US3192303A - Method of reducing overheating in melting troughs and similar devices in melting and holding furnaces - Google Patents

Description

Filed Aug. 3, 1962 June 29, 1965 o ss 3,192,303

METHOD OF REDUCING OVERHEATING IN MELTING TROUGHS AND SIMILAR DEVICES IN MELTING AND HOLDING FURNACES 3 Sheets-Sheet 1 INVENTOR E.R\K ALLAN oyssou ATTORNEYS E. A. OL$$ON June 29, 1965 3,192,303 METHOD OF REDUCING OVERHEATING IN MELTING TROUGHS AND SIMILAR DEVICES IN MELTING AND. HOLDING FURNACES Filed Aug. 3, 1962 I5 Sheets-Sheet 2 Fig.2

filllHNlliilllWllll) INVENTOR ER\K ALLAN LssoN ATTORNEYS June 29, 1965 o sso 3,192,303

METHOD OF REDUCING OVERHEATING IN MELTING TROUGHS AND SIMILAR DEVICES IN MELTING AND HOLDING FURNACES Filed Aug. 3, 1962 3 Sheets-Sheet 3 EPJK ALLAN 0L5 SON BY and ATTORNEYS United States Patent For a period of about 30 years two main types of electromagnetic induction furnaces have been employed for heating and melting, namely,

(1) The medium frequency furnace which is almost ideal from the lining point of view.

(2) The melting trough furnace, which is ideal from the electrotechnical point of view.

The high investment costs and energy consumption of medium frequency or crucible furnaces, due to the neces sity of using frequency conversion equipment, has stimulated efforts to develop the use of mains frequency for this type of furnace. This work produced rapid results, so much so that a large number of mains frequency crucible furnaces are now in operation. However, despite the advantage of considerably less expensive electrical equipment there remained certain disadvantages with this type of crucible furnace which limited its application. The performance of this type of furnace, for example, is very dependent on the charge quantity. Consequently, the electrical equipment must be designed to reduce this effect. At a certain minimum charge and tilting angle of the furnace the almost constant heat losses will exceed the heat energy that it is possible to induce in the melt. This disadvantage is very pronounced in holding furnaces, so much so that in some cases the electrical circuit must be designed to supply twice or more times as much heat for low charge than that required for 100% loading. During successive and slow emptying of the furnace (e.g., during continuous casting), there is a risk that a large part of the furnace contents must be left in the furnace, because due to the unfavourable charging of a tilted furnace and the consequential deterioration in efficiency, the temperature drops below a specified minimum value. Furnaces with melting troughs are not completely independent of the degrae of charge. This is due to the so-called pinch effect which can cause constriction of the melt in the trough by the electrical flux, unless the prevailing hydrostatic pressure in the melting trough counteracts this effect. However, even when the trough is only partly charged the heat losses are so small that sufficient heat can be generated to compensate for them. With crucible furnace the charge is heated by the transference of overheated melt from the trough whereas in electromagnetic induction furnaces heat energy arises within the charge. Thus larger demands are placed on the heat resistance of trough linings of electromagnetic induction furnaces than on the linings of crucible furnaces. The extent of overheating of the lining is dependent upon the electrical power supply and upon the flow of charge from the furnace space through the melting trough. Thus the lining not only determines the highest permissible temperature of the melt but also the maximum electrical energy fed in. Conversely, a reduced depth of lining reduces the required output from the electrical supply. The interaction between the magnetic field and the electric current causes circulation in the melting trough, but since the area of the trough in relation to its depth is very small the flow of the entering new cool melt is hardly influenced. The highest overtemperature is to be found in the lower part of the trough and rises to 100 C. in normal melting troughs with inputs of about 100 kw. By asymmetrical 3,192,303 Patented June 29, 1965 ICC connection of the branch channels leading from the trough to the furnace it would seem that a thermal current effect can be brought about, but since the distance between the channels is small the consequential eifect is negligible.

During melting and holding of certain metal alloys the maximum capacity of a furnace is determined by the evaporation temperature of the constituent elements in the alloy. Thus an excessive overtemperature can cause an interruption in the closed secondary circuit" of the melt due to the generation of vapour bubbles. Similar difficulties can be encountered in furnaces where the charge is re-heated by resistance heating in specially arranged supplementary channels and passed back into the furnace.

The present invention provides a method for bringing about an improved flow of melt through melting troughs or channels where (local) overheating of the melt occurs. The range of application for the above-mentioned furnace methods is thus considerably extended. The main principle of the invention is as follows. Gas, vapour, or a combination of both, is blown into the trough or its vicinity in such a manner that a supply of entering cool melt to the trough is promoted and also the removal of overheated melt is encouraged.

The invention will be explained in detail below. The attached drawings, FIGS. 13, show necessary arrangements for carrying out the method according to the invention.

FIG. 1 shows the induction unit 13 which is connected to the bottom of the furnace 6 on one side. The induction unit consists of a sheet metal cover 4 packed with refractory material 12. The trough 3 has been formed in the refractory material by using a pattern. The melt acts as a short-circuited secondary winding for the transformer of the induction unit. The primary winding is shown by reference number 7. The primary winding and secondary winding are arranged in such a way that a heavy electric current is induced within the melt in the trough 3 when a low frequency A.C. supply is connected to the primary winding. Heat is thus generated in the trough due to the electrical resistance of the melt. A yoke 8 encloses transformer lamination-s designed to accommodate for required magnetic flux. It is important, for the purpose of obtaining a high power factor, that the primary winding 7 lies as close as possible to the secondary circuit in the trough 3. The lining between the primary winding 7 and the melting trough 3 should therefore be made as thin as possible. The intermediate space 5 between this lining and the primary winding 7 can conveniently be used by devices for the cooling and protection of the primary windings. In order to improve the power factor of the unit the branch channels are joined by an opening 15 in the furnace lining. The previously mentioned pinch effect produces circulation that assists the melting in the trough. However, appreciable overheating in the trough, especially in its lower section, arise-s owing to the following factors:

(1) The depth of the trough is large in comparison to its cross-sectional area.

(2) The outflowing already overheated melt can easily flow back into the melting trough through the opening or throat 15 between the active sections of the trough.

(3) The furnace space is relatively large.

Employing the method of this invention gas may be asymmetrically blown into the throat 15. The existing hot melt in the throat is thereby forced to mix with the cooler melt in the furnace space, improving the supply of cooler melt to the melting trough.

With the arrangement shown in FIG. 2 it is possible to improve still further the direct flow of melt from the furnace space. Overheating can thus be reduced, which in turn reduces temperature stress on the lining and permits the application of a higher electrical power supply than that permitted previously. As shown .by FIG. 2 the branch channels are not connected to each other in the furnace lining as in FIG. 1, each channel being directly coupled by its pipe 9 to the furnace space. An electrically conducting rod 11 is inserted into the lining, bridging the branch channels at a short distance from the primary winding 7. The magnetic circuit within the melt in the trough is thus made as short as possible and the power factor is consequently improved. By means of a small pipe 16 or one of the pipes 9 gas may be blown into one branch channel which causes a forced circulation of the melt in the direction of the rising gas bubbles. The injected gas has a cooling effect on the melt. As only a small quantity of gas is necessary to produce satisfactory circulation, the cooling effect is, however, negligible. This is corroborated by the following simplified example.

On the assumption that (a) all gas which is introduced is heated to the temperature of the melt before leaving the pipe 9 and thereby displaces upwards the same volume of melt as that occupied by the gas after heating,

(b) the displaced melt is replaced only from below and new melt flows into the pipe on the opposite side of the trough,

then for l 111. (based on normal temperature and pressure) of gas blown in about 27.4 tons of melt are forced to move through the melting trough. About 531 kilocalories are carried away with the gas. It the gas is monomo'lceular, this corresponds to a maximum heat loss of A C. in the flow of melt.

It 100 kw. is effectively supplied to the melt, 27.4 tons/hr. of steel flowing through the trough would be given a temperature rise of about 18 C., which is only about /5 of the overtemperature normally encountered in other types of troughs. It must be noted that as good a through-flow as is assumed in the above example cannot be achieved in practice, partly due to the fact that the gas will not be completely heated up. But the heat carried away by the gas is so inappreciable that much larger gas flows can be employed without disadvantage.

Holding by means of resistance heating of melt which flows through channels arranged outside of the furnace space is schematically represented in FIG. 3. This arrangement, in common with the previous installation, incurs overheating of the melt the degree of overheating being dependent on the rate of flow of the melt and on the electrical power supplied. By blowing in gas via pipe 25 with the method described for the melting furnace, the melt in the channel 22 is drawn upwards. Heat is supplied to the melt by resistance heating using Water-cooled terminals 23 and 24. Any type of gas that is not deleterious to the melt may be used in this process. Moreover, a gas or vapour that actively promotes a desired reaction in the melt maybe used. To prevent the gas nozzle from clogging with slag or with solidified melt, gas flow must always be started before the furnace is charged and kept up until the trough is emptied and slag has run down from the walls of the furnace. In certain cases it is desirable to keep the melting trough full with melt between charges in which case continuous gas flow must be maintained. When an expensive type of gas must normally be used for processing, it is advisable to have change-over facilities so that cheaper gas can be used during these intermediate periods. Quick change-over from one gas to another can easily be eiiected by means of multi-way cocks connected to the various gas supply lines, which also facilitates the use of mixture of gases if so required.

I claim:

1. The process of treating metals which comprises the steps of enclosing a bath of molten metal in a furnace provided with a melting and holding chamber and an auxiliary heating trough having a molten metal circulating passage communicating with said holding chamber, electrically heating the metal contained in said melting trough, and directing a stream of gaseous material into said circulating passage of the auxiliary heating trough to cause asymmetric flow of the molten metal in said passage toward said holding chamber whereby to remove the overheated melt and reduce overheating in said auxiliary heating trough.

2. The process of treating metals according to claim 1 wherein said stream of gaseous material is preheated to approximately the temperature of the melt in said auxiliary heating trough.

3. The process of treating metals which comprises the steps of enclosing a bath of molten metal in a furnace provided with a melting and holding chamber and an auxiliary heating trough having a molten metal circulating passage including two legs connected at one end of each to each other and communicating with said holding chamber at their other ends, electrically heating the metal con- 7 tained in said melting trough, and directing a stream of gaseous material into at least one leg of said circulating passage of the melting trough to cause asymmetric flow of the molten metal in said passage toward said holding chamber, whereby to remove the overheated melt and reduce overheating in said auxiliary heating trough.

References Cited by'the Examiner UNITED STATES PATENTS 1,031,257 7/12 Greene '13434 X 1,940,622 12/33 Clamer 1334 X 1,977,388 10/34 Ilberg 13-29 2,975,224 3/61 Burch 13-6 X RICHARD M. WOGD, Primary Examiner,

Claims (1)

1. THE PROCESS OF TREATING METALS WHICH COMPRISES THE STEPS OF ENCLOSING A BATH OF MOLTEN IN A FURNACE PROVIDED WITH A MELTING AND HOLDING CHAMBER AND AN AUXILIARY HEATING TROUGH HAVING A MOLTEN METAL CIRCULATING PASSAGE COMMUNICATING WITH SAID HOLDING CHAMBER, ELECTRICALLY HEATING THE METAL CONTAINED IN MELTING TROUGH, AND DIRECTING A STREAM OF GASEOUS MATERIAL INTO SAID CIRCULATING PASSAGE OF THE AUXILIARY HEATING TROUGH TO CAUSE ASYMMETRIC FLOW OF THE MOLTEN IN SAID PASSAGE TOWARD SAID HOLDING CHAMBER WHEREBY TO REMOVE THE OVERHEATED MELT AND REDUCE OVERHEATING IN SAID AUXILIARY HEATING TROUGH.

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