Abstract:
Rotary kiln heat exchangers having precast hub and leg assemblies are disclosed. The hub and leg assemblies include interlocking features which secure the heat exchanger components together. A method of installing such heat exchangers in rotary kilns is also disclosed. Installation is relatively fast and simple, and the heat exchangers are capable of withstanding the harsh operating conditions of rotary kilns for extended periods of time.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
     More than one reissue application has been filed for the reissue of U.S. Pat. No. 6,688,884 B2. The reissue applications are application Ser. No. 11/069,643 (the present application) and application Ser. No. 12/152,878, which is a divisional reissue of the present application (application Ser. No. 11/069,643).  
     FIELD OF THE INVENTION 
     This invention relates to rotary kilns, and more particularly relates to heat exchangers installed in rotary kilns. 
     BACKGROUND INFORMATION 
     Rotary kilns are long, slightly inclined cylinders used for processing materials such as lime, limestone, dolomite, magnesite, petroleum coke and cement. The material to be treated is introduced at the higher end and heated air flowing counter-current to the material is introduced at the lower end. Rotary kilns generally operate on a twenty-four hour basis for several months between scheduled down periods. 
     Rotary kilns typically have a refractory brick interior and a steel shell exterior, and some have at least one heat exchanger. The heat exchanger divides the cross section of the kiln into three or more segments to enhance the heat transfer from the gas to the material and improve mixing of the material. A three-segment heat exchanger comprises three spokes or legs which extend from the axial center of the kiln to locations equally spaced around the interior circumference of the steel shell. Commercially available three-segment heat exchangers have been sold under the trademark Trefoil®. 
     Rotary kiln heat exchangers encounter harsh operating conditions. For example, internal gas temperatures may typically be 1,000 to 3,000° F. in a highly basic atmosphere in a rotary lime kiln, although temperatures outside of this range are possible depending on the particular application. The heat exchanger must take the structural loading and erosion, e.g., from several hundred tons per day of partially calcined rock that slides across or falls against the surfaces of the heat exchanger. Furthermore, the heat exchanger rotates continuously with the kiln, which subjects the components of the heat exchanger to varying compressive and tensile forces. The heat exchanger must also withstand the kiln shell deflection upon revolution over its roller supports. 
     Conventional rotary kiln heat exchangers are typically from 8 to 16 feet long along the longitudinal kiln axis, depending on the kiln diameter and other parameters, and have spokes or legs typically from 9 to 13.5 inches thick. The heat exchangers are usually formed from individual refractory bricks, although some have been formed in-situ from refractory materials which are cast and cured inside the kiln. Installation of conventional brick heat exchangers is labor-intensive and requires specially skilled artisans. The bricks also require complicated forms specific to a single rotary kiln size to support them during construction. Thus, brick heat exchangers are slow to install and are expensive. In-situ cast refractory heat exchangers also suffer from disadvantages such as premature wear, complicated forms and slower installation than brick. 
     Some examples of rotary kiln heat exchanger designs are disclosed in U.S. Pat. No. 3,030,091 to Wicken et al., U.S. Pat. No. 3,036,822 to Andersen, U.S. Pat. No. 3,169,016 to Wicken et al., U.S. Pat. No. 3,175,815 to Wicken et al., U.S. Pat. No. 4,846,677 to Crivelli et al, U.S. Pat. No. 5,330,351 to Ransom et al. and U.S. Pat. No. 6,257,878 to Marr et al. 
     Despite these prior designs, a need still exists for a rotary kiln heat exchanger that is relatively fast and simple to install, and can withstand the harsh operating conditions of rotary kilns for extended periods of time. The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to provide a precast monolithic rotary kiln heat exchanger hub comprising at least one recessed surface configured for engagement with a heat exchanger leg. 
     Another aspect of the present invention is to provide a rotary kiln heat exchanger hub comprising at least one portion configured for interlocking engagement with a heat exchanger leg, and at least one portion configured for slidable engagement with another heat exchanger leg. 
     A further aspect of the present invention is to provide a rotary kiln heat exchanger assembly comprising a heat exchanger hub including recesses, and heat exchanger legs received in the heat exchanger hub recesses. 
     Another aspect of the present invention is to provide a rotary kiln heat exchanger assembly comprising a heat exchanger hub, at least one precast heat exchanger leg interlocked with the trefoil hub, and at feast one precast heat exchanger leg slidably mounted in the trefoil hub. 
     A further aspect of the present invention is to provide a precast rotary kiln heat exchanger leg comprising an end configured for engagement with a heat exchanger hub. 
     Another aspect of the present invention is to provide a precast rotary kiln heat exchanger leg comprising a recess and/or protrusion extending along a side surface of the leg for engagement with a protrusion and/or recess of an adjacent heat exchanger leg. 
     A further aspect of the present invention is to provide a precast rotary kiln heat exchanger leg comprising an end including at least one recess or protrusion for engagement with an interior wall of a rotary kiln. 
     Another aspect of the present invention is to provide a precast rotary kiln heat exchanger leg comprising an end including means for adjusting the radial location of the heat exchanger in a rotary kiln. 
     A further aspect of the present invention is to provide a precast rotary kiln heat exchanger leg comprising a flared end for installation adjacent to an interior wall of a rotary kiln. 
     Another aspect of the present invention is to provide a rotary kiln comprising a refractory lining in the kiln, and a heat exchanger assembly in the kiln including precast heat exchanger legs and a central heat exchanger hub. 
     A further aspect of the present invention is to provide a rotary kiln comprising a refractory lining in the kiln, and a heat exchanger assembly in the kiln. The heat exchanger assembly includes a heat exchanger hub comprising recesses, and heat exchanger legs received in the heat exchanger hub recesses. 
     Another aspect of the present invention is to provide a method of installing a heat exchanger in a rotary kiln. The method comprises the steps of providing precast heat exchanger legs, providing a precast heat exchanger hub, and assembling the precast heat exchanger legs and precast heat exchanger hub in the rotary kiln. 
     A further aspect of the present invention is to provide a method of installing a heat exchanger in a rotary kiln. The method comprises positioning first and second heat exchanger legs in the kiln at initial positions, installing a hub between the first and second legs by moving the first and second legs from their initial positions to installed positions in which the first and second legs are engaged with the hub, and installing a third heat exchanger leg by engaging the third heat exchanger leg with the hub. 
     These and other aspects of the present invention will be more apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view shown with parts broken away of a conventional rotary kiln having a three-chamber brick heat exchanger. 
         FIG. 2  is an enlarged cross sectional view thereof illustrating a heat exchanger installed in the kiln in accordance with an embodiment of the present invention. 
         FIG. 3  is a detached elevational view of a heat exchanger hub in accordance with an embodiment of the present invention. 
         FIG. 4  is a right side view of the heat exchanger hub of  FIG. 3 . 
         FIG. 5  is a left side view of the heat exchanger hub of  FIG. 3 . 
         FIG. 6  is a bottom view of the heat exchanger hub of  FIG. 3 . 
         FIG. 7  is an elevational view of a heat exchanger leg in accordance with an embodiment of the present invention. 
         FIG. 8  is a right end view of the heat exchanger leg of  FIG. 7 . 
         FIG. 9  is a cross sectional view taken on section  9 - 9  of the heat exchanger leg shown in  FIG. 7 . 
         FIG. 10  is a top view of another heat exchanger leg in accordance with an embodiment of the present invention. 
         FIG. 11  is a right end view of the heat exchanger leg of  FIG. 10 . 
         FIG. 12  is an exploded isometric view illustrating the assembly of a heat exchanger hub and heat exchanger legs in accordance with an embodiment of the present invention. 
         FIG. 13  is a partial sectional view taken through section  13 - 13  of  FIG. 2 , illustrating raised courses of bricks between a heat exchanger leg and a refractory brick lining of a rotary kiln. 
         FIGS. 14-17  illustrate sequential steps for installing a heat exchanger in a rotary kiln shell in accordance with an embodiment of the present invention. 
       FIG. 17A is a view of a heat exchanger leg showing shims beneath the outer end thereof.  
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings wherein the showings are for the purpose of illustrating the preferred embodiment of the invention only, and not for the purpose of limiting same,  FIG. 1  shows a rotary kiln  10  including a heat exchanger  30  according to the present invention. The rotary kiln  10  has a steel shell  32  which is shown broken away so that the heat exchanger  30  is fully shown. The rotary kiln  10  may be, for example, 100 to 650 feet in length and 3 to 25 feet in diameter. The heat exchanger  30  occupies a longitudinal section in the middle of the kiln  10 . The heat exchanger  30  may be, e.g., from 8 to 30 feet in length. Although not shown in  FIG. 1 , the rotary kiln  10  may contain more than one heat exchanger sections  30 . 
     The rotary kiln  10  is mounted for rotation on trunions  16  with the influent end  18  elevated so that a charge of material to be processed can flow by gravity downstream within the kiln as it rotates. The rotary kiln  10  at the effluent end  20  discharges the dried and/or calcined material. Heated air and gaseous products of combustion, indicated by arrows  22 , are introduced at the effluent end  20  and flow in a countercurrent direction to the material being processed. Because the heat exchanger structure is subjected to extremely high torsional forces from the flowing materials charged, various means of construction are used to minimize the effect thereof. A retainer ring  24  may be constructed downstream from the heat exchanger  30 . The retainer ring  24  is secured adjacent to a brick lining  34 . A shaped refractory brick lining  34  is installed in the kiln  10  between legs  50 ,  50 a and  64  of heat exchanger  30 . 
     Referring now to  FIG. 2 , a rotary kiln heat exchanger  30  in accordance with a preferred embodiment of the present invention is shown in cross-section. The heat exchanger  30  is installed in rotary kiln shell  32 . Refractory brick lining  34  is installed inside the shell  32 . The heat exchanger  30  includes a heat exchanger hub  40  engaged with a first heat exchanger leg  50 , a second heat exchanger leg  50 a, and a third heat exchanger leg  64 . In the embodiment shown in  FIG. 2 , the inner ends of the legs  50 ,  50 a, and  64  fit with an interlocking arrangement within recesses in the hub  40 , as more fully described below. 
       FIGS. 3-6  illustrate details of the hub  40 . As shown most clearly in  FIG. 3 , the hub  40  includes recessed portions  41 . In this embodiment, the hub  40  includes three recesses  41  for receiving three heat exchanger legs. Alternatively, the hub  40  could include a different number of recesses depending upon the number of heat exchanger legs that are used. 
     As shown in  FIGS. 3-6 , the recessed portions  41  of the hub  40  include several protrusions  42  and recesses  43  which provide for interlocking engagement with the legs  50  and  50 a, as more fully described below. As shown most clearly in  FIGS. 3 and 5 , one of the hub recesses  41  includes pin slots  46  which are arranged for alignment with corresponding pin slots in the leg  64 , as more fully described below. 
     In the embodiment shown in  FIGS. 3-6 , the hub  40  may be formed of any precast monolithic refractory material having an alumina content of at least 70% by composition, and more preferably, a refractory material having an alumina content of about 80% to about 95% by composition. In one embodiment, hub  40  is formed of a dense, low cement/high alumina (80-85%) castable. The refractory material may optionally be reinforced with materials such as metal fibers, e.g., stainless steel, such as by way of example and not limitation,  430 ss,  310 ss and/or  304 ss. The length of the hub  40  may range from about 10 to about 24 inches, preferably from about 12 to about 18 inches. The thickness of the arms of the hub  40  may range from about 6 to about 15 inches, preferably from about 8 to about 13.5 inches. 
       FIGS. 7-9  illustrate details of the leg  50 . As shown in  FIG. 7 , the leg  50  includes a relatively narrow mid-section  51 , a flared inner end  52 , and a flared outer end  53 . The flared inner end  52  preferably has a thickness at least 20 percent greater than the thickness of the mid-section  51 , more preferably from about 25 to about 40 percent greater. The flared outer end  53  preferably has a thickness at least 40 percent greater than the thickness of the mid-section  51 , more preferably from about 55 to about 65 percent greater. The mid-section  51  may have a thickness of from about 6 to about 15 inches, preferably from about 8 to about 13.5 inches. The overall length “L” of the leg  50  may range from about 3 to about 8 feet, preferably from about 4 to about 6.5 feet. The depth “D” of the leg  50  may range from about 8 to about 18 inches, preferably from about 8 to about 12 inches. 
     As shown most clearly in  FIGS. 7 and 9 , the leg  50  includes a protrusion  54  which runs along a portion of the length of the leg  50 . A recess  55  is provided on the opposite side of the leg  50 . When multiple legs  50  are stacked together along the axial length of the rotary kiln, the protrusion  54  of one leg fits within the corresponding recess  55  of the adjacent leg. In this manner, the adjacent leg sections may be interlocked. 
     As shown most clearly in  FIGS. 7 and 8 , the inner end  52  of the leg  50  includes an extended tip  56 . The extended tip  56  fits within one of the recesses  41  of the hub  40 . Protrusions  57  and recesses  58  are provided at the inner end  52  of the leg  50 . In accordance with a preferred embodiment of the present invention, the protrusions  57  and recesses  58  provide for interlocking engagement with the corresponding recesses  43  and protrusions  42  of the hub  40 . 
     As shown in  FIG. 7 , a channel  60  is provided at the outer end  53  of the leg  50 . The channel may have any suitable dimensions, e.g., a depth of from about 1.5 to about 3 inches, and a width of from about 3 to about 7 inches. The outer end  53  of the leg includes a slot  61  that is dimensioned to receive a metal member, such as a bar, plate or channel (not shown) on the inner surface of the kiln shell to adjust and lock in place the radial position of the leg  50  within the rotary kiln. As shown in  FIG. 2 , the channel  60  is shaped to receive a bar  70  running longitudinally along the inner surface of the rotary kiln shell. The channel  60  and bar  70  arrangement helps secure the leg  50  in the desired location with respect to the shell  32 . The bar  70  may be made of steel or the like, and is attached to the shell  32  by any suitable means such as welding, mechanical fasteners, etc. Although a single bar  70  is shown in the embodiment of  FIG. 2 , multiple bars may alternatively be used. In addition to, or in place of, the bars  70  which run longitudinally along the inner surface of the shell  32 , other bar configurations may be used, such as bars forming rings around the inner circumference of the shell  32 . Basically, any means that adequately secures the leg  50  in the desired position against the interior of the shell  32  may be used. 
       FIGS. 10 and 11  illustrate details of the leg  64 . Many of the features of the leg  64  are the same as the features of the leg  50 . However, the inner end  52  of the leg  64  includes pin slots  66  instead of the protrusions and recesses  57  and  58  of the leg  50 . The pin slots  66  are arranged such that they line up with corresponding pin slots  46  of the hub  40 . As more fully described below, such a pin slot arrangement facilitates installation and securement of the leg  64  in relation to the hub  40 . 
     The legs  50 ,  50 a and  64  are preferably formed of a monolithic refractory material having an alumina content of at least 70% by composition, and more preferably, having an alumina content of about 80% to about 95% by composition. In one embodiment, legs  50 ,  50 a and  64  are formed of a dense, low cement/high alumina (80-85%) castable. The refractory material may be reinforced with metal fibers, e.g., stainless steel, such as by way of example and not limitation,  430 ss,  310 ss and/or  304 ss fibers. 
       FIG. 12  is an exploded isometric view illustrating the arrangement of the hub  40  and the legs  50 ,  50 a ,  64  and  64 a. The leg protrusions  57  fit within the hub recesses  43 . Similarly, the hub protrusions  42  fit within the leg recesses  58 . In this manner, the legs  50  and  50 a interlockingly engage with their respective hub recesses  41 . 
     As shown in  FIG. 12 , the extended tip  56  of the leg  64  fits within its corresponding recess  41  of the hub  40 . In the installed position, the pin slots  66 ,  66 a of the leg  64  are aligned with the pin slots  46  of the hub  40 . When the slots  66 ,  66 a and  46  are aligned, pins  67  may be inserted in the slots in order to provide additional securement between the leg  64  and hub  40 . The pins  67  preferably have diameters of from about 1 to about 2 inches, and lengths of from about 2 to about 6 inches. The pins  67  may be made of any suitable material such as Inconel 600 series or stainless steel 300 series alloys. 
     As shown in the embodiment of  FIG. 12 , the hub  40  has a height which is 50% greater than the height of each of the legs  50 ,  50 a and  64 . When multiple hubs  40  are installed along the axial length of the rotary kiln, and multiple legs are installed along the length of the kiln, the difference in height between the hubs and the legs results in an arrangement of two hubs for every three sets of legs. This interlocking staggered arrangement provides additional structural integrity for the heat exchanger. 
       FIG. 13  is a sectional view taken through section  13 - 13  of  FIG. 2 , illustrating a series of heat exchanger legs  50  (shown in cross section) installed in the refractory brick lining  34 . Two courses of raised bricks  76  are installed on each side of the legs  50 . Another course of raised bricks  74  is installed between the first two courses of raised bricks  76  and the refractory brick lining  34  on both sides of the legs  50 . The raised brick courses  74  and  76  are preferably staggered as shown in  FIG. 13  in order to prevent materials being treated in the kiln from infiltrating the joints between the bricks, and to reduce or eliminate fracturing of the bricks and legs. As shown most clearly in  FIG. 2 , the first raised brick course  76  is of greater height than the second raised brick course  74  which, in turn, is greater in height than the refractory brick lining  34 . The height of the first raised brick course  76  is preferably from about 9 to about 15 inches, while the height of the second raised brick course  74  is preferably from about 7 to about 12 inches. The height of the lining  34  preferably ranges from about 6 to about 9 inches. The raised brick courses  74  and  76  provide additional material to support the legs  50 ,  50 a and  64  in regions of high stress concentration, thereby reducing or eliminating cracking of the legs. 
       FIGS. 14-17  illustrate sequential steps for installing a heat exchanger in the rotary kiln shell  32  in accordance with an embodiment of the present invention. As shown in  FIG. 14 , the first leg  50  is positioned in the shell  32  with its outer end  53  located at a four o&#39;clock position  81 . The initial position of the leg  50  is designated as P 1  in  FIG. 14 . The axial center A of the rotary kiln shell  32  is shown in  FIG. 14 . The initial position P 1  of the first leg  50  is inclined at an angle, designated  85  in  FIG. 14 , with respect to the axial center A of the shell  32 .  FIG. 14  also illustrates an initial position P 1  of the second leg  50 a. The outer end  53  of the second leg  50 a is located at an eight o&#39;clock position  82  of the shell  32 . In its initial position P 1 , the second leg  50 a is inclined at an angle, designated  86  in  FIG. 14 , with respect to the center axis A of the shell  32 . The angles  85  and  86  preferably range from about 2 to about 8 degrees. 
     With the first and second legs  50  and  50 a located at their respective initial positions Pi, there is sufficient clearance between the legs for insertion of the hub  40 . The first and second legs  50  and  50 a and the hub  40  may be moved from the positions shown in  FIG. 14  to the interlocking positions shown in  FIG. 15 . The first leg  50  is rotated about a point that substantially corresponds with the four o&#39;clock position  81 . Similarly, the second leg  50 a rotates about a point substantially corresponding with the eight o&#39;clock position  82 . The hub  40  is moved from the elevated position shown in  FIG. 14  to the position shown in  FIG. 15 , at which the center of the hub  40  substantially corresponds with the axial center A of the shell  32 . The first and second legs  50  and  50 a and the hub  40  are thus moved from their initial positions as shown in  FIG. 14  to their installed positions as shown in  FIG. 15 . 
       FIG. 16  illustrates the subsequent installation of the third leg  64  in the shell  32 . The outer end  53  of the third leg  64  is installed at the twelve o&#39;clock position  83  with respect to the shell  32 . The inner end  52  of the third leg  64  is slid into place against the hub  40 . The pins  67  (as shown in detail in  FIG. 12 ) may be inserted between the third leg  64  and hub  40  to thereby form an interlocking engagement between the third leg  64  and the hub  40 . 
     As will be appreciated by those skilled in the art, kiln shells are not perfectly cylindrical. Thus, when forming legs  50 ,  50 a and  64 , it will be necessary to dimension such components to fit within the smallest cylindrical opening defined by the kiln shell. As a result, the insertion of shims 92 between the outer ends of legs  50 ,  50 a and  64  and kiln shell  32 , may be required for one or many of such legs  50 ,  50 a and  64 . 
     In one method of forming legs  50 ,  50 a and  64 , such legs are dimensioned shorter than necessary to fit within a given kiln shell, and the legs are then shimmed where necessary to account for areas of kiln shell  32  that are out of round. 
     As shown in  FIG. 17 , after installation of the first, second and third legs  50 ,  50 a and  64 , and the hub  40 , the refractory brick lining  34  is installed against the shell  32 , as well as the raised brick courses  74  and  76 . 
     The following example is intended to illustrate various aspects of the present invention, but is not intended to limit the scope of the invention. 
     EXAMPLE 
     A heat exchanger is installed in a rotary kiln as follows. After the internal surface of the kiln shell has been exposed and cleaned, the following sequence is carried out.
         1. enter kiln and establish a longitudinal centerline on the lowest segment of radius, or 6 o&#39;clock position;   2. measure interior circumference and divide circumference first by one-half and record, then divide the circumference by thirds and record;   3. from the first centerline on floor, measure one-half of the circumference and establish upper point at the 12 o&#39;clock position. From this line measure back down shell both to the left and right one-third of the circumference and establish these centerlines, at approximately the 4 o&#39;clock and 8 o&#39;clock positions;   4. at the 6 o&#39;clock position, set track segments for the rolling support table, the full length of work area;   5. set both monorail segments, approximately 20 degrees to the left and 20 degrees to the right of the upper or 12 o&#39;clock position centerline;   6. establish the starting point of the heat exchanger and mark kiln shell;   7. from each of the three centerlines, at 12 o&#39;clock, 4 o&#39;clock and 8 o&#39;clock positions, set the support channels and weld to shell;   8. set one leg on the left side of a support table, and second leg on the right side of the table, then raise table to up position;   9. set a hub in place on the support table and lower these three items into place;   10. with support table in the down position, set the remaining leg into place and install locking pins; and   11. lower table, roll forward to next position and repeat steps #8, #9 and #10.       

     This sequence is continued until the heat exchanger is completely installed. Then the support table track and monorail segments are removed and the remaining kiln brick lining is installed. 
     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.