Abstract:
A heat exchanger tube for conveying a heat transfer fluid, into which one or more turbulence-inducing elements are fixedly positioned on a supporting member extending in spaced relation along the central axis of the tube. The turbulence-inducing elements have a first portion facing upstream and a second portion facing downstream. The entire exterior surface of the first portion forms a continuous solid surface that blocks and deflects the path of the flowing fluid.

Description:
RELATED APPLICATIONS 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to tubular heat exchangers, and in particular to turbulence-inducing devices positioned in the tubes of the tubular heat exchanger that minimize or prevent fouling caused by the heat transfer fluids and enhance or maintain the overall heat transfer coefficient over the operational life of the tubular members. 
     Description of Related Art 
     Heat exchangers are found in many industrial and commercial applications. In the design of heat transfer equipment, an important factor includes the footprint of the exchanger relative to the capacity of fluid that is to be heated or cooled (the “receiving fluid”), as well as the requisite flow of the heating or cooling fluid (the “transferring fluid”). The heat transfer coefficient between the transferring fluid and the receiving fluid should be maximized to achieve the smallest allowable footprint of the heat exchanger. 
     Another factor that must be considered in designing heat exchangers is the tendency of heating or cooling fluids to foul in the tubes through which they pass. One detrimental effect of fouling is a lowering of the heat transfer coefficient. The thermal conductivity of the fouling layer is less than that of the tube material, which increases the heat transfer resistance, reduces the efficacy of the heat exchanger, and increases the tube skin temperature. Another negative effect of fouling is that the formation of depositions on the interior surface of the tubes reduces their cross-sectional area, causing increased resistance to the fluid flow and an increase in the pressure drop across the unit. 
     In refinery and petrochemical plants, problems caused by tube fouling are very expensive to remedy. Capital expenditures are higher due to the increased size of the heat exchanger (e.g., selecting heat exchangers with 10-50% greater surface area to accommodate conventional fouling expectations), the associated increase in requisite area within the plant, the higher strength and size foundations, and the extra transport and installation costs. Furthermore, the cost of operating the unit is increased due to additional fuel, electricity or process steam requirements. In addition, production losses occur during planned and emergency plant shutdowns due to fouling and associated system failures. 
     Various attempts to minimize or prevent fouling problems have been advanced. One common prevention technique is to use a fouling factor in the design phase of a heat transfer unit that includes increasing the heat transfer surface area, either by increasing the number of tubes or the tube length. Such a fouling factor is considered a necessary aspect of heat exchanger design, based on acceptance of the fact that fouling is inevitable. In addition to the aforementioned costs associated with selecting a larger heat exchanger, an additional concern is that the excess surface area calculated with a fouling factor can result in start-up complications and actually encourage more fouling. That is, it is common that at start-up, sludge and dirt migrate into dead zones and low velocity locations. The effect of increasing the number of tubes is to decrease the fluid flow velocity, thereby increasing the likelihood of fouling. Similarly, increasing the tube length results in lower fluid pressure, also increasing the likelihood of fouling. 
     Other known attempts to mitigate fouling problems involve the use of in-line mechanical cleaning devices to remove fouling build-up inside the tubes. These devices, which generally require direct physical contact with the inner tube surface, have not been especially successful in preventing fouling. 
     Deflection insertions are also another general category of fouling prevention or mitigation devices. For instance, U.S. Pat. No. 1,015,831 to Pielock et al. discloses a device that is inserted in a pipe to deflect the central and peripheral flow of liquid. Fluid along the side walls is directed toward the center of the pipe, and fluid moving along the longitudinal center line is directed towards the side walls. The device is constructed as a ring installed on the pipe&#39;s inner surface having a diametrically disposed web or a plurality of webs that form an apex pointed against the direction of fluid flow. However, the device described in Pielock et al. is mainly intended to diffuse central flow in multiphase fluid for equal distribution. Furthermore, in the context of a heat exchanger&#39;s transferring tube, fouling will predictably occur at the interface of the Pielock device and the tube&#39;s inner surface. 
     U.S. Pat. No. 3,995,663 to Perry describes a ferrule for insertion at the inlet of a vertical shell-and-tube heat exchanger, including a flange and shoulder to seat upon the tube sheet, a bore and a cylindrical portion as an extension of the bore to facilitate formation of a solid column of liquid entering the tube. The ferrule also includes an outwardly extending connecting wall that distributes fluid towards the apex of a conical member. Fluid entering the bore is directed to the side walls due to the shape of the conical member. Apparently, the purpose of the device is to distribute liquid to the walls of the ferrule rather than to the tube walls to provide liquid in the form of a falling film on the inner surfaces of the vertical tubes for evaporation. Therefore, application of this structure is necessarily limited to vertical shell-and-tube heat exchangers. 
     U.S. Pat. No. 5,311,929 to Verret and U.S. Pat. No. 4,794,980 to Raisanaen both disclose air-to-air heat exchangers that include cone-shaped elements disposed in each tube along a central rod. The cones serve as deflectors to create turbulence in the gases flowing through the tube. The elements disclosed in Verret are attached using a twisted strip of material bent inside the tubes to provide contact with the tube&#39;s internal surface. The conical elements described in Raisanaen are open on the downstream end, thus allowing fouling and sludge accumulation inside the cone. 
     The above-described references each have drawbacks that render them unsuitable for minimizing or preventing fouling. Additional known attempts to prevent fouling rely upon inserts fixed to the inner wall of the tube. However, fouling will eventually accumulate at, and proximate to the attachment points, which hinders removal of the inserts and thus complicates cleaning the inner surface of the tube. 
     Therefore, it is an object of the present invention to provide an apparatus for use in the tubes of heat exchangers that eliminates or minimizes fouling of the interior surfaces of the tubes. 
     It is another object of the present invention to provide an apparatus for use in tubes of heat exchangers that maintains the heat transfer coefficient over the operational life of the tubes. 
     It is still another object of the present invention to provide an apparatus for use in the tubes of heat exchangers that permits the designer to utilize the minimum theoretical heat exchanger size or capacity for a given application. 
     SUMMARY OF THE INVENTION 
     The above objects and further advantages are provided by the apparatus of the present invention for promoting turbulence in the tubes of a heat exchanger conveying the heat transfer fluid that in one embodiment comprehends a turbulence-inducing element formed with a conical upstream portion, from the base of which a second portion extends downstream. In one embodiment, the second portion is convex or hemi-spheroid in shape. In another embodiment, the second portion is conical in shape. In yet another embodiment, the second portion is shaped as a conical frustum. In yet another embodiment, the second portion is shaped as a truncated convex shape with a rounded edge surface. In another aspect of the present invention, longitudinal grooves and/or protrusions are formed on the exterior surfaces of the turbulence-inducing elements. The solid or closed downstream ends of the elements prevent accumulation of deposits. 
     A plurality of these turbulence-inducing elements are secured to a structural support member that is centrally positioned along the longitudinal axis of the tube. In a preferred embodiment, a plurality of the turbulence-inducing elements extend along substantially the entire length of the tube. The centrally-positioned support member can be a rigid member, such as a rod, or a flexible material, such as a solid or stranded wire or cable. Alternatively, a plurality of centrally-positioned links can be used to join the turbulence-inducing elements. 
     In a further aspect of the invention, springs can be provided at both ends of the centrally-positioned support member, to maintain the system in tension and absorb sudden load variations. 
     In the practice of the method of the invention, the apparatus including a plurality of turbulence-inducing elements mounted on the supporting member is inserted into one or more of the tubes of tube-type heat exchangers to induce turbulent fluid flow inside the tube, particularly at the inner wall of the tube. The supporting member is attached to the ends of the tube. The supported elements are dimensioned and configured so that they do not touch the adjacent inner wall of the tube in which they are mounted. During operation, the fluid in the tube flows across the symmetrically-shaped surfaces of the turbulence-inducing elements, which in turn applies tension to the supporting member and which thereby maintains the elements along the center of the tube. 
     Preventing formation of a quiescent boundary layer enhances the heat transfer coefficient and breaks down or prevents formation of the stagnant film on the inner surface of the tubes associated with the boundary layer. The apparatus and method of the invention also result in a thorough mixing of the heat transfer fluid as it passes through the tube, thereby enhancing its efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in further detail below and with reference to the attached drawings in which the same or similar elements are referred to by the same reference numerals, and in which: 
         FIG. 1  is a longitudinal cross-sectional view of a typical shell-and-tube heat exchanger of the prior art; 
         FIG. 2  is a longitudinal cross-sectional view of a prior art tube carrying heat transfer fluid in a tubular-type heat exchangers schematically illustrating the boundary layer phenomenon; 
         FIG. 3A  is a longitudinal cross-sectional view of a tube carrying heat transfer fluid in which the turbulence-inducing elements of the present invention are mounted; 
         FIG. 3B  is an end view of the tube shown in  FIG. 3A ; 
         FIG. 3C  is side perspective view of one embodiment in which each linking wire can be routed across a number of tube ends; 
         FIG. 3D  is side perspective view of one embodiment in which a tube sleeve can be inserted into the tubes; 
         FIGS. 3E and 3F  show a side perspective view and end view, respectively, of one embodiment showing a first linking wire routed across a row of tube ends and a second linking wire routed across a column of tube ends; 
         FIG. 3G  is a diagram used to describe relative dimensions according to one example; 
         FIG. 4  is a longitudinal cross-sectional view of a tube carrying heat transfer fluid according to the present invention schematically depicting the turbulent fluid flow within the tube; 
         FIGS. 5A, 5B, and 5C  are a series of front, side, and rear views, respectively, of one embodiment of a turbulence-inducing element of the present invention with a downstream portion in the form of a convex portion extending from the base of a conical portion; 
         FIG. 6  is a side perspective view of another embodiment of a turbulence-inducing element of the present invention with a downstream portion in the form of a truncated convex shape with a rounded edge surface; 
         FIG. 7  is a side perspective view of a further embodiment of a turbulence-inducing element of the present invention with a downstream portion having a shape that is conical with an apex; 
         FIG. 8  is a side perspective view of an additional embodiment of a turbulence-inducing element of the present invention with a downstream portion having a shape that is conical with a rounded apex; 
         FIG. 9  is a side perspective view of a still further embodiment of a turbulence-inducing element of the present invention with a frustoconical downstream portion; 
         FIG. 10  is a side perspective view of an embodiment of a turbulence-inducing element of the present invention having a generally conical upstream portion with a concave lateral outer surface; 
         FIG. 11  is a side perspective view of a further embodiment of a turbulence-inducing element of the present invention having an upstream portion having a pyramidal structure; 
         FIG. 12  is a side perspective view of another embodiment of a turbulence-inducing element of the present invention having an upstream portion with a star-shaped pyramidal structure; 
         FIGS. 13A, 13B, and 13C  are a downstream end view, side perspective view, and upstream end view, respectively, of another embodiment of a turbulence-inducing element of the present invention having surface grooves extending in the direction of fluid flow; 
         FIGS. 14A, 14B, and 14C  are a downstream end view, side perspective view, and upstream end view, respectively, of another embodiment of a turbulence-inducing element of the present invention in which the upstream conical surface portion is provided with a plurality of protruding stud elements; 
         FIGS. 15A, 15B, and 15C  are a downstream end view, side perspective view, and upstream end view, respectively, of an additional embodiment of a turbulence-inducing element of the present invention that has both grooves in the direction of fluid flow and protruding stud elements; 
         FIGS. 16-18  are longitudinal cross-sectional views of various embodiments of arrangements of turbulence-inducing elements according to the present invention including structures for accommodating expansion and contraction of the supporting member in the tube; and 
         FIGS. 19A and 19B  are a side perspective view and a downstream end view, respectively, of another embodiment of a turbulence-inducing element of the present invention; 
         FIG. 20  is a diagram used to describe relative dimensions according to the embodiment illustrated in  FIGS. 19A and 19B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is shown a longitudinal cross-sectional view schematically depicting the arrangement of elements in a typical shell-and-tube heat exchanger  20  of the prior art. A bundled tube heat exchanger is a well known configuration of a type of heat transfer equipment in which a plurality of tubes convey a heat transfer fluid. By means of the thermal conductivity of the tubes, heat is transferred to a receiving fluid that contacts the exterior surface of the tubes. 
     Exchanger  20  includes a shell  22  and a tube set  24  having a plurality of tubes  26 . The tubes  26  are supported at their ends by tube sheets  28 , also known as end plates. In the typical construction of a bundled tube heat exchanger, a series of baffles  30  are provided through which the plurality of parallel tubes  26  pass. 
     In operation, heat transfer fluid is introduced via a tube set inlet  38  proximate to the first end  34  of the shell-and-tube heat exchanger  20 , passes through the tubes  26 , and is discharged from a tube set outlet  40  proximate to the opposite end  36  of the heat exchanger  20 . While heat transfer fluid is passing through tubes  24 , receiving fluid is introduced into the shell inlet  42  proximate the end portion  36 . Receiving fluid contacts the outer surfaces of the tubes  26  as it passes over them and around the baffles  30 , thereby undergoing a temperature change. Heated or cooled fluid from the shell  22  is discharged via the shell outlet  44  proximate to the first end  34 . 
     As noted above, a common problem encountered in the tubes of shell-and-tube and other tubular heat exchangers is fouling of the inner walls and plugging of the tubes carrying the heat transfer fluid. This fouling leads to decreased cross-sectional area of the tubes, thus increasing the pressure drop across the tubes, and also causing decreased thermal conductivity. This phenomenon is schematically illustrated in  FIG. 2 , showing a boundary layer  46  formed on the inner surface of the tube  26 . As a result, the flow velocity of the boundary layer  46  is very low, reducing the heat transfer coefficient and promoting adhesion of impurities to the inner surface of the tube wall. 
     Heat transfer fluids can be gases or liquids, including high viscosity lube oil. The selection of the number, size and shape of turbulent-inducing elements depends on the allowable pressure; type of need; enhancement of heat transfer; and need for fouling mitigation. For example, if the pressure drop of a specific heat exchanger is small and more turbulence is required, a preferred embodiment would be to use a large number of turbulent-inducing elements, of relatively small size. 
     As will be apparent to one of ordinary skill in the art, although a shell-and-tube heat exchanger is depicted in  FIG. 1 , the turbulence-inducing elements of the present invention and their arrangement is applicable to other tubular heat exchangers including, but not limited to, double pipe heat exchangers and air-cooled heat exchangers. 
       FIGS. 3A and 3B  show a heat exchanger tube  126  according to the present invention including an apparatus  148  having a plurality of turbulence-inducing elements  150  positioned centrally and spaced apart along the length of tube  126  positioned along a structural support element  152 . There are a variety of ways to assemble the present invention, including casting them in place and/or “stringing” the turbulence-inducing elements  150  on the rod, by welding, by use of suitable adhesives, and the like. Note that while the figures show a plurality of identical turbulence-inducing elements  150 , turbulence-inducing elements of different shapes and types can be positioned on the structural support element  152 . Various embodiments of alternative shapes and types of turbulence-inducing elements are described below with respect to  FIGS. 5-15 . In addition, the total number of turbulence-inducing elements, the spacing between adjacent turbulence-inducing elements, the dimensions of the turbulence-inducing elements, including length and diameter relative to the tube diameter and other structural parameters, and/or the shape of turbulence-inducing elements, are determined by factors including, but not limited to, the heat transfer fluid flow rate and viscosity, increased back pressure that can result from a large diameter turbulence-inducing element blocking too much of the flow path, the maximum allowable pressure drop, and the target heat transfer coefficient. 
     The dimensions and spacing of the turbulence-inducing elements  150  relative to the size of the tube  126  are described according to the following formulas and with reference to  FIG. 3G , according to one example. 
     A minimum gap (g) is maintained between the inside diameter (ID) of the tube and the outer diameter (d) of the turbulence-inducing element, according to the following formula:
 
 g≧ 0.25*ID  (1)
 
     The diameter of the turbulence-inducing element (d) is determined relative to the inside diameter (ID) of the tube, according to the following formula:
 
 d=ID− 2 g   (2)
 
     The length (L) of the turbulence-inducing element is determined relative to the inside diameter (ID) of the tube, according to the following formula:
 
1.25( ID )&lt;= L &lt;=1.5( ID )  (3)
 
     The space (S) between adjacent turbulence-inducing elements is determined relative to the diameter (d) of the turbulence-inducing element and the gap (g) (described above), according to the following formula:
 
 S= 3.5* d/g   (4)
 
     The depth (h) of the second portion extending towards the downstream end of the tube is determined relative to the diameter (d) of the turbulence-inducing element, according to the following formula:
 
0≦h≦0.25d  (5)
 
     The above formulas used for calculating the dimensions and spacing of the turbulence-inducing elements are provided by way of example. In general, the relative dimensions and spacing of the turbulence-inducing elements can be modified in order to strike a balance between preventing or minimizing the formation of a boundary layer and the potential for erosion of the inner surface of the tube due to increased fluid flow rate against the inner surface walls. 
     Materials of construction suitable for the turbulence-inducing elements and the structural support element include: plastics, including PTFE (Teflon) and nylon; natural or synthetic rubbers; wood or wood-based composites; or relatively soft metals such as aluminum, titanium, and copper. 
     The ends  184  and  186  of the structural support element  152  are attached at the upstream end  154  and the downstream end  156 , respectively. The ends  184 ,  186  can include, for example, ball stops that are attached to a linking wire  155  at the upstream end  154  and a linking wire  157  at the downstream end  156  of the tube. 
     In one embodiment, as shown in  FIG. 3C , each linking wire  155  and  157  can be routed across a number of tube ends. 
     In a another embodiment shown in  FIG. 3D , a tube sleeve  190  can be inserted into the tubes, with a linking wire  192  attached, such as by welds  194 , to points on the inner wall of the tube sleeve that are 180 degrees apart. The end  185  of structural support element  152  is then connected to the center of linking wire  192 , such as with a ball stop. 
     In a further embodiment shown in  FIGS. 3E and 3F , linking wire  200  is routed across a row of tube ends, and linking wire  202  is routed across a column of tube ends. The end of structural support element  152  terminates in a threaded rod  208 . Structural element  152  can be a wire, in which case the threaded rod  208  can be attached such as by welding, by crimping or by ball stop. Alternatively, structural element  152  can be a rod, with threaded rod  208  merely being the end of structural element  152 , to which a thread has been applied, as with a chuck. The linking wires  200  and  202  cross at perpendicular angles at the centers of each tube  126 . A pair of internal guides  204  are provided for each tube  126  that linking wires  200  and  202  are routed across. Threaded rod  208  is then attached to the intersection of linking wires  200  and  202 , for example using internal nut  210 , internal washer  212 , external washer  214  and external nut  216 . Alternatively, linking wires  200  and  202  can be formed as a mesh, with washers at their intersecting points at the center of each tube. Threaded rod  208  can then be inserted through the central washer and secured with an external nut. 
     The turbulence-inducing elements  150  are configured and dimensioned to direct the flowing heat transfer fluid towards the inner surface of the tube wall. For example,  FIG. 4  schematically illustrates the turbulent flow that is created inside the inner tube  126  and, in particular, the flow that is created around the turbulence-inducing elements  150 . According to the present invention, fluid flow is directed toward the tube&#39;s inner wall surfaces to thereby disrupt the boundary layer that would otherwise form along the surface of tubes not having the turbulence-inducing elements  150 , with the result being that a region of turbulence is created downstream of the maximum diameter of the device. The likelihood of accumulation of impurities on the inner surface of the tubes is thereby eliminated or minimized because of the turbulent flow created by the apparatus of the present invention. 
     In addition,  FIG. 4  shows that as the fluid flow moves along the tube length, and additional downstream turbulence-inducing elements are encountered, the deflection of fluid by the turbulence-inducing elements is cumulative. For example, a first turbulence-inducing element generally receives a generally laminar flow of fluid from the upstream end of the tube, while a second turbulence-inducing element receives fluid with a flow path that has been deflected by the first turbulence-inducing element, and then a third turbulence-inducing element receives fluid with a flow path that has been deflected by both the first and second turbulence-inducing elements. 
       FIGS. 5A, 5B, and 5C  show a series of front, side, and rear views of one embodiment of a turbulence-inducing element  250 . Turbulence-inducing element  250  includes a first portion  260  which is positioned towards the upstream end of the tube and a second portion  270  towards the downstream end of the tube. The distal end  262  of the first portion  260  has a cross-sectional area smaller than the maximum cross-sectional area of the second end portion  270 . In general, the cross-sectional area of the first portion increases in the direction of fluid flow as arranged in the tube, and the cross-sectional area of the second portion decreases in the direction of fluid flow. Note, however, that the cross-sectional area of the distal end  262  of the first portion  260  should not be larger than the diameter of structural support element  252 , to prevent a perpendicular impingement of fluid particles on the distal end  262 . 
     Furthermore, in preferred embodiments of the present invention, the turbulence-inducing elements are symmetrical about their longitudinal axes, i.e., from the upstream portion to the downstream portion. Such an arrangement permits a balanced distribution of the transferring fluid within the tube and along the inner wall of the cylindrical tube as shown in  FIG. 4 . The thorough mixing of the heat transfer fluid increases the overall efficiency of the unit by disrupting the generally laminar flow of the liquid. 
     The first portion  260  of the turbulence-inducing element  250  is configured generally in the shape of a conical frustum, with the distal end  262  formed as a truncated apex or a truncated curved or rounded apex. In certain embodiments, the truncation can be minimized such that the distal end approaches an apex or rounded apex, depending on the diameter of the structural support element  252 . In a preferred embodiment, the distal end  262  is configured so as to minimize any energy loss associated with localized pockets of turbulence, which would otherwise deleteriously increase the pressure drop along the tube. 
     The turbulence-inducing element  250  can be attached to the structural support element  252  by any of a number of means. In a preferred embodiment, the turbulence-inducing element  250  can be cast on the wire or rod of the structural support element  252 . Alternatively, the turbulence-inducing element can be hollow or have a light-weight core between the distal end and the center of the convex second portion so that the rod can be inserted through and welded in place. Other examples include attaching the turbulence-inducing element  250  to structural support element  252  by crimping or pinning. 
     In one preferred embodiment, the shape of the second portion  270  facing the downstream end of the tube is generally convex. The edges  266  of the interface  264  between the imaginary transverse plane of the base of the first portion  260 , e.g., a plane characterized by a plurality of circumferential lines of a cone-shaped structure, and the imaginary base of the second portion  270  (shown in broken lines) are preferably rounded or partially rounded. 
     The configuration of the second portion can be any suitable shape that minimizes or eliminates edges, as this will minimize or eliminate the accumulation of material that can promote surface fouling of the second portion. 
     In preferred embodiments, the configuration of the second portion includes a closed outer surface to prevent heat transfer fluid from accumulating within the turbulence-inducing elements. 
     As shown in  FIG. 5 , the shape of the second portion can be a convex shape or a hemi-spheroid or other curvilinear shapes.  FIGS. 6-9  show various additional examples of suitable shapes for the second portion. In one embodiment, as shown in  FIG. 6 , a turbulence-inducing element  350  includes a first portion  360  in the configuration of a conical frustum and a second portion in the configuration of a truncated convex shape or a hemi-spheroid shape. In another embodiment, as shown in  FIG. 7 , a turbulence-inducing element  450  includes a first portion  460  in the configuration of a conical frustum and a second portion  470  comprising a small surface area truncated apex, e.g., with the area of the truncated portion approaching the cross-sectional area of the supporting member. In a further embodiment, as shown in  FIG. 8 , a turbulence-inducing element  550  includes a first portion  560  in the configuration of a conical frustum and a second portion  570  comprising a surface having a rounded apex. In still another embodiment, as shown in  FIG. 9 , a turbulence-inducing element  650  includes a first portion  660  in the configuration of a conical frustum and a second portion  670  comprising surface having a relatively large area truncated apex, e.g., with the area of the truncated portion many times larger than the cross-sectional area of the rod, as shown in  FIG. 9 . 
     One of ordinary skill in the art will appreciate that other configurations can be applied to the second portion of the turbulence-inducing elements according to the present invention, including a cross-sectional area that generally decreases in the direction of fluid flow. 
     The first portion of the turbulence-inducing elements can also be one of many shapes that have a cross-sectional area that generally increases along the direction of fluid flow, with the exception of embodiments shown in  FIGS. 14-15  in which protruding elements are provided on the lateral surface of the first portion to induce additional turbulence and, in one embodiment, to assist in maintaining the turbulence-inducing elements aligned with the longitudinal axis of the tube. For instance, as shown in  FIGS. 5-9 , the first portion can be a conical frustum. In another embodiment, and referring to  FIG. 10 , a turbulence-inducing element  750  includes a first portion  760  in the shape of a conical frustum having a concave lateral surface  768 . In a further embodiment, and referring to  FIG. 11 , a turbulence-inducing element  850  includes a first portion  860  in the shape of a pyramidal frustum. Embodiments of the first portion  860  preferably have bases with at least five sides to minimize pocket areas along the lateral surface of the pyramid, and more preferably have bases with an even number of sides to provide a symmetrical turbulence-inducing element. In an additional embodiment, and referring to  FIG. 12 , a turbulence-inducing element  950  includes a first portion  960  in the shape of a star pyramid frustum. In certain embodiments, including those described with respect to  FIGS. 5-12 , the first portion is configured so that energy loss is minimized along the direction of fluid flow. 
     One of ordinary skill in the art will appreciate that other configurations can be applied to the first portion of the turbulence-inducing elements according to the present invention that have a cross-sectional area that generally increases in the direction of fluid flow. 
     In further embodiments, and referring to  FIGS. 13-15 , one or more of the turbulence-inducing elements used in a tube can include additional features or extensions. In particular, with reference to  FIGS. 13A, 13B, and 13C , a turbulence-inducing element  1050  includes grooves  1072  distributed about the circumference of the element  1050 . The grooves generally begin at the halfway point of the first portion  1060  (which in the embodiment shown is in the configuration of a conical frustum), and extend downstream along its lateral surface to the base of the first portion  1060 . The grooves  1072  begin at a shallow depth, with the depth increasing as the grooves extend downstream, and the grooves end at the intersection between the base of the first portion and the second portion; upon encountering the solid second portion, the streams are directed out toward the tube wall. The grooves are preferably distributed evenly around the conical frustum to maintain the device at the tube center, i.e., to prevent fluid flow from creating asymmetrical forces that could displace the turbulence-inducing elements  150  towards the inner wall of the tube. In a preferred embodiments, the grooves  1072  are straight to avoid rotation-inducing forces on the turbulence-inducing elements, which could cause them to separate from the structural support element. In an alternate embodiment, the symmetrical grooves are curved, with mirrored curved grooves at complementary locations that prevent rotation of the turbulence-inducing elements. 
     In another embodiment, with reference to  FIGS. 14A, 14B, and 14C , a turbulence-inducing element  1150  includes a first portion  1160  having conical studs or spikes  1174  distributed on its lateral surface. These studs  1174  increase turbulence within the tube, thus providing a further enhancement to the anti-fouling and thermal mixing benefits of the turbulence-inducing element of the present invention. The studs can be conical, frustoconical, pyramidal, cylindrical, hemi-cylindrical, or of other suitable shapes. In one embodiment, these projections from the first portion  1160  can extend to the inner tube walls, maintaining the device at the tube center to avoid fouling accumulation. The studs may be cast or molded with the body of the turbulence-inducing element, or can be welded to the body, or can be inserted into holes designed for that purpose and pinned into place. 
     In a further embodiment, as shown in  FIGS. 15A, 15B, and 15C , a turbulence-inducing element  1250  includes grooves  1272  distributed evenly about the circumference of the element  1250  (as described with reference to  FIG. 13 ), and a first portion  1260  having conical studs  1274  distributed on its lateral surface (as described with reference to  FIG. 14 .) 
     The arrangement of the turbulence-inducing elements within a tube can follow the general configuration shown and described above with respect to  FIGS. 3A and 3B . In further embodiments, referring generally to  FIGS. 16, 17 and 18 , additional elements are included at certain locations on the structural support element to provide suitable tension and expansion capabilities to the apparatus of the present invention. In particular,  FIG. 16  is a schematic illustration of an apparatus  1348  including a plurality of turbulence-inducing elements  1350  arranged along the structural support element  1352 , similar to that described with respect to  FIGS. 3A and 3B . In addition, a portion of the structural support element  1352  proximate each end is provided with springs  1380 . The distal ends of the structural support element  1352  are attached to the linking wires at the tube ends in a manner similar to that described with respect to  FIGS. 3A and 3B . 
     The spring  1380  is preferably formed as helical extension spring having coils that are suitably dimensioned and spaced apart so as to minimize or prevent the likelihood of fouling inside the spring and/or on the tube&#39;s inner wall surface proximate the spring. In particular, the outer coil diameter is smaller than the inside tube diameter, with sufficient clearance to prevent scraping of the inner tube wall. Further, the coil spacing, known as the “pitch” of a spring, is sufficiently large to allow fluid to flow through the spring without substantial resistance to minimize or prevent the likelihood of fouling inside the spring. For example, each spring element  1380  can have an outer diameter one-half of the tube&#39;s inside diameter, and the spacing between coils of the spring can be between the tube&#39;s inside diameter and the tube&#39;s outer diameter. It will be appreciated that the spacing between coils depends upon the tension and the coil factor, in addition to any stop ball that may be in place. 
     Advantageously, including one or more spring elements on the turbulence-inducing apparatus of the present invention facilitates installation of the apparatus, allows for stresses to be absorbed thereby reducing the stress load on the structural support element and the end connections, and maintains tension in the apparatus  1348  even under conditions of transferring fluid flow surge. In addition, spring elements can minimize the tendency of the structural support element  152  to expand longitudinally during operation due to high temperatures, and also to minimize the tendency of the turbulence inducing devices  150  to sag toward the bottom surface of the tube due to gravity. The use of the spring elements can such sag and maintain the turbulence-inducing elements the longitudinal centerline of the tube. 
     Referring now to  FIG. 17 , an apparatus  1448  includes a plurality of turbulence-inducing elements  1450  arranged along the structural support element  1452 , with spring elements  1481  near each end. In particular, spring elements  1481  each include a first terminal end  1482  that extends through the coils of the spring elements to the turbulence-inducing element  1450 , and a second terminal end  1483  that also extends through the coils, in the opposite direction as terminal end  1482 , to each of the ends  1484 ,  1486  of the structural support element  1452 . Accordingly, in the event of forces that cause displacement of the turbulence-inducing elements within the tube, the coils of the spring elements  1481  compress and the overall length of the structural support element  1452  increases by the compression length of the spring elements  1481 . Such an arrangement allows for extension of the overall length of the structural support element  1452  while preventing overstretching of the spring elements  1481 . 
     Referring now to  FIG. 18 , a further embodiment of an apparatus for use in a heat exchanger tube for promoting turbulence of transferring fluid and minimizing fouling and other detrimental effects associated with boundary layer accumulation is shown. In particular, apparatus  1548  includes a plurality of turbulence-inducing elements  1550  arranged along a structural support element  1552 . A joint element  1590  is provided between a portion of the structural support element  1552  and a separate spring element  1580 . At each end, the structural support element  1552  extends from the separate spring element  1580  to end  1584 ,  1586 . An additional safety wire  1554  is connected at one end to the joint element  1590  and is connected at its other free end to a safety stop  1558 , which is shown in  FIG. 18  as a ball stop. The additional safety wire  1554  is inserted through the separate spring element  1580  and a sliding opening  1592  fixed to the structural support element  1552 . (For purposes of illustration only, in  FIG. 18  the additional safety wire  1554  is not inserted through the separate spring element  1580  and sliding opening  1592  and, therefore, is not shown in its operational position.) In  FIG. 18 , the sliding opening  1592  is shown as a ring. The benefit of the additional safety wire  1554  is that the safety stop  1558  can act as a brake or safety guard for preventing damage to the separate spring element  1580 , by preventing the spring from stretching beyond a predetermined distance. 
     Referring now to  FIGS. 19A and 19B , a further embodiment of an apparatus for use in a heat exchanger tube for promoting turbulence of transferring fluid and minimizing fouling and other detrimental effects associated with boundary layer accumulation is shown.  FIG. 19A  illustrates a side view of the embodiment in a heat exchanger tube  1610 , and  FIG. 19B  illustrates a downstream end view of the embodiment. In particular, apparatus  1600  includes turbulence-inducing elements that are formed from an assembly of two cones, namely, an outer cone  1620  and an inner cone  1630  nested inside. The two cones are arranged such that an annular gap  1650  is formed between the outer cone  1620  and the inner cone  1630 . 
     The outer cone  1620  is hollow. At the upstream end, the wall of the outer cone  1620  is relatively thin. At the downstream end, the wall of the outer cone  1620  is relatively thick. The inner cone  1630  includes a substantially closed outer surface and is affixed to the central wire in the same manner as described in the earlier embodiments. 
     The inner cone  1630  is connected to the outer cone  1620  via a plurality of longitudinal strips  1640  that are plate welded. It is preferable to use an even number of longitudinal strips to provide a symmetrical load which helps to maintain the cone assembly at the tube center. In a preferred embodiment, four longitudinal strips are utilized. 
     This embodiment will provide for more turbulence during fluid flow for more fluid mixing. In addition, because this configuration allows a portion of the transferring fluid to flow through the annulus gap between the two cones, it creates less erosion to the pipe&#39;s inner surface compared with the previously described embodiments. This embodiment is useful in situations where a large cone diameter is required for generating additional turbulence, which would otherwise cause erosion to the pipe&#39;s inner surface if some fluid was not permitted to flow through the inside of the cone assembly as described above. 
     Referring to  FIG. 20 , the dimensions and spacing of the turbulence-inducing elements illustrated in  FIGS. 19A and 19B  relative to the tube size are described according to the below formulas, according to one example. 
     A gap (g 1 ) is maintained between the inside tube diameter (ID) and the outer diameter of the outer cone  1620 , according to the following formula:
 
 g 1=0.1 *ID   (6)
 
     A gap (g 2 ) is maintained between the outer cone  1620  and the inner cone  1630 , according to the following formula:
 
 g 2=0.1 *ID   (7)
 
     The thickness (t) of the base of the outer cone  1620  is determined according to the following formula:
 
 t= 0.15 *ID   (8)
 
     The diameter (d) of the base of the inner cone  1630  is determined according to the following formula:
 
 d=ID− 2* g 1−2 *g 2−2* t   (9)
 
     The length (L 1 ) of the outer cone  1620  is the same as the length (L 2 ) of the inner cone  1630  and is determined by the following formula:
 
 L =1.5 *ID   (10)
 
     The spacing (S) between adjacent cone assemblies is determined by the following formula:
 
 S =3.5* d /( g 1 +g 2)  (11)
 
     The cone assembly of this embodiment does not include a second portion extending towards the downstream end of the tube as was described with some of the above embodiments. 
     Advantageously, the apparatus of the present invention can be integrated in new or existing heat transfer devices. Unlike prior art systems that attempt to impart turbulence to fluid flowing in a heat transfer device, the apparatus of the present invention can be installed in clean or fouled existing tubes of a heat transfer device. 
     In addition, the turbulence-inducing devices of the present invention are not designed to contact the tube&#39;s inner surface during operation, unlike prior art systems that attempt to impart turbulence to fluid flowing in a heat transfer device. 
     In an alternative embodiment one or more radial supporting devices are installed on, and extend radially from the longitudinal support element to contact the adjacent wall of the tube. The supporting device can be constructed from one or more pieces of wire tubing or other rigid material to provide three or four points of contact with the inner surface of the tube to thereby maintain the structural support element aligned with the longitudinal axis of the tube. The arms can be spaced from each other at intervals of 120° or 90°. The radial supporting devices can also be fabricated by casting metal or plastic materials with radial arms extending from a central hub. 
     Referring again to  FIG. 14 , in yet another alternative embodiment, one or more radially-extending supporting devices are installed on, and extend radially from the surface of, the turbulence-inducing devices to contact the adjacent wall of the tube. In this embodiment, it is preferred that the radially-extending supports are positioned in groups at one or more circumferential locations along the longitudinal axis of the turbulence-inducing element, wherein adjacent circumferential groups are spaced from each other along the central axis of the turbulence-inducing element. In one example, a single circumferential group includes four radial supports, being spaced at substantially 90 degrees from each other. Preferably, each circumferential group includes radial supports in multiples of four. The length of each radial support depends on the shape of the turbulence-inducing element and the location of the radial support on the surface of the turbulence-inducing element. For example, referring to the turbulence-inducing element shown in  FIG. 14 , radial supports in a first circumferential group have lengths that differ from radial supports in an adjacent circumferential group, such that all of the radial supports extend a substantially uniform distance from the longitudinal axis of the element. 
     The special geometry and studs serve to center the device in the tube during operation, preventing or reducing build-up of deposits inside the tubes. Existing turbulent devices that are held in position by contacting the tube inner surface lead to fouling at the contact points with the tube surface. This complicates maintenance because of the difficulty of first removing the turbulent devices without damaging them and then cleaning the tubes. 
     The method and apparatus of the present invention have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be defined by the claims that follow.