GB1599452A - Infra-red heating device - Google Patents

Description

(54) INFRA-RED HEATING DEVICE (71) We, THORN EMI LIMITED (FORMERLY THORN ELECTRICAL INDUSTRIES LIMITED,) a British Company, of Thorn House, Upper Saint Martin's Lane, London, WC2H 9ED, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to an infrared heating device.

In conventional infra-red heating an electrically heated resistance wire is used as a source on infra-red radiation. However, the relatively large mass of the wire and its refractory supports results in an appreciable warming-up and cooling-down time. This is inconvenient and can cause damage to continuously moving material being heated if the equipment is stopped with the material still exposed to the radiation emitted as the heater cools down. In an alternative arrangement the source of radiation is a tungsten or tungsten halogen filament lamp which has a much lower thermal mass and so reduce the warming-up and cooling-down time. This, however, only gives short wave infra-red radiation, since the fused silica envelope absorbs radiation beyond 4.5 to 5 microns and this limits the range of radiation available.

According to the present invention there is provided an infra-red heating device comprising a tubular, fused silica envelope encasing an elongated, incandescent filament, the combined emission and absorption characteristics of the filament and the envelope respectively being such as to cause the wavelengths of the energy emergent from the envelope to be restricted to a certain range, and wherein part only of the outer surface of said envelope, said part consisting of one or more complete, circumferentially extending bands, is rendered absorbent of energy having wavelengths in jid range, said bands being such as to respond to the absorption of said energy by emitting energy including wavelengths longer than those included in said range.

Preferably the absorbent part comprises a plurality of bands spaced along the length of the envelope.

Advantageously, the envelope is mounted with the filament at the principal focus of a reflector arranged to reflect substantially all the radiation onto a region to be heated.

This reflector may e a tubular reflector having an ellipsoidal cross-section, the filament being along one focus of the ellipes and the region to be heated along the other focus of the ellipse.

An infra-red heating device constructed according to the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a graph of energy against wavelength showing radiation emission waves at 26000K, 1600"K and 600 K and the transmission curve for fused silica; Figure 2 is a graph similar to Figure 1 showing a polymer absorption curve; Figure 3 is a diagrammatic perspective view of a device for heating a plastics fibre; and Figure 4 is a side elevation of a lamp for use in-the device of Figure 3.

Referring to Figure 1, the filament of a conventional tungsten-halogen lamp operates at approximately 2600"K and, as is shown, radiates most of its energy in the near infra-red (less than 3 microns wavelength) and visible spectrum. However, a siginficant proportion of the energy is radiated at longer wavelengths. In a tungsten-halogen lamp the envelope is made of fused silica which has a characteristic transmission curve as shown dotted in Figure 1. This begins to cut off radiation at about 3-5 microns and has total absorption of radiation beyond 4.5 to 5 microns. Thus, longer wavelength infra-red radiation is not available for heating using this lamp.

If radiation emitted by the filament is absorbed by a screen after passing through the fused silica envelope, this screen will be heated and will re-radiate energy in a spectrum determined by the screen temperature.

Figure 1 shows the energy spectrum curve for a screen heated to 600"K and it will be seen that a considerable proportion of the energy is emitted at wavelengths beyond 4.5 to S microns.

Referring to Figures 3 and 4, the heating device comprises a linear tungsten-halogen lamp 1 disposed in an ellipsoidal reflector 2.

The lamp is arranged with its filament 3 along one focus of the reflector and a polymer fibre 4 to be heated is arranged to move along the other focus of the reflector.

Thus the radiation emitted perpendicularly to the filament 3 is reflected by the reflector and focused onto the fibre 4.

The lamp 1 is generally of conventional construction having a fused silica envelope S and axially positioned filament 3. It differs from a conventional tungsten-halogen lamp in having a black coating 6 on the outside of the silica envelope 5. The coating 6 is in the form of annular bands around the envelope and is made of a black frit or glass which bonds to fused silica and is opaque, i.e.

absorbs substantially all the radiation incident on it from the filament 3.

In use, the filament 3 emits radiation in a curve generally corresponding to a black body radiation curve at approximately 2600ack. The visible radiation and infra-red radiation of wavelength below about 3.5 microns passes through the uncoated parts of the silica envelope without appreciable absorption. Radiation of wavelength beyond 4.5 to S microns is absorbed by the silica enveloPe which is heated by the absorbed radiation.The coated annular bands 6 absorb visible and short-wave infra-red radiation transmitted through the silica beneath the annular bands giving a further heating effect, which raises the envelope temperature to approximately 600 K. This black coating will then reradiate in a curve generally corresponding to a black body radiation curve at approximately 600"K which gives infra-red radiation beyond 4.5 to S microns extending up to and beyond 10 microns.

The two bands of radiation from the lamp filament and black coating respectively are reflected onto the fibre 4 by the reflector 2.

A typical absorption characteristic for a polymer fibre is shown in Figure 2. This extends from about 1 micron up to 15 microns with strong absorption bands correspond to the radiation bands from the lamp, which gives more efficient heating than for an uncoated lamp. Typically the lamp power can be reduced to one-third of that for an uncoated lamp and give the same heat transfer.

The relative amounts of radiation emitted in the two bands can be selected by choosing the relative areas of the coated and uncoated parts of the lamp envelope, and can be this matched to the relative absorptivities of the absorption bands of the material to be heated.

The wavelength of the peak on the radiation curve emitted by the coating is determined by the coating temperature, which can be varied to some extent by a suitable choice of the dimensions and design of the lamp. Thus the radiation band can be selected to match a corresponding absorb tion band of the material to be heated.

In an alternative arrangement more than one lamp may be used in a suitably shaped reflector to direct radiation onto a material.

The lamps need not all be the same.

The heating method can be used to heat other materials where an extended range of infra-red radiation is advantageous. In particular, it could be used in ink drying apparatus where the ink base material has an absorption band in the longer wave region The lamp or lamps can also be gas-filled filament lamps other than tungsten-halogen lamps, provided they radiate substantially in the infra-red region.

WHAT WE CLAIM IS: 1. An infra-red heating device comprising a tubular, fused silica envelope encasing an elongated, incandescent filament, the combined emission and absorption characteristics of the filament and the envelope respectively being such as to cause the wavelengths of the energy emergent from the envelope to be restncted to a certain range, and wherein part only of the outer surface of the envelope, said part consisting of one or more complete, circumferentially extending bands, is rendered absorbent of energy having wavelengths in said range, said bands being such as to respond to the absorption of said energy by emitting energy including wavelengths longer than those included in said range.

2. A heating device according to Claim 1, wherein the said envelope part comprises a plurality of bands spaced along the length owt the envelope.

3. A heating device according to any preceding claim, wherein the filament is made of tungsten and the envelope has a filling which in use maintains a tungstenhalogen regenerative cycle in the envelope.

4. A heating device according to any preceding claim, wherein the envelope is mounted with the filament at the principal focus of a reflector arranged to reflect substantially all the radiation onto a region to be heated.

5. A heating device according to Claim 4, wherein the reflector is a tubular reflector having an ellipsoidal cross-section, the filament is along one focus of the ellipse and the region to be heated is along the other focus of the ellipse.

6. A heating device substantially as

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. for a screen heated to 600"K and it will be seen that a considerable proportion of the energy is emitted at wavelengths beyond 4.5 to S microns. Referring to Figures 3 and 4, the heating device comprises a linear tungsten-halogen lamp 1 disposed in an ellipsoidal reflector 2. The lamp is arranged with its filament 3 along one focus of the reflector and a polymer fibre 4 to be heated is arranged to move along the other focus of the reflector. Thus the radiation emitted perpendicularly to the filament 3 is reflected by the reflector and focused onto the fibre 4. The lamp 1 is generally of conventional construction having a fused silica envelope S and axially positioned filament 3. It differs from a conventional tungsten-halogen lamp in having a black coating 6 on the outside of the silica envelope 5. The coating 6 is in the form of annular bands around the envelope and is made of a black frit or glass which bonds to fused silica and is opaque, i.e. absorbs substantially all the radiation incident on it from the filament 3. In use, the filament 3 emits radiation in a curve generally corresponding to a black body radiation curve at approximately 2600ack. The visible radiation and infra-red radiation of wavelength below about 3.5 microns passes through the uncoated parts of the silica envelope without appreciable absorption. Radiation of wavelength beyond 4.5 to S microns is absorbed by the silica enveloPe which is heated by the absorbed radiation.The coated annular bands 6 absorb visible and short-wave infra-red radiation transmitted through the silica beneath the annular bands giving a further heating effect, which raises the envelope temperature to approximately 600 K. This black coating will then reradiate in a curve generally corresponding to a black body radiation curve at approximately 600"K which gives infra-red radiation beyond 4.5 to S microns extending up to and beyond 10 microns. The two bands of radiation from the lamp filament and black coating respectively are reflected onto the fibre 4 by the reflector 2. A typical absorption characteristic for a polymer fibre is shown in Figure 2. This extends from about 1 micron up to 15 microns with strong absorption bands correspond to the radiation bands from the lamp, which gives more efficient heating than for an uncoated lamp. Typically the lamp power can be reduced to one-third of that for an uncoated lamp and give the same heat transfer. The relative amounts of radiation emitted in the two bands can be selected by choosing the relative areas of the coated and uncoated parts of the lamp envelope, and can be this matched to the relative absorptivities of the absorption bands of the material to be heated. The wavelength of the peak on the radiation curve emitted by the coating is determined by the coating temperature, which can be varied to some extent by a suitable choice of the dimensions and design of the lamp. Thus the radiation band can be selected to match a corresponding absorb tion band of the material to be heated. In an alternative arrangement more than one lamp may be used in a suitably shaped reflector to direct radiation onto a material. The lamps need not all be the same. The heating method can be used to heat other materials where an extended range of infra-red radiation is advantageous. In particular, it could be used in ink drying apparatus where the ink base material has an absorption band in the longer wave region The lamp or lamps can also be gas-filled filament lamps other than tungsten-halogen lamps, provided they radiate substantially in the infra-red region. WHAT WE CLAIM IS:

1. An infra-red heating device comprising a tubular, fused silica envelope encasing an elongated, incandescent filament, the combined emission and absorption characteristics of the filament and the envelope respectively being such as to cause the wavelengths of the energy emergent from the envelope to be restncted to a certain range, and wherein part only of the outer surface of the envelope, said part consisting of one or more complete, circumferentially extending bands, is rendered absorbent of energy having wavelengths in said range, said bands being such as to respond to the absorption of said energy by emitting energy including wavelengths longer than those included in said range.

2. A heating device according to Claim 1, wherein the said envelope part comprises a plurality of bands spaced along the length owt the envelope.

3. A heating device according to any preceding claim, wherein the filament is made of tungsten and the envelope has a filling which in use maintains a tungstenhalogen regenerative cycle in the envelope.

4. A heating device according to any preceding claim, wherein the envelope is mounted with the filament at the principal focus of a reflector arranged to reflect substantially all the radiation onto a region to be heated.

5. A heating device according to Claim 4, wherein the reflector is a tubular reflector having an ellipsoidal cross-section, the filament is along one focus of the ellipse and the region to be heated is along the other focus of the ellipse.

6. A heating device substantially as

herein described with reference to and as illustrated by the accompanying drawings.