GB2042703A

GB2042703A - Drying of Web Material

Info

Publication number: GB2042703A
Application number: GB8002767A
Authority: GB
Original assignee: Ciba Geigy AG
Current assignee: Novartis AG
Priority date: 1979-02-06
Filing date: 1980-01-28
Publication date: 1980-09-24
Also published as: CH621620A5; FR2448699A1

Abstract

This invention relates to a method of drying wet lengths of material by microwave power. The power is passed from a microwave generator 3 into a circulator 1 and thence to an obstacle 8 into one end of a closed elongate waveguide 7 tuned to come into resonance with the frequency of the generator when wet material is present in the waveguide. The material is passed through longitudinal slots 9 located along the centre lines of the broad faces of the elongate waveguide. A strong electric field of the given frequency is produced in the plane of the material. Power which is not absorbed by the material is returned to the circulator 1 and passed from it to a dummy load 5. In a preferred embodiment of the invention the material, after passing through a first section of waveguide, is passed through a second section of waveguide. The second section of waveguide is so disposed relative to the web and the first section of the wave guide that the nodes and antinodes of the standing wave pattern of microwaves in the second section corresponding to different positions on the material compared with the positions of the nodes and antinodes in the first section. <IMAGE>

Description

SPECIFICATION Drying of Web Material This invention relates to methods and apparatus for the drying of wet lengths of material.

Various method and apparatus have been devised and used for drying wet lengths of material which comprise passing the wet material through a high frequency electromagnetic field, thereby causing the liquid on or in the material to heat up and evaporate due to the dielectric heating effect of the high frequency electromagnetic waves. Such high frequency electromagnetic wave drying has in particular been proposed for drying wet paper webs and for drying moisture or solvent containing layers on plastics film material webs and for drying textiles. The method is particularly applicable to the drying of webs by removal therefrom of a liquid which shows a significant electrical loss angle at the high frequency concerned.In British patent specification No. 1079677 the use of high frequency electromagnetic radiation was proposed for removing the last 5 to 10% of moisture from photographic emulsion from which the majority of the water had been removed by conventional means. The frequency then proposed was 40 MHz. It was already well known (vide Kretzmann, "Industrial Electronics", p. 1 99, Philips Technical Library, 1953) that high frequency drying becomes more effective as the frequency is increased.Accordingly as microwave power generators have become readily available, microwave frequencies (e.g. 896 MHz and 2450 MHz) have been used for various industrial drying applications including the drying of undried thickly coated areas of photographic film material, the majority of the water having been removed by other drying means as proposed in British patent specification No.1508115. In B. P.1508115 various microwave driers are described, for example a zig-zag wave-guide type drier as disclosed in U.S. patent specification No.

3672066.

Various other types of microwave driers are described for example in U. S. patent specifications 3449836,3491457 and 3475827.

It has now been found that particularly good results can be achieved using a resonating closed elongate wave-guide having entry and exit slots therein for web material as the mircowave drier.

According to the present invention there is provided a method of drying wet lengths of material by microwave power which comprises passing power from a microwave generator into a circulator and thence through an obstacle into one end of a closed elongate waveguide tuned to come into resonance with the frequency of the said generator when wet material is present therein, passing the material through longitudinal slots located along the centre lines of the broad faces of the elongate waveguide, thereby producing in the plane of the material a strong electric field at said frequency, and power which is not absorbed by the said material is returned to the circulator and thence is passed into a dummy load.

Preferably the frequency of the microwaves generated by the microwave generator is approximately 2.45 GHz as this is one of the frequencies which has been allocated for industrial use. Microwaves at other frequencies may be used. At frequencies below 2.45 GHz, the efficiency of heating falls, as already mentioned.

Also the size of waveguide then becomes inconveniently large for many applications. At frequencies much above 2.45 GHz, microwave generators are not at present available capable of delivering power outputs sufficiently large for many purposes. Thus of the frequencies allocated for use for industrial purposes the most useful is 2.45 GHz.

Preferably the microwave generator is a magnetron but other microwave generators, such as solid state generators, could be used provided they were capable of delivering sufficient power.

Preferably the obstacle is an orifice plate. The obstacle serves to reflect the returning wave towards the closed end of the waveguide. The section of waveguide between the obstacle and the closed end of the waveguide then behaves as a resonant circuit, the Q-factor of which is partly determined by the obstacle (vide Walker and Straw, "Spectroscopy", p. 1 63, Chapman and Hall, 1961). Thus substantially all the microwaves entering the waveguide bounce back and forth between the obstacle and the closed end of the waveguide until their energy is dissipated drying the material. The electromagnetic field pattern within the waveguide is such that the maximum electric field is in the plane of the material which traverses the waveguide through the slotted broad faces of the waveguide.The wall currents within the waveguide are such that there is a minimum of electromagnetic radiation leakage when slots are cut in the broad faces of the waveguide.

Reflection of power between the closed end of the waveguide and the obstacle leads to production of a standing wave pattern in the electromagnetic field in the waveguide. Field maxima are produced at distances (4+n2/2) from the closed end where n has integral values e.g. 1, 2, 3, and A is the wavelength of the microwaves within the waveguide. When the distance between obstacle and closed end is an integral multiple of A/2, the waveguide resonates and the field maxima are most intense. Between the maxima (antinodes) are minima (nodes) and material passing through these minima will not be heated. In order to ensure that the material is dried evenly it is therefore preferred that the material passes also through a second waveguide so arranged that the maxima of the second waveguide correspond to the minima of the first waveguide.

Therefore according to a preferred method of the present invention there is provided a method of drying wet lengths of material using a microwave generator and a waveguide as just defined and wherein the material after passing through a first section of waveguide is passed through a second section of waveguide where it is further dried using a microwave generator and waveguide as just defined, the second section of waveguide being so disposed relative to the web and the first section of waveguide that the nodes and antinodes of the standing wave pattern of microwaves in the second section correspond to different positions on the material compared with the positions of the nodes and antinodes in the first section.

Conveniently in this preferred method the said second section is displaced relative to the said first section by one quarter of the wavelength within the waveguide of the microwave power supplied. Two separate waveguides may be employed but in one preferred method a single waveguide is employed which is folded into two sections through both of which the material is passed.

According to another aspect of the present invention there is provided an apparatus for drying wet lengths of material by microwave power which comprises (1) a microwave generator, (2) a dummy load and (3) a closed resonant waveguide incorporating a tuning device, respectively connected to three ports of a circulator so that power from the microwave generator can pass only through the circulator to the resonant waveguide and power reflected from the closed resonant waveguide can pass only through the circulator to be absorbed by the dummy load, the input to the resonant waveguide being provided with an obstacle to increase the Q factor of the combination of waveguide and tuning device, there being present in the said resonant waveguide entry and exit slots for the material to be dried, said slots being located along the centre lines of the broad faces of the elongate resonant waveguide.

Preferably in the apparatus the microwave generator is a magnetron, the obstacle is an orifice plate and the tuning device is a short circuit plunger.

Preferably there is provided in the apparatus a waveguide which is folded to comprise two sections through both of which the material is passed and most preferably the obstacle at one end of said first section of the said folded waveguide is displaced relative to the short circuit plunger at the end of the said second section of the folded waveguide by one quarter of the wavelength within the waveguide of the microwave power supplied measured in a direction transverse to the direction of movement of the wet material.

In another embodiment there is provided a system for drying wet lengths of material which comprises two apparatus as just defined, the said wet material passing through the waveguide of each apparatus, the resonant waveguide of the second apparatus being displaced relative to the resonant waveguide of the first apparatus by one quarter of the wavelength within the waveguide of the microwave power supplied, such displacement being measured in a direction transverse to the direction of movement of the wet material.

The method and apparatus of the present invention is of especial use for removing the last traces of moisture or solvent from wet lengths of material which have been rendered nearly dry by other means, e.g. hot gas impingement. In particular the method and apparatus of the present invention is of use when the last traces of moisture or solvent are not evenly distributed over the material. For example, it frequently happens when liquid based layers are coated on webs that coating faults occur. This means that in some places along the length of the coated web considerably more than the average amount of coating material is deposited on the web. Such areas which contain abnormal amounts of wet coating material are extremely difficult to dry satisfactorily because they often contain more than twice the average amount of solvent or moisture which is present on the web as a whole.

This means that in order to ensure that such areas are fully dry before the web is reeled prior to storage the drying path must be more than twice the length which would be required if such abnormal areas were not present or alternatively the energy supplied during the drying path must be more than twice the energy which would be required if such abnormal areas did not occur. On the one hand it is very expensive to provide a longer drying path than is required to dry the web apart from the abnormal areas whilst on the other hand it is both expensive and often impossible to provide twice the amount of energy required to dry the web except for the abnormal areas without damaging the coating on the web.

However when using the method and apparatus of the present invention it is possible to tune the waveguide so that it resonates strongly only when partly wet material is present in the waveguide. When the material passing through the waveguide is completely dry the microwaves entering the waveguide are reflected by the closed end of the waveguide out of the waveguide to the dummy load. The circulator prevents the microwaves being reflected back into the microwave generator and thus shortening its life. By use of a resonant waveguide which is tuned to resonate only when partly wet material is present in the waveguide it is possible to generate a great deal of energy in the wet areas of the material and to dry such areas very rapidly. By use of the method and apparatus of the present invention it is possible and indeed preferable to generate microwaves the whole time in the waveguide. When fully dried material is present in the waveguide the microwaves will be reflected back from the end of the waveguide to the dummy load. When an abnormal wet area enters the waveguide which has previously been tuned to resonate in such conditions, resonance will again occur and the microwaves will be reflected between the closed end of the waveguide and the obstacle, setting up a standing wave in the waveguide. The strong electric field so produced leads to heating of moisture in the abnormal area on the material until the wet area is dried. This continuous generation of microwaves obviates the need to detect wet material as it enters the waveguide and then to switch on the microwave generator. Such detection of wet material and switching on of the microwave generator, possibly several times per second, could give rise to almost insurmountable difficulties.

The method and apparatus of the present invention may be used to dry a great variety of materials. It is particularly useful for drying web material, for example paper webs and coated paper or film material webs. However it may be used to dry textile materials in web form or in filamentary form such as strands of silk or cotton, or it may be used to dry rope material. Further, it is of use in drying shaped materials in long lengths, e.g. long tubes of filled cigarette paper before cutting into individual cigarettes.

The method and apparatus of the present invention is of particular use for removing the last traces of moisture from coated photographic film material. Coating faults frequently occur when coating photographic film material and such faults are often manifest as areas on the coated surface which are abnormally thick compared with the remainder of the surface and such areas before drying contain abnormal amounts of moisture which must be removed before the coated film is reeled up. The usual coating liquid applied to photographic film material is an aqueous gelatin solution, e.g. an aqueous gelatin silver halide emulsion. Electromagnetic radiation can be used to heat wet areas in a partly dried coating without affecting the quality of product in areas already dry.

The accompanying drawings will serve to illustrate the invention.

Figure 1 is a side elevation of an apparatus for drying wet web material according to the present invention.

Figure 2 is a top plan view of two units shown in Figure 1.

Figure 3 is a top plan view of a modified apparatus.

In Figure 1 the apparatus comprises a circulator 1 to which is connected via port 2 a magnetron 3 which acts as a microwave generator. Connected to the circulator 1 via port 4 is a dummy load 5, and connected to the circulator 1 via port 6 is a closed resonant waveguide 7.

The closed resonant waveguide 7 comprises an orifice plate 8 which acts as an obstacle, a pair of slots 9 (only one of which is visible in this view) through which a web passes through the waveguide 7 and a movable short circuit plunger 11.

In Figure 2 the same numbers have the same significance as in Figure 1, but for ease of reference one set of numerals has been given the suffix a and the other b. A web 12 is shown passing through both waveguides 7a and 7b. It is to be noted that obstacle 6b is located further into the waveguide 7b than is obstacle 6a in waveguide 7a. In fact 6b is displaced relative to 6a by a distance equal to one quarter of the guide wavelength of the microwave power supplied.

This is to ensure that the nodes and antinodes of the reflected microwaves in the waveguides 7a and 7b will occur in different relative postions when in operation.

When using the pair of waveguides plus magnetrons shown in Figure 2 to dry web material which has been partly dried but which has on it wet patches at irregular intervals it is first necessary to tune both waveguides so that they resonate only when web material having a wet patch thereon is present in the waveguide.

This tuning is carried out by turning on both microwave generators 3a and 3b, passing wet web material through both waveguides and operating each short circuit plunger 11 a and 11 b until a resonant state is present in each waveguide. The wet web is then withdrawn and the apparatus is now in a condition in which it can dry partly dried web material of the same composition which has wet patches thereon in which the liquid is of the same composition as that of the liquid in the wet web used to tune the waveguides.

If the composition of the web is different and more importantly if the liquid on the web is altered then the apparatus must be reined to ensure that full resonance in each waveguide occurs only when wet web material is present in each waveguide.

In operation partly wet web material is passed through both waveguides 7a and 7b with both the microwave generators 3a and 3b switched on.

When wet web material is not present in either waveguide there is no resonance therein and the microwave power is dissipated in the dummy loads 5a and 5b. As soon as wet web material is present in a waveguide resonance is set up and the microwave power heats up the wet patch thus evaporating the liquid and drying the patch. If insufficient microwave power is focussed on the wet patch in one waveguide because the position of the patch coincides with a node in the wave formation, then sufficient power will be generated in the other waveguide because the waveguides have been so arranged that the nodes and antinodes do not occur in the same relative positions.

Figure 3 illustrates a modification to the arrangement shown in Figure 2 in that only one waveguide is used. In Figure 3 a microwave generator 1 5 supplies microwaves via a circulator to a folded waveguide 1 7. A dummy load 18 is mounted at the third port of the circulator 16.

Mounted in the waveguide 1 7 is an orifice plate 19 which acts as an obstacle and a short circuit plunger 20. The web 22 is passed through the folded waveguide 1 7 via slots (not shown in this view). The relative position of the nodes and antinodes in each leg of the waveguide is determined by the position of the orifice plate 1 9 and the short circuit plunger 20. As in Figure 1 the resonance of the waveguide is controlled by the position of the short circuit plunger and at resonance the active length of waveguide between plate 19 and plunger 20 is an integral multiple of > 1/2. The position of plate 1 9 relative to the bend 21 in the waveguide must be chosed so that plate 19 and plunger 20 are separated from the edge of the web 22 by distances differing by A/4.

Claims

1. A method of dryirig wet lengths of material by microwave power which comprises passing power from a microwave generator into a circulator and thence through an obstacle into one end of a closed elongate waveguide tuned to come into resonance with the frequency of the said generator when wet material is present therein, passing the material through longitudinal slots located along the centre lines of the broad faces of the elongate waveguide, thereby producing in the plane of the material a strong electric field at said frequency, and power which is not absorbed by the said material is returned to the circulator and thence is passed into a dummy load.

2. A method according to claim 1 wherein the frequency of the microwaves generated by the generator is approximately 2.45 GHz.

3. A method according to either claim 1 or claim 2 wherein the microwave generator is a magnetron.

4. A method according to any one of claims 1 to 3 wherein the obstacle is an orifice plate.

5. A method of drying wet lengths of material using a microwave generator and a waveguide according to any one of claims 1 to 4 wherein the material after passing through a first section of waveguide is passed through a second section of waveguide where it is further dried using a microwave generator and waveguide as just defined, the second section of waveguide being so disposed relative to the web and the first section of waveguide that the nodes and antinodes of the standing wave pattern of microwaves in the second section correspond to different positions on the material compared with the positions of the nodes and antinodes in the first section.

6. A method according to claim 5 wherein the said second section is displaced relative to the first section by one quarter of the wavelength within the waveguide of the microwave power supplied. ~~~~~~~~~~~~~~~

7. An apparatus for drying wet lengths of material by microwave power which comprises (1) a microwave generator, (2) a dummy load and (3) a closed resonant waveguide incorporating a tuning device, respectively connected to three ports of a circulator so that power from the microwave generator can pass only through the circulator to the resonant waveguide and power reflected from the closed resonant waveguide can pass only through the circulator to be absorbed by the dummy load, the input to the resonant waveguide being provided with an obstacle to increase the 0 factor of the combination of waveguide and tuning device, there being present in the said resonant waveguide entry and exit slots for the material to be dried, said slots being located along the centre lines of the broad faces of the elongate waveguide.

8. Apparatus according to claim 7 wherein the microwave generator is a magnetron.

9. Apparatus according to either claim 7 or claim 8 wherein the obstacle is an orifice plate.

10. Apparatus according to any one of claims 7 to 9 wherein the tuning device is a short circuit plunger.

11. Apparatus according to any one of claims 7 to 10 wherein the waveguide is folded to comprise two sections through both of which the material is passed.

12. An apparatus according to claim 11 wherein the obstacle at one end of said first section of the said folded waveguide is displaced relative to the short circuit plunger at the end of the said second section of the folded waveguide by one quarter of the wavelength within the waveguide of the microwave power supplied measured in a direction transverse to the direction of movement of the wet material.

13. A system for drying wet lengths of material which comprises two apparatus as claimed in any one of claims 7 to 10, the said wet material passing through the waveguide of each apparatus, the resonant waveguide of the second apparatus being displaced relative to the resonant waveguide of the first apparatus by one quarter of the wavelength within the waveguide of the microwave power supplied, such displacement being measured in a direction transverse to the direction of movement of the wet material.

14. A method of drying wet lengths of material according to claim 1 substantially as hereinbefore described.

1 5. Apparatus for drying wet lengths of material substantially as hereinbefore described with reference to the accompanying drawings.