US2519941A - Installation for the measurement and the control of the temperature in a metal melting and particularly in a submerged resistor type induction furnace - Google Patents

Description

IN V EN TOR M. TAMA Filed May 27, 1947 MAR/o EMA BY fi ?ru/W i mram/Ey IN A SUBMERGED RESISTER TYPE INDUCTION FURNACE INSTALLATION FOR THE MEASUREMENT AND THE CONTROL OF THE TEMPERATURE IN A METAL MELTING AND PARTICULARLY Aug. 22, 1950 Patented A e. 22, 1950 i INSTALLATION FOR THE MEASUREMENT AND THE CONTROL OF THE TEMPERA- TURE IN A METAL MELTING AND PAR- TICULARLY IN A SUBMERGED BESISTOR TYPE INDUCTION FUBNACE Mario Tama, Morrisville, Pa., asignor to Ajax Engineering Corporation, Trenton, N. J.

Application May 27, 1947, Serial NO. 750,'762

2 Claims. 1 v

The invention relates to the indication, meas urement and control 'of the temperature in metal melting and particularly submerged resistor type induction furnaces. These furnaces are used at a rapidly increasing rate for the melting and casting of metal alloys and for similar Operations which necessitate a very exact temperature control.

The induction furnaces answer this requirement satisfactorily because their Operating 'conditions can be easily adjusted to the dsired processing temperatures with a high degree of accuracy.

In order to maintain the temperature of the molten metals within a predetermined narrowly limited range the continuous uninterrupted measuring and control of the temperature is an important requisite.

submerged resistor type indicator furnaces are, as commoniy known, composed of a hearth and a secondary melting loop which are housed in a refractory lined metal casing.

The melting loop consists in modern induction furnaces as for instance disclosed in prior U, S. Patents No. 2339964 of Manuel Tama, No. 2,342,617 of Manuel Tama and Mario Tama, Reissue Patent No. 22,602 of Mario Tama, No. 2,381,523 of Manuel Tama and Mario Tama and No. 2,375,049 of Manuel Tama and Mario Tema. of a channel, which is spaced from the hearth, for instance, a bottom channel and of channel sections connecting this spaced channel and the hearth; the melting loop is at its outlet or outlets through the iurnace wall closed by a removable refractory plug.

The metals to be molten and to be prooessed, for instance, cast from the furnaces are charged in the solid state through an upper opening into the hearth.

The hitherto customary temperature measuring method consists in immersing the measuring instrument, !or instance, a thermocouple having a protecting tube through the upper charging The instrument 2 I the iron is quickly dissolved and contaminates the molten metals.

The protective tubes are coated with refractory washes to prevent contamination of the !nolten charge, but the renewal of the coatings which has to be eflected at least every 24 hours is asource of continuous trouble and expense.

The substitution of iron tubes by refractory materials, for instance, by silicon carbide tubes has its merits. Silicon carbide is a good heat conductor and responds to slight temperature changes, silicon carbid is not attacked by the molten 'metals. for instance, mclten aluminum, but is, however, easily breakable.

In order to avoid these various disadvantages and to protect the thermocouple tubes against breakage and wear it has been suggested to introduce the measuring instrument into downwardly inclined passages provided in the furnace wall in such a manner that the protective tube does not extend into the hearth proper but ends within the passage short ,of the inside face of the furnace wall. The inner surface of the furnace wall is provided with a protrusion or bulge tapering into the walls of the furnace having a recess therein at the point where the passage opens into the hearth.

Consequ ently the instrument will only measure the temperature of the very small fraction of the stagnant molten charg which ascends into the inclined passage. The temperature thus measured by the instrument is different from the temperature of the bulk of the charge.

It is the primary object of this invention to provide temperature measuring and controlling means for use in metal melting and particularly in submerged resistor type induction furnaces which permit a continuous, exact and reliable indication of the true Operating temperature of the :l'urnace or the bath respectively.

It is also an important object of the invention to so locate the measuring instrument, such as a thernocouple. in the furnace that it is not contacted by the fresh charge and therefore protected against its damaging action.

It is a further object of the invention to place the thermocouple within the furnace in direct contact with that main portion of the eharge which is continuously moved and circulated through the furnace and therefore has the true average Operating temperature.

It is a further object of the invention to avoid repair and renewal of the thermocouple and to eliminate the hitherto customary unusually high costs of temperature indication in metal melting and particularly in 'submerged resistor type induction furnaces With these and additional' objects in view which will become apparent as. this speciflcation proceeds, this invention comprises in its broad aspect' the placementofthe measuring instrument and particularly a thermocouple provided with a' refractory protection tube in a section of the metal melting furnace which is spaced from the metal charging opening and the hearth and is interposed in the flow movement of the molten metal from the charge opening and/or the hearth; accordingly the instrument is protected 'against damage by the fresh charge but still kept in contact with that main portion of the metal bath which has the true temperature to be measured. In its application to a submerged resistor type induction furnace the thermocouple tube is placed in a closing plug of the furnace in such a manner that it projects into the metal conducted through the loop; a recess is preferably provided in the plug adjoining furnace wall to protect the projecting tube end against damage.

\ The invention is particularly well applicable to induction furnaces of the above mentioned type, where the melting loop is composed of a channel spaced from the hearth which, for instance, may be a bottom channel and channel portions connecting the channel with the hearth.

If used in connection with these furnaces the thermocouple tube projects into a lateral extension of the bottom channel; the plug as well as the plug accommodating furnace wall is recessed so that the projecting tube end is protected against the cleaning tools which are introduced from above into the melting loop. However, the invention is equally. well applicable to any other type of metal melting or induction furnaces as long as the metal bath is kept in movement.

several modes of mounting the thermocouple in submerged resistor type induction furnaces are shown in the accompanying drawings.

In the drawings,

Fig. 1 is a vertical sectional elevation of an induction furnace disclosed in the above mentioned patents and having the thermocouple installed in conformity with this invention,

Fig. 2 is a sectional view on line 2-2 of Fig. 1,

Fig. 3 is a vertical sectional View showing the application of the invention to an induction furnace provided with an arcuated melting loop.

The furnace shown in Fig. 1 comprises a hearth l and a secondary melting loop located underneath the hearth and composed of channel 2, two lateral channels 3 and a center chanel 4, the lateral and the center rchannel connecting channel 2 and hearth l.

The furnace is housed in a casing provided with a refractory lining 6.

The secondary blocks 'l accommodate the prir'nary and the secondary loop, the primary being composed of an iron core 20 and a coil 2l which is insulated by an asbestos sleeve 22.

Channel 2 is provided with plugs ll, IZ. Insofar the furnace is similarly built to the furnaces which are disclosed in the above mentioned patents of applicants.

In conformity with this invention the bottom plug ll is provided with a bore and this bore accommodates the thermocouple tube l3 which consists of a suitable refractory material, such as for instance siliconcarbide or graphite; the tube is sealed in the plug bore by a refractory cement layer I4; the inside of the tube which ment.

A space l 8 is recessed from the refractory furnace wall which space in this case forms an extension of the bottom channel 2. As the length of this recess is substantially equal or slightly smaller than the lengths of the projecting tube end, the instrument is protected against the cleaning tools which are inserted from above into the channels 3.

The location of the thermocouple !3 in Fig. 3 is the same 'as in Fig. 1; the only difference is that the thermocouple is here associated with the arcuated melting loop l5.

In order to fully apprehend the significance of this invention a short explanation of the manner of measuring the temperature in metal melting induction furnaces will be helpful.

The readings of the thermocouple are connected to a temperature controller, which in turn regulates the operation of the contactors leading the power to the furnace. There are two methods of control.

One is called the on-off control and the other the high-low control.

In the on-off control only one contactor is used which .connects full power of the furnace as soon as a minimum temperature is reached and disconnects the furnace entirely when the maximum temperature is reached.

In the high-low control, two contactors are used, one for full power and the other for low power. The low powered tap does not provide enough power to hold the temperature of the furnace, while the high powered tap is sufiicient to raise the temperature rapidly. When maximum temperature is reached, the high power contactor is disconnected and the low power contactor is connected. When minimum temperature is reached, the operation is reversed; the high tap goes on and the low tap goes ofl.

With both of these temperature controls it is necessary that the control instrument is located at the proper place. If the thermocouple extends into the bath located in the hearth, as hitherto customary, it will come in contact with the cold metal charged into the furnace and show a temperature which is too low.

However, if it is placed to extend into the melting loop or a melting channel in conformity with this invention, it will show the correct average temperature, because due to the energetic circulation easily obtainable in these furnaces, the thermocouple will measure the desired true bath temperature and still be fully protected against breakage and other 'injuries.

This great advantage is particularly apparent in furnaces where a unidirectional flow of the molten metal is obtained, as for instance disclosed in U. S. application Serial No. '735,851.

Whereas the temperature in the section of the hearth into which the solid metal is charged greatly differs from the true temperature of the bath, the temperature of the metal into which the thermocouple projects according to this invention is the actual Operating temperature of the furnace or of the bath because it is the temperature of the metal which is in constant movement and circulation through the melting loop and the major portion of the hearth.

Temperature measurements carried-out in a 60 kw. twin-coil furnace of the type disclosed in U. S. Reissue Patent No. 22,602, prove the measuring accuracy and reliability of this invention.

When the furnace was operated with the full power of 60 kw. the temperature difference of the metal in the hearth and in the melting loop was between 40 to 50* F., when operated with 20 kw. the difierence was only between to F. The same furnace equipped with the unidirectional flow means according to U. S. application Serial No. I35,851 only shows a difference of 10 F., which is well within the range of error of customary thermocouples.

From the above it is apparent that the invention is particularly well applicable to induction furnaces provided with a thorough circulation of the molten charge and that in this manner for the first time an automatic temperature control has been achieved in induction furnaces.

The very small temperature differences in induction furnaces which are equipped to produce a thorough metal circulation demonstrate that the location of the thermocouple in conformity with this invention satisfactorily corresponds to the requirements of a reliable temperature measurement, whereas its application in prior art furnaces having considerable temperature difierences in the hearth and in the melting channels is of minor avail.

By the placement of the thermocouple in the loop closing plug and the recessing of a space into which the tube end projects, the instrument is adequately protected against breakage and contact with the cleaning tools which are introduced from above into the furnace.

If the instrument must be repaired or the tube must be replaced which may happen after an extended period of use, this is without any difflculty or inJury to the furnace wall efiected by the easy removal of the plug and the location thereon of another thermocouple.

The invention, therefore, assures a continuous automatic temperature measurement and control in metal melting furnaces and eliminates at the same time breakage or Injury of the measuring instrument; moreover, reliable measurements are obtained of the true Operating temperature of the furnace and of the molten charge.

What I claim is:

1. In a submerged resistor type induction fur nace a hearth, a secondary melting loop composed of a bottom channel and channels connecting the bottom channel with the hear-th, a primary transformer assembly threading said secondary loop adapted to hold metal in the molten state and to cause its circulation through the hearth and the melting loop, a refractory lined casing housing the furnace, an outlet extending from the bottom channel through the lower portion of the casing, a plug ending short of the furnace lining to close said outlet whereby a recess directly connected with the melting loop is formed which is protected against mechanical attack by cleaning tools and a thermocouple located in said plug projecting with the end of its measuring tube into said recess.

2. In a submerged resistor type induction furnace for melting metals a hearth to hold a molten metal bath a secondary melting loop to circulate the molten metal through said hearth, a primary transformer assembly threading said melting loop, a refractory casing enclosing said hearth, said melting loop and said transformer assembly, an outlet extending from said melt ing loop through the lower portion of the casing, a plug to close said outlet and a thermocouple having a protective refractory tube being inserted into said plug and reaching with its protected measuring end into said melting loop.

MARIO TAMA.

REFEREN CES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,602 Tama Feb. 13, 1945 764.592 Johnson July 12, 1904 2,071,531 Hulme Feb. 23. 1937 2,102,955 Hulme Dec. 21, 1937 2,303,704 Oseland Dec. 1, 1942 2,358,09O Longoria Sept. 12, 1944 FOREIGN PATEN'S Number Country Date 98,312 switzerland Mar. 12, 1940

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