US2888506A - Salt bath furnace - Google Patents

Description

May 26, 1959 G. V. HARRIS SALT BATH FURNACE Filed June 17, 1957 .EA/70H: CQ

ATTORNEYS.'

United States Patent O SALT BATH FURNACE George V. Harris, Racine, Wis., assignor to Lindberg Engineering Company, Chicago, Ill., a corporation of Illinois v Application June 17, 1957, Serial No. 665,979

Claims. (Cl. 13-26) This invention relates to salt bat-h furnaces and more particularly to an electrically energized furnace for maintaining a bath of salt in molten condition for treating of articles therein.

Salt bath furnaces, as heretofore used, have been fuel heated or heated electrically by insertion of electrodes into the salt between which current is passed. Electrically heated salt bath furnaces are subject to several objections, including rapid erosion of the electrodes, but principally the diculty of starting up 'from a cold condition. When the furnace cools, the salt solidies and is a poor conductor. Additionally, initial current flow tends to melt the salt around the electrodes and the molten salt flows away from the electrodes interrupting the circuit. To start up such a furnace it has generally been necessary to apply heat 4from a separate source such as a burner of some sort and this is an extremely difficult operation. It has therefore been the usual practice to maintain electrically heated salt bath furnaces in heated condition at all times which results in a substantial waste of power and deterioration of the salt.

It is one of the objects of the present invention to provide a salt bath furnace in which heating current is induced in a separate annular ring in contact with the salt so that the furnace can be started up easily and quickly.

Another object is to provide a salt lbath furnace in which the furnace chamber is annular and is continuously rotated so that the articles may be loaded and unloaded at a single point and exact timing may be obtained.

According to a feature of the invention, the annu-lar ring is in the form of a metal lining for the furnace chamber when carburizing type salts are used which are not highly corrosive to metal and cannot be retained by yrefractory. When neutral salts are used, which are highly corrosive to metal and not to refractory, the ring is preferably in the form of a separate annular metallic ring laid loosely in the annular furnace chamber.

A further object of the invention is to provide a salt bath Ifurnace in which a stationary, partially annular hood, is fitted over the body and is interrupted at the loading and unloading point.

According to a feature of the invention, a cross member extends over the body centrally of the gap in the hood and carries a temperature responsive device projecting down into the salt in the chamber which device, at that position, will not interfere with loading or unloading of the furnace.

The above and other objects and features of the invention will be more readily apparent from the following description when read in connection with the accompanying drawing, in which:

Figure 1 is a perspective view of a furnace embodying the invention with portions of the framework omitted;

Figure 2 is a top plan view;

Figure 3 is a section on the line 3-3 of Figure 2; and

Figure 4 is a section similar to Figure 3 of an alternative construction.

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The furnace, as shown, comprises an annular body indicated generally at 10 which, as best seen in Figure 3, may be formed by an annular channel section shell 11 lined with refractory material 12. The refractory material is recessed centrally of the shell throughout the circumference thereof to define an annular furnace chamber in which the molten salt, as indicated at 13, is adapted to be held. It will be understood that the metal shell 11 is interrupted at at least one point in its circumference so that it will not constitute a closed current conducting element. v

The body is supported for rotation about a vertical axis centrally of the annulus on vertically extending frame members 14 carrying rollers 15 rotatable about horizontal axes to support the body for rotation and rollers 16 rotatable about vertical axes to guide the body for rotation about the desired axis. According to the present invention, the body is continuously rotated for which puipose one or more drive rollers 17 driven through motors 18 and suitable speed reducers 19 engage the body to turn it. The rate of turning may be adjusted according to the desired retention time of pieces in the furnace, but is generally a relatively slow rate requiring several minutes per revolution.

The salt in the chamber is adapted to be heated by induced current ow and for this purpose a primary unit is provided magnetically coupled to the body. The primary unit as shown, comprises a rectangular closed core 21 extending radially of the body and completely spanning the body. The core carries a winding 22 preferably lying within the body, as shown, which may be energized from any desired source of alternating current.

Since the salt, especially in solid condition, is not highly conductive there is provided, according to the present invention, an annular ring of conductive material in contact with the salt which will be heated by induced current to heat the salt. In the form shown in Figure 3, the ring takes the foim of a metallic liner 23, channel shaped in section, and mounted within the chamber. The salt is held directly in the ring 23 in contact with the walls thereof so that the heat generated in the ring will be transferred to the salt rapidly and with a high degree of efficiency. This type of ring construction is preferably employed when the salt used is a carburizing type which is not highly corrosive to metal, but which is erosive to refractory and is not well retained thereby. The ring may be formed of an anti-corrosive alloy which lis also a good material for an electric heating unit such as a nickel chrome alloy.

When the furnace is to be used with neutral type salts which are corrosive to metal and which may be retained adequately by refractory, the furnace may take the form shown in Figure 4. In this construction, the salt 13 is retained directly by the refractory lining 12 and the conductive ring may be in the form of one or more annular rings of metal, as shown at 24. The ring or rings 24 may be formed of a nickel chrome alloy, or similar material, which will generate heat in response to flow of electric current and which olfer as good resistance as possible to corrosion by the salt. These rings may be dropped loosely in the annular chamber in the body and when an existing ring becomes eroded to the point where it is no longer effective, it may be removed and a new ring may be placed in the body. Since the rings are of simple construction they are relatively inexpensive so that this type of operation is completely feasible.

Since the salt when heated ygives olf undesirable fumes, the furnace is preferably hooded to the maximum extent and may be connected to a stack, or the like, to disperse the fumes. As shown, an arcuate hood 25 stack action.

partially annular and of channel section is provided. The hood is placed over the body with the channel inverted so that the flanges of the channel will overlap the sides of the metal shell 11, as shown in Figure 3, with the web of the channel spaced above the open top of the annular chamber. At one or more points the hood may be connected to a vent 26 from which air and fumes may be withdrawn by a fan or by' natural With this construction, the fumes generated by the molten salt are removed without interfering with rotation of the body. The hood may be supported, as shown, by the upper ends of certain of the supporting frame members 14 in the desired position relative to the furnace body.

The opening left by interruption of the hood may be utilized as a loading and Iunloading station for articles to be treated. While the articles may be handled in any desired manner, one conventional method is illustrated by which articles of any desired type,-as indicated at 27 in Figure 3, are suspended by wires from cross rods 28 which may simply be laid across the top of the furnace body spanning the annular chamber therein, as shown. As the body turns, an operator standing at the unloading and loading station may remove rods 28 passing out from under the hood and may place additional rods 28 loaded with articles to be treated on the body near the point at which it is entering the hood.

In order to control or indicate the temperature of the bath a temperature responsive device is provided, as shown at 29, projecting into the bath. The device 29 is carried by a cross member 31 which may be in the form of an extension of one of the uprights 14, as

shown, and which extends radially across the top of the furnace body centrally of the gap in the hood 25. ln this position, rods 28 may be removed from one side of the cross member and placed on the body on the other side of the cross member so that no rods will be carried by the furnace body as it passes under the cross member. In this way, the temperature responsive device may project into the salt in the chamber without interfering with loading or unloading thereof. The temperature responsive device is preferably a suitably enclosed thermocouple or like temperature sensing instrument which may supply a signal to a controller for regulating primary current flow thereby to maintain the temperature of the salt accurately within the desired It has been found that with a furnace, as shown, temperature can be raised from room temperature to 1400" in approximately one hour and without the addition of any external heat. When starting up, even though the salt is solidified, the annular ring 23 or 24 will heat rapidly and all of the heat generated thereby, with the exception ofthe small amount lost by radiation will be transferred to the salt. The salt will therefore be heated regardless of its own conductivity whether it is in molten or solid form.

While two embodiments of the invention have been shown and described herein, it will be understood that they are illustrative only and not to be taken as a detinition of the scope of the invention, reference being had for this purpose to the appended claims.

What is claimed is:

1. A salt bath furnace comprising a horizontal body formed with an annular chamber adapted to hold molten salt, an annular member of electrically conductive material in the chamber in contact with the salt therein, means mounting the body for rotation about a vertical axis coaxial with the center of the annular body, means for continuously rotating the body, and an electrical primary unit magnetically coupled with the annular member to induce a ow of current therethrough, the primary unit being stationary and the body lturning relative thereto.

2. I'he furnace of claim 1 in which the annular member is a metal liner for the annular chamber to hold the salt.

3. The furnace of claim 1 in which the body is formed of refractory non-conducting material and the annular member is a metal ring lying in the chamber.

4. The furnace of claim 1 including a stationary circular channel section hood fitting over the body and interrupted at one point for loading and unloading parts to be treated.

5. The furnace of claim 4 including a temperature responsive device mounted intermediate the interrupted portion of the hood and projecting downward into the salt in the chamber.

6. A salt bath furnace comprising an annular body formed of refractory material and having an annular open topped chamber therein to hold molten salt, an electrical primary :unit including a closed section core around the body at one side thereof and an energizing winding on the core, means mounting the body for rotation through the core, means for rotating the body, and an annular metal ring in the chamber in contact with the salt therein.

7. A salt bath furnace comprising an annular body formed of refractory non-conductive material and having an annular open topped chamber therein to hold molten salt, an electrical primary unit including a closed section core around the body at one side thereof and an energizing winding on the core, means mounting the body for rotation through the core, means for rotating the body, a partially annular hood of channel section fitting over the body and leaving an opening at one side for loading vand unloading articles to be treated, and an annular metal ring in the chamber in contact with the salt therein.

8. A salt bath furnace comprising an annular body formed of refractory non-conductive material and having `an annular open topped chamber therein to hold molten salt, an electrical primary unit including a closed section core around the body at one side thereof and an energizing winding on the core, means mounting the body for rotation through the core, means for rotating the body, a partially annular hood of channel section fitting over the body and leaving an opening at one side for loading and unloading articles to be treated, a temperature responsive device mounted centrally of the opening in the hood, and projecting into the salt in the chamber, and an annular metal ringin the chamber in contact with the salt therein.

9. The furnace of claim 8 in which the ring forms a channel section liner for the chamber.

10. The furnace of claim 8 -in which the ring lies loosely in the bottom of the chamber.

References Cited in the le ofthis patent UNITED STATES PATENTS France June 23,

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