Abstract:
The present invention comprises a novel heat exchanger configuration which preferably uses the axial flow direction for the shell-side fluid and in which dead zones and areas of stagnation are significantly minimized or eliminated and in which inlet region tube erosion is addressed by providing a sacrificial portion of tube length so as to make repair and replacement of the eroded portion of tubes significantly cheaper, easier and with minimal process interruption. Because axial flow is employed with respect to the shell-side fluid according to a preferred embodiment of the present invention, tube vibration problems are generally eliminated.

Description:
RELATED APPLICATION 
     This patent application claims priority to Provisional Application Ser. No. 60/366,776, filed on Mar. 22, 2002. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to heat exchangers and more particularly to design aspects of heat exchanger components. 
     2. Background of the Invention 
     Although heat exchangers were developed many decades ago, they continue to be extremely useful in many applications requiring heat transfer. While many improvements to the basic design of heat exchangers have been made over the course of the twentieth century, there still exist tradeoffs and design problems associated with the inclusion of heat exchangers within commercial processes. 
     In particular, one of the most problematic aspects associated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the various deposits and coatings which form on the surfaces of heat exchangers as a result of process fluid flow and heat transfer. There are various types of fouling including corrosion, mineral deposits, polymerization, crystallization, coking, sedimentation and biological. In the case of corrosion, the surfaces of the heat exchanger can become corroded as a result of the interaction between the process fluids and the materials used in the construction of the heat exchanger. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop in connection with the fluid flowing on the inside of the exchanger. 
     One type of heat exchanger which is commonly used in connection with commercial processes is the shell-and-tube exchanger. In this format, the device is designed such that one fluid flows on the inside of the tubes, while the other fluid is forced through the shell and over the outside of the tubes. Typically, baffles are placed to support the tubes and to force the fluid across the tube bundle in a serpentine fashion. 
     Fouling can be decreased through the use of higher fluid velocities. In fact, one study has shown that a reduction in fouling in excess of 50% can result from a doubling of fluid velocity. It is known that the use of higher fluid velocities can substantially decrease or even eliminate the fouling problem. Unfortunately, sufficiently high fluid velocities needed to substantially decrease fouling are generally unattainable on the shell side of conventional shell-and-tube heat exchangers because of excessive pressure drops which are created within the system because of the baffles. Also, when shell-side fluid flow is in a direction other than in the axial direction and especially when flow is at high velocity, flow-induced tube vibration can become a substantial problem in that various degrees of tube damage may result from the vibration. 
     Higher fluid velocities associated with tube-side flow may also be problematic. For example, in the traditional shell-and-tube arrangement, the higher fluid velocities associated with tube-side flow tend to cause erosion of the tube&#39;s inner surface particularly at the tube inlet. At a fluid velocity of, for example, 8 feet per second, the inner surface of a brass tube may erode over the length beginning at the inlet and extending for 6 inches or more into the tube. As fluid velocities increase, the problem worsens both in terms of the length of tube subject to erosion and the speed at which erosion occurs. 
     Tube erosion could eventually undermine the integrity of the tube-to-tubesheet joints. At the extreme, erosion can cause perforation of the tube which ultimately results in mixing between fluids on the shell side and tube side of the exchanger. 
     Inner surface tube erosion is especially problematic in the shell-and-tube arrangement since once a significant amount of erosion takes place, it becomes necessary to replace or repair the tube. Since, in conventional shell-and-tube heat exchangers, the majority of the tube length subject to erosion is embedded within the interior of the tubesheet, repairs and replacement of the tubes are costly and time consuming. For example, it may be necessary to cut the tube adjacent to the interior surface of both tubesheets, extract the remaining pieces within the interior of the tubesheets, extract the middle portion of the tube (between the two tubesheets), and then clean the surfaces and install a new tube. As is known in the art, this is an arduous process which generally results in significant process downtime. 
     In addition to the tube erosion problem discussed above, existing shell-and-tube heat exchangers suffer from the fact that “dead zones” and areas of fluid stagnation exist on the shell side of the exchanger. These dead zones and areas of stagnation generally lead to excessive fouling as well as reduced heat-transfer performance. One particular area of fluid stagnation which exists in conventional shell-and-tube heat exchangers is the area near the tubesheet proximate to the outlet nozzle for the shell side fluid to exit the heat exchanger. Because of known fluid dynamic behavior, there tends to exist a dead zone or stagnant region which is located in the region between the each tubesheet and each nozzle. This area of restricted fluid flow on the shell side can cause a significant fouling problem in the area of the tubesheet because of the nonexistent or very low fluid velocities in this region. As is known in the art the same problem as described above also exists within the region adjacent to the inlet nozzle. 
     SUMMARY OF THE INVENTION 
     According to a representative embodiment, the present invention comprises a novel heat exchanger configuration which preferably uses the axial flow direction for the shell-side fluid and in which dead zones and areas of stagnation are significantly minimized or eliminated and in which inlet region tube erosion is addressed by providing a sacrificial portion of tube length so as to make repair and replacement of the eroded portion of tubes significantly cheaper, easier and with minimal process interruption. Because axial flow is employed with respect to the shell-side fluid according to a preferred embodiment of the present invention, tube vibration problems are generally eliminated. 
     In one embodiment of the present invention, a novel heat exchanger is provided such that each of the plurality of tubes contained within the heat exchanger extends a predetermined distance beyond the exterior surface of the tubesheet. The extension of the tubes in this manner permits a length of the tubes located near the inlet portion of the tubes to be employed as a sacrificial section which may be easily replaced prior to the point in time at which inner surface erosion reaches a problematic level. Further, in the event tube erosion does occur in the sacrificial section according to the teachings of the present invention, it is not as significant a cause for concern from the operational standpoint. 
     In still another embodiment of the present invention, a cone section which connects the shell to the tubesheet assembly is provided in order to allow shell side fluid traveling towards the tubesheet to uniformly and circumferentially exit the tube bundle while minimizing low-flow zones. 
     In yet another embodiment of the present invention, the novel heat exchanger is formed to include a shell extension which is located such that the shell in the heat exchanger of the present invention extends beyond where the heat exchanger cone meets the shell and further towards the shell-side face of the tubesheet located near the shell side fluid outlet. This shell extension serves to force shell side fluid flow toward the tubesheet in order to further minimize dead zones and regions of low or non-existent fluid flow at or around the center-facing surface of the tubesheet in the region located near the shell side fluid outlet and shell side fluid inlet. The shell extension also limits and/or eliminates shell-side erosion problems because it provides a 360-degree entry and exit path for shell-side fluid flow instead of a configuration where shell-side fluid flows directly against the tube bundle. 
     In another embodiment, the heat exchanger tubesheet is formed such that a conical extension which is preferably centered at the center of the shell-side face of the tubesheet is present. This conical section serves to further reduce and/or eliminate a small region of stagnation which would otherwise be present in the heat exchanger of the present invention as a result of directional flow caused by the aforementioned cone section and shell extension of the present invention. 
     In yet another aspect of the present invention, standard size “off-the-shelf” heat exchanger modules are employed to maximize the benefits of the fouling reducing aspects of the present invention and to allow for very significant reductions in design time when preparing to implement processes. According to this aspect of the present invention, several smaller standard size heat exchangers may be employed in parallel or in series or in both parallel and series to achieve the desired process characteristics including meeting the necessary heat-transfer requirements. 
     As will be recognized by one of skill in the art, and as will be explained in further detail below, the present invention provides many advantages including a significant reduction of dead zones and low-fluid-velocity regions which would otherwise lead to significant fouling problems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation cutaway view of a single-tube-pass heat exchanger having a non-removable tube bundle and representing a first embodiment of the present invention; and 
     FIG. 2 is a side elevation cutaway view of a two-tube-pass heat exchanger having a removable tube bundle and representing a second embodiment of the present invention; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a heat exchanger  100  constructed according to the teachings of the present invention. In the figure, the shell portion is broken away to more clearly illustrate the tube bundle construction. While FIG. 1 shows a shell-and-tube exchanger in the form of a single-pass embodiment, the teachings of the present invention are equally applicable to many other forms of shell-and-tube exchangers such as, for example, multi-pass and U-shaped implementations. The heat exchanger  100  of the present invention includes a shell  150  and a tube bundle  160  contained therein. 
     In a preferred embodiment, tube bundle  160  includes a pair of tubesheets  180  and  190  located, respectively, at each end of the tube bundle  160 . The tubes contained in tube bundle  160  are fastened to apertures contained within tubesheets  180  and  190  by means known in the art such as by welding or by expanding the tubes into tubesheets  180  and  190 . Tube side inlet  140  and corresponding tube side outlet  130  provide a means for introducing a first fluid into the tubes in tube bundle  160 , and for expelling the first fluid from exchanger  100 , respectively. Shell side inlet  110  and shell side outlet  120  provide a means for a second fluid to enter and exit the shell side of heat exchanger  100 , respectively, and thus pass over the outside of the tubes comprising tube bundle  160 . 
     Preferably, the tubes in tube bundle  160  are supported by the novel coil structure which is disclosed in the assignee&#39;s co-pending patent application entitled “Heat Exchanger Flow Through Tube Supports” and which eliminates the need for baffles and allows for high-velocity fluid flow. Alternatively, the tubes in tube bundle  160  may be supported by conventional means such as by “rod baffles”, “twisted tubes” or “egg crate” style tube supports. Segmental baffles are not preferable according to the teachings of the present invention because they generally do not allow high-velocity fluid flow and they further create dead zones. 
     In a preferred embodiment of the present invention, axial flow is used for the shell side fluid. In addition, it is also preferable that a countercurrent flow arrangement be employed as between the two different fluids although a non-countercurrent (i.e. cocurrent) flow may also be implemented according to the teachings of the present invention. 
     As will be noted in FIG. 1, the tubes in tube bundle  160  extend some length beyond the surface of tubesheet  180  in the direction of and towards tube side inlet  140 . In a preferred embodiment of the present invention, the extension is at least 6 inches beyond the surface of tubesheet  180  and possibly more depending upon the intended fluid velocities and the tube metallurgy. The extended tube length employed in connection with the present invention serves as a sacrificial length which may be easily replaced when necessary or desirable so as to avoid the effects of inlet tube erosion which is most prevalent at higher fluid velocities. As will be understood by one of skill in the art, the more rapid the intended fluid velocities, the longer the tube length extension should be. The only practical limitation on the tube length extension is the requirement that the tube length not extend so much such that unfavorable velocity profiles are created within channel  125 . 
     In one embodiment of the present invention, the tube length extension is 6″ beyond the surface of tubesheet  180 . This length of extension is satisfactory for tube materials such as carbon steel, copper nickel and other metals or other materials which are subject to erosion at levels that can cause perforation problems. In the case of brass or other tube materials which are especially susceptible to erosion, tube lengths are preferably extended beyond 6″. Of course, varying extension lengths may be used without departing from the scope or spirit of the present invention. As will be understood by one of skill in the art, the extension length should increase as the tube material&#39;s susceptibility to erosion increases. 
     Although not shown in FIG. 1, the tubes in tube bundle  160  may also be extended in the direction of outlet nozzle  130  and through tubesheet  190 . By extending the tubes and providing a sacrificial section that extends beyond both tubesheet  180  and tubesheet  190 , a sacrificial section is available if flow direction is reversed and outlet nozzle  130  is employed as an inlet nozzle. 
     The teachings of the present invention, particularly the use of extended tube lengths, allow for periodic replacement of the sacrificial tube section as erosion occurs or at selected time intervals. The sacrificial section may be cut off and a new sacrificial section may be welded on or otherwise fastened by expanding a new section within the remaining portion of the tube length which extends outward from the tubesheet. Other welding and other techniques may also be employed in order to replace sacrificial tube lengths as may be required. 
     Yet another aspect of the present invention which serves to eliminate dead zones and low-flow areas and which allows consistent high-velocity fluid flow throughout the heat exchanger  100  of the present invention is also illustrated in FIG.  1 . As can be seen in the figure, shell extensions  115  are included so as to extend shell  150  laterally past the point at which the shell  150  meets cone  135  extending from the outer periphery of tubesheets  180  and  190  towards shell  150  and including nozzles  120  and  110 , respectively. By extending the shell  150  through the use of shell extensions  115  as indicated in FIG. 1, shell side fluid flow is directed towards the tubesheets  180  and  190  without the fluid having the opportunity to immediately enter or leave the region immediately adjacent to the inlet and outlet nozzles  110  and  170 , respectively, where fluid velocity would otherwise be slowed significantly. Further, shell extensions  115  minimize shell-side erosion problems due to the fact that they prevent shell-side fluid from directly flowing against tube bundle  160  upon entry or upon exiting from heat exchanger  100 . 
     Another aspect of the present invention is the inclusion of cone  135  at either or both of the ends of shell  150 . Cone  135  preferably extends from the outer surface of shell  150  to tubesheet  180  and/or tubesheet  190 . The size and shape of cone  135  is selected based upon fluid modeling studies but in most cases standard parts which are readily available may be selected for use as cone  135 . Cone  135 , together with shell extension  115 , serves to direct fluid flow towards tubesheets  180  and  190  rather than permitting fluid to immediately exit outlet nozzle  170  or to immediately enter the interior of tube bundle  160  from inlet nozzle  110 , as applicable. By doing so, the low-velocity fluid zones which would otherwise exist in the vicinity of tubesheets  180  and  190  are eliminated. 
     FIG. 1 also illustrates the novel conical tubesheet extension of the present invention. As can be seen in the figure, tubesheets  180  and  190  include a conical shaped extension which protrudes toward the interior of the heat exchanger cavity and away from inlet nozzle  140  and outlet nozzle  130  respectively. In one preferred embodiment of the invention, the complete diameter of tubesheets  180  and  190  form the base for the conical protrusion extending from the surface of tubesheets  180  and  190 . In another embodiment, only a portion of the diameter of tubesheets  180  and  190  form the base for the conical protrusion. For example, according to this embodiment, the conical protrusion may be formed to have a base diameter of 4″-6″ while the diameter of the tubesheets  180  or  190  may be on the order of 12″-24″. It is preferable in this embodiment for the center point of the conical protrusion to be the same as the center point of the tubesheets themselves. In other words, the conical protrusion is preferably centered on the circular surface of the tubesheets  180  and  190 . 
     The inclusion of conical protrusions as described above results in the reduction and/or elimination of a small dead zone and low-flow area which would otherwise tend to be present in the present heat exchanger adjacent to the center of the interior tubesheet surface facing the heat exchanger cavity. The particular low-flow area which otherwise would be present in the heat exchanger of the present invention results from the inclusion of the shell extension  115  and cone  135  components of the present invention. By including the tubesheet protrusions in the heat exchanger  100  of the present invention, the spaces in heat exchanger  100  which are taken up by the protrusions which would otherwise be “dead zones” or low-flow areas are filled up with solid material so that the low-flow areas and “dead zones” are eliminated with negligible or no loss of heat-transfer capability. 
     As will be readily understood by one of skill in the art, the sizing and detailed shape of the conical protrusion may vary from the examples provided above while still remaining within the scope and the spirit of the present invention. Fluid modeling methodologies as are known in the art may be employed if desired to determine the particular sizes and shapes that meet the desired criteria for the specific design. Of course, the conical protrusion on one tubesheet need not be the same in terms of size or shape as another conical protrusion on another tubesheet within a particular heat exchanger. Sizing and shaping between and among protrusions on tubesheet surfaces may vary according to expected specific fluid flow velocities and tendencies. 
     As can be seen in FIG. 1, the preferable embodiment in which tube supports  170  are included is illustrated. Tube supports  170  are preferably metal coil structures as more fully disclosed in assignee&#39;s co-pending patent application entitled “Heat Exchanger Flow Through Tube Supports”. By using these novel metal coil structures as tube supports  170 , conventional baffles may be eliminated and higher fluid velocities may be employed. 
     Turning now to FIG. 2, another embodiment of the present invention is illustrated wherein the novel features discussed above are employed in another heat exchanger configuration. As can be seen in the figure, the heat exchanger  200  illustrated in FIG. 2 is a two-tube-pass configuration with U-shaped tubes. In addition, as opposed to the configuration of heat exchanger  100  in FIG. 1 wherein tubesheet  180 , conical section  135  and shell  150 , for example, are welded together, the configuration of heat exchanger  200  is such that channel  225 , tubesheet  280  and tube bundle  260  are easily removed from the heat exchanger shell body through the use of bolts  230 . 
     In a preferred embodiment, tube bundle  260  includes tubesheet  280  which is located at the end of the tube bundle  260  adjacent to channel  225 . Tube side inlet  240  and corresponding tube side outlet  210  provide a means for introducing a first fluid into the tubes in tube bundle  260 , and for expelling the first fluid from exchanger  200 , respectively. As can be seen in FIG. 2, pass partition plate  245  prevents fluid from entering exchanger  200  through inlet  240  and exiting exchanger  200  through outlet  210  without passing through the tubes in tube bundle  260 . Shell side inlet  210  and shell side outlet  220  provide a means for a second fluid to enter and exit the shell side of heat exchanger  200 , respectively, and thus pass over the outside of the tubes comprising tube bundle  260 . 
     As is the case with the FIG. 1 embodiment, it is preferable for the tubes in tube bundle  260  to be supported by the novel coil structure which is disclosed in the assignee&#39;s co-pending patent application entitled “Heat Exchanger Flow Through Tube Supports” so that baffles may be eliminated and so that high-velocity fluid flow may be achieved. Alternatively, the tubes in tube bundle  260  may be supported by conventional means such as by rod baffles, twisted tubes or egg crate style tube supports. Again, in this embodiment as in the FIG. 1 embodiment, segmental baffles are not preferable according to the teachings of the present invention because they generally do not allow high-velocity fluid flow and they further create dead zones. 
     Because the FIG. 2 embodiment involves a “U-tube” and thus two tube passes, one of the two passes will be cocurrent with the shell-side flow. Axial flow is preferably used for the shell side fluid in the FIG. 2 embodiment. 
     As is the case in the FIG. 1 embodiment, the tubes in tube bundle  260  of the FIG. 2 embodiment extend some length beyond the surface of tubesheet  280  in the direction of and towards tube side inlet  240 . In the FIG. 2 embodiment of the present invention, the extension is at least 6 inches beyond the surface of tubesheet  280  and possibly more depending upon the intended fluid velocities and the tube metallurgy. 
     In the FIG. 2 embodiment, the tube length extension may be, for example, 6″ beyond the surface of tubesheet  280 . Of course, varying extension lengths may be used in the FIG. 2 embodiment without departing from the scope or spirit of the present invention. As will be understood by one of skill in the art, the extension length should increase as the tube material&#39;s susceptibility to erosion increases. 
     Yet another aspect of the present invention which serves to eliminate dead zones and low-flow areas and which allows consistent high-velocity fluid flow throughout heat exchanger  200  of the present invention is also illustrated in FIG.  2 . As can be seen in the figure, a first shell extension  215  (on the left side of FIG. 2) extends shell  250  laterally past the point at which the shell  250  meets cone  235  extending from the outer periphery of tubesheet  280  towards shell  250 . Cone  235  may also include a flange or ring portion which abuts tubesheet  280  as is shown in FIG. 2. A second shell extension  215  (on the right side of FIG. 2) extends shell  250  laterally past the point at which shell  250  meets cone  235  and towards shell cover  295 . Shell cover  295  may be welded to shell  250  as shown in FIG. 2 or it may be attached to shell  250  through the use of bolts or other fastening techniques known in the art. By extending shell  250  through the use of shell extensions  215  as indicated in FIG. 2, shell side fluid flow is directed towards the tubesheet  180  and shell cover  295 , respectively, without the fluid having the opportunity to immediately enter the region immediately adjacent to shell-side inlet nozzle  210  and outlet nozzle  220 , respectively, where fluid velocity would otherwise be slowed significantly. As in the FIG. 1 embodiment, this arrangement also service to minimize shell-side erosion problems. 
     Another aspect of the present invention is the inclusion of cones  235  at either or both of the ends of shell  250 . Cones  235  preferably extend from the outer surface of shell  250  to tubesheet  280  and/or shell cover  295 . The size and shape of cones  235  are selected based upon fluid modeling studies, but in most cases standard parts which are readily available may be selected for use as cones  235 . Cones  235  serve to direct fluid flow towards tubesheet  280  and shell cover  295  rather than permitting fluid to flow toward inlet nozzle  210  or outlet nozzle  220  as applicable. By doing so, the low-velocity fluid zones which would otherwise exist in the vicinity of tubesheet  280  and shell cover  295  are eliminated. 
     FIG. 2 also illustrates the novel conical tubesheet extension of the present invention. As can be seen in the figure, tubesheet  280  includes a conical shaped extension which protrudes toward the interior of the heat exchanger cavity and away from channel  225 . In one preferred embodiment of the invention, the complete diameter of tubesheet  280  forms the base for the conical protrusion extending from the surface of tubesheet  280 . In another embodiment, only a portion of the diameter of tubesheet  280  forms the base for the conical protrusion. For example, according to this embodiment, the conical protrusion may be formed to have a base diameter of 4″-6″ while the diameter of tubesheet  280  may be on the order of 12″-24″. It is preferable in this embodiment for the center point of the conical protrusion to be the same as the center point of tubesheet  280  itself. In other words, the conical protrusion is preferably centered on the circular surface of the tubesheet  280 . 
     As will be readily understood by one of skill in the art, the sizing and detailed shape of the conical protrusion for the FIG. 2 embodiment may vary from the examples provided above while still remaining within the scope and the spirit of the present invention. 
     As can be seen in FIG. 2, the preferable embodiment in which tube supports  270  are included is illustrated. Tube supports  270  are preferably metal coil structures as more fully disclosed in assignee&#39;s co-pending patent application entitled “Heat Exchanger Flow Through Tube Supports”. By using these novel metal coil structures as tube supports  270 , conventional baffles may be eliminated and higher fluid velocities may be employed. 
     It is preferable that in connection with the use of the heat exchanger of the present invention, a strainer of some form is employed at some point in the process line prior to reaching the heat exchanger. This is important in order to avoid any debris becoming trapped within the heat exchanger of the present invention either in a tube or on the shell side of the heat exchanger. If debris of a large enough size or of a large enough amount were to enter the heat exchanger of the present invention (or, in fact, any currently existing heat exchanger) fluid velocities can be reduced to the point of rendering the heat exchanger ineffective. 
     The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims, and by their equivalents.