GB1604282A - Elements which heat by convection and radiation - Google Patents

Description

(54) ELEMENTS WHICH HEAT BY CONVECTION AND RADIATION (71) We, RHONE-POULENC INDUSTRIES, a French Body Corporate, of 22 Avenue Montaigne, 75 Paris 8eme, France, 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 heating elements, in particular radiant elements as well as heating elements which operate by convection and heaters involving their use.

Numerous devices for heating by convection are known and are used, in particular, in residential premises.

There are even standards which attempt to lay down rules for the characteristics of heating apparatuses. Amongst the characteristics which tend to be imposed on heating apparatuses, it is necessary a) that the hot air leaving the apparatus (generally at its upper part) should not be at an excessive temperature (for example, it should be less than 1200C) and b) that no part of the apparatus which can be touched with the fingers should be too hot.

Although such constraints are beneficial for comfort and safety, they nevertheless, by their very nature, impose severe limitations on the available power per unit volume of equipment. The present invention sets out to overcome these problems while providing heating equipment of reduced volume, which is nevertheless capable of emitting a high calorific power.

The heating process with which the present invention is concerned is heating by convection and radiation, that is to say involving at least one wall arranged so that a stream of rising hot air in contact with the said wall is produced naturally, such that the wall dissipates part (preferably a large part) of its energy (or calorific power) in the orm of rays (or radiant heat).

The heating elements according to the invention are characterised in that they possess one or more tubular portions or channels, the walls of which comprise: (a) an electrically insulating material which consists of the combination of a reinforcing filler, the particles of which are of elongate structure; and a polyimide resin, and (b) an electrical resistance which consists of at least one electrically conductive and electrically resistant wire or filamentary material, this filamentary material being at least partially embedded in said insulating material and being covered with a heatstable, electrically insulating varnish of different chemical nature to the said polyimide resin, the ends of the filamentary material being provided with means to allow them to be connected to a source of electrical energy.It should be noted that in our Specification No 1579194 we describe and claim a heating element which is substantially flat and which comprises (a) as above, in the form of a layer and (b) an electrical resistance consisting. of a said filamentary material which has been wound around and at least partially embedded in the insulating material such that the portions of the turns on a given side are parallel to one another and the portions of the turns on one side run at an angle to the portions of the turns on the other side.

The tubes (which are, of course, closed) or channels generally have an approximately cylindrical or conical shape, that is to say that they are in the shape of a cylinder or cone having a small opening.

Although the cylindrical shape is the more common, the conical shape can sometimes be adopted because certain advantages arise. For example a cone can with withdrawn from the mould during the preparation of the channels more easily and it is more easy to regulate the stream of rising hot air which because it expands, naturally tends to accelerate its movement.

With a conical shape, the preferred angle at the apex of the cone is less than 30 , and preferably less then 100.

The channels therefore generally have the shape of a cylinder or a cone, the directrix of which is open curvilinear or polygonal, as will be apparent from the Figures discussed below.

For a given tube or channel, the directrix should be relatively completely closed.

These conditions are generally satisfied as follows: if A and B denote the ends of the directrix of the tube or channel in question (thus A and B coincide for a closed tube), if d is the distance AB, if S is the surface area of the region enclosed by the directrix going from A to B and the rectilinear segement AB, and if L is the length of the directrix between A and B, it is generally the case that L d < 3 and preferably L d < 4 (L+d)2 S > 25 and preferably (L+d)2 S > 15 The invention also provides heating elements which compnse a plurality of juxtposed tubes or channels, pairs of tubes or channels preferably having a common wall.Examples of such tubes and channels are shown in Figures la, Ib and lc of the accompanying drawings, which further illustrate the present invention, in these Figures; only the directrix (that is to say the cross-section of the channels) has been shown) Figure la shows a series of channels having an essentially rounded cross-section.

Figures lb and Ic show a series of channels having a triangular cross-section. The series of channels la and lb can be produced by shaping a single initial surface (or plate); the series of channels lc can be produced by combining 2 planar plates with one zig-zag plate.

The polyimide resins used in forming the tubes are very advantageous in that, when used under suitable conditions, they emit a substantial amount of energy in the form of rays (i.e. radiant heat).

The electrical resistance carried bv the walls of the tubes and channels of the invention can comprise one or two windings of conductive wires or filamentary material wound round the tubes. It is preferred for the filamentary material to be embedded in one surface of the insulating material. These conductive wires or filaments are metallic wires; they are covered with a heat-stable, electrically insulating varnish of a different -chemical nature from that of the polyimide resin which forms part of the material defined under a).

The ends of the electrical wires are of course equipped with means which enable them to be connected to a source of electrical energy, these means being placed either on each tube or, in the case of a series of tubes, on the end tubes.

The reinforcing filler of elongate structure which is a constituent of component a) generally has a flake structure or a fibrous structure. A fibrous material may consist of simple fibres or of fibres in the form of a woven or non-woven fabric. The filler can be inorganic or organic.

The proportion by weight of reinforcing filler of elongate structure, relative to the combination (in material a) of polyimide resin + reinforcing filler, is generally from 40 to 90% and preferably from 55 to 80%.

Examples of such reinforcing fillers include mica flakes (or splits), asbestos fibres, glass or ceramic fibres, woven or non-woven fabric (especially mats) of glass fibres, non-woven fabrics (especially felts) of asbestos fibres, and woven or non-woven fabrics of heat-stable synthetic fibres, for example of an aromatic polyamide or a polyamide-imide.

Various types of polyimide resins which are a constituent of component a) can be used. Amongst the most advantageous types, the polyimides resulting from the reaction of bis-imides of unsaturated dicarboxylic acids with amino or unsaturated compounds are used. More particularly, the products resulting from the reaction of bis-imides with polyamides or with N-vinylpyrrolidone are used.

The polyimide resins obtained by reacting a bis-imide with a polyamine can be in the form of a prepolymer (which may still be soluble in certain solvents) for an intermediate stage in the production of the heating element, or in the completely polymerised or polycondensed (totally insoluble) form in the heating elements as used normally. The products resulting from the reaction of a bis-imide and with a diamine are described in, for example, French Patent 1,555,564, French Certificate of Addition No. 96,189, U.S.

Patent Nos. 3,562,223 and 3,658,764 and in U.S. Reissue No. 29,316.

The polyimide resins obtained by reacting a bis-imide with N-vinylpyrrolidone are described in, for example, French Patent No. 2,297,684. These resins are very convenient because one can produce pre -impregnated materials by impregnating a fibrous material (for example a woven fabric) with the mixture of monomers (bisimide, N-vinylpyrrolidone and, optionally, unsatuated polyester) without solvent, and then by subsequently polymerising.

The electrically insulating material a) therefore consists of the combination of reinforcing filler of elongate structure with a polyimide resin i.e. the filler is impregnated with the resin. It is thus possible to impregnate the filler dry by powdering, in a molten medium, or by means of an aqueous solution or dispersion of a prepolymer obtained by reacting a bisimide of an unsaturated dicarboxylic acid with a polyamine. The preparation of such prepolymers is described, for example, in French Patent 1,555,564. The preparation of aqueous suspensions of such prepolymers, and the impregnation of a web of fibres therewith, is described in French Patent 2,110,619.

These processes lead to the production of a pre-impregnated material consisting of a reinforcing filler of elongate structure and a prepolymer. During subsequent treatments (pressing and heating), these preimpregnated materials are converted into an impregnated material of a tppe sometimes referred to as a laminate or felt.

As a heat-stable varnish for the electrically resistant wires, there may principally be mentioned varnishes of the polyester-imide, polyimide or, preferably, polyamide-imide type. Typical such polyamide-imides are described in French Patent 1,498,015 and U.S. Patent No.

3,541,038. The polyamide-imides are preferably those obtained by reacting trimellitic anhydride with aromatic isocyanates, it being possible for this basic formula to be modified in numerous ways, for example by adding polymeric or nonpolymeric adjuvants or by adding comonomers capable of co-polycondensing with trimellitic anhydride and the diisocyanate.

According to an advantageous aspect of the invention, the varnished electrical wires are encusted with the electrically insulating material a). The degree of encrusting is generally from 50 to 100% and preferably from 80 to 100%. The term "degree of encrusting" means the porportion of the diameter of the electrical wire (measured linearly) which is below the surface of the material a). If the degree of encrusting is 100%, the varnished metallic electrical wire can be covered with a layer of polyimide resin (originating, for example, from flow during a pressing operation).The thickness of this layer is generally very low, of the order of a few microns, usually less than 50 ,a and preferably less than 10 y. In the degree of encusting is less than 100%, it is possible for the surface of the heating element not to be rigorously planar locally, but to exhibit undulations caused by the wires. The resin which has flowed forms a connecting zone between the substrate and the resistant wire. In order to achieve such a shape, the pressing surfaces advantageously have a certain degree of flexibility during the pressing operations.

In general, the heating elements of the invention are rigid or semi-rigid. The term "semi-rigid" means that the material can withstand a non-permanent elastic deformation by curving down to a radius of curvature 3 cm.

It is preferred to use a metallic electrical wires having a diameter of 0.05 to 0.8 mm, spaced by, say, 1 to 10 mm.

According to a variant of the invention, the walls of the heating elements furthermore comprise: (c) one or two additional layers of electrically insulating material, for example of the same material as that used for component (a) which layers are located against one or both of the faces of this material a) (and joined thereto).

These various constituents (a), (b) and (c) may be joined to one another permanently by chemical bonds or physical adhesive bonds.

The elements can be manufactured in various ways.

According to a first process, which is preferred, an article is produced which has an approximately cylindrical shape and consists of a cylindrical pre-impregnated material possessing, on its external surface, a helical coil of one or more enamelled conductive wires (this pre-impregnated material itself consisting of a fibrous material or a material in the form of flakes, impregnated with a polyimide prepolymer), and the cylinder is then directly pressed on the mandrel. Pressures of 5 to 100 bars are generally very suitable; pressing is generally carried out hot so as to soften the polyimide prepolymer and advantageously to polycondense this polyimide completely; the wires become encrusted under the pressure and the softening of the prepolymer.Since the prepolymers obtained from a bismaleimide and a diamine generally have a softening point of 80 to 2000 C, the pressing temperature is generally 100 to 2500 C. In order to make it possible to anchor (or assemble) the various components efficiently, the temperature is preferably greater than 1500C. It is of course possible to reheat the whole, for example for a few hours at 200"C or above.

According to a less preferred process, the cylindrical pre-impregnated material possessing on its surface the helical coil is withdrawn from the mandrel and then flattened and pressed to give it the desired shape. During pressing, the cylinder possessing the coil is compressed to provide the desired shape such that the portions of the turns of the filamentary material on one side are arranged approximately parallel to one another, the portion of filamentary material on the other side runnine crosswise or at an angle to those on the first side.

Such processes make it possible to obtain heating elements which simply comprise constituents a and b. In order to obtain more complex heating elements, a stack is produced which comprises, firstly, the cylindrical article as defined above and, secondly, one or optionally two, pre impregnated materials, generally substantially planar (which are intended to produce layers c). This stack is then pressed as described above.

Another process for the production of heating elements according to the invention produces, more conveniently, a heating tube consisting of a felt, for example of asbestos, impregnated with polyimide pre polymer, on the surface of which is en crusted the enamelled (varnished) conductive wire; according to this process, the asbestos felt containing the polyimide prepolymer is prepared, in accordance with the so-called papermaking technique, by directly incorporating all the ingredients into the mixer (called a "beater" by the paper manufactures), that is to say by simultaneously introducing the water, the fibres (preferably asbestos) and the binder (polyimide prepolymer) in powder form.A felt is then formed on a papermaking machine and the water is extracted from the felt, firstly by draining and applying a vacuum and secondly by drying at a temperature of, say, 70 to 1000C, generally by placing the -felt in a ventilated oven.

The enamelled electrical conductor is wound around the foil or strip thus prepared prior to curing. In view of the flexibility of the foil or strip, it is advisable to guide the foil or strip between rigid elements.

In this felt, the binder is at the pre polymer stage, that is to say it can be softened by heating. The felt thus prepared usually has a density of 0.5 to 1.2 g/cc whilst, at the final stage, that is to say after pressing the felt and curing the polyimide, the density of the material is usually from 1.5 to 1.6 g/cc.

After the enamelled wire has been positioned, the asbestos felt is pressed hot whilst giving it the shape desired; the aim of this operation is threefold: to cause the encrusting of the enamelled wire, to increase the density of the material and to cause the softening of the polyimide prepolymer. As a general rule, this pressing is carried out at a temperature of 100 to 250"C and preferably 160 to 220"C. The pressure is generally 5 to 100 bars.

If necessary, the heating element thus prepared can be finished by hot-pressing wit a pre-impregnated material; moreover, it is -not necessary to separate the various pressing/heating phases which can be combined in a single phase.

The ends of the conductive wires of the articles according to the invention, obtained in accordance with one or other of the procedures described, can then be connected by the usual means to a source of electrical energy, including appropriate control or regulating devices as required. If several wires have been used, it is of course, possible, by connecting them separately, to form elements having several heating rates (that is to say several heating powers).

The pressing operation used in the various processes can itself take place in several stages, depending in particular on the shape chosen. Thus, in order to obtain a series of channels as illustrated in Figure lc, it is preferred to carry out a first pressing to produce a zig-zag pre-impregnated material and then to press this zig-zag foil (in which the polyimide is still in the prepolymer stage) together with two pre-impregnated materials having a planar shape.

In order to obtain heating elements, the pressing operations defined above can be carried out by means of a pressing roller whilst the various constituents a, b and optionally c are being wound on to a mandrel.

One variant of such a process consists in producing the constituents (a), and optionally (c), not by winding preimpregnated filamentary material but by winding a pre-impregnated rove or roving.

The polyimide prepolymer flows during the subsequent heat and pressure treatments and produces a layer as defined for (a) and (c).

The heating elements according to the invention are particularly valuable because of their numerous qualitites; they offer all the guarantees from an electrical point of view, in particular safety of use; the fact that a varnish which is different from the polyimide resin can be used for the wires results in increased safety; the heating tubes are particularly well suited for use in a wide variety of residental premises. The problem of the rapid heating of cold and poorly insulated premises is especially well solved by the use of these partially radiant devices.

Naturally, the technique described above, which will be illustrated by the following Examples, makes it possible to produce articles of very varied dimensions.

The Examples which follow further illustrate the present invention.

EXAMPLES I to 4 Production of a pre-impregnated material.

A satin-type glass fabric weighing 200 g/m2 is impregnated with polyimide prepolymer; this prepolymer is prepared from N,N1 - 4,4' - diphenylmethane - bis maleimide and bis - ( - 4 - aminophenyl)methane (molar ratio of bis imide/diamine=2.5) and has a softening point of 1000C. It is used in the form of a solution in N-methylpyrrolidone (50g of prepolymer per 100g of solution) and the glass fabric is impregnated by dipping. The pre-impregnated material is then dried (1 hour at 1500C). The amount of prepolymer deposited on the glass fabric is about 40 g per 100 g of pre-impregnated material.

Production of the heating element.

A rectangle of dimensions 32 cm x 12.2 cm is cut out of this pre-impregnated material and wound around a rotating mandrel (3.9 cm diameter).

The pre-impregnated material is coated on the mandrel with a thin layer of adhesive in order to make it easier to wind the electrically resistant wire. The enamelled metallic wires are then wound onto the mandrel. These wires are made of "kanthal" (Registered Trade Mark for an iron/nickeUchromium alloy); they have a diameter of 0.2 mm and a resistance of 44 ohms/cm. They have previously received 6 layers of a polyamide-imide varnish (a product obtained from bis - (4 isocyanatophenyl)- methane and trimellitic anhydride in a molar ratio of about 1), the varnish being employed in the form of a solution in a mixture of N - methylpyrrolidone and xylene.

The ends of the wires are provided with means for connecting them to the mains electricity supply.

A second rectangle of pre-impregnated material, similar to the first, is wound on to this winding and held very tightly thereto.

The whole is withdrawn from the mandrel and heated in an oven at 2000C for T hour.

Examples I to 4 below differ from one another in the nature of the winding of electrical wires and, obviously, in the nature of the results obtained.

EXAMPLE 1 The wire has a length of 10.5 m.

Only one wire is used. A heating element is obtained which is in the shape of a cylinder with a circular base and has a length of 32 cm and a diameter of 3.9 cm.

Under 220 volts, the current intensity is 0.48 amps and the power emitted is 105 watts. The temPeratUre of the air at the upper outlet of the tube (in the vertical position) is about 55"C. The temperature of the surface of the element is about 110 C.

EXAMPLE 2 The winding is produced using 2 wires each of length 16.5 m.

A heating element is obtained as in Example 1.

Under 220 volts, the current intensity is 0.61 amps, the power emitted is 134 watts, the temperature of the air leaving the apparatus is about 650C and the temperature of the surface reaches 1500C.

EXAMPLE 3 The procedure of Example 2 is followed but using wires having a length of 13.5 m.

Under 220 volts, the current intensity is 0.73 amps, the power emitted is 161 watts, the temperature of the air leaving the apparatus is about 750C and the temperature of the surface is about 185"C.

EXAMPLE 4 5 elements, identical to that of Example 3, are connected up in parallel.

Under 220 volts, the overall current intensity is 3.7 amps, the total power emitted is 806 watts, the temperature of the air leaving the apparatus is about 800C and the temperature of the surface of the elements is about 185"C.

After functioning for 1,500 hours, there is no notable variation in the characteristics.

EXAMPLE 5 (A) A conductive wire made of kanthal (diameter: 0.2 mm; resistance: 44 ohms/cm; length: 26 m) is arranged between 2 combs 1 m apart, having teeth, 3, as shown in Figure 2 of the accompanying drawings.

A meander winding having 26 turns (or reaches), which are parallel to one another, 5 mm apart and connected in series, is thus obtained, (B) Production of a pre-impregnated material.

A glass fabric, similar to that of Example 1, is dipped in a mixture, heated to 750C, of: 75 g of N,N' - 4,4' - diphenyl - methane bis - maleimide, 12.5 g of N- vinylpyrrolidone - 2 and 12.5 g of a solution of polyester resin composed of 40% of allyl phthalate and 60% of a polyester prepared from maleic acid, trans-nadic acid, propylene glycol and ethylene glycol.

The fabric thus impregnated is heated at 100"C for 3 minutes.

(C) Production of the wall of the heating element.

The winding of electrically resistant wires prepared in paragraph A is deposited on the pre-impregnated material prepared in paragraph B, the extreme edges of the preimpregnated material being folded back on to the winding and the ends being clipped as shown in Figure 3 of the accompanying drawings, in which (1) represents the winding of electrically resistant wires; (2) represents the pre-impregnated material and (4) represents a clip.

(D) Production of the heating element.

The whole made up in this way is shaped on a set of metallic cylindrical bars coated with (non-stick) Teflon (Registered Trade Mark for polytetrafluoroethylene).

Figure 4 of the accompanying drawings shows the arrangement of the various elements during assembly. (5) represents the bars.

The bars of each vertical set are then drawn towards one another by exerting a tension on the ends of the heating windmg and the whole is heated at 2000C for 12 hours.

A rigid structure similar to that shown in Figure la is obtained. The heating structure consists of 18 juxtaposed heating channels having a common wall. These tubes together formed a rectangular parallel ipiped having dimensions of 16 cm x 14 cm x 3 cm.

The power is 45 watts; this unit functioned for 10,000 hours without any change in the characteristics.

WHAT WE CLAIM IS: 1. A heating element comprising one or more tubular portions or channels, the walls of which comprise: (a) an electrically insulating material consisting of a reinforcing filler, the particles of which are of elongate structure and a polyimide resin, and (b) an electrical resistance consisting of at least one electrically conductive and electrically resistant filamentary material, this filamentary material being at least partially embedded in said insulating material and being covered with a heatstable electrically insulating varnish of different chemical nature to the said polyimide resin, the ends of the filamentary material being provided with means to allow them to be connected to a source of electrical energy.

2. An element according to claim 1 in which the tubular portions or channels have a substantially conical or cylindrical shape.

3. An element according to claim 1 or 2 in which the consecutive tubular portions or channels are arranged in juxtaposed manner having a common wall.

4. An element according to any one of claims 1 to 3 in which the polyimide resin is one obtained by reacting a bis-imide of an unsaturated dicarboxylie acid with an unsaturated compound.

5. An element according to claim 4 in which the unsaturated compound is Nvinylpyrrolidone.

6. An element according to any one of claims 1 to 3, in which the polyimide resin is one obtained by reacting a bis-imide of an unsaturated dicarboxylic acid with a polyamine.

7. An element according to any one of the preceding claims in which the wires are varnished with a polyamide-imide.

8. An element according to claim 7 in which the polyamide-imide is one obtained by reaction of trimellitic anhydride with a diisocyanate.

9. An element according to any one of claims I to 8 in which the reinforcing filler is mica flakes or a woven or non-woven glass or asbestos fibre material.

10. An element according to any one of claims 1 to 9 in which the walls additionally comprise one or two additional layers of an electrically insulating material as defined under a) which layer or layers are located against, and joined to, one or both faces of material a).

11. An element according to claim 1 substantially as hereinbefore described with reference to Figures la, lb or ic of the accompanying drawings.

12. Process for the preparation of an element as claimed in any one of the preceding claims which comprises encrusting varnished electrically conducting and electrically resistant filamentry material in the surface of a material or felt comprising fibres, pre-impregnated with a polyimide resin or a prepolymer thereof, the varnish being electrically insulating and of a different chemical nature to the said polyimide resin, and hot pressing the whole into the desired shape.

13. Process according to claim 12 in which the pressing is carried out with a pressing roller whilst the constituents a), b) and, optionally, c) are being wound on a mandrel.

14. Process according to claim 12 substantially as hereinbefore described.

15. An element as defined in claim 1

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

Claims (16)

**WARNING** start of CLMS field may overlap end of DESC **. prepared in paragraph A is deposited on the pre-impregnated material prepared in paragraph B, the extreme edges of the preimpregnated material being folded back on to the winding and the ends being clipped as shown in Figure 3 of the accompanying drawings, in which (1) represents the winding of electrically resistant wires; (2) represents the pre-impregnated material and (4) represents a clip. (D) Production of the heating element. The whole made up in this way is shaped on a set of metallic cylindrical bars coated with (non-stick) Teflon (Registered Trade Mark for polytetrafluoroethylene). Figure 4 of the accompanying drawings shows the arrangement of the various elements during assembly. (5) represents the bars. The bars of each vertical set are then drawn towards one another by exerting a tension on the ends of the heating windmg and the whole is heated at 2000C for 12 hours. A rigid structure similar to that shown in Figure la is obtained. The heating structure consists of 18 juxtaposed heating channels having a common wall. These tubes together formed a rectangular parallel ipiped having dimensions of 16 cm x 14 cm x 3 cm. The power is 45 watts; this unit functioned for 10,000 hours without any change in the characteristics. WHAT WE CLAIM IS:

1. A heating element comprising one or more tubular portions or channels, the walls of which comprise: (a) an electrically insulating material consisting of a reinforcing filler, the particles of which are of elongate structure and a polyimide resin, and (b) an electrical resistance consisting of at least one electrically conductive and electrically resistant filamentary material, this filamentary material being at least partially embedded in said insulating material and being covered with a heatstable electrically insulating varnish of different chemical nature to the said polyimide resin, the ends of the filamentary material being provided with means to allow them to be connected to a source of electrical energy.

2. An element according to claim 1 in which the tubular portions or channels have a substantially conical or cylindrical shape.

3. An element according to claim 1 or 2 in which the consecutive tubular portions or channels are arranged in juxtaposed manner having a common wall.

4. An element according to any one of claims 1 to 3 in which the polyimide resin is one obtained by reacting a bis-imide of an unsaturated dicarboxylie acid with an unsaturated compound.

5. An element according to claim 4 in which the unsaturated compound is Nvinylpyrrolidone.

6. An element according to any one of claims 1 to 3, in which the polyimide resin is one obtained by reacting a bis-imide of an unsaturated dicarboxylic acid with a polyamine.

7. An element according to any one of the preceding claims in which the wires are varnished with a polyamide-imide.

8. An element according to claim 7 in which the polyamide-imide is one obtained by reaction of trimellitic anhydride with a diisocyanate.

9. An element according to any one of claims I to 8 in which the reinforcing filler is mica flakes or a woven or non-woven glass or asbestos fibre material.

10. An element according to any one of claims 1 to 9 in which the walls additionally comprise one or two additional layers of an electrically insulating material as defined under a) which layer or layers are located against, and joined to, one or both faces of material a).

11. An element according to claim 1 substantially as hereinbefore described with reference to Figures la, lb or ic of the accompanying drawings.

12. Process for the preparation of an element as claimed in any one of the preceding claims which comprises encrusting varnished electrically conducting and electrically resistant filamentry material in the surface of a material or felt comprising fibres, pre-impregnated with a polyimide resin or a prepolymer thereof, the varnish being electrically insulating and of a different chemical nature to the said polyimide resin, and hot pressing the whole into the desired shape.

13. Process according to claim 12 in which the pressing is carried out with a pressing roller whilst the constituents a), b) and, optionally, c) are being wound on a mandrel.

14. Process according to claim 12 substantially as hereinbefore described.

15. An element as defined in claim 1

whenever prepared by a process as claimed in any one of claims 12 to 14.

16. A heater which comprises at least one heating element as claimed in any one of claims I to 11 and 15.