US3419254A

US3419254A - High temperature multiple hearth furnace structures

Info

Publication number: US3419254A
Application number: US613832A
Authority: US
Inventors: Jr Charles F Von Dreusche
Original assignee: Nichols Engineering and Research Corp
Current assignee: Nichols Engineering and Research Corp
Priority date: 1967-02-03
Filing date: 1967-02-03
Publication date: 1968-12-31
Anticipated expiration: 1985-12-31

Description

3 1968 c. F. VON DREUSCHE. JR 3,419,254

HIGH TEMPERATURE MULTIPLE HEARTH FURNACE STRUCTURES Filed Feb. 5. 1967 l N VEN TOR.

EWA fi/QFWLWAMR Sheet BY maqy a i II I/ Dec. 31, 1968 c. F. VON DREUSCHE. JR 5 HIGH TEMPERATURE MULTIPLE HEARTH FURNACE STRUCTURES Filed Feb. 3, 1967 Sheet 2 of 6 j &

m INVENTOR.

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,477 A Eys Dec. 31, 1968 3,419,254

HIGH TEMPERATURE MULTIPLE HEARTH FURNACE STRUCTURES c. F. vow DREUSCHE, JR

Sheet Filed Feb. 3, 1967 I N VEN TOR. CHARLES I/GNMEMEC/R BY %Q; 1/4

Dec. 31, 1968 HIGH TEMPERATURE MULTIPLE HEARTH FURNACE STRUCTURES Filed Feb. 5, 1967 Sheet 5 of 6 Dec. 31, 1968 c. F. VON DREUSCHE, JR

HIGH TEMPERATURE MULTIPLE HEAR'IH FURNACE STRUCTURES Sheet Filed Feb. 5, 1967 iqz El.

United States Patent 3,419,254 HIGH TEMPERATURE MULTIPLE HEARTH FURNACE STRUCTURES Charles F. von Dreusche, Jr., Ramsey, N.J., assignor to Nichols Engineering & Research Corporation, New York, N.Y., a corporation of Delaware Filed Feb. 3, 1967, Ser. No. 613,832 18 Claims. (Cl. 263-26) ABSTRACT OF THE DISCLOSURE Multiple hearth furnace with air ducting inside center shaft and rabble arms; insulative arrangements comprising a fibrous layer and solid outer layer secured to rabble arms and center shaft; ceramic rabble teeth formed separate from rabble arm; hearth brick free of glass formers; and canopy-type shields covering surrounding buttstraps.

This invention relates to furnace structures and more particularly it concerns novel arrangements for permitting higher temperature furnace operation.

The present invention is particularly well suited in con nection with multiple hearth type furnaces. These furnaces are characterized by a plurality of vertically aligned hearths, or hearth chambers, down through which material being processed is moved. The temperatures and atmospheres in the various hearth chambers can be controlled 7 individually and, depending upon the particular construction of the furnace, the rate of movement of material through it can be controlled for each hearth.

The multiple hearth furnace includes an elongated center shaft which extends up through the center of the furnace, passing through each hearth floor. Rabble arms are secured to the center shaft and extend radially outward therefrom over each hearth floor. These rabble arms are provided with rabble teeth which extend down into the material being processedon the hearth. As the center shaft rotates, these rabble arms move over the material being processed which the ra'bble teeth plow through it. Depending upon the angle of inclination of the rabble teeth, the mat rial will be moved radially inwardly toward the center shaft or outwardly therefrom. Drop holes are provided in the floor of each hearth, either in toward the center shaft or out toward the furnace walls so that as the material completes its movement over a hearth, it will drop down onto the next lower hearth and move across this hearth in the opposite direction. Thus the material is caused to move slowly in serpentine fashion through the furnace.

The multiple hearth furnace possess certain advantages over other solid material processing furnaces such as rotatory kilns and the like.

Because these furnaces permit close control of individual hearth atmospheres and temperatures it is possible to perform very delicate operations such as the regeneration of the spent bone char from the refining of cane and beet sugars. Various other adsorptive agents can also be regenerated in these furnaces. US. Patent 3,153,633 to Von Dreusche, for example, shows the use of a multiple hearth furnace for the regeneration of certain granular activated carbon absorptive agents.

Another advantage of multiple hearth furnaces for the processing of dry solid materials lies in their ability to maintain such materials in mixed condition throughout their passage through the furnace. In inclined rotary kilns on the other hand, particles of the material being processed tend to segregate according to size, with the result that certain portions of the material enjoy longer exposure to the furnace atmospheres and temperatures and consequently become processed to a different degree than do other portions. The multiple hearth furnace avoids these difficulties and ensures that all sizes of particles being processed receive substantially the same treatment.

In the past there have been certain drawbacks to multiple hearth type furnaces. Among these, one of the most severe was their limited maximum operating temperature. In general, these furnaces had been limited to about 1800 F. At higher temperatures the rabble teeth may become subject to excessive wear, the rabble arms would sag, the center shaft became liable to buckle and the hearth structure itself would lose rigidity and would tend to collapse.

The present invention makes it possible to operate a multiple hearth furnace for prolonged periods at temperatures in excess of 2200 F.

According to one aspect of the present invention there is provided a novel heat shielding arrangement with a heat dissipating arrangement which cooperate to ensure that the temperaures of the metallic portions of the furnace remain sufliciently low so that they retain their basic strength. The novel heat shielding arrangement serves to maintain a Wide temperature differential between the furnace interior and the metallic elements within the furnace while the heat dissipating arrangement serves to remove heat from the metallic elements at a rate equal to that at which it passes through the heat shield.

The heat dissipating arrangement of the present invention comprises an air ducting system which is integrated with the center shaft and rabble arms of the furnace. The center shaft and rabble arms are hollow and are provided with internal partitions which define air ducts extending along the center shaft and rabble arms. More particularly, the hollow core of the center shaft is provided with an inner tubular shell which divides its interior into two concentric longitudinally extending air passageways. Each rabble arm is hollow and is provided with an internal septum along its length dividing it also into two longitudinally extending air passages. These passages communicate with each other at the outer tip of the arm. At the opposite end, however, where the arm joins the center shaft, the two rabble arm air passageways each communicate with a different one of the two concentric center shaft air passageways. Air is blown up through one of the center shaft air passageways and passes in parallel paths through each of the individual rabble arms. As a result heat is removed efiiciently and evenly from each of the rabble arms and from the center shaft.

The novel heat shielding arrangement of the present invention, which cooperates with the heat dissipation arrangement to maintain a wide temperature drop between the furnace interior and the structural metal portions of the rabble arms and center shaft involves first the use of special ceramic rabble teeth formed to interlock loosely with the rabble arms along their bottom surface. These ceramic teeth, in addition to providing heat shielding between the hot mix being processed and the metal portions of the rabble teeth, further provide improved abrasion resistance at elevated temperatures. Batts of fibrous insulation are placed in the loose interlock between the rabble arms and rabble teeth. This insulation in addition to providing further temperature protection also serves to provide a cushioning effect to avoid any sharp shocks that may take place when the tooth strikes a foreign object on the hearth.

The novel heat shielding arrangement of the present invention also involves special supporting means which securely holds heat resistant material in place so that it fully covers and fully protects the metal parts such as the center shaft and rabble arms; and yet at the same time accommodates the uneven dimensional changes between the metal parts and the heat resistant material which occurs at elevated temperatures.

In the past where heat resistant material, which has a low coefficient of thermal expansion, was secured to a metal structure having a much higher coefficient of thermal expansion, difiiculties developed at higher temperatures because the heat resistant material would crack and break away by the expansion of the metal structure. The heat resistant material could not successfully be reinforced by embedding a layer of steel mesh into it because this merely increased the surface area where differences in expansion coefiicients caused cleavages. Matts of fibrous insulation were also unsuccessful for they were easily dislodged by gaseous blasts from the furnace burners. Also they rapidly filled with hearth dust and lost their insulative characteristics.

According to the present invention novel means are provided for securing heat resistant material to the center shaft and rabble arms of a multiple hearth furnace. This novel means includes a plurality of metal protrusions which are attached to and extend out from the center shaft and rabble arms. These metal protrusions serve to support the outer solid heat resistant elements at a finite distance away from the surfaces of the center shaft and rabble arms. Disparities in thermal expansion are thus absorbed in this finite space and in the flexing or movement of the metal protrusions. In one case L-shaped stainless steel hooks are welded to the center shaft and are embedded at their outer ends into insulating firebricks. In another case a castable heat resistant material is held displaced from the center shaft and/ or rabble arms by the interposition of a batt of fibrous insulative material. Again wire-like members may be welded to the metal element being protected and may pass through the fibrous batt into the cast or solid heat resistant material to hold it in place. In the case of the rabble arms the metal protrusions need not extend into the solid heat resistant material. Instead a batt of fibrous insulative material may be laid on top of the rabble arm and a ceramic-like heat resistant cover may rest on the fibrous material, and with the fibrous material it may extend down over the sides of the rabble arms. The cover is provided with internal projections capable of loosely engaging the projections on top of the rabble arms to prevent the heat shield from moving off the rabble arm.

According to a further aspect of the present invention the furnace itself is specially constructed to withstand higher furnace temperatures. This aspect of the present invention involves in part the discovery that certain constituents common to most firebrick used in hearth construction are of glass-like quality. That is at higher temperatures, in the order of 2200 F., they exhibit an apparent reduction in viscosity and actually act more in the nature of a lubricator than a binder. Accordingly the bricks lose their structural strength. While for kilns and other types of furnace where bricks are subjected solely to compressive stresses, this effect is negligible or unnoticeable, nevertheless in multiple hearth type furnaces where the hearth floors are cantilevered from the furnace walls and depend upon the bending resistance of the tirebrick itself this problem becomes serious. The present invention overcomes this problem by using in multiple hearth construction, hearth firebrick which is high in alumina and which contains a minimum of glass formers such as TiO Fe O CaO, MgO, Na O, K 0, Li O.

The furnace hearth itself is also made to withstand higher temperatures by means of a special butt strap construction. In order to permit the hearth bricks to maintain themselves in the configuration of a slightly dome-shaped hearth capable of sustaining significant loads, steel band or butt straps are placed about the girth of the furnace in the vicinity of each hearth floor thereby to provide radial support. Unfortunately, these butt straps must be of metal because they are subjected to tensile stresses. Because of this they are subject to thermal dimensional changes due to changing ambient conditions. This expansion and contraction of the butt straps produces undue stresses on the hearth floor. The present invention overcomes this situation by providing hood-like shields extending out over each butt strap. These shields trap rising warm air from along the exterior of the furnace and subject the butt straps to this rather constant temperature air. At the same time they protect the butt strap from changing ambient conditions, thereby causing the butt strap to follow the hearth temperatures rather than the ambient temperatures. As a result dimension disparities between the hearth and butt straps are reduced and consequently the overall hearth strength is maintained.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent constructions as do not depart from the spirit and scope of the invention.

Specific embodiments of the invention has been chosen for purposes of illustration and description, and are shown in the accompanying drawings, forming a part of the specification, wherein:

FIG. 1 is a section view, taken in elevation, of a furnace embodying the present invention;

FIG. 2 is an enlarged fragmentary portion of FIG. 1;

FIG. 3 is section view taken along line 33 of FIG. 2;

FIGS. 48 are enlarged fragmentary views illustrating the development of the center shaft insulative arrangement in the furnace of FIG. 1;

FIG. 9 is a view similar to FIG. 8 and illustrating an alternate furnace center shaft insulative arrangement;

FIGS. 10-14 are cross sectional views of a rabble arm in the furnace of FIG. 1, and illustrating the development of the rabble arm insulative arrangement;

FIG. 15 is a longitudinal section view of a rabble arm of the furnace of FIG. 1;

FIG. 16 is an enlarged fragmentary view illustrating one form of juncture insulative arrangement between a rabble arm and the center shaft of the furnace of FIG. 1;

FIG. 17 is a view similar to FIG. 16 illustrating an alternate form of juncture insulative arrangement;

FIG. 18 is a cross sectional view of a rabble arm showing an alternate insulative arrangement;

FIG. 19 is a longitudinal section view of the rabble arm of FIG. 18;

FIG. 20 is an enlarged fragmentary view illustrating a further rabble arm and center shaft insulative arrangement;

FIG. 21 is an enlarged fragmentary view illustrating the manner of securing a rabble arm to the center shaft in the furnace of FIG. 1;

FIG. 22 is a fragmentary view showing a butt-strap and shield arrangement used in the furnace of FIG. 1.

As shown in FIG. 1, a furnace 30 of generally cylindrical configuration is mounted upright on top of a plurality of support pillars 31. The pillars 31 in turn are secured on pedestals 32 which rest on a working floor 34.

The furnace 30 is constructed of a tubular outer steel shell 36 which is lined, as indicated at 38, with firebrick or a similar heat resisting material. At the ends of the steel shell 36 there are provided upper and lower lateral support structures 42 and 44 of structural steel.

A rotatable vertical center shaft 46 extends axially through the furnace and is secured by upper bearing means 48 to the upper lateral support structure 40. The lower end of the center shaft 46 passes down through the lower lateral support structure 42 to a lower bearing means 50 which is supported on the working floor 34.

The center shaft 46 is of composite construction and comprises an inner tubular member 52 arranged coaxially within an outer tubular member 54. A novel outer insulative arrangement 56, to be described more fully hereinafter surrounds the outer tubular member 54. The outer tubular member 54 extends only within the furnace 30 itself between the upper and lower lateral support structures 40 and 42. Upper and lower water or sand seals 58 and 60 are arranged between the upper and lower lateral support structures 40 and 42 respectively and the center shaft 46 to permit rotation of the center shaft while preventing gas leakage into or out from the furnace.

The center shaft 46 is rotated by a drive means (not shown) which operates through a speed reducer 62 to turn a pinion gear 64. The pinion gear in turn meshes with a large bevel gear 65 mounted on the portion of the inner tubular member 52 which extends below the furnace 30.

A blower 66 is mounted below the furnace 30. The output of the blower 66 is connected via a fitting 68 and openings 70 in the inner tubular member 52 to blow cool ambient air up through the inner tubular member.

The furnace is closed at its upper and lower ends by means of a refractory ceiling 72 and a refractory floor 74. The ceiling 72 is of refractory material such as high duty firebrick and is arranged in generally arched configuration to be self supporting just under the upper lateral support structure 40. The floor 74 is of a special double thickness insulation to resist the higher temperatures produced near the bottom of the furnace; and it is supported on the lower lateral support structure 42.

The interior of the furnace 30 is divided, by means of hearth floors 76, 80, 82 and 84 into a plurality of vertically aligned hearths designated respectively as HEARTH A, HEARTH B, HEARTH C, HEARTH D, HEARTH E, and HEARTH F. (For purposes of illustration, the portion of furnace 30 which would include another hearth fioor 78, separating HEARTH B and HEARTH C, has been cut away.)

Each of the hearth floors 7684 is made of refractory material and is of slightly arched configuration to be self supporting within the furnace. Central drop holes 86 are formed in the alternate hearth floors 76, and 84 near the center shaft 46. Outer drop holes 88 are formed in the intermediate hearth floors 78 and 82 near the outer shell 36 of the furnace. A material feed inlet opening 90 is formed near the outer periphery of the ceiling 72, and is connected to an outer feed tube 92 on top of the furnace. At the bottom of the furnace there is provided a material outlet opening 94 and an air lock 95 near the outer periphery of the furnace floor 74. A gas exhaust opening 96 and chimney 98 are provided in the ceiling 72 to permit exhaustion of gases and products of combustion formed inside the furnace 30 during operation.

A plurality of rabble arms 100 extend radially out from the center shaft 46 in each hearth over the hearth floor. The rabble arms have rabble teeth 102 formed thereon which extend downwardly nearly to the hearth floor. The rabble teeth are inclined with respect to the longitudinal axis of their respective rabble arms so that as the rabble arms 100 are carried around by rotation of the center shaft 46 the rabble teeth 102 will continuously rake through the material being processed on the associated hearth floor and gradually urge the material toward the drop holes 86 or 88 in the hearth floor.

Each hearth is provided with a plurality of burner nozzles 104 which extend both radially and tangentially into the hearths. The burner nozzles form the outputs of various individually controlled burner assemblies (not shown) distributed about the furnace 30; and they serve to produce and/ or maintain proper temperatures and atmospheres within different regions of the furnace to carry out the particular processing desired. There are also provided special working doors 105 and windows 106 for monitoring the operation of the furnace operation at each of the hearth levels.

For structural support purposes the furnace 30 is girded by a steel butt strap 108 at each juncture of a hearth floor 76, 78, 80, 82 and 84 with the outer furnace wall. These butt straps act in tension to resist the tendency of the arched hearth floors to expand outwardly under the loading of the material being carried by them. Special novel butt strap shields 110, to be described more fully hereinafter, extend over the butt straps on the hot hearths and serve to retard the tendency toward unequal expansion and contraction between the butt straps and the hearth floor material under changing ambient and internal furnace conditions.

Operation of the furnace as thus far described takes place in the following manner. Material to be processed is supplied continuously via the outer feed tube 92 and the feed inlet opening 90 and falls upon the floor 76 of HEARTH A near its outer periphery. As the center shaft 46 rotates the rabble arms 100 in HEARTH A cause their associated rabble teeth to rake through the material and gradually urge it in toward the central drop hole 86 in the hearth floor. The material then drops down onto the floor of HEARTH B (not shown) where associated rabble arms and teeth gradually work the material back toward the outer periphery where it passes through drop holes to the floor 80 of HEARTH C. This gradual back and forth movement takes place among the various hearths while the burner nozzles 104 supply heat to maintain proper temperature to carry out the processing operation. Ultimately the material passes out of the outlet opening 94 near the bottom of the furnace and through the air lock 95.

In a particular processing operation the temperatures among the various hearths of the furnace of FIG. 1 may coincide with the following schedule:

Because of the lower temperatures encountered in HEARTH A and HEARTH B, they may be lined with conventional high duty firebrick. Also the rabble arms and rabble teeth in these arms may be integrally formed; although in HEARTH B they should be of alloy steel while in HEARTH A they may be of cast iron. The remaining hearth floors and associated furnace walls should be of a super duty firebrick. Typical examples of such super duty firebrick those sold under the trademarks Ufala, H-W Mullite, and Korundal XD.

The center shaft and rabble arm insulation arrangements in those hearths below HEARTH C are specially designed in accordance with the present invention to withstand the higher temperatures and corrosive atmospheres which prevail in these hearths. Moreover the interiors of the center shaft and rabble arms are specially constructed to effect rapid dissipation of heat from the metallic portions thereof so as to maintain their temperatures below that at which they have structural rigidity.

As shown in FIGS. 2 and 3, the inner tubular member 52 of the center shaft 46 cooperates with the outer tubular member 54 to divide the interior of the center shaft into inner and outer concentrically arranged longitudinally extending air passageways and 122. The inner air passageway 120, which communicates via the openings 70 in the bottom thereof with the blower 66, is closed at the upper end of the center shaft 46 by a partition 124 which covers the inner tubular member 52. The outer air passageway 122 on the other hand is closed toward the bottom of the center shaft 54 near the lower seal 60; but opens via an air outlet passage 128 out through the top of the furnace.

As shown in FIGS. 2 and 3, each of the rabble arms 100 is of hollow construction and is formed with a longitudinally extending internal web 130. The web 130 divides the interior of the rabble arm into two

separate air passageways

132 and 134. These passageways communicate with each other via an opening 136 in the web 130 toward the outer end of the rabble arm 100. At the inner end of each rabble arm, i.e. where it is secured to the center shaft 46, each

air passageway

132 and 134 communicated with a different one of the inner and outer air passageways 120 and 122 in the center shaft.

It will be appreciated that the above described center shaft and rabble arm construction provides air flow regions along the interior surfaces of all metal elements within the furnace 30. Moreover the air flow system provides for continuous flow of cool ambient air up through the center shaft 46 and separately into and along the interior of each of the rabble arms 100 during operations of the furnace. None of the cooling air which passes through any one rabble arm must pass through any other rabble arm. As soon as the quantity of air which has cooled one rabble arm returns to the center shaft 46, it passes directly up through the outer air passageway 122 and is exhausted out through the air outlet passage 128. In an actual six hearth furnace having a temperature schedule as above described and having a 21 foot inside diameter, coolant air at 65 F. was supplied by the blower 66 at a rate of 3000 cubic feet per minute. It was found that after passing through the rabbling system the air exhausted from the center shaft was only 200 F. to 250 F.

The ability of the furnace 30 to withstand the high temperatures involved depends upon the cooperation of the above described air fiow system with the special center shaft insulative arrangement 56 and a special rabble arm and rabble tooth construction to be described.

As shown in FIG. 2, the steel outer tubular member 54 of the center shaft 46 is covered with a dual layered insulation. There is provided immediately against the outer surface of the tubular member 54 an inner layer of fibrous insulative material 140. Such insulative materials are well known to the trade; a typical example being that sold under the name Cerafelt by Johns-Manville, New York, N.Y. This inner layer is surrounded by an outer layer 142 of cast or similarly formed solid, highly refractive insulation. A typical example of such solid castable insulation is that sold under the name H-W Lightweight Castable 28, made by Harbison-Walker Refractories Co., Pittsburgh, Pa. The solid outer layer is secured to the center shaft by means of several stainless steel wire-like prongs 144 which are welded at one end to the steel outer tubular member 54 and which are embedded at their other end into the solid insulative outer layer 142.

The above described arrangement, in addition to maintaining a high temperature differential between the steel portions of the center shaft 46 and the furnace interior, additionally serves to accommodate the ditferenetial expansions and contraction which occur between the insulative outer layer 142 and the steel outer tubular member 54 as these elements are subjected to temperature changes. The greater rates of expansion and contraction of the steel outer tubular member 54 merely contract or expand the inner layer 140 of fibrous insulative material without stressing the outer layer 142. The wire-like steel prongs 144 simply bend in response the differential expansions and contractions and thus prevent the transmittal of stress between the outer tubular member 54 and the insulative outer layer 142.

FIGS. 48 illustrate the manner in which the above described insulative arrangement is provided on the cen- 8 ter shaft 46. As shown in FIG. 4, the wire-like stainless steel prongs 144 are first welded at one end to the outer surface of the steel outer tubular member 54. Thereafter, as shown in FIG. 5, the insulative inner layer of fibrous material is forced over the prongs 144 and up against the surface of the outer tubular member. The prongs 144 are then bent, as shown in FIG. 6, to form acute angles with the tubular member 54. A casting form is then placed about the center shaft as shown in FIG. 7 and then castable insulation is poured into the annular space between the form 150 and the inner insulative layer 140. After the castable insulation has hardened to form the outer layer 142, the form 150 is removed as shown in FIG. 8.

An alternate center shaft insulative arrangement is illustrated in FIG. 9. According to this arrangement, wirelike stainless steel prongs 152, similar to the prongs 144 described above, are welded to the outer surface of the outer tubular member 54 as is the preceding arrangement. The outer tips of these prongs are pointed, as illustrated at 154. The prongs 152 are then bent at right angles at a finite distance out from the outer tubular member 54; and insulating firebricks 156 are then forced onto the bent portions of the prongs so that their tips 154 become embedded into the brick. No portion of the prongs 152 are exposed to the high hearth temperatures and at the same time they securely support the insulating firebricks 156 in place about the center shaft. Additionally, any expansion or contraction of the outer tubular member 54 will be accommodated without any loss in the integrity of the insulating system or its support.

It will be noted that in both above described insulative arrangements, the prongs 144 and 152 permit the free conduction of any heat they receive back to the center shaft for dissipation via its air flow system. Thus the temperature of the prongs never reaches a level where they lose their structural strength.

The novel insulative arrangements used in connection with rabble arms 100 in the higher temperature hearths such as HEARTH D, HEARTH E, and HEARTH P, will now be described. Reverting briefly to FIG. 2, it will be seen that the rabble teeth 102 are separate from their associated rabble arms 100. Moreover, the rabble arms 100 are made up of a metallic inner core structure 160 which is covered with a special insulative arrangement 162. The special insulative arrangement 162 comprises an inner layer of fibrous insulation 164 which is placed over the metallic core 160, and an outer solid insulative cover 166, which rests on top of the fibrous insulation 164. The fibrous insulation 164 may be formed batts of high temperature resistant flexible material. For example, it may be the same Cerafelt material described above and used as the inner insulative layer 140 on the center shaft 46. The outer solid insulative cover 166 may be of pre-formed ceramic material. An example of such material is that which is sold under the mark Korundal XD, by Harbison-Walker Refractories Co., Pittsburgh, Pa. As shown in FIG. 2, there are provided spaced upstanding lugs 170 on top of the metallic core 160 of the rabble arms 100. These lugs mesh with corresponding internal lugs 172 on the inside of the outer insulative cover 166. This prevents the cover 166 from moving longitudinally with respect to the metallic core 160.

FIGS. 10-15 inclusive illustrate the development of the insulative arrangement 162 and the manner of securing the teeth 102 to the rabble arms 100. As shown in FIG. 10 the metallic core 160 is hollow and is formed with its web 130 cast in place. Additionally, the metallic arm 160 is provided with a pair of lower flanges 174 upon which the rabble teeth 102 are mounted. As shown in FIG. 10, the lugs 170 are cast integrally into the top of the core 160.

The first step in preparing the insulative arrangement 162, is to provide a batt of the fibrous insulation 164 and to place it over the top of the metallic core 160 so that 9 it extends down on each side thereof toward, but not all the way down, to the flanges 174. The lugs 170, it will be noted, protrude slightly up through the fibrous insulation 164.

As shown in FIG. 12, the outer solid insulative cover 166 of ceramic material, which was pre-formed, is placed on top of the fibrous insulation 164. As can be seen, the internal lugs 172 on the outer cover 166 protrude slightly into the fibrous insulation 164 and cooperate with the lugs 170 and the metallic core 160 to secure the outer cover 166 so that it does not slide lengthwise with respect to the metallic core 160. As can be seen in FIG. 15, the

lugs

170 and 172 cooperate to prevent longitudinal sliding of the outer cover 166.

It will be noted that should the metallic core 160' expand or contract due to temperature changes within the furnace, such expansion and contraction is easily accommodated by compression and expansion of the fibrous insulation 164; and accordingly there is no imposition of undue stresses upon the outer solid insulative cover 166, even though the cover does not expand or contract to the same extent as the metallic core 160 under varying temeratures.

As shown in FIG. 13, a further batt of fibrous insulative material 180 is placed across the bottom of the metallic core 160 and is bent up and around the lower flanges 174. This material may be cemented in place with a commercial cement used for such purposes. An example of such cement is that known as JM Cera-Kote furnished by the supplier of the Cerafelt insulation.

As shown in FIG. 14, the rabble teeth 102, which are formed of a ceramic material (e.g. such as Korundal), are shaped along their upper portion with a pair of inwardly facing hook-like formations 182. These hook-like formations are dimensioned to fit loosely over the lower flanges 174 of the metallic core 160 in interlocking relationship therewith. However, when the further insulative batt 180 is in place between thetooth 102 and the flanges 174, the interlock is quite snug. It will be noted that the rabble teeth 102 do not directly contact the metallic portions of the rabble arm 100 but instead are insulated from it by means of the further insulative batt 180.

An important feature of the above described tooth-arm interconnection is that the interposition of the insulative batt 180 provides a cushioning effect whereby should the rabble tooth 102 be subjected to sudden shocks, these shocks will be absorbed in the batt 180* and will not be transmitted through to the rabble arm 100.

FIG. 16 illustrates in detail the manner in which the junction between the above described rabble arm 100 and the center shaft 46 is insulated when the center shaft 46 has been provided with the insulative inner and

outer layers

140 and 142. As shown in FIG. 16, the outer insulative layer 142 is terminated at a point 190 just above the rabble arm 100. After the rabble arm is secured in place to the center shaft 46, the space between the rabble arm and the center shaft insulation arrangements is filled with an insulating paste 192 which swells prior to setting. An example of such paste is that known as Insulag, and sold by Quigley Company, New York, NY. After the insulating paste 192 has set, it is covered with a layer of castable insulation 194 which extends between the outer castable layer 142 of the center'shaft 46 and the outer cover 166 of the rabble arm 100.

FIG. 17 illustrates a center shaft-rabble arm juncture insulation arrangement suitable for use in situations where the center shaft 46 is provided with firebrick insulation as shown in FIG. 9. As illustrated in FIG. 17, a batt of fibrous insulative material 200 is forced up under the firebrick immediately adjacent to the opening in the center shaft 46 for rabble arm 100. The batt 200 extends from the firebrick 202 to the rabble arm 100 and is forced just under the solid outer cover 166.

FIGS. 18 and 19 show an alternate arrangement for insulating the rabble arms 100. As shown in FIGS. 18 and 19, the rabble arms are formed with an inner metallic core 210 having a flat bottom 212 extending outwardly in the shape of lower flanges 214. A plurality of wire-like prongs 216 are welded at various locations to the metallic core 210 along its sides and top. Thereafter, a layer of fibrous insulative material 218 is pressed down over the prongs 216 so that it lays snugly over the top of the core 210. Thereafter, castable insulation 220, such as that used to form the outer insulative layer 142 of the center shaft 46, is cast about the exterior of the rabble arm 100'. As the insulation sets, it becomes secured to the prongs 216 and is thereby held in place on the metallic core 210.

The ceramic teeth 102 are secured in place in a manner similar to that described above. That is, there is provided a layer 222 of fibrous insulative material which surrounds the bottom 212 of the metallic core 210 and extends up and around the flanges 214 thereof. The rabble teeth 102 are thereafter fitted into place. The spaces between the rabble tooth hook-like formations 182 and the bottom edges of the solid insulative layer 220 on the rabble arms, is filled by forcing thereinto further batts 224 of fibrous insulative material.

As shown in FIG. 19 there is provided a pin 227 at the tip of the rabble arm for securing the teeth 102 so they do not move longitudinally along the arm. Similar arrangements (not shown) are provided for the rabble teeth in the other embodiments.

FIG. 20 illustrates the arrangement of the rabble arms 100 and center shaft '46 when the center shaft is insulated by means of an inner fibrous layer and a cast outer layer 142, and when the rabble anrns 100 are insulated as shown in FIGS. 18 and 19. As shown in FIG. 20; the uppermost rabble arm 106a is of the type which would be used in HEARTH A, HEARTH B, and H'EARTH C. That is, it is not provided with a special insulative arrangement such as described in connection with the rabble arms to be used in the lowermost hearths. Accordingly, the center shaft insulation extends down to a line 230. The space between the line 230 and the rabble arms 100 is then filled with a paste-like insulative material 232 such as the Insulag material described above. The dual layered insulation comprising the fibrous inner layer 140 and the cast solid outer layer 142 continues along the center shaft 46 down toward the next lower rabble arm 10017. As can be seen in dotted outline 234, there may be provided in the cast insulative outer layer 142, a reinforcing screen so as to aid in avoiding cracking of the outer insulative layer 142. The outer insulative layer 142 terminates at a line 236, just above the rabble arm 10%. The space between the line 236 and the rabble arm 10% is then filled, as illustrated at 23 8, with an insulative paste material such as Insulag material, which expands prior to setting. Thereafter, a layer of castable insulation 240, which may be of the same material as the castable insulation 142 used in the center shaft 46, is provided to extend between the outer insulation 142 of the center shaft and the outer insulative layer 220 on the rabble arm 1001). As shown in FIG. 20, the next lower rabble arm 100s is also insulated in the same manner; and similarly, its juncture with the center shaft 46 is provided with the arrangement described above.

FIG. 21 illustrates the manner of securing the rabble arms 100 to the center shaft 46. As shown in FIG. 21, the rabble arm core is shaped with a tapered inner end 260, the tip of which joins the inner tubular member 52 of the center shaft 46 and places one of the rabble arm air passageways into communication with the center shaft inner air passageway 120. The outer end of the tapered portion 260 is secured into a flanged opening 262 formed in the outer tubular member 54 of the center shaft 46. The other rabble arm air passageway is thus placed into communication, via the tapered portion 260, with the center shaft outer air passageway 122. A pin 264 secures the rabble arm 100 in place to the center shaft. The arm and center shaft junction insulative arrangements are as above described.

It will be appreciated that in all of the above insulative arrangements there is provided room for expansion and contraction of a metallic element without the imposition of undue stresses upon the solid outer insulating shields. Moreover, the solid outer insulating shields are held securely in place and they serve to protect the fibrous inner insulative layers from contamination due to dust etc. within the furnace. Thus the fibrous inner layers are maintained at maximum insulating efficiency.

Referring now to FIGS. 2 and 22, it will be noted that the butt straps 108 extend around the outside of the furnace 30 juncture of each hearth floor with the furnace wail. As pointed out above, the butt straps act in tension to maintain the hearth floors in their arched configuration so that they are capable in supporting themselves as well as the weight of the material resting upon them. A difficulty occurs 'when the butt straps 108 are subjected to changing conditions. When this occurs, the amount of tensile stress thy impose upon the hearth floors, changes due to the thermal expansion and/ or contraction of the butt straps themselves. The shields 110 are of canopylike configuration. As shown in FIG. 22 one edge of the shields is attached to the outer furnace wall; and the shield extends from there to cover the top and outside of the butt straps 108. In so doing, the shields 110 provide an air space 110a along the outer surface of the butt straps 10 8. This air space is filled with air which has been war-med along the outer shell 36 of the furnace. The air which is contact with this outer shell is warmed by the shell and rises directly upwardly where it is caught by the shield 11.0 and is held in the space 110a immediately adjacent to butt strap 10 8. As a consequence, the butt strap is continuously subjected to substantially constant temperatures. Should the ambient conditions change, the butt strap 108 will be protected by the warm air surrounding it and trapped in place about it. Moreover, the canopy-like shield 110 protects the butt strap from wind, rain and snow, all of which tend to produce expansion and/or contraction of the butt strap by changing the temperature adjacent thereto.

The strength of the various arched hearth floors at high temperatures has been improved according to the present invention, by constructing these floor-s out of a firebrick having a minimum of glass formers. These glass formers are elements which, although capable of resisting high temperatures, nevertheless serve as a lubricant at high temperatures thereby permitting the refractory material to distort. The glass formers which produce this effect in high aluminum firebrick are TiO F CaO, MgO, Na O, K 0, and Li O By providing a hearth floor material made up of high alumina brick and characterized by an absence of appreciable amounts of these glass formers (i.e. less than the amount which would produce individual brick distortion in excess of 0.30% under a load of 200 pounds per square inch at 2200" F. for 100 hours), a suitable hearth floor can be obtained.

Materials which have performed satisfactorily in this regard have compositions within the following ranges:

Percent Silica (SiO 10.2-37.3 Alumina (A1 0 59-893 Titanium dioxide (TiO Below 3 Iron oxide (Fe O Below 1.4 Lime (CaO) Below 0.1 Magnesia (MgO) Below 0.1 Alkalides (Na O, LiO, K 0) Below 0.3

Having thus described the invention with particular reference to the preferred form thereof, it will be obvious to those skilled in the art to which the invention pertains, after understanding the invention, that various changes and modifications may be made therein without departing from the spirit and scope of the invention, as defined by the claims appended thereto.

What is claimed as new and desired to be secured by Letters Patent is:

1. A rabble arm and rabble tooth construction for use in multiple hearth furnaces, said construction comprising an elongated metallic rabble arm, a ceramic rabble tooth, said arm and said tooth being cooperatively configurated such that said tooth loosely interlocks with said arm to hang downwardly therefrom and a batt of fibrous insulative material interposed between said rabble arm and said rabble tooth where they interlock to provide an insulating and shock absorbing tight connection between the two.

2. A construction as in claim 1 wherein said rabble arm is provided with a laterally extending bottom flange and wherein said ceramic rabble tooth is shaped with a pair of mutually facing hook-like formations at the top thereof which slip over and accommodate said bottom flange.

3. In a multiple hearth furnace, the combination of an upright cylindrical outer wall, a plurality of hearth floors secured to said wall at various heights, said hearth floors being made of refractory material and having a slightly arched configuration to permit self support as well as support of material thereon to be processed in said furnace, a plurality of metallic butt straps extending around the outside of said outer wall in the vicinity of said hearth floors and stressed in tension to exert radial forces on said hearth floors for reinforcing same and canopy like elements extending over said butt straps to protect same from changing ambient conditions.

4. A combination as in claim 3 wherein said canopy like elements are open at the bottom to trap upwardly rising air which has been warmed by the furnace walls.

5. A combination as in claim 4 wherein said canopylike elements are secured along one edge to the outer furnace wall immediately above each butt strap and extend out and down over the butt strap.

6. In a multiple hearth furnace, the combination of a rotatable center shaft, a plurality of rabble arms extending radially out from said center shaft in each hearth, an inner layer of fibrous insulative material covering said center shaft and said rabble arms, an outer solid insulative material covering the fibrous material on said center shaft and on said arms, and at the intersection of said arms and said center shaft, a batt of fibrous insulative material extending under the outer solid insulative material on each and held in place thereby.

7. A combination as in claim 6 and including a castable solid outer insulative material covering the exposed portions of said batt.

8. In a multiple hearth furnace a hearth floor construction comprising a slightly arched floor secured to the outer walls of the furnace and extending across the interior thereof, said floor being formed of high alumina firebrick and characterized by an absence of appreciable amounts of Ti, 0 Fe O CaO, MgO, Na O, K 0 and Li O.

9. In a multiple hearth furnace, the combination of a rotatable center shaft, a layer of fibrous insulative material covering at least a portion of said center shaft, a solid heat resistive refractory outer shell covering said insulative material, and a plurality of wire-like projections extending out from said center shaft through said fibrous insulative material and entirely embedded in said refractory outer shell whereby uneven expansion is allowed between said center shaft and said refractory outer shell.

10. A combination as in claim 9 wherein said outer shell is cast over said fibrous insulative material.

11. A combination as in claim 9 wherein said fibrous insulative material is in batt form.

12. In a multiple hearth furnace, the combination of a rotatable center shaft, an outer insulative covering surrounding said center shaft, and a plurality of wirelike projections extending out from said center shaft and entirely embedded in said outed insulative covering whereby uneven expansion is allowed between said center shaft and said outer insulative covering.

13. A combination as in claim 12 wherein said outer insulative covering comprises an array of firebricks, each having one of said wire-like projections embedded into it.

14. A combination as in claim 13 wherein said wirelike projections extend directly out from said center shaft and are bent downwardly at right angles at a finite distance from said shaft and wherein the bent down portions are embedded into said firebrick.

15. An insulative rabble arm for use in a multiple hearth furnace, said rabble arm comprising a metal core, a layer of fibrous insulative material covering said metal core, an outer shell of a solid heat resistant refractory material covering and contacting said fibrous insulative material and a plurality of wire-like projections extending out from said metal core through said fibrous insulative material and entirely embedded in said outer shell whereby uneven expansion is allowed between said metal core and said outer shell.

16. A rabble arm as in claim 15 wherein said outer shell is cast over said fibrous insulative material.

17. A rabble arm as in claim 15 wherein said fibrous insulative material is in batt form.

18. An insulative rabble arm for use in a multiple hearth furnace, said rabble arm comprising a metal core having a bottom configuration shaped to interlock loosely with detachable rabble teeth, a layer of fibrous insulative material covering said metal core including a batt of fibrous insulative materal secured about the surfaces of said bottom configuration, and an outer shell of a solid heat resistant refractory material covering and contacting a portion of said fibrous insulating material.

References Cited UNITED STATES PATENTS 1,444,209 2/1923 Pike 263-26 1,687,935 10/1928 Fowler 26326 2,269,580 1/ 1942 Connolly 26326 3,226,101 12/1965 Balaz et al 263-6 JOHN L. CAMBY, Primary Examiner.