GB2194623A - Furnace - Google Patents

Abstract

A furnace comprising thermal insulation 15 within a housing, a tube 16 through the housing, a belt 12 of fused silica glass fibres to convey items through the tube, and heating means 14 to heat the tube. The tube 16 may be of fused silica or mullite. A water-cooled chilling plate 19 is provided at the exit. An atmosphere of nitrogen and hydrogen is maintained in tube 16 by supplying ammonia to a catalytic cracker embedded in the insulation 15. Controls for the cracker and heating means are disclosed. <IMAGE>

Description

SPECIFICATION Furnace This invention relates to a furnace. The furnace of the invention is suitable for heat treating jewelry.

Manufacture of many items of jewelry involves a heat treatment step in which parts of the item which are previously press-fitted or otherwise held together become joined by solder. For this to occur, it is usual to provide a jewelry furnace which may be of a type through which the items of jewelry are conveyed on an endless belt and in which a protective atmosphere is maintained under slight overpressure.

Existing jewelry furnaces are large and heavy and not particularly economical to run. Thus, they can be 3 metres long, weight 2 tons and consume 6 kilowatts of power. They usually have an endless belt of chain mail like construction on which the jewelry items are carried. This needs to be removed when the furnace is turned off otherwise it will contract and fracture or stretch. Also, the belt has apertures in it in which very small items of jewelry may become stuck and occasionally items become soldered to the belt instead of to each other.

The aim of this invention is to provide a furnace improved in at least some of the above respects.

According to a first aspect of the present invention, there is provided a furnace comprising a housing, thermal insulation in the housing, a tube through the housing, a belt made of fused silica glass fibres to convey items through the tube and a heating means to heat the tube.

The use of fused silica glass fibres for the belt of the furnace of the invention greatly improves the furnace relative to previously known furnaces. The belt will normally be woven, relatively tightly, so that while being flexible it has no apertures through which small items might pass. The prospects of items being inadvertently soldered to the fused silica glass fibre belt do not arise, and the belt, having a coefficient of thermal expansion of zero or almost zero does not need to be removed when the furnace is turned off.

Preferably, the tube in the furnace is of rectangular cross-section and preferably of a cast ceramic material such as mullite or fused silica. In this respect as well the invention offers great improvements over the known constructions because the invention allows the tube to be of considerably greater cross-section, thus increasing the capacity of the furnace.

Preferably also the thermal insulation within the housing is a mass of ceramic fibres. Previously known furnaces have employed brick insulation which is far heavier and more bulky for the same effect as the ceramic fibres proposed for the present invention.

The heating means can be resistance heater elements embedded in such a mass of ceramic fibres which can very simply be machined to provide recesses in which the elements can be located. Thus, the elements can be located very close to the tube and provide very effective transfer of heat to the interior of the tube and the jewelry conveyed therethrough upon the belt.

A further preferred feature of the present invention is the provision of a cooling zone extending from the housing and through which the belt passes on leaving the tube and the provision of a chiller plate beneath the run of the belt in the cooling zones. Because the fused silica glass fibre belt has very low thermal conductivity, it does not suffer from the problem encountered with metal belts that they need to travel a considerable distance from the heated zone in order to cool down.

In contrast, with the belt of the present invention, and a chiller plate therebeneath, a cooling zone of sufficient length to allow the temperature of the jewelry items to be reduced to that at which they can conveniently be handled is quite short.

It is normal with jewelry furnaces to obtain the necessary inert atmosphere within the furnace by cracking ammonia to provide nitrogen together with hydrogen as a reducing agent.

This is a cheaper way of obtaining nitrogen than directly from a nitrogen gas supply, while the provision of hydrogen as a reducing atmosphere means that fluxes do not have to be provided. In previously proposed furnaces it has been necessary to provide a large subsidiary plant for the cracking operation. With the present invention, however and particularly with the use of the feature of the ceramic mass insulation, it is possible to perform the cracking operation within the same housing as the furnace which greatly simplifies and cheapens and reduces the weight of the overall construction.

It should be understood that the invention has aspects in addition to the above described first aspect involving the provision of a belt of fused silica glass fibres. In another aspect the invention provides a furnace comprising a housing, thermal insulation within the housing, a tube through the housing, a belt to convey items through the tube and heating means to heat the tube, wherein the tube is a mullite tube. Preferably the tube is of rectangular cross-section. It could alternatively be of fused silica.

In another aspect the invention provides a furnace comprising a housing, thermal insulation within the housing, a tube through the housing, a belt to convey items through the tube and heating means to heat the tube, wherein the thermal insulation comprises a mass of ceramic fibres. The mass of ceramic fibres may be shaped to accommodate a heat ing element which is the heating means at a position adjacent the tube.

In another aspect the invention provides a furnace comprising a housing, thermal insula tion within the housing, a tube through the housing, a belt to convey items through the tube, heating means to heat the tube and a cooling zone extending from the housing and through which the belt passes on leaving the tube, with a chiller plate beneath the run of the belt in the cooling zone. Preferably the chiller plate is water cooled.

The above aspects can, according to the invention, be provided singly or in any combi nation of two or more.

In order that the invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a transverse cross-section through a furnace of the invention, taken on the line X-Y of Figure 2; Figure 2 is a longitudinal section of the same furnace and Figure 3 is a schematic illustration of the control system for a furnace of the invention.

As shown in Figures 1 and 2, the furnace comprises a housing having thermal insulation 15 which comprises a mass of ceramic fibres.

Located within that mass is a ceramic tube 16 of rectangular cross-section and preferably cast of mullite. Above and below the tube and adjacent thereto are resistance heating ele ment 14 used to heat the tube. These ele ments can be of zig-zag formation and with a distribution along the length of the tube se lected so as carefully to control the tempera ture profile and temperatures experienced by items being conveyed through the tube.

Numeral 12 indicates an endless belt woven of fused silica glass fibres passing over a driven roller 20 and a free running roller 11.

The belt, preferably, has a thickness of 1 to 3 mm, most preferably 2 mm. It may have its edges turned over and stitched down. A pinch roller 21 on a spring-urged arm urges the belt against the driven rqller 20. The top run of the belt moves from left to right in Figure 2.

Shown- particularly- in Figure 1 is a catalytic cracker with an inner tube 22 surrounded by a heating element 23 and to contain a nickel oxide catalyst. Shown at 10 is a control valve for the supply of ammonia via tube 24 to the cracker in which, under heat, it will be sepa rated into nitrogen and hydrogen to be sup plied via a pipe 27 to the interior of the tube 16 Thermocouples 25 and 26 respectively sense the temperatures in the tube and cracker.

As shown in Figure 2, downstream of the tube is a cooling zone through which the belt passes and in which there is provided, beneath the belt, a chiller plate 19. This may take the form of a water cooled plate being for instance a plate with a pipe for the water attached to its lower side, or a construction with bores to provide a water path. The chiller plate contacts the belt and rapidly cools it because little heat is conducted along the belt from the furnace.

The atmosphere within the tube and cooling zone is kept at slight overpressure so as to prevent the ingress of air and the gas is continuously burned off at the end of a flare tube 17. An adjustable gate 18 is provided at the downstream end of the passage through the cooling zone and tube and there is a similar gate at the other end where insulation material 13 is shown, this being the introduction zone to the furnace.

The belt will normally be provided with a variable speed drive likely to offer speeds between 10 and 1000 millimetres per minute.

The temperature within the tube will of course be controllable because some jewelry making operations require temperatures of around 900"C while others are conducted at lower temperature. The greatly reduced size of the furnace compared with prior furnaces means that the fuel consumption can be very significantly reduced and the furnace can be run off normal mains supply, which was not the case with the prior art. Further, as can be appreciated, the furnace is compact and light due to the use of improved insulation and the fused silica glass belt, while its capacity can be improved due to the use of the cast ceramic tube which is preferably of rectangular crosssection.In the prior art, where the present invention has a cast ceramic tube, there were two tubes, an inner one of Inconel, a nickel alloy on which an heating element was mounted surrounded by a ceramic tube. This was complicated and expensive to make and did not provide very much capacity.

Figure 3 is a schematic view of circuitry appropriate for a furnace of the invention. It will be seen that this is simply connectable to mains and the first item is a double motor switch 30 controlling the supply to a motor control board 31 to which are connected a motor M and a potentiometer 32 to act as a speed controller.

Next is a circuit for the cracker 22, 23 having a double cracker switch 33 to control, via a fuse, supply to a thyristor stack 34, operation of which is controlled by a temperature controller 35. The temperature controller, which is active whenever connection is made to the mains, is responsive to a selectable temperature and to the temperature detected by the thermocouple 26 adjacent the cracker heater element 23 to control the operation of the thyristor stack, and thus the time at which current is applied, during each cycle, to the cracker heater element.

The cracker circuit also includes a control for the gas supply, in particular for controlling the valve 10. The valve 10 is only opened to allow passage of ammonia when the cracker switch is closed and when the temperature sensed by the thermocouple 26 is within a given range. It is a function of the temperature controller, therefore, to so control the relay 7,8 that the valve 10 is only opened at these appropriate conditions, and in particular that valve is closed in the event of the cracker switch being opened or the- temperature being wrong. This is an important safety feature.

The final element of the circuit is for the oven heater 14 which has a temperature control 36 operable whenever the furnace is connected to mains, and which controls a thyristor stack 37 in response to the selected temperature and the temperature detected by the thermocouple 25, so as to control the proportion of the cycle for which current is supplied to the heating elements 14, when the double oven switch shown at 38 is closed.

Claims (18)

1. A furnace comprising a housing, thermal insulation within the housing, a tube through the housing, a belt of fused silica glass fibres to convey items through the tube, and heating means to heat the tube.

2. A furnace according to claim 1, wherein the belt is 1 to 3 mm, preferably 2 mm thick.

3. A furnace according to claim 1 or 2, wherein the belt is woven and has its edges turned over and stitched down.

4. A furnace according to claim 1, 2 or 3, wherein the tube is of substantially rectangular cross-section.

5. A furnace according to claim 1, 2, 3 or 4, wherein the tube is of ceramic material.

6. A furnace according to any one of claims 1 to 5, wherein the tube is of mullite.

7. A furnace according to any one of claims 1 to 5, wherein the tube is of fused silica.

8. A furnace according to any preceding claim, wherein the thermal insulation comprises a mass of ceramic fibres.

9. A furnace according to claim 8, wherein the heating means comprises one or more resistance heating elements embedded in the mass of ceramic fibres.

10. A furnace according to claim 9, wherein the elements are located in grooves machined in the mass of ceramic fibres.

11. A furnace according to any preceding claim, which includes a cooling zone extending from the housing and through which the belt passes on leaving the tube and a chiller plate beneath the run of the belt in the cooling zone.

12. A furnace according to claim 11, wherein in use the chiller plate is cooled by water passed therethrough.

13. A furnace according to any preceding claim, wherein a catalytic cracker is positioned in the housing within the insulation to supply appropriate atmosphere to the interior of the tube.

14. A furnace according to claim 13, including a solenoid operated valve and logic circuitry which only permits supply to the cracker in the event of the cracker obtaining the correct temperature and being switched on.

15. A furnace comprising a housing, thermal insulation within the housing, a tube through the housing, a belt to convey items through the tube and heating means to heat the tube, wherein the tube is a mullite tube.

16. A furnace comprising a housing, thermal insulation within the housing, a tube through the housing, a belt to convey items through the tube and heating means to heat the tube, wherein the thermal insulation comprises a mass of ceramic fibres.

17. A furnace comprising a housing, thermal insulation within the housing, a tube through the housing, a belt to convey items through the tube, heating means to heat the tube and a cooling zone extending from the housing and through which the belt passes on leaving the tube, with a chiller plate beneath the run of the belt in the cooling zone.

18. A furnace constructed and arranged substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.