wax so as to have a loss factor equal to or greater than that of the material of whichthe mass In is composed. A mold having a loss factor equal to or greater than the material to be dielectrically heated is disclosed and claimed in my copending application .Serial No. 678,217 filed June 21, 1946, and entitled Method of Controlling Dielectric Heating (Prevention of Heat Loss). Dielectric heating equipment including a mold having a wax impregnant such as ceresin is disclosed and. claimed in th copending application of George W. Scott, Jr., Serial No. 678,215, filed June 21, 1946, and entitled Method and Apparatus for Dielectric Heating. In the curing of cork composition, the coated cork particles are compressed between the electrode plates and the latter are held in position by through pins 15 inserted through holes inthe mold wall.

A voltage divider in the form. of a helix l6 of any suitable conductor is disposed about themold i4 and'preferably in contact therewith or. partly or wholly embedded therein. The ends of the helix are electrically connected to the electrode plates. In the illustrated embodiment, the end turns of the helix-are in contact with -the.pins 15 which are of metal and are also in. contact with the electrode plates l l and 12. A source of high-frequency current I! suchas-a vacuumtube oscillator is connected across the. electrode plates and the coil iii in parallel therewith by conductors attached to the pins [5. .The coil and plates may, of course, be separately connected in parallel to the source if desired. -If theinductance of the coil 16 is greater than needed to balance the capacity of the mass lil,:in ordertoproduce a condition of parallel resonance, avariable inductance 18 may be connected inparallelwith the load and the coil It to reduce the overall inductance for the purpose of tuning the circuit to resonance.

The coil H5 connected to the source of highfrequency H for dielectrically heating the mass constitutes a very much higher impedance than the mass. At the relatively high frequencies employed in dielectric heating work, the stray capacitance between the mass Ill and the coil [6 capacitatively couples all points of the coil [6 to the mass [0, and there is thus provided a voltage divider which is capacitatively coupled to the mass at an infinite number of points.

The coil 16 performs two important functions.

In the first place, it serves to maintain substantially uniform distribution of the electric lines of force through the mass In between the electrode plates H and I2. This improves the uniformity of heating throughout all portions of the mass. This action results from the fact that the voltage divider coil, 16 confines within its turns the highfrequency electric field of the condenser formed of the electrodeplates H and I2 separated by the mass Into be dielectrically heated. The second function results from the fact that, because of the impedance constituted by various sections of the voltage divider It all connectedin series, there isbrought about a uniform voltage gradient therealong whenthe turns are uniformly spaced, and the voltage gradient in the coil determines the position of the equipotential surfaces through the mass. This follows from the fact that the voltage divider. establishes a high-frequency boundary potential distribution in the space closely adjacent to and surrounding the mass; and Where the turns of the voltage divider coil [6 are equally spaced, its impedance value will be uniform along its length, and it will possess a uniform voltage gradient which will establish a similargradient'within the mass, for the voltage dividerv controls the electric field between the electrodesin accordance with its, voltage gradient. This gradient may bevaried by changing the spacing of the coil turns in various portions of its length to produce a substantially uniform temperature in the mass. Where the turns of the voltage divider coil I6 are spaced closer together, the impedance value. will be higher, and a proportionately greater voltagedrop will occur Thus, the voltage gradient will be nonuniform and a higher heatingrate will be obtained in the mass in the zones where the voltage drop in the voltage divider coil 16 is greater because, as mentioned above, the voltage gradient of .the coil controls the electric field within the mass. Thus, if it is desired to heat one portion of the mass to a higher temperature than another, this result may be accomplished by properly distributing the turns of the. coil,,thereby changing the voltage increased or the thickness of ,the mold wall decreased. The latter is particularly desirable to reduce the amount of energy consumed in heating the mold itself as contrasted with the contents thereof.

The coil N5, of course produces a highaitaii frequency electromagnetic field longitudinally through the mold ID. This field tends to heat the electrode plates H and I2 by induction. While some heating of the electrode plates is desirable to prevent excessive loss of heat thereto from the mass 10, overheating of the electrode plates is undesirable because of the danger of overcuring the top and bottom of the mass or even charring it. To avoid this, I provide the electrode plates II and I2 which may conveniently be composed of steel, with peripheral rings or bands IQ of copper or other suitable material having a conductivity much greater than that of the metal of which the plates themselves are composed. The copper rings act in the known manner to shield the electrode plates from the electromagnetic field. The alternating electromagnetic field thus induces current largely in the rings id rather than in the electrode plates themselves.

In using the apparatus described above for performing the method of my invention, the mold M is filled by the aid of a so-called charging boot with the electrode I I and upper insulating layer l3 removed. The insulating layer and electrode plate are then placed on top'of the charge and it is compressed to the desired degree after which the upper pins I5 are inserted to hold the plate in place. High-frequency voltage is then applied as by closing a switchzil. When the mass has been heated sufiiciently, the switch is opened, the plate H and upper insulating layer are removed and the mass is then pushed out of the mold.

Figure 3 illustrates a modification of the apparatus already described. The modified apparatus is adapted to be used between the'platens 2| and 22 of a press instead of relyi'ngon through pins to hold and confine the material. As shown in Figure 3, a bottom electrode plate 23 is disposed on the platen 22. A top electrode plate 2t carried on the upper platen 2| is adapted'to enter a mold 25 and compress a charge of material 26 therein. Insulating layers are disposed "between the electrode plates and the material'to be heated. The mold 25 may be similar in construction to the mold l4 and is provided with a surrounding helix 2'! which serves as an inductance to balance the load capacity, having its ends connected to the electrode plates 23 and 2d, respectively. In other respects, the arrangement shown in Figure 3 is similar to that of Figure 2.

Figure 4 illustrates the embodiment shown'in Figures 1 and 2 with, however, closer spacingoi the successive turns of the coil Hi along thetop and bottom portions of the mass it! than the spacing of the turns adjacent the intermediate portions of the mass. By such'spacing, a proportionately greater voltage drop with consequently greater dielectric heating is achieved in the transverse portions of themass at the top and bottom within the space surrounded by the closer spaced turns. In this way, a more uniform temperature of the mass Ill is obtained in areas of uneven and possibly greater heat loss as by conduction through the electrodes. Moreover, by

presetting such spacing as andwhere desired along the length of the mass, variations from the uniform which may be wanted under special circumstances can also be achieved.

desired distributionof lines offorce or position of the equipotential surfaces through the mass. In the second place the voltage divider controls the electric field within the mass between the.

electrode plates in accordance with its voltage gradient and permits controlled-heating of the.

mass to obtaina substantially uniformtemperature in the mass or a higher temperature in one.

portion of the mass thanin another. In addition, the helix forming the inductance serves mechanically to reinforce the-mold wall, permitting a re duction-in the sectionthereofwithout seriously weakening it.. Amore uniform product is thus obtainable by the use of the invention withoutv for supplying high-frequency alternating electric.

current tosai'd-w electrode. plates froma source to establish a high-frequency electric field within said mass between said electrode plates, and meanslfor controlling the. voltage gradient within said mass. between said electrode plates comprising a voltage divider coupled to said high-frequency current source and disposed closely ad.- jacent to and surroundin said mass between said electrode plates.

2. Apparatus for dielectrically heating a mass of dielectric material comprising a pair of substantially parallel electrode plates adapted to be positioned on opposite sides of said mass, means for supplying high-frequency alternating electric current to said electrode plates from a source to establish a high-frequency electric field within said mass, and means for controlling the voltage gradient within said mass between said plates comprising a voltage dividerv coil having a plurality of turns disposed closely adjacent to and surrounding. said mass between said electrode plates and coupled to said high-frequency current source.

3. Apparatus for dielectrically heating a mass of dielectric material comprising a pair of substantially parallel electrode plates adapted to be positioned on opposite sides of said mass, means for supplying high-frequency alternating electric current to said electrode plates from a source to establish a high-frequency electric field within said mass, and means for limiting the distribution of the electric lines of force of said field essentially to within said mass between said electrode plates and for controlling the voltage gradient within said mass between said electrode plates comprising a voltage divider providing a plurality of closely spaced conducting surfaces disposed closely adjacent to and surrounding said mass between said electrode plates and coupled to said high-frequency current source.

4. Apparatus for dielectrically heating a mass of dielectric material comprising a pair of substantially parallel electrode plates adapted to be positioned on opposite sides of said mass, means for supplying high-frequency alternating electric current to said electrode plates from a source to establish a high-frequency electric field within 15 said-mass, and means .for limiting the .distrlbue' tion -of thev electric lines offence-of said field essentially to within said mass between saidelectrode plates comprising a voltage divider. coil. having a-plurality of closely spaced turns dis,-

posed closely adjacent. to. and surrounding. said mass-between. said electrode plates and con.-.

nected to said electrode platesin parallel therewith.

5-. Apparatusfor dielectrically heating a mass of dielectric material comprising a nonconductive container for said mass, electrode plates closing said container and adapted to confine said mass within said container, means for supplying high-frequency alternating electric currentquency alternating electric current to said: elec-.

trode plates from a. source to establish a highfrequency electric field within said mass, and;

means for. limitingsaid field essentially to within said mass and for controlling the voltage gradient within said mass between said electrode plates comprising a voltage divider coil. having aplu-- rality or closely spaced turns between said electrode plates. which are disposed closely adjacent to and. surround. said mold and are connected to.

saidelectrode plates in parallel therewith.

7...Apparatusfor dielectrically heating a mass of dielectricmaterial comprising a pair of substantially parallel electrode plates adaptedto be positioned on oppositesides of. said mass, means for supplying high-frequency alternating electric current to said electrode plates from a source to establish a high-frequency electric field within said. mass, and means for controlling the voltage gradient within said mass between said electrode plates comprising a voltage divider coupled to said high-frequency current source and disposed closely adjacent to and surrounding said mass between said electrode plates, said voltage divider having its conducting surfaces nonuniformly spaced to provide a nonuniform voltage gradient in the voltage divider and in the electric field.

8. Apparatus for dielectrically heating a mass of dielectric material comprising a pair of substantially parallel electrode plates adapted to be positioned on opposite sides of said mass, means for supplying high-frequency alternating electric current to said electrode plates from a source to establish a high-frequency electric field within said mass, and means for controlling the voltage gradient within said mass between said electrode plates comprising a voltage divider coil having a plurality of turns disposed closely adjacent to and surrounding said mass between said electrode plates, and means connecting said voltage divider coil to said electrode plates in parallel therewith to establish a tuned high-frequency resonant circuit with said voltage divider coil and the capacitance between said electrode plates.

9, Apparatus for dielectrically heating a mass of dielectric material comprising, a pair of substantiallysparallel electrode plates adapted to be positioned on opposite sides of said mass, means for supplying high-frequency alternating electric current to said electrode plates from a sourceto establish a high-frequency electricfield within said 1 mass, means for controlling the voltage gradient within said mass between said electrode plates comprising a voltage divider coupled to said high-frequency current source and disposed closely adjacent to and surrounding said mass between said electrode plates, and a variable inductance connected in parallel with said electrode'plates for tuning the high-frequency alternating electric current circuit to resonance.

10. Apparatus for dielectrically heating a mass of dielectric material comprising a pair of substantially parallel electrode plates adapted to be positioned on opposite sides of said mass, means for supplying high-frequency alternating electric current to said electrode plates from a source to establish a high-frequency electric field within said mass, means for controlling the voltage gradient within said mass between said electrodeplates comprising a voltage divider coil having..'a plurality of closely spaced turns between saidelectrode plates which are disposed closely adjacent to and surrounding said mass and connected to said electrode plates in parallel therewith, and a metal conductor of lower electrical impedance than said electrode plates disposed adjacent to and surrounding each of said electrode plates and electrically connected thereto.

11. In a process of dielectrically heating a mass of dielectric material disposed between spaced electrode plates, the steps of establishing a highfrequency alternating electric field within said mass between said electrode plates to dielectrically heat the mass and controlling the electric field within the mass between said spaced electrode plates by establishing a high-frequency boundary potential distribution in the space around said mass between said electrode plates closely adjacent to said mass.

12. In a process of dielectrically heating a mass of dielectric material disposed between spaced electrode plates, the steps of establishing a highfrequency alternating electric field within said mass between said electrode plates to dielec-a trically heat themass and controlling the electric field within the mass between said electrode plates by establishing a high-frequency boundary potential distribution in the space around said mass between said electrode plates closely adjacent to said mass having a voltagegradient therealong which is predetermined in accordance with the voltage gradient desired within said mass between said electrode plates.

13. In a process of dielectrically heating a mass of dielectric material disposed between spaced electrode plates, the steps of establishing a highfrequency alternating electric field withinsaid mass between said electrode plates to dielectrically heat the mass and controlling the electric field within the mass between said electrode plates by establishing a nonuniform high-frequency boundary potential distribution in the space around said mass between said electrode plates closely adjacent to said mass to effect predetermined voltage'drops within said mass in accordance with the voltage drops in said boundary potential distribution.

14. In a process of dielectrically heating a mass of dielectric material disposed between spaced electrode plates, the steps of establishing a highfrequency alternating electric field within said mass between said electrode plates to dielectrically heat the mass and confining and controlling the electric field within the mass between said electrode plates by positioning a voltage divider electrically connected to said electrode plates closely adjacent to and surrounding said mass between said electrode plates to establish an arbitrary high-frequency boundary potential distribution closely adjacent to said mass.

- GEORGE E. GARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,572,873. Allcutt Feb. 16, 1926 1,839,801 Northrup Jan. 5, 1932 1,839,802 Northrup Jan. 5, 1932 2,325,637 Stewart Aug. 3, 1943 2,426,053 Roberds Aug. 19, 1947

Images