US3711674A

US3711674A - T-ring microwave drying apparatus

Info

Publication number: US3711674A
Application number: US00149548A
Authority: US
Inventors: L Admiraal
Original assignee: MacMillan Bloedel Ltd
Current assignee: MacMillan Bloedel Ltd
Priority date: 1971-06-03
Filing date: 1971-06-03
Publication date: 1973-01-16
Anticipated expiration: 1990-01-16
Also published as: CA922382A

Abstract

Apparatus for drying moisture-laden dielectric materials by microwave energy including a waveguide system shaped to direct microwave energy from a microwave generator to opposite sides of material to be dried. The waveguide system is in the form of a closed ring and is designed so that all microwave energy reflected by the material is directed back thereto and cannot reach the generator. The apparatus can 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 includes means for measuring energy transmitted through the material, and means for controlling the output of the generator inversely relative to the level of the transmitted energy.

Description

United States Patent [191 Admiraal T-RING MICROWAVE DRYING APPARATUS Inventor: Lambertus Adniii-aal, Coij uitlam,

British Columbia, Canada MacMillan Bloedel Limited, Van- Assignee: couver, British Columbia, Canada Filed: June 3, 1971 Appl. No.: 149,548

Reierences Cited UNITED STATES PATENTS 4/195! Nelson ..2l9/l0.55

Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney- Fetherstonhaugh & Co.

[57] ABSTRACT Apparatus for drying moisture-laden dielectric materials by microwave energy including a waveguide system shaped to direct microwave energy from a microwave generator to opposite sides of material to be dried. The waveguide system is in the form of a closed ring and is designed so that all microwave energy reflected by the material is directed back thereto and cannot reach the generator. The apparatus can 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 includes means for measuring energy transmitted through the material, and means for controlling the output of the generator inversely relative to the level of the transmitted energy.

16 Claims, 7 Drawing Figures Jan. 16, 1973 PATENTEBJAH 16 mm 3.71 l, 674

sum 2 BF 2 LAMBERTUS ADMIRAAL Tall/E Y5 This invention relates to apparatus for drying moisture-laden dielectric materials, such a lumber, wood based materials, bagasse, paper and the like by means of microwave energy, and particularly to apparatus including a T-ring waveguide system.

This apparatus may be used for drying many different materials, but for the sake of convenience it is described herein in connection with lumber.

It has not been practical in the past to dry lumber by means of microwave energy in view of the difficulty of controlling the operation because of the wide variations in moisture content of different portions within some boards, and between different adjacent boards, as well as the high cost of the equipment involved and of the electrical power consumed. A percentage of the microwave power generated for drying purposes is reflected by the lumber, and this results in an economical loss, as well as danger to the microwave generator. Applicant has set out in a co-pending application apparatus and a method for protecting microwave generators in this type of equipment, and for utilizing some of the reflected microwave energy. The co-pending apparatus requires a four-port hybrid to protect the generator from reflected microwave energy.

The present invention protects the microwave generator, and returns all of the reflected energy back to the lumber, and this is accomplished in a simpler manner than in the apparatus of the co pending application without the necessity of a four-port hybrid.

The apparatus according to the present invention directs microwave energy to areas of the lumber directly opposite to each other. Means is provided for preventing the reflected microwave energy from reaching the generator, and for directing this reflected power back to the lumber. The output of the generator can be controlled inaccordance with the moisture content of the lumber exposed to the microwave energy so that this energy increases and decreases in accordance with the moisture content. Means is provided for measuring the microwave energy transmitted through the lumber, and this is used to control the output of the generator inversely relative to the transmitted energy.

It is possible with this invention to arrange a series of these drying units so that lumber passes successively through them. The arrangement is such that each board is exposed to the microwave energy at each unit, and this exposure is in accordance with the moisture content of the board at that unit. In other words, if the moisture content is high, the maximum available amount of microwave energy is directed to it, and this can be varied right down to the point where the board does not require any further drying, in which case the microwave generator is practically shut off. For example, when a board that is wet at one end and dry at the other end is directed through the apparatus, the wet end will be exposed to the drying action of microwave energy at each drying unit until its moisture content is lowered to the desired level, while the dry end practically will not be exposed to microwave energy or will be exposed to energy at a relatively low level until its moisture content reaches the desired level.

Microwave drying apparatus according to the present invention comprises a waveguide system including two waveguides arranged in a closed ring, and a tubular leg communicating with and extending away from the ring, said leg being adapted to be connected to a microwave generator. A slot is formed in the ring through which lumber can be moved with the longitudinal center line thereof on an electromagnetic center line. The length of one of said waveguides equalling a and the length of the other of said waveguides equalling b from the tubular leg to the electromagnetic center line, the relationship of the lengths of said channels to each other is where the symbol A is the wavelength of the microwaves in the waveguides and m an integer.

This waveguide system is preferably in the form of a rectangular waveguide ring having a first pair of opposite sides interconnecting with a second pair of opposite sides, in which case the tubular leg is connected to one of the first pair of sides and is equallyv spaced from both sides of the second pair forming with said one side a T-junction. -It is preferable to provide a directional power meter or measurer in the tubular leg for microwave energy transmitted through the lumber. This is used to operate a controller which regulates the microwave output of the generator in accordance with the moisture content of the portion of the lumber subjected to the microwave energy. I

The microwave energy transmitted by the lumber relates to the moisture content of the latter. As the amount of moisture in the lumber increases so increases the amount of microwave energy absorbed in the microwave process, and vice versa. The transmitted power is 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 transmitted power, automatically receives maximum available microwave energy, while lumber of low moisture content receives less energy. This results in 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 apparatus of the present invention will be understood from the following description in connection with the accompanying drawings, in which FIG. 1 illustrates a preferred form of T-ring drying apparatus in accordance with this invention,

FIG. 2 diagrammatically illustrates the operation of this apparatus in preventing reflected microwave energy from reaching the generator,

FIG. 3 is a view similar to FIG. 2, illustrating the operation of the apparatus relative to the transmitted power,

FIG. 4 diagrammatically illustrates a control system for the microwave generator,

FIG. 5 illustrates a simple and effective alternative form of T-ring drying apparatus,

FIG. 6 diagrammatically illustrates drying apparatus incorporating a plurality of these microwave drying units and FIG. 7 is an alternative to the arrangement shown in FIG. 6.

Referring to FIG. 1 of the drawings, 10 is one form of microwave drying apparatus in accordance with this invention, and this apparatus is made up of a tubular waveguide system 11 in the form of two

waveguides

15 and 16 arranged in a closed circuit, as shown. The'ring includes a pair of

opposed sides

20 and 21 interconnected by another pair of

opposed sides

24 and 25. A tubular leg 28 communicates with and is connected to side 20 midway between

sides

24 and 25 of the waveguide ring. Leg 28 and waveguide side 20 form a T with said side forming the crossbar which connects the tubular leg to the

ring sides

24 and 25; A microwave generator 32 is coupled to the outer end of leg 28.

A slot 35 is formed in waveguide side 24, through which lumber 37 to be dried can travel. Suitable means,

not shown, is provided for causing the lumber to travel with its center line coinciding with an electromagnetic center line 39 of the waveguide.

Waveguides l5and 26 of the ring extend from leg 28 to opposite sides of the electromagnetic center line 39 or to the sides 42 and 43 of the lumber or boards being dried. The length of channel 15 is represented by a while the length of channel 16 is represented by b. As

the center line of board 37 is always on the electromagnetic center line of the waveguide system, the lengths of these channels can be considered from the tubular leg to the opposite sides of the board or to the electromagnetic center line. In any case, the relationship of

channels

15 and 16 to each other is as follows:

where the symbol )t is the wavelength of the microwaves in the waveguides and m an integer.

lf the lengths of the channels or paths a and b have the above relationship, all microwave power reflected by the lumber faces C and D will be prohibited from entering leg 28 of the T-junction due to cancellation, but all of the microwave power transmitted through the lumber will flow back to generator 32.

If the reflection coefficient of the lumber at location C and D equals g, see FIG. 2, then lumber face C will reflect Pl 6 P/2 and lumber face D a similar amount P2 but P2 has travelled (by the time it reaches the T- junction) a distance which is 2 X (2 m l (M4) longer than Pl has travelled by the time it reaches the T-junction. Therefore P2 is 2 X (2 m l)()t/4)= (2 m l) )t/2) m )t'l' (M2) out of phase. Since m is an integer the term m )t has no influence and the phase shift between P1 and P2 is therefore (A/2or l80. FIG. 2 shows the situation at the center of the T-junction. Due to the 180 phase shift, P1 and P2 cannot enter the leg of the T because of cancellation. Thus non of thereflected power reaches the microwave generator. Therefore Pl has to travel into waveguide leg b, and P2 will continue into waveguide leg a. When P1 and P2 reach the lumber again, a fraction g is being reflected, but since Pl travels leg b it receives a 180 phase shift, therefore, upon arrival at the T-junction you have These two small power fractions are now in phase and they therefore can enter the leg of the T and flow to the generator. I

FlG.-3 illustrates the flow of the power transmitted through the lumber. If the transmission coefficient for section CD of the lumber is h and if(l =3) h =k then the transmitted power at face D is P3 K H2) and at face C P4 K (H2) Since P3 and P4 have travelled the same distance (a b CD) by the time they reach the T-junction no phase shift has taken place, therefore, they can enter the leg of the T-junction together and combine into P5 kP and flow to the microwave generator. This P5 must be so low that the generator is not harmed or affected by it.

PS can be measured by a meter 46, see FIG. 1, before entering generator 32 and can be used to control the drying process, i.e., the generator output will be regulated so that P5 s a'preset A P.

FIG. 4 diagrammatically illustrates apparatus for controlling the output of microwave generator 32. Meter 46 is connected to a controller 48 for the generator. The microwave energy transmitted through the lumber is measured by meter 46, and this causes controller 48 to regulate the output of the microwave generator in accordance with the moisture content of the wood exposed to the original microwave energy from the generator.

H0. 5 illustrates a simple form of waveguide system 52. This waveguide system comprises an elongated casing 54 having a central wall 55 therein and extending longitudinally thereof. This wall terminates short of the opposite ends 58 and 59 of the casing. The wall forms two waveguides 62 and 63 which are joined at their opposite ends at 65 and 66 so that they in effect form a closed ring. A tubular leg 69 is connected to and opens into casing 52 centrally of its end 58. The longitudinal center line 70 of leg 69 is aligned with said central wall 55. This leg and the adjacent end portion of the casing form a T-junction. A suitable microwave generator 72 is connected to the free end of leg 69.

A slot 75 is formed in the casing at its channel 62 so that lumber 77 can move through this slot to be dried, the longitudinal center line of said lumber coinciding with an electromagnetic center line 79 of the waveguide.

Apparatus 50 functions in the same manner as apparatus 10 described above.

FIG. 6 illustrates a plurality of waveguide units 10 or 50, for example units 50, aligned so that lumber can travel successively through the waveguide slots thereof. The tubular legs 69 of these units instead of being connected to generator 72 are, in this example, connected by means of tube 93 to a common microwave generator 87. Directional powermeters 89 on these legs measure the power transmitted through the lumber and operate control valves 91 and limit the amount of the transmitted power to the previously mentioned limit of A P. In this way each unit 50 receives the amount of power required by the lumber in each unit individually. The output of generator 87 is sufficient to supply all of these drying units 50.

Tube 93 is at one end connected to a waterload 95 and a directional powermeter 96 measures the power reaching the waterload. This power meter 96 is adapted to control the output of generator 87 within the range thereof relative to the amount of power reaching the water load. The power reaching the water load will depend upon the amount of power being utilized by the drying units.

FlG. 7 illustrates an alternative arrangement for the multiple drying units of FIG. 6. In this alternative power meter 96 and water load 95 are omitted and the outer end of tube 93 is connected directly to the tubular leg 69 of the last unit 50, with reference to the direction of movement of the material being dried. A directional power meter 97 is located in tube 93 between the microwave generator 87 and the tubular leg 69 of the first unit 50. This power meter is operationally coupled to an output controller 98 for the generator 87. Controller 98 can be set in such a way that the total amount of transmitted power from all units 50 which is flowing back to the generator is limited to the previously mentioned limit A P multiplied by the number of units 50.

Iclaim:

1. Microwave drying apparatus for moisture-laden dielectric materials, comprising a waveguide system in the form of two waveguides arranged in a closed ring, and a tubular leg communicating with and extending away from the ring, said leg being adapted to be connected to a microwave generator, and a slot in the ring V through which material to be dried can be moved with the longitudinal center line of said material on an electromagnetic center line of the waveguides, said electromagnetic center line being a location in the waveguide slot for which the length of one of said waveguides equaling a, and the length of the other of said waveguides equaling b, where a and b are measured from the tubular leg through the respective waveguides to said location, the relationship of a and b to each other being where A is the wavelength of the microwaves in the waveguide and m an integer.

2. Microwave drying apparatus as claimed in claim 1 wherein said waveguide is in the form of a substantially rectangular ring having a first pair of opposite sides interconnected with a second pair of opposite sides, said tubular leg being connected to one of said sides and forming with the latter side a T-junction.

3. Microwave drying apparatus as claimed in claim 2 in which said slot is in one of the sides of said second pair.

4. Microwave drying apparatus as claimed in claim 1 including a meter in said tubular leg for microwave energy transmitted through the material.

5. Microwave drying apparatus as claimed in claim 1 including a microwave generator coupled to the tubular leg, a meter in said tubular leg for microwave energy transmitted through said material, and a controller for the generator and operated by said meter to increase and decrease the microwave energy directed to the material inversely relative to the transmitted microwave energy.

6. Microwave drying apparatus as claimed in claim 1 in which said waveguide system is in the form of an elongated casing having a central wall extending longitudinally thereof and terminating short of opposite ends of the casing, said wall forming said two waveguides on opposite sides of the wall, and said tubular leg being connected to one of said ends of the casing and having a longitudinal center line aligned with the central wall.

7. Microwave drying apparatus as claimed in claim 6 in which said slot isformed in the casing through one of said waveguides on one side of the central wall.

8. Microwave drying apparatus as claimed in claim 6 energy transmitted through said material.

9. Microwave drying apparatus as claimed in claim 6 including a microwave generator coupled to the tubular leg, a meter in said tubular leg for microwave energy transmitted through said material, and a controller for the generator and operated by said meter to increase and decrease the microwave energy directed to the material inversely relative to the transmitted microwave energy.

10. Microwave drying apparatus comprising a plurality of spaced-apart and aligned waveguide systems; each waveguide system comprising two waveguides arranged in a closed ring, and a tubular leg communicating with and extending away from the ring, a slot in the ring through which material to be dried can be moved with the longitudinal center line of said material on an electromagnetic center line of the waveguides, said electromagnetic center line being a location in the waveguide slot for which the length of one of said waveguides equaling a and the length of the other of said waveguides equaling b, where a and b are measured from the tubular leg through the respective waveguides to said location, the relationship ofa and b to each other being where A is the wavelength of the microwaves in the waveguides and m an integer; said slots of said waveguide rings being aligned and having a common electromagnetic center line, and a common microwave generating means connected to each of the tubular legs of said waveguide rings.

11. Microwave drying apparatus as claimed in claim 10 including a meter in each tubular leg for microwave energy transmitted through said material at the waveguide of said each leg.

12. Microwave drying apparatus as claimed in claim 10 including a meter in each tubular leg for microwave energy transmitted through said material at the waveguide of said each leg, and controller means operated by said meter for the flow of microwave energy from said generator means to said material to in crease and decrease the microwave energy directed to the material inversely relative to the transmitted microwave energy in each tubular leg.

13. Microwave drying apparatus as claimed in claim 12 including a terminal power meter for measuring and a terminal absorption means for absorbing that portion of the microwaves-generated by said generating means that is unable to enter any of said tubular legs.

14. Microwave drying apparatus as claimed in claim 13 including a controller means for said generator means and operated by said terminal power meter to increase and decrease the microwave energy relative to the energy reaching the terminal absorption means.

15. Microwave drying apparatus as claimed in claim 12 including a directional power meter located between the said generating means and all of said waveguide systems, said directional power meter measuring the total power transmitted through said material in said waveguide systems, said transmitted power flowing back to said generating means and that portion of the generated microwaves which is excluded from increase and decrease the microwave energy generated by said generating means inversely relative to the energy flowing back to said generating means, as measured by said directional power meter 1

Claims

1. Microwave drying apparatus for moisture-laden dielectric materials, comprising a waveguide system in the form of two waveguides arranged in a closed ring, and a tubular leg communicating with and extending away from the ring, said leg being adapted to be connected to a microwave generator, and a slot in the ring through which material to be dried can be moved with the longitudinal center line of said material on an electromagnetic center line of the waveguides, said electromagnetic center line being a location in the waveguide slot for which the length of one of said waveguides equaling a, and the length of the other of said waveguides equaling b, where a and b are measured from the tubular leg through the respective waveguides to said location, the relationship of a and b to each other being b a + (2m + 1) ( lambda /4) where lambda is the wavelength of the microwaves in the waveguide and m an integer.

7. Microwave drying apparatus as claimed in claim 6 in which said slot is formed in the casing through one of said waveguides on one side of the central wall.

8. Microwave drying apparatus as claimed in claim 6 including a meter in said tubular leg for microwave energy transmitted through said material.

10. Microwave drying apparatus comprising a plurality of spaced-apart and aligned waveguide systems; each waveguide system comprising two waveguides arranged in a closed ring, and a tubular leg communicating with and extending away from the ring, a slot in the ring through which material to be dried can be moved with the longitudinal center line of said material on an electromagnetic center line of the waveguides, said electromagnetic center line being a location in the waveguide slot for which the length of one of said waveguides equaling a and the length of the other of said waveguides equaling b, where a and b are measured from the tubular leg through the respective waveguides to said location, the relationship of a and b to each other being b a + (2 m + 1) ( lambda /4) where lambda is the wavelength of the microwaves in the waveguides and m an integer; said slots of said waveguide rings being aligned and having a common electromagnetic center line, and a common microwave generating means connected to each of the tubular legs of said waveguide rings.

12. Microwave drying apparatus as claimed in claim 10 including a meter in each tubular leg for microwave energy transmitted through said material at the waveguide of said each leg, and controller means operated by said meter for the flow of microwave energy from said generator means to said material to increase and decrease the microwave energy directed to the material inversely relative to the transmitted microwave energy in each tubular leg.

13. Microwave drying apparatus as claimed in claim 12 including a terminal power meter for measuring and a terminal absorption means for absorbing that portion of the microwaves generated by said generating means that is unable to enter any of said tubular legs.

15. Microwave drying apparatus as claimed in claim 12 including a directional power meter located between the said generating means and all of said waveguide systems, said directional power meter measuring the total power transmitted through said material in said waveguide systems, said transmitted power flowing back to said generating means and that portion of the generated microwaves which is excluded from entering any of said tubular legs also flowing back to the generating means.

16. Microwave drying apparatus as claimed in claim 15 including a controller means for said generator means and operated by said directional power meter to increase and decrease the microwave energy generated by said generating means inversely relative to the energy flowing back to said generating means, as measured by said directional power meter.