U.S. patent number 3,670,133 [Application Number 05/149,651] was granted by the patent office on 1972-06-13 for microwave drying apparatus and method.
This patent grant is currently assigned to MacMillan Bloedel Limited. Invention is credited to Lambertus Admiraal.
United States Patent |
3,670,133 |
Admiraal |
June 13, 1972 |
MICROWAVE DRYING APPARATUS AND METHOD
Abstract
Apparatus and method for drying moisture-laden dielectric
materials by microwave energy and including directing microwave
energy from a generator to opposite sides of material to be dried.
Means is provided for protecting the generator from microwave
energy reflected by the material. It is preferable to include means
for controlling the output of the generator in accordance with the
moisture content of the portion of the material exposed to the
microwave energy. This is accomplished by measuring energy
transmitted through the material, and controlling the output of the
generator inversely relative to the level of the transmitted
energy.
Inventors: |
Admiraal; Lambertus (Coquitlam,
British Columbia, CA) |
Assignee: |
MacMillan Bloedel Limited
(Vancouver, British Columbia, CA)
|
Family
ID: |
22531244 |
Appl.
No.: |
05/149,651 |
Filed: |
June 3, 1971 |
Current U.S.
Class: |
219/693; 34/264;
219/695; 219/709 |
Current CPC
Class: |
F26B
3/34 (20130101); H05B 6/78 (20130101); H05B
2206/046 (20130101) |
Current International
Class: |
F26B
3/32 (20060101); F26B 3/34 (20060101); H05B
6/78 (20060101); H05b 009/06 () |
Field of
Search: |
;34/1,DIG.19
;219/10.55 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Jaeger; Hugh D.
Claims
I claim:
1. Microwave drying apparatus for moisture-laden dielectric
material comprising a microwave generator, means for dividing
microwave energy from the generator into two equal parts, and
separate channels for directing said two energy parts to opposite
sides of material to be dried, the length of one of said channels
to the longitudinal center line of the material being equal to the
length of the other of said channels plus a whole number of half
wavelengths of the microwaves from said generator.
2. Microwave drying apparatus as claimed in claim 1 in which said
channels are located to direct said two energy parts against
directly opposite areas on said material.
3. Microwave drying apparatus as claimed in claim 1 including means
for detecting the microwave energy that is transmitted through said
material, and controlling means for regulating the output of the
microwave generator inversely relative to the level of the
transmitted energy.
4. Microwave drying apparatus for moisture-laden dielectric
material comprising a waveguide having two channels extending to
outer ends on opposite sides of a path along which material to be
dried travels, said path having an electromagnetic center line and
said material being guided to travel with its center line on said
electromagnetic center line and between said two channel ends, a
microwave generator, a four-port hybrid coupling the waveguide
channels to the generator, the length of one of said channels to
the electromagnetic center line being equal to the length of the
other of said channels plus a whole number of half wavelengths of
the microwaves from said generator, said waveguide channels
directing microwave energy from the generator to opposite sides of
material moving along said path.
5. Microwave drying apparatus as claimed in claim 4 in which said
two channel ends are located close to each other.
6. Microwave drying apparatus as claimed in claim 4 in which said
channel ends are located directly opposite from each other.
7. Microwave drying apparatus as claimed in claim 4 including
control means operated by microwave energy transmitted through said
material to control the output of the microwave generator inversely
relative to the level of the transmitted energy.
8. Microwave drying apparatus as claimed in claim 4 in which said
channel ends are located directly opposite from each other whereby
microwave energy transmitted through said material travels back
through said channels, and control means operated by said
transmitted energy to control the output of the microwave generator
inversely relative to the level of the transmitted energy.
9. Microwave drying apparatus as claimed in claim 4 including
microwave energy absorption means coupled to one of the ports of
said hybrid.
10. Microwave drying apparatus for moisture-laden dielectric
material comprising a hybrid having a common central section and a
longitudinal center line, a first pair of arms projecting from one
end of the central section on opposite sides of the center line, a
second pair of arms projecting from an opposite end of the central
section on opposite sides of the hybrid center line, a microwave
generator coupled to one arm of said second pair of arms so that
microwave energy therefrom is directed through both of the arms of
the first pair, a waveguide having two channels, one connected to
each arm of said first pair, said two channels extending to outer
ends on opposite sides of a path along which said material travels,
said path having an electromagnetic center line and said material
being guided to travel with its center line on said electromagnetic
center line and between said two channel ends, the length of one of
said channels to the electromagnetic center line being equal to the
length of the other of said channels plus a whole number of half
wavelengths of the microwaves from said generator, said waveguide
channels directing microwave energy from the generator to opposite
sides of the material moving along said path.
11. Microwave drying apparatus as claimed in claim 10 in which said
two channel ends are located close to each other.
12. Microwave drying apparatus as claimed in claim 10 in which said
channel ends are located directly opposite from each other.
13. Microwave drying apparatus as claimed in claim 10 including
control means operated by microwave energy transmitted through the
said material to control the output of the microwave generator
inversely relative to the level of the transmitted energy.
14. Microwave drying apparatus as claimed in claim 10 in which said
channel ends are located directly opposite from each other whereby
microwave energy transmitted through said material travels back
through said channels, and control means operated by said
transmitted energy to control the output of the microwave generator
inversely relative to the level of the transmitted energy.
15. Microwave drying apparatus as claimed in claim 10 including
microwave energy absorption means coupled to the other of said
second pair of arms.
16. The microwave drying method which comprises dividing microwave
energy from a microwave generator into two equal parts, and
directing said energy parts through two separate channels to
opposite sides of material to be dried, the distance of travel
through one of said channels to the longitudinal center line of the
material being equal to the distance of travel through the other of
said channels to said center line plus a whole number of half
wavelengths of the microwaves from said generator.
17. The method according to claim 16 in which said two microwave
energy parts are directed by the channels to directly opposite
areas on said material.
18. The method according to claim 16 including detecting the
microwave energy transmitted through said material, and controlling
the output of the microwave generator inversely relative to the
level of the transmitted microwave energy.
Description
This invention relates to apparatus and methods for drying
moisture-laden materials, such as lumber, wood based materials,
bagasse, paper and the like by means of microwave energy.
Different types of materials can be dried by the present apparatus
and methods, but for the sake of convenience the invention is
described herein relative to the drying of lumber.
Heretofor the drying of lumber by means of microwave energy has
been difficult and not practical in a commercial sense. The main
difficulty is caused by the fact that the moisture content of
lumber varies greatly from piece to piece and very often throughout
the length of each piece. As the cost of microwave generating
equipment and the amount of power consumed are very high, it has
not been thought practical economically to dry by means of
microwave energy. Another problem is caused by the fact that some
microwave energy is reflected by the lumber back to the generator,
and this causes serious problems.
The present invention greatly reduces these problems by directing
microwave energy from a generator to opposite sides of the lumber,
preferably in directly opposed areas. This means that the microwave
generator "sees" portions of the board which have substantially the
same moisture content. If the output of the generator is controlled
in accordance with the moisture content of the lumber exposed to
the microwave energy, this energy will increase and decrease in
accordance with the moisture content. This is contemplated by the
present invention, and includes means for measuring the microwave
energy transmitted by the lumber and controlling the output of the
generator inversely relative to the transmitted energy. The
invention also includes a stopper or means for protecting the
microwave generator from microwave energy reflected by the
lumber.
The microwave drying method according to this invention comprises
dividing microwave energy from a microwave generator into two equal
parts, and directing said energy parts through two separated
channels to opposite sides of the lumber to be dried, the distance
of travel through one of said channels to the longitudinal
centerline or to a side of the lumber being equal to the distance
of travel through the other of said channels to said centerline or
lumber side plus a whole number of half wavelengths of the
microwaves from said generator. It is preferable according to this
method to direct the two parts of the microwave energy to directly
opposite areas of the lumber. The method also includes measuring
the microwave energy transmitted through the lumber and controlling
the output of the microwave generator inversely relative to the
level of the transmitted microwave energy.
The microwave energy from the generator is preferably divided by a
four-port hybrid which directs the energy to the two channels and,
at the same time, protects the generator from microwave energy
reflected by the lumber.
Apparatus according to the present invention comprises a microwave
generator, means for dividing microwave energy from the generator
into two equal parts, and separate channels for directing said two
energy parts to opposite sides of lumber to be dried, the length of
one of said channels to the longitudinal centerline or the side of
the lumber being equal to the length of the other of said channels
to the center line or lumber side plus a whole number of half
wavelengths of the microwaves from said generator. These channels
are preferably located to direct the two energy parts against
directly opposite areas of the lumber. The apparatus also includes
means for measuring the microwave energy transmitted through the
lumber, and controlling means for regulating the output of the
generator inversely relative to the level of the transmitted
energy.
In the preferred form of the apparatus, a waveguide system having
two channels is used, and the channels of this waveguide system are
coupled to the generator by a four-port hybrid. In the preferred
form of the invention, the two waveguide channels are in the form
of a closed loop or ring, and the lumber travels through a slot in
the loop or ring. Guide means is provided for maintaining the
longitudinal center line of the lumber on an electromagnet center
line. The length of one of the channels to this electromagnetic
center line is equal to the length of the other of the channels
plus a whole number of half wavelengths of the microwaves from the
generator. With this arrangement, the microwave energy transmitted
through the lumber travels back through the channels towards the
hybrid. This transmitted energy is measured and the output of the
generator is controlled inversely relative to the transmitted
energy.
It has been found that the transmitted energy levels are mainly
related to the moisture content of the transmitting lumber. As the
amount of moisture in the lumber increases so increases the amount
of microwave energy absorbed in the drying process and vice versa.
The transmitted power represents the amount of power or energy
which the lumber was unable to absorb. In this way, lumber of high
moisture content and therefore very low in transmitting power,
receives automatically maximum microwave energy, while lumber of
lower moisture content receives less energy. This ensures maximum
power being applied to wet areas without applying too much power to
dryer areas, even though these areas are in the same piece of
lumber or in adjacent different pieces.
The method and apparatus of the present invention will be
understood from the following description in connection with the
accompanying drawings, in which
FIGS. 1, 2 and 3 diagrammatically illustrate the operation of a
four-port hybrid,
FIG. 4 illustrates one form of microwave drying apparatus in
accordance with this invention,
FIG. 5 illustrates an example of guide means for lumber being dried
in this apparatus,
FIG. 6 illustrates a preferred form of microwave drying apparatus
according to this invention,
FIGS. 7, 8 and 9 illustrate the operation of the four-port hybrid
when used in the drying apparatus of FIG. 4 or FIG. 6, and
FIG. 10 illustrates a control system for the drying apparatus.
When a microwave generator is directly coupled to a slotted
waveguide through which lumber travels to be dried, all microwave
power or energy reflected from the lumber face returns to the
generator. This endangers the generator and interferes with its
proper operation. This situation is avoided by the use of a
four-port hybrid between the generator and the slotted
waveguide.
FIG. 1 illustrates the basic working of a four-port hybrid 10. This
hybrid is formed with a central section 12 and has a longitudinal
center line 13. A first pair of arms 16 and 17 project from one end
of the central section on opposite sides of center line 13 at an
angle to each other, and a second pair of arms 20 and 21 project
from the opposite end of the central section of the hybrid on
opposite sides of said center line and at an angle to each other.
These arms constitute the four ports of the hybrid. A microwave
generator 24 is coupled to arm 20, and arm 21 is connected to
suitable microwave energy absorbing means, such as a waterload or
another system utilizing microwave energy. Channels 27 and 28 are
connected to arms 16 and 17, respectively, and for the purpose of
this illustration, a reflector plate 30 extends across the two
channels 27 and 28.
When microwave energy or power P comes from generator 24 into port
20 then P1 equalling P/2 continues through port 16, and P2
equalling P/2 will cross the center line 13 and continue through
port 17. The crossing of the center line gives an automatic
90.degree. phase shift to P2. If P1 and P2 both travel the same
distance d before being reflected by reflector 30, their phase
relationship does not change by the time they re-enter ports 16 and
17.
The powers P1 and P2 in FIG. 2 represent their returning or
reflected powers, and each one has the tendency to split again into
two equal parts, each one of them being equal to P/4. If P3 is that
half of P1 which does not cross over the center line, and P4 is
that half of P2 which does cross over the center line and therefore
has a further 90.degree. phase shift, then there are in port 20 two
electromagnetic waves of equal power but with a phase shift of
180.degree. or half a wavelength. These two waves cancel each other
completely and therefore prohibit propagation in port 20, thereby
protecting the microwave generator.
FIG. 3 illustrates the results of the above-mentioned cancellation.
Since no power from ports 16 and 17 can enter port 20, and since it
has to go somewhere, the only way open is port 21, that is, P2 does
not cross over the centerline therefore maintains its 90.degree.
phase shift while P5 has crossed over the center line and therefore
has a 90.degree. phase shift. There are now in port 21 two
electromagnetic waves of equal power and of identical phase and
therefore they reinforce each other and travel as one wave of power
P6 which equals P through port 21 towards the absorption means.
If the total reflection surface 30 is replaced by a surface with a
partial reflection coefficient of g, such as the surface of moist
lumber, then the same reasoning holds as above, the only difference
being in the various power levels, for example, P6 equalling g
.times. P.
FIG. 4 illustrates one form of microwave drying apparatus 35 in
accordance with this invention. This apparatus includes a four-port
hybrid 37 similar to hybrid 10 described above, and which couples a
microwave generator 39 to a waveguide system 40. Suitable microwave
absorption means, such as a waterload 42, is coupled to the
hybrid.
Waveguide system 40 has a pair of arms or channels 45 and 46 which
extend to opposite sides of a path 48 having an electromagnetic
center line 49 and along which lumber 50 travels with its
longitudinal center line coinciding with centerline 49. Channels 45
and 46 of the waveguide have slots 54 and 55 therein through which
path 48 extends. As far as this invention is concerned, the
effective parts of channels 45 and 46 terminate in ends 57 and 59
substantially at center line 49. However, the channels 45 and 46
may continue on beyond the lumber path in section 61 and 62 so that
microwave energy transmitted through the lumber may be used for any
desired purpose.
If the path lengths of FIG. 4 indicated by a and b are not both
equal to d of FIG. 1, the distance from the hybrid to a reflecting
surface, but is
a = d, and b = d + m .lambda./2 where m is an integer and .lambda.
is the wavelength of the microwaves, then the above reasoning still
holds because the distance
b - a = m .lambda./2 is being traveled twice, therefore a phase
shift of
2 .times. m .lambda./2 =m .lambda. takes place so that no extra
phase shift has been introduced.
If the condition b - a =m .lambda./2 is realized for a board of
width w then the center line of that board coincides with the
electromagnetic center line (EMCL) 49. If now a board of any other
width is placed with its center line on the EMCL, then the
pathlengths of a and b are changed into a.sup.1 and b.sup.1
respectively but in identical ways therefore the term b.sup.1 -
a.sup.1 = m .lambda./2 still holds.
Summarizing, it can be said that independent of power level P,
reflection coefficient g or lumber width w, the microwave generator
will receive no reflected power if all boards are centered on the
EMCL. As an example, with a frequency of 915 MHz, a wavelength
results in the guide system of 17.2 inches, there is an EMCL every
4.3 inches (or .lambda./4) across the width of the waveguide slot.
Therefore within approximately 2 inches of the physical center line
of the system there is an EMCL.
In order the comply with this reasoning, the effective length of
channel 46 to the edge 65 of lumber 50 or center line 49 equals the
effective length of channel 45 to edge 66 of the lumber or center
line 49 plus a whole number of half wavelengths of the microwaves
from generator 39.
It is clear that when drying lumber of varying widths in apparatus
35, it is not possible to use a one-sided guide rail, and
therefore, it is necessary to provide means for keeping the
longitudinal center lines of the lumber on the electromagnetic
center line 49. FIG. 5 illustrates one way of accomplishing this. A
scissor type guide 70 can be used for this purpose. Guide 70
includes a pair of arms 72 and 73 pivotally connected at 74 to each
other said arms carrying rollers 76 and 77 at upper ends thereof
equally spaced from pivot 74, and weights 79 and 80 at lower ends
thereof, also equally spaced from the pivot. Guide 70 has a
vertical center line 82 which extends through pivot 74 and
coincides with the electromagnetic center line 49 of apparatus
35.
Lumber 50 to be dried by this apparatus is directed between rollers
76 and 77 which apply substantially equal pressure to opposite
edges of the lumber and so positions the longitudinal centerline
thereof on the electromagnetic center line.
As stated above, a certain percentage of the microwave energy
directed against the opposite edges of the lumber is transmitted
theretrough, and the amount transmitted depends upon the moisture
content of the lumber. As the moisture content of the lumber is
constantly changing, it is desirable to control the power from the
microwave generator in accordance with the changed moisture
contents. This can be accomplished by positioning detectors 86 and
87 on the outer end sections 61 and 62 of waveguide channels 45 and
46, respectively. These detectors also known as directional power
meters are well known and do not need description herein. These
detectors are connected to a controller for the microwave generator
39 in the manner described hereinafter.
FIG. 6 illustrates a preferred form of drying apparatus 35a in
accordance with this invention. In this example, the arms or
channels 45 and 46 of waveguide system 40 are formed in a closed
loop 90 having a single slot 91 therein through which path 48
extends. The same relationship as above exists between the
effective lengths of channels 45 and 46 to the edges 66 and 65 of
lumber 50 or to the electromagnetic center line 49. If in this
case, the detector 86 for channel 45 were located in arm 46 and the
detector 87 for channel 46 were located in arm 45, as indicated in
broken line in FIG. 6, then there would be completed the
transformation of FIG. 4 to FIG. 6 by squeezing all waveguide
system components into one plane. However, if this were done a
difficulty arises because the detector 86 not only measures that
part of the power flowing through arm 45 that is transmitted
through the lumber from C to D and beyond but also that part of the
power flowing through arm 46 that is reflected at the lumber face
D. Therefore the detectors 86 and 87 have been replaced by a single
detector 96 in arm 20 which detector will measure, as will be shown
later, the sum of the two transmitted powers flowing back to
generator 39. For convenience another detector 97 can be added in
arm 21 for the measurement of the reflected power flowing to
waterload 42.
As stated above, all of the reflected microwave power or energy is
directed back to waterload 42 and so the microwave generator 39 is
protected against this. In actual practice, this happens only if
the areas C and D of the lumber have the same reflection
coefficient. Since C and D in FIG. 4 are spaced apart, the
generator is protected only if the moisture content of the boards
to be dried is relatively constant. If the moisture content in
different areas of a given board or if the moisture content of
successive boards is different, the reflection coefficient in
apparatus 35 will be different, and the microwave generator can be
exposed to reflected power. However, apparatus 35 can be used for
lumber in which the moisture content does not vary greatly within
boards or successive boards.
Apparatus 35a eliminates this problem since C and D are directly
opposite each other or, in other words, the microwave generator
"sees" substantially the same portion of each board.
When the energy or power transmitted through the lumber is used to
control the output of the generator, the same problem arises with
apparatus 35 since the power transmitted in channel 45 may be
different from that transmitted in channel 46 at any given
instance. Apparatus 35a also eliminates this problem.
The loop or ring waveguide system of apparatus 35a in FIG. 6 is
considered to be much better than apparatus 35 for general
purposes. If, in apparatus 35a, the path lengths a and b fulfill
the following condition:
b = a + m .lambda./2 (1)
where .lambda. is the MW wavelength in the waveguide and m is an
integer, then all reflected power from the lumber faces C and D
will flow to the waterload and all transmitted power exiting from
the lumber faces D and C will flow to the generator.
FIG. 7 shows how the energy P flowing from the generator through
port 20 is split into two equal parts P1 and P2, but P2 also
obtained a 90.degree. phase shift because it crossed the center
line of the four-port hybrid. After travelling through the
respective waveguides a and b, part of the energy is reflected
back. Since both P1 and P2 "see" the same portion of the lumber
(albeit from two different sides) the same part g of P1 and P2 is
reflected back as P3 and P4, respectively. Since both arms a and b
have been travelled twice by the time the four-port hybrid is
reached, any additional phase shift due to b .noteq. a amounts to a
whole number of full wavelengths therefore has no influence. The
only phase shift between P3 and P4 is that P4 is still 90.degree.
ahead of P3 (as was the case between P2 and P1).
In FIG. 8 illustrates what happens when P3 and P4 enter the
four-port hybrid and try to split into two equal parts. In port 20
you then have P5 (or half of P3) and P8 (or the crossed over half
of P4 which gets another 90.degree. phase shift for crossing).
These are now two electromagnetic wave systems of equal magnitude
but with a 180.degree. phase shift between them, therefore
cancellation between the two wave systems will take place, thus
prohibiting propagation towards the generator. In port 21 the two
wave systems P6 and P7 from ports 16 and 17 are of the same
magnitude and phase when they meet, therefore, they will reinforce
each other and flow to the water load.
Since P5 and P8 have to go somewhere, they will act and reinforce
each other in a manner identical to P6 and P7, respectively, and
will also flow through port 21 to the water load. Therefore the
amount of reflected power ending up in the water load is P9 = P3 +
P4 = gP or the total amount of the original P which has been
reflected by the lumber.
As far as the transmitted power is concerned, there is a completely
different result. In FIG. 9 P results in P1 flowing through arm a
and P2 flowing through arm b (P2 has already a 90.degree. phase
shift). Those equal parts of P1 and P2 that are not reflected at
the lumber faces C and D will enter the lumber where a certain
fraction will be absorbed (the fraction depending on the lumber
portion CD in the wave guide) and the remainder P1' and P2' will
continue on their way through the wave guides b respectively a. If
the transmission coefficient for the portion CD of the lumber is h
and (1-g) h = k then you have:
P .fwdarw. P1 = (P/2) through arm a .fwdarw. P1' = k P/2 through
arm b
and
P .fwdarw. P2 = (P/2) + 90.degree. through arm b .fwdarw. P2' =
k (P/2) + 90.degree. through arm a
In FIG. 9 you find now (through an identical exercise as in FIG. 8)
that the only way the transmitted power P2' and P1' can go is
through port 20 towards the generator since port 21 is a prohibited
area due to cancellation.
Since the transmitted power P' represents the amount of power which
the lumber was unable to absorb and therefore is going to waste (in
the generator) it should be kept to a tolerable minimum .DELTA. P,
depending on the wood properties. Therefore P' should be measured
with detector 96 and used to control the output of the generator so
that P' .ltoreq. .DELTA. P. In this way lumber of high moisture
content and therefore very low in transmitted power, receives
automatically maximum power while lumber of lower moisture content
gradually receives less power, thus resulting in a slowdown of the
drying process when the lumber reaches its desired final moisture
content.
FIG. 10 diagrammatically illustrates a system for controlling the
power output of microwave generator 39 in accordance with the
moisture content of the lumber being subjected to the drying action
of microwaves at any given instant. The detector or measurer 96 of
transmitted microwave energy P' is operatively connected to an
output control unit 100 for generator 39. The arrangement is such
that as the moisture content of the wood being "viewed" increases,
the transmitted energy decreases and the output of the generator is
increased, and vice versa.
* * * * *