WO1994001982A1

WO1994001982A1 - Radiant heating apparatus

Info

Publication number: WO1994001982A1
Application number: PCT/GB1993/001424
Authority: WO
Inventors: David Aubrey Plumptre
Original assignee: Severn Furnaces Limited
Priority date: 1992-07-07
Filing date: 1993-07-07
Publication date: 1994-01-20
Also published as: GB9214380D0

Abstract

A radiation transmitting apparatus (20) includes at least one radiation emitting element (11) enclosed in an envelope (12). The radiation emitting element (11) emits radiation including radiation in a first wavelength range and the envelope (12) absorbs and is heated by radiation in the first wavelength range. The apparatus (20) further includes means to absorb radiation in the first wavelength range which means may comprise a reflecting structure (10) which reflects radiation other than that in the first wavelength range. Dissipating means are included to dissipate the heat generated in the means to absorb radiation in the first wavelength range by absorption of radiation in the first wavelength range. The apparatus (20) is intended to be embodied as a radiant bulb furnace.

Description

RADIANT HEATING APPARATUS

The present invention relates to a radiation transmitting apparatus which includes a radiation emitting element enclosed in an envelope.

One example of such a radiation transmitting apparatus is a radiant bulb furnace for heating specimens, the or each bulb therein comprising a heating filament enclosed in an envelope.

Although the filament of a bulb in such a radiant bulb furnace can reach temperatures of up to 3000^βC most of the energy is radiated from it in the form of light of varying wavelength.

Most of the radiation emitted by the filament passes through the bulb envelope and can be used to heat any specimen which absorbs the radiation. Radiation whose wavelengths are in a first range, however, are absorbed by and heat the bulb envelope. Some of this heat, though, is lost due to the heated envelope re-radiating in the first wavelength range.

To ensure that most of the radiation from the or each bulb of a radiant bulb furnace is transmitted to the specimen it has been found advantageous to place the or each bulb in a reflective cavity within the furnace. However, this results in a large amount of the radiation in the first wavelength range emitted by the envelope being reflected back into and absorbed by itself and catastrophic overheating of the envelope occurs. Therefore, to ensure an adequate bulb life the heating effect of the radiation in the first wavelength range must be limited to below a critical value. It is therefore an aim of the present invention to alleviate the above-mentioned problem, and according to a first aspect of the invention there is provided a radiation transmitting apparatus including at least one radiation emitting element enclosed in an envelope, the radiation emitting element emitting radiation including radiation in a first wavelength range, wherein the envelope absorbs and is heated by radiation in the first wavelength range and wherein the apparatus further includes means to absorb radiation in the first wavelength range.

In one embodiment, the means to absorb radiation in the first wavelength range comprises a reflecting structure which may comprise a facing sheet to absorb and be heated by radiation in the first wavelength range and a backing sheet in contact with a surface of the facing sheet remote from the envelope to reflect radiation other than that in the first wavelength range.

The facing sheet may be of polished quartz or other glass and is preferably of polished water-free quartz glass.

The backing sheet is made of a highly reflective material which may be a coating and is preferably a coating of silver with a protective layer of copper on a surface of the silver remote from the facing sheet.

In an embodiment of the invention according to its first aspect, means are included in the radiation transmitting apparatus to dissipate the heat generated in the means to absorb radiation in the first wavelength range. In a specific example, the dissipating means comprises a base structure of heat conducting material in contact with the backing sheet of the reflecting structure and means to cool the base structure.

In another embodiment of the invention according to its first aspect, the envelope enclosing the radiation emitting element is of quartz glass.

According to a second aspect of the invention there is provided a reflecting structure to absorb radiation in a first wavelength range.

Embodiments of the first and second aspects of the invention will now be described by way of example with reference to the accompanying illustrative drawings in which:-

Figure 1 illustrates a perspective view of a radiant bulb furnace embodied as a radiation transmitting apparatus according to the first aspect of the invention;

Figure 2 is a schematic cross-sectional view of one half of the furnace shown in Figure 1; and

Figure 3 is an exploded view of the reflecting surface shown in the broken-line circle in Figure 2 which includes a reflecting structure according to the second aspect of the invention.

With reference to Figures 1 and 2, there is shown a radiant bulb furnace 20 comprised of two sections 1 and 2 which can be closed around a specimen, not shown, by virtue of a common hinge 3.

Each furnace section 1 and 2 consists of a water-cooled metal container 4, the water-cooling being effected by water flowing through conduits 13 internally in the metal containers 4. The walls of each metal container 4 define a cavity or recess 9 in which are mounted tubular quartz- halogen radiant bulbs 5 behind a window comprised in this embodiment of a clear polished water-free quartz glass plate 6. The interior surfaces of each cavity or recess 9 are smooth finished.

It will be appreciated, however, that the glass plate 6 comprising the window does not necessarily have to be polished, water-free or of quartz.

Each quartz-halogen bulb 5 comprises a thin tungsten filament 11 enclosed in a quartz glass envelope 12 containing inert gas and a halogen gas 14. The electrical connections for each bulb 5 are at either end of the bulb 5 outside the cavity or recess 9.

In operation, the filament 11 of each quartz-halogen bulb 5 typically emits light of wavelength ranging from the visible region right through to the infra-red region. This corresponds to wavelengths from about 300 nanometres through to and beyond 3000 nanometres depending on the temperature of the filament 11.

Most of the radiation emitted by the filament 11 of each bulb 5 passes through the quartz glass envelope 12 and the quartz glass plate 6 and can be used to heat the specimen. Those wavelengths beyond about 3000 nanometres, however, are absorbed by the quartz glass envelope 12 of the bulb and heat it. As the quartz glass envelope 12 heats up, though, it itself emits radiation of wavelengths beyond about 3000 nanometres. Therefore, when the bulb 5 is placed in a reflective cavity a large amount of this long wavelength radiation is reflected by the interior walls of the cavity back towards the quartz glass envelope 12 where it is absorbed.

The cumulative effect of the quartz glass envelope 12 absorbing the long wavelength radiation emitted by itself together with the long wavelength radiation emitted by the filament 11 is to lead to catastrophic overheating of the quartz glass envelope 12.

To ensure an adequate bulb life the heating effect of the long wavelength radiation must therefore be controlled to limit to a maximum of about 1000°C the quartz glass envelope 12 temperature in order to prevent failure of the quartz glass envelope 12 and the consequent oxidation of the hot tungsten filament 11 in air.

Towards this end, the metal interior surfaces of the cavity or recess 9 are covered with a reflecting structure 10 which is polished so as to make intimate contact therewith.

Referring now to Figure 3, the reflecting structure 10 in this embodiment consists of a polished water-free quartz glass facing sheet 7, typically 1.5mm thick, and a thin backing sheet 8 of reflecting material on the surface of the facing sheet 7 remote from the bulbs 5. The backing sheet 8 in this embodiment is a coating made of silver with a protective layer of copper on the surface of the silver remote from the facing sheet 7. It will be apparent to those skilled in the art, however, that the backing sheet can be just as easily be of, for example, aluminium or gold.

The polished water-free quartz glass facing sheet 7 of the reflecting structure 10 absorbs the radiation having wavelengths beyond about 3000 nanometres emitted from the hot walls of the quartz glass envelopes 12. Although the facing sheet 7 is heated in the process of absorption, this heat is conducted through the reflecting structure 10 to the water-cooled metal container 4 before any substantial re-radiation of wavelengths beyond about 3000 nanometres can occur.

In this way, the long wavelength radiation emitted by the quartz glass envelopes 12 which leads to catastrophic overheating of the quartz glass envelopes 12 is removed from the apparatus.

With regard to the radiation of wavelength less than about 3000 nanometres, this shorter wavelength radiation passes through the facing sheet 7 of the reflecting structure 10 without damaging it. This radiation is then reflected by the backing sheet 8 of the reflecting structure 10 and passes through the clear glass plate 6 to be absorbed by, and heat, the specimen.

When heated sufficiently, the specimen will also emit radiation at wavelengths beyond about 3000 nanometres. This long wavelength radiation, together with the long wavelength radiation emitted by the quartz glass envelopes 12 in the direction of the specimen, will be absorbed by and heat the clear glass plate 6. The clear glass plate 6 so heated will then in turn re-radiate over the entire cavity or recess 9, with a substantial proportion of the long wavelength radiation emitted in this way again being absorbed and dissipated by the reflecting structure 10 in the manner as hereinbefore described.

In an alternative embodiment of the invention hereinbefore described with reference to the accompanying drawings, the window behind which the radiant bulbs 5 of each furnace section 1 and 2 are mounted is comprised of a plurality of clear glass plates separated and linked by water-cooled metal elements or members. Preferably, the clear glass plates are of polished water-free quartz glass.

In this alternative embodiment, the exterior surfaces of the metal elements or members are covered with the previously described reflecting structure 10. Accordingly, a significant proportion of the radiation emitted by the heated specimen in the long wavelength range will be absorbed and dissipated by the reflecting structure covering the exterior surface of the metal elements or members facing the specimen.

In addition, a significant proportion of the long wavelength radiation emitted by the quartz glass envelopes in the direction of the specimen will be absorbed and dissipated by the reflecting structure covering the exterior surfaces of the metal elements or members facing the bulbs.

In consequence, heating of the glass plates in this alternative embodiment is reduced due to them not absorbing as much long wavelength radiation as the glass plates 6 in the embodiment described with reference to the drawings.

Furthermore, overheating of the glass plates in the alternative embodiment is prevented by the juxtaposition of the glass plates with the water-cooled metal elements or members. Towards this end, the edges of the glass plates comprising the windows of both the alternative embodiment and the embodiment described with reference to the drawings can be polished to ensure intimate contact with the juxtaposed water-cooled metal and thereby conduction of heat from the glass plates. An additional benefit of this practice is that it gives rise to the phenomena of Total Internal Reflection.

It will be appreciated by those skilled in the art that heating of the glass plates in the alternative embodiment will still be reduced even if the exterior surfaces of the metal elements or members are not covered with the aforesaid reflecting structure or only selected parts of the exterior surfaces of the metal elements or members are covered with the reflecting structure.

In a modification of the alternative embodiment of the invention, the need for use of the aforedescribed reflecting structure is obviated by so dimensioning the plurality of glass plates comprising each window that a high proportion of the long wavelength radiation emitted by the quartz glass envelopes and the specimen is absorbed by the glass plates and the heat generated thereby dissipated by conduction to the water-cooled metal elements or members.

The advantages of the present invention are that it not only features a means of making the surfaces around a radiation emitting element enclosed in an envelope reflecting but at the same time reduces the amount of reflected radiation whose wavelengths are damaging to the bulb envelope. This enables the temperature of each bulb envelope to be held well below its critical temperature thereby ensuring an adequate bulb life.

As an example, the present invention enables quartz- halogen bulbs to be used for long periods, in both air and vacuum, with such high power densities that specimen temperatures in excess of 2000°C can be sustained without the use of forced air cooling.

Claims

1. A radiation transmitting apparatus including at least one radiation emitting element enclosed in an envelope, the radiation emitting element emitting radiation including radiation in a first wavelength range, wherein the envelope absorbs and is heated by radiation in the first wavelength range and wherein the apparatus further includes means to absorb radiation in the first wavelength range.

2. Apparatus as claimed in claim 1 wherein the means to absorb radiation in the first wavelength range comprises a reflecting structure.

3. Apparatus as claimed in claim 2 wherein the reflecting structure comprises a facing sheet to absorb and be heated by radiation in the first wavelength range and a backing sheet in contact with a surface of the facing sheet remote from the envelope to reflect radiation other than that in the first wavelength range.

4. Apparatus as claimed in claim 3 wherein the backing sheet is a coating on the surface of the facing sheet remote from the envelope.

5. Apparatus as claimed in claim 3 or claim 4 wherein the facing sheet is of polished glass.

6. Apparatus as claimed in claim 5 wherein the glass is a polished quartz glass.

7. Apparatus as claimed in claim 5 wherein the glass is a polished water-free quartz glass.

8. Apparatus as claimed in any of claims 3 to 7 wherein the facing sheet is generally 1.5mm in thickness.

9. Apparatus as claimed in any of claims 3 to 8 wherein the backing sheet is of a highly reflective material.

10. Apparatus as claimed in any of claims 3 to 9 wherein the backing sheet is of silver with a protective layer of copper on a surface of the silver remote from the facing sheet.

11. Apparatus as claimed in any preceding claim wherein means are included to dissipate the heat generated in the means to absorb radiation in the first wavelength range.

12. Apparatus as claimed in claim 11 as appendent on any of claims 2 to 10 wherein the dissipating means comprises a base structure of heat conducting material in contact with the backing sheet of the reflecting structure.

13. Apparatus as claimed in claim 12 wherein means are included to cool the base structure.

14. Apparatus as claimed in any preceding claim wherein the envelope is of quartz glass.

15. Apparatus as claimed in any preceding claim wherein the apparatus is a radiant bulb furnace including at least one radiant bulb.

16. Apparatus as claimed in claim 15 wherein the furnace includes a cavity to receive the at least one bulb .

17. Apparatus as claimed in claim 16 wherein the means to absorb radiation in the first wavelength range covers at least part of the interior surfaces of the cavity.

18. Apparatus as claimed in claim 17 wherein the means to absorb radiation in the first wavelength range covering at least part of the interior surfaces of the cavity is the reflecting structure specified in any of claims 2 to 10.

19. Apparatus as claimed in claim 16 wherein the means to absorb radiation in the first wavelength range comprises a window between the at least one bulb and the position in the furnace where a specimen is to be located for heating, the window comprising a plurality of clear glass sheets separated and linked by metal elements, whereby means are provided to cool the metal elements so that the heat generated in the clear glass sheets is dissipated by conduction to the metal elements.

20. Apparatus as claimed in claim 17 and claim 18 wherein a window is located between the at least one bulb and the position in the furnace where a specimen is to be located for heating, the window comprising at least one clear glass sheet.

21. Apparatus as claimed in claim 19 wherein the at least one bulb is positioned on one side of the cavity and wherein at least one other bulb is positioned on another side of the cavity and wherein a further window is located between the at least one other bulb and the position in the furnace where the specimen is to be located for heating, the further window comprising a plurality of clear glass sheets separated and linked by metal elements, whereby means are provided to cool the metal elements of the further window so that the heat generated in the clear glass sheets of the further window is dissipated by conduction to the metal elements.

22. Apparatus as claimed in claim 20 wherein the at least one bulb is positioned on one side of the cavity and at least one other bulb is positioned on another side of the cavity and wherein a further window is located between the at least one other bulb and the position in the furnace where a specimen is to be located for heating, the further window comprising at least one clear glass sheet.

23. Apparatus as claimed in claim 20 or claim 22 wherein the windows comprise more than one clear glass sheet and wherein the clear glass sheets of each window are separated and linked by metal elements and further wherein means are provided to cool the metal elements of each window.

24. Apparatus as claimed in any of claims 19, 21 or 23 wherein at least a part of the exterior surfaces of the metal elements are covered with the reflecting structure specified in any of claims 2 to 10.

25. Apparatus as claimed in any of claims 19 to 24 wherein the clear glass sheets are of polished quartz glass.

26. Apparatus as claimed in claim 25 wherein the clear glass sheets are of polished water-free quartz glass.

27. Apparatus as claimed in any of claims 15 to 26 wherein the or each bulb is a quartz-halogen bulb.

28. A reflecting structure to absorb radiation in a first wavelength range.

29. A structure as claimed in claim 28 comprising a facing sheet of material to absorb radiation in the first wavelength range and a backing sheet of reflecting material in contact with a surface of the facing sheet to reflect radiation other than that in the first wavelength range.

30. A structure as claimed in claim 29 wherein the backing sheet is a coating.

31. A structure as claimed in claim 29 or claim 30 wherein the facing sheet is of polished water-free quartz glass.

32. A structure as claimed in any of claims 29 to 31 wherein the backing sheet is of silver with a protective layer of copper on a surface of the silver remote from the facing sheet.

33. A radiation transmitting apparatus substantially as hereinbefore described with reference to Figures l, 2 and 3.

34. A reflecting structure substantially as hereinbefore described with reference to Figure 3.