US3491457A - Microwave drying method and apparatus - Google Patents

Description

1970 H. SCHREIBER ETAL 3,491,457

MICRQWAVE DRYING METHOD AND APPARATUS Filed Oct. 10, 1967 4 Sheets-Sheet l INVENTORS HENRY SHIRE/BER 34 DAV/0 J; GOERZJK.

JEROME W HANK/N A may;

Jan. 27, 1970 H. SCHREIBER ETAL 3,491,457

MICROWAVE DRYING METHOD AND APPARATUS' Filed Oct. 10' 1967 4 Sheets-Sheet 2 INVENTORS HENRY SHIRE/Bf? DAV/D J. GOEEZ, JR.

F 1 JEROME yZN AT 0 NEYS MICROWAVE DRYING METHOD AND APPARATUS Filed 001:. 10, 1967 4 Sheets-Sheet 5 g INVENTORS \66 may fiiHkf/Bf/P 6 6 DAVID J. 606/82, JR.

I EF 7 JEA0ME w HANK/N Jan. 27, 1970 H. SCHREIBER ET AL 3,491,457

MICROWAVE DRYING METHOD AND APPARATUS 4 Sheets-Sheet 4 Filed Oct. 10, 1967 ZNW EDAA wm m C6 v m 5 WO mm 5 nDJ I r1 M 1 M 1 n J. m w, W W I a m .04,

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ATTORNEYS FIE--5- United States Patent f 3,491,457 MICROWAVE DRYING METHOD AND APPARATUS Henry Schreiber, Port Washington, N.Y., and David J. Goerz, Jr., Menlo Park, and Jerome W. Hankin, Morega, Califi, assignors, by mesne assignments, to Bechtel International Corporation, San Francisco,

Calif., a corporation of Delaware Filed Oct. 10, 1967, Ser. No. 674,293 Int. Cl. F26b 5/02, 3/04, 13/02 U.S. Cl. 34-1 Claims ABSTRACT OF THE DISCLOSURE A drying device for a web wherein the solvent to be removed from the web is subjected to microwave heating and-high speed gas jets, the gas jets breaking down the solvent boundary layer and carrying otf the solvent released from the web by the absorption of microwave energy. Apparatus is provided for carrying out the method in an eflicient manner.

BACKGROUND OF THE INVENTION Dryer for paper and other materials containing solvents or moisture wherein microwave energy is combined with high velocity gas impingement.

DESCRIPTION OF THE PRIOR ART Heretofore microwave energy has been employed for drying paper and similar materials and gas has been used to absorb and carry away the released solvent. Boundary layer conditions adjacent to the web being dried act as a high resistance insulator, sharply limiting mass transfer from the web. The low velocity ambient gas is therefore limited in its ability to carry away released solvent to the rate at which solvent particles can leave the liquid surface on the web and cross the boundary layer adjacent to the surface of the web.

In the present invention, microwaves energy is absorbed by the solvent in the web. This causes an increase in the thermal energy of the solvent by virtue of' directly imparting thermal agitation to its molecules. In a web of finite thickness or a solvent layer containing solvent uniformly distributed throughout it, microwave energy is absorbed by the solvent throughout the web or solvent layer. The solvent nearer the surface of the web experiences mass and heat transfer to its ambient surroundings, thus causing the temperature of the remaining liquid solvent at the surface to be lower than the remaining liquid solvent in the interior of the web or solvent layer. Since vapor pressure is a function of temperature, the vapor pressure of the solvent within the web or solvent layer will be higher than that at the surface, causing a solvent mass and thermal energy migration to the surface. At the surface, a high velocity jet of gas is made to impinge upon the surface of the web or solvent layer, disrupting the solvent boundary layer at the surface. Since the boundary layer acts as a resistance to mass transfer away from the surface, mass transfer is enhanced by disruption of the boundary layer. The solvent molecules so released are subsequently mechanically entrained and absorbed in the gas stream and exhausted from the apparatus.

SUMMARY OF THE INVENTION Microwave drying resulting from microwave heating in combination with high velocity gas jets offers a number of advantages over other drying methods. For instance, there is three dimensional moisture leveling without the necessity of overdrying, thus improving product quality. The moisture leveling results from the fact that ICC the wettest areas in the web automatically and instantaneously absorb the greatest amount of microwave energy. It is therefore possible to achieve a more uniform product and more easily reproducible machine operating conditions on a fresh start-up. This combination of drying principles requires little space, has a high degree of reliability and is capable of achieving extremely high drying rates. Since surface contact of the dryer apparatus with the web is not required to achieve the drying effect, it is possible to set surface coatings on both sides of a web before any physical contact with the web is made. Binder migration problems in surface coatings are minimized by this method. The combination of microwave energy with high velocity gas impingement results in a highly efiicient energy transfer means. The system described in this invention is controllable to a high degree with respect to all operating parameters, therefore, it is possible to optimize the combination of microwave energy. Thermal energy and mechanical energy to achieve maximum economy and efficiency.

The use of high velocity gas provides means to sweep moisture from the high electric field areas thus permitting the use of high electric field gradients with a minimum of RF corona and hence permit the highest possible field concentration in the web and hence highest possible drying rate. The process of the present invention is primarily adapted for the removal of solvents from continuous flexible webs, such as partially dried paper, particularly when the moisture content of the paper has been brought down to about 15' to 20% by conventional drying methods known to those skilled in the art and coatings and printing placed on a substrate. For convenience the invention will be largely described in connection with the drying of paper, although it will be understood that the invention is one of broad applicability and can be used for the removal of any polar compound such as water, alcohol, acetone or other solvents from a web, sheet or coating.

In accordance with the present invention, a high velocity gas jet is directed at the paper in conjunction with the application of microwave energy whereby the boundary layer is broken and the solvent molecules which have been driven to the surface of the web by the absorption of microwave energy are forceably removed from the web. By high speed is meant a velocity of from 10,000 to 20,000 feet per minute. Although the invention is primarily applicable with air, any gaseous medium can be used such as nitrogen or superheated steam. Thus the invention comprises the employment of the combination of microwave heating in conjunction with a high speed gas jet disrupting the boundary layer.

In accordance with one embodiment of the invention, a meander line form of waveguide system is employed. However other systems can be used to apply the microwave energy such as an asymmetric stripline or interdigital slow wave structure which serves as a single sided microwave radiator to distribute the energy to the web.

In accordance with one embodiment of the present invention a split waveguide is employed having a substantial separation over at least a part of the waveguide length between these two elements so as to reduce the possibility of arcing due to the accumulation of lint or the like. In accordance with another embodiment of the invention, gas flows through the waveguide to prevent condensation of solvent or the accumulation of gas borne solids therein.

Preferably the gas is recirculated in the system and only a sufficient percentage is exhausted to atmosphere to carry away the released solvent to prevent condensation in the system. The purpose of this method is to minimize heat loss. This is possible since the gas is not used primarily as an absorbent but is used primarily as a mechanical means of stripping released solvent from the boundary layers and secondarily of conveying away the moisture evolved.

In acocrdance with another embodiment of the invention, conductive tabs surround the gas nozzles, which are made of a non-conducting, low loss tangent material, the tabs prevent arcing at the close spacing required at the air nozzles. Preferably the waveguides which are used to convey the microwave energy to the paper also form part of the duct system, whereby, gas is carried to and from the web.

In accordance with another embodiment of the invenrected at the moving web to break up the solvent boundary layer adjacent to the web surface and to minimize the carry over of released solvent into adjacent waveguides.

The invention is primarily adapted for use in the microwave bands designated UHF 9 and SHF 10, i.e. frequencies of about 300 mHz. to 30,000 mHz. As a practical matter it is preferred to operate on the frequencies assigned by the FCC for ISM use at 915 mHz., 2450 mHz., 5800 mHz. and 22,125 mHz.

In accordance with one aspect of the invention, an improved power input matching structure is provided wherein microwave power is supplied to the split waveguide structure from one side through a split right angle waveguide ell with an impedance matching means. Further, the invention provides a dummy load at the terminal end of the waveguide so that if the web should break, or if all the energy is not absorbed by the web and structure there would be no substantial reflection of power and a consequent destruction of the equipment but the power is harmlessly absorbed in the dummy load.

Other features of the invention are set forth in the balance of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective, fragmentary view of a device embodying the present invention.

FIGURE 2 is a diagramatic plan view of a complete drying apparatus embodying the present invention.

FIGURE 3 is an end view of a dryer with some of the parts in section.

FIGURE 4 is a plan view of the dryer with some of the parts in section.

FIGURE 5 is an enlarged section on the line 5-5 of FIGURE 3.

FIGURE 6 is an enlarged section on the line 66 of FIGURE 5.

FIGURE 7 is a section on the line 77 of FIGURE 6.

FIGURE 8 is a perspective view, along the lines of FIGURE 1, showing an alternate embodiment of the invention.

FIGURE 9 is a perspective view showing still another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the description which follows it is assumed that a partially dry web containing, for instance, from 15 to 20% moisture is being further dried by the combined action of microwave energy and high speed air jets which are applied alternately to the web.

The web 10 is drawn through the dryer by means well known to those skilled in the art and not illustrated. This can be in the direction shown by the arrow or in the opposite direction. In the embodiment illustrated, the web is subjected to three successive microwave and air drying stages although it will be understood that any number of stages might be employed. Air under pressure is supplied by a fan 12 which forces air through duct 14 through R.F. filter 15 to the inlet plenums 16. If desired, a heater 18 may be included in the air inlet circuit. Air is exhausted through the outlet pleniums 20 through line 22 which may be provided with an exhaust fan 24 to aid in the flow. A return line 26 may be provided so that at least a portion of the air can be recirculated to conserve heat. When a return line is employed, dampers 28, 29, 30, 31, and 32 are employed in the lines as illustrated to regulate the air flows in the system.

A source of microwave energy 36 is provided which feeds the waveguide, described in detail hereinafter through line 38. The generator 36 can be any well known source of RF energy and in one practical embodiment of the invention consisted of a klystron and associated power supply operating at 2450 mHz. Suitably the RF generator operates at a frequency from about 300 to 30,000 mHz. Thus the ISM allocated frequencies of 915; 2,450; 5,800 and 22,125 mHz. are suitable.

The energy is fed into waveguide generally designated 40. At the terminal end of the waveguide 40 a dummy load 42 is provided to absorb excess energy and in case of a web break, to absorb total energy input. The waveguide, described hereinafter in detail is split and power is introduced to the structure through a split microwave tee, one half of tee is on the upper half and one half on the lower half of line 38 as is best seen in FIGURE 3. The opposite half of the waveguide is provided with an impedance matching stub 44 tuned with a piston 46 which provides a shorting plane and which is provided with an adjusting screw 48. At the outlet end, the previously mentioned dummy load 42 is provided on one half of the waveguide and the dummy load can take the form of a water filled tube 50 set at an angle near the center of the guide.

The opposite terminal half of the waveguide is provided with an adjustable matching stub 52 which can be of the same structure as stub 44.

The waveguide proper in the embodiment illustrated in FIGURES 1 through 7 consists of two complementary halves so that only One half will be described in detail. The waveguide includes a back wall 54 and sidewalls 56 and 58. The side wall 58 can be made as a continuation of the wall 60 which divides the inlet and outlet plenums 16 and 20. Thus, the Web 10 passes between the two halves of the waveguide and absorbs the microwave energy from the electric field within waveguide. After passing through the waveguide, the web is subjected to a jet of high speed gas directed through the nozzle openings 62. These openings 62 are preferably set at an angle to the plane of the paper as is shown to break up the boundary layer and arranged so as to minimize moisture entering adjacent waveguide. These jet openings 62, as is shown in the enlarged views of FIGURE 6 and 7, may form in a strip of plastic or other suitable non-conductive low loss tangent material 64 and are supported by conductive straps 66 which electrically connect the walls 20 and 56 keeping them at the same potential and thus prevent any possible arcing. This mechanical construction is particularly advantageous since if one desires to change the size of the nozzle for any reason, it is easy to replace the small plastic strips 64. Alternately, the jet plate may be conductive and can be formed as an integral part of the structure.

It will be noted that the two halves which together constitute the waveguide are separated by a substantial distance over at least a portion of the length to minimize the tendency for dust or lint which accumulates between the two halves to cause arcing. In the embodiment illustrated they are completely separated. It is preferred that this separation be at least inch.

In order to provide some circulation of air through the waveguides and to prevent possible condensation of moisture therein, the back wall 54 of the waveguide are provided with a series of small holes 68. These are spaced at intervals along the waveguide, the spacing being at about one half waveguide wavelength. These do not contribute substantially to the drying effect but are only provided to maintain the circulation of gas through the waveguide and minimize possiblecondensation of moisture therein.

In FIGURE 8 there is illustrated another embodiment of the invention which operates on the same general principles but which is slightly different from a mechanical standpoint. Here a gas inlet plenum 70 and a gas outlet plenum 72 are provided on each side of the device. The waveguides 74 are formed of two halves, as previously described, and gas outlet tubes 76 are provided at suitable intervals leading from the waveguide to the outlet plenum 72. The gas jets are formed between one wall of a waveguide and the U-shaped structure 78 which is provided on each side of the web. Welds 79 are provided at intervals to prevent arcing and provide structural strength and dimensional stability. Tubes 80 lead from the interior of the members 78 to the outlet plenum 72. It will be noted that the jets are of two types, one formed between the wall of the waveguide 74 and the adjacent wall of the -U-shaped member 78 as at 82 and also between two adjacent U-shaped members as at 84. Although the principle of operation is generally the same as heretofore described, it should be noted that in this instance, a small amount of gas is bled out of the waveguide rather than being bled into it as previously described.

The devices heretofore described have been symmetrical. Although this is the preferred configuration, other configurations can be used such as the asymmetric arrangement shown in FIGURE 9. Here, instead of the usual waveguide which operates with the sheet in its center, a stripline or interdigital guide slow wave structure is employed which radiates the web from one side. This comprises metallic duct 90 which is provided with the strips or fingers 92 which results in a electromagnetic fiold with an intensity distribution as approximately shown by the dash line 94. Within the guide 90 an inlet plenum 96 is formed by the members 98, the jets 100 being provided by the walls of the U-shaped non-conductive members 98. The U-shaped members serve as outlets for the spent air. At the opposite side of the web a cover 102 may be provided and this may be of a non-conductive material since it does not serve to confine the RF energy. An air system is provided on the opposite side by ducts 104.

Although air has been described as the gaseous medium, a gas such as nitrogen or superheated steam might be employed. It is highly important that the gas have a high velocity at the jet and must be at least 10,000 feet per minute. Velocities of 20,000 feet per minute and up can be used although it is generally uneconomic to go much higher than 20,000 feet per minute. Since the gas is not employed to absorb moisture in the usual sense of the word but is employed to break the boundary layer and to mechanically convey off the moisture, it is not necessary to provide extremely dry gas for carrying out the purposes of the invention, thus a substantial amount of the air can be recirculated by the dampers previously shown and described resulting in a great heat economy. It is only necessary that the gas be sufliciently dry to prevent condensation within the various ducts and waveguides.

For application to wide webs, it can be assumed that there will be several air distribution modules of the type described along the paper width and that multiple power inputs may exist in the direction of paper flow.

Although various preferred embodiments of the invention have been shown, it will be understood that these are for purposes of illustration only and are not intended as limitations on the invention which is one of broad applicability.

We claim:

1. An apparatus for drying a web comprising in combination:

(a) means for moving a web through said apparatus,

(b) a plurality of conductors of electromagnetic energy across the path of the web which produces a series of electromagnetic fields in said web,

(0) a first gas jet in said apparatus, said jet impinging on said web between electromagnetic fields (d) means for supplying gas through said jet at a velocity of at least 10,000 feet per minute (e) means for supplying a portion of gas through said conductors whereby a circulation of gas through the conductors is provided to prevent condensation of moisture therein and (f) means for supplying energy to said conductors at a frequency of from 300 mHz to 30 gHZ.

2. The apparatus of claim 1 wherein the wave conductor is a split waveguide, with one half of the guide on each side of the web, said halves being separated through at least part of their length by at least A-inch.

3. The apparatus of claim 1 wherein the jets are formed by apertures in a non-conductive material with conduc tive straps spaced at intervals across the non-conductive material.

4. The structure of claim 2 wherein each half waveguide forms a portion of a gas conduction system for conveying gas to and from the jets.

5. The structure of claim 2 wherein microwave energy power is supplied to the split waveguide structure from one said through a split right angle waveguide ell with an impedance matching means.

6. The structure of claim 2 wherein the waveguide is provided with a dummy load at its terminal end.

7. The structure of claim 2 wherein the waveguide crosses the web a plurality of items.

8. The structure of claim 1 wherein the jets are supplied with gas from a pressure source and adjustable means provides whereby part of the gas is recirculated.

9. The structure of claim 7 wherein the gas source includes controlled heating means.

10. The structure of claim 2 wherein gas is admitted to the waveguide through holes spacer at /2 waveguide length intervals.

References Cited UNITED STATES PATENTS 2,042,145 5/1936 Darrah 341 2,226,871 12/1940 Nicholas 34l XR 2,560,903 7/1951 Stiefel 21910.55

3,237,314 3/1966 Smith 344 3,293,765 12/ 1966 Winkler et a1. 34-1 OTHER REFERENCES 1,014,117 12/1965 Great Britain. 1,463,338 11/1966 France.

KENNETH W. SPRAGUE, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent NO, 3,491,457 Dated January 27, 1970 Inv t HENRY SCHREIBER, et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 41; cancel "microwaves" and insert microwave Column 2, line 19; after "energy" insert -combine--.

Column 3, line 11; after "inven-" insert the words tion the gas jets are di am i 11, cancel "another embodiment of".

Column 3, line 75; cancel "pleniums" and insert --plenums Column 6, line 36; change "said" to side Column 6, line 41; cancel "items" and insert times Column 6, line 44,- cancel "provides" and insert are provided Column 6, line 48; change "spacer" to spaced SIGNEB AND Fl 3 .2.5.9

Attach EdwlrdMFlctcher, Ir.

mm I. x m' a a a, S Om flomissioner of Pawn ta-

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