US2274287A - Crucible furnace - Google Patents

Description

Feb. 24, 1942. R. D. YORK CRUCIBLE FUR Filed Sept. 50, 1940 ET Al. 2,2 74,287

NACE

2 Sheets-Sheet 2 sli Vonne,"

Patented Feb. 24, 1942 Raymond D. York, University, and yCharles M. Walker, Charlottesville, Va., assignors to Raymond D. York, trustee 'Apnlicatlon september 30, 1940, serial No. 359,114

13 claims. (o1. 263-11) 'I'his inventionrelates to Crucible furnaces, and

more particularly to al forced` draft furnace ofthe'coke-red type.

For many years, cokeered Crucible furnaces` have been displaced by other Atypes of furnaces such asoil fired and electric furnaces because of many advantages which were considered to reside inzthe latter, even though coke-redfurnaces are lower in cost and more economical to operate. It has been found in recent years that later types c of furnaces have a substantial percentage efrfailures in providing metals for many dilerent uses. Animportantobject of the present invention is to provide a` furnace which possesses the ad,

vantages of a coke-liredfurnace as to cost :of

manufacture-and economy in the production of` metal and which, because of the high temperatures and uniform4 heats which it provides has` been found to verymaterially reduce the percentages of failures in producing metal formany different uses.

- A further object is: to provide a furnace of the coke-fired type wherein the air for supporting combusti-on is preheated in a novel manner, thus assisting in maintaining high temperatures withinthefurnace.'` n.

.A further object is to provide a, coke-firedfur nace of the type referred to wherein the' absorption of heat into the airy fed' through the grate is utilized for maintaining at lower temperatures .the portionsfof the furnace walls'which are or;

dinaruy most highly heated, thus greatly 1engthening the lifeof the furnace.

"A further object is to provide'a furnace of the type referred to wherein the walls of the furnace are provided with novel preheating ducts for heating the air which supports combustion, and

wherein the location, structure and functioning of the ducts is such that the air passing there-y through maintains the normally hottest portions of the furnace wall at uniformly lower `temperaturesto l'minimize the burning-out of such por-f tions of the furnace walls', thus greatlylengthening the life of the furnace.

' A 4further objectv `is to provide a metaljacket around the `furnace and to provide novel'means for assemblingl the insulating and re brick ele-` ments within such jacket to form` the fire boX of the furnace. I c

A further object is to provide, in conjunction with the preheating ducts referred to within the furnace wa1l,'an auxiliary duct surrounding the metal jacket of the furnace in such a manner as to initially absorb heat'therefrom so as to pass f air `intovthe interior heatingducts at a temperature higherthan room temperature. y

A further object is to provide, in conjunction with a furnace of the type` referred to, a/novel form of Crucible supporting block wherein a por,- tion of the air for supporting the combustion of theV fuel passes through ducts in the supporting block to tend to carry heat away from the block itself to lengthen the life thereof, the air passing through the block absorbing yheat units fromthe block to assist in maintaining the latter at a lower temperature.

VA further object is to provide in conjunction withlthe block structure novel .meansk for Iksupporting the grate and the Crucible supporting block and to supply to the latter the desired air for passage rthrough the openings therein.

Other objects and advantages of the invention will become apparent during the coursey of rthe following description. n y c n I In, thedrawlngs we have shown one'embodiment of the invention. In this showing-'-I Figurel isa centralverticalsectional view throughthe furnace, taken onka plane'indicated bythe line I-c-I'in Figure 2, l v f f ligure'Z is a horizontal sectional view 'on line 2-2 of Figure 1, y l I `h Figure 3 is a similar view on line 3'-3 of Fig- Figure 4 is a sectional view taken arcuately as indicated by the line 4'-4 'in Figure'B, and,

yFigure 5 is a 'fragmentary vertical sectional view taken'online 5-5 of `Figure 2.

' Referring to Figure 1 the numeral I I) designates the base of the furnace, which is ca'st relatively heavy as shown, and is preferably provided with a depending annular'flange I I to support the Abase on thefsurrounding surface I2. The base is prof vided with a central frustro-conicalsupporting portion I3 'having a flat-'circular top I4 from which an annular flange `I5 projectslupwardly, this flange being slightly smaller in. diameter than the top portion- I4. The top portion) I4-supports a tubular column I6, the lower end of v,which ts over theflanger I5 to maintain itproperly centered with respectto the furnace. `The column `I 6. is provided withair inlet openings Il fora purpose to be described. I ,I

The furnace further comprises 4a cylindrical shell I8 the lower end of which ts downwardly over the flange I I, as shownin Figure `1. `Within theshell I8is arranged a horizontal shelfmember` I9 suitably supported by radially inwardly projecting bracket elements 20 welded or othery Wise secured to the shell Ill.` These brackets are arranged at spaced points around the furnace, as will be understood. The shelf I9 is arranged a substantial distance above the base I to provide an air 0r draft chamber 2|. 'I'he shelf I9 is of a diameter approximately equal to the internal diameter of the shell I8 to t therewithin.

The shelf I9 is provided with a central circular opening 22 and the upper surface of the shelf around such opening supports the peripheral portion of a grate 23. The grate is provided with a central circular opening 24 which tapers to decrease in diameter upwardly, and the inner peripheral portion of the grate rests upon and is supported by a plate 25 resting upon the upper end of the column I6 and provided with adepending annular flange 26 fitting within the upper end of the column I6. The plate 25 is provided with a central opening 2l for the passage of air upwardly therethrough.

A Crucible 28 is adapted to be arranged in the furnace and to be supported by a block indicated as a whole by the numeral 29.' This block is preferably formed as described and -claimed in our copending application Serial No. 272,658, filed May 9, 1939. The block comprises a permanent element 39 and' a replaceable element 3l, the lower portion of the permanent element resting upon theplate 25 to be directly supported thereby and having its outer surface tapered to correspond to the taper of the opening 24 to nt therein, as shown in Figure 1.

The replaceable block elementV 3I is provided with an axial tapered opening 32 which surrounds a similarly tapered portion 33 formed integral with the block element and projecting upwardly therefrom, the diameter of the upper end of the portion 33 being slightly less than the internal diameter of the lower end of the'opening in the block element 3I to permit the assembly of the elements. In assembling the elements of the block structure, a layer of material 34, such as paper, is arranged around the upwardly projecting portion 33 yof Athe block 30 and over the upper face of the body portion of such Ablock element. The block element 3| is then placed in position and the space between the surface of the opening 32 and the adjacent layer of paper 34 is filled with a suitable cement as indicated by the numeral 35.

The lpermanent block element 38 is provided in the bottomv thereof with a recess 36 communieating with the opening 21, and the walls of the block element 30 are provided with upwardly inclined openings 31 extending therethrough and preferably decreasing in cross-sectional area radially outwardly of the block, as shown in Figure 1. These openings feed air to the fire bed around the block and serve the two purposes of assisting in the combustion of the fuel and in maintaining the block structure at relatively lower temperatures than ordinarily would occur, thus tending to substantially lengthen the life of the block structure.

A circular casing 38 surrounds the furnace outwardly ofthe draft chamber V2l to form a preheating chamber 39 which preferably extends somewhat above the horizontal level of the shelf I9, as shown in Figure l. The preheating chamber serves several purposes, for example, in preheating the air, protecting workmen from possible injuries from burns by coming into contact with the bottom portion of the furnace, and in improving the supplying of the air to the furnace to support combustion. The casing 38 may be welded or otherwise secured in position around the shell I 8.

An inlet opening 49 is provided in the casing 38 and receives air from the blower 4l of any suitable type. As will become apparent, the air divides upon being pumped through the opening 48 and passes circumferentially through the chamber 39 to the opposite side of the furnace, whereupon it passes through an opening 42 formed in the shell I 8, it being apparent that the openings 48 and 42 are preferably diametrically opposite each other. The cross-sectional continuity of the chamber 39 is interrupted to permit the cleaning out of ashes from the ash pit or draft chamber 2| and to render uniform the flow of air around the chamber 39 at the desired velocity. As shown in Figure 4, the shell 38 is provided with upper and lower walls 43 and 44, the former of which is continuous around the furnace at all portions of such wall lying in a horizontal plane. The wall 44 is offset upwardly to one side of the opening 42, as at 45, to provide space for a clean-out door 46 which is accessible for the cleaning out of the ash pit.

The offsetting of the bottom wall 44 as at 45 obviously restricts the flow of air around the chamber 39 and this is desirable in order to maintain an ample volume of air in the chamber 39 a sufficient length of time to permit proper preheating while permitting a sufficiently rapid flow of air to and through the opening 42 to the furnace. In fact it has been found desirable to add a baille 46 adjacent one vertical wall of the offset to somewhat further impede the passage of air through the space 41 above the offset 45.

The offset 45 and baie 48 obviously retard the ow of air to the opening 42 from one side thereof and it is desirable to provide means at the opposite side of the opening 42 to similarly retard the flow of air` to the opening 42 to equalize the iiow of air passing to this opening from opposite sides thereof. To this end, a vertical bafe 48 is arranged adjacent the opening 42 and extends from the top wall 43 to a point spaced from the bottom wall 44 to provide a somewhat restricted passage 49. A horizontal baffle 58 is arranged at a point spaced from the top wall 43 to provide a somewhat restricted passage 5I, and one end of the baille 59 is spaced from the ballie 48 to provide a somewhat restricted passage 52. The opposite end of the horizontal baffle 50 is connected to a vertical baffle 53 the upper and lower ends of which are spaced from the top and bottom walls 43 and 44 respectively to provide restricted passages 54 and 55. With the construction shown, there will be a properly retarded flow of air at a uniform rate to the opening 42 from both sides thereof while permitting free access to the ash pit.

The structure built up within the shell I8 to provide the re box of the furnace is of particular importance. The re box proper is formed of a plurality of high grade refractory bricks 56 laid to provide a cylindrical fire box forming the lower portion of the combustion chamber 51. The refractory bricks 56 may be of the type having lug and recess or tongue and groove engaging faces as indicated by the numeral 58.

The lowermost layer vof the nre bricks 56 rests upon a fire brick structure indicated by the numeral 59. Such structure rests directly upon and is supported by the shelf I9 and is constructed of a plurality of re bricks laid so as to form a circular unit the diameter of the opening traer-152m'y 6U of which is somewhat greaterthan the diameter of the opening 22 to allow sufcient space on the upper surfaceiof the'shelf I9 to support the grate 23. The radiallyl outerfsurface of the fire brick unit 59 is circular, which is also `true of the cross-sectional shape of the shell I8. However, it is impossible in practice to make both of these units true circles and in assembling the structure, there ,would be undue pressure caused by the' shell I8 againstV the re brick structure 59 if thelatter were made of such size as to fit tightly within the shell I8. To eliminate such difficulty the outside diameter of the re brick unit 59 is made appreciably smaller than the internal diameter of the shell I8 and "after the re brick unit 59 has ben located in position, the space between such unit and the shell I8 is rammed with crushed fire brick and high temperature cement, as indicated by the numeral 6I.

The fire brickV unit 59 is provided approximately in the4 axial plane of the opening 42 with an air inlet opening Ii2of substantial `size as shown in Figure 3, and such opening is in registration with an opening 63'of similar shape and sizeformed in the shelf I9. A casingA 64 is arranged beneath the shelf I9 to connect the opening 42 to the opening 63, the air passing through the opening "42 thus passing upwardly through the openings 62 and`63 without flowing into the draft chamber or ash pit 2|. Diametrically opposite the opening 62, the rebrick unit 59` is provided with a pairvof openings 55 havinga block 65 (Figure 5) therebetween, and the openings 65 register with openings 66. of similar shape and size formed in the shelf or plate I9 to discharge air into the draftA chamber `2I in a manner to bedescribed.

Radially outwardly of the lowermost re brick 56, a plurality of insulating bricks' 61 is laid in circular form against the inner surface of the shell I8 and defining with the lowermost row of fire bricks 56 an annular air passage 68 communicating at one side with the opening 62 and late around the lwermost row of lire bricks 56 and thence downwardly through the openings 65 In view of the fact' that the annularl and 66. chamber 68 is arranged within the bricks 61, the latter need not 'del-lne a circle smaller in diameter than the internal diameter of the shell I8,v since the bricks61` are capable of whatever slight horizontal movement is necessary to adjust its shape to that of the shell I8.

'Above the insulating'bricks 61 another layer of insulating bricks 69is laid, 4and such layer of bricks has' itsradially inner 4and outer surfaces respectively circular .in form except that the structure as a whole is provided atdiametrically opposite points respectively with an opening 18 and a pair of openings 1I, formed in the inner peripheryof the structure, to communicate with the annular passage 68, the openings 1I being separated by a block 1`I'.` Since the structure made up of the fire bricks 69 cannot shift horizontally to accommodate its outer periphery tothe shape of the shell I8, its outside diameter is made smaller than the internal diameter of the shell I8 to provide an annular space which, after the bricks 69'have been assembled in position, is filledfwith, a'mixture of crushed fire brick and high temperature cement V12, similar to that employed around the fire brick structure 59. For a purpose to be described, the opening '10 is preferably approximately one-half the area of the opening 62, and the sum of the areas of the openings 1I is slightly greater than the area of the openings 65. Y l

Above the re bricks 69 a series of insulating bricks 13, similar to the bricks 61, is laid up within the shell I8, a single circumferential series of the bricks `13 being arranged with theirradially outer surfaces in contactwith the shell I8. The series of bricks 13 has its radially inner surface spaced from .the adjacent re bricks 61 to form an annular space or chamber 14 similar to the space 68. The space 14 being annular, the bricks 13 are free to shift horizontally to accommodate themselves to the shape of the shelll, and no space need be left between thebricks 13 and the shell I8. The space 14 communicates at one side with the opening 1U and diametrically opposite .such opening a block 13 is arranged above the block `1I for a purpose to be described.

Above `the bricks 13 another layer of fire bricks 14' is laid in circular form with the inner periphery thereof contacting fwith the outer periphery of the vadjacent series of fire bricks 56. The ,bricks 14 are similar to the bricks 69 except that no gaps are left therein for openings, the bricks 14 forming a solid horizontal structure dening the top of the passage 14. Since the bricks 14 are incapable of shifting horizontally to accommodate themselves to the shape of the shell I8, a space is left between the bricks 14 and shell I8 to be referred to later.

Above the bricks 14' a solid circular layer of insulating bricks 15 is laid, the radially inner surface of the series of bricks 15 contacting with radially outer surfaces of the adjacent fire bricks A space is left between the radially outer surfaces of the bricks 15 and the shell I8, as in the case of the bricks 14. The upper surfaces of the bricks 15 and the upper surfaces of the uppermost layer of bricks 56 preferably lie in a m common plane.

Above the bricks 56 and 15 several circular layers'of re bricks 16 are laid up, the lowermost layer of such bricks extending across the uppermost bricks 56 and across a substantial portion of the bricks 15, as shown in Figure 1.

Accordingly the bricks 16 tend to bind the bricks 56 and 15 (together. At least for a portion of the height of the bricks 16, a circular series of insulating bricks 11 is laid up as shown in Figure 1 'to minimize passage of heat radially fromy the upper portion ofthe combustion chamber. These bricks have their radially outer surfaces spaced from 'the shell I8. After the assembly of the elements above the bricks 13, a mixture of crushed ilrebrick and high temperature cement 18 is rammed into the spaces radially outwardly of the bricks 14, 15 and 11 and` above the latter bricks to the upper limits of the shell I8 and the uppermost bricksy 16. The assembly of these elements completes `the furnace except for the top thereof which may be of any suitable structure (not shown) such, for example, as the structure shown in our copending application Serial No. 272,658, referred to above.

The operation of the apparatus 'is as follows:

Assuming that a crucible- 28 is arranged in the furnace surrounded by a bed of burning coke, the combustion of the coke will be supported by air pumped by the blower 4I. This blower pumps air into the chamber 39 at the right hand side It is desired to maintain a lsubstantial volume of air in the chamber 39 and to move this air relatively slowly to permit it to take full advantage ofthe heat units present in the adjacent portion of the shell I8. The baiiie means shown in Figure 4 extends around approximately half the circumference of vthe chamber 39 and the structure of such baiiie means is such as to provide access to the clean-out opening 46 while at the same time retarding the flow of airk to the inlet opening 42. Free 110W of air is impeded on one side of the opening 42 by the structure 45 and baffle 46, and is impeded to an equal extent on the opposite side of the opening 42 by the baies 48, 50 and 53. While thesebailes provide air passages exceeding in cross-sectional area the area of the opening 42, they none the less impede the free iiow of air more than if they were eliminated, and the air permitted to flow directly vto the opening 42, the directfiow of air being interrupted and eddy currents being set up as will be apparent.

Air entering the opening 42 flows through the casing 64 and thence upwardly through the opening 62 (Figures 1 and 3). 'Ihe opening 1llhas an area of approximately one-half the area of the opening 62. Thus the opening 62 admits sufficient air for circulation through both of the annular passages 68 and 14, and the relative areas of the openings 62 and 10 are such that one-half of the air flowing through the opening 62 will flow through the opening 10 and thus through the annular passage 14. The remaining one-half of the air will iiow through the annular passage 68.

The air flowing through the passages B8 and 14 absorbs heat from the elements forming such passages, and particularly from the adjacent fire bricks 56, thus the air becomes elevated to a higher temperature and the absorption' of heat units from the fire bricks 56 maintains such bricks at a lower temperature than otherwise would be present. The preheating of the air in this manner assists in maintaining a high temperature in the re bed, as will become apparent, and the absorption of heat lunits from the fire bricks B greatly lengthens the life of these elements, the re bricks lasting much longer periods of time without replacement because of the oper-- ation referred to.

The air passing upwardly through the opening 1li divides and ows in opposite directions through the passage 14. It will be obvious that the air owing in opposite directions around the passage 14 would meet at a point diametrically opposite the opening 10, if the passage 14 were continuous. It has been found that with such arrangement, however, the meeting, surging and eddying of the air sets up such resistance to the flow of air as to require substantial pressure for pumping air through the system. Accordingly the block 13 is arranged diametrically opposite the opening to stop and direct downwardly the flow of air through each half of the passage 14, this arrangement greatly reducing resistance to the flow of the air. It will be noted that the openings 1| are separated from each other by the block 1|' which is equal in thickness to the block 13. The air from the opposite sides of the passage 14 thus flows downwardly through the openings 1l. These openings are preferably.` equal in area and the sum of their areas is equal to and preferably-slightly greater than the area of the opening 10 to take care of the expansion of the air occurring through the fiow thereof around opposite sides of the passage 14.

Thus the air passing around the chamber 14 inopposite directions is directed downwardly forr discharge through the openings with the air flowing around the annular passage 68. The ends of the passage 68 are blocked off as at 68 diametrically opposite the opening E2 for the same reason that the corresponding portion of the passage 14,is blocked off, namely, to prevent the generation of air resistance by directing the flow of -air downwardly from the ends of the passage 68 through the openings E5. The openings 65 are preferably identical in shape and size and the sum of the areas of the openings 65 preferably exceed the area of the opening 62 to take care of expansion of the air passing around the passages 68 and 14, all of this air being discharged into the ash pit through the openings 65.

The air entering the chamber 2i will be highly heated by the means referred to and is ready for passage upwardly through the fire bed. Most of the air in the chamber 2l ows upwardly through the grate 23 and through the coke burning in the combustion chamber. Some of the air in the chamber 2| enters the tubular support I6 through the openings I1and blows upwardly through the opening 21 and into the chamber 36 to be discharged upwardly and radially outwardly through the openings 31 in the lower block element .'ill.v The air flowing through the openings 31 is, not only employed' for assisting in supporting the combustion of the fuel but also serves to lengthen the life of J the block 30. This is accomplished in two ways. While the preheated air is at a relatively `high temperature, it obviously passes through the opening 31 at a lower temperature than the block 30 and accordingly tends to maintain this block element at a lower temperature. In the second place, the flow of air tends to carry the heat of combustion away from the supporting block and thus assists in maintaining the supporting block at a lower temperature than otherwise would be present.

With the structure shown and described it has been found that the supplying of the preheated air enables the combustion to take place at a higher temperature than is true when cold air is introduced vthrough the grate, and accordingly higher temperatures maybe maintained at a relatively uniform point. The intense heat and uniformity of the temperature permits the melting of the metal in a shorter length of time and improves the quality of the metal by greatly improving its degree of uniformity. It has been found that metal melted inthe present furnace is of such quality thatthe percentage of rejected castings made therefrom is very greatly reduced.

The higher temperatures maintained in the. furnace ordinarily would tend to shortenthe life of the fire bricks, but the utilization of the chambers surrounding the re bricks in the zone of the highest combustion temperature has been found to lower the temperature of the fire bricks to such a point that the life yof these elements is very materially increased. `The passage of the,-

Vand in practice substantially complete combustion takes place. It is obvious that the structure provides vmaterial advantages over prior constructions and the design, cross-sectional area;

etc.,- of the various air passages is suchthat a relatively small blower 4I will maintain an am` ple flow of air for supporting the combustion of `the fuel. y f

It will be noted that the grate 23 is not'forced to support the weight of the block 29l and the Such weight is dil crucible and its contents. n rectly supported by the block 29, plate 25 and pedestal I6. VThe grate is called upon to support y `only the weight of the bed of fuel and the grate,

in turn, is effectively supported by the shelf or plate I9 and by the plate 25. Thisfeature, per

se, forms no part of the present invention, being vcopending application referred to also discloses the provision of a: two-part block 28 with the use of the paper or similar material 34 which, under ciated elements of the furnace provide greatly the' furnace.

improved results and also ylengthen the life of The provision of two passages 68 and 14 instead of one passage is advantageous for the reason that in actual practice it has been found that the provision of a single passage of suiicient cross-sectional area to carry the necessary flow of air leaves the lower fire bricks 56 the influence of the heat in the combustion the crucible upon removal of the latter, a dovetail connection between 1 the two block elements being maintained after the paper layer 44 has been burned. The portion of the block represented by the element 3l is subjected to the heaviest damage from heat and when burned beyond usefulness it maybe broken and removed, whereupon a new element 3|,y may be placed in position without renewing the entire block.

The blockstructure shown is an improvement over that disclosed in our copending application since it provides the air passages 31l which serve to lengthen the life of the block element 30 by .the absorption of heat units therefrom and by tending to blow the heat of combustion away from such block element. v

` possible from .a practical standpoint by the use of the block elements 59, 69, 14', 15 and 11 which are spaced inwardly from the shell I8 `and with the space filled with the mixture of the crushed fire brick and high temperature cement. With the use of such construction it is notl necessary that the outer-surfaces of the layers of bricks which extend from the fire bricks 56 to the shell I8 be truly circular in order to fit within the shell I8, and uneven pressures which would occur through an effort to fit the brick elements within the shell I8 are eliminated, thus further increasing the life of the bricks by the 'elimination of internal strains.

The provision of the air passages 68 and 14 and their arrangement with respect to the assounsupported radially outwardly thereof with the result that such re bricks will buckle during the operation of the furnace. By 'dividing such passage into the two passages 68 and 14, with the arrangement shown, it will be apparent that every fire brick 56 in the furnace is supported radially outwardly thereof by the outer blocks 59, 51,59, `13, 14' or 15. Thus the re bricks56 are all braced againstradially outward buckling.

In experimental work in connection with the furnaceV it Awas naturally assumed that it would be advisable to extend preheating passages, such ,as the passages 68 and 14throughout the height `of the furnace. In actual practice this was found to be wholly impracticable.

The result of using such construction wasl that the air absorbed so much heat `from the inner fire brick structure as to prevent the generation in the fire box of suiiicient heat for heating the metal in the crucible. yIn addition, this absorption of heat was so great that the temperature of the air reaching the grate'resulted in the burning out of the grate in a relatively short period of time. The use of the passages 68 and 14 adjacent the lower portion of the fire box is highly advantageous for the reason that it absorbs only so much heat as is necessary to protect the lire bricks from burning out rapidly as is usually true, and in the second place results in the preheating of the air to a suiciently high temperature to assistin the proper maintenance of combustion inthe re box without the air .being sufficiently highly heated to burn out the grate.

As a result of the features referred to the highest temperature of combustion occurs at the lower `end of the iire box and accordingly the crucible will be most highly heated adjacent the bottom thereof. This is a highly desirable result which is not obtained with other crucible furnaces and isthe reason why higher quality metals are obtained. n Instead of providing for the highest temperature adjacent the upper portion of the crucible, resulting in occluded gases and impurities in the metal in the bottom of the crucible, the present construction provides, in effect, a boiling actionfrom the bottom of the crucible which results in the constant agitation of the entire massof metal in the Crucible with the driving off of gases and the forcing of impurities to the surface of the metal where they can be readily skimmed off. f

i `From the Lforegoing it will be apparent that the present furnace provides a highly effective and efficient means for melting metal in a crucible through the use of relatively cheap coke as the fuel. The present construction, therefore, permits the melting of high grade metal at a much We claim:

1.1 A crucible furnace comprising a wall structure, a grate within said wall structure, a draft chamber. below said grate, saidl wall structure having a continuous horizontal passage formed therein above the level of said grate and provided with an openingcommunicating directly with said draft chamber, a preheating chamber surrounding said draft chamber and communicating with said passage solely at a point horif zontally remote from said opening, and means for supplying air to said preheating chamber at a point remote from its point of connection with said passage 2. A Crucible furnace comprising a wall structure part of which is made up of an annular lining of re brick material, a grate within said wall structure, a draft chamber beneath said grate, said wall structure being provided with a pair of horizontal annular passages spaced one above the other and each having its radially inner wall formed by said nre brick material, said wall structure being provided with an inlet opening for the lower of said passages communicatingv between said passage and said draft chamber, said wall structure being further provided with an inlet opening for the upper of 'said passages communicating be tween such passage and the lower passage above said first named opening, said wall structure being further provided with outlet openings remote from said inlet openings, one communicating between the upper and lower of said passages and the other communicating between the lower passage and said draft chamber, and means for supplying air.

to` said first named inlet opening.

3. A crucible furnace comprising a wall structure part of which is made up of an annular lining of fire brick material, a grate within said Wall structure, a draft chamber beneath said grate, said wall structure being provided with a pair of horizontal annular passages spaced one above the other and each having its radially inner wall formed 'by said re brick material, said wall structure being provided with an inlet opening for the lower of said passages communicating between said passage and said draft chamber, said wall structure being further provided with an inlet opening for the. upper of said passages communicating between such passage and the lower passage above said rst named opening, said wall structure being further provided with outlet openings remote from said inlet openings, one communicating between the upper and lower of said passages andthe other communicating between the lower passage and said draft chamber, and means for supplying air to said first named inlet opening, said second named inlet opening being of a cross-sectional area approximately one-half the cross-sectional area of the rst named inlet opening whereby approximately one-half the air passing through said iirst named opening will f.l ow through each of said'passages.

` 4.*A crucible furnace comprising a wall structure part of which is made up of an annular lining of re brick material, a grate within said wall structure, a draft chamber beneath said grate, vsaid wall structure being provided with a pair of horizontal annular passages spaced one `above the other and each having its radially inner wall'formed by said re brick material, said wall structure being provided with an inlet opening for the lower of said passages communicating between said passage and said draft chamber, said wall structure being further provided with an inlet opening for the upper off said passages communicating between such passage and the lower passage above lsaid first named opening-said wall structure being further provided with outlet openings remote from said inlet openings, one communicating. between the upper and lower of said passages and the other communicating between the lower passage and said draft chamber, a preheating chamber surrounding said draft chamber and communicat- 'ture part of which is made up of an annular lining of fire brick material, a grate-within said wall structure, a draft chamber beneathsaid grate, a Crucible supporting block arranged centrally of said grate and projecting thereabove, said block being provided with upwardly and outwardly diverging'openings therethroughv communicating at their lower ends with saiddraft chamber, said wall structure being provided with a pair of horizontal annular passages spaced one above the other and each having its radially inner wall formed by said fire brick material, said wall structure being provided with an inlet opening for the lower of said passages communicating between said passage and said draft chamber, said wall structure being further provided with an inlet opening for the upper of said passages communicating between such passage and the lower passage above said first named opening, said wall structure being further provided with outlet openings remote from said inlet openings, one communicating between' the upper and k-lower of said passages and the other communicating between the lower passage and said draft chamber, and means for supplying air to said iirst named inlet opening.

6. A Crucible furnace comprising a wall structure part of which is made up of an annular lining of re brick material, a grate within said wall structure, a draft chambg beneath said grate,` a Crucible supporting block arranged centrally of said grate and projecting thereabove, said block being provided with upwardly and outwardly diverging openings therethrough communicating at their lower ends with said draft chamber, said wall structure being provided with a pair of horizontal annular passages spaced one above the other and each havingits radially inner wall formed by said re brick material, said wall structure being provided with an inlet opening for the lower of said passages communicating between said passage and said draft chamber, said wall' structure being further provided with an inlet opening for the upper of said passages communicating between such passage and the lower passage above said firstnamed opening, said wall structure being further provided with outlet openings remote from said inlet openings, one. communicating between the upper and lower of said passages and the other communicating between the lower passage and said draft chamber, a preheating chamber surrounding said draft chamber and communicating with said rst named opening, and means OI` Supplying Vair to said preheating'chamber at a point remote from the point of connection of "said'preheating chamberwith said first named opening.

' `7. Acrucible furnace comprising a Wall structure, a cylindrical metal jacket surrounding said wall structure and extending therebelow, a grate supported in said wall structure adjacent the lower end thereof, the space within said jacket beneath said grate forming a draft chamber, said wall structure being provided with an annular passage above the level of said grate, said passage vbeing `provided at diametrically opposite points `with openings one of which is an inlet opening and the other of which is an outlet open- Y ing communicating with said draft chamber, a

casing surrounding said jacket and forming therewith an annular preheating chamber communicating with said inlet opening, and means for supplying air to said preheating chamber at a point remote from the point of connection of such chamber with said inlet opening.

8. A crucible furnace comprising a'cylindrical wall structure at least a portion of the interior of the lower end of which is formed annular of fire brick material, a metal shell surounding said Wall structure and extending therebelow, a she1f structure supported by said shell and on which said wall structure rests, a grate arranged within said wall structure, said shelf extending slightly inwardly radially of said wall structure to support the peripheral portion of said grate, the space within said shell below said shelf 'forming a draft chamber, said wall structure being provided with a pair of annular passages spaced one above the other and `having their radially inner wall surfaces formed by said flre brick material, said wall structure being provided with a pair of inlet openings one of which communicates with the lower passage and the other of which communicates between said passages, said wall structure being further provided with outlet openings one of which communicates between said passages and the other of which communicates between the lower passagel and said draft chamber diametrically opposite said inlet openings, and means for supplying air to said first named inlet opening.

9. A crucible furnace comprising a cylindrical wall structure at least a portion of the interior of the lower end of which is formed annular of re brick material, a metal shell surrounding said wall structure and extending therebelow, a shelf structure supported by said shell and on which said wall structure rests, a grate arranged within said wall structure, said shelf extending slightly inwardly radially of said wall structure to support the peripheral portion of said grate, the space within said shell below said shelf forming a draft chamber, said wall structure being provided with a pair of annular passages spaced one above the other and having their radially inner wall surfaces formed by said re brick material, said wall structure being provided with a pair of inlet openings one of which communicates with the lower passage and the other of which communicates between said passages, said wall structure being further provided with outlet openings one of which communicates between said passages and the other of which communicates between the lower passage and said draft chamber diametrically opposite said inlet openings, a casing surrounding said shell radially outwardly of said draft chamber and forming with said shell a preheating chamber communicating with said first named inlet opening, and means for supplying air to said preheatingchamber at a'point remote from its point of connection with said rst named inlet opening.

10. A crucible furnace comprising a cylindrical wall structure, a circular grate arranged within said wall structure adjacent the lower end thereof and provided with a central opening, a draft chamber beneath said grate, said wall structure being provided with an annular passage above the level of said grate and a pair of diametrically opposite openings one of which is an inlet opening and the other of which is an outlet opening communicating between said passage and said draft chamber, a crucible supporting block having its lower end arranged in the opening in said grate, a supporting structure beneath said grate for supporting the latter adjacent the opening therein and for supporting said block, said supporting structure comprising a tubular pedestal having openings communicating with'said draft chamber, said block having upwardly and outwardly diverging openings communicating at their upper ends with the space within said wall structure above the grate and at their lower ends with the interior of said tubular pedestal, and means for supplying air to said inlet opening.

11. A crucible furnace comprising a cylindrical wall structure, a circular grate arranged within said wall structure adjacent the lower end thereof and provided with a central opening, a draft chamber beneath said grate, said wall structure being provided with an annular passage above the level of said grate and a pair of diametrically opposite openings one of which is an inlet opening and the other of which is an outlet opening communicating between said passage and said draft chamber, a crucible supporting block having its lower end arranged in the opening in said grate, a supporting structure beneath said grate for supporting the latter adjacent the opening therein and for supporting said block, said supporting structure comprising a tubular pedestal having openings communicating with said draft chamber, said block having upwardly and outwardly diverging openings communicating at their upper ends with the space within said wall structure above the grate and at their lower ends with the interior of said tubular pedestal, a preheating chamber surrounding said draft chamber and communicating with said inlet opening, and means for supplying air to said draft chamber at a point remote from the point of connection of the latter with said inlet openmg.

12. A crucible furnace comprising a relatively thick wall structure having a re brick lining of substantial height within the lower portion thereof and made up of layers of brick laid in anlnular form, a metal shell surrounding said wall structure further comprising a pair of rows of vertically spaced bricks each laid in annular form in contact with said shell and having their radially inner faces spaced from said fire bricks to form annular passages, and a plurality of annular rows of brick above, below and between said last named rows of bricks, the bricks of said plurality of rows having their radially inner faces contacting with said fire bricks and their` radially outer faces spaced radially inwardly of said shell, a cementitious mixture lfilling the spaces between said shell and the radially outer faces of the bricks of said plurality of rows, said Wall structure having diametrically opposite openings communicating between said passages, a grate arranged Within the lower row of fire bricks, a draft chamber formed beneath said grate, said Wall structure having diametrically opposite inlet and outlet openings the latter of which communicates between the lower annular passage and said draft chamber, and means for supplying air to said inlet openings.

13. A crucible furnace comprising a Wall structure part of which is -made up of an annular lining of re-brick material, a grate Within said lining, a draft chamber below said grate, said lining having a continuous horizontal passage formed therein above the level of said grate and provided with an opening communicating with said draft Chamber, a preheating chamber surrounding said draft chamber and communicating with said passage at a point remote from said opening, and means for supplying air to said preheating chamber at a point remote from its point of connection with said passage.

RAYMOND D. YORK. CHARLES M. WALKER.

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