Abstract:
The present invention comprises a novel tube support system for a heat exchanger that serves to replace the baffles present in typical shell-and-tube heat exchangers. The shell-and-tube heat exchanger of the present invention employs helically coiled wires to form a support structure for the tubes contained within the heat exchanger shell. The elimination of baffles and the use of the coil support structure according to the present invention permits axial fluid flow for the shell side fluid and significantly minimizes fouling problems and tube damage resulting from flow-induced tube vibration.

Description:
RELATED APPLICATION  
       [0001]    This patent application claims priority to Provisional Application Serial No. 60/366,914, filed on Mar. 22, 2002. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to heat exchangers and more particularly to support structures for heat exchanger tubes within heat exchanger devices.  
           [0004]    2. Background of the Invention  
           [0005]    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 available in the twentieth century have been made, there still exist tradeoffs and design problems associated with the inclusion of heat exchangers within commercial processes.  
           [0006]    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.  
           [0007]    Many heat exchangers in use today also contain baffles. Baffles are interposed in the fluid path in order to ensure that the fluid flowing on the outside the tubes flows across the tubes. Unfortunately, however, baffles serve to increase the fouling problem because they create dead zones on the shell side of the exchanger.  
           [0008]    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.  
           [0009]    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, higher fluid velocities 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.  
           [0010]    Another problem that often arises in connection with the use of heat exchangers is tube vibration damage. Tube vibration is most intense and damage is most likely to occur in cross flow implementations where fluids flow is perpendicular to the tubes, although tube vibration damage can also occur in non-crossflow (i.e. axial) implementations in the case of very high fluid velocities.  
           [0011]    Existing shell-and-tube heat exchangers suffer from the fact that they must typically use baffles to maintain the required heat transfer. This, however, results in “dead zones” within the heat exchanger where flow is minimal or even non-existent. These dead zones generally lead to excessive fouling. Other types of heat exchangers may or may not employ baffles. If they do, the same increased fouling problem exists. Further, in heat exchangers fitted with baffles, for example, the cross flow implementation results in the additional problem of potential damage to tubes as a result of flow-induced vibration. In the case of such damage, processes must often be interrupted or shut down in order to perform costly and time consuming repairs to the device.  
         SUMMARY OF THE INVENTION  
         [0012]    According to a representative embodiment, the present invention comprises a novel tube support system that serves to replace the baffles present in typical shell-and-tube heat exchangers. The shell-and-tube heat exchanger of the present invention employs helically coiled wires to form a support structure for the tubes contained within the heat exchanger shell.  
           [0013]    In one embodiment of the present invention, the wire coil has a diameter substantially equal to the space between the heat exchanger tubes.  
           [0014]    In another embodiment, the wire coil has a diameter equal to one-half of the space between the tubes.  
           [0015]    In a preferred embodiment of the present invention, the coils in the support structure alternate between a clockwise and a counterclockwise rotation within the support structure.  
           [0016]    In one embodiment of the present invention, the coils forming the support structure overlap with one another while in an alternative embodiment, the coils make point contact with another.  
           [0017]    In a preferred embodiment of the present invention, high velocity axial flow is used in order to eliminate dead zones and related fouling problems.  
           [0018]    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 flow-induced tube vibration that can lead to tube damage, thermal expansion problems and dead zones that promote rapid fouling. Furthermore, the present invention provides axial flow on the shell side thereby eliminating the presence of dead zones which cause fouling and which are typically contained within prior art heat exchangers.  
           [0019]    Additionally, the heat exchanger design according to the present invention permits operation at high fluid velocities on the shell side of the exchanger in order to substantially reduce fouling. Velocities are essentially only limited by erosion limits and pump size. The use of the tube support system of the present invention also makes it easier to predict the performance of the heat exchanger as the flow geometry is simple and has no bypass or leakage streams. As a result, simpler calculations may be used in order to design exchangers using the teachings of the present invention.  
           [0020]    The above and other objects of the present invention are achieved through the use of a tube support system which supports the tubes in a novel way and in a way in which baffles are not required to obtain the necessary heat transfer characteristics. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a side elevation view of a single-pass heat exchanger as constructed according to the teachings of the present invention;  
         [0022]    [0022]FIG. 2 is a cross-sectional view of the heat exchanger of the present invention according to a first embodiment wherein the coil wire thickness is substantially equal to the inter-tube spacing and the tubes are placed in an in-line pitch;  
         [0023]    [0023]FIG. 3 is a close up view of tubes and the coil structure according to the first embodiment of the present invention as also illustrated by FIG. 2.  
         [0024]    [0024]FIG. 4 is a cross-sectional view of the heat exchanger of the present invention according to a second embodiment wherein the coil wire thickness is substantially equal to one-half of the inter-tube spacing;  
         [0025]    [0025]FIG. 5 is a cross-sectional view illustrating the weld between two coils within the support structure framework in the case of an embodiment of the present invention wherein the coil wire thickness is equal to any amount greater than one-half the inter-tube spacing and up to a full inter-tube spacing and the coils overlap with one another; and  
         [0026]    [0026]FIG. 6 is a cross-sectional view of the heat exchanger of the present invention according to a third embodiment wherein the coil wire thickness is equal to the inter-tube spacing and the tubes are placed in a triangular pitch. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    [0027]FIG. 1 illustrates a heat exchanger 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, two or more tube passes, U-shaped tubes, removable tube bundle designs, and exchangers known as multi-tube double pipes. The heat exchanger  100  of the present invention includes a shell  150  and a tube bundle  160  contained therein.  
         [0028]    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 and/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 .  
         [0029]    The novel coils  170  of the present invention are shown in FIG. 1. As will be discussed in greater detail below, coils  170  contain tubes within their internal periphery and also serve to provide a support structure to allow tubes to be inserted between the outside peripheries of the coils  170 . According to the teachings of the present invention, coils  170  may extend fully from tubesheet  180  all the way to tubesheet  190 , or alternatively, one or more coil structures may be intermittently spaced along the tubes. For example, a coil structure may begin twelve inches from tubesheet  180  and then extend approximately eight inches. This could be followed by a gap of approximately two feet followed by another length of coil structure and so on. However, it is possible for the coil structure to extend the full length of the tubes without gaps. The support structures of the present invention may be preferably welded to tie rods or, in the alternative or in addition, to several tubes at the outer periphery of tube bundle  160  in order to prevent the support structure from moving.  
         [0030]    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 or a combination of cocurrent and countercurrent flow may also be implemented according to the teachings of the present invention.  
         [0031]    Turning now to FIG. 2, the novel support structure employed to support the tubes contained within tube bundle  160  is described. In a first embodiment as reflected by FIG. 2, coiled wires which have a diameter that is substantially equal to the space between the tubes comprising tube bundle  160  are used. The wire material is preferably comprised of erosion-resistant material such as stainless steel, titanium or other materials with similar metallurgical characteristics. In connection with the description herein, it will be understood by one of skill in the art that the term “wire” may encompass any or all of a wire, rod, strip or bar, all of which may be implemented in constructing the support structure of the present invention. As can be seen in FIG. 2, in the finished product, the wire material is wrapped around the tubes  230  to form coils that overlap with one another.  
         [0032]    The coils structure is preferably constructed as follows. Coils  170  are prefabricated according to the specified diameter, tube pitch and coil pitch requirements. Coil pitch represents the axial distance along the tube length associated with one complete 360° turn around the tube. In a preferred embodiment the coil makes at least two complete turns around the length of the tube. Such prefabricated coils are generally available from coil manufacturers. Individual coils  170  are placed in a jig and adjacent coils are preferably fused together by welding. For example electrical arc welding may be used. According to the teachings of the present invention, coils  170  may be comprised of various wire cross-sections such as circular, square, elliptical, rectangular, or other suitable geometric shapes. FIG. 2 is an example of the use of circular cross-section for coils  170 . It will be appreciated by one of skill in the art that in connection with the fabrication process, the coil outer diameter must not exceed the tube pitch plus one intertube space and further that the inside diameter of the coils  170  must have sufficient clearance to allow for insertion of tubes  170 .  
         [0033]    In the FIG. 2 embodiment of the present invention, tubes  230  are aligned with one another in horizontal rows and also in vertical rows thus comprising the known in-line arrangement for tubes. As will be understood by one of skill in the art, other tube positioning arrangements are also possible without departing from the scope or spirit of the teachings of the present invention.  
         [0034]    A series of coils  170  are connected together by welding to form the support structure of the present invention. As shown in FIG. 2, the coil wire thickness is substantially equal to the space that would otherwise exist between the tubes  230 . This results in an overlapping arrangement as between the coils forming the framework of the support structure. It is preferable in this embodiment for various portions of the support structure to alternate as between counterclockwise and clockwise wrappings (illustrated in FIG. 2 as “CC” and “C” respectively). For example, in FIG. 2, the coil at the top left corner has a clockwise wrap while all coils in contact with that coil have a counterclockwise wrap.  
         [0035]    As can be seen in FIG. 2, it is preferable that in the in-line embodiment, all tubes are contained within the interior surface of a coil  170 . In other words, no tubes are located between the outer peripheries of two or more coils  170 . It will also be understood by one of skill in the art that the outer edge of tube bundle  160  will preferably be fitted with sealing strips, rings or bands which are fastened to tube bundle  160  and extend toward the inner surface of shell  150  in order to avoid flow bypassing.  
         [0036]    According to the teachings of the present invention, tubes  230  are interposed into the interior of coils  170  but tubes  230  are not physically attached (e.g. by welding) to each other. This provides the advantage that it is easier to fabricate the exchanger as well as service the exchanger by replacing damaged tubes.  
         [0037]    [0037]FIG. 3 is a close up side view of the tube support structure of the present invention including the tubes  230  and the coils  170 . Coils  170  extend in the inter-tube space and coils  170  themselves overlap with one another when viewed from the axial direction as in FIG. 2. However, when viewed from the front as in FIG. 3, the coils  170  do not overlap with one another but instead make contact with one another via weld  310 . In FIG. 3, the top coil  170  is wound in a clockwise fashion when viewed from the right while the bottom coil  170  is wound in a counterclockwise fashion when viewed from the right.  
         [0038]    Turning now to FIG. 4, an axial view of the heat exchanger  100  of the present invention according to a second embodiment is illustrated. In this embodiment, the thickness of coils  410  is substantially equal to one-half of the inter-tube spacing size. As a result, in this configuration, rather than overlapping with one another, coils make point contact with one another, for example at point  430 . It is preferable in this embodiment, as it is in the first embodiment, for the wrapping of coils to alternate as between clockwise and counterclockwise for adjacent coils.  
         [0039]    As will be readily understood by one of skill in the art, the two embodiments provided, namely using coil thicknesses of approximately 100% of the inter-tube spacing and approximately 50% of the inter-tube spacing are not the exclusive possibilities. In fact, any coil thickness which is at least 50% but no more than approximately 100% of the inter-tube spacing amount may be used in connection with the teachings of the present invention.  
         [0040]    [0040]FIG. 5 illustrates the trimming requirements which may be undertaken in any embodiment of the present invention wherein the coil thickness is equal to any amount greater than one-half of the inter-tube spacing amount (i.e. any embodiment other than the above-described second embodiment). In such cases, it is possible to trim coil wire  510  so that it may make planar contact with its neighboring coil wire, for example in FIG. 5, coil wire  520 . By employing trimming, and thus providing planar contact between coil wires  510  and  520 , it is possible to create a larger contact area and thus provide a stronger weld. According to the teachings of the present invention, coil wires should be trimmed down to approximately one-half of the inter-tube space. For example, if the coil thickness of coil wires  510  and  520  were 70% of the inter-tube space, each of coil wires  510  and  520  should be trimmed down to approximately 50% of the inter-tube space at the contact point at weld  530 .  
         [0041]    [0041]FIG. 6 is an end view of a third embodiment of the present invention wherein the tubes  610  are arranged in triangular pitch. According to the teachings of the present invention, in this case, some tubes  610  will be contained within the interior of coils  620  and others will not. The tubes  610  that are not contained within the interior of individual coils  620  are nonetheless supported by the exterior of the coils  620  which are adjacent to the relevant tube  610 . Again, in this embodiment, it is preferable that coils which are adjacent to one another be wound in opposite directions (i.e. clockwise adjacent to counterclockwise).  
         [0042]    In FIG. 6, the coil thickness is equal to the inter-tube spacing which results in an overlap as between the adjacent coils when viewed from the end as in the FIG. 6 view. Alternatively, but not shown, coil thickness in the triangular pitch case can be anywhere from 50% of inter-tube spacing to 100% of inter-tube spacing. As discussed above, in the case of 50% of inter-tube spacing, the coils will make point contact and not overlap with one another.  
         [0043]    The tubes on the left half of FIG. 6 represent the same tubes as is shown on the right half of FIG. 6. Thus, for example, the tube  610  at the upper left hand corner of the left side coil structure and tubes is the same tube as is shown in the upper left hand corner of the right side coil structure and tubes illustrated in FIG. 6. In a preferred embodiment of this invention, rather than extending from one tubesheet all the way to the other tubesheet, multiple sections of coil structures are interspersed along the length of the tubes  610  with gaps between such coil structures. However, it is possible for the coil structure to extend the full length of the tubes without gaps. In this case, it is preferable that the coil structure be produced such that individual segments with alternating designs are placed end to end to form a coil structure extending the full length of the tubes.  
         [0044]    It is preferable that each successive coil structure along the tube alternate with respect to which tubes are contained within the interior of the coils and which tubes are not. Thus, for example, the tube at the upper left corner illustrated in the left side of FIG. 6 is contained within a coil  610  at one point during the length of the tube while further down the tube at the next successive coil structure segment (as shown on the right side of FIG. 6), that same tube is supported by the exterior surfaces of the adjacent coils. It is preferable to form each coil structure such that successive coil structures alternate with respect to which tubes are enclosed internally and which are not as described above.  
         [0045]    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.  
         [0046]    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.