GB1603077A - Generation of infra-red radiation - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE GENERATION OF INFRA-RED RADIATION (71) I, PETER DOUGLAS FRANCIS, a British Subject, of 40 Lower Field Road, Westminster Park, Chester, do hereby declare the invention for which I pray that a patent may be granted to me and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention is concerned with generating infra-red radiation. Typical sources of infra-red radiation comprise radiating bodies which are raised to elevated temperatures by heating, for example, with an electric element or by application of a gas flame. The heated body radiates electromagnetic radiation in a manner similar to the theoretical "black body".Probably the most common source of infra-red radiation is the electrically powered source in which the radiating body is heated by an electric current in a resistance winding.

According to the present invention, an infra-red element comprises at least one body of carbon felt and a skin or envelope of electrically non-conducting material completely enveloping the body or bodies to isolate it or them both physically and electrically from the surrounding atmosphere.

When using such an element to generate infra-red radiation, the body or bodies of carbon felt are heated to an elevated temperature at which they will emit infra-red, by exposing the bodies to micro-wave radiation. Carbon felt is a very good absorber of micro-wave radiation, and with sufficient micro-wave power in relation to the thermal capacity of the carbon felt body or bodies the bodies can be heated to a very high temperature, for example 2500"C, by dissipation of micro-wave energy in the carbon felt.

One of the main advantages of the infrared element of this invention is that there is no need for any physical connection between the element and the source of power for heating the element. The micro-wave energy can be directed at the element to be absorbed in the carbon felt from a distance away from the element. This feature can be espcially useful when the infra-red elements are used, to irradiate with infra-red articles being manufactured as they are being moved on a conveyor system, e.g. on a production line. For example, it is a normal requirement when manufacturing tins or cans for containing beverages or foodstuffs to provide a coating of lacquer on the interior surface of the can. This lacquer coating is typically cured after application to the can surface.The curing requires the lacquer to be heated to a predetermined temperature and maintained at that temperature for a certain length of time. Hitherto it has been the practice to pass the coated cans through a tunnel oven which raises the temperature of the complete can to the appropriate curing temperature of the lacquer. This process can be relatively wasteful of energy, both because the whole can is heated to the desired temperature and because an airflow is necessary through the oven to remove solvent vapours produced in the curing process.

Infra-red elements in accordance with the present invention can be employed to irradiate only the internal surfaces of the can with infra-red radiation. One element is positioned in each can to be cured and microwave radiation is directed into the can. It will be understood that the cans are openended at one end during this process. The micro-waves are absorbed in the carbon felt body or bodies of each element, so that the bodies are heated until they emit infra-red radiation. The infra-red radiation impinges directly on the interior surfaces of the can.

Furthermore, infra-red radiation not absorbed in the can surfaces will normally be reflected back onto an opposite surface portion. Thus, the presence of the infra-red element within the can permits a high level of infra-red flux to be generated inside the can, with relatively little escape of radiation.

Because there is no physical connection between the infra-red element and the source of energy, the problems of providing electrical connections to the element are obviated. This is especially valuable when the element is placed in a can whilst it is moving on a conveyor system.

The skin or envelope of electrically nonconducting material enveloping the body or bodies of carbon felt has two main functions. It will be understood that the solvent vapour produced when curing the lacquer coating can produce an explosive mixture in air. The skin or envelope prevents such a potentially explosive solvent vapour/air mixture from coming directly into contact with the carbon felt of the element. It has been observed that when placed in a strong micro-wave field, carbon felt appears to scintillate. This is thought to be as a result either of corona discharges produced by the electric field at the tips of the very fine fibres in the felt or by incandescent heating of the fibres.

On the other hand, the skin or envelope prevents the lacquer coating from being contaminated by any vapourised material generated by the carbon felt.

As mentioned previously, it is desirable when curing lacquer coatings to provide a flow of air to remove the solvent vapour as it is produced. The skin or envelope of electrically non-conducting material additionally isolates the carbon felt from the convection cooling effects of such a solvent vapour removal airflow in the interior of the can.

As a result, the carbon felt can be raised to a high temperature.

In one embodiment, the skin or envelope is a coating of electrically non-conducting refractory material directly on the or each body of carbon felt. The coating is conveniently a cured ceramic paste, such as that known by the trade name Fiberfrax ceramic paste which is manufactured by Carborundum Company Limited, of St. Helens, Merseyside. This paste is stable up to temperatures of 12500C.

In another embodiment, the skin or envelope is a sealed envelope of electrically non-conducting material enclosing the body or bodies. Preferably the envelope is of quartz glass. The envelope may be evacuated or may contain an inert gas.

The present invention further envisages apparatus for generating infra-red radiation comprising one or more infra-red elements as described above and means for generating and directing micro-wave electromagnetic energy at the or at least some of the elements, whereby to raise the temperature of the element or elements so that they emit infra-red radiation.

Examples of the present invention will now be described with reference to the accompanying drawings in which: Figures I and 2 illustrate different embodiments of infra-red element; Figure 3 illustrates schematically apparatus for irradiating the interior of cans, using infra-red elements as illustrated in Figure 1; Figure 4 illustrates part of a production line version of the apparatus of Figure 3 and Figure 5 illustrates a complete production line apparatus for curing lacquer coatings on the inside surfaces of cans.

Referring to Figures 1 and 2, there is shown in each Figure an infra-red element comprising a rod or tube 10 of dieletric or electrically insulating material, conveniently quartz glass. Towards one end of the rod 10 there are mounted, in the embodiment of Figure 1, a plurality of discs 11 of carbon felt. The discs are formed of different diameters ranging from a smallest diameter disc 12 positioned approximately in the centre of the rod or tube 10 to a largest diameter disc 13 positioned adjacent one end 14 of the rod. The other end 15 of the rod is used, in operation, for supporting the element in an operative position. For example, the end 15 of the rod may be gripped and the discs 11 inserted into the open end of a can which has a lacquer coating on its interior surface. Micro-wave energy is then directed into the can.It can be seen that with this arrangement the larger disc 13 of the element is nearest to the closed end of the can into which the element is inserted, that is to say most distant from the source of mirco-wave radiation. Correspondingly, the smallest disc 12 is nearest the open end of the can, and nearest to the source of micro-wave radiation. By making the discs 11 graded in size between smallest disc 12 and largest disc 13 absorption of micro-wave radiation is distributed between the various discs, so that they are heated to an approximately uniform temperature and the resulting infra-red emission provides substantially uniform irradiance over the interior surface of the can.

In one example, all the discs 11 are 5 mm thick and are fixed to the rod 10 with spacings between each of 10 mm. The smallest disc 12 is 10 mm in diameter and the largest disc 13 is 50 mm in diameter.

The Figure 2 illustrates a different arrangement of the carbon felt on the rod 10. In this different arrangement, a disc 16 of carbon felt is provided near the end 14 of the rod 10 and a disc 17 of an electrically insulating material is provided approximately midway along the rod. Between discs 16 and 17 are strung a plurality of strips 18 of carbon felt. Once again, this arrangement provides for a progressive absorption of micro-wave energy directed at the carbon felt substantially parallel with the rod 10 towards the end 14 thereof.

In each of the embodiments illustrated in Figures 1 and 2, the carbon felt discs and/or strips are enveloped in a skin of electrically non-conducting material so as to isolate the carbon felt from the surrounding atmosphere. The skin may be a coating of a cured ceramic paste applied directly to the carbon felt. A suitable ceramic paste is that known by the trade name Fibrefrax ceramic paste, available from Carborundum Company Limited, St. Helens, Merseyside. Such a paste is stable up to 12500C enabling the carbon felt to be heated, by exposure to micro-wave radiation, to a corresponding temperature.

However, a preferred type of isolating skin is illustrated in Figure 1. This comprises an envelope or bulb 19 of electrically insulating material, preferably quartz glass, completely enclosing the discs 11 and fused to the rod 10 at a point 20 near the smallest disc 12. The bulb 19 is either evacuated or filled with an inert gas, thereby reducing any tendency for the carbon felt to oxidise when heated by exposure to micro-wave radiation.

As explained previously, the skin of insulating material, whether the coating of ceramic paste or the enveloping bulb 19, effectively isolates the carbon felt from the exterior atmosphere. When the infra-red element is used for curing lacquer coatings on the inside of cans, this isolation prevents the coating from being contaminated by any material vapourised from the carbon felt.

Further, the possibility of a combustible mixture of air and solvent vapour from the lacquer coating being ignited by the carbon felt is obviated.

Apart from these advantages the bulb 19 as illustrated in Figure 1 has two further advantages when the element is used for curing lacquer coatings in cans. It will be appreciated that when the element is exposed to micro-wave radiation it is the discs or other bodies of carbon felt that absorb the micro-wave energy. The carbon felt discs have a relatively low thermal capacity and, therefore, the discs can heat up rapidly to an infra-red emitting temperature.

However, a proportion of the infra-red radiation emitted by the discs is absorbed in the bulb 19 of quartz glass, thereby heating the bulb 19. The bulb has a relatively large thermal capacity and therefore heats up relatively slowly compared to the carbon felt discs 11. Furthermore. whereas when the bulb 19 is evacuated the carbon felt discs only lose heat by radiation, the bulb 19 loses heat both by radiation and conduction with the surrounding atmosphere.Accordingly, in a typical mode of operation, for example with the element inserted in a can, on first exposure to micro-wave radiation, the carbon felt discs rapidly heat to a relatively high temperature, up to 25000C, at which they radiate relatively short wavelength infra-red (xmas approximately equals 1.3 lem). However, the quartz glass bulb 19 heats up relatively slowly to a lower temperature, 600 to 9000C, at which it emits infra-red radiation with a longer wavelength max ,,, approximately equal to 2.5 to 3.5 Fm) Typical lacquer coating formulations for use in coating the inside of cans usually absorb infra-red radiation in spectral regions between 3 and 3.5 microns and between 5 and 15 microns wavelength.Thus, infra-red of relatively long wavelength may be favourably absorbed in the lacquer coating, thereby directly heating the coating rather than relying on heat conducted into the coating from the metal wall of the can.

The element with the quartz bulb 19 may, therefore, enable the lacquer coating to be cured more efficiently, without requiring the wall of the can to be heated to the same extent as hitherto. However, as is well known, the intensity of radiation from sources of relatively long wavelength infrared is very much less for the same radiating area. To some extent this problem is reduced by using the quartz glass bulb 19 because the bulb may be shaped suitably in relation to the can in which it is to be inserted, to provide "close coupling" for the long wavelength infra-red between the bulb and the interior walls of the can. Thus, preferably, the bulb 19 has a diameter only slightly less than the interior diameter of the can.When choosing a suitable diameter of bulb 19, consideration must be given to provision of an airflow circulating between the bulb 19 and the wall of the can for removal of solvent vapours from the lacqeur coating.

It is also possible to provide a degree of close coupling between an element in which the carbon felt bodies are coated with the ceramic paste and the interior surfaces of the can. This may be done by forming or positioning the carbon felt bodies so that they are close to the interior walls of the can. In this case it may be desirable to employ rings or annulae of carbon felt instead of the discs 11, so that even the rings nearest the open end of the can may be formed to be close to the interior can surface, without excessively shielding rings behind it further into the can.

Figure 3 illustrates schematically an arrangement for production line curing of lacquer coatings in cans, using several infrared elements similar to that illustrated in Figure 1. A conveyor belt 30 carries a line of cans 31 open end up in the direction indicated by arrow 32. Inserted into the open ends 33 of the cans 31 are infra-red elements 34, themselves supported on a belt 35. The belt 35 is arranged to move in synchronism with the conveyor belt 30 so that the elements 34 are carried along positioned in the cans 31. A horn antenna 36 is supported above the conveyor belt 30 and the element supporting belt 35 and is arranged for directing micro-wave energy downwardly at the open ends 33 of the cans 31. Micro-wave energy is generated by a magnetron 37 connected to the horn antenna 36 by means of a waveguide.A stub tuner 38 is positioned in the wave-guide for optimising the matching of the antenna 36 to minimise reflections. The belt 35 is made of nylon or a similar dielectric material to be substantially transparent to micro-wave energy. It will be appreciated that as the cans 31 with the inserted elements 34 travel in the direction of arrow 321 they pass into and then turn out of a region beneath the antenna 36 in which they are irradiated by micro-wave energy.

Referring now to Figure 4, there is illustrated in plan view part of an apparatus for curing lacquer coatings on the inner surfaces of cans. Cans are fed to the apparatus along a conveyor 40. In the apparatus, the cans are conveyed around a semi-circular path 41 by means of a drum conveyor 42. Throughout the semi-circular path 41, the cans are irradiated with microwave energy by means of a semi-circular array of horn antennae 43 provided above the semi-circular path 41 directing microwave energy downwards onto the cans transported thereby. In the array illustrated there are twenty horn antennae. Before entering the semi-circular path 41 where the cans are irradiated with micro-wave energy, at a point 44 infra-red elements, similar to that illustrated in Figure 1, are inserted into the open tops of the cans.Thus, throughout the semi-circular path 41, the cans are irradiated with micro-wave energy from the horns 43, which energy is absorbed in the carbon felt of the infra-red elements and re-radiated as infra-red. A suitable speed of rotation of the drum 42 is 2 revs per minute so that each can spends approximately 15 seconds in the irradiated region.

Figure 5 illustrates a more complete curing apparatus in which a conveyor 50 carries cans to a first semi-circular conveyor portion 51 which is linked to a second conveyor portion 52, to form an S-shaped conveyor. Cans 53 are conveyed to the curing apparatus by a conveyor 50 in the direction of arrow 54. Infra-red elements, such as described with reference to Figure 1, are inserted in the cans 53 at a point 55 before the cans enter the first semi-circular region 51. Each of semi-circular regions 51 and 52 are provided with a plurality of micro-wave horn antennae, substantially as described with reference to Figure 4.During the passage of the cans with the inserted elements along the semi-circular conveyor portion 51, the micro-wave energy absorbed by the elements is re-radiated as infra-red and progressively raises the temperature of the lacquer coating inside the can to that temperature required to cure the coating.

While the cans are conveyed in the semicircular portion 52, the cans are maintained at the desired curing temperature. It will be understood that a lesser amount of microwave energy may be necessary in the portion 52, merely to maintain the can temperature, compared with that required in the portion 51 to bring the temperature of the can up to the curing temperature. Accordingly, the power density of the micro-waves irradiating the cans in the portion 52 may be somewhat reduced compared to that irradiating cans 51. This may be achieved either by reducing the power of the magnetron or other generating device or by reducing the number of horn antennae, so that there are greater intervals between adjacent antennae, passing through which the infra-red elements in the cans absorb little or no micro-wave energy.

In order to make efficient use of the micro-wave energy radiated by the horn antennae, micro-wave absorbing bodies may be provided in the interstices between adjacent cans on the conveyor. These bodies will then absorb micro-waves not entering the open ends of the cans and they will heat up in response, becoming themselves sources of infra-red. The infra-red radiation produced, though impinging upon the outer surfaces of the cans tends to encourage the heating of the cans, which in turn more quickly raises the interior lacquer coating to the desired curing temperature.

On leaving the second semi-circular portion 52, the interior lacquer coating in the cans is fully cured and the inserted infra-red elements are withdrawn at a point 57, permitting the cured cans 58 to be transported away in the direction of arrow 59 for further operations in the manufacturing process.

The infra-red elements withdrawn from the cans at point 57 are conveyed by means of a return line 60 back to the point 55 where they are again inserted in fresh cans approaching the curing apparatus.

Generally, micro-wave energy propagated inside the cans and not absorbed by the carbon felt bodies will tend to be absorbed in the interior surface layers of the metal walls of the cans, thereby heating the walls and aiding the curing process.

WHAT WE CLAIM IS: 1. An infra-red element comprising at least one body of carbon felt and a skin or

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. indicated by arrow 32. Inserted into the open ends 33 of the cans 31 are infra-red elements 34, themselves supported on a belt 35. The belt 35 is arranged to move in synchronism with the conveyor belt 30 so that the elements 34 are carried along positioned in the cans 31. A horn antenna 36 is supported above the conveyor belt 30 and the element supporting belt 35 and is arranged for directing micro-wave energy downwardly at the open ends 33 of the cans 31. Micro-wave energy is generated by a magnetron 37 connected to the horn antenna 36 by means of a waveguide. A stub tuner 38 is positioned in the wave-guide for optimising the matching of the antenna 36 to minimise reflections. The belt 35 is made of nylon or a similar dielectric material to be substantially transparent to micro-wave energy.It will be appreciated that as the cans 31 with the inserted elements 34 travel in the direction of arrow 321 they pass into and then turn out of a region beneath the antenna 36 in which they are irradiated by micro-wave energy. Referring now to Figure 4, there is illustrated in plan view part of an apparatus for curing lacquer coatings on the inner surfaces of cans. Cans are fed to the apparatus along a conveyor 40. In the apparatus, the cans are conveyed around a semi-circular path 41 by means of a drum conveyor 42. Throughout the semi-circular path 41, the cans are irradiated with microwave energy by means of a semi-circular array of horn antennae 43 provided above the semi-circular path 41 directing microwave energy downwards onto the cans transported thereby. In the array illustrated there are twenty horn antennae. Before entering the semi-circular path 41 where the cans are irradiated with micro-wave energy, at a point 44 infra-red elements, similar to that illustrated in Figure 1, are inserted into the open tops of the cans.Thus, throughout the semi-circular path 41, the cans are irradiated with micro-wave energy from the horns 43, which energy is absorbed in the carbon felt of the infra-red elements and re-radiated as infra-red. A suitable speed of rotation of the drum 42 is 2 revs per minute so that each can spends approximately 15 seconds in the irradiated region. Figure 5 illustrates a more complete curing apparatus in which a conveyor 50 carries cans to a first semi-circular conveyor portion 51 which is linked to a second conveyor portion 52, to form an S-shaped conveyor. Cans 53 are conveyed to the curing apparatus by a conveyor 50 in the direction of arrow 54. Infra-red elements, such as described with reference to Figure 1, are inserted in the cans 53 at a point 55 before the cans enter the first semi-circular region 51. Each of semi-circular regions 51 and 52 are provided with a plurality of micro-wave horn antennae, substantially as described with reference to Figure 4.During the passage of the cans with the inserted elements along the semi-circular conveyor portion 51, the micro-wave energy absorbed by the elements is re-radiated as infra-red and progressively raises the temperature of the lacquer coating inside the can to that temperature required to cure the coating. While the cans are conveyed in the semicircular portion 52, the cans are maintained at the desired curing temperature. It will be understood that a lesser amount of microwave energy may be necessary in the portion 52, merely to maintain the can temperature, compared with that required in the portion 51 to bring the temperature of the can up to the curing temperature. Accordingly, the power density of the micro-waves irradiating the cans in the portion 52 may be somewhat reduced compared to that irradiating cans 51. This may be achieved either by reducing the power of the magnetron or other generating device or by reducing the number of horn antennae, so that there are greater intervals between adjacent antennae, passing through which the infra-red elements in the cans absorb little or no micro-wave energy. In order to make efficient use of the micro-wave energy radiated by the horn antennae, micro-wave absorbing bodies may be provided in the interstices between adjacent cans on the conveyor. These bodies will then absorb micro-waves not entering the open ends of the cans and they will heat up in response, becoming themselves sources of infra-red. The infra-red radiation produced, though impinging upon the outer surfaces of the cans tends to encourage the heating of the cans, which in turn more quickly raises the interior lacquer coating to the desired curing temperature. On leaving the second semi-circular portion 52, the interior lacquer coating in the cans is fully cured and the inserted infra-red elements are withdrawn at a point 57, permitting the cured cans 58 to be transported away in the direction of arrow 59 for further operations in the manufacturing process. The infra-red elements withdrawn from the cans at point 57 are conveyed by means of a return line 60 back to the point 55 where they are again inserted in fresh cans approaching the curing apparatus. Generally, micro-wave energy propagated inside the cans and not absorbed by the carbon felt bodies will tend to be absorbed in the interior surface layers of the metal walls of the cans, thereby heating the walls and aiding the curing process. WHAT WE CLAIM IS:

1. An infra-red element comprising at least one body of carbon felt and a skin or

envelope of electrically non-conducting material completely enveloping the body or bodies to isolate it or them both physically and electrically from the surrounding atmosphere.

2. An infra-red element as claimed in claim 1 wherein the skin or envelope is a coating of electrically non-conducting refractory material directly on the or each body of carbon felt.

3. An infra-red element as claimed in claim 2 wherein the coating is a cured ceramic paste.

4. An infra-red element as claimed in claim 1 wherein the skin or envelope is a sealed envelope of electrically nonconducting material enclosing the body or bodies.

5. An infra-red element as claimed in claim 4, wherein the envelope is of quartz glass.

6. An infra-red element as claimed in claim 4 or claim 5 wherein the envelope is evacuated or contains an inert gas.

7. Apparatus for generating infra-red radiation comprising one or more infra-red elements as claimed in any preceding claim and means for generating and directing micro-wave electro-magnetic energy at the or at least some of the elements, whereby to raise the temperature of the element or elements so that they emit infra-red radiation.

8. An infra-red element substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

9. Apparatus for irradiating the interior of cans with infra-red radiation, as claimed in claim 7 and substantially as hereinbefore described with reference to Figures 3 to 5 of the accompanying drawings.