Abstract:
An insert for a radiant tube of a furnace including a first section adapted to absorb heat from combustion gases passing through the radiant tube and radiantly transfer the heat to a wall of the radiant tube and a second section for directing heat and gases in the radiant tube toward the first section of the insert and a system including a radiant tube and one or more such inserts. Also, a method of improving heat transfer from a radiant tube of a furnace to the material being heated including supplying an insert as described above and placing the insert into the radiant tube such that the first section corresponds to a portion of the radiant tube that is closest to the material being heated. Also, an insert for a radiant tube of a furnace including a ceramic body and a metal deposited on the surface of the ceramic body.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application No. 61/879,902 filed Sep. 19, 2013 entitled “Radiant Heat Insert” and U.S. Provisional Patent Application No. 61/879,912 filed Sep. 19, 2013 entitled “Radiant Heat Insert”, the entire disclosures of which are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an insert for placement in the tubes of a radiant tube furnace to increase heat transfer to the material being heated and to improve the fuel efficiency of the furnace. More particularly, it relates to a radiant tube insert that transfers more heat to the portion of the radiant tube that is closest to the material being heated. 
         [0004]    2. Description of the Related Art 
         [0005]    Radiant tube combustion furnaces are commonly used to heat materials such as ferrous and non-ferrous metals including steel and aluminum. Such radiant tube furnaces may be continuous furnaces where the material being heated is continuously passed through the furnace or may be batch furnaces where a large load of material is placed in the furnace. As an example a radiant tube continuous combustion furnace  10 , as illustrated in  FIGS. 1 and 2 , is a generally rectangular box-shaped structure having a roof  11 , a floor  13 , and two side walls  15 . Two sets of radiant tubes  12  are located in the furnace, an upper set that is closer to the roof and a lower set that is closer to the floor. The material  14  being heated is passed between the upper set of radiant tubes  12  and the lower set of radiant tubes  12 . Each radiant tube  12  has a burner section  16  attached to a combustion burner  17  and an exhaust section  18  through which the combustion gases, commonly referred to as flue gas, exit the radiant tube  12 . While  FIGS. 1 and 2  show a U-shaped radiant tube, the radiant tube may take other suitable shapes including straight and W-shaped. Heat generated by the combustion burner  17  is transferred through the walls of the radiant tube  12  to the material  14  being heated. In the burner section  16 , thermal energy from the inside of the radiant tube  12  can be transmitted from the radiant tube  12  through convection from the high temperature combusted gas passing through the tube  12  and, near the burner end, by radiation from the bright combustion flame. In the exhaust section  18 , thermal energy can only be transmitted through convection from the remaining combustion gas passing through the tube  12 . Further, the available heat in the exhaust section  18  is lower because the combustion gas loses energy while traveling down the radiant tube  12 , as indicated by directional arrows  20 , causing the combustion gas at the exhaust end of the tube  12  to be at a significantly lower temperature than at the burner end. 
         [0006]    Because of this configuration, more thermal energy is transmitted from the burner section  16  relative to the exhaust section  18 . This creates uneven heat transfer to the material  14  that is being heated, which, in this case, is travelling in a direction perpendicular to the radiant tubes  12  as indicated by arrows  22  and, in the case of a batch furnace, is stationary. A large amount of thermal energy is also wasted in the exhaust section  18 , as most of the thermal energy exits the furnace  10  without any means to direct it to the material  14  being heated. 
         [0007]    Inserts that can be arranged inside of the exhaust section  18  have been previously created in order to increase the overall heat transfer to the material  14  being heated, as well as, to more evenly distribute the amount of energy given off by the burner section  16  and the exhaust section  18 . This is accomplished by mixing and forcing more exhaust gas to the interior surface  24  of the radiant tube  12 , as well as by transmitting radiant energy that the insert collects. These designs have been proven to increase furnace efficiency by 5-20%, which reduces costs of continuous furnace operation. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to an insert for a radiant tube of a furnace including a first section adapted to absorb heat from the combustion gases passing through the radiant tube and radiantly transfer the heat to a wall of the radiant tube and a second section for directing heat and gases in the radiant tube toward the first section of the insert. The shape of at least a portion of the first section may approximate the shape of the radiant tube and may include a tubular member having a first end, a second end, and a sidewall extending between the first end and the second end and defining at least one central passageway. The second section may include at least one wing extending from an exterior surface of the tubular member. At least a portion of the sidewall of the tubular member may be flat and at least a portion of the tubular member may be curved. The cross-section of the sidewall of the tubular member may be a semi-circle or a sector of a circle. The insert may further include at least one projection extending from an exterior surface of the curved portion of the sidewall of the tubular member. 
         [0009]    The at least one wing may have a first end corresponding to the first end of the tubular member and a second end corresponding to the second end of the tubular member, and an exterior surface of the wing may slope in a downward direction from the first end of the wing to the second end of the wing. Further, a laterally outer edge of the second end of the wing may be closer to the tubular member than a laterally outer edge of the first end of the wing. The shape of the at least one wing may be adapted to direct heat and gases in an outward and downward direction toward an external surface of the tubular member. 
         [0010]    The maximum width of the second section of the insert may be equal to or smaller than the maximum width of the first section, and the maximum length of the second section may be equal to or shorter than the maximum length of the first section. 
         [0011]    The insert may further include a connection channel. 
         [0012]    The insert may be constructed from a ceramic. The ceramic may be silicon carbide and may also include a metal deposited on its surface. 
         [0013]    The invention is also directed to a method of improving heat transfer from a radiant tube of a furnace to the material being heated in the furnace including supplying an insert having a first section for radiantly transferring heat to a wall of the radiant tube and a second section for directing heat and gases in the radiant tube toward the first section of the insert and placing the insert into the radiant tube such that the first section corresponds to a portion of the radiant tube that is closest to the material being heated. A gap may be provided between an outer surface of the first section of the insert and an inner surface of the radiant tube and the second section of the insert may direct heat and gases into the gap. The first section of the insert and the second section of the insert may have the features described above. 
         [0014]    The invention is also directed to an insert for a radiant tube of a furnace including a ceramic body and a metal deposited on the surface of the ceramic body. The metal may be at least one of palladium and platinum. 
         [0015]    The invention is also directed to a system for radiantly conducting heat to a material, the system including a radiant tube having a fluid passageway and one or more inserts provided in the fluid passageway, wherein the inserts comprise a first section adapted to absorb heat from the combustion gases passing through the radiant tube and radiantly transfer the heat to a wall of the radiant tube and a second section for directing heat and gases in the radiant tube toward the first section of the insert. The first section of the insert and the second section of the insert may have the features described above. The system may include a plurality of inserts that are connected to one another within the fluid passageway. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a top plan view of a radiant tube combustion furnace with the roof removed; 
           [0017]      FIG. 2  is a side elevation view of the radiant tube combustion furnace of  FIG. 1  with a portion of the side wall removed; 
           [0018]      FIG. 3  is a perspective front view of one radiant tube insert according to the present invention; 
           [0019]      FIG. 4  is a side elevation view of the radiant tube insert of  FIG. 3  installed in the radiant tube combustion furnace of  FIGS. 1 and 2 , with a portion of the side wall removed; 
           [0020]      FIG. 5  is a top plan view, partially in section, of the radiant tube insert of  FIG. 3  installed in the radiant tube combustion furnace of  FIGS. 1 and 2 , with the roof of the combustion furnace and the upper portion of the top set of radiant tubes removed; 
           [0021]      FIG. 6  is a perspective front view of another radiant tube insert according to the present invention; 
           [0022]      FIG. 7  is a perspective top view of the radiant tube insert of  FIG. 6 ; 
           [0023]      FIG. 8  is a perspective side view of the radiant tube insert of  FIG. 6 ; 
           [0024]      FIG. 9  is a side elevation view of the radiant tube insert of  FIG. 6  installed in the radiant tube combustion furnace of  FIGS. 1 and 2  with a portion of the side wall removed; and 
           [0025]      FIG. 10  is a top plan view, partially in section, of the radiant tube insert of  FIG. 6  installed in the radiant tube combustion furnace of  FIGS. 1 and 2 , with the roof of the combustion furnace and the upper portion of the top set of radiant tubes removed. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0026]    For purposes of the description hereinafter, the words “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”, “longitudinal”, “proximal”, “distal” and like spatial terms, if used, shall relate to the described embodiments as oriented in the drawing figures. However, it is to be understood that many alternative variations and embodiments may be assumed except where expressly specified to the contrary. It is also to be understood that the specific devices and embodiments illustrated in the accompanying drawings and described herein are simply exemplary embodiments of the invention. 
         [0027]    The present invention is directed to an insert for placement in the radiant tube of a furnace to increase heat transfer from the radiant tube to the material being heated. As shown in FIGS.  3  and  6 - 8 , the insert  26 ,  126  includes a first section  28 ,  128  and a second section  30 ,  130 , respectively. 
         [0028]    The respective first sections  28 ,  128  include a tubular member  32 ,  132  having a first end  34 ,  134 , a second end  36 ,  136 , and a sidewall  38 ,  138  extending between the first end  34 ,  134  and the second end  36 ,  136 . The sidewall  38 ,  138  defines at least one central passageway  40 ,  140 . The sidewall  38 ,  138  may include at least one flat portion  42 ,  142  and at least one curved portion  44 ,  144 . 
         [0029]    A connection channel  46 ,  146  may extend from the first end  34 ,  134  to the second end  36 ,  136  of the tubular member  32 ,  132  and be incorporated in or between the flat portions  42 ,  142  of the tubular member  32 ,  132 . As shown in  FIG. 3 , the flat portions  42  of the tubular member  32  may extend from either side of the connection channel  46  in a substantially horizontal direction such that the cross-section of the sidewall  38  of the tubular member  32  is a semicircle. Alternatively, as shown in  FIGS. 6-8 , the flat portions  142  of the tubular member  132  may extend from either side of the connection channel  146  in a downward sloping manner such that the angle α between the flat portions  142  is less than 180° and the cross-section of the sidewall  138  of the tubular member  132  is a sector of a circle. 
         [0030]    As shown in  FIGS. 4 and 9 , the curved portion  44 ,  144  of the sidewall  38 ,  138  may approximate the shape of the interior surface  24  of the radiant tube  12 . 
         [0031]    As shown in  FIGS. 5 and 10 , the connection channel  46 ,  146  is adapted to receive a rod  52  within its central passageway  50 ,  150 . The insertion of the rod  52  through the central passageway  50 ,  150  acts to connect a series of inserts  26 ,  126  together and may act to limit rotation of the insert  26 ,  126  when it is placed in the radiant tube  12 . The central passageway  50  of the connection channel  46  may have a cross-section that is D-shaped to accept a rod  52  having a similarly D-shaped cross section ( FIG. 3 ), or may have a circular cross section that accepts a rod  52  having a circular cross section ( FIGS. 6-8 ). As shown in  FIGS. 6-8 , a lip  154  may extend from each end of the connection channel  146 . On one end (first end  156 ), the lip  154  extends from the top of the connection channel  146  and has a semi-circular shape, and on the other end (second end  158 ), the lip  154  extends from the bottom of the connection channel  146  and also has a semi-circular shape. When the inserts  126  are placed in the radiant tube  12 , the lip  154  on the first end  156  of one insert  126  engages the lip  154  on the second end  158  of the adjoining insert  126  to form a continuous passageway and to limit rotation of the adjoining inserts  126  relative to one another. Other suitable engagements may be employed to limit rotation of adjoining inserts  26 ,  126  relative to one another. 
         [0032]    At least one support beam  160  may extend from the bottom surface of the connection channel  146  ( FIGS. 6-8 ) or the interior surface of the flat portions  142  through the central passageway  140  of the first section  128  to the interior surface of the curved portion  144  of the second section  130 . 
         [0033]    At least one projection  62 ,  162  may extend from the exterior surface  64 ,  164  of the curved portion  44 ,  144  of the tubular member  32 ,  132 . When inserted into the radiant tube  12 , the projection  62 ,  162  acts to provide a gap  66  between the exterior surface  64 ,  164  of the sidewall  38 ,  138  of the tubular member  32 ,  132  and the interior surface  24  of the radiant tube  12 . The projections  62 ,  162  may take any size, shape, orientation, and number as long as they act to provide a gap between the exterior surface  64 ,  164  of the sidewall  38 ,  138  of the tubular member  32 ,  132  and the interior surface  24  of the radiant tube  12 . There may be two projections  62  having rectangular cross-sections and extending from the first end  34  of the tubular member  32  to the second end  36  of the tubular member  32  as shown in  FIG. 3 , or the projections  162  may only extend for a portion of the distance between the first end  134  of the tubular member  132  to the second end  136  of the tubular member  132  as shown in  FIGS. 6-8 . 
         [0034]    The shape of the first section  28 ,  128  is adapted to absorb the heat from the combustion gases passing through the radiant tube  12  and the central passageway  40 ,  140  of the tubular member  32 ,  132  and transfer this heat to the portion of the radiant tube  12  that is closest to the material  14  being heated. To accomplish this, as shown in  FIGS. 4 and 9 , the insert  26 ,  126  is placed in the radiant tube  12  such that the curved portion  44 ,  144  of the tubular member  32 ,  132  corresponds to the portion of the radiant tube  12  that is closest to the material  14  being heated. For example, in a furnace such as the one shown in  FIGS. 1 and 2 , where the material  14  being heated passes between two radiant tubes  12  with the direction  20  of the flow of gases through the radiant tubes  12  being perpendicular to the direction  22  of travel of the material  14  being heated, the curved portion  44 ,  144  of the tubular member  32 ,  132  is positioned to correspond to the lower half of the circumference of the upper radiant tube  12  and the upper half of the circumference of the lower radiant tube  12  ( FIGS. 4 and 9 ). 
         [0035]    By adapting the shape of the curved portion  44 ,  144  of the tubular member  32 ,  132  to closely approximate the shape of the radiant tube  12 , a radiant view factor ratio (which is determined by the angle at which the thermal radiation contacts the lower temperature surface, i.e., the radiant tube  12 ) of nearly 1:1 is provided in the portion of the radiant tube  12  that is closest to the material  14  being heated. The flat portion  42 ,  142  of the tubular member  32 ,  132  faces the portion of the radiant tube  12  that is farthest from the material  14  being heated and provides a poor view factor to this portion of the radiant tube  12 . In this way, the first section  28 ,  128  maximizes the amount of surface area of the insert  26 ,  126  that transmits its collected energy to the portion of the radiant tube  12  that is closest to the material  14  being heated and minimizes the heat transferred to the portion of the radiant tube  12  that is farthest from the material  14  being heated. Further, if the flat portions  142  of the tubular member  132  are sloped in a downward direction such that the angle α between the flat portions  142  is less than 180°, as shown in  FIGS. 6-8 , the hot gases on the outside of the tubular member  132  are also directed toward the portion of the radiant tube  12  that is closest to the material  14  being heated. 
         [0036]    The second section  30 ,  130  may include at least one wing  68 ,  168  extending from an exterior surface of the flat portion  42 ,  142  of the tubular member  32 ,  132  or from the top exterior surface of the connection channel  46 ,  146 . While the embodiments specifically described and shown herein have a pair of wings, it is to be recognized that the insert may have only a single wing or more than two wings. 
         [0037]    As shown in  FIG. 3 , two wings  68  may extend from a first surface  69  of the connection channel  46 . The wings  68  may share a vertical base portion  70  having a first end  72  and a second end  74 . The first end  72  of the vertical base portion  70  of the wings  68  may be connected to the exterior surface of the flat portion  42  of the tubular member  32  or to the top exterior surface of the connection channel  46 . Each wing  68  extends laterally from the second end  74  of the vertical base portion  70 . The first exterior surface  76  of each wing  68  may be convex, and the second interior surface  78  of each wing  68  may be concave. Each wing  68  slopes in a downward direction from the first end  80  of the wing  68  to the second end  82  of the wing  68  such that the distance between the second end  82  of the wing  68  and the flat portion  42  of the tubular member  32  is smaller than the distance between the first end  80  of the wing  68  and the flat portion  42  of the tubular member  32 . 
         [0038]    As gases flowing through the radiant tube  12  contact the first end of the insert  26  and flow towards the second end of the insert  26 , the curved shape and angled surface of the wings  68  cause swirling and/or turbulence of the gas, as shown by the arrows  22  in  FIG. 4 , which is believed to prevent laminar fluid flow through the radiant tube  12  and direct gases and energy toward the shell of the insert. This mixing of the gas inside of the radiant tube  12  eliminates the hot core of gasses that form when the gas flow is uninterrupted, as in exhaust sections of radiant tubes without inserts installed. 
         [0039]    The angle and shape of the wings  68  are also configured to direct the gas to be in contact with the interior surface  24  of the radiant tube and the first section  28  of the insert  26  by directing the gas toward the exterior surface of the curved portion  44  of the insert  26  and into the gap  66  between the first section  28  and the portion of the radiant tube  12  that is closest to the material  14  being heated as shown by the arrows in  FIG. 4 . 
         [0040]    Alternatively, the wings  168  may be attached directly to the top exterior surface of the flat portions  142  of the tubular member  132  or the top exterior surface of the connection channel  146 , as shown in  FIGS. 6-8 . Each wing  168  may have a generally rectangular shape that may be twisted such that the laterally outer edge  184  of the second end  182  of the wing  168  is closer to the flat portion  142  of the tubular member  132  than the laterally outer edge  184  of the first end  180  of the wing  168 . The first end  180  of each wing  168  is attached near the top portion of the connection channel  146  and the second end  182  of each wing  168  is attached on a side portion of the connection channel  146  such that the laterally inner edge  186  of the first end  180  of the wing  168  is farther from the flat portion  142  of the tubular member  132  than the laterally inner edge  186  of the second end  182  of the wing  168 . The angle β between the second ends  182  of the wings  168  is larger than the angle γ between the first ends  180  of the wings  168 . Each wing  168  slopes in a downward direction from the first end  180  of the wing  168  to the second end  182  of the wing  168 . 
         [0041]    Like the previously described wings  68  of  FIG. 3 , the wings  168  shown in  FIGS. 6-10 , are shaped to cause swirling and/or turbulence of the gas, as shown by the arrows in  FIG. 9 , which is believed to prevent laminar fluid flow through the radiant tube  12 . The angle and shape of the wings  168  are also configured to force more gas to be in contact with the interior surface  24  of the radiant tube and the first section  128  of the insert  126  by directing the gas toward the exterior surface of the curved portion  144  of the insert  126  and into the gap  66  between the first section  128  and the portion of the radiant tube  12  that is closest to the material  14  being heated. 
         [0042]    As shown in  FIG. 3 , the maximum width of the second section  30  may be slightly smaller than the maximum width of the first section  28 . Alternatively, the second section  130  may have the same maximum width as the maximum width of the first section  128  (as shown in  FIGS. 6-8 ) or a slightly larger maximum width than the maximum width of the first section  28 ,  128 . 
         [0043]    The length of the second section  30 ,  130  may be equal to or less than the length of the first section  28 ,  128 . If the length of the second section  30 ,  130  is less than the length of the first section  28 ,  128 , the second section  30 ,  130  may be attached to the first section  28 ,  128  at any position between the first end and the second end of the insert. For example, as shown in  FIG. 3 , the second section may be attached nearer the first end  34  of the tubular member  32  or, as shown in  FIGS. 6-8 , the second section  130  may be attached approximately mid-way between the first end  134  and the second end  136  of the tubular member  132 . 
         [0044]    The insert  26 ,  126  may be constructed from any suitable ceramic having good heat transfer, for example, silicon carbide or siliconized silicon carbide. The insert  26 ,  126  may also include a metal, such as palladium and/or platinum, deposited on the surface of the insert  26 ,  126 , which reacts with and/or catalyzes exhaust gases such as NO x  to reduce harmful emissions. 
         [0045]    Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.