GB1583771A - Electric heaters having resistance wire embedded in ceramic - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRIC HEATERS HAVING RESISTANCE WIRE EMBEDDED IN CERAMIC (71) We, FAST HEAT ELEMENT MANU- FACTURING COMPANY, INC. a corporation organised under the laws of the State of Illinois, United States of America, of 442 West Fullerton Avenue, Elmhurst, Illinois 60126, United States of America, 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: This invention relates generally to electric heaters and particularly to such heaters having electric resistance wire embedded in ceramic.

In previously proposed mica band heaters, a wire wound mica heating element is assembled between mica insulator strips. The resultant mica sandwich is then encased in a sheet metal enclosure and formed into a desired shape.

The electrical mica insulators used are of relatively low thermal conductivity and thus limit the heat transfer efficiency. Also these insulator strips undergo physical and chemical changes upon exposure to temperatures in excess of 1 200 F, which consist of dehydration or the baking out of the water of hydration.

This change further decreases thermal conductivity and also reduces electrical insulating properties.

The presence of air voids and undesirable expansion under elevated temperature inherent in such mica heaters reduce heat transfer capability and result in loss of heater efficiency.

These factors cause the previously proposed heater to operate at relatively higher than most efficient internal temperatures, resulting in premature heater failure. Additionally, where clamp force must be applied to maintain the heater in a given position, for example, around the nozzle of a tube having contents which must be heated as they pass therethrough, expansion of the heater under elevated temperatures causes loss of clamping force, resulting in heater inefficiency because the heater must be hotter to achieve a given surface temperature, and the higher temperature of the heater induces further expansion as the temperature is elevated.

In a previously proposed ceramic band heater, coils of element wire are strung through ceramic insulator blocks which are shielded by a light sheet metal cover. The assembly so formed is then strapped around an object to be heated. The resulting assembly can be likened to an oven assembly wherein heat transfer to the heated object is principally by convection rather than conduction. Such a heating system is not capable of high wattage because the inefficient convection that heat transfer will not remove heat from the element wire fast enough, and thus would lead to overheating of the wire and premature element failure. This limitation of wattage thus increases heat-up time of any object to be heated. Due to the open design of the casing for such ceramic heaters, carbon forming materials can enter the heater, causing grounding type failures, which also may constitute a safety hazard.Also, inherent bulk requirements for such heaters present their use in some applications where space is critical.

In accordance with the invention, there is provided a method of fabricating an electric heater, comprising the steps of assembling resistance wire on an uncured sheet of ceramic particles impregnated to a high density and bound together in heat dissipatable binder material, arranging the assembled wire and uncured sheet between uncured sheets of electrical insulator particles bound together in heat dissipatable binder material to form an assembly, compressing the assembly to substantially eliminate air voids between the sheets, and heating the compressed assembly at a temperature sufficient to substantially dissipate the binder materials and cure the ceramic and insulator particles into an integral mass.

In a particular embodiment of the invention described in more detail below, organically bound ceramic particle strips each comprise a thin pliable 'green' sheet of ceramic particles, pressed and rolled to a high density, and bonded together with binder materials, usually organic in nature, to an overall thickness upwards of .018 inch. The ceramic particles in the sheets are typical powdered ceramic materials, such as particles of aluminium oxide, magnesium oxide, boron nitride, or silicone dioxide. The binders for the ceramic particles are typically silicone resin, rubber, varnish, or glyptal. These bonded 'green' or unbaked ceramic particle sheets conventionally are used in the fabrication of ceramic underlayment for printed circuits, the end product when baked out being referred to as "ceramic substrata", but in their 'green' state before baking they are pliable and bendable.

In fabricating the heater, a lower organic ceramic strip is laid over the bottom wall of a U-shaped metal housing, and the core organic - ceramic strip which has been wound with Nichrome (Registered Trade Mark) or other resistance wire is placed over the lower strip.

A second or upper organic - ceramic insulator strip is placed over the wire wound core strip, and a metal pressure plate is installed over the upper strip to close the housing. The edges of the housing are bent over the pressure plate, and the assembly is then rolled and flattened, thereby eliminating air voids between the elements and amalgamating and unifying the structure .

The assembly may then be shaped, for example bent into a curved band heater.

When the heater assembly is in its final finished shape, the entire assembly is fired at an elevated temperature above the vaporization point of the binder materials in the strips, above the sintering temperature of the ceramic and below the melting point of the sheath covering, preferably in an oxygen atmosphere, to vaporize and carbonize the binders and oxidize the carbon, which is vented from the heater in the form of carbon dioxide. As a result of this process, the ceramic materials of the strips sinter into an integral heatconducting and electrically-insulating mass.

Leads may then be connected to the heater element terminals and any desired heater mounting members may then be attached.

Two embodiments of the invention in the form of a band heater and a strip heater will now be described, by way of example, with reference to the accompanying drawings in which : Figure I is a perspective view of the curved band heater, Figure 2 is a perspective exploded view of the component parts of a heater assembly embodying the invention, Figure 3 is a perspective view of the strip heater, Figure 4 is a sectional view of assembled parts of the heater before closing the housing and compression and heating of the assembly, and Figure 5 is a sectional view of a completed heater assembly.

With reference to the drawings, the curved band heater 10(as shown in Figure 1) or a strip heater 11 (as shown in Figure 3), is fabricated, preferably, from a sheet metal channel 12, having a flat base 19 and upstanding sides 14, into which is laid, successively, a thin flat pliable insulator sheet 15 of bound ceramic particles, a resistance wire wound core 16 of bound ceramic particles, a second or upper insulator sheet 17 of bound ceramic particles, and a metal pressure plate 18, all of which may be held together and centered during initial assembly by means of suitable tape or adhesive.

The margins 29 of the upstanding sides 14 on channel 12 are bent over the pressure plate 18 to close the assembly and bind the pressure plate thereover. The closed assembly is then rolled flat or is formed into a curved finished shape to compress the parts together and eliminate air voids between the elements, as shown in Figure 5.

Core strip 16 is wound with Nichrome (Registered Trade Mark) or other resistance wire 20, and the ends of the wire may be attached to terminal pads 21. The pressure plate 18 and the insulator strips 15 and 17 are of about the same length and width as the base 19 to fit snugly within the channel 12, but the core strip 16, while about the same length as the base 19, is substantially narrower than the strips 15 and 17, to provide a gap 24 for electrical clearance between the core strip 16, its winding of wire 20, and the channel sides 14.

The bound ceramic particle insulator sheets 15 and 17 and the core strip 16 each comprise high density ceramic particles bound together by a binder, usually organic material, which has been fabricated by pressing and rolling the material together. While the strips are green, i.e. before heating to the vapor point of the binder material and sintering of the ceramic particles, the strips are pliable and bendable, but after heating to a temperature above the vapor point of the organic or inorganic binder material and after sintering of the ceramir particles, the strips become semi-brittle and hard and amalgamate into a unitary mass to insulate the resistance wire 20 embedded therein, while providing efficient heat transfer and low expansion characteristics when a current is applied to the resistance wire.

Before heating and sintering, the assembly is bendable and formable without damaging the core 16 and insulator strips 15 and 17, so the assembly may be shaped, for example into the configuration of a curved band heater 10, shown in Figure 1, or left in its extended form to be completed as a strip heater 11, shown in Figure 3. After the forming step, the assembly is fired at an elevated temperature preferably in an oxygen atmosphere, sufficient to vaporize and bake out the binder materials of the strips its, 16 and 17 and to sinter the ceramic particles, binding them together into a single mass. The applied temperature for vaporization and sintering should be less than the melting point of the metal numbers, so as to weaken those parts.

Electric leads 25 and 16, respectively, may be connected to each of the terminal pads 21, connecting the heater wires 20 to a power source. A slight extension 13 may be provided on each edge of the channel to support the lead wires, and the channel edges may be potted with suitable electrical cement 35 to close the ends and finish the connection to the heater assembly .

Means for mounting or clamping the heater assembly to or about a surface to be heated may also be connected to the finished heater assembly. Such means may comprise a band 27, which may be spot welded to the pressure plate 18, having turned and apertured ends 30, through which apertures a bolt 30 may be inserted, and clamping may be accomplished by tightening a nut 3 1 on the bolt.

In the above-described embodiment, during fabrication of the heater, instead of a formed mica core and mica insulation strips, as in the previously proposed mica heater, and instead of a preformed wire strung ceramic block, as in a conventional ceramic heater, resistance wire is wound on a core strip of organically bound ceramic particles, which is sandwiched between similar organically bound ceramic strips, and the assembly is rolled or pressed in a metal housing to eliminate air voids between the elements, whereupon formation of the heater is completed and the entire assembly is heated to bake out the binders and sinter the ceramic particles into a unitary mass embedding therein the heater wire.

The above described electric heater assembly may be conveniently formed to a desired thickness and shape without damaging its ceramic components. It has low expansion and high heat transfer characteristics and is easy to manufacture and very efficient and economical in use.

It will be appreciated that modifications of the above-described assemblies are possible.

For example, terminal connections and lead wire arrangements other than as shown may be utilized and various other details may be changed or modified without departing from the scope of the invention as defined by the append

Claims (9)

ed claims. Reference is directed to our co-pending Patent Applications Nos. 7939995 and 7942365 (Serial no. 1583772 and 1583773) whose descriptions are substantially the same as the present case, but whose claims relate respectively to the heater per se and to the intermediate core and insulator body. WHAT WE CLAIM IS:

1. A method of fabricating an electric heater, comprising the steps of assembling resistance wire on an uncured sheet of ceramic particles impregnated to a high density and bound together in heat dissipatable binder material, arranging the assembled wire and uncured sheet between uncured sheets of electrical insulator particles bound together in heat dissipatable binder material to form an assembly, compressing the assembly to substantially eliminate air voids between the sheets, and heating the compressed assembly at a tempreture sufficient to substantially dissipate the binder materials and cure the ceramic and insulator particles into an integral mass.

2. A method according to claim 1 wherein the compressed assembly is bent into a curved sllape.

3.

wherein the assembly is placed in a housing before the assembly is compressed, and the housing compressed together with the assembly.

4. A method according to claim 3, wherein the housing is closed before the compression step.

5. A method according to any of claims 1 to 4, wherein the compressed assembly is formed before it is heated.

6. A method according to any of claims 1 to 5, wherein leads for electrically connecting the resistance wire to a source of power are attached to the resistance wire after the assembly is heated.

7. A method according to any of claims 1 to 6, wherein means for mounting the assembly on a selected surface is connected to the assembly after the assembly is compressed and heated.

8. A method of fabricating an electric heater, according to claim 1 and substantially as herein described.

9. A heater fabricated by the method accord ingtoanyofclaims 1 to 8.