Patent Publication Number: US-2010119986-A1

Title: Multiple hearth furnace

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to a multiple hearth furnace (MHF). 
     BRIEF DESCRIPTION OF RELATED ART 
     Multiple hearth furnaces (MHFs) have been used now for about one century for heating or roasting many types of material. They comprise a plurality of hearth chambers arranged one on top of the other. Each of these hearth chambers comprises a circular hearth having alternately a central material drop hole or a plurality of peripheral material drop holes therein. A vertical rotary shaft extends centrally through all these superposed hearth chambers and has in each of them a rabble arm fixing node. Rabble arms are connected in a cantilever fashion to such a rabble fixing node (normally there are two to four rabble arms per hearth chamber). Each rabble arm comprises a plurality of rabble teeth extending downwards into the material on the hearth. When the vertical rotary shaft is rotated, the rabble arms plough material on the hearth with their rabble teeth either towards the central drop hole or towards the peripheral drop holes in the hearth. Thus, material charged into the uppermost hearth chamber is caused to move slowly downwards through all successive hearth chambers, being pushed by the rotating rabble arms over the successive hearths alternately from the periphery to the center (on a hearth with a central material drop hole) and from the center to the periphery (on a hearth with peripheral material drop holes). Arrived in the lowermost hearth chamber, the roasted or heated material leaves the MHF through a furnace discharging opening. 
     It will be appreciated that the vertical rotary shaft and the rabble arms are not only subjected to severe mechanical stresses, but they also have to withstand high temperatures and very corrosive atmospheres. Consequently, it is particularly important to warrant that structural rigidity of these elements is not affected by overheating, and that high temperature corrosion (in particular accelerated chloride corrosion due to overheating) as well as low temperature corrosion (in particular corrosion due to acid condensation as a direct consequence of overcooling) are reliably avoided. Furthermore, non-uniform temperature distributions may result in mechanical stresses causing deformations or even mechanical ruin of the shaft or the rabble arms. 
     In documents describing very early multiple hearth furnaces it is sometimes mentioned that rabble arms may either be water or gas cooled. Nevertheless, operating hearth furnaces exclusively include—as far as applicants know—gas cooled rabble arms. Indeed, if there is a leakage in a water cooled rabble arm, the whole furnace has to be shut down in order to find and repair the leakage, whereas a leakage in a gas cooled rabble arm does not necessarily require a direct intervention. However, gas cooled MHFs have serious drawbacks too. For example, a gas cooling circuit is not always capable of warranting a precise control of surface temperature. It follows that some surfaces of the vertical rotary shaft or the rabble arms or may be overheated or overcooled, which leads to the drawbacks mentioned above. 
     In most MHFs, the vertical rotary shaft as well as the rabble arms are tubular structures that are cooled by a gaseous cooling fluid, generally pressurized ambient air. (For the sake of simplicity, the gaseous cooling fluid will be called herein “cooling gas”, even if it may be a mixture of several gases, such as e.g. air). The vertical rotary shaft includes a cooling gas distribution channel for supplying the cooling gas to the rabble arms. From this cooling gas distribution channel, the cooling gas is channeled through the connection between the rabble arm and the rabble arm fixing node into the tubular structure of the rabble arm. As the cooling system of the rabble arm is normally a closed system, the cooling gas returning from the rabble arm must be channeled through the connection between the rabble arm and the rabble arm fixing node into an exhaust gas channel in the vertical rotary shaft. 
     In the last hundred years, there have been described various embodiments of such gas-cooled vertical rotary shafts and cantilever rabble arms for a MHF. For example: 
     U.S. Pat. No. 1,468,216 discloses a vertical hollow shaft of a MHF, in which a central partition wall separates a cooling gas distribution duct from an exhaust duct, each of them having a semicircular cross-section. In each hearth chamber, a cooling gas flow is branched off from the cooling gas flow in the cooling gas distribution duct to be rerouted through a rabble arm cooling system and to be thereafter evacuated into the exhaust duct. It follows that in the cooling gas distribution duct the flow rate and, consequently, the velocity of the gas strongly diminish from the bottom to the top and in the exhaust duct strongly they strongly increase from the bottom to the top. This results in a very un-uniform cooling of the vertical rotary shaft as well in a lengthwise as in a circumferential direction. 
     U.S. Pat. No. 3,419,254 discloses a double-shell gas-cooled vertical rotary shaft. The central space within the interior shell constitutes an intake duct and the annular space between the outer shell and the inner shell an exhaust duct. While this system warrants a more uniform cooling of the vertical rotary shaft in a circumferential direction of the shaft, cooling in the lengthwise direction of the shaft is still very uniform. 
     U.S. Pat. No. 2,332,387 also discloses a double-shell gas-cooled vertical rotary shaft. In this shaft, the annular space between the outer shell and the inner shell constitutes an intake duct and the central space within the interior shell an exhaust duct. The outer shell is—except at the rabble arm supports—of substantially the same diameter from the bottom to the top. In order to have a more uniform cooling gas flow within both ducts, U.S. Pat. No. 2,332,387 teaches to increase the diameter of the interior shell from the bottom to the top. A first disadvantage of this system is that the cooling gas strongly heats up from the bottom to the top of the annular intake duct, which results in a poorer cooling of the shaft and the rabble arms in the upper hearth chambers. A further disadvantage of this system is that the geometry of the shaft must be different in each hearth chamber, which makes its manufacturing of course more expensive. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a MHF with a more uniform gas cooling of the shaft and the rabble arms. 
     More particularly, the present invention proposes a multiple hearth furnace comprising, in a manner known per se: a plurality of hearth chambers arranged one on top of the other; a hollow vertical rotary shaft extending centrally through the hearth chambers and including an outer shell; in each of the hearth chambers, at least one rabble arm secured to the shaft; a gas cooling system for the shaft and the rabble arms including, within the outer shell, an annular main distribution channel for supplying a cooling gas to the rabble arms and a central exhaust channel for evacuating the cooling gas leaving the rabble arms; and a connecting means for connecting the rabble arms to the shaft including cooling gas supply means in direct communication with the annular main distribution channel and cooling gas return means in direct communication with the central exhaust channel. In accordance with the present invention, the gas cooling system further comprises an annular main supply channel surrounding the annular main distribution channel and being outwardly delimited by the outer shell. A cooling gas inlet is connected to the annular main supply channel. A cooling gas passage between the annular main supply channel and the annular main distribution channel is spaced from the cooling gas inlet, so that cooling gas supplied to the cooling gas inlet has to flow through the annular main supply channel through several hearth chambers before it flows through the cooling gas passage into the annular main distribution channel. It will be appreciated that with such a system, the whole main supply flow of cooling gas is first used to provide an efficient and uniform cooling of the outer shell of the vertical rotary shaft in several hearth chambers. The constant, high flow rate in the annular main supply channel warrants a relatively small temperature increase of the cooling gas between the cooling gas inlet and the cooling gas passage in the annular main distribution channel. In this inner annular distribution channel, the flow of the cooling gas—which now diminishes from hearth chamber to hearth chamber—is relatively well protected against additional warming up, so that the rabble arms in all superposed hearth chambers are supplied with a cooling gas at substantially the same temperature. All this results in a very efficient and uniform cooling of the shaft and the rabble arms. 
     The gas cooling system can e.g. comprise a single cooling gas inlet connected either to the lower or to the upper end of the vertical rotary shaft, i.e. the cooling gas supplied to the cooling gas inlet has to flow through the annular main supply channel through all hearth chambers before it flows through the cooling gas passage into the annular main distribution channel. However, in a preferred embodiment, the gas cooling system further comprises partition means partitioning the annular main supply channel and the annular main distribution channel in a lower half and an upper half. A lower cooling gas inlet is then connected to the lower half of the annular main supply channel at the lower end of the shaft, and an upper cooling gas inlet is connected to the upper half of the annular main supply channel at the upper end of the shaft. A lower cooling gas passage is arranged between the lower half of the annular main supply channel and the lower half of the annular main distribution channel and located near the partition means, so that cooling gas supplied to the lower cooling gas inlet has to flow upwards though the lower half of the annular main supply channel up to the partition means before it can flow through the lower cooling gas passage into the lower half of the annular main distribution channel. An upper cooling gas passage is arranged between the upper half of the annular main supply channel and the upper half of the annular main distribution channel and located near the partition means, so that cooling gas supplied to the upper cooling gas inlet has to flow downwards though the upper half of the annular main supply channel down to the partition means before it can flow through the second cooling gas passage into the upper half of the annular main distribution channel. It will be appreciated that this system results in a further improvement of the cooling system of the shaft and the rabble arms. With this split system, it is e.g. easier to equilibrate gas supply for the rabble arms in the superposed hearth chambers. 
     A preferred embodiment of the outer shell comprises: shaft support tubes and cast rabble arm fixing nodes interconnecting the shaft support tubes, wherein at least one rabble is fixed to each of the rabble arm fixing nodes. In this shaft, the rabble arm fixing node and the shaft support tubes are advantageously welded together. The shaft support tubes are advantageously made of thick walled stainless steel tubes and are dimensioned as structural load carrying members between the rabble arm fixing nodes. It will be appreciated that such a shaft can be easily manufactured at relatively low costs using standardized elements. It provides however a strong, long-lasting support structure that has a very good resistance with regard to temperature and corrosive agents in the hearth chambers. 
     A preferred embodiment of a rabble arm fixing nodes advantageously comprises a ring-shaped cast body made of refractory steel. It will be appreciated that such a rabble arm fixing node is a particularly compact, strong and reliable connection means for connecting the rabble arm to the vertical rotary shaft. 
     A preferred embodiment of a rabble arm rabble arm includes a tubular structure for circulating therethrough a cooling gas and plug body connected to the tubular structure of the rabble arm received in a socket on the vertical rotary shaft. It will be appreciated that such a plug body, which can be manufactured without necessitating complicated casting moulds, is a particularly compact, strong and reliable connection means for connecting the rabble arm to the vertical rotary shaft. 
     A further preferred embodiment of a rabble arm fixing nodes comprises a ring-shaped cast body including: at least one socket for receiving therein the plug body of the rabble arm. A central passage forms the central exhaust channel for the cooling gas within the rabble arm fixing node. First secondary passages are arranged in a first ring section of the cast body, so as to provide gas passages for cooling gas flowing through the annular main distribution channel. Second secondary passages are arranged in a second ring section of the cast body, so as to provide gas passages for cooling gas flowing through the annular main supply channel. The cooling gas supply means is arranged in the cast body so as to interconnect the annular internal supply channel for the cooling gas with at least one gas outlet opening within the socket and advantageously comprises at least one oblique bore extending through the ring-shaped cast body from the second ring section into a lateral surface delimiting the socket. The cooling gas return means is arranged in the cast body so as to interconnect the central passage with at least one gas inlet opening within the socket and advantageously comprises a through hole in axial extension of the socket. This embodiment of an arm fixing node combines a low pressure drop cooling gas distribution in the shaft and a solid fixing of the rabble arm on the shaft with a very compact and cost saving design. With its integrated gas passages, it substantially contributes to the fact that the vertical rotary shaft, which includes three co-axial cooling channels therein, can be manufactured using a very small number of standardized elements. It also essentially contributes to warranting a strong, long-lasting shaft support structure with a very good resistance with regard to temperature and corrosive agents in the hearth chambers. 
     In a preferred embodiment, a section of the shaft extending between two adjacent hearth chambers comprises: a shaft support tube arranged between two arm fixing nodes to form the outer shell of the section of the shaft, the shaft support tube delimiting the annular main supply channel to the outside; an intermediate gas guiding jacket arranged within the shaft support tube so as to delimit the annular main supply channel to the inside and the annular main distribution channel to the outside; and an inner gas guiding jacket arranged within the intermediate gas guiding jacket so as to delimit the annular main distribution channel to the inside and the central exhaust channel to the outside. In this preferred embodiment, the intermediate gas guiding jacket advantageously comprises: a first tube section with a first end fixed to the first fixing node and a free second end; a second tube section with a first end fixed to the second fixing node and a free second end; a sealing means providing a sealed connection between the free second end of the first tube section and the free second end of the second tube section, while tolerating relative movement in the axial direction of both free second ends. Similarly, the inner gas guiding jacket advantageously comprises: a first tube section with a first end fixed to the first fixing node and a free second end; a second tube section with a first end fixed to the second fixing node and a free second end; a sealing means providing a sealed connection between the free second end of the first tube section and the free second end of the second tube section, while tolerating relative movement in the axial direction of both free second ends. The sealing means advantageously comprises a sealing sleeve fixed to the free second end of one of the first or second tube sections and engaging in a sealed manner the free second end of the other tube section. It will be appreciated that such a shaft section can be easily manufactured at relatively low costs using standardized elements. 
     The rotary hollow shaft further advantageously comprises: an outer thermal insulation on its outer shell, the outer thermal insulation including an inner refractory layer of micro porous material, an intermediate refractory layer of insulating castable material and an outer refractory layer of dense castable material. 
     A preferred embodiment of a rabble arm advantageously comprises: an plug body for fixing the rabble arm to the rotary hollow shaft; an arm support tube fixed to the plug body; and a gas guiding tube arranged inside the arm support tube and cooperating with the latter to define between them a small annular gap for channeling the cooling gas from the shaft to the free end of the rabble arm, wherein the interior section of the gas guiding tube forms a return channel for the cooling gas from the free end of the rabble arm to the shaft. In this embodiment, the plug body is advantageously a solid cast body including at least one cooling gas supply channel and at least one cooling gas return channel. The at least one cooling gas supply channel and the at least one cooling gas return channel are then advantageously provided as bores in the solid cast body. 
     Such a rabble arm further advantageously comprises: an arm supporting tube; a micro porous thermal insulation layer arranged on the arm supporting tube; and a metallic protecting jacket covering the micro porous thermal insulation. In a preferred embodiment, metallic rabble teeth fixed to the metallic protecting jacket by welding, wherein anti-rotation means are arranged between the arm supporting tube and the metallic protecting jacket. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the present invention will be apparent from the following detailed description of a preferred but not limiting embodiment with reference to the attached drawings, wherein: 
         FIG. 1  is three dimensional view of a multiple hearth furnace in accordance with the invention, with a partial section; 
         FIG. 2  is schematic diagram illustrating the flow of cooling gas through the rotary hollow shaft and the rabble arms. 
         FIG. 3  is a section through a rotary hollow shaft, drawn as a three dimensional view; 
         FIG. 4  is three dimensional view of a rabble arm fixing node, with four rabble arms fixed thereto; 
         FIG. 5  is a first section through a socket in a rabble arm fixing node with a plug body of a rabble arm received therein (the section is drawn as a three dimensional view); 
         FIG. 6  is a second section through a socket in a rabble arm fixing node with a plug body of a rabble arm received therein (the section is drawn as a three dimensional view); 
         FIG. 7  is a section through a free end of a rabble arm (the section is drawn as a three dimensional view). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a multiple hearth or roasting furnace  10 . Both the construction and operation of such a multiple hearth furnace (MHF)  10  are known in the art and are therefore described herein only as far as they are relevant for the illustration of the inventions claimed herein. 
     The MHF as shown in  FIG. 1  is basically a furnace including several hearth chambers  12  arranged one on top of the other. The MHF shown in  FIG. 1  includes e.g. eight hearth chambers numbered  12   1 ,  12   2  . . .  12   8 . Each hearth chamber  12  includes a substantially circular hearth  14  (see e.g.  14   1 ,  14   2 ). These hearths  14  alternately have either several peripheral material drop holes  16  along their outer periphery, such as e.g. hearth  14   2 , or a central material drop hole  18 , such as e.g. hearth  14   1 . 
     Reference number  20  identifies a vertical rotary hollow shaft coaxially arranged with the central axis  21  of the furnace  10 . This shaft  20  passes through all hearth chambers  12 , wherein a hearth without central material drop hole  18 —such as e.g. hearth  14   2  in FIG.  1 —has a central shaft passage opening  22  to allow the shaft  20  to freely extend therethrough. In a hearth with a central material drop hole  18 —such as e.g. hearth  14   1  in FIG.  1 —the shaft  20  extends through the central material drop hole  18 . It will be noted in this context that the central material drop hole  18  has a much bigger diameter than the shaft  20 , so that the central material drop hole  18  is indeed an annular opening around the shaft  20 . 
     Both ends of the shaft  20  comprise a shaft end with a journal rotatably supported in a bearing (not shown in  FIG. 1 ). Rotation of the shaft  20  about its central axis  21  is accomplished by means of a rotary drive unit (not shown in  FIG. 1 ). As such a rotary drive unit for the shaft  20  as well as shaft bearings are known in the art and furthermore not relevant for the understanding of the inventions claimed herein, they will not be described with greater detail hereinafter. 
       FIG. 1  also shows a rabble arm  26  that is secured in hearth chamber  12   2  to a rabble arm fixing node  28  on the shaft  20 . Such an arm fixing node  28  is principally arranged in every hearth chamber  12 , wherein it normally supports more than one rabble arm  26 . In most MHFs, such an arm fixing node  28  normally supports four rabble arms  26 , wherein the angle between two successive rabble arms  26  is 90°. Each rabble arm  26  includes a plurality of rabble teeth  30 . These rabble teeth  30  are designed and arranged so as to move material on the hearth either towards its center or towards its periphery when the shaft  20  is rotated. In a hearth chamber with peripheral material drop holes  16  in its hearth  14 , such as e.g. hearth chamber  12   2 , these rabble teeth  30  are designed and arranged so as to move material on the hearth  14  towards the peripheral material drop holes  16  when the shaft  20  is rotated. In a hearth chamber with a central material drop hole  18  in its hearth  14 , such as e.g. hearth chamber  12   1 , these rabble teeth  30  are however designed and arranged so as to move the material on the hearth  14  towards the central material drop hole  18  when the shaft  20  is rotated in the same direction. 
     Now follows a brief description of material flow through the MHF  10 . In order to heat or roast material within the MHF  10 , this material is discharged from a conveying system (not shown) through a furnace charging openings  32  into the uppermost hearth chamber  12   1  of the MHF. In this chamber  12   1  material falls onto the hearth  14   1 , which has a central material drop hole  18 . As the shaft  20  is continuously rotated, the four of rabble arms  26  in the hearth chamber  12   1  push the material with their rabble teeth  30  over the hearth  14   1  towards and into its central material drop hole  18 . Through the latter material falls onto the hearth  14   2  of the next hearth chamber  12   2 . Here, the rabble arms  26  push the material with their rabble teeth  30  over the hearth  14   2  towards and into its peripheral material drop holes  16 . Through the latter, material falls onto the next hearth (not shown in  FIG. 1 ) that has again a central material drop hole  18 . In this way, material entering the MHF  10  through the furnace charging opening  32  is passed over all eight hearths  14   1  . . .  14   8  by the rotating the rabble arms  26 . Arrived in the lowermost hearth chamber  12   8 , the roasted or heated material finally leaves the MHF  10  through a furnace discharging opening  34 . 
     As known in the art, both the shaft  20  and the rabble arms  26  have internal channels through which is circulated a gaseous cooling fluid, usually pressurized air, which will be called hereinafter for the sake of simplicity “cooling gas”. The object of this gas cooling is to protect the shaft  20  and the rabble arms  26  against damage due to the elevated temperatures in the hearth chambers  12 . Indeed, in the hearth chambers  12  ambient temperature may be as high as 1000° C. 
     The flow diagram of  FIG. 2  gives a schematic overview of a new and particularly advantageous gas cooling system  40  for the shaft  20  and the rabble arms  26 . The big dashed rectangle  10  schematically represents the MFH  10  with its eight hearth chambers  12   1  . . .  12   8 . A schematic representation of the rotary hollow shaft  20  illustrates the flow paths of the cooling gas within the shaft  20 . Reference numbers  26 ′ 1  . . .  26 ′ 8  identify in each hearth chamber  12   1  . . .  12   8 , a schematic representation of the cooling system of a rabble arm arranged in the respective hearth chamber. The small dashed rectangles  28   1  . . .  28   8  are schematic representations of the rabble arm fixing nodes in the shaft  20 . 
     Reference number  42  in  FIG. 2  identifies a cooling gas supply source, e.g. a fan pressurizing ambient air. As is know in the art, the fan  42  is connected by means of a lower cooling gas supply line  46 ′ to a lower cooling gas inlet  44 ′ of the shaft  20 . This lower cooling gas inlet  44 ′ is arranged outside the furnace  10  below of the lowermost hearth chamber  12   8 . However, in the MHF of  FIG. 2 , the fan  42  is also connected by means of an upper cooling gas supply line  46 ″ to an upper cooling gas inlet  44 ″ of the shaft  20 . This upper cooling gas inlet  44 ″ is arranged outside the furnace  10  above the uppermost hearth chamber  12   1 . It follows that the flow rate from the fan  42  is split between the lower cooling gas inlet  44 ′, to be supplied to lower half of the shaft  20 , and the upper cooling gas inlet  44 ″, to be supplied to upper half of the shaft  20 . It remains to be noted that—as the shaft  20  is a rotary shaft—both cooling gas inlets  44 ′ and  44 ″ must be rotary connections. As such rotary connections are known in the art and as their design is furthermore not relevant for the understanding of the inventions claimed herein, the design of the upper and lower cooling gas inlets  44 ′,  44 ″will not be described with greater detail hereinafter. 
     The shaft  20  includes three concentric cooling gas channels within an outer shell  50 . The outermost channel is an annular main cooling gas supply channel  52  in direct contact with the outer shell  50  of the shaft  20 . This annular main supply channel  52  surrounds an annular main distribution channel  54 , which finally surrounds a central exhaust channel  56 . 
     It will be noted that between hearth chambers  12   4  and  12   5 , i.e. approximately in the middle of the shaft  20 , a partition means, as e.g. a partition flange  58 , partitions the annular main supply channel  52  and the annular main distribution channel  54  in a lower half and an upper half. This partitioning does however not affect the central exhaust channel  56 , which extends from the lowermost hearth chamber  12   8  through all hearth chambers  12   8  to  12   1  to the top of the shaft  20 . If it is necessary hereinafter to make a distinction between the lower and upper half of the annular main supply channel  52 , respectively between the lower and upper half of the annular main distribution channel  52 , the lower half will be identified with the superscript (′) and the upper half with the superscript (″) 
     The lower cooling gas inlet  44 ′ is directly connected to the lower half  52 ′ of the annular main supply channel  52 . The cooling gas supplied to the lower cooling gas inlet  44 ′ consequently enters beneath the lowermost hearth chamber  12   8  into the lower annular main supply channel  52 ′ and is then channeled through the latter up to the partition flange  58  between hearth chambers  12   5  and  12   4 , wherein the flow rate of the cooling gas remains unchanged over the whole length of the lower annular main supply channel  52 ′. This constant flow rate of cooling gas over the whole length of the lower annular main supply channel  52 ′ warrants that the outer shell  50  of the shaft  20  is efficiently cooled in the four lower hearth chambers  12   8  . . .  12   5 . 
     Just below the partition flange  58 , there is a lower cooling gas passage  60 ′ between the lower annular main supply channel  52 ′ and the lower annular main distribution channel  54 ′. Through this lower cooling gas passage  60 ′, the cooling gas enters into the lower annular main distribution channel  54 ′. Via at least one cooling gas supply channel  62   5  . . .  62   8  in its rabble arm fixing node  28   5  . . .  28   8  each rabble arm cooling system  26 ′ 5  . . .  26 ′ 8  in the lower half of the MHF  10  is in direct communication with the lower annular main distribution channel  54 ′. Via at least one cooling gas exhaust channel  64   5  . . .  64   8  in its rabble arm fixing node  28   5  . . .  28   8 , each rabble arm cooling system  26 ′ 5  . . .  26 ′ 8  in the lower half of the MHF  10  is also in direct communication with the central exhaust channel  56 . Consequently, in the rabble arm fixing node  28   5 , a secondary cooling gas flow is branched off from the main cooling gas flow in the lower main distribution channel  54 ′ and rerouted through the rabble arm cooling system  26 ′ 5  to be thereafter directly evacuated into the central exhaust channel  56 . In the rabble arm fixing node  28   6 , another part of the gas flow in the annular main distribution channel  54 ′ passes through the rabble arm cooling system  26 ′ 6  and is thereafter also evacuated into the central exhaust channel  56 . Finally, in the last rabble arm fixing node  28   8 , all the remaining gas flow in the lower main distribution channel  54 ′ passes through the rabble arm cooling system  26 ′ 8  and is thereafter evacuated into the central exhaust channel  56 . 
     The flow system in the upper half of the shaft  20  is very similar to the flow system described above. The upper cooling gas inlet  44 ″ is directly connected to the upper half  52 ″ of the annular main supply channel  52 . The cooling gas supplied to the upper cooling gas inlet  44 ″ consequently enters into the upper annular main supply channel  52 ″ above the uppermost hearth chamber  12   1  and is then channeled through the latter down to the partition flange  58  between hearth chambers  12   4  and  12   5 , wherein the flow rate of the cooling gas remains unchanged over the whole length of the upper annular main supply channel  52 ″. This constant flow rate of cooling gas over the whole length of the upper annular main supply channel  52 ′ warrants that the outer shell  50  of the shaft  20  is efficiently cooled in the four upper hearth chambers  12   1  . . .  12   4 . 
     Just above the partition flange  58 , there is an upper cooling gas passage  60 ″ between the upper main supply channel  52 ″ and the upper annular main distribution channel  54 ″. Through this upper cooling gas passage  60 ″, the cooling gas enters into the upper main distribution channel  54 ″. The connection of each rabble arm cooling system  26 ′ 4  . . .  26 ′ 1  in the upper half of the furnace  10  to the upper main distribution channel  54 ″ and the central exhaust channel  56  is as described above for rabble arm cooling systems  26 ′ 4  . . .  26 ′ 1  in the lower half. Consequently, in the rabble arm fixing node  28   4 , a secondary cooling gas flow is branched off from the main cooling gas flow in the upper main distribution channel  54 ″ and rerouted through the rabble arm cooling system  26 ′ 4  to be thereafter directly evacuated into the central exhaust channel  56 . In the rabble arm fixing node  28   3  another part of the gas flow in the upper main distribution channel  54 ″ passes through the rabble arm cooling system  26 ′ 3  and is thereafter also evacuated into the central exhaust channel  56 . Finally, in the uppermost rabble arm fixing node  28   1  all the remaining gas flow in the upper main distribution channel  54 ″ passes through the rabble arm cooling system  26 ′ 1  and is thereafter evacuated into the central exhaust channel  56 . From the central exhaust channel  56  the exhaust gas stream is then either directly evacuated into the atmosphere or evacuated by means of a rotary connection into a pipe for a controlled evacuation of the gas (not shown). 
       FIG. 3  illustrates a particularly advantageous embodiment of the rotary hollow shaft  20  of the furnace. This  FIG. 3  shows more particularly a longitudinal section through the central part of shaft  20 . This central part includes the aforementioned partition flange  58 , which partitions the annular main supply channel  52  and the annular main distribution channel  54  in a lower half  52 ′,  54 ′ and an upper half  52 ″,  54 ″. 
     The outer shell  50  of the shaft consists mainly of intermediate support tubes  68  interconnected by the rabble arm fixing node  28 . Such a rabble arm fixing node  28  comprises a ring-shaped cast body  70  made of refractory steel. The intermediate support tubes  68  are made of thick walled stainless steel tubes and are dimensioned as structural load carrying members between successive rabble arm fixing nodes  28 . The intermediate support tubes  68  interconnected by massive rabble arm fixing nodes  28  constitute the load bearing structure of the shaft  20 , which supports the rabble arms  26  and allows to absorb important torques when the rabble arms  26  are pushing the material over the hearths  14 . It will further be noted that—in contrast to prior art shafts—the outer shell  50  described herein is advantageously a welded structure, the ends of the intermediate support tubes  68  are welded to the rabble arm fixing nodes  28 , instead of being flanged thereon. 
     As explained above, the section of the shaft extending between adjacent hearth chambers  12   4  and  12   5  (i.e. the central shaft section) is rather particular because it comprises the partitioning flange  58 , as well as the cooling passages  60 ′,  60 ″ between the annular main supply channel  52  and the annular main distribution channel  54 . Before describing this particular central shaft section, a “normal” shaft section will now be described, also with reference to  FIG. 3 . Such a “normal” shaft section extending between two other adjacent hearth chambers, as e.g. hearth chambers  12   3  and  12   4 , comprises the intermediate support tube  68  welded between two arm fixing nodes  28   3  and  28   4  to form the outer shell  50  of the shaft  20 . The intermediate support tube  68  also delimits the annular main supply channel  52  to the outside, which warrants a very good cooling of the intermediate support tube  68 . An intermediate gas guiding jacket  72  is arranged within the intermediate support tube  68  so as to delimit the annular main supply  52  channel to the inside and the annular main distribution channel  54  to the outside. An inner gas guiding jacket  74  is arranged within the intermediate gas guiding jacket  72  so as to delimit the annular main distribution channel  54  to the inside and the central exhaust channel  56  to the outside. The intermediate gas guiding jacket  72  comprises a first tube section  72   1  and a second tube section  72   2 . The first tube section  72   1  is welded with one end to the fixing node  28   4 . The second tube section  72   2  is similarly welded with one end to the fixing node  28   3  (not shown in  FIG. 3 ). The first tube section  72   1  and the second tube section  72   2  have opposite free ends that are arranged opposite one another. A sealing sleeve  76  is fixed to the free end of first tube section  72   1  and sealingly engaging the free end of the second tube section  72   2 , while simultaneously tolerating relative movement of both tube sections  72   1  and  72   2  in the axial direction. It follows that an expansion joint is formed in the intermediate gas guiding jacket  72 . This expansion joint allows to compensate for differences in thermal expansion of the intermediate support tube  68  and the intermediate gas guiding jacket  72 , because the latter remains generally cooler than the intermediate support tube  68 . The inner gas guiding jacket  74  similarly comprises a first tube section  74   1  and a second tube section  74   2 . The first tube section  74   1  is welded with one end to the fixing node  28   4 . The second tube section  74   2  is similarly welded with one end to the fixing node  28   3  (not shown in  FIG. 3 ). The first tube section  74   1  and the second tube section  74   2  have opposite free ends that are arranged in opposite one another. A sealing sleeve  78  is fixed to the free end of first tube section  74   1  and sealingly engaging the free end of the second tube section  74   2 , while tolerating relative movement of both tube sections  74   1  and  74   2  in the axial direction. It follows that an expansion joint is formed in the inner gas guiding jacket  74 . This expansion joint allows to compensate for differences in thermal expansion of the intermediate support tube  68  and the inner gas guiding jacket  74 , which remains generally cooler than the intermediate support tube  68 . It will furthermore be appreciated that the solution with the two sealing sleeves  76 ,  78  renders assembling by welding of the shaft sections much easier. 
     As can be seen in  FIG. 3 , the section of the shaft extending between adjacent hearth chambers  12   4  and  12   5  distinguishes from the “normal” section described in the preceding paragraph by several features. The intermediate support tube  68  consists e.g. of two halves  68   1  and  68   2  that are assembled at the level of the partition flange  58  (in fact, each tube half  68   1  and  68   2  includes a terminal ring flange  58   1  and  58   2  and both ring flanges  58   1  and  58   2  are welded together). The intermediate jacket  72 ′ simply consists of two tube sections  72 ′ 1  and  72 ′ 2 , wherein a first end of each tube section  72 ′ 1  and  72 ′ 2  is welded to one of both arm fixing nodes  28   3  and  28   4 , and the second end is a free end spaced apart from the partitioning flange  58  to define the gas passages  60 ′ and  60 ″ between the lower annular main supply channel  52 ′ and the lower annular main distribution channel  54 ′, respectively the upper annular main supply channel  52 ″ and the upper annular main distribution channel  54 ″. The inner jacket  74 ′ consists of four tube sections  74 ′ 1 ,  74 ′ 2 ,  74 ′ 1 ,  74 ′ 2 , wherein the first tube section  74 ′ 1  is welded with one end to the arm fixing node  28   4 , the second tube section  74 ′ 2  is welded with one end to the flange  58   1 , the third tube section  74 ′ 3  is welded with one end to the flange  58   2  and the fourth tube section  74 ′ 4  is welded with one end to the arm fixing node  28   3 . A first sealing sleeves  80  provides a sealed connection and axial expansion joint between the opposite free ends of the first tube section  74 ′ 1  and the second tube section  74 ′ 2 . A second sealing sleeves  82  provides a sealed connection and axial expansion joint between the opposite free ends of the third tube section  74 ′ 3  and the fourth tube section  74 ′ 4 . The sealing sleeves  80  and  82  just work as the sealing sleeves  76  and  78  and render assembling of the central shaft section much easier. 
     To complete thermal protection of the shaft  20 , the latter is advantageously recovered with a thermal insulation (not shown). Such an insulation of the shaft  20  is advantageously a multilayer insulation including e.g. an inner refractory layer of micro-porous material, a thicker intermediate refractory layer of insulating castable material and an even thicker outer refractory layer of dense castable material. 
     A preferred embodiment of a rabble arm fixing node  28  is now describe with reference to  FIG. 3  and  FIG. 4 . As said already above, the rabble arm fixing node  28  comprises a ring-shaped cast body  70  made of refractory steel. The central passage  90  in this ring shaped body  70  forms the central exhaust channel  56  for the cooling gas within the rabble arm fixing node  28 . First secondary passages  92  are arranged in a first ring section  94  of the ring shaped body  70  around the central passage  90 , so as to provide gas passages for cooling gas flowing through the annular main distributi0on channel  54 . Second secondary passages  96  are arranged in a second ring section  98  of the ring shaped body  70  around the first ring section  94 , so as to provide gas passages for cooling gas flowing through the annular main supply channel  52 . For each rabble arm  26  to be connected to rabble arm fixing node  28 , the ring shaped body  70  includes furthermore a socket  100 , i.e. a cavity extending radially into the ring shaped body  70  between the aforementioned first and second secondary passages  92  and  96 . The rabble arm fixing node  28  includes four sockets  100 , wherein the angle between the central axis of two consecutive sockets  100  is 90°. Oblique bores  102  in the ring shaped body  70  (see  FIG. 5 ), which have an inlet opening  102 ′ in the second ring section  98  of the ring shaped body  70  and an outlet opening  102 ″ in a lateral surface of the socket  100 , form the cooling gas supply channels  62 , which have already been mentioned within the context of the description of  FIG. 3 . A through hole  104  in the ring shaped body  70 , in axial extension of the socket  100 , forms the cooling gas return channel  64 , which has already been mentioned within the context of the description of  FIG. 3 . 
     Considering now more particularly  FIG. 3 ,  FIG. 5  and  FIG. 6 , it will first be noted that the rabble arm  26  includes a plug body  110  that form a coupling end of the rabble arm  26  received in the socket  100  of the rabble arm fixing node  28  (see  FIGS. 3 &amp;5 ). The plug body  110  is cast solid body with several bores therein, which is advantageously made of refractory steel. The socket  100  has therein two concave conical seat surfaces  112 ,  114  separated by a concave cylindrical guiding surface  116 . The plug body  110  has thereon two convex conical counter-seat surfaces  112 ′,  114 ′ separated by a convex cylindrical guiding surface  116 ′. All these conical surface  112 ,  114 ,  112 ′,  114 ′ are ring surfaces of a single cone, i.e. have the same cone angle. This cone angle should normally be greater than 10° and smaller than 30° and is normally within the range of 18° to 22°. When the plug body  110  is axially inserted into the socket  100 , the convex conical counter-seat surface  112 ′ is pressed against the concave conical seat surface  112  and the convex conical counter-seat surfaces  114 ′ is pressed against the concave conical seat surfaces  114 . 
     When securing a new rabble arm  26  to the shaft  20 , the plug body  110  of the rabble arm  26  has to be introduced into the socket  100  of the rabble arm fixing nod  110 . During this introduction movement, the outer concave conical seat surface  114  first guides the plug body  110  into axial alignment with the cylindrical guiding surface  116 . Thereafter both cylindrical guiding surfaces  116  and  116 ′ cooperate with one another for axially guiding the plug body  110  into its final seat position in the socket  100 . It will be appreciated that axial guidance provided by the two cylindrical guiding surfaces  116  and  116 ′ considerably reduces the risk of damaging the plug body  110  or the socket  100  during the final coupling operation. 
     The rabble arm  26  further comprises an arm support tube  120  welded with one end to a shoulder surface  122  on the rear side of the plug body  110 . This arm support tube  120  has to withstand the forces and torques acting on the rabble arm. It advantageously consists of a thick walled stainless steel tube extending over the whole length of the rabble arm  26 . A gas guiding tube  124  is arranged inside the arm support tube  122  and cooperates with the latter to define between them a small annular cooling gap  126  for channeling the cooling gas to the free end of the rabble arm  26 . The interior section of the gas guiding tube  124  forms a central return channel  128  through which the cooling gas flows back from the free end of the rabble arm  26  to the plug body  110 . 
     It will be noted that one end of the gas guiding tube  124  is welded to a cylindrical extension  130  on the rear side of the plug body  110 . The diameter of this cylindrical extension is smaller than the internal diameter of the arm support tube  120 , so that an annular chamber  131  remains between the cylindrical extension  130  and the arm support tube  120  surrounding the cylindrical extension  130 . This annular chamber  131  is in direct communication with the small annular cooling gap  126  between the gas guiding tube  124  and the arm support tube  122 . 
     As already explained above, the plug body  110  is a solid cast body comprising several bores that will now be described. In  FIG. 6 , reference number  132  identifies an central hole extending axially through the plug body  110 , from an end face  134  on the cylindrical extension  130  to a front face  136  on the front end of the plug body  110 . The purpose of this central hole  132  will be described later. Reference number  140  in  FIG. 6  identifies gas return bores arranged in the plug body  110  around the central hole  132  and having inlet openings  140 ′ in the end face  134  and outlet openings  140 ″ in the front face  136  of the plug body  110  (there are four of such gas return bores  140  arranged around the central hole  132 ). These gas return bores  140  form communication channels between the return channel  128  in the rabble arm  26  and a gas outlet chamber  142  remaining in the socket  100  between the front face  136  of the plug body  110  and a bottom surface  144  of the socket  100  when the plug body  110  is seated therein. From this gas outlet chamber  142 , the cooling gas returning from the rabble arm  26  overflows through the through hole  104  into the central passage  90  of the rabble arm fixing node  28 , i.e. into the central exhaust channel  56  of the shaft  20 . Reference number  146  in  FIG. 5  identifies four gas supply bores arranged in the plug body  110 . These gas supply bores  146  have inlet openings  146 ′ in the convex cylindrical guiding surface  116 ′ of the plug body  110  and outlet openings  146 ″ in the cylindrical surface of the cylindrical extension  130 . It will be noted that the inlet openings  146 ′ in the convex cylindrical guiding surface  116 ′ are overlapping with the gas outlet openings  102 ″ of the oblique bores  102  in the ring shaped body  70 . It is recalled in this context that these oblique bores  102  form the cooling gas supply channels  62  for the rabble arm  26  in the rabble arm fixing node  28 . Consequently, when the plug body  110  is seated in its socket  100 , the gas supply bores  146  form communication channels in the plug body  110  between the annular chamber  131 , which is in direct communication with the small annular cooling gap  126  in the rabble arm  26 , and the cooling gas supply for the rabble arm  26  in the rabble arm fixing node  28 . It will be appreciated that a positioning pin  148  in the front end of the plug body  110  co-operates with a positioning bore in the bottom surface  144  of the socket  100  to warrant an angular alignment of the inlet openings  146 ′ in the convex cylindrical guiding surface  116 ′ of the plug body  110  with the gas outlet openings  102 ″ in the concave cylindrical guiding surface  116  in the socket  100  when the plug body  110  is inserted into the socket  100 . For sealing off the gas passages between the rabble arm fixing node  28  and the plug body  110  in the socket  100 , the convex conical counter-seat surfaces  112 ′,  114 ′ of the plug body  110  are advantageously equipped with one or more temperature resistant seal rings (not shown). Furthermore, for improving the sealing function of the convex conical counter-seat surfaces  112 ′,  114 ′ in the socket  100 , the latter are advantageously recovered with a temperature resistant sealing paste. 
     Referring now to  FIG. 6 , novel preferred securing means for securing the plug body  110  in its socket  100  will be described. This novel securing means comprises a clamping bolt  150 . The latter comprises a cylindrical bolt shank  152  loosely fitted in the central hole  132  of the plug body  110 . This bolt shank  152  supports on the front side of the plug body  110  a bolt head  154 , which advantageously has the form of a hammer head defining a shoulder surface  156 ′,  156 ″ on each side of the shank  152 . On the rear side of the plug body  110 , the bolt shank  152  has a threaded bolt end  158 . The preferred securing means shown in  FIG. 6  further comprises a threaded sleeve  160  (or a standard nut) that is screwed onto the threaded bolt end  158  protruding out of the central hole  132  of the plug body  110  on the rear side of the latter. 
       FIG. 6  shows the axial clamping device in a clamping position in which it firmly presses the plug body  110  into the socket  100 . In this clamping position the threaded sleeve  160  bears against an abutment surface on the rear side of the plug body  110 . This abutment surface corresponds e.g. to the end surface  134  of the cylindrical extension  130  of the plug body  110 . On the other side of the plug body  110 , the bolt shank  152  extends through the gas outlet chamber  142  and the through hole  104  in the bottom of the socket  104  into the central passage  90  of the rabble arm fixing node  28 . Here, the hammer head  154  of the bolt  150  is in hooking engagement with an abutment surface  162  in the arm fixing node  28 , wherein its two shoulder surface  156 ′,  156 ″ bear against the abutment surface  162 . It will be appreciated that the clamping bolt  150  is sufficiently preloaded, i.e. the threaded sleeve  160  is tightened with a predetermined torque, to warrant that the plug body  110  is always firmly pressed into the socket  100  during operation of the MHF. 
     When one of the rabble arms  26  is dismounted, the clamping bolt  150  is extracted with rabble arm  26 , i.e. it remains in the plug body  110  of the rabble arm  26 . In order to be able to extract the hammer head  154  through the through hole  104  in the bottom of the socket  100 , this through hole has the form of a key hole having a form corresponding roughly to the cross-section of the hammer head  154 . It follows that by rotating the hammer head  154  by 90° about the central axis of the bolt shank  152 , the hammer head  154  can be brought from the “hooked position” shown in FIG.  6 ”, into an “unhooked position”, in which it can be axially extracted through the keyhole  104  into the socket  100 . Similarly, when a new rabble arm  26  is mounted, the hammer head  154  is first in a position in which it can axially pass through the key hole  104 . Once the plug body  110  is seated in its socket  100 , the hammer head  154 , which is now located on the other side of key hole  104 , can be brought into the “hooked position” shown in  FIG. 6  by rotating the hammer head  154  by 90° about the central axis of the bolt shank  152 . It will further be appreciated that in the “hooked position” of the clamping bolt  150  shown in  FIG. 6 , the hammer head  154  leaves a quite large outlet opening for the cooling gas flowing through the through hole  104  into the central gas passage  90 . 
     The clamping device shown in  FIG. 6  also comprises actuation and positioning means for tightening/losing and positioning it from a safe position outside the MHF. This actuation means will now be described with reference to  FIG. 6  and  FIG. 7 . In  FIG. 6 , reference number  170  identifies an actuation tube that is secured (e.g. welded) with one end to the threaded sleeve  160 . Reference number  172  identifies a positioning tube that is secured with one end to the bolt shank  152  (e.g. by means of a bolt  173  welded to the rear end of the positioning tube  172  as shown in  FIG. 6 ). Referring now to  FIG. 7 , it will be seen that both the actuation tube  170  and the positioning tube  172  axially extend through the intermediate support tube  120  up to the free end of the latter. Here, both the front end of the actuation tube  170  and the front end of the positioning tube  172  include a coupling head  174 ,  176  for coupling thereto an actuation key (not shown). Both coupling heads  174 ,  176  may e.g. include a hexagonal socket as shown in  FIG. 7 . The coupling head  174  of the actuation tube  170  is rotatably supported in a central through-hole  178  of an end-cup  180  and sealed within this through-hole  178 . The end-cup  180  comprises on its rear side a first flange  182  closing the front end of the intermediate support tube  120  and on its front side a second flange  184  closing the front end of an outer metallic protecting jacket  186 , which will be described later. The positioning tube  172  is rotatably supported with the actuation tube  170 . A blind flange  188  is flanged on the front face of the second flange  184  of the end-cup  180 , so as to close the central through-hole  178  in the end-cup  180 . A thermally insulating plug is inserted between the coupling head  174  and the blind flange  188 . Reference number  192  identifies a positioning pin fixed to the blind flange  188 . This positioning pin  192  extends through the insulating plug  190  to bear with one end onto the coupling head  174 , thereby avoiding a loosening of the threaded sleeve  160 . 
     After removing the blind flange  188  and the thermally insulating plug  190 , one has access to the coupling heads  174 ,  176  of the actuation tube  170  and the positioning tube  172 . The actuation tube  170  is used to tighten the threaded sleeve  160 . The positioning tube  172  mainly serves as an indicator of the position the hammer head  154  has with regard to the key-hole  104 . Its coupling head  176  is therefore provided with an adequate positioning mark. It will be noted that the positioning tube  172  may also be used for fixing the clamping bolt  150  while loosening the threaded sleeve  160  by means of the actuation tube  170 . Finally, the coupling head  174  of the actuation tube  170  may also have marks thereon, which in combination with the marks on the coupling head  176  of the positioning tube allow to check whether a sufficient tightening torque has been applied to the clamping device. It remains to be noted that the blind flange  188  may be removed during operation of the cooling system without a substantial gas leakages. Indeed, the threaded sleeve  160  seals the rear end of the actuation tube  170  and the front end of the actuation tube is sealed within the central through-hole  178  in the end-cup  180 . 
     The aforementioned metallic protecting jacket  186 , which is seen on  FIGS. 4 to 7 , recovers a micro porous thermal insulation layer  194  arranged on the intermediate support tube  120 . Anti-rotating means, as e.g. identified with reference number  196  in  FIG. 6 , interconnect the metallic protecting jacket  186  and the intermediate support tube  120  and avoid any rotation of the protecting jacket  186  about the central axis of the rabble arm  26 . It will be appreciated that in a preferred embodiment of the rabble arm  26 , the protecting jacket  186  is made of stainless steel, wherein the rabble teeth  30 , which are also are made of stainless steel, are welded directly onto the protecting jacket  186  (see e.g.  FIG. 7 , showing one of these rabble teeth  70 ).