Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1337
~ `~ sackground o~ the Invention
.
This invention relates to microwave power applicators,
i.e., to devices for applying microwave power to workpieces. ;
More particularly, this invention relates tQ microwave power
applicators designed so as to substantially uniformly dis-
tribute hot spots which occur due to reflection standing wave
patterns.
From a commercial point of view microwave energy
usually is applied in one of three ways, namely resonant
cavity ovens, waveguide applicators and horn type antenna
applicators. `
In resonant cavity ovens energy in the form of micro-
wave radiation is fed into a totally enclosed metal box having
dimensions suitable for supporting standing waves. The material
to be heated is placed in the applicator and necessarily has -~
smaller dimensions than those of the box. This type of appli-
cator suffers from the disadvantages that heating lS not
even, pressure cannot be applied conveniently to the workpiece,
and the workpiece cannot be added continuously to or removed
20 continuously from the oven while it is operating. -
In a waveguide applicator a slot is cut in the
side of the waveguide in such a way that the workpiece may be
introduced into the wa~eguide. Usually slots are provided on
either side of the waveguide so that the workpiece may be
passed through the waveguide continuously. This type of
device is useful for heating thin webs of preformed material
but cannot be used conveniently to apply pressure to the
workpiece. In addition there is a limit to the size of the
workpiece that can be passed through the slots, and devices
of this type generally are not useful for heating thick work-
pieces. . .
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In horn type antenna applicators a horn type antenna
is used to convert the microwave energy from a transverse
electric or magnetic mode to a free space or transverse
electromagnetic mode. With this type of applicator it is
difficult to control or contain the energy, and it is not
possible to heat thick wor~pieces evenly or to apply
pressure to the workpiece during heating.
In Canadian Patent 836,140 issued March 3, 1970
to Her Majesty in right of Canada as represented by the
National Research Council of Canada there is disclosed a
microwave power applicator having a basic configuration which
resembles an embodiment of the instant invention. However,
in the microwave power applicator disclosed in this patent
no attempt is made to substantially uniformly distribute
across the central tube which carries the workpiece hot spots
which will occur as a result of reflection standing wave
patterns within the central tube. !
Summary of the Inve~ntion
In accordance with this invention there is provided
apparatus for applying microwave power to a workpiece compris-
ing a first hollow tube having an inlet end and an outlet
end, the first tube being adapted to receive through its
inlet end a workpiece to which microwave power is to be
applied, the workpiece passing through the first tube where
microwave power is applied thereto and exiting through the out-
let end of the first tube; first and second hollow waveguide
sections located on opposite sides of the first tube and comm-
unicating with the first tube to introduce microwaves into
it; and means for supplying microwaves to the first and
sec~nd sections to be propagated through the first and second
sections for introduction into the first tube; the
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33t7
frequency of the microwaves, the width of the first tube mea-
sured between the first and second sections and the angles of
inclination of the first and second sections with respect to
the first tube being chosen such that the peak amplitudes of
reflection standing waves created by introduction of microwaves - -
into the first tube via the first and second sections respect-
ively are interleaved, thereby substantially uniformly distri-
buting hot spots created by the standing waves across the width
of the first tube. ~-
BRIEF DESCRIPTION OF THE DRAWINGS
This invention will become more apparent from the
following detailed description taken in conjunction with the - ;
appended drawings, in which:
FIGURE 1 is a perspective view of a microwave power
applicator embodying the instant invention, -
FIGURE 2 illustrates another and more preferred embodi- -
ment of the invention, and
FIGURE 3 illustrates schematically the interleaving
of "hot spots" in a microwave power applicator embodying the
instant invention.
DETAILED DESCRIPTION OF THE INVENTION INCLUDING
THE PREFERRED EMBODIMENT ' -
:.. . .
Referring to Figure 1, there is shown a source 10 of
microwave energy. Source 10 may be any conventional microwave
energy source and, for industrial microwave applications, nor- ~ -
mally will operate at 915 MHz or 2450 MHz in accordance with ~-
D.O.T. and F.C.C. regulations. It may be, for example, an
EIMAC ttrade mark) Power Pack PPL-25. There is also shown what
could be referred to as an applicator 11, the latter consisting
of a hollow central tube 12 and two, hollow waveguide sections
13 and 14 located on opposite sides of central tube 12.
Central tube 12 has an inlet end 15 and an outlet end 16
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and is adapted to receive through its inlet end a workpiece to ;
which microwave power is to be applied, the workpiece passing
through central tube 12 where microwave power is applied there-
to and exiting through outlet end 16. Central tube 12 conven-
iently may be a waveguide section itself but, in any event, is
constructed so as to receive m~crowavesfrom waveguide sections
13 and 14 which are capable of propagating into the workpiece
(not shown) being passed continuously through central tube 12.
In the embodiment of the invention shown in Figure 1 central
tube 12 and waveguide sections 13 and 14 all are rectangular,
each having two longer sides and two shorter sides. The
longer sides of the central tube and the two waveguide
sections lie in the same two planes, and waveguide sections
13 and 14 are affixed to the shorter sides of central tube 12.
It would be possible, however, for central tube 12 to be
rotated 90 so that sections 13 and 14 would be affixed to its
longer sides. In a preferred embodiment of the invention,
and as shown in the Figures, waveguide sections 13 and 14 are -
each inclined at an acute angle to the longitudinal axis of
central tube 12 and are located directly opposite each other.
In a less preferred embodiment waveguide sections 13 and 14
each could be located perpendicular to the longitudinal axis -~
of central tube 12. In another embodiment of the invention
waveguide sections 13 and 14, rather than being located imme-
diately opposite each other, could be located in staggered
relationship with respect to each other. Moreover, if
desired, more than one waveguide section could be affixed to
each shorter side of central tube 12.
Waveguide sections 13 and 14 each are connected to
microwave power source 10, this type of connection being
conventional and being omitted for the sake of clarity,
and microwaves may be supplied alternately or continuously
from source 10 to waveguide
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sections 13 and 14. The embodiment of the invention illustrated
in Figure 2 differs from the embodiment of Figure 1 in that
two microwave sources lOa and lOb are provided, the latter
being connected to waveguide section 14 and the former to
waveguide section 13. In this embodiment of the invention,
which is the preferred embodiment, the frequencies of the
microwaves produced by sources lOa and lOb are different. The
ramifications of this will be outlined hereafter.
- .. ..
As noted hereinbefore, waveguide sections 13 and
14 preferably eAch are inclined at an acute angle to the
longitudinal axis of central tube 12 but may be perpendicular
A thereto. The former constitutes the preferred embodiment of
the invention because the dimension A, as seen in Figure X, is
considerably greater than the dimension B, so the energy `
density over the area constituted by A times the depth of
central tube 12 is considerably less than the energy density -
over thearea constituted by B times the depth of central tube
12, so that any tendency for arcing to take place would be
reduced with waveguide sections 13 and 14 each inclined at -
an acute angle to the longitudinal axis of central tube 12.
Moreover, with waveguide sections 13 and 14 each inclined at
an acute angle to the longitudinal axis of central tube 12, there
is a reduction in the tendency for microwave energy passing through ~ -
the workpiece to be propagated through the waveguide on the
opposite side of central tube 12, as would be the case if
waveguide sections 13 and 14 were directly opposite each
other and perpendicular to the longitudinal axis of central ,
tube 12.
In practising this invention it is necessary for
the microwaves to be propagated in waveguides 13 and 14 in
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l.t~ 3~7
the TE mode, where n is any number. In experiments which
have been conducted to demonstrate the practicability of the
invention, the TElo mode has been successfully utilized. It
is to be understood with reference to the aforementioned
nomenclature that in the TE mode the electric field vector
no
is parallel to the shorter sides of waveguide sections 13 and
14 and hence perpendicular to the longer sides thereof. The
desired objective is to obtain a field configuration within
central tube 12 that is as uniform as possible. The micro-
wave energy being supplied to the workpiece in central tube 12from waveguide sections 13 and 14 is attenuated as it passes
through the workpiece,and the magnitude of the E vector
decreases logarithmically as the microwave energy propagates
through the workpiece. There are, of course, two logarithmic
attenuation curves, since microwave energy is applied to both
sides of central tube 12. As far as the total-heating effect
is concerned, these two curves add together to produce sub-
stantially uniform heating across the workpiece.
Central tube 12 preferably is made sufficiently long
so that at its outlet end 16 there will be substantially no
energy to be reflected. This contributes to minimizing the
occurrence of standing waves within central tube 12 caused
by microwave propagation therein. Wotwithstanding central tube
12 being designed so as to minimize reflections and thus the
occurrence of standing waves, reflection standing wave patterns
will necessarily be present within central tube 12,and there
will be two such reflection standing wave patterns, one
associated with each waveguide section 13 and 14. The location
of the points of peak amplitude of the reflection standing
wave pattern associated with waveguide section 14 are shown
by the points designated 17 in Flgure X, while the locations
of the peak amplitudes of the reflection standing wave pattern
associated with waveguide section 13 are shown by the points
designated 18 in Figure X. The location o~ the points 17 and
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18 which are closest to the side walls of central tube 12
opposite to waveguide sections 14 and 13 respectively will occur
one quarter of a wavelength from the respective side walls of -~
central tube 12 measured along the lines containing the points,
the remaining points 17 and 18 occurring at half wavelength
spacings. At the points where the reflection standing wave : -
patterns have peak amplitude, hot spots will be created in the . :
workpiece being processed. In accordance with this invention ~-
it has been discovered that by choosing the frequency of the
10 microwaves, the width of central tube 12 as measured between .
waveguide sections 13 and 14 and the angles of inclination of
waveguide sections 13 and 14 with respect to the longitudinal
axis of central tube 12, the peak amplitudes of the reflection
standing wave patterns created by introduction of microwaves ~
into central tube 12 via waveguide sections 13 and 14 can be - - :
interleaved, thereby substantially uniformly distributing the ~
hot spots created by the reflection standing wave patterns ~ .
across the width of central tube 12. The location of the hot
spots may be determined by experimental observation of
temperature variations across central tube 12. Theoretically,
when the frequency of the microwaves, the dielectric constant -~
,,. ~ .,
of the workpiece, and the geometry of the applicator are known, `~ :
the location of the points 17 and 18 which are closest to the-
side walls of central tube 12 opposite to waveguide sections
14 and 13 will be known, as will the locations of the remain-
ing points 17 and 18, and by varying either the angles of incli-
nation of waveguide sections 13 and 14 with respect to the
longitudinal axis of central tube 12 or the width of central
tube 12, the locations of points 17 can be made to fall midway
between two adjacent points 18 and vice versa when the points
17 and 18 are projected parallel to the longitudinal axis of
central tube 12 onto a plane which is perpendicular to this
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337
longitudinal axis, as best shown in Figure 3. In practice,
experimental observation may be required to locate the hot
spots and their changing position as the parameters noted
above are ~aried. Thus the hot spots created by the
reflection standing wave patterns are distributed uniformly
across central tube 12 and hence across the
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337
workpiece. It should be understood that the nature of the
workpiece itself has some effect on the location of the
reflection standing wave patterns, and it may be necessary to
take this into consideration to achieve the desired inter-
leaving. If desired, waveguide sections 13 and 14 may be
affixed to central tube 12 by means of flexible sections, so
that the angles of inclination of waveguide sections 13
and 14 with respect to the longitudinal axis of central tube
12 may be varied. Likewise, central tube 12 may be provided -
with an expansion joint to permit its width to be varied.
In using the apparatus shown in Figure 1, it is
first adjusted to uniformly distribute the hot spots across
the width of central tube 12. The workpiece is inserted into -
inlet 15 and passes continuously through central tube 12
emerging via outlet 16. Microwave power is applied thereto
from source 10 or sources lOa and lOb via waveguide sections
13 and 14. A wide variety of workpieces may be heated by
passage through central tube 12. Preferably the cross-
sectional area of the workpiece should be the same as the
cross-sectional area of central tube 12 or the E field will
be distorted. To this end, plywood sections have been pro-
cessed in apparatus of the type described, the microwave
energy being used to cure the resin used to laminate the veneers
of the plywood, but many other applications are possible. For
example, the apparatus could be used for curing rubber.
In the manufacture of plywood it is necessary to -
apply pressure to the plywooa before the resin is cured. This
can be achieved with apparatus embodying the instant invention
by incorporating a press within central tube 1~. The press
may take the form of two moving belts suitably backed up
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337
and defining a nip between the belts into which the plywood
is inserted. In fact, the belts could be fabricated of
metal and could constitute a part of central tube 12.
In the preferred embodiment of the invention two
microwave power sources lOa and lOb are employed each
operating at a different frequency. Whenever microwaves
are applied to central tube 12 from both sides thereof, an
interference standing wave pattern will be set up. If two
microwave sources are employed operating at even slightly
different frequencies, the interference standing wave pattern
will not be static but will move ~Ye ~~ ~e_~racross central
tube 12. The hot spots created by this interference stand-
ing wave pattern thus will not be static and will not create
any problem. In the case where microwave energy is applied
alternately to waveguide sections 13 and 14 from a single
source 10, there also will be no interference standing wave
pattern produced. However, in the embodiment of the invention
where microwaves of the same frequency, e.g. from a common
source, are applied simultaneously to waveguide sections 13
and 14, a static interference standing wave pattern will
result that cannot be compensated for. Consequently this is
a less preferred embodiment of the invention.
While preferred and other embodiments of the
invention have been described herein, those skilled in the
art will appreciate that changes and modifications may be
made therein without departing from the spirit and scope
of this invention as defined in the appended claims.
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