Construction For Tuning Microwave Heating Applicator

Abstract

A microwave heating applicator that operates in the TM modes having a substantially cylindrical resonant cavity in which elongate material is heated while being processed therethrough. A microwave heating applicator having a perforated end cap on one end, said end cap having perforations for admitting heated air into the cavity, the size of each perforation being sufficiently small relative to its length to attenuate the transmission of microwave energy, a manifold for receiving heated air and guiding the air toward the perforations in the end cap; and a cylindrical metal sleeve mounted in openings through the end cap and manifold, one end of said sleeve projecting into the resonant cavity and the other end being accessible for adjusting its axial position as to tune the cavity.

Description

This invention generally relates to microwave heating applicators and more particularly to applicators for continuously processing and heating a threadlike substance. The invention particularly relates to applicators which operate in the TM modes and which may be tuned while in use.

Various microwave heating applicators are known which provide means for continuously processing a threadlike substance and which also have the capacity for being tuned while in operation. An exemplary teaching of this type is described in U.S. Pat. No. 3,461,261 in which tuning is accomplished by partially loading the cavity using a quartz sleeve mounted in one end of the applicator. However, none of the previously known microwave applicators are capable of being operated in the TM modes and tuned while simultaneously passing an airstream through a resonant cavity. In brief, the present invention comprises a microwave heating applicator having a cylindrical resonant cavity which is tuned with a metallic sleeve, the axial position and portion of the sleeve within the resonant cavity determining the effective electrical length of the applicator as a whole as determined by the formula

.theta.=2.pi.l.sub.1 /.lambda.g.sub.1 + 2.pi.l.sub.2 /.lambda.g.sub.2

Where

.theta. = effective electrical length of applicator

1.sub.1 = physical length of that portion of resonant cavity which is occupied by the metallic sleeve.

1.sub.2 = physical length of the remaining portion of resonant cavity not occupied by the metallic sleeve.

.LAMBDA.g.sub.1 = wavelength in coaxial section of resonant cavity occupied by the metallic sleeve.

.LAMBDA.g.sub.2 = wavelength in remaining portion of resonant cavity not occupied by metallic sleeve.

In addition, a further feature of the present invention is in providing an applicator construction which allows a stream of heated air to be passed through the resonant cavity while also allowing the cavity to be tuned.

It is to be understood, therefore, that one object of the present invention is to provide a microwave heating applicator with means for tuning a cylindrical resonant cavity by changing its effective electrical length and while simultaneously processing a threadlike substance through the applicator in a heated airstream.

Another object is to provide a microwave heating applicator of the kind described which may be disassembled for cleaning and then reassembled with a minimum of down time and labor.

Other objects of this invention will become apparent in view of the following detailed description and the accompanying drawings.

In the drawings forming a part of this application and in which like parts are identified by like reference numerals throughout the same.

FIG. 1 is a perspective view of a preferred embodiment of this invention in a microwave heating applicator, a portion of the applicator being broken away;

FIG. 2 is a vertical section of a portion of the applicator shown in FIG. 1;

FIG. 3 is an enlarged vertical section of the lower end of the applicator;

FIG. 4 is a section taken on lines 4-4 of FIG. 3; and

FIG. 5 is a perspective view of the tuning sleeve mounted in the perforated end cap forming part of the applicator.

Referring to FIG. 1 in particular, there is shown a preferred embodiment in a microwave applicator 10 connected to a source of microwave energy 11 through a three-port circulator 12. One port of the circulator connects with a water load 13 which absorbs reflected energy, the other two ports connecting the applicator with the source of microwave energy, all in a manner well known in this art.

The present invention is directed more especially to those structural details of applicator 10 which allow tuning while elongated, threadlike material T is being processed. In general, applicator 10 comprises a cylindrical waveguide 20 that operates in the TM modes for heating the material T as it is drawn through the waveguide. Heated air is introduced into the waveguide through a bottom manifold 21 secured by set screw 22 to an axially segmented sleeve 23, said sleeve forming an integral part of a perforated end cap 24 having a plurality of openings 24a for admitting heated air into the resonant cavity. The cross section or size of each opening 24a is restricted by design so that microwave energy is not lost through the openings. This involves a selection and design of opening size and length as required to attenuate microwave energy of predetermined wavelength. In effect, the perforated end cap serves as a conducting screen in which openings 24a are too small to permit radiation leakage but offer only slight restriction to the flow of heated air.

End cap 24 provides a cylindrical wall section 25 that defines a portion of the resonant cavity, a relatively large flange 26 being provided at the end of the wall section for attaching the end cap to the bottom flange 27 of wave guide 20. The two flanges are joined together by a rapid disconnect coupling 28 of conventional design.

It will be especially noted that the end cap is designed as a component part of the applicator so that the interface between flanges 26 and 27 is a quarter wavelength from the end of the resonant cavity. This construction creates a condition such that the E field between flanges is at a minimum. Also, the relatively large and flat surfaces of contact between flanges makes the joint relatively insensitive to frequency variations.

End cap 24 further comprises an inner segmented sleeve 29 that extends into the resonant cavity a distance of approximately one quarter wavelength, and the ends of the sleeve are preferably tapered to avoid arcing. The tapered ends also aid in preventing dust or other materials introduced into the waveguide from collecting in a region of high E field intensity.

Inner segmented sleeve 29, as well as exterior sleeve 23, define an inner opening and mounting for a metallic tuning sleeve 30. The segments of sleeve 29 are particularly formed so that their tapered ends contact the tuning sleeve with a resilient gripping force. This force is sufficient to support the sleeve itself but allows the sleeve to be moved coaxially of the resonant cavity into various positions of adjustment. Once the resonant cavity has been properly tuned, sleeve 30 is clamped in a fixed axial position by means of an expansible band 31 mounted around a segmented portion of sleeve 23.

Applicator 10 further comprises an upper end cap 40 having a lateral connection 41 that communicates with a vacuum source, not shown. In addition, a choke device 42 having a pair of quarter wave recesses 43 and 44 spaced approximately one-half wavelength apart is employed to attenuate microwaves and prevent energy losses through the upper end of the waveguide.

In operation, applicator 10 provides a cylindrical resonant cavity that operates in essentially the TM modes for drying a threadlike material. The moisture and other gases which is expelled from the threadlike material is removed by a stream of heated air. This air is introduced through bottom manifold 21, passed into the resonant through openings 24a of end cap 24 and then removed from the cavity through the upper end cap 40. Unlike conventional microwave heating applicators, the resonant cavity of applicator 10 may be tuned during normal processing of the threadlike material by simply adjusting the axial position of tuning sleeve 30.

Although a preferred embodiment of this invention is illustrated and described, various modifications and changes may be made in the applicator construction without departing from the spirit of the invention or the scope of the attached claims, and each of such modifications and changes is contemplated.

Claims

What we claim is:

1. In a microwave heating applicator having a substantially cylindrical resonant cavity, a perforated end cap mounted to one end of said applicator, said end cap having perforations for admitting heated air into the cavity, the size of each perforation being sufficiently small relative to its length to inhibit the transmission of microwave energy therethrough, means defining a central opening through said end cap and coaxial relative to said cavity; means defining a plenum at the one end of said applicator for receiving heated air and guiding the air through the perforations in said end cap, said plenum means having an opening coaxial with the opening of said end cap; and a cylindrical tuning sleeve mounted in the openings through said end cap and plenum means, one end of said cylindrical sleeve extending into said cavity, the other end of said sleeve projecting out from said plenum means.

2. The microwave heating applicator of claim 1, the opening through said end cap being defined by a substantially cylindrical mounting sleeve having a segmented end that extends into said cavity and terminates at a distance approximately one-fourth wavelength from the near end of the cavity.

3. The microwave heating applicator of claim 2, said segmented end comprising a plurality of fingers that taper axially inward and provide resiliency for griping said tuning sleeve.

4. The microwave heating applicator of claim 1, said end cap comprising a cylindrical wall section that defines a portion of the resonant cavity and having a connecting flange for attachment to the applicator, the axial length of said wall portion being approximately one-fourth wavelength.

5. A microwave heating applicator having a resonant cavity adapted to operate in the TM modes and comprising: a cylindrical waveguide; an end cap mounted at one end of said waveguide, said end cap being perforated and having a central opening coaxial of said waveguide the size of each perforation being sufficiently small relative to its length to inhibit the transmission of microwave energy therethrough; a plenum in communication with said perforations and having an opening therethrough and a cylindrical metal tuning sleeve extending through the opening of said end cap and axially adjustable therein, one end of said sleeve projecting into the resonant cavity and the other end being accessible to adjust the axial position of the sleeve and tune the cavity.

6. The microwave heating applicator of claim 5, said end cap comprising a substantially cylindrical mounting sleeve for supporting said metal tuning sleeve, and further comprising a mainfold mounted to said end cap for receiving heated air and guiding the air through the perforated end cap into the resonant cavity, said manifold having an opening therethrough coaxial with the mounting sleeve and the central opening of said end cap, said mounting sleeve extending through the opening of said manifold.