Patent Publication Number: US-2023147084-A1

Title: Mitigation of Shell-Side Liquid Maldistribution in Coil Wound Heat Exchanger Bundles

Description:
BACKGROUND 
     The present invention relates to coil wound heat exchangers and methods of cooling and/or liquefying streams of fluid using said coil wound heat exchangers. 
     A coil wound heat exchanger (CWHE) is a type of heat exchanger that is well known in the art. A coil wound heat exchanger comprises one or more tube bundles enclosed within a pressure vessel, the or each tube bundle comprising a plurality of tubes suitable for conveying one or more streams of fluid in heat exchange with a fluid flowing through the pressure vessel externally to the tubes. The tubes of the or each tube bundle are helically wound about a mandrel that extends in an axial direction of the bundle such that said tubes form a plurality of tube layers around said mandrel, each of said tube layers comprising one or more of said helically wound tubes. Typically, each tube layer is spaced from adjacent tube layers (i.e. separated in the radial direction of the bundle) by spacers, which may for example take the form of spacer wires or rods. The one or more tube bundles can be installed in the pressure vessel with appropriate headers and piping for introducing one or more streams of fluid into the tubes and withdrawing said stream(s) from the tubes, and additional piping may be used for fluid flow between bundles. In many instances, coil wound heat exchangers are used to cool and/or liquefy one or more streams of fluid, with said stream(s) of fluid being passed through the “tube-side” of the CWHE, i.e. through the tubes of the one or more tube bundles, and with a cold refrigerant being passed through the “shell-side” of the CWHE, i.e. through the pressure vessel externally to the tubes. Often in such arrangements the refrigerant that is used on the shell-side of the CWHE is a liquid or two-phase refrigerant that vaporizes as it passes through the shell-side over the outside of the tubes. 
     The tube bundle(s) of a CWHE can be fabricated by methods known in the art of CWHE fabrication. The tube bundle is typically constructed from aluminum, stainless steel, copper, or another metal with suitable thermal and mechanical properties. The tube bundle is formed by helically winding groups of tubes, that are typically of constant diameter and similar length, about the mandrel. The mandrel may be a cylindrical pipe having a length, outer diameter, and wall thickness which impart the required structural strength to support the desired size and number of tube layers. In one method of fabrication, solid rods may be wound helically about and in contact with the mandrel, spacers may be installed on the wound rods parallel to the mandrel axis, and then tubes may be helically wound in a first layer in contact with the spacers. Numerous additional tube layers are then formed in the radial direction, each layer typically being separated from adjacent layers by spacer wires or rods that run parallel to the mandrel axis or helically around the mandrel axis. Winding of the tubes can be done with the mandrel axis oriented vertically in a fixed position while the tubes are wound onto the coil bundle from reels adapted to move circumferentially about the axis, and also to move upward and downward parallel to the axis. Alternatively, the tube bundles can be built by rotating the mandrel and bundle on a lathe about a fixed horizontal axis while tubes are wound onto the coil from reels adapted to move axially, i.e. from side to side. 
     The or each tube bundle of a CWHE may therefore be characterized by a number of dimensional parameters, such as mandrel outer diameter, spacer thickness (and resulting radial spacing between tube layers), number of spacers, number of tubes, tube inner diameter, tube outer diameter, bundle outer diameter, tube length, tube pitch (distance between tubes in a given layer), and tube winding angle. Certain of these parameters also influence the basic fluid flow and heat transfer characteristics of the tube bundle. 
     Coil wound heat exchangers are often used in the process industries for heating or cooling fluid streams at high heat transfer rates, which requires a large heat transfer area. For example, they are often used in the production of liquefied natural gas (LNG), where a large heat transfer area is needed for indirect heat transfer between the natural gas feed stream, which is passed through the tube-side of the CWHE, and the cold refrigerant, which is passed through the shell-side of the CWHE. 
     By way of example,  FIG.  1    shows a coil wound heat exchanger arranged for liquefaction of natural gas in a known manner. This particular arrangement utilizes a CWHE having two tube bundles for the cooling and liquefaction of a natural gas feed (which may have been pre-treated and/or pre-cooled prior to being introduced into and cooled and liquefied in the coil wound heat exchanger). The CWHE  1  comprises a pressure vessel  3 , containing a warm heat exchange zone  5 , and cold heat exchange zone  9 . A first tube bundle is utilized in the warm heat exchange zone  5  in which a natural gas feed stream, introduced into the tube bundle via line  11 , is initially cooled in tube circuit  13  against a refrigerant (later described) flowing over the first tube bundle through the shell side of the CWHE. Tube circuit  13  is formed of multiple tubes which are part of the first tube bundle, wherein said bundle also includes tube circuits  31  and  39  as described later (tube circuits  13 ,  31  and  39  together representing the tube side of the first tube bundle). Cooled and at least partially condensed natural gas withdrawn via line  15  from the first tube bundle may optionally be reduced in pressure, for example across throttling valve  17 . The natural gas then flows via line  19  into tube circuit  21  of a second tube bundle in the cold heat exchange zone  9 , in which the natural gas is further cooled before being withdrawn as LNG product via line  23 . 
     The refrigerant that is in used in this example is a refrigerant that is condensed before being vaporized in the shell side of the CWHE, such as for example a mixed refrigerant (a multicomponent refrigerant comprising light hydrocarbons and optionally nitrogen, said light hydrocarbons typically comprising methane, ethane, ethylene and/or propane). A two-phase stream of compressed refrigerant is supplied via line  25  from a refrigerant compression system (not shown) and flows into phase separator  27 . Liquid refrigerant is withdrawn via line  29 , subcooled in tube circuit  31 , and reduced in pressure across throttling valve  33 . Optionally, a hydraulic expansion turbine may be used to extract work from the refrigerant liquid prior to throttling valve  33 . The refrigerant from throttling valve  33  is combined with refrigerant flowing downward from cold heat exchange zone  9  (described later) and the combined refrigerant is distributed via distributor  35 . The combined refrigerant flows downward over the first tube bundle through the shell side of the CWHE while vaporizing and warming to provide refrigeration for cooling the natural gas in tube circuit  13  as earlier described. In addition, the vaporizing refrigerant provides refrigeration for subcooling the liquid refrigerant in tube circuit  31  as earlier described, and refrigeration for cooling vapor refrigerant in tube circuit  39  (described below). 
     Vapor refrigerant is withdrawn from separator  27  via line  37 , is cooled and may be partially condensed in tube circuit  39  in warm heat exchange zone  5 , and finally passes through tube circuit  41  of the second tube bundle in the cold heat exchange zone  9 , wherein it is liquefied and optionally subcooled. This refrigerant is reduced in pressure across throttling valve  43  and distributed via distributor  45  in cold heat exchange zone  9 . This refrigerant flows downward over the second tube bundle through the shell side of the CWHE and vaporizes to provide refrigeration for cooling the natural gas in tube circuit  21  as earlier described. In addition, the vaporizing refrigerant provides refrigeration for cooling the refrigerant in tube circuit  41 . Distributor  45  is shown schematically and may include means for phase separation and distribution of separate vapor and liquid refrigerant streams to heat exchange zone  9 . Two-phase refrigerant flowing downwards from the shell side of cold heat exchange zone  9  enters warm heat exchange zone  5  and joins with the refrigerant discharged from throttling valve  33 , whereupon the combined refrigerant is distributed via distributor  35  and flows downward over the first tube bundle through the shell side of the warm heat exchange zone  5 , as described earlier. The refrigerant reaching the bottom of heat exchanger pressure vessel  3  is typically totally vaporized, and is withdrawn as vapor via line  47 . This vapor is compressed in the refrigerant compression system (not shown) and optionally precooled to provide the two-phase cooled compressed refrigerant via line  25  as earlier described. 
     As noted above, tube circuits  13 ,  31 , and  39  are parts of the first tube bundle that is installed in the warm heat exchange zone  5  of the pressure vessel  3 , and tube circuits  21  and  41  are parts of the second tube bundle which is installed in the cold heat exchange zone  9  of heat exchanger pressure vessel  3 . The tubes in each of tube circuits  13 ,  31 , and  39  typically are aggregated at each end, for example by gathering the multiple tubes from each circuit into one or more tube sheets which can be connected to inlet and outlet lines. Likewise, the tubes in each of tube circuits  21  and  41  are typically aggregated at each end, for example by gathering the multiple tubes from each circuit into one or more tube sheets which can be connected to inlet and outlet lines. The first tube bundle and second tube bundle can each be fabricated by methods known in the art of coil wound heat exchanger fabrication as discussed above. 
     An improvement to a CWHE of the type described above and depicted in  FIG.  1    is described in EP1367350B2. As vaporizing refrigerant flows downward through the shell side over the tube bundle in warm heat exchange zone, the net vapor fraction increases and the heat transfer mechanism changes gradually from predominantly two-phase boiling heat transfer at the cold or top end to single-phase vapor heat transfer at the warm or bottom end, such that the nature of the heat transfer mechanism changes significantly from top to bottom of the bundle. In order to address this, EP1367350B2 proposes splitting the tube bundle in warm heat exchange zone  5  into at least two smaller coil wound tubing bundles that differ from each other in terms of one or more dimensional parameters, selected from the mandrel outer diameter, spacer thickness, number of spacers, number of tubes, tube inner diameter, tube outer diameter, tube length, tube pitch, and tube winding angle, in order that each bundle can be designed to match more closely the nature of the heat exchange and fluid flow phenomena which occur across each bundle. 
     International patent application WO 2020/074117 notes that conventional CWHE tube bundles are designed and manufactured so that the tube(s) in each tube layer have a uniform tube pitch throughout the layer (i.e. the distance between tubes in a given layer remains constant and the same throughout said layer), resulting in an evenly distributed heating surface and tube bundle weight over the axial length of the tube bundle. It furthermore notes, that the heating power requirements at different positions of the tube bundle may, however, differ depending on the flow regime of the fluid on the shell side of the CWHE space, and that structural-mechanical problems may also arise at the end of the tube bundle from the load during the winding process. In order to address these issues it proposes that the tube bundle instead be designed and manufactured with a tube pitch that increases (or decreases) monotonically in the axial direction in at least a section of the tube bundle. 
     US patent application US 2019/0011191 A1 describes an arrangement for addressing the issue of liquid maldistribution on the shell side of the CWHE that utilizes a vaporizing refrigerant. US 2019/0011191 A1 notes that, in many cases, the liquid phase of the refrigerant becomes diverted in the direction of the outer tube layers of the tube bundle as the refrigerant descends through the bundle, which can lead to a local deficit in the supply of liquid (vaporizing) refrigerant to the tube bundle in the region of the inner tube layers of the tube bundle, which in turn results in reduced heat transfer performance. In order to address this, it suggests that the CWHE is provided with a gas discharge device for discharging vapor phase refrigerant from the shell side of the CWHE in the region of the inner tube layers, and/or is provided with a gas supply device for introducing vapor phase refrigerant into the shell side of the CWHE in the region of the outer tube layers, so as to reduce or avoid pressure drop in the radial direction, thereby reducing or avoiding the deflection of the liquid. 
     BRIEF SUMMARY 
     Disclosed herein are coil wound heat exchangers in which one or more tube layers of the tube bundle are provided with a non-uniform tube winding angle and tube pitch in order to facilitate the equalization of radial pressure imbalances on the shell side of the coil wound heat exchanger (CWHE), thereby reducing radial maldistribution of fluid on the shell side of the CWHE and improving the heat transfer efficiency of the CWHE in operation. More specifically, the CWHE may have a greater tube winding angle and tube pitch at a middle location (axially) of the tube bundle than at locations towards the axial ends of the bundle. For example, in a cold end up CWHE the tube winding angle and tube pitch may increase from the bundle top to a maximum in the middle of the bundle and then decrease toward the bottom of the bundle. Also disclosed herein are methods of cooling and/or liquefying streams of fluid using such coil wound heat exchangers. 
     Several preferred aspects of the systems and methods according to the present invention are outlined below. 
     Aspect 1: A coil wound heat exchanger (CWHE) comprising a tube bundle enclosed within a pressure vessel, the tube bundle comprising a plurality of tubes for conveying one or more streams of fluid in heat exchange with a fluid flowing through the pressure vessel externally to the tubes, said tubes being helically wound about a mandrel extending in an axial direction of the bundle so as to form a plurality of tube layers around said mandrel, each of said tube layers comprising one or more of said helically wound tubes, wherein at least one of the tube layers has a first tube winding angle and a first tube pitch at a first axial location of the layer towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial location of the layer towards the opposite axial end of the bundle, and a third tube winding angle and third tube pitch at a third axial location of the layer between the first axial location and second axial location, the third tube winding angle being greater than the first tube winding angle and greater than the second tube winding angle and the third tube pitch being greater than the first tube pitch and greater than the second tube pitch. 
     Aspect 2: A CWHE according to Aspect 1, wherein all or substantially all of the tube layers have a first tube winding angle and a first tube pitch at a first axial location of the layer towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial location of the layer towards the opposite axial end of the bundle, and a third tube winding angle and third tube pitch at a third axial location of the layer between the first axial location and second axial location, for each of said layers the third tube winding angle being greater than the first tube winding angle and greater than the second tube winding angle and the third tube pitch being greater than the first tube pitch and greater than the second tube pitch. 
     Aspect 3: A CWHE according to Aspect 2, wherein the third axial locations of each of said tube layers are at the same or substantially the same axial position of the bundle. 
     Aspect 4: A CWHE according to in any one of Aspects 1 to 3, wherein in at least one of or all or substantially all of the tube layers the third tube winding angle at the third axial location is equal to or larger than any tube winding angle at any other axial location of the layer between the first axial location and the second axial location of the layer, and the third tube pitch at the third axial location is equal to or greater than any tube pitch at any other axial location of the layer between the first axial location and the second axial location of the layer. 
     Aspect 5: A CWHE according to any one of Aspects 1 to 4, wherein in at least one of or all or substantially all of the tube layers the third axial location of the layer is at a point that falls between 10% and 90% of the distance in the axial direction of the bundle between the first axial location and second axial location. 
     Aspect 6: A CWHE according to any one of Aspects 1 to 5, wherein in at least one of or all or substantially all of the tube layers the third tube winding angle is at least 5% greater than the first tube winding angle and at least 5% greater than the second tube winding angle. 
     Aspect 7: A CWHE according to any one of Aspects 1 to 6, wherein in at least one of or all or substantially all of the tube layers the third tube winding angle is at least 0.5 degrees greater than the first tube winding angle and is at least 0.5 degrees greater than the second tube winding angle. 
     Aspect 8: A CWHE according to any one of Aspects 1 to 7, wherein in at least one of or all or substantially all of the tube layers the third tube pitch is at least 5% greater than the first tube pitch and is at least 5% greater than the second tube pitch. 
     Aspect 9: A CWHE according to any one of Aspects 1 to 7, wherein in at least one of or all or substantially all of the tube layers the third tube pitch is at least 10% greater than the first tube pitch and is at least 10% greater than the second tube pitch. 
     Aspect 10: A CWHE according to any one of Aspects 1 to 9, wherein the CWHE is a vertically oriented CWHE with the axial direction of the bundle being vertical or substantially vertical, the CWHE further comprising a refrigerant dispenser located above the tube bundle for dispensing a refrigerant into the pressure vessel externally to the tubes so that the refrigerant flows downwards through the pressure vessel over and through the tube bundle externally to the tubes. 
     Aspect 11: A CWHE according to Aspect 10, wherein the CWHE has an inlet configured to introduce the one or more streams of fluid into the plurality of tubes of the tube bundle at the bottom of the tube bundle and an outlet configured to withdraw the one or more streams of fluid from the tubes at the top of the tube bundle such that the one or more streams flow upwards through the tubes of the tube bundle. 
     Aspect 12: A CWHE according to Aspect 10 or 11, wherein the CWHE is configured to use a liquid or two-phase refrigerant that vaporizes as it flows downwards through the pressure vessel over and through the tube bundle externally to the tubes. 
     Aspect 13: A method of cooling and/or liquefying one or more streams of fluid via heat exchange with a refrigerant using a CWHE according to any one of Aspects 1 to 12, wherein the method comprises passing the one or more streams of fluid through the plurality of tubes of the tube bundle and passing the refrigerant through the pressure vessel externally to the tubes. 
     Aspect 14: A method according to Aspect 13, wherein the one or more streams of fluid that are to be cooled and/or liquefied comprise a natural gas stream. 
     Aspect 15: A method according to Aspect 13 or 14, wherein the refrigerant comprises a mixed refrigerant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic illustration of a coil wound heat exchanger (CWHE) arranged for liquefaction of natural gas in accordance with the prior art. 
         FIG.  2    is a graph plotting tube pitch versus location along the tube bundle axis for a tube layer of a CWHE in accordance with the prior art. 
         FIG.  3    is a graph plotting tube winding angle versus location along the tube bundle axis for the tube layer of  FIG.  2   . 
         FIG.  4    is a perspective section view of a middle portion of the tube layer of  FIGS.  2  and  3   . 
         FIG.  5    is a top-down section view of the middle portion of the tube layer depicted in  FIG.  4   . 
         FIG.  6    is a graph plotting tube pitch versus location along the tube bundle axis for a tube layer of a CWHE in accordance with an embodiment of the present invention. 
         FIG.  7    is a graph plotting tube winding angle versus location along the tube bundle axis for the tube layer of  FIG.  6   . 
         FIG.  8    is a perspective section view of a middle portion of the tube layer of  FIGS.  6  and  7   . 
         FIG.  9    is a top-down section view of the middle portion of the tube layer depicted in  FIG.  8   . 
         FIG.  10    is a graph plotting tube pitch versus location along the tube bundle axis for a tube layer of a CWHE in accordance with another embodiment of the present invention. 
         FIG.  11    is a graph plotting tube winding angle versus location along the tube bundle axis for the tube layer of  FIG.  10   . 
         FIG.  12    is a graph schematically depicting the tubes of a tube layer, of CWHE in accordance with another embodiment of the present invention, when “unwound”, with the location along the tube bundle axis plotted on the vertical axis and multiples of the tube layer circumference plotted on the horizontal axis. 
         FIG.  13    is a graph schematically depicting the tubes of the tube layer of  FIG.  12    when “unwound”, with the location along the tube bundle axis plotted on the vertical axis and fractions of the tube layer circumference plotted on the horizontal axis. 
         FIG.  14    is a graph schematically depicting the tubes of a tube layer, of CWHE in accordance with another embodiment of the present invention, when “unwound”, with the location along the tube bundle axis plotted on the vertical axis and multiples of the tube layer circumference plotted on the horizontal axis. 
         FIG.  15    is a graph schematically depicting the tubes of a tube layer, of CWHE in accordance with another embodiment of the present invention, when “unwound”, with the location along the tube bundle axis plotted on the vertical axis and multiples of the tube layer circumference plotted on the horizontal axis. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are coil wound heat exchangers, and methods of cooling and/or liquefying streams of fluid using said coil wound heat exchangers, in which one or more tube layers of the tube bundle are provided with a non-uniform tube winding angle and tube pitch in order to facilitate the equalization of radial pressure imbalances on the shell side of the coil wound heat exchanger (CWHE), thereby reducing radial maldistribution of fluid (particularly liquid) on the shell side of the CWHE and improving the heat transfer efficiency of the CWHE in operation. 
     As used herein and unless otherwise indicated, the articles “a” and “an” mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used. 
     Where used herein to identify recited features of a method or system, the terms “first”, “second”, “third” and so on, are used solely to aid in referring to and distinguishing between the features in question, and are not intended to indicate any specific order of the features, unless and only to the extent that such order is specifically recited. 
     As used herein, the “tube winding angle” of a tube of a tube layer at a particular location refers to the included (acute) angle formed between the tube axis of the tube at that location and a plane perpendicular to the tube bundle axis. The tube winding angle of a tube layer at a particular axial location refers, where the layer is formed of a single tube, to the tube winding angle of the tube of the layer at that axial location or refers, where the layer is formed of more than one tube, to the average (arithmetic mean) of the tube winding angles of the tubes of the layer at that axial location. 
     As used herein, the “tube pitch” of a tube of a tube layer at a particular location refers to the average (arithmetic mean) of the center-to-center distances between the tube axis of the turn of the tube at that location and tube axes of each of the two adjacent turns of tube in the layer, the center-to-center distance between two turns of tube being measured perpendicular to the axes of the turns of tubes in question. Depending on the number of tubes that form the tube layer, the two turns of tube that are adjacent to the turn of tube at the location in question may be turns of the same tube or turns of a different tube or pair of tubes. More specifically, where the tube layer is formed of a single tube, the two turns of tube that are adjacent to the turn of a tube at a particular location will be the two adjacent turns of the same single tube that forms the layer; where the tube layer is formed of two tubes, the two turns of tube that are adjacent to the turn of a tube at a particular location will be the two adjacent turns of the other tube forming the layer; and where the tube layer is formed of three or more tubes, the two turns of tube that are adjacent to the turn of a tube at a particular location will be the adjacent turns of the adjacent tubes in the layer. The tube pitch of a tube layer at a particular axial location refers, where the layer is formed of a single tube, to the tube pitch of the tube of the layer at that axial location or refers, where the layer is formed of more than one tube, to the average (arithmetic mean) of the tube pitches of the tubes of the layer at that axial location. 
     As used herein, the “tube axis” of a tube or the turn of a tube refers to the central longitudinal axis of said tube or turn of tube. 
     As used herein, the “tube bundle axis” refers to the central longitudinal axis of the tube bundle. Typically, the tube layers of the tube bundle form a set layers positioned coaxially about the mandrel, such that the tube bundle and mandrel share the same central longitudinal axis (within manufacturing tolerances). Typically, the pressure vessel that contains the tube bundle is a generally cylindrical vessel that is likewise positioned coaxially with the tube layers and mandrel. 
     As used herein, the “axial direction of the bundle” refers to the direction along the tube bundle axis; an “axial location” or “axial position” of the bundle refers to a location or position that is at a specific point along the tube bundle axis, but (unless otherwise indicated) does not refer to any specific circumferential location of the tube bundle at that axial point along the tube bundle axis; and the “axial ends” of the bundle refer to the ends of the bundle at either end of the tube bundle axis. 
     Solely by way of example, certain exemplary embodiments of the invention will now be described with reference to the Figures. 
     As described above, a coil wound heat exchanger (CWHE) is a type of heat exchanger that is well known in the art and that can be fabricated by methods that are well known in the art. A CWHE comprises one or more tube bundles that are enclosed within a pressure vessel, the or each tube bundle comprising a plurality of tubes suitable for conveying one or more streams of fluid in heat exchange with a fluid flowing through the pressure vessel externally to the tubes. The tubes of the or each tube bundle are helically wound about a mandrel that extends in an axial direction of the bundle such that said tubes form a plurality of tube layers around said mandrel, each of said tube layers comprising one or more of said helically wound tubes. Typically, each tube layer is spaced from adjacent tube layers (i.e. separated in the radial direction of the bundle) by spacers, which may for example take the form of spacer wires or rods. At each axial end of the tube bundle the ends or “tails” of the one or more helically wound tube or tubes forming each tube layer are typically aggregated or gathered together into one or more groups with the end or “tails” of the tubes forming the other layers so that each tail in the group can be inserted and fixed into a tube sheet, so as to thereby form a group of inlets or outlets. The one or more tube bundles can be installed in the pressure vessel with appropriate headers and piping for introducing one or more streams of fluid into the tubes and withdrawing said stream(s) from the tubes, via said inlets and outlets, and additional piping may be used for fluid flow between bundles. 
     In many instances coil wound heat exchangers are used for cooling and/or liquefying one or more streams of fluid, with said stream(s) of fluid being passed through the tubes of the one or more tube bundles of the CWHE (i.e. through the “tube-side” of the CWHE), and with a cold refrigerant being passed through the pressure vessel externally to the tubes (i.e. through the shell-side of the CWHE). Often, the CWHE is employed in a “cold end up” orientation, whereby the CWHE is vertically oriented with the tube bundle axis being vertical or substantially vertical, the CWHE having a refrigerant dispenser located above the or each tube bundle (or located above at least the top most bundle in the event that more than one bundle is present) for dispensing a refrigerant into the pressure vessel externally to the tubes so that the refrigerant flows downwards through the pressure vessel over and through the tube bundle(s) externally to the tubes, and the CWHE being configured such that the one or more streams of fluid that are to be cooled and/or liquefied are introduced at the bottom of the tube bundle(s), flow upwards through the tubes of the tube bundle(s), and are withdrawn from the top of the tube bundle(s). Often the CWHE is configured to use a liquid or two-phase refrigerant that vaporizes as it flows downwards through the pressure vessel over and through the tube bundle externally to the tubes. 
     Such coil wound heat exchangers can be used in a variety of different processes, but in certain preferred embodiments of the present invention they may be used for cooling and/or liquefying a natural gas stream using a mixed refrigerant. A suitable exemplary arrangement of a CWHE for liquefying a natural gas stream is shown in  FIG.  1   , described above. 
     In the typical prior art CWHE, the tube(s) in each tube layer have a uniform tube winding angle and tube pitch throughout the layer, as is illustrated in further detail by  FIGS.  2  to  5   . 
     More specifically,  FIG.  2    is a graph plotting tube pitch versus location along the tube bundle axis and  FIG.  3    is a graph plotting tube winding angle versus location along the tube bundle axis for a tube layer of a tube bundle of a CWHE in accordance with the prior art. For the sake of simplicity, in this example the tube layer consists of a single helically wound tube, although in practice a tube layer will often comprise a plurality of helically wound tubes. Location along the tube bundle axis is plotted on the X-axis (horizontal axis) of both Figures, with 0 representing a first axial location of the tube layer towards one axial end of the bundle, 1 representing a second axial location of the layer towards the opposite axial end of the bundle, and the numbers between 0 and 1 representing the fractions of the total axial distance (i.e. distance as measured in the axial direction of the bundle) from the first axial location to the second axial location. Normalized values of tube pitch are plotted on the Y-axis (vertical axis) of  FIG.  2   , the normalized tube pitch of a tube at a location being defined herein as the ratio of the tube pitch at said location over the tube pitch of the tube at the first axial location. Normalized values of tube winding angle are plotted on the Y-axis (vertical axis) of  FIG.  3   , the normalized tube winding angle of a tube at a location being defined herein as the ratio of the tube winding angle at said location over the tube winding angle of the tube at the first axial location. As can be seen, as both the winding angle and pitch of the helically wound tube forming the tube layer are kept constant throughout the layer, the tube pitch remaining the same at each axial location between the first and second axial locations, and the tube winding angle remaining the same at each axial location between the first and second axial locations. 
       FIG.  4    and  FIG.  5    are, respectively, perspective and top-down section views of a middle portion (axially) of the tube layer of  FIGS.  2  and  3   . The views show the tube layer, having a diameter D, cut-away along a plane containing the tube bundle axis Z, the center-to-center distances (measured parallel to the tube bundle axis) between the turns of the tube at the angular coordinate ⊖ about the tube bundle axis corresponding to the point on the right hand side of the tube layer (as viewed in  FIGS.  4  and  5   ) at which the tube layer has been cut-away being labelled as p 1  to p 7 . As can be seen, because the tube winding angle remains constant at each axial coordinate along the tube bundle axis Z (and thus also at every angular coordinate ⊖ about the tube bundle axis Z), the center-to-center distances p 1  to p 7  between each of the turns of the tube likewise remain constant and equal, and likewise the tube pitch remains constant at each axial coordinate along the tube bundle axis Z (and thus also at every angular coordinate ⊖ about the tube bundle axis Z). It should be noted that although the center-to-center distances p 1  to p 7  depicted in  FIG.  4    and  FIG.  5    do not correspond exactly to the tube pitches of the turns of tube (since distances p 1  to p 7  are measured parallel to the tube bundle axis, whereas tube pitch is measured perpendicular to the axes of the turns of tube), the two sets of values are closely correlated. 
     In a CWHE tube bundle, heat transfer occurs between the tube side stream(s) and shell side stream (as noted above), and uniform distribution of streams in both shell-side and tube-side is necessary in order to achieve the best heat transfer performance. However, it is known that radial maldistribution of fluid may occur in the shell-side of typical prior art CWHEs where the shell-side stream has two-phase flow (i.e. contains both liquid and vapor phases). More specifically, pressure imbalance can occur in the shell-side of the CWHE in the radial direction of the tube bundle when shell-side vapor and liquid concurrently flow downward over the tubes of the tube bundle, and this radial pressure imbalance can divert liquid on the shell-side in the radial direction of the tube bundle, resulting in liquid maldistribution on the shell-side of the CWHE. Such maldistribution not only reduces the heat transfer efficiency of the CWHE in operation, but also may force the use of a larger CWHE with a greater heat transfer surface area than theoretically required in order to allow for losses in heat transfer efficiency in practice. 
     The present inventors have now found that such radial pressure imbalances on the shell side of the CWHE can be equalized, and radial maldistribution of liquid on the shell side of the CWHE thereby reduced and the heat transfer efficiency of the CWHE thereby improved, by modifying the design of the CWHE tube bundle such that the CWHE uses instead tube bundle(s) in which one or more (and preferably all or substantially all) of the tube layers of the tube bundle have a non-uniform tube pitch, whereby the CWHE has a greater tube pitch at a middle location (axially) of the tube bundle than at locations towards the axial ends of the bundle. This non-uniform tube pitch is achieved by forming the or each of said tube layers from one or more tubes that have a non-uniform tube winding angle, whereby the tube layer(s) have a greater tube winding angle at a middle location (axially) of the tube bundle than at locations towards the axial ends of the bundle. This is because, for a given helically wound tube, increasing the winding angle of said tube increases the center-to-center distance between the turns of that tube. Thus, a greater tube winding angle at a particular location can provide a greater tube pitch at that location. 
     Accordingly, the coil wound heat exchangers according to the present invention differ from the typical prior art CWHE in that in a CWHE according to the present invention at least one of (and preferably all or substantially all of) the tube layers of the tube bundle have a first tube winding angle and a first tube pitch at a first axial location of the layer towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial location of the layer towards the opposite axial end of the bundle, and a third tube winding angle and third tube pitch at a third axial location of the layer between the first axial location and second axial location, the third tube winding angle being greater than the first tube winding angle and greater than the second tube winding angle and the third tube pitch being greater than the first tube pitch and greater than the second tube pitch. 
     The non-uniform tube pitch in the coil wound heat exchangers according to the present invention takes account of the variation in liquid to vapor ratio in the two-phase shell side fluid as it flows through the shell side in the axial direction of the tube bundle. In a vertically oriented CWHE, the two-phase shell side flow at the top of bundle contains little vapor and a large amount of liquid, while the two-phase shell side flow at the bottom of bundle contains little or no liquid and large amounts of vapor. The strongest vapor and liquid interaction occurs in the middle of bundle where the two-phase shell side flow contains comparable amounts of vapor and liquid. This strongest vapor and liquid interaction with heat transfer can generate the greatest radial pressure imbalance in the shell-side, which, in turn, drives the most severe liquid maldistribution therein. However, a larger tube pitch and hence a larger spacing between adjacent turns of tube has been found to equalize the radial pressure imbalance more effectively. Thus, a larger tube pitch in the middle of bundle has been found to better equalize the greater radial pressure imbalance at the middle of the bundle, hence providing better performance. 
       FIGS.  6  to  9    illustrate the variation in tube pitch and tube winding angle in a tube layer of a CWHE in accordance with one embodiment of the present invention. 
     More specifically,  FIG.  6    is a graph plotting tube pitch versus location along the tube bundle axis and  FIG.  7    is a graph plotting tube winding angle versus location along the tube bundle axis for the tube layer in accordance with this embodiment. For the sake of simplicity, in this embodiment the tube layer consists of a single helically wound tube, although in practice a tube layer comprising a plurality of helically wound tubes may equally be used. Location along the tube bundle axis is plotted on the X-axis (horizontal axis) of both Figures, with 0 representing a first axial location of the tube layer towards one axial end of the bundle (preferably towards the bottom end of the bundle in a vertically oriented CWHE), 1 representing a second axial location of the layer towards the opposite axial end of the bundle (i.e. preferably towards the top end of the bundle in a vertically oriented CWHE), and the numbers between 0 and 1 representing the fractions of the total axial distance (i.e. distance as measured in the axial direction of the bundle) from the first axial location to the second axial location. Normalized values of tube pitch are plotted on the Y-axis (vertical axis) of  FIG.  6   , and normalized values of tube winding angle are plotted on the Y-axis (vertical axis) of  FIG.  7   . The first axial location may in particular be an axial location at one end of the tube layer and the second axial location may in particular be an axial location at the other end of the tube layer such that  FIGS.  6  and  7    depict the tube pitch and tube winding angle over the entire axial length of the tube bundle excluding only the ends of the tube bundle where the tails of the tubes are gathered together for insertion and fixing into tube sheets. 
     As can be seen, in this exemplary embodiment both the winding angle and pitch of the helically wound tube forming the tube layer gradually increase moving (in the axial direction bundle) from the first axial location of the tube layer to a third axial location at which the tube winding angle and tube pitch reach a maximum, which third axial location falls, in this particular example, at a point that is 0.4 (i.e. 40% or ⅖ th ) of the total axial distance from the first axial location to the second axial location. Both the winding angle and pitch of the helically wound tube forming the tube layer then gradually decrease moving (in the axial direction bundle) from the third axial location of the tube layer to the second axial location of the tube layer. In this particular example the normalized tube pitch at the first axial location (i.e. the first tube pitch) is by definition 1.0 and the normalized tube winding angle at the first axial location (i.e. the first tube winding angle) is also 1.0, the normalized tube pitch at the second axial location (i.e. the second tube pitch) is 0.925 and the normalized tube winding angle at the second axial location (i.e. the second tube winding angle) is nearly the same value, and the normalized tube pitch at the third axial location (i.e. the third tube pitch) is 1.11 and the tube winding angle at the third axial location (i.e. the third tube winding angle) is nearly the same value. The normalized winding angle is close to the normalized tube pitch because at the small winding angles used for this example, less than 20°, the relationship between pitch and winding angle is nearly linear. At larger winding angles the value of normalized winding angle will differ by greater amount from the normalized tube pitch, but the shape of one curve will be similar to the other. 
     It should be noted that the specific distance of the third axial location from the first and second axial locations and specific tube pitch and tube winding angles in the above embodiment represent only one exemplary set of values, and other values may suitably be employed. However, it is preferred, in this and other embodiments of the invention: that the third axial location of the layer at which the tube winding angle and tube pitch are each at a maximum is at a point that falls between 10% and 90%, and more preferably between 20% and 80% or between 30% and 70%, of the distance in the axial direction of the bundle between the first axial location and second axial location; that the third tube winding angle is at least 5% greater than the first tube winding angle and at least 5% greater the second tube winding angle, and/or is at least 0.5 degrees greater than the first tube winding angle and is at least 0.5 degrees greater the second tube winding angle; and that the third tube pitch is at least 5% greater, and more preferably at least 10% greater than the first tube pitch and is at least 5% greater, and more preferably at least 10% greater than the second tube pitch. Typically, the tube layer will have (at any location of the between the first axial location and the second axial location) a minimum tube winding angle of 2°, a maximum tube winding angle of 25°, and a maximum tube pitch of 2.0 times the tube diameter. 
       FIG.  8    and  FIG.  9    are, respectively, perspective and top-down section views of a middle portion (axially) of the tube layer of  FIGS.  6  and  7   , which portion encompasses the third axial location of said layer. The views show the tube layer, having a diameter D, cut-away along a plane containing the tube bundle axis Z, the center-to-center distances (measured parallel to the tube bundle axis) between the turns of the tube at the angular coordinate ⊖ about the tube bundle axis corresponding to the point on the right hand side of the tube layer (as viewed in  FIGS.  8  and  9   ) at which the tube layer has been cut-away being labelled as p 1  to p 7 . As can be seen, travelling along the tube bundle axis in the direction of the indicated arrow the tube winding angle increases, reaches a maximum (at the aforementioned third axial location, not labelled in  FIGS.  8  and  9   ), and then decreases, as a result of which the center-to-center distances p 1  to p 7  between each of the turns of the tube likewise increases, reaches a maximum, and then decreases (such that p 1 &lt;p 2 &lt;p 3 &lt;p 4 &gt;p 5 &gt;p 6 &gt;p 7 ), and likewise travelling along the tube bundle axis in the direction of the indicated arrow the tube pitch also increases, reaches a maximum (at the aforementioned third axial location), and then decreases. It should again be noted that although the center-to-center distances p 1  to p 7  depicted in  FIG.  8    and  FIG.  9    do not correspond exactly to the tube pitches of the turns of tube (since distances p 1  to p 7  are measured parallel to the tube bundle axis, whereas tube pitch is measured perpendicular to the axes of the turns of tube), the two sets of values are closely correlated. It should also be noted that the specific sizes of the center-to-center distances p 1  to p 7 , and correspondingly the tube pitches, depicted in  FIG.  9    are purely illustrative and are not necessarily to scale. 
       FIGS.  10  and  11    illustrate the variation in tube pitch and tube winding angle in a tube layer of a CWHE in accordance with another embodiment of the present invention. 
     More specifically,  FIG.  10    is a graph plotting tube pitch versus location along the tube bundle axis and  FIG.  11    is a graph plotting tube winding angle versus location along the tube bundle axis for the tube layer in accordance with this embodiment. For the sake of simplicity, in this embodiment the tube layer again consists of a single helically wound tube, although in practice a tube layer comprising a plurality of helically wound tubes may again be used. Location along the tube bundle axis is plotted on the X-axis (horizontal axis) of both Figures, with 0 representing a first axial location of the tube layer towards one axial end of the bundle (preferably towards the bottom end of the bundle in a vertically oriented CWHE), 1 representing a second axial location of the layer towards the opposite axial end of the bundle (i.e. preferably towards the top end of the bundle in a vertically oriented CWHE), and the numbers between 0 and 1 representing the fractions of the total axial distance from the first axial location to the second axial location. Normalized values of tube pitch are plotted on the Y-axis (vertical axis) of  FIG.  10   , and normalized values of tube winding angle are plotted on the Y-axis (vertical axis) of  FIG.  11   . The first axial location may in particular be an axial location at one end of the tube layer and the second axial location may in particular be an axial location at the other end of the tube layer such that  FIGS.  10  and  11    depict the tube pitch and tube winding angle over the entire axial length of the tube bundle excluding only the ends of the tube bundle where the tails of the tubes are gathered together for insertion and fixing into tube sheets. 
     As can be seen, in this exemplary embodiment both the winding angle and pitch of the helically wound tube forming the tube layer initially remain constant, staying the same from the first axial location of the tube layer to a fourth axial location that is, in this particular example, at a point that is 20% of the total axial distance from the first axial location to the second axial location. At this fourth axial location, both the winding angle and pitch of the tube undergo a step change to a higher (maximum) value, and then remain at this higher (maximum) value until a fifth axial location that is, in this particular example, at a point that is 60% of the total axial distance from the first axial location to the second axial location. At this fifth axial location, both the winding angle and pitch of the tube undergo another step change, this time to a lower value, and remain at this lower value from the fifth axial location to the second axial location of the tube layer. Accordingly, in this embodiment, the third axial location of the tube layer, at which the tube winding angle and tube pitch are each at a maximum, can be considered to be any of the axial locations between the fourth axial location and fifth axial location. In this particular example the normalized tube pitch at the first axial location (i.e. the first tube pitch) is 1.0 and the normalized tube winding angle at the first axial location (i.e. the first tube winding angle) is 1.0, the normalized tube pitch at the second axial location (i.e. the second tube pitch) is 1.0 and the normalized tube winding angle at the second axial location (i.e. the second tube winding angle) is 1.0, and the normalized tube pitch at the third axial location (i.e. the third tube pitch) is 1.07 and the normalized tube winding angle at the third axial location (i.e. the third tube winding angle) is about 1.07. 
     It should be noted that the specific distance of the fourth and fifth axial locations (and hence third axial location) from the first and second axial locations, and the specific tube pitch and tube winding angles in the above embodiment represent only one exemplary set of values, and other values may suitably be employed, as discussed above in relation to the embodiment of the invention described with reference to  FIG.  6  to  9   . 
     It should also be noted that in further embodiments of the invention a CWHE can be used that has a tube layer with a variation in tube pitch and tube winding angle that is a mixture of the variations in tube pitch and tube winding angle depicted in  FIGS.  6  and  7    and  FIGS.  10  and  11   . For example, in one embodiment, the tube pitch and winding angle could each initially remain constant from the first axial location to a fourth axial location (similar to  FIGS.  10  and  11   ), then each gradually increase to a maximum at a third axial location before decreasing back to a lower value (in a similar manner to  FIGS.  6  and  7   ) at a fifth axial location, and then remain at said lower value from the fifth axial location to the second axial location (similar to  FIGS.  10  and  11   ). In another embodiment, the tube pitch and winding angle could each gradually increase (in a similar manner to the gradual increase in  FIGS.  6  and  7   ) moving from the first axial location to a fourth axial location at which a maximum value is reached, then each remain constant at said maximum value from the fourth axial location to a fifth axial location (similar to  FIGS.  10  and  11   ) before gradually decreasing (in a similar manner to the gradual decrease in  FIGS.  6  and  7   ) moving from the fifth axial location to the second axial location. Various other combinations and permutations are equally possible, as will be apparent to the person of ordinary skill in the art. 
       FIGS.  12  and  13    illustrate the variation in tube pitch and tube winding angle in a tube layer of a CWHE in accordance with another embodiment of the present invention. In this embodiment, the tube pitch and tube winding angle vary in a similar manner to the embodiment described with reference to  FIGS.  10  and  11   . However, in this embodiment the tube layer is formed of, by way of example, fourteen helically wound tubes. 
       FIG.  12    is a graph schematically depicting the helically wound tubes of the tube layer as if unwound and laid flat but otherwise maintaining the tube pitch and tube winding angle of the tubes, and  FIG.  13    is a graph schematically depicting the helically wound tubes of the tube layer as if cut along a line parallel to the tube bundle axis and unwound and laid flat but otherwise maintaining the tube pitch and tube winding angle of the tubes. In each of these Figures each of the tubes is depicted by a differently shaded line. In each of the Figures, location along the tube bundle axis is plotted on the Y-axis (vertical axis), with 0 representing a first axial location of the tube layer towards one axial end of the bundle (preferably towards the bottom end of the bundle in a vertically oriented CWHE), 1 representing a second axial location of the layer towards the opposite axial end of the bundle (i.e. preferably towards the top end of the bundle in a vertically oriented CWHE), and with the numbers between 0 and 1 representing the fractions of the total axial distance from, respectively, the first axial location to the second axial location. In  FIG.  12    multiples of the tube layer circumference are plotted on the X-axis (horizontal axis), and in  FIG.  13    fractions of the tube layer circumference are plotted on the X-axis. The first axial location may in particular be an axial location at one end of the tube layer and the second axial location may in particular be an axial location at the other end of the tube layer such that  FIGS.  12  and  13    depict the tubes of the layer over the entire axial length of the tube bundle excluding only the ends of the tube bundle where the tails of the tubes are gathered together for insertion and fixing into tube sheets. 
     As can be seen from  FIGS.  12  and  13   , at the first axial location all of the tubes at that location have the same tube winding angle and the same tube pitch (said winding angle representing therefore the first tube winding angle of the layer and said pitch representing therefore the first tube pitch of the layer), and both the winding angle and the pitch of all of the tubes remain constant, staying the same from the first axial location of the tube layer to a fourth axial location that is, in this particular example, a third (33%) of the total axial distance from the first axial location to the second axial location. At this fourth axial location the winding angle and pitch of the tubes undergo a step change to a higher, maximum value that is the same for all of the tubes, and all of the tubes remain at this maximum winding angle (i.e. the third winding angle of the layer) and maximum pitch (i.e. the third tube pitch of the layer) until a fifth axial location is reached that is, in this particular example, two-thirds (66%) of the total axial distance from the first axial location to the second axial location (accordingly, in this embodiment the third axial location of the tube layer, at which the tube winding angle and tube pitch of the layer are each at a maximum, can be considered to be any of the axial locations between the fourth axial location and fifth axial location). Finally, at the fifth axial location both the winding angle and pitch of the tubes undergo a step change to a lower value that is the same for all of the tubes, the tubes then remaining at this lower winding angle and lower pitch from the fifth axial location to the second axial location of the tube layer (said winding angle representing therefore the second tube winding angle of the layer and said pitch representing therefore the second tube pitch of the layer). 
     It should be noted that although the tube layer in this exemplary embodiment is formed of fourteen tubes, in other embodiments higher or lower numbers of tubes could equally be used. Likewise, the specific distance of the fourth and fifth axial locations (and hence third axial location) from the first and second axial locations, and the specific tube pitch and tube winding angles used in the above embodiment represent only one exemplary set of values, and other values may suitably be employed, as discussed above in relation to the embodiment of the invention described with reference to  FIG.  6  to  9   . 
       FIG.  14    illustrates the variation in tube pitch and tube winding angle in a tube layer of a CWHE in accordance with yet another embodiment of the present invention, in which embodiment the tube layer is again formed of, by way of example, fourteen helically wound tubes. 
       FIG.  14    is a graph schematically depicting the helically wound tubes of the tube layer as if unwound and laid flat but otherwise maintaining the tube pitch and tube winding angle of the tubes, each of the tubes being depicted by a differently shaded line. Location along the tube bundle axis is plotted on the Y-axis (vertical axis), with 0 representing a first axial location of the tube layer towards one axial end of the bundle (preferably towards the bottom end of the bundle in a vertically oriented CWHE), 1 representing a second axial location of the layer towards the opposite axial end of the bundle (i.e. preferably towards the top end of the bundle in a vertically oriented CWHE), and with the numbers between 0 and 1 representing the fractions of the total axial distance from, respectively, the first axial location to the second axial location. Multiples of the tube layer circumference are plotted on the X-axis (horizontal axis). The first axial location may in particular be an axial location at one end of the tube layer and the second axial location may in particular be an axial location at the other end of the tube layer such that  FIG.  14    depicts the tubes of the layer over the entire axial length of the tube bundle excluding only the ends of the tube bundle where the tails of the tubes are gathered together for insertion and fixing into tube sheets. 
     As can be seen from  FIG.  14   , at the first axial location all of the tubes have the same tube winding angle and the same tube pitch (said winding angle representing therefore the first tube winding angle of the layer and said pitch representing therefore the first tube pitch of the layer), and both the winding angle and the pitch of all of the tubes remain constant, staying the same from the first axial location of the tube layer to a fourth axial location that is, in this particular example, a third (33%) of the total axial distance from the first axial location to the second axial location. Starting at this fourth axial location, the winding angle and pitch of each of the tubes gradually increase moving in the axial direction of the bundle from the fourth axial location to a third axial location at which the tube winding angle and tube pitch of each of the tubes reach a maximum, said maximum winding angle of each of the tubes being the same (and representing therefore the third tube winding angle of the layer) and said maximum pitch of each of the tubes being the same (and representing therefore the third tube pitch of the layer). In this particular example, the third axial location falls at a point that is half (50%) of the total axial distance from the first axial location to the second axial location. From the third axial location the tube winding angle and tube pitch of each of the tubes then gradually decrease moving in the axial direction of the bundle from the third axial location to a fifth axial location that is, in this particular example, two-thirds (66%) of the total axial distance from the first axial location to the second axial location. At this fifth axial location all of the tubes have the same tube winding angle and the same tube pitch, and all of the tubes then remain at this winding angle and this pitch from the fifth axial location of the tube layer to the second axial location of the tube layer (said winding angle representing therefore the second tube winding angle of the layer and said pitch representing therefore the second tube pitch of the layer). 
     Again, although the tube layer in this exemplary embodiment is formed of fourteen tubes, in other embodiments higher or lower numbers of tubes could equally be used. Likewise, the specific distance of the third, fourth and fifth axial locations from the first and second axial locations, and the specific tube pitches and tube winding angles used in the above embodiment represent only one exemplary set of values, and other values may suitably be employed, as discussed above in relation to the embodiment of the invention described with reference to  FIG.  6  to  9   . 
       FIG.  15    illustrates the variation in tube pitch and tube winding angle in a tube layer of a CWHE in accordance with yet another embodiment of the present invention, in which embodiment the tube layer is again formed of, by way of example, fourteen helically wound tubes. 
       FIG.  15    is a graph schematically depicting the helically wound tubes of the tube layer as if unwound and laid flat but otherwise maintaining the tube pitch and tube winding angle of the tubes, each of the tubes being depicted by a differently shaded line. Location along the tube bundle axis is plotted on the Y-axis (vertical axis), with 0 representing a first axial location of the tube layer towards one axial end of the bundle (preferably towards the bottom end of the bundle in a vertically oriented CWHE), 1 representing a second axial location of the layer towards the opposite axial end of the bundle (i.e. preferably towards the top end of the bundle in a vertically oriented CWHE), and with the numbers between 0 and 1 representing the fractions of the total axial distance from, respectively, the first axial location to the second axial location. Multiples of the tube layer circumference are plotted on the X-axis (horizontal axis). The first axial location may in particular be an axial location at one end of the tube layer and the second axial location may in particular be an axial location at the other end of the tube layer such that  FIG.  15    depicts the tubes of the layer over the entire axial length of the tube bundle excluding only the ends of the tube bundle where the tails of the tubes are gathered together for insertion and fixing into tube sheets. 
     As can be seen from  FIG.  15   , at the first axial location all of the tubes have the same tube winding angle and the same tube pitch (said winding angle representing therefore the first tube winding angle of the layer and said pitch representing therefore the first tube pitch of the layer), and both the winding angle and the pitch of the tubes remain constant from the first axial location of the tube layer to a fourth axial location that is, in this particular example, a third (33%) of the total axial distance from the first axial location to the second axial location. At this fourth axial location the winding angle of a first half of the tubes of the layer is increased, and at a fifth axial location a very small distance further along tube bundle axis the winding angle of the other half the tubes is increased to match the winding angle of the first half of the tubes. This results in the fourteen tubes of the layer being grouped into seven pairs of tubes at the fifth axial location, whereby center-to-center distance between the tubes of each pair of tubes is smaller than the center-to-center distance between the tube of one pair and the adjacent tube of the adjacent pair, the tube pitch of each tube at the fifth axial location being, therefore, the average of the center-to-center distances between that tube and each of its adjacent tubes. Nevertheless, at this fifth axial location both the tube pitch of the tube layer (being the average of the tube pitches of all the tubes at this axial location) and the tube winding angle of the tube layer are at their maximum values (said pitch representing therefore the third tube pitch of the layer and said winding angle representing therefore the third tube winding angle of the layer). The tube pitch and tube winding angle of the layer remain at these maximum values until a sixth axial location is reached that is, in this particular example, two-thirds (66%) of the total axial distance from the first axial location to the second axial location (and accordingly in this embodiment the third axial location of the tube layer, at which the tube winding angle and tube pitch of the layer are each at a maximum, can be considered to be any of the axial locations between the fifth axial location and sixth axial location). At this sixth axial location the winding angle of the first half of the tubes of the layer is decreased, and at a seventh axial location a very small distance further along tube bundle axis the winding angle of the other half the tubes is decreased to match the winding angle of the first half of the tubes. This results in the “un-grouping” of the fourteen tubes of the layer, so that at the seventh axial location all of the tubes have the same tube winding angle and the same tube pitch, the tube winding angle and tube pitch of the tube layer at the seventh axial location being smaller than the tube winding angle and tube pitch of the tube layer at the axial locations between the fifth and sixth axial locations. Both the tube winding angle and the pitch of the layer then remain constant from the seventh axial location of the layer to the second axial location of the tube layer (said tube winding angle and tube pitch representing therefore the second tube winding angle of the layer and second tube pitch of the layer). 
     It should be noted that although the tube layer in this exemplary embodiment is formed of fourteen tubes, in other embodiments higher or lower numbers of tubes could equally be used. Likewise, the specific distances of the fourth, fifth, sixth and seventh axial locations (and hence third axial location) from the first and second axial locations, and the specific tube pitch and tube winding angles used in the above embodiment represent only one exemplary set of values, and other values may suitably be employed, as discussed above in relation to the embodiment of the invention described with reference to  FIG.  6  to  9   . Furthermore, although in this excemplary embodiment the tubes are grouped into pairs in the middle section of the bundle (i.e. between the fifth and sixth axial locations) in other embodiments the tubes could be grouped into triplets, quadruplets, or other groupings. 
     In each of the embodiments of the invention described above with reference to  FIG.  6  to  15   , only a single tube layer of the CWHE has been discussed. However, as noted above, it is preferred all or substantially all of the tube layers of the tube bundle have a non-uniform tube pitch, whereby the CWHE has a greater tube pitch at a middle location (axially) of the tube bundle than at locations towards the axial ends of the bundle, and hence it is preferred that all or substantially all of the tube layers (e.g. at least 90%, and more preferably at least 95%) of the tube bundle have a first tube winding angle and a first tube pitch at a first axial location of the layer towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial location of the layer towards the opposite axial end of the bundle, and a third tube winding angle and third tube pitch at a third axial location of the layer between the first axial location and second axial location, the third tube winding angle being greater than the first tube winding angle and greater than the second tube winding angle and the third tube pitch being greater than the first tube pitch and greater than the second tube pitch. Thus, it is likewise preferred that all or substantially all of the tube layers (e.g. at least 90%, and more preferably at least 95%) of the tube bundle according to the embodiment of the invention in question have the same or substantially the same configuration as the tube layer that has been described. In particular, it is preferred that the third axial location of each of said tube layers is at the same or substantially the same axial position of the bundle (e.g. each of the third axial locations is at the same axial position or all fall within an axial region representing 5% or less of the total axial length of the bundle). Likewise, it is preferred that the fourth, fifth, sixth and/or seventh axial positions, where present, of each of said tube layers are at the same or substantially the same axial positions of the bundle. Likewise, it preferred that the first, second and third winding angles of each of said layers are the same or substantially the same. 
     It will be appreciated that the invention is not restricted to the details described above with reference to the preferred embodiments but that numerous modifications and variations can be made without departing from the spirit or scope of the invention as defined in the following claims.