US3475827A - R.f. seal in microwave drier - Google Patents

Description

NOV. 4, 1969 v J GQERZ, JR 3,475,827

R.F.SEAL IN MICROWAVE DRIER Filed Dec. 6, 1967 v 2 Sheets-Sheet 2 INVENTOR,

DAVID J. GOERZ, JR.

FIE--5- B ATTMNEYS United States Patent 3,475,827 R.F. SEAL IN MICROWAVE DRIER David J. Goerz, Jr., Menlo Park, Califi, assignor, by

mesne assignments, to Bechtel International Corporation, San Francisco, Calif., a corporation of Delaware Filed Dec. 6, 1967, Ser. No. 688,468 Int. Cl. F26b 3/34; Hb 9/06 US. 'Cl. 34-1 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION R.F. shielding and air sea] in microwave drier for a web such as a paper web.

7 Description of the prior art Although shielding methods are known for R.F. energy, ordinarily such shields are large and bulky and the present invention provides a shield of relatively small dimensions. Further, since the R.F. energy which would otherwise escape is absorbed, an air seal can also be provided which would otherwise be diflicult because of arcing normally characteristic inherent when bringing together two portions of a split waveguide.

SUMMARY OF THE INVENTION In a microwave drier for paper and similar webs, there must be a substantial separation between the halves of the split waveguide to prevent arcing and concurrently to permit the insertion of the web into the drier. This provides a large path for the escape of R.F. energy and, since the power levels are very high in such driers, would constitute a health hazard to workmen utilizing the equipment. Althought methods have been known in the past for shielding such structures, ordinarily large heavy metal shields or ornate and costly provisions would be necessary. In accordance with the present invention, instead of employing a large metal shield, a relatively small shield of a lossy R.F. material is employed to absorb the energy. Preferably the lossy material is provided in a tapered form at the edges of the waveguide so that the energy will be gradually absorbed over a substantial area. After the energy has been absorbed, there is no danger of arcing so it is quite easy to provide in such a structure an eflicient gas seal. Further, since the lossy material absorbs rather than reflects the R.F. energy, a substantial amount of heat is absorbed in the shield so that preferably the R.F. shields are provided with fluid cooling means. If the fluid is water, it further contributes to the absorption of the R.F. energy.

In a preferred embodiment of the invention, the fluid cooling tubes also serve the purpose of providing an efficient gas seal between the split halves of the waveguide, material contributing to the efficiency of the system.

At the entry and exit ends of the drier, it is impossible for obvious reasons to provide a complete air seal since this would be inconsistent with the necessity of moving the web at a high rate of speed through these openings. However, either or both of these openings can be protected against the escape of R.F. energy by employing the same types of means as are used at the sides of the device, although no gas seal is combined with the R.F. shield.

In the past, such driers which have utilized an air or other gas flow have suffered from the difficulty that the paper tends to flutter as it enters or leaves the drier and in accordance with the present invention, simple means is provided for stabilizing the web position by blowing air onto the web at the entrance and exit forks of the drier.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a partial perspective view, with certain parts in section, of a drier embodying the present invention.

FIGURE 2 is an enlarged section on the line 2-2 of FIGURE 1.

FIGURE 3 is an enlarged section on the line 3-3 of FIGURE 1.

FIGURE 4 is an enlarged portion of FIGURE 3.

FIGURE 5 is a plan view of a drier embodying the present invention, generally on the line 55 of FIG- URE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to improvements and modifications of the equipment shown in patent applications Ser. No. 674,293, filed Oct. 10, 1967 and 677,914, filed Oct. 25, 1967, and reference is made to these applications for the details of the structure and operation of a microwave drier of the type to which the present invention pertains.

Suflice it to say that a web 7 is drawn between the two halves 9 and 11 constituting a split waveguide which is supplied with microwave energy through waveguides 13 or 14 from a suitable source, not illustrated. As the web is drawn between the halves of the waveguide, it is also subjected to jets of high velocity air or other gas from the plenum chambers 15 acting through the openings as at 17 and 19 and the hot, moist gas is drawn off through the plenum chambers 21. The split halves of the waveguide may be wholly or partially covered with a plastic or other suitable material as is described in the second application identified above. The description thus far has been in the way of brief background and constiutes no part of the present invention.

At the edges of the waveguide which must be split to accommodate the drying process, there must be a substantial opening or otherwise arcing difliculties would be encountered. If conventional shielding were used on these openings, it would have to be very large or complex to protect workers around the equipment. In accordance with the present invention, the edges of the waveguides are filled with a lossy R.F. material as at 23. Although the material is shown as extending only from one side of the waveguide since this is simplest from a mechanical standpoint, the material could be applied to both sides of the waveguide, meeting someplace near the center. Further, the lossy material is preferably tapered toward the center of the drier as at 25 so that the R.F. energy which would otherwise escape is absorbed over a substantial distance.

Suitable lossy materials are well known to those skilled in the art and generally consist of a mixture of soft iron and carbon such as the material sold under the name of Ferroxcube. Other suitable lossy materials are Carborunwhere it may becooled and recirculated. 'The'uibescam" be formed in pairs with a suitable connecting means 35 held in place by means of a screw 37 into the lossy material. Obviously other suitable fastening means can be employed.

As is best seen in FIGURE 3, the tubes 27 and 29 are not only held against the lossy material but are also in contact with the opposite wall 39 of the waveguide extension. Thus, the tubes not only function to cool the lossy material and as an auxiliary absorber for RF. energy, but also constitute an efficient gas or air seal.

At the entry and exit portions of the drier, it is obvious that no physical contact'between the two halves is possible since the web must pass through this space. However, sufficient R.F. shielding is provided at these points by means of the plates 41 and 43 of lossy material which lie at the entry and or exits of the drier, forming an efficient shielding for the RF. energy at these points. Further these plates can be cooled by means of tubes 45 which are similar to the tubes previously described and which can be in the same circulatory system as is shown in FIGURE 5 It was previously mentioned that the web frequently tends to wave or flutter as it enters or leaves the drier. This is for the reason that in the drier described in the above identified application, the paper was first subjected to a vacuum chamber and, the air flowing intended to cause a flutter or instability of the paper. This can be largely alleviated by the means shown in FIGURE 2. wherein the inlet air plenums 15 lie on each side of the web 7 and on each side of the lossy plates 41 and 43. These plates are provided with a series of apertures 47 so that the web on entering or leaving the drier is subjected to positive air pressure on both sides thereof. "1 his "substantially prevents flutter 0f the paper.

It is obvious that many variations can be made in the exact structure shown and described Without departing from the spirit of this invention.

1. In a microwave drier having-a split waveguide structure, the improvement comprising at least one plate of a solid, lossy R.F. material, said plate. lying along, at least one edge of the split waveguide whereby R.F. energy which would otherwise escape from'the edge of the guide is absorbed and a tube passing through said plate and means for passing a fluid through said tube to cool said lossy material. p

2. The structure of claim 1 wherein the fluid is water.

3. The structure of claim 1 wherein the solid plate of lossy material is provided betweenthe two 'split halves of the waveguide, with saidtube being partly imbedded in the exposed edge of thelossy R.F:. material, said tube contacting the opposite edge of the waveguide,'providing agas seal. 7 References Cited UNITED STATES PATENTS KENNETH w. SPRAGUE, Primary Examiner US. (:1. X.R. 219-1055

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