GB2161591A

GB2161591A - Coreless induction furnace

Info

Publication number: GB2161591A
Application number: GB08417952A
Authority: GB
Inventors: Robert Stanley Ellis
Original assignee: IPW Ltd
Current assignee: IPW Ltd
Priority date: 1984-07-14
Filing date: 1984-07-14
Publication date: 1986-01-15
Also published as: GB8417952D0

Abstract

The furnace has an outer jacket 1 including a power input winding 3 and flux guides 4 and containing a separate and removable crucible 2. The crucible includes a non- magnetic metal, e.g. stainless steel, casing 5 and an inner layer of refractory material 10. The casing 5 is longitudinally divided into hemi- cylindrical parts 5a, 5b, and a disc- shaped base 5c, which are all electrically insulated from one another to prevent circulation of appreciable heating currents in the casing. An expansion-absorbing layer 8 is interposed between the walls of the casing and the refractory lining 10, and the base 5c is covered by heat-insulating bricks 9 and has an electrically insulated central hole through which an earthing spider can be inserted into the crucible chamber 11. The casing 5 is provided with pipework 6a, 6b, through which coolant can be pumped. <IMAGE>

Description

SPECIFICATION Coreless induction furnaces Induction furnaces of the so-called coreless type rely on electrical currents induced in a charge of alloy or other metallic material to produce working temperatures of up to 1650"C or more.

Such furnaces conventionally comprise a cylindrical power input winding which is intimately bonded to an internal refractory lining by an intermediate layer known as mudding.

However, with continued use the refractory lining eventually deteriorates to the point where it has to be knocked out and replaced.

This process frequently results in damage to the winding itself making the repair job very expensive. It has also been found that the thermal expansion to which the lining is subject in use can itself result in damage to the winding.

The present invention is primarily aimed at alleviating these longstanding problems.

There is now proposed a coreless induction furnace comprising an outer jacket including a power input winding, and a separate and removable crucible located within the outer jacket, the crucible including a non-magnetic metal casing which is divided to prevent circulation of appreciable heating currents therein, and a lining of refractory material.

Thus, when the refractory lining requires replacement the crucible can be removed for repair without disturbance to the winding.

Furthermore, a replacement crucible can be inserted immediately so that the furnace is not out of commission for a prolonged period.

This feature is also advantageous in situations where different metals are required to be melted since the crucible can be changed over between melts to remove the danger of contamination. This arrangement also permits the induction winding to be relieved of many of the thermal expansion forces to which it is subject in conventional designs.

Within the broad idea of the invention the casing could be round, square, rectangular or conical for example. The important thing is that it should be divided to form an electrical break that substantially prevents heating currents from being induced in the casing walls.

It will be appreciated that the major currents tend to be induced in a path around the axis of the winding and lying substantially within its length. The division should therefore be arranged so that no such path exists.

According to a preferred arrangement, the casing is generally cylindrical and is longitudinally divided into two or more parts and includes a separate disc-shaped base which lies outside the length of the input winding, the said parts and the base all being electrically insulated from one another.

There is preferably an expansion-absorbing layer interposed between the casing and the refractory lining.

The invention includes a crucible for use in a furnace as described above and including a non-magnetic metal casing which is divided to prevent circulation of appreciable heating currents therein, and a lining of refractory material.

The invention will now be exemplified with reference to the accompanying drawings in which: Figure 1 is a vertical section through a coreless induction furnace, Figure 2 is a plan view of the furnace, partly in section, and Figure 3 is an exploded perspective view of a casing which forms part of the furnace crucible.

For convenience the furnace is shown with its axis vertical.

Referring to Figs. 1 and 2, the furnace comprises an outer jacket 1 and a separate and removable inner crucible 2.

The outer jacket 1 is held by a rigid steel supporting frame which for clarity has been omitted from the drawings. The frame may be static or tilting. The jacket comprises an upright cylindrical input winding 3 formed of a metal tube through which a liquid coolant can be pumped. Around the outside of the winding are placed eight C-shaped laminated iron flux guides 4. Each guide comprises an axial section 4a and a pair of radial end sections 4b directed towards the interior of the furnace.

Each guide is radially adjustable.

The crucible comprises a non-magnetic stainless steel casing 5 which, as shown in Fig. 3, includes two generally hemi-cylindrical parts 5a, 5b, and a disc-shaped base Sc lying beyond the length of the winding. The meeting edges of the three parts are flanged and are separated by mica-based high temperature electrical insulation material. The three parts are joined by nuts and bolts (not shown) inserted through the flanges and provided with bushes and washers of similar insulating material. There is therefore no electrical connection between the three parts. Each hemicylindrical part 5a, 5b, is provided with external pipework 6a, 6b, respectively, mounted in intimate thermal contact with the case walls to provide cooling channels through which a suitable coolant can be pumped. The upper rim of the case carries a two-part annular flange 7.

The inside walls of the case are lined by a layer of asbestos mill board 8, and the floor of the case is lined by a double layer of heatinsulating bricks 9. Within this is a layer of refractory heat-insulating material 10 which defines the melting chamber 11 of the crucible. The purpose of the layer 8 is to take up any relative expansion between the refractory lining 10 and the casing 5.

The base of the casing 5 contains an electri cally insulated central hole 12 through which an earthing spider (not shown) can be inserted projecting through the layers 9 and 10 into the chamber 11 to earth the contents of the chamber.

The casing of the crucible, which is normally electrically floating, carries an earth leakage sensor (not shown) connected to an external monitor to give warning of any breakthrough in the refractory lining. The outside wall of the crucible also carries a temperature sensor to control the flow of coolant through the winding 3 and/or the cooling channels 6a, 6b.

In use, the charge of alloy or other metallic material to be melted is placed in the chamber 11 and the winding 3 is coupled up to a suitable a.c. power source, which may be at normal 50/60 Hz mains frequency or at a higher or lower frequency.

The lines of magnetic flux, which are directed into the interior of the furnace by the guides 4, tend to ignore the divided casing 5 and induce an electrical heating current in the charge, as in a conventional induction furnace.

When the refractory lining eventually becomes worn the crucible can be extracted from the outer jacket enabling the lining to be repaired or replaced without risk of damage to the winding 3. The winding is also less liable to damage due to expansion of the refractory lining than in a conventional furnace. It is a simple job to exchange one crucible for another so that the risk of contamination with material from a previous melt can be eliminated.

It will be appreciated that the input winding 3 may be a single winding as shown or it may be a multiple winding, e.g. three, fed from either a single or multi-phase supply. Similarly, forced air cooling may be employed in place of the cooling system illustrated.

Although eight flux guides 4 are shown in the drawings the number could be varied as required. It is normally desirable for there to be fairly close contact between the ends of the flux guides and the walls of the crucible 2, although a small gap is desirable to allow for thermal expansion.

An auxiliary layer of thermal insulation could be provided between the crucible and the winding 3 to protect the winding, particularly in high temperature work.

Claims

1. A coreless induction furnace comprising an outer jacket including a power input winding, and a separate and removable crucible located within the outer jacket, the crucible including a non-magnetic metal casing which is divided to prevent circulation of appreciable heating currents therein, and a lining of refractory material.

2. A furnace according to Claim 1, in which the metal casing is generally cylindrical and is longitudinally divided into two or more parts and includes a separate disc-shaped base which lies outside the length of the input winding, the said parts and the base all being electrically insulated from one another.

3. A furnace according to Claim 2, in which the meeting edges of the component parts of the metal casing are flanged, the flanges being separated by a high temperature electrical insulation material.

4. A furnace according to Claim 3, in which the flanges are joined by fastenings inserted through the flanges from which the fastenings are insulated by bushes and washers of high temperature electrical insulation material.

5. A furnace according to any preceding claim, in which the metal casing is provided with cooling channels through which a coolant can be pumped.

6. A furnace according to any preceding claim, in which there is an expansion-absorbing layer interposed between the metal casing and the refractory lining.

7. A furnace according to Claim 6, in which the expansion-absorbing layer is of asbestos millboard.

8. A furnace according to any preceding claim, in which the base of the crucible is covered by heat-insulating bricks interposed between the metal casing and the refractory lining.

9. A furnace according to any preceding claim, in which the base of the crucible contains an electrically insulated hole through which an earthing spider can project into the melting chamber of the crucible to earth its contents.

1 0. A furnace according to Claim 9, in which the metal casing carries an earth leakage sensor connected to an external monitor to give warning of any breakthrough in the refractory lining.

11. A coreless induction furnace which is substantially as described above with reference to the drawings.

1 2. A crucible as defined in any of Claims 1 to 10, for use in a furnace in accordance with Claim 1.

1 3. A crucible which is substantially as described above with reference to the drawings.

14. Any novel feature or novel combination of features described above and/or shown in the drawings.