U.S. patent number 3,557,334 [Application Number 04/803,374] was granted by the patent office on 1971-01-19 for method and apparatus for regulating heating in a microwave resonant cavity.
This patent grant is currently assigned to E. I. duPont deNemours and Company. Invention is credited to Richard William Lewis.
United States Patent |
3,557,334 |
|
January 19, 1971 |
METHOD AND APPARATUS FOR REGULATING HEATING IN A MICROWAVE RESONANT
CAVITY
Abstract
The method of operation of a microwave resonant cavity system in
a preselected undercoupled or overcoupled mode and with preselected
frequency offset provides stable heating of dielectric material
advancing through the cavity for the purposes of moisture or
solvent removal or heat treating. The system includes a three-port
circulator coupled between a microwave power source, a resonant
cavity and a water load. Means are provided between the circulator
and cavity to establish the preselected coupling mode and between
the circulator and water load to feed a portion of reflected power
from the cavity due to detuning and decoupling to the power source
to regulate the shift in frequency of the power source (frequency
offset) according to variations in resonant frequency of the load
cavity.
Inventors: |
Richard William Lewis (Derby,
KS) |
Assignee: |
E. I. duPont deNemours and
Company (Wilmington, DE)
|
Family
ID: |
25186369 |
Appl.
No.: |
04/803,374 |
Filed: |
February 28, 1969 |
Current U.S.
Class: |
219/693; 219/694;
219/695 |
Current CPC
Class: |
H05B
6/666 (20130101); H05B 6/782 (20130101) |
Current International
Class: |
H05B
6/78 (20060101); H05B 6/66 (20060101); H05b
009/06 (); H05b 001/02 () |
Field of
Search: |
;219/10.55 ;333/1.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: J. V. Truhe
Assistant Examiner: L. H. Bender
Attorney, Agent or Firm: Howard P. West, Jr.
Claims
1. A method comprising the steps of: a. advancing a dielectric mass
axially through a microwave resonant cavity said mass presenting a
changing load to said cavity; b. transmitting microwave frequency
energy to said cavity from a magnetron source to heat said mass,
said cavity being purposefully decoupled and detuned in a
preselected direction from a critically coupled and matched
frequency relationship with said source for operation at the
decoupled and detuned condition selected, a portion of said energy
being reflected away from said cavity; and c. absorbing a part of
said portion, the remainder being reflected back to
2. A method of drying a wet tow having a changing moisture content
comprising: a. passing the tow axially through a microwave resonant
cavity; b. transmitting microwave frequency energy to said cavity
from a magnetron source, said cavity being in an overcoupled
relationship with said source and being tuned to a frequency above
a matched frequency relationship with said source for operation at
said relationships, a portion of said energy being reflected away
from said cavity; and c. absorbing a part of said portion, the
remainder being reflected back to
3. The method as defined in claim 2, said tow comprising
multifilament
4. The method as defined in claim 3, about 11 percent of said
portion being
5. In a microwave dielectric heating apparatus that includes a
cylindrical resonant cavity heater, a microwave frequency energy
source for supplying energy to the cavity, a water load and a
circulator, said circulator being coupled to said source, said
heater and said water load by respective first, second, and third
waveguide sections, there being a first iris positioned in said
second waveguide section, the improvement comprising: means coupled
with said heater for varying the transverse cross section thereof
to provide tuning capability for said heater; and a second iris
6. The apparatus as defined in claim 5, said source being a
magnetron.
Description
This invention relates generally to an improved method and
apparatus for heating dielectric material by means of microwave
energy in which a low loss resonant cavity is employed as a heating
chamber. More specifically, it relates to maintaining stability of
operation in spite of significant changes in the heating load
within the resonant cavity.
One type of resonant cavity to which the present invention applies
is disclosed in copending application Ser. No. 590,917, filed Oct.
31, 1966, in the name of Lewis and White and now U.S. Pat. No.
3,461,261.
It is well known to heat various types of materials through
exposure to high frequency electromagnetic fields. Conductive or
metallic materials are usually heated by placing them in a high
frequency magnetic field. Here the predominant heating effects
usually result from induced electric currents, but additional
heating may result from magnetic hysteresis or other magnetic
effects depending on the magnetic characteristics of the material.
In contrast, dielectric heating results for many materials which
are considered to be nonconductors or dielectrics when they are
exposed to a high frequency electric field.
The rate of dielectric heating follows the relationship P= CE.sup.2
f .epsilon.", where P is the rate of energy absorption per unit
volume, C is a constant, E is the electric field strength, f is the
frequency of the electric field, and .epsilon." is the loss factor
of the material being heated. In continuous processes such as the
drying of a continuously moving textile tow or the heat setting of
a running yarn line, the rate P at which heat is required depends
on processing conditions, such as the throughput of the material
and the amount of moisture to be removed or the required
temperature rise. To achieve the desired heating rate, it is
necessary to select values of frequency and field strength
depending on the loss factor of the material being heated.
Breakdown and arcing within the cavity puts a practical limit on
the field strength that can be employed. Thus for a given set of
process conditions, the heating rate is known and, if the
dielectric properties are known so that the values of the loss
factor and the maximum electric field can be determined, an
appropriate range of frequencies can be found from the above
equation. However, when the inlet moisture content fluctuates with
time and/or the final temperature of material is to be maintained
in an assigned range, the novel method and apparatus of this
invention are particularly important and useful. Useful dielectric
heating has been achieved at frequencies from 500 kHz.
(kilocycles/second) to 500 MHz. (megacycles/second) for such
applications as drying of foundry sand molds and pharmaceuticals
and the curing of glue lines in plywood. However, when desired
heating rates are high and/or the materials to be heated have
dielectric properties for which the loss factor is quite low,
development of heating equipment in the microwave frequency range
of 1,000 MHz. and above has been found necessary. The use of such
high frequencies in microwave ovens has been successful in the
fields of rapid food cooking, foaming and curing of large foamed
polymer mattresses and cushions, and the like. However, these
devices produce relatively low electric field strengths and are
therefore not practical for heating advancing yarn lines, tow or
other process streams.
Extremely high electric field strengths have been achieved in
resonant microwave structures with a wide variation of spatial
electric field concentration arrangements. A resonant microwave
cavity specifically suited for yarn or tow heating is described in
the above-referenced Lewis & White copending application. It is
to resonant cavities of this type that the present invention is
particularly applicable.
The dielectric properties define the loss factor .epsilon." for any
given material. The loss factor is the product of the dielectric
constant (.epsilon.') and the loss tangent (tan .delta.). The
dielectric constant is a measure of the energy stored in the
electric field that passes through the material compared to the
energy that would be stored in the same electric field passing
through a vacuum. The loss tangent is a measure of the ratio of
energy absorbed to energy stored that occurs in the material in an
alternating electric field.
The term dielectric constant is misleading, since it is rarely a
constant. It is, in fact, a function of both temperature and
frequency for the material in question. For example, at a microwave
frequency of about 3,000 MHz., the dielectric constant of water
changes slightly downward with increasing temperature, e.g., from
76.7 at 25 .degree. C. to 52 at 95 .degree. C. The loss tangent,
however, for water drops from 0.157 at 25.degree. C. to 0.047 at
95.degree. C. Values for 6/10 nylon (polyhexamethylene sebacamide)
at 3,000 MHz. are 2.84 at 25.degree. C. and 2.94 at 84 .degree. C.
for the dielectric constant, whereas the loss tangent varies from
0.128 at 25 .degree. C. to 0.356 at 84 .degree. C., an increase of
nearly threefold. A similar change has been found for nylon 6-6
(polyhexamethylene adipamide).
Thus, variations of these types can significantly influence the
operation of dielectric heating systems. Since the dielectric
constant is related to the energy stored in the electric field that
will pass through the material, an increase or decrease in the
dielectric constant can cause a corresponding increase or decrease
of the capacitance of the load that can cause a change in reactance
of the load. For a fixed value of dielectric constant, an increase
or decrease in the loss tangent will cause a corresponding increase
or decrease in heating since the loss tangent is a direct measure
of energy absorbed by the material at constant field strength.
In the past, it has been normal practice to operate resonant
cavities for dielectric heating at the resonant frequency for the
power source and at maximum level of energy coupling to the power
source. However, when such a system is used for removal of moisture
from moving tow, for example, a sudden increase in the moisture
content in the cavity will change the loading of the resonant
cavity. This change in loading will change the impedance of the
cavity and cause a shift in coupling, which will reduce the level
of power available to be absorbed by the load. In such a case, the
heating will decrease when increased heating is needed and the
desired drying level will not be achieved. On the other hand, when
heating relatively dry polymeric material for heat treating
purposes, the dielectric loss factor of the polymer is increased as
temperature increases causing an increase in the rate of energy
absorption by the material from the electric field that may result
in "run-away" heating if the heating is performed at the maximum
level of energy coupling.
Although solutions have been disclosed in the prior art for
problems of load changes in dielectric heating systems in which the
resonant structure is an inductance-capacitance tank circuit for an
oscillator tube (the capacitance section being the heating section)
none would apply when a microwave resonant cavity is used as the
dielectric heating chamber. In this case, there is no known route
to automatic stable operation. This is true primarily because the
resonant cavities receive microwave power directly from magnetrons
or similar power sources which in themselves are generally resonant
systems. Effective energy transfer from one resonant system to
another requires a close match in the frequency of the two systems
and a close match in impedance. Shifting of cavity resonant
frequency and/or cavity impedance due to load change normally
results in a reflection of power back to the power source. Although
this reflected power will in some cases be beneficial in shifting
the frequency of the power source (if a magnetron), the reflected
power if too great can cause damage to the power source.
Additionally, the reflected power is not available within the
cavity for heating.
The key to the successful operation of microwave resonant cavity
heating systems was the discovery of a method of coupling the
resonant cavity and a magnetron power source that permits their
combined operation in a controlled and stable manner with a
preselected range of frequency mismatch and reflected power. The
ability to preselect a frequency and impedance mismatch between a
microwave resonant cavity and the magnetron supplying power to it
makes possible the choice of an inherently stable mode of operation
by which polymeric materials can be heated.
Inherent stability of heating of dielectric material is achieved by
this invention in a microwave resonant cavity which is purposefully
decoupled and detuned from a critical coupled and matched frequency
relationship with the power source in a direction and to an extent
depending upon the characteristics of the material being heated.
For moisture or solvent removal, the cavity is overcoupled and
tuned to a resonant frequency slightly above power source
frequency. On the other hand, when heating relatively dry polymeric
material primarily for the purpose of raising its temperature to a
predetermined level, the cavity is undercoupled and tuned to a
resonant frequency slightly below that of the power source.
Operation of a microwave resonant cavity in the preselected
undercoupled or overcoupled mode and with the preselected direction
of frequency offset of this invention has been achieved in a system
in which a preselected percentage of the power reflected from the
cavity is purposefully directed back to the magnetron. The
percentage of power is chosen so that if total reflection occurs at
the cavity, the amount of power reaching the magnetron is below the
level which will damage the magnetron. The percentage of power is
chosen large enough to provide the required increase in the
"pulling" range of the magnetron to accommodate the preselected
range of frequency offset.
The preferred embodiment of a system or apparatus to achieve the
objects of this invention comprises a tunable microwave resonant
cavity heater, a magnetron source of microwave energy, a circulator
and a water load. The circulator is coupled to the source, the
heater and the water load by respective first, second and third
waveguide sections. A first iris is positioned in the second
waveguide section to decouple the heater from a critical coupled
relationship with the source and reflect a portion of the energy
from the source back through the circulator toward the water load.
The heater is also detuned from a matched frequency relationship
with the source which causes additional energy reflection. A second
iris is positioned in the third waveguide section to reflect a
predetermined part of the portion of energy being reflected toward
the water load back through the circulator to the source to provide
partial pulling of the source frequency which in turn provides
required frequency offset. The novel combination of an iris and a
water load in one leg of the system has been found to be a very
simple yet versatile combination for preselecting the percentage of
power which is permitted to return to the magnetron. In the
extremes, the absence of the second iris permits essentially all of
the reflected power to be dissipated or absorbed in the water load
and essentially no power will be fed back to the magnetron to
"pull" its frequency in the direction of any shift of resonant
frequency of the cavity. On the other hand, a complete reflection
at the water load directs all of the reflected power back to the
magnetron so that maximum "pulling" of its frequency towards that
of the cavity is produced. This latter case nullifies the purpose
of the circulator and water load combination which is to permit
frequency offset and allow for magnetron protection. Furthermore,
operation such as in the latter case is as if there were a direct
connection of magnetron to cavity as described in the
aforementioned copending application. Operation then would in
effect be in a matched frequency condition.
FIG. 1 is a schematic drawing in partial section showing the
component arrangement of the microwave heating system;
FIG. 2 is a partially sectioned end view of a resonant cavity and
cavity tuning frame, this view taken in the plane perpendicular to
FIG. 1 along lines 2-2 with view rotated 90 .degree.
counterclockwise and enlarged resonant cavity and cavity tuning
frame;
FIG. 3 is a schematic drawing showing the system arrangement with
instrumentation for use in setting system operation for optimum
control when heating a continuously running length of material;
FIG. 4 is a graphical representation of power transferred into a
resonant cavity as a function of frequency;
FIG. 5 is a graph of power transferred to a cavity as a function of
the dissipating load in the cavity for a coupling iris of fixed
construction having notations to depict the effect of moisture
content and temperature variations of the material treated on
load;
FIG. 6 is a graph of power vs. frequency as in FIG. 4 except that
notations are added to assist in describing effects of both
moisture level and temperature of material treated on tuning and
consequently on stability; and
FIG. 7 is the graph of FIG. 5 with notations employed to describe
effects of overcoupling on stability in drying and undercoupling on
stability in polymer heating.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The novel method of heating regulation of this invention is
achieved by means of a simple apparatus system in which the desired
coupling mode is established through the selection of the cavity
coupling iris. The required amount of frequency offset between
power source and cavity to provide stability is achieved by
selecting magnetrons with a "pulling" range as wide as possible
(5--8 MHz. is typical although a magnetron with a range of 20 MHz.
has been found) and then adjusting the amount of "pulling" by
preselecting the percentage of reflected power that can return to
the magnetron to cause frequency "pulling."
Referring to FIG. 1 a magnetron 10 (such as Amperex YJ 1160 fitted
with a modified Eimac No. EW3- TC 2 Laucher for 15/8 inch coax to a
wave guide) energized by a conventional high voltage DC power
supply (not shown) is connected through a first waveguide section
11 (Type WR340) to the first port of a three-port circulator 12
(Airtron No. 336359 or Eimac No. EW 3- C1). The second port of
circulator 12 is coupled by means of a second waveguide section 13
to resonant cavity heater 15 through a first impedance transforming
apertured iris 14. The third port of circulator 12 is coupled by
means of a third section of waveguide 16 through a second apertured
iris 17 into a water load 18 (Eimac No. EW 3- WL2.5). Iris 17 is
positioned in waveguide 16 across its juncture with water load 18.
In the event of frequency mismatch between magnetron 10 and
resonant cavity 15 or impedance mismatch of the transmission
waveguide 13 and cavity 15 as transformed by iris 14, power is
reflected back through waveguide 13 and into circulator 12. The
function of circulator 12 is to prevent excessive reflected power
from being directed back to the magnetron 10 where it could cause
damage through arcing or overheating. Thus, circulator 12 directs
reflected power from cavity 15 through waveguide section 16 and
iris 17 into water load 18. The amount of this reflected power
which is absorbed, and dissipated in water load 18 is dependent on
the choice of the aperture of iris 17.
Waveguide sections 11, 13 and 16 are chosen of the shortest length
which will ensure the proper phase relationship for each component.
Using short lengths not only keeps the system compact but also
helps alleviate phase shift problems which arise during load
variations. It was possible using short lengths of WR340 waveguide
in the system shown to eliminate the need for a phase shifter in
waveguide section 16.
The choice of iris 17 is also the key to determining the proportion
of reflected power entering waveguide section 16 which will be
directed back through waveguide 16 into circulator 12 and then into
magnetron 10 to produce frequency " pulling." The greater the
proportion of reflected power from iris 17 the greater the "
pulling," which in turn reduces the achievable frequency offset
between magnetron and cavity. Conversely a reduction in the
proportion of reflected power reduces " pulling" and increases the
achievable frequency offset.
As mentioned previously, too great a proportion of reflected power
may exceed the safe level for the magnetron in the case where all
power is reflected from the cavity. In addition, when the system is
being turned on and tuned in (start up), excessive reflected power
can cause operating difficulty by forcing the magnetron to
oscillate in an undesired mode and not couple to the cavity.
In the preferred embodiment, a tow or yarn 22 which is to be dried
or heated is moved from a supply source (not shown) by means of
pull rolls 25, 25', passed through tuned resonant cavity 15, and
withdrawn by means of rolls 26, 26'. A longitudinal slot 28 is
provided in cavity 15 along one side in order to facilitate
stringup of the cavity. When drying a wet tow, this slot is located
along the bottom edge of cavity 15 in order that excess liquid from
the material being dried may run out from the cavity. Cavity 15 may
be tuned to different frequencies by the method disclosed in the
Lewis & White reference.
The preferred embodiment of the cavity tuner is shown in FIG. 1 and
FIG. 2. Lips 30, 31 on each side of slot 28, respectively, are
provided on which tuning frame 32 is brought to bear. Tuning frame
32 comprises two curved pieces 38, 38' separated and held by a bar
40 at one end of each. Pieces 38, 38' are thus arranged to fall on
either side of flange 41 for attachment of waveguide section 13 and
iris 14. The other end of each curved piece carries a plate 42,
42'. A nut 44 is welded to the center of bar 40 and positioned to
receive an adjusting screw 34 so that screw 34 may be brought to
bear on lip 30. Similarly, nuts 46, 46' are welded to each plate
42, 42' and positioned to receive thread positioning pins 36, 36'.
Lock nuts 48, 48' serve to hold the adjustment of pins 36, 36'
which bear on lip 31. A third pair of nuts 50 are welded to the
edges at the uppermost points of curved pieces 38, 38'. Set screws
52, fit into nuts 50 and provide vertical supports to tuning frame
32 by bearing on the top of cavity 15. A short rod 35 is welded to
the head of screw 34 to provide an adjusting handle.
In operation frame 32 is positioned around cavity 15 as shown in
FIGS. 1 and 2. By rotation of screw 34, the transverse cross
section of cavity 15 is changed. This changes the resonant
frequency of the cavity.
The experimental system for empirically selecting optimum drying
system conditions is shown in FIG. 3. This is the same as the
system of FIG. 1 with instrumentation added. Thus a dual
directional coupler 19, (Microwave Devices No. WRB3 or Eimac EW 3-
DPM3) including two power indicators such as 20, 20' (1 N21 crystal
and DC milliammeter), is inserted in wave guide section 13. In the
alternative a coupler 19 could be located in waveguide sections 11
and 16. Indicators 20, 20' can be arranged to measure P.sub.m, the
power from magnetron 10 or P.sub.r, the power reflected from cavity
15. An indicator 20" with a sampling loop (M. E. Porter Co.) may
also be placed as shown in FIG. 3 so as to measure cavity power. In
addition a moisture meter 21, of conventional design (Decker Corp.
capacitive moisture meter with capacitor plates sized to
accommodate tow to be dried), is arranged and positioned as in FIG.
3 to monitor moisture content of material coming from cavity
15.
For this type of system, many modes of resonance may occur in a
microwave resonant cavity of fixed dimension. These resonances
involve the store energy of both the electric and magnetic fields
in the enclosed volume of the cavity. Energy at any of several
frequencies, quite close together, can resonate the cavity
depending on how energy is coupled into the cavity and on slight
changes within the cavity. As disclosed in the previously
referenced copending application by Lewis & White, the cavity
can be constructed and arranged in such a way that the particular
resonant field configuration which has a strong axial electric
field is maintained in the face of disturbances which would
normally cause shifting to an unwanted resonant field
distribution.
Even for this cavity structure, a significant change in dielectric
constant or in the amount of material which is placed in the axial
region of the cavity will modify the field distribution by
concentrating more field lines in the axial regions if, for
example, the dielectric constant change is an increase. Such an
increase lowers the resonant frequency of the cavity. Microwave
power is usually supplied from magnetrons, amplitrons, klystrons,
or the like which supply power at a characteristic frequency with a
relatively narrow band width or tunable range. Depending on the
type of power source used, a shift in load which changes cavity
frequency may or may not be able to " pull" the power source very
far in frequency. With power sources with a narrow bandwidth and
narrow " pulling" range, the detuning of a cavity due to a small
load change could easily be enough so that the cavity would not
receive sufficient energy from the power source.
FIG. 4 shows a representative graph of the power that is
transferred into a resonant cavity as a function of the frequency
of power supplied, with f.sub.o being the cavity resonant
frequency, and f.sub.1 and f.sub.2 being the half power bandwidth
of the cavity. Since energy transport losses in this type system
can be made almost negligible, when the power source frequency is
also f.sub.o, the power transferred into the cavity is essentially
the output of the microwave power source. When conditions change so
that the cavity resonance f.sub.o is higher than the power source
frequency indicated as f.sub.a then operation moves to point A in
which case the power coupled into the cavity is reduced to a value
P.sub.c. The remaining power P.sub.r, in the absence of protective
devices will be reflected back to the power source. In some power
sources, this reflected power will " pull" the power source
frequency from f.sub.a to a value approaching f.sub.o. In other
cases, this reflected may cause failure of the power source. As
discussed later, the present invention serves to alleviate or
eliminate this effect and in addition, uses the " pulling" effect
to provide stability in the face of load upsets.
Energy may be introduced into microwave cavity 15 by either a loop
or an iris. The preferred coupling for the stable side fed cavity,
as explained in the above-referenced Lewis & White application,
is through an inductive iris-type coupling. Inductive rather than
capacitive irises are preferred since an inductive iris operates
with lower voltages across the iris opening and thus there is less
likelihood of breakdown and arcing at the iris. The dimensions of
the iris affect the amount of microwave power that can be
transferred into the cavity 15. FIG. 5 depicts the relationship
between power transferred into the cavity and the dissipating load
in the cavity for a given iris 14. From this graph, it is obvious
that as the load is increased, the power coupled into the cavity
increases to a maximum at point N and then, for further increased
load, the coupled power decreases.
When the cavity is operated in such a manner that the load draws
all of the power from the source, it is said to be critically
coupled. Operation to the left of the critical coupling peak N in
FIG. 5 such that an increase in load will draw more power into the
cavity, is defined as overcoupling. When available power decreases
with increased load, i.e., when operating to the right of the peak
N in FIG. 5 the system is said to be undercoupled.
The known practice is to operate the microwave resonant cavity at
design frequency (f.sub.o in FIG. 4) and critically coupled so that
small load changes have minimum effect on available power and
reflected power is kept to a minimum. This type of operation
generally results in unstable system performance when appreciable
changes in conditions of the load are encountered.
The operation of resonant cavities which are critically coupled at
normally expected load is also illustrated by FIG. 5 using, for
example, a process of drying wet tow by heating the tow as it is
being transported through a resonant cavity wherein the cavity is
critically coupled at the normal moisture level of the tow entering
the cavity, operation will be that as indicated by point N. Tow
which is drier than normal entering the cavity will present a
reduced load to the cavity and require less power. This condition
follows arrow D and is a stable operating condition, for if
moisture level returns to normal, the available power will follow
the increase in load requirements back to point N. However, if tow
wetter than normal enters the cavity, operation will follow arrow
W. Now it is obvious that the increased load is supplied with
reduced power and, therefore, a drying rate below normal will exist
when a higher drying rate is required.
In addition to the above effect, the effect of changes in load on
resonant frequency of the cavity are important as illustrated in
FIG. 6. Considering again the process of drying tow in a resonant
cavity, if usual conditions are selected for tow of average
dryness, the system will be operated with power source and resonant
cavity matched in frequency at f.sub.t and the maximum power will
be transferred to the cavity as indicated by point T. If drier tow
enters the cavity, the cavity is detuned to a higher resonant
frequency (for convenience still represented by point T) but the
magnetron frequency is " pulled" only to point D.sub.t and the
amount of power coupled into the cavity is reduced. Reduced power
is required in this case and, therefore, the operation under these
conditions is stable. However, when wetter tow enters the cavity,
it is detuned to a lower resonant frequency. If the cavity resonant
frequency in this case is again represented by point T, the
magnetron frequency will be " pulled" to follow part way to point
W.sub.t and again the amount of power coupled into the cavity is
reduced which in turn results in a reduction of drying rate at a
time when an increased drying rate is required.
Thus, when drying continuously moving material in a conventional
system which is critically coupled and tuned to a matched frequency
of cavity and power source based on normal expected moisture
content of the material, an unstable system results with material
of increased moisture whereas a stable system results for
conditions of reduced moisture. This unstable system condition for
increased moisture may be alleviated by purposefully detuning the
cavity so that a small increment of frequency between the cavity
resonant frequency and the " pulled" frequency of the magnetron
exists in the properly selected direction. If this increment is set
such that the magnetron " pulled" frequency is lower than the
cavity resonant frequency as illustrated by point A of FIG. 6,
increased moisture would decrease the cavity resonant frequency and
operation would shift to W.sub.a which increases the power coupled
into the cavity and thus increases the drying rate. Decreased
moisture would increase the cavity resonant frequency and operation
would shift to D.sub.a at which point power coupled into the cavity
is decreased resulting in a reduced drying rate. Thus, stability is
enhanced for both wetter and drier than average material when the
system is adjusted so that the cavity resonant frequency for
average moisture conditions is slightly above magnetron " pulled"
frequency. However, operation at critical coupling (FIG. 5)
continues to present unstable operating system conditions and
detuning alone is not completely satisfactory for inherent system
stability. Detuning the cavity to a frequency below magnetron "
pulled" frequency such as represented by point B on FIG. 6 results
in instability for both wetter (W.sub.b) and drier (D.sub.b)
material.
For the process of microwave heating of a moving polymeric (e.g.,
nylon) yarn, a situation is presented which is similar in many
respects but different in others to the above-discussed drying
process. If effects of denier variability are neglected, variations
in entering yarn temperatures will have the predominating effect on
cavity operation. As noted hereinabove, a rise in temperature
generally increases the loss tangent significantly for a polymeric
material, but has little effect on the dielectric constant. Thus,
the major shift in cavity operation with temperature would be an
increase in load with increased temperature rather than any effect
on tuning. Thus, if the temperature of the yarn entering the cavity
becomes higher than normal (corresponding to arrow H of FIG. 5),
the load on the cavity will increase. However, as indicated by this
graph, the available power would decrease in this situation and
therefore, the heating would stabilize and therefore, not "
run-away." On the other hand, if the entering yarn temperature were
lower than normal (arrow C of FIG. 5), the available power would be
decreased. Now to achieve the desire end temperature, the heating
rate should be increased but this cannot be achieved with the
decrease in power available as indicated by this graph. Thus, as
with drying, critical coupling is not satisfactory for heating a
running yarn if inherent temperature control is desired.
For moisture removal the greatest inherent operating stability of a
microwave resonant cavity is achieved by the mode of operation
comprising overcoupling for normal load and detuning of the cavity
to a frequency slightly above the microwave power supply frequency.
The effect of operation with overcoupling is shown in FIG. 7 at
point 0. If higher moisture content material enters the cavity, the
load will increase (arrow W.sub.o) and the available power will
increase to compensate for this system upset. Reduced moisture
(arrow D.sub.o) with reduced load will require and receive reduced
power. Both conditions, therefore, tend to be stable. Similarly,
when the cavity is detuned to a frequency slightly higher than the
magnetron " pulled" frequency (point A on FIG. 6) the stabilizing
influences for both wetter and drier entering material (as
explained earlier) combine with the stabilizing effects of
overcoupling to provide inherent system stability for moisture
removal.
For heating a running yarn of polymeric material, such as nylon,
the greatest inherent operating stability of a microwave resonant
cavity system is attained by a mode of operation comprising
undercoupling for normal load and detuning of the cavity to a
frequency slightly lower than the microwave power source frequency.
As evident from FIG. 7, this undercoupled mode (point U) of
operation for heating nylon yarn is inherently stable since
material with higher than normal or desired temperature would
increase the load presented to the cavity which in turn would
decrease the power available (arrow H.sub.u) whereas cooler yarn
entering the cavity would receive increased heating due to the
greater power available (arrow C.sub.u). For the former situation,
heating rate would decrease and the temperature rise would level
out and not " run-away." In the latter situation, heating rate
would increase and the increased temperature rise would cause
system operation to move back towards the condition represented by
point U. Now if the only effect on cavity operation for nylon yarn
heating were the influence of changes in loss factor with
temperature, the cavity would be inherently stable for all
three-frequency situations, points A, T and B considered in FIG. 6.
However, there is a slight increase in dielectric constant with
temperature which has a slight frequency effect. An examination of
FIG. 6 shows that operation of the cavity tuned to a resonant
frequency slightly lower than the power source frequency (Point B)
adds to the inherent stabilizing influence of undercoupling, using
the analogy of hotter (H.sub.b) and cooler (C.sub.b) yarn entering
the cavity, and thus is important to the preferred mode of
operation for heating polymeric yarns. It is obvious from FIG. 6
that detuning the cavity to operate at a resonant frequency
slightly higher than the power source frequency (Point A) would
result in adverse conditions in that increases in dielectric
constant would tend to lead to a " run-away" condition (H.sub.a)
which would tend to counteract some of the stabilizing effect of
the undercoupled mode, while cooler (C.sub.a) yarn entering the
cavity would cause available power and heating rate to
decrease.
An important advantage of the apparatus of the present invention is
that it provides the means for achieving the desired modes of
control with a minimum of auxiliary equipment while providing
excellent protection to the power source. The use of tunable
magnetrons with frequency measuring devices providing feedback for
achieving the desired frequency offset to compensate for load
disturbances would require much more complex systems than the one
described.
The use of a ferrite isolator in place of the circulator-water
load-iris combination (12, 16, 17 and 18, FIG. 1) to provide
protection to the magnetron against excessive reflected power for
the magnetron which will give the desired " pulling" range has
several disadvantages at present. Each process for which control is
desired will require that the isolator provide a specific degree of
isolation and possess a specific power dissipation capability which
in many cases will require the production of a special isolator
which will be far more expensive than an "off the shelf" component.
Then once the system is in operation, process changes are not
easily accommodated. Design of the preferred embodiment of this
invention requires merely the choice of a water load of sufficient
power handling capacity (wide range of "off the shelf" items) and
selection of an iris size; and since process changes might require
system component changes, this is easily effected with the
availability of water loads of a wide power handling range and the
simplicity of an iris change (an easily machined, slotted aperture
plate).
EXAMPLE
The following illustrates the use of this invention to dry a
Teflon*
FOOTNOTE: -registered Du Pont Trademark. yarn, which is coated with
a water dispersion, to a specified moisture level content with a
minimum fluctuation in moisture levels.
A Teflon yarn coated with a Teflon water dispersion at a process
speed of 56 yards per minute then is passed through a microwave
system similar to that as shown in FIG. 1. Water load iris 17 was
chosen to provide a coefficient of coupling of 2 (11 percent
reflection) to provide a half power pulling range of 4--5 MHz.
##SPC1##
Normal steady state system requirements for desired drying, i.e.,
4--5 percent residual water content to give best dispersion
retention indicated a required power level of 1,300 watts.
Variations in dispersion application had shown that power
requirements would vary as much as .+-.300 watts. The results of
providing the power variations needed by coupling and/or frequency
pulling of varying combinations (items A--E) is shown in table I.
##SPC2##
These results show that overcoupling yields a far more uniform
drying operation than with critical coupling and this uniformity is
further enhanced by the 1 MHz. frequency offset.
* * * * *