US3907491A - Molten metal superheater and method of using the same - Google Patents

Abstract

A heat exchanging device is provided for superheating molten metals, such as aluminum, which are reflective of radiant heat. Conductive channel walls are provided in a superheating furnace, said walls absorbing, conducting, and transferring heat from a high-temperature burner to the molten metal, which passes between said channel walls at high flow rates. Such superheated molten metal may be utilized for melting additional solid metal stock, for casting, or as desired.

Description

United States Patent Morse 1 Sept. 23, 1975 1 MOLTEN METAL SUPERHEATER AND 1,825,556 9/1931 Stevenson 432/210 METHOD OF USING THE SAME 2,024,048 12/1935 Kyriacou 432/28 3,249,347 5/1966 Wille 432/20 X [75] Inventor: David V. Morse, Niagara Falls, NY.

[73] Asslgneez 32%;??? Company Primary Examiner.l0hn J. Camby g Attorney, Agent, or Firm-David E. Dougherty;

[22] Filed: May 22, 1974 Raymond W. Green; Herbert W. Mylius Appl. No.: 472,394

Related US. Application Data Continuation of Ser. No. 300,924, Oct. 26, 1972, abandoned.

References Cited UNITED STATES PATENTS 2/1931 Kutchka 432/210 [57] ABSTRACT A heat exchanging device is provided for superheating molten metals, such as aluminum, which are reflective of radiant heat. Conductive channel walls are provided in a superheating furnace, said walls absorbing, conducting, and transferring heat from a hightemperature burner to the molten metal, which passes between said channel walls at high flow rates. Such superheated molten metal may be utilized for melting additional solid metal stock, for casting, or as desired.

16 Claims, 2 Drawing Figures US Patent Sept. 23,1975 Sheet 1 of2 3,907,491

FIG. 2

FIG. I

US Patent Sept. 23,1975 Sheet 2 of2 3,907,491

FIG. 2

MOLTEN METALSIlPERI-IEATER METHOD BA CKGR OUND OF THE INVENTION In the preparation of useful shapes from metals, such as aluminum, it is necessary to reduce the metal to a liquid state. It has been customary in the art of melting metals to maintain a bath of molten metal, into which solid metals are submerged to effect a rapid heat trans fer from the molten metal to the solid metal, so that the solid metal is reduced to a liquid state. Large pieces of metal to be thus melted have been handled without undue difficulty, since "they sink'rapidly in the melt. Small metal particles, however, are difficult to immerse, and accordingly tend to be exposed for an exa relatively large body of molten metal, and is poured ordirected over, through, or into solid metal pieces, so as to rapidly immerse or engulf the solid metal to thereby effect'a rapid heat transfer between the melt and the solidmetal. While this process is successful in many instances, its success is to a great degree dependent upon the'size and condition of theparticles, and the rate of flow of the molten metal over the particles. The flow rate may not be too high however, since splashing and severe oxidationmay result. Further, the presence of contaminated cut particles, such as oilsoaked cuttings and turnings, further slows the melting of such'materials.

An improvement in the handling of molten metals is set forth in U.S. Pat. No. 3,272,619, to Sweeney et al., issued Sept. 13', 1966. Said Sweeney et al patent discloses a method and apparatus for melting metal, particularly light metals such as aluminum, wherein a stream of molten metal is continuously withdrawn from a body of molten metal and poured or directed at a relatively constant rate along a curved path, thus forming a vortex or whirlpool into which solid metal particles are introduced. The intensity of the vortex or whirlpool is controlled so as to immerse the solid metal in the molten metal at any desired rate. By this means,'contaminated solid metal particles may be introduced into the bath by maintaining the contaminated particles on or in the slag layer until the contaminants have been burned off, and then quickly immersed in the bath prior to oxidation of the particles. I

One disadvantage of prior art techniques has been the limited rate at which metal stock may be reduced to molten metal. It has been known for some time that one means to increase the capacity of melting equipment is to increase the temperature of the molten metal which is employed to melt the solid material. It has been further known that onemeans to accomplish this would be by increasing, the rate of heat transfer to the molten metal. Since the rate of heat-transfer is a function of overall heat transfercoefficient, the area'of the heating surface, and the temperature differential,*variattempts have been made to increase melt rates by varying the individual factors involved. For example, the temperature differential may be varied by such means as the use ofpreheated fuels and air. and the choice of various fuels. Attempts have been made to vary the area of heating surface, and furnace areas have been increased a great deal. However, there is a practical limit to the size of furnaces, both because of melt loss and volume of metal, and the relative costs involved. Little attempt has been made to vary the third factor, however, theoverall heat transfer coefficient which has, prior to the present invention, been considered a constant value, due to the lack of essentials to change it.

SUMMARY OF THE INVENTION It has now been determined that by the use of a superheating furnace as disclosed herein, high melt rates may be achieved. These improved melt rates are achieved in an economical, adaptable, and efficient manner.

The present invention comprises a technique for absorbing and transmitting heat which would normally be reflected by a molten metal such as aluminum. This is accomplished by incorporating channel walls into a superheating furnace. The channel walls absorb, conduct, and transfer heat from a high-temperature burner to the molten metal through intimate contact (conduction). It has further been discovered that the heat transfer is enhanced if the molten metal is passed rapidly across the channel walls, so that the Reynolds number defines transition or turbulent flow.

Thus, one object of this invention is to provide a process and apparatus for the rapid and efficient superheating of light metals such as aluminum.

Another object of this invention is to provide apparatus whereby various metals and alloys may be rapidly and efficiently reduced from the solid state to the liquid state.

Still another object of this invention is to provide an economical means for rapidly melting large volumes of metal. These and other objects of the invention are accomplished by the process and equipment herein set forth.

BRIEF. DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an overall view of a crucible melt system and superheating furnace embodying the principlesof this invention. I

FIG. 2 illustrates, in cross section, the novel superheating furnace of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention provides an improved method for superheating molten metal. The superheated metal may then be used, as in the preferred embodiment, to melt additional solid metal. In accordance with this aspect of the invention, molten metal is transferred, preferably without any contact with an oxidizing influence such as air, from beneath the surface of a body of molten metal to an elongated conduit which passes through a superheating section. The molten metal is heated inthe superheating section to a temperature substantially higher than the melting temperature of the molten metal, and then transferred to a crucible wherein solid metal is contacted by said superheated liquid metal to thereby cause the dissolution or. melting of said metal.

In one embodiment of the invention, the moltenrnetal is withdrawn from a holding crucible, passed through a superheating furnace, and directed into a separate melting chamber such as disclosed by Sweeney et al, U.S. Pat. No. 3,272,619. It is also contemplated, however, that the crucible melt system may comprise but a single melting crucible. from which molten metal is directly withdrawn for passage to the superheating chamber. It is to be recognized, of course, that molten metal is to be withdrawn from the system at a rate commensurate with the rate at which solid metal is added.

it is also to be recognized that the superheating furnace of this invention may be utilized for any purpose for which a substantial degree of superheating of molten metal is desirable.

As previously noted, a number of attempts to increase heat-transfer rates have been made, but little has been done to vary the coefficient of heat transfer between a fluid metal and a heating surface.

It may be shown mathematically, however, that the heat-transfer coefficient is velocity dependent. This may be seen emperically through a consideration of the temperature distribution of a fluid film in contact with a solid heat exchange medium. As one thins the fluidwall interface, i.e., decreases the distance from the measuring point to the wall, the temperature approaches that of the wall. That is, the smaller the distance from the heat exchange surface, the smaller the differential in temperature between the fluid and the heat exchange surface. One means for accomplishing this thinning of the interfacial barrier is to increase the rate of fluid flow. Accordingly, it may thus be seen that an increase in velocity of a molten metal along the surface of a heat exchange material will result in an increased capacity for heat exchange.

In conjunction with the above, one must also consider the efficiency of the heat exchange media employed. It is well-known that some metals, such as aluminum, are highly reflective of radiant heat energy. For such metals, it is essential that conductive heat exchange be employed to as great an extent as possible. Thus, one wishes to minimize the surface area of the fluid metal which is subjected to radiant heat,.while maximizing the surface area subjected to conductive heating, i.e., metal to heated surface contact.

It has now been found that this may best be achieved by devising a furnace in which the molten metal is directed, by means of partition walls, through a series of radiantly heated channels. Although it is possible to completely enclose said molten metal in externally heated conduits. it is more economical to employ a hearth having longitudinal partitions, forming open channels which may be radiantly heated from above by an open flame.

For greatest efficiency, as previously indicated, it is desirable to maximize the area of conductive heat exchange while reducing the area of radiant heat exchange. This may be achieved by so proportioning the channels so as to have a high channel wall, with a minimal width of molten metal. It has been found that a depth to width ratio of at least 1:1 is essential, with a ratio of 3:1 or greater being preferred, up to about l:l.. It is of course to be recognized that nonrectangular channel cross-sections may be employed, such as truncated triangles, or U-shaped channels, without departing from the tenor of the invention. In such cases, it should be recognized that the ratio of conductively heated areato radiantly heated area. must be maximized.

The longitudinal shape of the channel may be varied, but it has been found that a zigzag pattern offers an efficient utilization of channel-forming material, in that adjacent channels contact opposite sides of a single channel wall. If desired, alternative forms are possible, such as a decreasing-diameter spiral, but at higher cost. It is also possible to tilt the hearth in the desired directionof flow, although at the flow rates contemplated for the superheating furnace of this invention a level hearth floor is acceptable.

As previously indicated, a high flow rate of molten metal is desirable, both to increase heat exchange effectiveness and to insure that the channels are kept clean and free from pools, or dead spots, particularly at turning or reversal points of flow. It has been found that while lower flow rates do achieve an improvement over priorart techniques, flow rates in excess of 200 pounds per minute are to be preferred, with rates in excess of 1,500 pounds per minute particularly suitable.

It should be readily apparent that the channel forming material should be selected from those materials possessing high heat conductivity and resistance to molten metals, such as metals having high melting temperatures, and refractories. Further, since the channelforming material is heated primarily by radiation, and by contact with the combustion products of hightemperature burners, this material should be capable of withstanding high temperatures and absorbing great amounts of radiant energy. Exemplary of such materials are bonded silicon carbide refractories.

As previously indicated, heat is provided to the molten metal flowing through the superheating furnace by such means as burning an external fuel therein. Since a highly oxidizing atmosphere may be detrimental to a molten metal bath, it is preferred to produce a reducing atmosphere in the superheating furnace zone by maintaining an oxygen deficiency therein. It is to be understood, of course, that conventional furnace practices and materials may be employed in the practice of the present invention, to the extent that they are applicable. For example, standard furnace making techniques and materials maybe employed for the floor, walls,and ceiling of the furnace. The choice of high-temperature burners is of course, within the skill of the art, although gas burners are preferred.

Applicants invention may be. better understood by a closer examination of FIGS. 1 and 2. FIG. 1 represents an overall view of a crucible melt system and the superheating furnace of applicant's invention. The pump, 1, in the pump well 2, picks up molten metal and discharges it by way of furnace or heat exchanger 4 into melting crucible 9, into which scrap or solid metal is introduced by a conveyor or other means, not shown. More specifically, the molten metal is transported by pump 1 through conduit 3a into melting furnace 4, which has insulated walls 5 and 6. The superheating furnace 4 is sub-divided into a zigzag flow pathby the presence of channel wall elements 7 constructed of a suitable heat exchange medium as set forth herein. Upon completion of its passage through the length of channel 3, during which heat is. absorbed from the channel wall elements 7, the superheated molten metal iswdischarged through conduit 8 into the crucible 9,

whereinit is employed to melt solid metal being simultaneouslyaddedto said crucible. Said crucible is provided with means for skimming, and overflowfrom said crucible passes under the skim arch by way of conduit 11, back into the pumping well, 2. Molten metal is discharged by means of conduit 12 for purposes such as molding or metal fabrication. It may thus be seen that one aspect of the present invention comprises a unitary system whereby scrap or solid metal may be added to a superheated flow of molten metal, with a portion of the total flow being discharged for utilization, while another portion of the metal is recirculated through a superheating furnace to regain heat utilized in melting said metal and to provide heat for melting additional solid metal.

In FIG. 2, a cross section is shown of the superheating furnace element for further understanding. Superheating furnace 4, having therein channels 3 the walls of which are formed by heat exchange elements 7, is heated from above by a reducing flame caused by the burning of suitable fuels by flat flamed burners, 15, the fuel being provided through a suitably insulated conduit 14. The exhaust gas from said combustion leaves the furnace chamber by means of stack 13. Thus, molten metal flows through channel 3 in contact with heat exchange elements 7, which are heated directly by the reducing flame of the burners, 15.

The following information, based upon the use of this invention for the melting of aluminum, may be taken as exemplary of the utility of applicants inventive concept. It is known from the literature that the specific heat of aluminum over the range from C to 660C is 5.78 gc/ga, while the specific heat at temperatures of 660C and above equals 6.70 gc/ga. The heat of fusion for aluminum is known to be 2,550 gc/ga. The heat content of aluminum at 760C (1400F), may be calculated from the above data and the atomic weight of aluminum (26.97), as being 250 cal/g, or 461 BTU per pound of aluminum at a temperature 100C above the melting point, i.e., 100C of superheating. The amount of excess heat energy available per pound of aluminum as a result of 100C superheating may be calculated as being 45 BTU per pound. Thus, one can see that ten pounds of molten aluminum at 760C 1400F has the excess heat content necessary to melt one pound of aluminum and return both to the bath without crossing the liquidus-solidus line. By applying even a very conservative safety factor of 100 percent, one may then calculate that a system which achieves 100C superheating is capable of melting one pound of aluminum for each 20 pounds of molten bath. By employment of a liquid metal pump delivering 600,000 pounds per hour, for

. example, oneis enabled to have a solid metal charge rate of 30,000 pounds per hour, a most unexpectedly high rate. The key factor in achieving this charge rate is in the ability to successfully superheat the molten metal to the desired degree at the extremely high flow velocities required, made possible by the present invention.

The following results may be obtained from a superheating furnace in accordance with this invention, utilizing silicon nitride bonded silicon carbide channel walls. Aluminum is passed through a 24 foot long channel, having a depth of 6 inches and a width of 2 inches, at a flow rate of 1,700 pounds/minute (102,000 pounds/hour). Using a gas fed burner yielding an average temperature of combustion products above the alu minum of about 2,000F, and an overall heat transfer rate of approximately 968,000 BTU/hour, a temperature rise of 40F is obtained in the aluminum.

As previously indicated, the channel walls which serve as the heat exchange media of the superheating furnace set forth herein, may be made from highly heat conductive materials, such as Refrax, a silicon nitride bonded silicon carbide refractory available from The Carborundum Company. However, it should be recognized that other materials may be used which are suited to the specific molten metal being processed. or the op erating temperature of the furnace. For example, it is contemplated that cast iron channels may be employed in a furnace employed for melting lead.

As previously indicated, it is desirable to have a depth to width ratio in the channels of greater than 1:1, and preferably about 3: 1. When employing 6-inch high Refrax bricks, a 3:1 ratio yields a channel width of approximately 2 inches. With a flow rate in excess of 1,500 pounds per minute of molten aluminum passing through this channel, it is obvious that this furnace design is inherently clean, in that the channel walls are constantly swept by a very rapidly moving stream of molten metal.

The furnace construction for the superheating furnace itself is of standard practice and nature. The burner system employed is so selected and controlled as to yield the greatest possible thermal efficiency. Fuel to air ratio, which largely governs fuel economy and atmosphere control is of particular importance, since most furnaces go to an oxidizing atmosphere at low fire. This, of course, creates oxides which not only increases melt loss, but also decreases the heat transfer capabilities of the metal surface, which in turn decreases overall fuel efficiency and raises stack temperatures, affecting refractory life. This may be controlled in a number of ways. Obviously, the furnace must be as pressure tight as possible. Further, the furnace must have pressure control or a small flue so as to provide a positive furnace pressure. The fuel to air ratio may be controlled by a direct pressure control system, which comprises a means for sensing a static pressure change in the controlled air flow to the burner, which in turn controls the gas flow proportionately throughout an 8:1 turndown. This method is subject to variability of from 5 to 7 percent resulting from changes in orifice area, gas density, and air density, which result from furnace heat. A preferred method is the use of hydraulic air/fuel ratio control, accomplished by sensing the pressure drop across an orifice which is remote from the burner itself. By this means, when the stoichiometric ratio of fuel to air is set, the hydraulic control system will maintain this ratio throughout a turndown ratios of 8:1. Such a system permits the furnace to operate at about 45 percent fuel efficiency. Using a 9.5:1 fuel to air ratio, a flue gas analysis reveals no oxygen remaining after combustion. Thus, since there is no available oxygen remaining in the combustion chamber, the possibility of generating oxides is negated.

The system of this invention is quite adaptable, and is suitable for use with a great number of metals, such as aluminum, zinc, lead, magnesium, and alloys such as brass, bronze, etc. Alloy conversions may be made readily, by making the furnace unit small, with a large holding capcity, so that any small amount of metal remaining in the furnace can be diluted in the larger holding capacity. Such a unit is a very economical unit to build. The holding capacity need be little more than an the metal forthe size of pour required for the given use,

and may serve as .the pump well. 7

The systemof this invention is adaptable to many types of scrap, maybeequipped forautomated hand ling,.and is quite economical to operate. The melting cnuciblelis preferably one such as taught by Sweeney et al U.,S. Pat. N6. 3,272,619. although other suitable melting systems rnaybeemployed. The crucible should bejoff sufficient size ,thatall normallyused forms of scrap, or raw material may be charged into it.

It should be noted that theheating capacity of the system described herein is dependent upon a number of factors, which may be varied to achieve the desired results. For example, the degree ofsuperheat achieved in the molten metal is dependent upon flow rate, and the amount of contact with the heat exchange medium. Thus, it may be seen that an increase in superheating ability may be achieved by merely increasing the length of channel through which the molten metaltravels. Thus, Within limits, it is possible to vary the melting capacity of the system described herein. One may increase the capacity of the pump employed to move the molten metal through the channels, or alternatively, one. may increase the channel length through which said molten material flows.

' It is also possible to vary the heating capacity of the burners employed to provide heat to the channel walls, either by fuel regulation or selection.

' it is to be recognized that modifications and variations in th e above disclosed invention may be made without departing from the broad spirit andscope of this invention, as definedin the following claims.

l. A heat exch angelunit for transmitting heat to molten metals which reflect radiant heat, said unit comprisinghighly' heat conductive surfaces fonning an elon gated generally horizontally disposed channel for the flowof said molten metal, and means for radiantly heating said conductive surfaces to a temperature range above'the temperature of said'molten metal.

"A heat exchange unit as set forth in claim 1 wherein said heat conductive surfac'es'a're a refractory materiall i i ""3."A' heat exchange unit as set forth in claim 2 wherein sa'id r'efr'actorymaterial is bonded silicon car- "4ZA' heat exchange unit as set forth in claim 1 wherein saide'longated'chan'nel has a depth to width ratio of from about 1:1 to about l:l. V

5. A heat'exchanger comprising walls formed of heat insulating material; an inlet means for introducing a tween said inlet means andioutlet means; heat conductive means disposed within saidv flow channel for contact with said molten metal; and heat source means for providing heat to said heat conductive means for conductive transfer of said heat to said molten metal.

6. A heat exchanger as defined in claim 5 wherein said heat conductive mcansinclude spaced channel wall means. having a surface thereof extending above said flow channel,

7. A heat exchanger as defined in claim 5 wherein said heat source meansis comprised of a source of radiant-heat. I,

8. A heat exchanger as defined in claim 7 wherein said heat conductive means include spaced channel wall means having a surface thereof extending above said flow channel. N i

9. A heat exchanger as defined in claim 7 wherein said heatconductive means comprise a series of heat exchange elements including walls defining said flow channel and having portions of said walls extending above said flow channeLsaid portionsbeing directly exposed to said, source of radiant heat. 1

10 A heat exchanger as defined in claim 9 wherein said elements are elongated and extend generally horizontally within said heat exchanger and define said flow channel as a tortuous path beneath said source of radiant heat. v i A IL A method of heating molten material having a heat reflective surface, said method comprising the steps of providing acontainer for said molten material, radiantly heating a body which is disposed within said container and partially immersed within said molten material and which is partially aboveisaid reflective surface and exposed to a source-of radiant heat, theportion of. said body extending above said reflective surface absorbing heat from said source of radiant heat, and the portion of said body immersed in said molten material conducting said heat to said molten material beneath said reflective surface. i

12.1 The method of claim 11 including the additional step of causing acontinuous flow of molten material into and out'of said container, I I I p 13, The method of claim l2 including the additional step of introducing solid materialinto said molten material. i I

p 14. The method'of claim 13 including the additional step of maintaining a reducing atmosphere above the flow of molten metal into said heat exchanger, an outlet 7 surface of said molten material. 7

IS. The method'of claim 12 wherein the flow rate of molten material is in excess of 200 pounds per minute. 16. The method of claim 11 including the additional step of maintaining a reducing atmosphere above the surface of said molten material.

Claims (16)

1. A heat exchange unit for transmitting heat to molten metals which reflect radiant heat, said unit comprising highly heat conductive surfaceS forming an elongated generally horizontally disposed channel for the flow of said molten metal, and means for radiantly heating said conductive surfaces to a temperature range above the temperature of said molten metal.

2. A heat exchange unit as set forth in claim 1 wherein said heat conductive surfaces are a refractory material.

3. A heat exchange unit as set forth in claim 2 wherein said refractory material is bonded silicon carbide.

4. A heat exchange unit as set forth in claim 1 wherein said elongated channel has a depth to width ratio of from about 1:1 to about 10:1.

5. A heat exchanger comprising walls formed of heat insulating material; an inlet means for introducing a flow of molten metal into said heat exchanger, an outlet means for removing molten metal from said heat exchanger, and a flow channel for said molten metal between said inlet means and outlet means; heat conductive means disposed within said flow channel for contact with said molten metal; and heat source means for providing heat to said heat conductive means for conductive transfer of said heat to said molten metal.

6. A heat exchanger as defined in claim 5 wherein said heat conductive means include spaced channel wall means having a surface thereof extending above said flow channel.

7. A heat exchanger as defined in claim 5 wherein said heat source means is comprised of a source of radiant heat.

8. A heat exchanger as defined in claim 7 wherein said heat conductive means include spaced channel wall means having a surface thereof extending above said flow channel.

9. A heat exchanger as defined in claim 7 wherein said heat conductive means comprise a series of heat exchange elements including walls defining said flow channel and having portions of said walls extending above said flow channel, said portions being directly exposed to said source of radiant heat.

10. A heat exchanger as defined in claim 9 wherein said elements are elongated and extend generally horizontally within said heat exchanger and define said flow channel as a tortuous path beneath said source of radiant heat.

11. A method of heating molten material having a heat reflective surface, said method comprising the steps of providing a container for said molten material, radiantly heating a body which is disposed within said container and partially immersed within said molten material and which is partially above said reflective surface and exposed to a source of radiant heat, the portion of said body extending above said reflective surface absorbing heat from said source of radiant heat, and the portion of said body immersed in said molten material conducting said heat to said molten material beneath said reflective surface.

12. The method of claim 11 including the additional step of causing a continuous flow of molten material into and out of said container.

13. The method of claim 12 including the additional step of introducing solid material into said molten material.

14. The method of claim 13 including the additional step of maintaining a reducing atmosphere above the surface of said molten material.

15. The method of claim 12 wherein the flow rate of molten material is in excess of 200 pounds per minute.

16. The method of claim 11 including the additional step of maintaining a reducing atmosphere above the surface of said molten material.

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