BACKGROUND ART In industrial furnaces working at high temperatures of several hundred degree C (e. g. melting furnaces, holding furnaces, induction furnaces for the heating or melting of metals or hot-air ovens e. g. for the heating and containing of materials subject to chemical reactions, etc. ), the state of the linings containing the material to be heated or melted or subject to chemical reactions, etc. are subject to rough conditions regarding temperature, temperature gradients, pressure, aggressive chemicals, etc. leading to wear and risk of failure. Consequently, there is a need for monitoring of vital parts of a furnace, in particular such parts which are affected e. g. by heat, the heated or melted material, or for an induction furnace by heat from the induction coil itself.

In an induction furnace, induction coils are used for inductive heating and melting or stirring of electrically conductive material by means of electric currents generated in the material by the electromagnetic fields from the coil.

The coils typically comprise several mutually electrically insulated turns. The

temperature stability of the insulating material is typically considerably lower than that of the electrical conductor of the coil. A fault in the insulation due to overheating may therefore result in a short of neighbouring turns of the coil or a short to ground with considerable security problems such as danger to the operating staff due to voltage carrying housing parts or a fire released due to the overheating or steam'explosions'in case of thermal overheating of water-cooled induction coils.

A thermal overheating of the insulation may occur 1) due to an insufficient disposal of the heat generated in the coil due to current losses as well as 2) an insufficient thermal insulation against outer sources of heat located in the neighbourhood of the coil. A source of heat may e. g. be the crucible of an induction furnace comprising a melted material whose refractory lining is abraded or locally faulty or which for other reasons is insufficiently insulated.

Specifically in induction melting furnaces, the ceramic lining is subject to severe thermal, chemical and mechanical stresses during operation. These stresses lead to abrasion of the lining, i. e. to a decreased strength of the wall, so that at regular intervals (e. g. six to eight weeks), the used lining must be removed and a new lining inserted. If this is not done in time, damage to the induction coil may result.

Measurement procedures for evaluating the physical condition or state of preservation of the lining are known. One temperature measurement system uses a magnetic contact thermometer placed on the outer cover. Another temperature measurement method is based on pyrometric principles.

Another measurement method is described in EP-B1-519231. This principle is based on the determination of the conductivity of the refractory lining. For this purpose, a net of electrodes is applied by means of which abrasion associated change of conductivity of the ceramic material can be measured.

The net provides a surface picture of the conductivity of the lining based on which conclusions concerning the temperature conditions may be made.

An electric system has the disadvantage of being sensitive to electro- magnetic noise. The conductivity measurement system is strongly dependent

on materials, which necessitates costly calibration. The pyrometric system has the disadvantage that the point determination of temperature is possible only from the outside and only appropriate for the determination of the surface temperature of the areas considered. It is not possible directly to measure temperature conditions inside the refractory lining.

DD-A-240 947 discloses a system for monitoring melting furnaces by means of optical waveguides mounted on or near the outer surface of the vessel holding the melt. The system aims at detecting wear and weak spots in the walls of the vessel. A change in the wall thickness due to spotty wear results in an increased temperature at the location of the optical waveguide located near the weak spot, which again results in an increased intensity of light radiated by the optical waveguide. This increased intensity is detected by an optoelectronic receiver. If a net of crossing waveguides are mounted and each waveguide end provided with a photo-detector, an indication of the location of a weak spot may be derived by decoding the combination of photo-detectors that are activated. A disadvantage of the system is that an array of photo-detectors are needed to get a precise location of the weak spot US-5,356, 220 discloses a method and an apparatus for monitoring the temperature of a blast furnace by laying an optical fibre on a surface of the iron skin of a hot-air oven. A temperature distribution of the optical fibre is measured by an apparatus based on the intensity of Raman back scattering.

A disadvantage of this system is that only the temperature of the outer surface of the furnace, here an iron shell, is monitored. This makes it difficult to predict the condition of e. g. the lining, and to protect essential parts of the furnace.

DISCLOSURE OF INVENTION In an aspect, it is the object of the present invention to provide a furnace for which the condition of its vital parts can be monitored.

It is a further object of the present invention to provide a furnace for which the condition of its vital parts can be monitored for fast temperature changes, caused e. g. by short circuits, damages of coil insulation and accidental loss coil cooling, hot spots or cracks of refractory lining.

It is a further object of the present invention to provide a furnace for which the condition of its vital parts can be monitored for long term changes, caused e. g. by material fatigue or corrosion.

It is a further object of the present invention to provide a furnace for which the condition of its vital parts can be monitored without being dependent on material properties of the vital parts.

It is a further object of the present invention to provide a furnace for which the condition of its vital parts can be monitored in presence of an electro- magnetic field.

It is a further object of the present invention to provide a furnace for which the condition of its vital parts can be monitored with high spatial accuracy.

Further, objects appear elsewhere in the description.

In an aspect the present invention fulfils this object by providing a furnace, the furnace comprising: (a) a heat source; (b) a container containing said heat source, said container comprising a refractory lining comprising a first refractory lining layer adjacent to said heat source, and a container wall comprising at least one intermediate layer and an outer wall having an outer surface distal from said heat source, said heat source providing a temperature profile in said refractory lining and in said container wall ; and

(c) at least one optical waveguide adapted for monitoring said temperature profile by optical back scattering measurement; said at least one optical waveguide being located between said first refractory lining layer and said outer surface of said outer wall ; whereby vital parts of the furnace, e. g. the refractory lining, intermediate layers, such as permanent linings, and the outer wall, optionally comprising an induction coil for an induction furnace, can be monitored.

Also, the location of the at least one optical waveguide close to the vital part of the furnace allows for a higher sensitivity to temperature changes thereby ensuring that fast temperature changes, caused e. g. by short circuits, damages of coil insulation and accidental loss coil cooling, hot spots or cracks of refractory lining can be obtained.

Also, the location of the at least one optical waveguide close to the vital part of the furnace allows sensitive detection of slow processes such as corrosion whereby long term changes can be monitored.

Further, the use of optical back scatter measurements it is obtained that it is the condition of vital parts can be monitored without being dependent on material properties of the vital parts.

Also, the use of an optical waveguide ensures that the condition of vital parts can be monitored in presence of an electromagnetic field.

Further, the location of the optical waveguide close to the vital part of the furnace allows several optical waveguides to be located near the vital part whereby a high spatial accuracy can be obtained and e. g. hot spots can be accurately determined.

Additionally, a number of advantages can be obtained, including: 1. Improved determination of the maximum operational time (or time to failure) of the monitored parts (e. g. coil and/or lining of an induction furnace)

leading to improved utilization of the parts and increased operational time of the induction furnace, leading to decreasing production costs.

2. Improved security by accurate localization of weak hot spots, the computer assisted processing of the data captured by the system making possible process visualization and various control and alarm functions.

3. Faster start-up after the replacement of components (e. g. the lining or a coil or parts thereof) because time consuming and costly calibrations are unnecessary compared to the protection system based on conductivity measurements.

In an embodiment, the refractory lining is made of a heat-resistant material such as e. g. a ceramic material, e. g. an oxidized ceramic material such as SiO2/AI203/MgO, etc.

In a preferred embodiment, said outer wall comprises an induction coil whereby monitoring of induction coil in induction furnaces can be obtained.

In an embodiment, an optical fibre is mounted directly on the surface of the coil by fixing it with fibreglass-tape wound around the conductor. In an embodiment of the invention, the induction coil is further provided with a high temperature varnish. In another embodiment of the invention, an optical fibre is inserted in a steel tube insulated by a polymer-sheath, preferably a fluoropolymer. In a further embodiment, an insulated steel tube with the fibre is fixed to the induction coil fibreglass-tape wound around the conductor.

In a preferred embodiment, said at least one optical waveguide is located in a refractory lining layer whereby it is obtained that the optical fibre can easily be exchanged when the refractory lining is being replaced. Further only one back scattering measurement equipment is necessary contrary to prior art techniques which needs several (adjusted) detectors. Also, easy installation, commissioning, maintenance etc. can be obtained.

In a preferred embodiment, said at least one optical waveguide is located in an intermediate layer whereby it is obtained that the optical fibre can be maintained while the refractory lining is being replaced thereby providing low operation cost, easy handling, appropriate response time to hot spots coming from a weak refractory lining and/or the induction coil. Also, an easy check of the condition of specific elements of the furnace, e. g. the top or bottom of the furnace (taken in an axial direction of the furnace), or e. g. the protective layers of the refractory lining or the insulation layer of the induction coil can be obtained.

In another preferred embodiment, said at least one optical waveguide is located in the outer wall whereby it is obtained that a risk of heat damage of critical parts in the outer wall, e. g. the coils of an induction furnace, can be monitored thereby providing low operation cost, easy handling, appropriate response time to hot spots coming from a weak refractory lining and/or the induction coil. Also, temperature profiles close to the coil at least on the surface of the coil can be monitored when using optical waveguides which are insensitive to electromagnetic fields.

In another preferred embodiment, said at least one optical waveguide is located in a combination of said refractory lining layer, said intermediate layer, and said outer wall whereby it is obtained that more critical parts of the furnace can be monitored simultaneously thereby allowing simultaneous measurements of temperature profiles at several different locations of the furnace in a radial direction from the centre axis of the furnace and thereby providing a picture of the thermal gradients through the wall and/or in an axial direction of the furnace. Also, different locations in different axial locations (or heights of the furnace, e. g. the bottom and top) along the direction of the centre axis can be monitored.

In another preferred embodiment, said optical waveguide is located inside an insulation surrounding the induction coil whereby a prolonged life time of the optical waveguides (compared to a solution close to the crucible) is obtained.

In another preferred embodiment, the optical waveguide is located inside an insulation surrounding individual turns of the induction coil whereby it is

obtained that a heating of the electrical conductor will be imparted to the optical waveguide very fast and thus permits a short forecast lead time for corrective actions.

In another preferred embodiment, said at least one optical waveguide consists of a net whereby improved resolution can be obtained.

In a preferred embodiment, more optical fibre nets are located in different layers whereby a combined evaluation of the temperature profiles of more critical parts of a furnace (e. g. the lining and an induction coil of an induction furnace) can be monitored simultaneously in three dimensions, i. e. infor- mation about both vertical and horizontal coordinates, and better spatial resolution in 3 dimensions can be achieved.

In another preferred embodiment, said at least one optical waveguide is arranged in at least one meander, or arranged in as least one helix, whereby an accurate monitoring can be obtained. In case the optical waveguide, e. g. an optical fibre, is enclosed in a metallic jacket, e. g. a tube, the spiral shape of the optical waveguide results in induced eddy currents inside the metallic jacket thereby leading to"self-heating"and consequently misleading tem- perature profile measurements. To avoid or reduced induced eddy currents the optical waveguide (and the metal jacket) should be arranged ortho- gonally to the inductive fields as in the case of the meander shaped arrangement.

In another preferred embodiment, said intermediate layer consists of a flexible mat made of temperature resistant material whereby a thermal and electrical insulation and protective effect of the intermediate layers, e. g. a permanent layer and induction coil can be obtained. Further, this allows easy maintenance as the mat can be a prefabricated element which is easy to install.

In a preferred embodiment, the material of the mat is adapted to have a low friction to adjacent layers whereby an easier and smoother removal and/or insertion of the mat adjacent to e. g. a refractory lining can be obtained. Also, this allows easy maintenance.

In another preferred embodiment, at least a part of said at least one optical waveguide is embedded in a mat in at least one of said locations whereby easy mounting and dismounting and low friction effect to adjacent layers can be obtained.

In a preferred embodiment relating to induction furnaces, said induction coil comprises at least one turn arranged around at least a part of said container.

In a preferred embodiment, said optical waveguide consists of an optical fibre.

Suitable optical fibres are known in the art. In an embodiment, the optical fibre is a polymer coated quartz fibres, 62.5 core diameter, large core because we need high energy density inside the fibre, polyimid coating, Al, Au (e. g. Pyrocoat high temperature fibre, PCU/MDU062H from Spectran, Connecticut, USA).

In an embodiment, the furnace comprises more than one optical waveguide, each having an end being adapted for being coupled to the optical measurement system, e. g. via an optical multiplexer. One or more optical waveguides, e. g. optical fibres, may be arranged in a layer in the form of a network. The network may e. g. take the form of one or more meanders.

Alternatively one or more optical fibres may be formed as one or more helixes. Alternatively, several crossing networks may be arranged.

In a preferred embodiment, said optical fibre is provided with a jacket, or a sheath whereby protection regarding mechanical stress and heat of said optical fibre is obtained, e. g. also when incorporated in a mat.

In an embodiment, the at least one optical waveguide is embedded in a waveguide-hosting layer comprising a heat resistant filler such as a heat resistant concrete whereby the waveguide, and optionally its covering jacket, is protected and fixed.

In a preferred embodiment, said optical back scattering measurement comprises Raman back scattering measurements based on the principle of optical time domain reflectometry, or on optical frequency domain reflecto- metry whereby a distributed measurement of temperature, a temperature profile is obtained.

Suitable Raman back scattering measurement equipment is known in the art.

In an embodiment, an appropriate OFDR-measuring equipment could e. g. be a DTS-system (DTS=distributed temperature sensing) such as a controller OTS 20P from LIOS Technology, Kiln, Germany.

In a preferred embodiment, said at least two optical waveguides are serially coupled whereby different parts-e. g. the upper part and the lower parts of the furnaces-could be monitored simultaneously with the same back scatter measuring equipment is obtained.

In a preferred embodiment, said optical waveguides are serially coupled whereby temperature profiles for several furnaces may be obtained with one back scattering and evaluation unit.

In another aspect, the object of the present invention is fulfilled by providing a mat, the mat comprising said at least one optical waveguide adapted for monitoring a temperature profile by optical back scattering measurement, said at least one optical waveguide consisting of a net, and being adapted to be located between a first refractory lining layer and the outer surface of the outer wall of a furnace as defined for the furnace according to the invention whereby easy mounting and dismounting and low friction effect to adjacent layers can be obtained.

The mat or waveguide-hosting layer has-besides its measuring function- further advantageous functions, in particular because of its location between lining and outer furnace wall : Buffering of the increased heat load due to heat induced wear of the refractory lining to minimize damage of the heating element (e. g. the electrical insulation of an induction coil) ;

An additional electrical insulation layer between an induction coil and a melt, and Easier removal of the lining after abrasion because the mica-mat works as a"sliding-layer" In still another aspect, the object of the present invention is fulfilled by providing a method of thermally monitoring the condition of its vital parts of a furnace by measuring a temperature profile thereof, the method comprising (a) providing a heat source; (b) providing a container containing said heat source, said container comprising a refractory lining comprising a first refractory lining layer adjacent to said heat source, and a container wall comprising at least one intermediate layer and an outer wall having an outer surface distal from said heat source, said heat source providing a temperature profile in said refractory lining and in said container wall ; and (c) providing at least one optical waveguide adapted for monitoring said temperature profile by optical back scattering measurement; said at least one optical waveguide being located between said first refractory lining layer and said outer surface of said outer wall. d) coupling said at least one optical waveguide to an optical back scattering measurement equipment; e) measuring a temperature distribution along said at least one optical waveguide by an optical back scattering measurement; f) determining the temperature profile for the vital parts based on said measured temperature distribution; and g) assessing the condition of the vital parts based on said determined temperature profile.

In a preferred embodiment, said optical back scattering measurement equipment is adapted to perform Raman back scattering measurements based on the principle of optical time domain reflectometry, or optical frequency domain reflectometry.

In another preferred embodiment, a correlation between the length coordinates of said at least one optical waveguide on its path in the furnace and the corresponding polar spatial coordinates is established.

In a preferred embodiment, the method further comprises determining a first temperature profile to be recorded as a reference profile.

In a preferred embodiment, the method further comprises comparing the current temperature profile and the reference profile and provide a trigger signal when a critical difference occurs.

In still another aspect, the object of the present invention is fulfilled by providing a system of thermally monitoring the condition of its vital parts of a furnace by measuring a temperature profile thereof, the system comprising (a) a heat source; (b) a container containing said heat source, said container comprising a refractory lining comprising a first refractory lining layer adjacent to said heat source, and a container wall comprising at least one intermediate layer and an outer wall having an outer surface distal from said heat source, said heat source providing a temperature profile in said refractory lining and in said container wall ; (c) at least one optical waveguide adapted for monitoring said temperature profile by optical back scattering measurement; said at least one optical waveguide being located between said first refractory lining layer and said outer surface of said outer wall ;

d) an optical back scattering measurement equipment coupled to said at least one optical waveguide, said optical back scattering measurement equipment being adapted to measure a temperature distribution along said at least one optical waveguide; e) said optical back scattering measurement equipment further being adapted to determine the temperature profile for the vital parts based on said measured temperature distribution; and f) a display for displaying the condition of the vital parts based on said determined temperature profile.

In a preferred embodiment, the optical measurement equipment is adapted to perform Raman back scattering measurements based on the principle of optical time domain reflectometry or optical frequency domain reflectometry.

In another preferred embodiment, the at least a part of the length of said at least one optical waveguide is adapted to be mounted in physical contact with and along the periphery of turns of an induction coil.

In a further aspect, the present invention provides a method of monitoring the state of preservation of a refractory lining of melting furnaces comprising a temperature sensor arranged in the furnace wherein at least one optical fibre is arranged between the wall and the lining of the furnace and where temperatures are detected by a fibre-optic back scattering measuring system.

In a preferred embodiment, the temperature sensor is at least one optical fibre, the optical fibre functioning as a sensor and where the temperatures are spatially distributed over the optical fibre.

In a further preferred embodiment, the spatially distributed temperatures are detected by optical time domain reflectometry or optical frequency domain reflectometry.

In a preferred embodiment, the detected temperatures in an electronic evaluation system are put together to a spatial or time based temperature profile for the lining.

In a preferred embodiment, the method further comprises a) after a replacement of the lining a first temperature profile is recorded as a reference profile from the spatially distributed temperatures; b) specific differences in the thermal parameters from those of the reference profile are defined as critical; c) during the operation of the furnace a comparison between the current temperature profile and the reference profile is performed; and d) a warning and/or a switch operation is triggered by the occurrence of a critical difference.

In a further aspect, the present invention provides a system for monitoring the condition of a refractory lining of melting furnaces comprising a temperature sensor arranged in the furnace wherein an optical fibre functioning as a sensor is arranged in a layer between the wall and the lining of the furnace and where the optical fibre is coupled to a fibre-optic back scattering measuring system.

In a preferred embodiment, the temperature sensor is at least one optical fibre, the optical fibre functioning as a sensor and where the temperatures detected by the fibre-optic back scattering measuring system are spatially distributed over the optical fibre.

In a preferred embodiment, the measuring system is based on optical time domain reflectometry or optical frequency domain reflectometry.

In another preferred embodiment, at least one optical fibre is laid out in a layer in the form of a net.

In a preferred embodiment, the net is formed as at least one meander or at least one helix from the at least one optical fibre.

In a preferred embodiment, the optical fibre is provided with a jacket or sheath.

In a preferred embodiment, the layer is formed as flexible mat made of temperature resistant material.

In a preferred embodiment, the material of the mat comprises a low friction material.

In a further aspect, the present invention provides a system for monitoring the temperature of induction coils wherein an optical waveguide is arranged in spatial vicinity of the induction coil so that the temperature of the induction coil or the temperature of a heat source located in the vicinity of the induction coil is transferred to the optical waveguide and that an optical measurement system is connected to the optical waveguide for detecting the heating of the optical waveguide is provided.

In a preferred embodiment, at least one optical waveguide is located relative to the part of the furnace that is to be monitored in such a way that the temperature distribution of the optical waveguide enables conclusions regarding the temperature distribution of the induction coil and/or of a heat source located in the vicinity of the induction coil to be drawn.

In another further aspect, the present invention provides a measuring system for detecting the heat distribution of the optical waveguide and connected to the optical waveguide is provided and wherein the optical waveguide itself is the temperature sensing element.

In a preferred embodiment, the spatially distributed temperatures are detected by optical time domain reflectometry or optical frequency domain reflectometry.

In another preferred embodiment, the induction coil comprises a copper profile.

In another preferred embodiment, the induction coil comprises a bundle of copper wires.

In another preferred embodiment, the optical waveguide is located inside an insulation surrounding the induction coil.

In another preferred embodiment, the optical waveguide is located inside an insulation surrounding individual turns of the induction coil.

In another preferred embodiment, the optical waveguide is located along the periphery of the turns of the induction coil.

In another preferred embodiment, the location of the optical waveguide is displaced in a direction of an external heat source with respect to the location of the induction coil.

In another preferred embodiment, the optical waveguide is formed as mesh or net.

In another further aspect, the present invention provides use of a monitoring system according to the invention for monitoring an inductively heated crucible.

In another further aspect, the present invention provides use of a system according to the invention for monitoring the condition of the refractory lining of an induction furnace.

BRIEF DESCRIPTION OF DRAWINGS The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

FIG. 1 shows a typical profile of a turn of an induction coil equipped with an optical waveguide, FIG. 1. a being a view along the direction of the current flow and FIG. 1. b being a cross sectional view taken along line AA', FIG. 2 shows a profile of a turn of an induction coil equipped with an optical waveguide and an electrically insulating bandage, FIG. 2. a being a view along the direction of the current flow and FIG. 2. b being a cross sectional view taken along line AA', FIG. 3 shows a partial perspective view of an induction coil with a number of turns equipped with a meander formed optical waveguide positioned on the interior side of the coil, FIG. 4 shows a partial perspective view of an induction furnace according to the invention with an induction coil with a number of turns equipped with a meander formed optical waveguide positioned on the interior side of the coil between the coil and heating container of an induction furnace, FIG. 5 shows an example of the structure of a wall of an induction furnace according to the invention, FIG. 6 shows a flexible mat for a furnace, the mat containing an optical waveguide connected (schematically) to an optical back scattering measuring system for determining a temperature profile of a part of the waveguide, FIG. 7 shows an induction melting furnace according to the invention with two optical waveguide nets for monitoring the lining and the induction coil, respectively, and FIG. 8 shows an example of measuring equipment for optical back scattering for determining a temperature profile of an induction furnace.

The figures are schematic and simplified for clarity, and they just show details which are essential to the understanding of the invention, while other

details are left out. Throughout, the same reference numerals are used for identical or corresponding parts.

MODE (S) FOR CARRYING OUT THE INVENTION FIG. 1 shows a typical profile of a turn of an induction coil equipped with an optical waveguide, here an optical fibre embedded in the outer wall (not shown), FIG. 1. a being a view along the direction of the current flow and FIG.

1. b being a cross sectional view taken along line AA'.

FIG. 1 shows an optical fibre 2 mounted on a turn 1 of an induction coil. The coil is preferably consists of a metal tube, e. g. made of Cu or another metal or alloy with an electrical conductivity sufficient to keep current losses low.

The tube may e. g. host a fluid coolant (e. g. water) for cooling the coil. The optical fibre is located on the outer surface of the turns of the coil in thermal contact with the coil.

Fig. 2 is identical to FIG. 1 except that a sheet 3 of an electrically insulating material (e. g. a fibreglass-tape) is wound around the turn 1 of a coil, including the optical fibre 2 thereby locating the optical fibre inside an insulation surrounding individual turns of the induction coil. The bandage 3 of insulating material is shown partially open, indicating a possible outtake of the optical fibre 2 (e. g. for connection to an optical back scattering system or for being relocated to another part of the furnace). Apart from electrical insulation of the coil (i. e. insulation of the turns from each other and insulation of the coil from the surroundings), the bandage has the purpose of fixing the optical fibre to the coil and to protect the fibre mechanically. The process of arranging the electrically insulating bandage on the induction coil is preferably combined with the provision of a means for protecting the coil with a high-temperature stable insulating sheath or layer.

FIG. 3 shows a partial perspective view of an induction coil with a number of turns equipped with a meander formed optical waveguide positioned on the interior side of the coil, the detailed form and distance between individual

'branches'of the meander contributing to providing an adjustable spatial resolution of the temperature measurement.

The induction coil 4 comprises a number of turns. The cross sectional profile 7 of each turn made of a hollow Cu-tube is illustrated. An optical waveguide 5 is shown in a meander structure positioned a distance s from the induction coil (the distance s preferably being taken in a direction of the axis of the coil (cf. 301 in FIG. 7) ). The net constituted by the meander formed optical waveguide covers the entire inner surface of the coil (at a distance s from the coil following a direction from the coil periphery towards the axis of the coil).

This allows a temperature profile for the coil to be monitored which profile is induced mainly by the heating source (cf. 80 in FIG. 5). The waveguide may e. g. be incorporated in a layer hosting the coil (cf. e. g. W1 in FIG. 5) or in a separate layer (e. g. W2 or W3 in FIG. 5) thereby allowing the waveguide to be located any convenient distance s from the coil.

FIG. 4 is identical to FIG. 3, except that FIG. 4 additionally indicates the outer surface 6 of a refractory lining or crucible wherein heated or melted metal is situated. Ceramic material in various layers is typically located between the interior of the crucible and the induction coil. The optical waveguide is thus fully or partially surrounded by ceramic materials. The distance s is preferably chosen with a view to the thermal properties of the ceramic materials so that thermal events originating from the coil as well as from the interior of the crucible (e. g. the melt) may be detected with appropriate sensitivity. In an embodiment of the invention, the distance s is in the range 0-20 cm, such as 10 cm.

In an embodiment of the invention, the optical waveguide is a high- temperature optical fibre coated with polyimide, aluminium or gold. In a preferred embodiment of the invention, the optical fibre is further provided with a jacket or a sheath or a tube (cf. 12 in FIG. 6), here in a stainless steel tube. The tube dimension is designed for high pressure purpose (e. g. 1.8 mm in diameter with a thickness of 0.15 mm for a 62. 5/125 um optical fibre) and provides a certain"over length"of the fibre to compensate often large differences in material expansion (e. g. 4.5-5% 0, i. e. the length of the fibre has to be around 5% 0 longer than the stainless steel tube). Due to inductive

effects in an electrically conductive jacket, the optical fibre is preferably arranged in meander shaped net with an orientation ensuring a minimum inductive coupling. A spiral shape of the jacket allows induction of eddy currents inside the jacket and leads to a"self heating"of the jacket thereby affecting the temperature profiles. It is preferred that a major part of the length of the jacket is orthogonal to the inductive field.

The connection of one end of the optical waveguide 5 to an optical back scattering measurement equipment 14 via an optical connection cable 16 and optical connectors 17 is schematically indicated in FIG. 4.

FIG. 5 shows an embodiment of an induction furnace according to the invention.

Although the following example concerns an induction furnace, the structure of the wall and location of the optical fibres and the associated the use of the optical measuring principle may be used for other furnaces and for other vital parts thereof than the lining and the induction coil.

FIG. 5 shows a schematic structure of the wall of an induction furnace from the inner surface 31 of the first refractory lining layer AO to the outer surface 41 of the outer wall 40. The wall of the furnace comprises a number of different layers (A0, A1, A2, W1, W2, W3) with various functions between the refractory lining 30 (e. g. the first refractory lining layer AO) and the wall 40 (e. g. the outer wall W1) of the furnace, including at least one optical waveguide located in a separate layer.

The coil or coils of an induction furnace typically comprise water cooled Cu- tubes 60 (only one turn is included in the cross sectional view of FIG. 5 for simplicity). In the embodiment in FIG. 5, the outer wall W1 of the furnace is constituted by the coil and a filling material. Between the refractory lining 30 (e. g. the first refractory lining layer AO) and the wall 40 (e. g. the outer wall W1) a mat according to the invention (or another waveguide-hosting layer) is located as indicated by W3. An optical fibre (10, cf. FIG. 6) functioning as a temperature sensor is integrated in the mat (20, cf. FIG. 6). The mat is constructed as an insulating and sliding (low friction) layer W3 of mica

whereby the removal of the mat during replacement of the (worn out) lining is eased. The Layers A1 and A2 may e. g. be formed as special ceramic inner protective layers. The layer W2 may be made of further insulating sheets or be a part of the mat (20 in FIG. 6) in the form of sheet material (e. g. ceramic non-woven fabric or staple tissue).

In FIG. 5, the heat source, here a melt 80, is located in the left part of the cross section and the inductor assembly is located to the right. In the embodiment shown in FIG. 5, the refractory lining 30 is made of known sinter materials (such as a ceramic material), and implemented in one or more layers (A0, A1,...).

In addition to the above described location of the optical waveguide in a mat in the intermediate layer W3, there are preferably three main locations for the optical waveguide for monitoring the temperature profile: 1. Directly on the coil surface (cf. e. g. FIG. 2 or FiGs. 3,4 with s=0) or integrated in the same layer as the coil (cf. W1 on FIG. 7) 2. Buried inside the wall 40 (termed the permanent lining) of a heat resistant concrete (cf. e. g. layers W2 or W3 on FIG. 7). This may be useful for new induction furnaces. The life time of the fibre in this case may e. g. be up to 2 years and its replacement be correlated with the maintenance of the induction coil.

3. Between the first refractory lining layer (AO) and the wall (40), in the layers A1 or A2. A1 may e. g. be a Mica-mat (e. g. Phlogopit)) and A2 a layer carrying the optical fibre. Typically the jacket is mounted between the mica- mat and the permanent lining (e. g. layers W3, W2, W1) or within the mica- mat (A1). This may be useful for mounting on existing furnaces. Due to the location of the fibre or fibre jacket close to the heat source (80), the fibre or fibre jacket has to be replaced relatively frequently, e. g. coordinated with the replacement of the abraded refractory lining every 6-8 weeks.

FIG. 6 shows a flexible mat for a furnace, the mat containing an optical waveguide connected (schematically) to an optical back scattering

measuring system for determining a temperature profile of a part of the waveguide.

FIG. 6 illustrates a flexible mat 20 made of a temperature resistant material.

The mat is prepared and installed in connection with the switch of the refractory lining. As key ingredient of the mat, a heat resistant mica-art (e. g.

Phlogopit, KMg3 [(OH, F) 2lAISi3010]) may be used. The mat may additionally be enforced by glass-or carbon-fibres. The mat may cover the whole surface area of the crucible wall or only temperature critical parts of the wall. FIG. 6 indicates that the mat comprises a first 21 and a second 22 half part, which (along a longitudinal edge or fold 24) are assembled by folding one part on the other. The optical waveguide is situated in the interlayer (or layer groove) 23. Before the assembly of the two mat half parts 21,22, the optical waveguide 10 may be fixed in the interlayer 23 with an appropriate temperature resistant adhesive (e. g. silicone).

The optical waveguide, her an optical fibre 10 is laid out in the mat 20 in the form of a meander net 100. The main direction of the straight pieces of the meander 100 is denoted x in FIG. 6. A second optical fibre may be arranged in a meander form perpendicular to the main direction x so that the straight pieces of the second meander run in the direction denoted y in FIG. 6.

Preferably, the main direction x is parallel to the furnace or crucible axis (e. g.

301 in FIG. 7). In case of a helix layout of one or more optical waveguides, the helixes may be left or right oriented and fully or partially overlapping or be located beside each other.

The optical waveguide 10 may be provided with a jacket or sleeve 12 (e. g. of stainless steel). The jacket may extend over the full length of the optical waveguide. The optical waveguide 10 is connected to an optical temperature measuring system 14 for fibre-optic back scattering measurements via an optical connection 16 (e. g. established via a pigtail or an optical connector or any other suitable connection).

The optical waveguide for the present purpose is typically an optical fibre comprising a high index core for guiding light and a low index cladding for confining the light to the core and a primary coating for serving as a

mechanical protection. The optical waveguide may e. g. be a high- temperature stable optical fibre. An optical fibre (e. g. made of quartz) with a coating of polyimide (a heat resistant polymer) or the like may e. g. be used at temperatures up to 380-420 °C. A large core, polyimide coated quartz fibre (adapted to sustain a high energy density inside the fibre and to tolerated elevated surrounding temperatures) may preferable be used, e. g. the'Pyrocoat'high temperature fibre TCU/MDU062H from Spectran, Connecticut, USA).

In an embodiment of the invention aluminium or gold coated fibres are used, which extend the temperature range up to 600°C. A relatively brittle high- temperature optical fibre may additionally be protected by a secondary coating. This provides a considerable increase in the mechanical load capacity of the optical fibre, which is an advantage during embedding of the optical waveguide in the mica-mat and for its handling in general. High- temperature resistant fluorpolymers or polyimides are known and useful as materials for such coating. A non-metallic jacket (e. g. of Capton foil, Kevlar0, or carbon fibre web) in which the optical waveguide is loosely inserted may be used as an additional protective jacket (or used without a secondary coating).

The length of the optical waveguide may be adapted to the surface area of the mat and thus to the size of the furnace and to the layout of the waveguide. For a 6 ton melting furnace with a meander form layout of the optical fibre with a distance 25 of around 6 cm between neighbouring straight pieces of waveguides in the meander, around 80 m of optical fibre is needed.

The remote (relatively remote to the optical measuring system 14) end 27 of the waveguide may be located inside the mat. The local (relatively close to the optical measuring system 14) end 26 of the waveguide is provided with a normal optical connector or an optical pigtail. The pigtail or connector, respectively, is taken out of the furnace at an appropriate location, e. g. inside the insulation rings for insulating the induction coil at the top or bottom of the furnace between the yokes (i. e. the strong iron or steel bars for fixing the furnace during operation).

With the above described layout of the optical fibre and a sufficient spatial (length) resolution of the evaluation unit (optimally 50 cm) an in-situ observation of slowly developing abrasion of the lining due to a slow increase in the basic temperature as well as local fissures or rifts and'metal tongues'is possible through local temperature peaks. With the above example of a 80 m length of waveguide and a 50 cm difference between measuring points, the temperature of 160 individual points along the waveguide are monitored. This may of course be individually designed to a specific application by configuring the layout of the optical waveguide and the density (along the waveguide) of the measuring points according to the requested spatial precision of the application in question. In an embodiment of the invention, the precision of the temperature measurement is around 1 K. Thus a real online monitoring of the state of the electrical insulation of an induction coil, the refractory lining and/or other vital parts of an induction furnace is available. It is obvious that state of the art electronic and data processing units may be utilized to extract, store, recall and display a spatial and/or time based temperature profile.

The electronic evaluation unit may also be refined to a warning or alarm system. In connection with the heating of an object to be melted after the insertion of a new lining, a basic temperature profile is recorded and stored as a reference profile. During operation, every temperature of each data point within the area to be monitored (e. g. the lining area or the induction coil area) is compared to the temperature of the reference profile for the corresponding data point. If the temperature difference exceeds a predetermined value, a warning or an alarm is issued and a specific action may be initiated (e. g. disconnection of the induction coil from its electrical power supply). Based on the number of determined'critical'temperature values it is possible to estimate whether a (coherent) surface abrasion or single rifts or tongues are present.

FIG. 7 shows an induction melting furnace 300 according to the invention with two optical waveguide nets 100, 101 for monitoring the lining 30 and the induction coil 4, respectively.

Two independent waveguide nets 100 and 101 are located in layers W1 and W3, respectively, of the container wall of the furnace and adapted for monitoring the induction coil 4 and the lining 30 of the crucible, respectively.

The local end 26 of each of the waveguide nets are provided with an optical connector 17 and is connected to an optical back scattering and evaluation system 14 (possibly through and optical multiplexer for both nets to utilize the same equipment) via an optical connecting cable 16. Due to the considerable length range of operation (several km) of OTDR-or OFDR- measuring systems, it is possible to monitor several furnaces with one single measuring and evaluation unit 14 by serially connecting several waveguides 100,102 by means of an optical connecting cable 16. In this case, the remote end 27 of the optical waveguide of a first waveguide hosting layer W3 is also provided with an optical connector 17 or an optical pigtail by means of which the first waveguide is optically connected to the start end 26 of a second optical waveguide 102 of a second waveguide hosting layer 200 (e. g. for monitoring another lining or induction coil or some other vital part of another furnace). If a splice of the optical waveguide is located inside the mat or other layer in which it is mounted, the protection or covering of the splice must be free of any metals or other electrically conducting materials.

Candidates for this purpose are plastic splice protection solutions based on shrinkable tubing techniques (e. g. heat-shrinkable tubing).

In the embodiment in FIG. 7, waveguide net 101 for monitoring the induction coil is located in the outer wall layer W1 comprising the induction coil 4 in a position inside the coil (in a direction towards the centre axis 301 of the furnace). The layer W1 comprises an electrically insulating filler (such as concrete) around the induction coil and in which the optical waveguide is located. It may, however, be advantageous to host the optical waveguide in a separate layer e. g. in the layer W2 in FIG. 7 between the coil and the layer W3 in the form of a thin layer of concrete in which the fibre is embedded. The fibre net is held fixed in place by the hardened concrete. The layer W2 may alternatively be a separate layer of a thermally and/or electrically insulating material. Alternatively, the optical fibre may be positioned along the periphery of each turn of the coil as illustrated in FIG. 2. In an embodiment of the invention, the waveguide net 101 for monitoring the induction coil is fastened to the inner surface of the layer W1 hosting the coil (i. e. the surface

of the layer facing the centre axis 301). Waveguide net 100 for monitoring the refractive lining 30 is located in a separate waveguide-hosting layer W3.

The waveguide net 100 may, however, alternatively be integrated with the liner, e. g. by fastening the waveguide to the surface of the lining with an adhesive.

Measurement principles: The optical temperature measurement based on the well-known Raman effect. A laser beam coupled into the optical fibre generates two additional signals in the spectrum of the scattered light (Stokes-and Antistokes lines) caused by excitation of electrons and relaxation/transition in different vibration states. The amount of transition and therefore the intensity of Raman scattering light is a function of the temperature.

For the locally distributed measurement of the Raman scattered light one requires an optical backscatter process. The most well known back scatter process is the OTDR process (OTDR: Optical Time Domain Reflectometry).

The OTDR measurement process uses the pulse-echo process and the scattering level and scatter location are determined from the difference in the time it takes the light impulse to travel from the moment it was sent to the moment it is detected.

The OFDR Raman temperature sensor (OFDR = Optical Frequency Domain Reflectometry) does not use time like the OTDR technology, instead it uses frequency.

The OFDR process provides a statement about the local course but only once the backscatter signal detected during the entire measurement period has been measured in a complex fashion as a function of frequency and then has been Fourier transformed.

Instead of using a pulsed laser technology (OTDR-technique) the OFDR- technique uses a continuous-wave laser technology.

One advantage of using the OFDR technique for this application as opposed to the OTDR technique is that the OTDR-technique has a high energy density locally limited to the duration of the pulse, which may give problems with reflections in connectors, which again may lead to distorted temperature profiles especially in connection with small measurement ranges like in induction furnaces (60-100 m, minimum 3 connectors for one furnace- device/supply line/upper mat/lower mat). In view of the replacement of the fibre jacket due to thermal wear (e. g. every 6-8 weeks in case the optical fibre is located close to the lining of a furnace), where the connectors between the optical fibre and the optical measuring system are repeatedly connected and disconnected-each resulting connection yielding in a different reflection loss-it is also an advantage to use a less critical measurement technique regarding the connector issue. This also speaks in favour of OFDR, because with the OFDR technique the energy density is distributed over the whole length of the fibre (and not concentrated in a high energy pulse yielding relatively high reflection losses).

The optical back scattering technique (OFDR-technique), is e. g. described in the following references: 1), Verfahren zur Auswertung optisch ruckgestreuter Signale zur Bestim- mung eines streckenabhangigen MeRprofils eines Ruckstreumediums", EP 0692705,1995, Dr.-Ing. U. Glombitza 2)"Sensorische Nutzung integrierter Lichtwellenleiter", Dr.-Ing. U. Glombitza, "Innovationen in der Mikrosystemtechnik"Band 58, VDI/VDE-IT 1997 3) "Faseroptische Temperaturmessung zur Überwachung elektrischer Betriebsmittel", Dr.-Ing. U. Glombitza,"Sensorik und prozessnahe Informa- tionsverarbeitung in der Energietechnik"ETG-Fachbericht, VDE-Verlag Band 64, S. 27 FIG. 8 shows an example of a measuring equipment for optical back scattering for determining a temperature profile of an induction furnace.

FIG. 8 shows the schematic set-up of an OFDR Raman temperature measurement system 14. The temperature measurement system is composed of a controller (frequency generator, laser source, optical module, HF mixer, reception and microprocessor unit) and a quartz glass fibre 5 (optical waveguide) as line shaped temperature sensor. Its structure consists of three channels as in addition to the two measurement channels (Antistokes and Stokes), an additional reference channel is required. In accordance with the OFDR process, the output performance of the laser is passed through the frequency in a sinus shape from the start frequency to the end frequency (100 MHz) within a measurement time interval with the help of the HF modulator. The resulting frequency lift is a direct measurement for the local resolution of the reflectometer. The frequency modulated laser light is coupled via the optical module into the optical waveguide (see figure above). The Raman light which is continuously backscattered from every local element of the fibre is spectrally filtered in the optical module and converted into electrical signals via photodetectors. Then the measurement signals are amplified and mixed into the low frequency spectral range (NF range). The Fourier transform of the averaged NF signals result in the two Raman backscatter curves in the local area. The amplitudes of these backscatter curves are proportional to the intensity of the Raman scattering of the local element observed. The fibre temperature along the sensor cable is a result of the amplitude relationship of the two measurement channels.

FIG. 8 schematically shows the position of the temperature monitoring optical waveguide 5 between the induction coil 4 and the crucible lining 30 of an induction furnace.

To visualize the data, a standard PC with a software which is able to decode the controller-protocol may be used.

An appropriate OFDR-measuring equipment could e. g. be a DTS-system (DTS=distributed temperature sensing) such as a controller of the type OTS 20P from LIOS Technology GmbH, Kiln, Germany.

The invention is defined by the features of the independent claim (s).

Preferred embodiments are defined in the dependent claims.

Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims. For example, the invention may be used in other areas where temperature induced wear of vital components is an issue, e. g. in chemical plants where the insensitivity to electromagnetic fields and the absence of electric voltage sources in measuring equipment (to avoid the risk of explosions) are important elements.