CN116086212A

CN116086212A - Reducing maldistribution of shell-and-tube liquid in coil heat exchanger tube bundle

Info

Publication number: CN116086212A
Application number: CN202211374936.1A
Authority: CN
Inventors: 金波; J·D·布考斯基; P·A·霍顿; C·M·奥特; M·J·罗伯茨; M·R·皮拉雷拉
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 2021-11-05
Filing date: 2022-11-04
Publication date: 2023-05-09
Also published as: US20230147084A1; CN219624540U; EP4177556A1

Abstract

A coil heat exchanger (CWHE), and a method of using the CWHE to cool and/or liquefy a fluid stream, are described herein, wherein one or more tube layers of a tube bundle are provided with non-uniform tube wrap angles and tube pitches to facilitate balancing radial pressure imbalance on a shell side of the CWHE, thereby reducing radial maldistribution of fluid on the shell side and improving heat transfer efficiency of the CWHE.

Description

Reducing maldistribution of shell-and-tube liquid in coil heat exchanger tube bundle

Background

The present invention relates to coil heat exchangers and methods of cooling and/or liquefying a fluid stream using the coil heat exchangers.

Coiled tube heat exchangers (CWHEs) are one type of heat exchanger known in the art. A coil heat exchanger includes one or more tube bundles enclosed within a pressure vessel, the or each tube bundle including a plurality of tubes adapted to carry one or more fluid streams for heat exchange with fluid flowing through the pressure vessel outside the tubes. The tube of the or each tube bundle is helically wound around a mandrel extending in the axial direction of the bundle such that the tubes form a plurality of tube layers around the mandrel, each tube layer comprising one or more of the helically wound tubes. Typically, each tube layer is spaced apart from an adjacent tube layer by a spacer (i.e. separated in the radial direction of the bundle), which may for example take the form of a spacer wire or rod. One or more tube bundles may be installed in a pressure vessel with appropriate headers and tubing for introducing one or more fluid streams into and withdrawing the streams from the tubes, and additional tubing for fluid flow between the bundles. In many cases, coil heat exchangers are used to cool and/or liquefy one or more fluid streams that pass through the "tube pass" of the CWHE, i.e., through the tubes of one or more tube bundles, and cold refrigerant passes through the "shell pass" of the CWHE, i.e., outside the tubes through the pressure vessel. Typically in this arrangement the refrigerant used on the shell side of the CWHE is a liquid or two-phase refrigerant that will vaporize as it passes through the shell side over the outside of the tube.

The tube bundle of the CWHE may be manufactured by methods known in the CWHE manufacturing arts. The tube bundle is typically constructed of aluminum, stainless steel, copper, or other metals having suitable thermal and mechanical properties. The tube bundle is formed by helically winding a tube set, typically of constant diameter and similar length, around a mandrel. The mandrel may be a cylindrical tube whose length, outer diameter and wall thickness impart the required structural strength to support the desired size and number of tube layers. In one method of manufacture, a solid rod may be helically wound around and in contact with a mandrel, a spacer may be mounted on the wound rod parallel to the mandrel axis, and then the tube may be helically wound in a first layer in contact with the spacer. A number of additional layers of tubing are then formed in the radial direction, each layer being separated from the adjacent layers, typically by spacer wires or rods running parallel to or helically around the mandrel axis. The winding of the tube may be performed with the mandrel axis oriented vertically in a fixed position, while the tube is wound onto the tube bundle from a reel adapted to move circumferentially about the axis and also move up and down parallel to the axis. Alternatively, the tube bundle may be constructed by rotating the mandrel and bundle on a lathe about a fixed horizontal axis while the tube is wound onto the coil from a spool adapted for axial movement (i.e., side-to-side).

Thus, one or more or each tube bundle of the CWHE may 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 wrap angle. Some of these parameters also affect the basic fluid flow and heat transfer characteristics of the tube bundle.

Coil heat exchangers are commonly used in the process industry for heating or cooling fluid streams at high heat transfer rates, which require a large heat transfer area. For example, they are commonly used for the production of Liquefied Natural Gas (LNG) where a large heat transfer area is required to indirectly transfer heat between the natural gas feed stream passing through the tube side of the CWHE and the cold refrigerant passing through the shell side of the CWHE.

For example, fig. 1 shows a coil heat exchanger arranged in a known manner for liquefied natural gas. This particular arrangement uses a CWHE with two tube bundles for cooling and liquefying of the natural gas feed (which may have been pre-treated and/or pre-cooled prior to being introduced into the coil heat exchanger and cooled and liquefied therein). CWHE 1 comprises a pressure vessel 3 containing a warm heat exchange zone 5 and a cold heat exchange zone 9. A first tube bundle is used in the warm heat exchange zone 5, wherein the natural gas feed stream introduced into the tube bundle via line 11 is initially cooled in the tube loop 13 by a refrigerant (described later) flowing over the first tube bundle through the shell side of the CWHE. The tube circuit 13 is formed of a plurality of tubes as part of a first tube bundle, wherein the bundle further includes

tube circuits

31 and 39 as described later (

tube circuits

13, 31 and 39 together represent tube passes of the first tube bundle). The cooled and at least partially condensed natural gas withdrawn from the first tube bundle via line 15 may optionally be depressurized, such as through a throttle valve 17. The natural gas then flows via line 19 into the tube loop 21 of the second tube bundle in the cold-heat exchange zone 9, wherein the natural gas is further cooled before being withdrawn as LNG product via line 23.

The refrigerant used in this example is one that is condensed prior to vaporization in the shell side of the CWHE, such as, for example, a mixed refrigerant (a multi-component refrigerant comprising light hydrocarbons, typically comprising methane, ethane, ethylene, and/or propane, and optionally nitrogen). The two-phase flow of compressed refrigerant is supplied from a refrigerant compression system (not shown) via line 25 and flows into phase separator 27. Liquid refrigerant is drawn through line 29, subcooled in tube circuit 31, and depressurized through throttle valve 33. Optionally, a hydraulic expansion turbine may be used to extract work from the refrigerant liquid prior to throttle 33. The refrigerant from the throttle valve 33 is combined with the refrigerant flowing downward from the heat-and-cold exchange region 9 (described later), and the combined refrigerant is distributed via the distributor 35. The combined refrigerant flows downwardly over the first tube bundle through the shell side of the CWHE while vaporizing and heating to provide refrigeration for cooling the natural gas in the tube loop 13, as previously described. In addition, the vaporized refrigerant provides refrigeration for the liquid refrigerant in the subcooling tube circuit 31, as previously described, as well as for the vapor refrigerant in the cooling tube circuit 39 (described below).

Vapor refrigerant is withdrawn from separator 27 via line 37, cooled and possibly partially condensed in the tube loop 39 of the warm heat exchange zone 5, and finally passed through the tube loop 41 of the second tube bundle of the cold heat exchange zone 9, where it is liquefied and optionally subcooled. This refrigerant is depressurized through the throttle valve 43 and distributed into the heat-exchanging area 9 via the distributor 45. This refrigerant flows down the second tube bundle through the shell side of the CWHE and vaporizes to provide refrigeration for cooling the natural gas in the tube loop 21, as previously described. Further, the vaporized refrigerant provides refrigeration for the refrigerant in the cooling tube circuit 41. The distributor 45 is schematically shown and may comprise means for phase separating and distributing the separated vapor and liquid refrigerant streams to the heat exchange zone 9. The two-phase refrigerant flowing downwards from the shell side of the cold-heat exchange zone 9 enters the warm-heat exchange zone 5 and merges with the refrigerant discharged from the throttle valve 33, whereupon the combined refrigerant is distributed via the distributor 35 and flows downwards over the first tube bundle through the shell side of the warm-heat exchange zone 5, as previously described. The refrigerant reaching the bottom of the heat exchanger pressure vessel 3 is typically completely vaporized and withdrawn as vapor via line 47. This vapor is compressed in a refrigerant compression system (not shown) and optionally pre-cooled to provide a two-phase cooled compressed refrigerant via line 25, as previously described.

As described above, the

tube circuits

13, 31 and 39 are parts of a first tube bundle installed in the heat exchange zone 5 of the pressure vessel 3, and the

tube circuits

21 and 41 are parts of a second tube bundle installed in the cold heat exchange zone 9 of the heat exchanger pressure vessel 3. The tubes of each of the

tube loops

13, 31 and 39 are typically aggregated at each end, for example by aggregating the multiple tubes from each loop into one or more tube sheets that may be connected to inlet and outlet lines. Likewise, the tubes of each of the

tube circuits

21 and 41 are typically aggregated at each end, for example by aggregating the multiple tubes from each circuit into one or more tube sheets that may be connected to the inlet and outlet lines. The first tube bundle and the second tube bundle may each be manufactured by methods known in the art of coil heat exchanger manufacture as discussed above.

An improvement to CWHE of the type described above and depicted in fig. 1 is described in EP1367350B 2. As the vaporized refrigerant flows down through the shell side over the tube bundle of the warm heat exchange zone, the net vapor fraction increases and the heat transfer mechanism gradually transitions from two-phase boiling heat transfer at the cold end or top end to single-phase vapor heat transfer at the warm end or bottom end, causing the properties of the heat transfer mechanism to change significantly from top to bottom of the bundle. To solve this problem, EP1367350B2 proposes to divide the tube bundles in the warm heat exchange zone 5 into at least two smaller coiled tube bundles, which differ from each other in one or more dimensional parameters selected from the group consisting of 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, so that each bundle can be designed to more closely match the heat exchange properties and fluid flow phenomena occurring on each bundle.

International patent application WO 2020/074117 teaches that conventional CWHE tube bundles are designed and manufactured such that the tubes in each tube layer have a uniform tube pitch throughout the layer (i.e. the distance between the tubes in a given layer remains constant and the same throughout the layer), resulting in a uniform distribution of heating surface and tube bundle weight over the axial length of the tube bundle. However, it also indicates that the heating power requirements of the tube bundle at different locations may vary depending on the flow conditions of the fluid on the shell side of the CWHE space, and that structural-mechanical problems may also occur at the ends of the tube bundle due to loading during the winding process. To address these problems, it is proposed that the tube bundle should alternatively be designed and manufactured with a tube pitch that increases (or decreases) monotonically in the axial direction in at least a portion of the tube bundle.

U.S. patent application US2019/0011191A1 describes an arrangement for solving the problem of maldistribution of liquid on the shell side of a CWHE with vaporized refrigerant. US2019/0011191A1 notes that in many cases, as the refrigerant descends through the bundle, the liquid phase of the refrigerant will turn in the direction of the outer tube layers of the bundle, which can result in a localized under-supply of liquid (vaporized) refrigerant flowing into the bundle within the inner tube layer region of the bundle, thereby resulting in reduced heat transfer performance. In order to solve this problem, it is proposed to provide a gas discharge means at the CWHE for discharging the gas-phase refrigerant from the shell side of the CWHE in the region of the inner tube layer and/or to provide a gas supply means for introducing the gas-phase refrigerant into the shell side of the CWHE in the region of the outer tube layer in order to reduce or avoid a pressure drop in the radial direction, thereby reducing or avoiding deflection of the liquid.

Disclosure of Invention

Disclosed herein are coil heat exchangers in which one or more tube layers of a tube bundle are provided with non-uniform tube wrap angles and tube pitches to facilitate balancing radial pressure imbalance on the shell side of the coil heat exchanger (CWHE) to reduce radial maldistribution of fluid on the shell side of the CWHE and to improve heat transfer efficiency of the CWHE in operation. More specifically, the CWHE may have a larger tube wrap angle and tube pitch at an intermediate position of the tube bundle (axial direction) than at a position toward the axial end of the bundle. For example, in a cold end up CWHE, the tube wrap angle and tube pitch may increase from the top of the bundle 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 a fluid stream using such coil heat exchangers.

Several preferred aspects of the system and method according to the present invention are summarized below.

Aspect 1: a coil 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 fluid streams for heat exchange with fluid flowing through the pressure vessel outside the tubes, the tubes being helically wound around a mandrel extending in an axial direction of the bundle so as to form a plurality of tube layers around the mandrel, each of the tube layers comprising one or more of the 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 position of the layer towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial position of the layer towards an opposite axial end of the bundle, and a third tube pitch winding angle and a third tube pitch at a third axial position of the layer between the first axial position and the second axial position, the third tube winding angle being greater than the first tube winding angle and greater than the second tube pitch angle and the third tube pitch being greater than the first tube pitch.

Aspect 2: the CWHE of aspect 1 wherein all or substantially all of the tube layers have a first tube wrap angle and a first tube pitch at a first axial location of the layers toward one axial end of the bundle, a second tube wrap angle and a second tube pitch at a second axial location of the layers toward an opposite axial end of the bundle, and a third tube wrap angle and a third tube pitch at a third axial location of the layers between the first axial location and the second axial location, the third tube wrap angle being greater than the first tube wrap angle and greater than the second tube wrap angle and the third tube pitch being greater than the first tube pitch and greater than the second tube pitch for each of the layers.

Aspect 3: the CWHE of aspect 2 wherein the third axial position of each tube layer is at the same or substantially the same axial position of the bundle.

Aspect 4: the CWHE of any one of aspects 1-3 wherein in at least one or all or substantially all of the tube layers, the third tube wrap angle at the third axial position is equal to or greater than any tube wrap angle at any other axial position of the layer between the first axial position and the second axial position of the layer, and the third tube pitch at the third axial position is equal to or greater than any tube pitch at any other axial position of the layer between the first axial position and the second axial position of the layer.

Aspect 5: the CWHE according to any one of aspects 1 to 4 wherein in at least one or all or substantially all tube layers, the third axial position of the layers is located at a point between 10% and 90% of the distance in the axial direction of the bundle that falls between the first axial position and the second axial position.

Aspect 6: the CWHE of any one of aspects 1-5 wherein in at least one or all or substantially all of the tube layers, the third tube wrap angle is at least 5% greater than the first tube wrap angle and at least 5% greater than the second tube wrap angle.

Aspect 7: the CWHE of any one of aspects 1-6 wherein in at least one or all or substantially all of the tube layers, the third tube wrap angle is at least 0.5 degrees greater than the first tube wrap angle and at least 0.5 degrees greater than the second tube wrap angle.

Aspect 8: the CWHE according to any one of aspects 1 to 7 wherein in at least one or all or substantially all of the tube layers, the third tube pitch is at least 5% greater than the first tube pitch and at least 5% greater than the second tube pitch.

Aspect 9: the CWHE of any one of aspects 1-7 wherein in at least one or all or substantially all of the tube layers, the third tube pitch is at least 10% greater than the first tube pitch and at least 10% greater than the second tube pitch.

Aspect 10: the CWHE according to any one of aspects 1 to 9 wherein the CWHE is a vertically oriented CWHE, the axial direction of the bundle is vertical or substantially vertical, the CWHE further comprising a refrigerant distributor located above the tube bundle for distributing refrigerant into the pressure vessel outside the tubes such that the refrigerant flows downwardly through the pressure vessel on and through the tube bundle outside the tubes.

Aspect 11: the CWHE of aspect 10 wherein the CWHE has an inlet configured to introduce the one or more fluid streams into the plurality of tubes of the tube bundle at a bottom of the tube bundle and an outlet configured to withdraw the one or more fluid streams from the tubes at a top of the tube bundle such that the one or more streams flow upward through the tubes of the tube bundle.

Aspect 12: the CWHE of either aspect 10 or 11 wherein the CWHE is configured to use a liquid or two-phase refrigerant that vaporizes as it flows down the pressure vessel over and through the tube bundle outside the tubes.

Aspect 13: a method of cooling and/or liquefying one or more fluid streams via heat exchange with a refrigerant using the CWHE according to any of aspects 1 to 12, wherein the method comprises passing the one or more fluid streams through the plurality of tubes of the tube bundle and passing the refrigerant outside the tubes through the pressure vessel.

Aspect 14: the method of aspect 13, wherein the one or more fluid streams to be cooled and/or liquefied comprise a natural gas stream.

Aspect 15: the method of aspects 13 or 14, wherein the refrigerant comprises a mixed refrigerant.

Drawings

Fig. 1 is a schematic diagram of a coil heat exchanger (CWHE) arranged for lng according to the prior art.

Fig. 2 is a graph depicting tube pitch versus position along the tube bundle axis for tube layers of a CWHE according to the prior art.

Fig. 3 is a graph depicting the tube wrap angle of the tube layers of fig. 2 versus position along the tube bundle axis.

Fig. 4 is a perspective cut-away view of the intermediate portion of the tube layer of fig. 2 and 3.

Fig. 5 is a top down cross-sectional view of the intermediate portion of the tube layer depicted in fig. 4.

Fig. 6 is a graph depicting tube pitch versus position along the tube bundle axis for tube layers of a CWHE in accordance with an embodiment of the invention.

Fig. 7 is a graph depicting the tube wrap angle of the tube layers of fig. 6 versus position along the tube bundle axis.

Fig. 8 is a perspective cut-away view of the intermediate portion of the tube layer of fig. 6 and 7.

Fig. 9 is a top down cross-sectional view of the intermediate portion of the tube layer depicted in fig. 8.

Fig. 10 is a graph depicting tube pitch versus position along the tube bundle axis for a tube layer of a CWHE in accordance with another embodiment of the invention.

Fig. 11 is a graph depicting the tube wrap angle of the tube layers of fig. 10 versus position along the tube bundle axis.

Fig. 12 is a graph schematically depicting the tubes of a tube layer of a CWHE according to another embodiment of the invention when "deployed", wherein the position along the tube bundle axis is plotted on the vertical axis and the multiple of the tube layer circumference is plotted on the horizontal axis.

Fig. 13 is a graph schematically depicting the tubes of the tube layer of fig. 12 when "unrolled" with positions 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 a CWHE according to another embodiment of the invention when "deployed", wherein the position along the tube bundle axis is plotted on the vertical axis and the multiple of the tube layer circumference is plotted on the horizontal axis.

Fig. 15 is a graph schematically depicting the tubes of a tube layer of a CWHE according to another embodiment of the invention when "deployed", wherein the position along the tube bundle axis is plotted on the vertical axis and the multiple of the tube layer circumference is plotted on the horizontal axis.

Detailed Description

Coil heat exchangers, and methods of cooling and/or liquefying a fluid stream using the same, are described herein in which one or more tube layers of a tube bundle are provided with non-uniform tube wrap angles and tube pitches to facilitate balancing radial pressure imbalance on a shell side of a coil heat exchanger (CWHE) to reduce radial maldistribution of fluid (particularly liquid) on the shell side of the CWHE and to improve 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.

The terms "first," "second," "third," and the like, when used herein to identify enumerated features of a method or system, are merely used to aid in referencing and distinguishing between the features discussed and are not intended to indicate any particular order of the features, unless and only to the extent that such order is explicitly recited.

As used herein, the "tube wrap angle" of the tubes of a tube layer at a particular location refers to the angle (acute angle) formed between the tube axis of the tube at that location and a plane perpendicular to the tube bundle axis. The pipe winding angle of the pipe layer at a specific axial position refers to the pipe winding angle of the pipe of the layer at that axial position in the case where the layer is formed of a single pipe or refers to the average (arithmetic average) of the pipe winding angles of the pipe of the layer at that axial position in the case where the layer is formed of more than one pipe.

As used herein, the "tube pitch" 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 tube ring and the tube axis of each of the adjacent two tube rings in the layer, the center-to-center distances between the two tube rings being measured perpendicular to the axis of the tube ring in question. The two turns adjacent to the turns at the location in question may be turns of the same tube or turns of different tubes or different pairs of tubes, depending on the number of tubes forming the tube layers. More specifically, where a tube layer is formed from a single tube, two tube turns adjacent to a tube turn at a particular location will be two adjacent turns of the same single tube forming the layer; in the case where a tube layer is formed of two tubes, the two tube turns adjacent to the tube turn at a particular location will be the two adjacent turns of the other tube forming the layer; in the case of a tube layer formed of three or more tubes, two tube turns adjacent to a tube turn at a particular location will be adjacent turns of adjacent tubes in the layer. Tube pitch of a tube layer at a particular axial position refers to the tube pitch of the tube of the layer at that axial position in the case where the layer is formed of a single tube or to the average (arithmetic average) of the tube pitches of the tube of the layer at that axial position in the case where the layer is formed of more than one tube.

As used herein, the "tube axis" of a tube or tube collar refers to the central longitudinal axis of the tube or tube collar.

As used herein, "tube bundle axis" refers to the central longitudinal axis of the tube bundle. Typically, the tube layers of the tube bundle form a set of layers coaxially positioned about the mandrel such that the tube bundle and the mandrel share the same central longitudinal axis (within manufacturing tolerances). Typically, the pressure vessel containing the tube bundle is a generally cylindrical vessel, which is also positioned coaxially with the tube layers and the mandrel.

As used herein, "axial direction of the bundle" refers to a direction along the axis of the bundle; "axial position" or "axial position" of a bundle refers to a position or location (location) at a particular point along the axis of the bundle, but (unless otherwise specified) does not refer to any particular circumferential position of the bundle at that axial point along the axis of the bundle; and the "axial ends" of the tube bundle refer to the ends of the bundle at either end of the tube bundle axis.

By way of example only, certain exemplary embodiments of the present invention will now be described with reference to the accompanying drawings.

As described above, a coil heat exchanger (CWHE) is one type of heat exchanger known in the art and may be manufactured by methods known in the art. A CWHE comprises one or more tube bundles enclosed within a pressure vessel, the or each tube bundle comprising a plurality of tubes adapted to convey one or more fluid streams for heat exchange with fluid flowing through the pressure vessel outside the tubes. The tube of the or each tube bundle is helically wound around a mandrel extending in the axial direction of the bundle such that the tubes form a plurality of tube layers around the mandrel, each tube layer comprising one or more of the helically wound tubes. Typically, each tube layer is spaced apart from an adjacent tube layer by a spacer (i.e. separated in the radial direction of the bundle), which may for example take the form of a spacer wire or rod. At each axial end of the tube bundle, the one or more helically wound tubes or ends or "tails" of the tubes forming each tube layer are typically gathered or clustered together to form one or more groups, with the ends or "tails" of the tubes forming the other layers such that each tail of the group may be inserted and secured in the tube sheet to thereby form a set of inlets or outlets. One or more tube bundles may be installed in a pressure vessel with appropriate headers and piping for introducing one or more fluid streams into and withdrawing the streams from the tubes via the inlet and outlet ports, and additional piping may be used for fluid flow between the bundles.

In many cases, coil heat exchangers are used to cool and/or liquefy one or more fluid streams, wherein the fluid streams pass through the tubes of one or more tube bundles of the CWHE (i.e., through the tube side of the "CWHE"), and wherein cold refrigerant passes outside the tubes through the pressure vessel (i.e., through the shell side of the CWHE). Typically, the CWHE is employed in a "cold end up" orientation whereby the CWHE is oriented vertically with the tube bundle axis vertical or substantially vertical, with a refrigerant distributor located above the or each tube bundle (or at least the topmost tube bundle if there is more than one) for distributing refrigerant into the pressure vessel outside the tubes such that the refrigerant flows down through the pressure vessel on and through the tube bundles outside the tubes, and the CWHE is configured such that one or more fluid streams to be cooled and/or liquefied are introduced at the bottom of the tube bundle, flow up through the tubes of the tube bundle, and are withdrawn from the top of the tube bundle. Typically, CWHE is configured to use a liquid or two-phase refrigerant that vaporizes as it passes down through the pressure vessel on and through the tube bundle outside the tubes.

Such coil heat exchangers can be used in a variety of different processes, but in certain preferred embodiments of the invention they can be used to cool and/or liquefy a natural gas stream using a mixed refrigerant. A suitable exemplary arrangement of CWHE for liquefying a natural gas stream is shown in fig. 1, as described above.

In a typical prior art CWHE, the tubes in each tube layer have a consistent tube wrap angle and tube pitch throughout the layer, as shown in further detail in fig. 2-5.

More specifically, for the tube layers of the tube bundle according to the CWHE of the prior art, fig. 2 is a graph depicting the tube pitch versus position along the tube bundle axis, and fig. 3 is a graph depicting the tube wrap angle versus position along the tube bundle axis. For simplicity, in this example, the tube layer consists of a single helically wound tube, although in practice the tube layer may typically comprise a plurality of helically wound tubes. The position along the axis of the tube bundle is plotted on the X-axis (horizontal axis) of the two figures, where 0 represents the first axial position of the tube layers towards one axial end of the bundle, 1 represents the second axial position of the layers towards the opposite axial end of the bundle, and the number between 0 and 1 represents the fraction of the total axial distance from the first axial position to the second axial position (i.e. the distance measured in the axial direction of the bundle). The normalized value of the tube pitch is plotted on the Y-axis (vertical axis) of fig. 2, with the normalized tube pitch of the tube at one location being defined herein as the ratio of the tube pitch at that location to the tube pitch of the tube at the first axial location. The normalized value of the tube winding angle is plotted on the Y-axis (vertical axis) of fig. 3, with the normalized tube winding angle of the tube at one location being defined herein as the ratio of the tube winding angle at that location to the tube winding angle of the tube at the first axial location. It can be seen that since the winding angle and pitch of the helically wound tube forming the tube layer remains constant throughout the layer, the tube pitch remains the same at each axial position between the first and second axial positions, and the tube winding angle remains the same at each axial position between the first and second axial positions.

Fig. 4 and 5 are a perspective view and a top-down cross-sectional view, respectively, of an (axial) intermediate portion of the tube layers of fig. 2 and 3. These views show the tube layer having a diameter D, taken along a plane containing the tube bundle axis Z, with the center-to-center distance between the tube turns at an angular coordinate θ about the tube bundle axis (measured parallel to the tube bundle axis) corresponding to the point on the right-hand side of the tube layer (as viewed in fig. 4 and 5) where the tube layer has been taken apart, labeled p1 through p7. It can be seen that since the tube wrap angle remains constant at each axial coordinate along the tube bundle axis Z (and thus also at each angular coordinate θ about the tube bundle axis Z), the center-to-center distances p1 through p7 between each tube turn likewise remain constant and equal, and the coupon pitch remains constant at each axial coordinate along the tube bundle axis Z (and thus also at each angular coordinate θ about the tube bundle axis Z). It should be noted that although the center-to-center distances p1 through p7 depicted in fig. 4 and 5 do not correspond exactly to the tube pitch of the tube coil (since the distances p1 through p7 are measured parallel to the tube bundle axis and the tube pitch is measured perpendicular to the axis of the tube coil), these two sets of values are closely related.

In CWHE bundles, heat transfer occurs between the tube side flow and the shell side flow (as described above), and for optimum heat transfer performance, a uniform distribution of flow in both the shell side and tube side is necessary. However, it is known that radial maldistribution of fluid may occur in the shell side of a typical prior art CWHE, where the shell side flow has a two-phase flow (i.e., contains both liquid and gas phases). More specifically, as the shell-side vapor and liquid simultaneously flow downwardly through the tubes of the tube bundle, pressure imbalance occurs in the shell side of the CWHE in the radial direction of the tube bundle, and such radial pressure imbalance diverts the liquid on the shell side in the radial direction of the tube bundle, resulting in maldistribution of the liquid on the CWHE shell side. This maldistribution not only reduces the heat transfer efficiency of CWHE in operation, but may also necessitate the use of larger CWHEs with larger heat transfer surface areas than theoretically required in order to account for losses in heat transfer efficiency in practice.

The inventors have now found that by modifying the design of the CWHE tube bundle such that the CWHE instead uses a tube bundle in which one or more (and preferably all or substantially all) tube layers of the tube bundle have non-uniform tube pitch, whereby the CWHE has a larger tube pitch at a (axial) intermediate position of the tube bundle than at a position towards the axial ends of the bundle, it is possible to equalize such radial pressure imbalance on the shell side of the CWHE and thereby reduce liquid radial maldistribution on the shell side of the CWHE and thereby improve the heat transfer efficiency of the CWHE. Such non-uniform tube pitch is achieved by forming one or more or each of the tube layers from one or more tubes having non-uniform tube winding angles, whereby the tube layers have a greater tube winding angle at a (axial) intermediate position of the tube bundle than at a position towards the axial ends of the bundle. This is because, for a given helically wound tube, increasing the winding angle of the tube increases the center-to-center distance between turns of the tube. Thus, a larger tube wrap angle at a particular location may provide a larger tube pitch at that location.

Thus, the coil heat exchanger according to the invention differs from a typical prior art CWHE in that in the CWHE according to the invention at least one of the tube layers of the tube bundle (and preferably all or substantially all of the tube layers) has a first tube winding angle and a first tube pitch at a first axial position of the layer towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial position of the layer towards the opposite axial end of the bundle, and a third tube winding angle and a third tube pitch at a third axial position of the layer between the first axial position and the second axial position, 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 heat exchanger according to the present invention accounts for the variation in the liquid-vapor ratio of 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 the tube bundle contains little or no liquid and a large amount of liquid, while the two-phase shell-side flow at the bottom of the tube bundle contains little or no liquid and a large amount of vapor. The strongest vapor and liquid interaction occurs in the middle of the beam, where the two-phase shell-side flow contains a significant amount of vapor and liquid. This strongest interaction of vapor and liquid with heat transfer creates the greatest radial pressure imbalance in the shell side, which in turn results in the most severe maldistribution of liquid therein. However, it has been found that a larger tube pitch and thus a larger spacing between adjacent tube turns more effectively equalizes radial pressure imbalance. Thus, it has been found that a larger tube pitch in the middle of the bundle can better equalize the larger radial pressure imbalance in the middle of the bundle, thereby providing better performance.

Fig. 6 to 9 show variations in tube pitch and tube winding angle in the tube layer of the CWHE according to one embodiment of the invention.

More specifically, for the tube layers according to this embodiment, fig. 6 is a graph depicting the relative relationship of the tube pitch to the position along the tube bundle axis, and fig. 7 is a graph depicting the relative relationship of the tube winding angle to the position along the tube bundle axis. For 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. The position along the axis of the tube bundle is plotted on the X-axis (horizontal axis) of the two figures, where 0 represents a first axial position 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 represents a second axial position 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 number between 0 and 1 represents a fraction of the total axial distance from the first axial position to the second axial position (i.e. the distance measured in the axial direction of the bundle). The normalized value of the tube pitch is plotted on the Y-axis (vertical axis) of fig. 6, and the normalized value of the tube winding angle is plotted on the Y-axis (vertical axis) of fig. 7. The first axial position may be, in particular, an axial position at one end of the tube layer and the second axial position may be, in particular, an axial position at the other end of the tube layer, such that fig. 6 and 7 depict the tube pitch and tube wrap angle over the entire axial length of the tube bundle, excluding only the ends of the tube bundle, with the tail portions of the tubes bunched together for insertion and fixation into the tube sheet.

It can be seen that in this exemplary embodiment, both the winding angle and pitch of the helically wound tube forming the tube layer gradually increases moving from a first axial position (along the axial direction of the bundle) of the tube layer to a third axial position where the tube winding angle and tube pitch reach a maximum, in this particular example the third axial position falls at a point of 0.4 (i.e. 40% or 2/5) of the total axial distance from the first axial position to the second axial position. Then, both the winding angle and the pitch of the spirally wound tube forming the tube layer gradually decrease, moving from the third axial position (in the axial direction of the bundle) of the tube layer to the second axial position of the tube layer. In this particular example, the normalized tube pitch at the first axial position (i.e., the first tube pitch) is by definition 1.0, and the normalized tube winding angle at the first axial position (i.e., the first tube winding angle) is also 1.0, the normalized tube pitch at the second axial position (i.e., the second tube pitch) is 0.925, and the normalized tube winding angle at the second axial position (i.e., the second tube winding angle) is approximately the same value, and the normalized tube pitch at the third axial position (i.e., the third tube pitch) is 1.11, and the tube winding angle at the third axial position (i.e., the third tube winding angle) is approximately the same value. The normalized winding angle is close to the normalized tube pitch because the relationship between pitch and winding angle is nearly linear at the smaller winding angles of less than 20 ° used in this example. At larger winding angles, the value of the normalized winding angle will differ significantly from the normalized tube pitch, but the shape of one curve will be similar to the other curve.

It should be noted that the specific distance of the third axial position from the first and second axial positions and the specific tube pitch and tube wrap angle in the above embodiments represent only one set of exemplary values, and that other values may be employed as appropriate. However, in this and other embodiments of the invention, it is preferred that: the third axial position of the layer where the tube winding angle and the tube pitch are both at maximum is at a point falling between 10% and 90%, more preferably between 20% and 80% or between 30% and 70% of the distance between the first axial position and the second axial position in the axial direction of the bundle; 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, and/or at least 0.5 degrees greater than the first tube winding angle and at least 0.5 ° greater than the second tube winding angle; the third tube pitch is at least 5% greater than the first tube pitch, more preferably at least 10% greater than the first tube pitch, and at least 5% greater than the second tube pitch, more preferably at least 10% greater than the second tube pitch. Typically, the tube layers will have (at any location between the first axial location and the second axial location) a minimum tube wrap angle of 2 °, a maximum tube wrap angle of 25 °, and a maximum tube pitch of 2.0 tube diameters.

Fig. 8 and 9 are a perspective view and a top-down cross-sectional view, respectively, of an (axial) intermediate portion of the tube layers of fig. 6 and 7, which portion covers a third axial position of said layers. These views show the tube layer having a diameter D, taken along a plane containing the tube bundle axis Z, with the center-to-center distance between the tube turns at an angular coordinate θ about the tube bundle axis (measured parallel to the tube bundle axis) corresponding to the point on the right-hand side of the tube layer (as viewed in fig. 8 and 9) where the tube layer has been broken away, labeled p1 through p7. It can be seen that the tube wrap angle increases (at the third axial position described above, not marked in fig. 8 and 9) to a maximum value and then decreases, proceeding in the direction of the indicated arrow along the tube bundle axis, as a result of which the center-to-center distance p1 to p7 between each tube turn likewise increases, reaches a maximum value and then decreases (such that p1< p2< p3< p4> p5> p6> p 7), and likewise the tube pitch also increases (at the third axial position described above) to a maximum value and then decreases as proceeding in the direction of the indicated arrow along the tube bundle axis. It is also noted that although the center-to-center distances p1 through p7 depicted in fig. 8 and 9 do not correspond exactly to the tube pitch of the tube coil (as the distances p1 through p7 are measured parallel to the tube bundle axis and the tube pitch is measured perpendicular to the axis of the tube coil), the two sets of values are closely related. It should also be noted that the particular sizes of the center-to-center distances p 1-p 7 and corresponding tube pitches depicted in fig. 9 are merely illustrative and are not necessarily drawn to scale.

Fig. 10 and 11 show variations in tube pitch and tube wrap angle in a tube layer of a CWHE according to another embodiment of the invention.

More specifically, for the tube layers according to this embodiment, fig. 10 is a graph depicting the relative relationship of the tube pitch to the position along the tube bundle axis, and fig. 11 is a graph depicting the relative relationship of the tube winding angle to the position along the tube bundle axis. For simplicity, in this embodiment the tube layer is also composed of a single helically wound tube, although in practice a tube layer comprising a plurality of helically wound tubes may equally be used. The position along the axis of the tube bundle is plotted on the X-axis (horizontal axis) of the two figures, where 0 represents a first axial position 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 represents a second axial position 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 a number between 0 and 1 represents a fraction of the total axial distance from the first axial position to the second axial position. The normalized value of the tube pitch is plotted on the Y-axis (vertical axis) of fig. 10, and the normalized value of the tube winding angle is plotted on the Y-axis (vertical axis) of fig. 11. The first axial position may be, in particular, an axial position at one end of the tube layer and the second axial position may be, in particular, an axial position at the other end of the tube layer, such that fig. 10 and 11 depict the tube pitch and tube wrap angle over the entire axial length of the tube bundle, excluding only the ends of the tube bundle, with the tail portions of the tubes bunched together for insertion and fixation into the tube sheet.

It can be seen that in this exemplary embodiment, both the winding angle and the pitch of the helically wound tube forming the tube layer are initially kept constant, from the first axial position to the fourth axial position of the tube layer, i.e. in this particular example at a point of 20% of the total axial distance from the first axial position to the second axial position. At this fourth axial position, both the winding angle and the pitch of the tube undergo a step change to a higher (maximum) value and then remain at this higher (maximum) value until the fifth axial position, i.e. in this particular example at a point of 60% of the total axial distance from the first axial position to the second axial position. At this fifth axial position, both the winding angle and the pitch of the tube undergo another step change, this time to a lower value, and remain at this lower value from the fifth axial position to the second axial position of the tube layer. Thus, in this embodiment, the third axial position of the tube layer where both the tube winding angle and the tube pitch are maximum may be considered as any axial position between the fourth axial position and the fifth axial position. In this particular example, the normalized tube pitch at the first axial position (i.e., the first tube pitch) is 1.0, and the normalized tube winding angle at the first axial position (i.e., the first tube winding angle) is 1.0, the normalized tube pitch at the second axial position (i.e., the second tube pitch) is 1.0, and the normalized tube winding angle at the second axial position (i.e., the second tube winding angle) is 1.0, and the normalized tube pitch at the third axial position (i.e., the third tube pitch) is 1.07, and the normalized tube winding angle at the third axial position (i.e., the third tube winding angle) is about 1.07.

It should be noted that the specific distances of the fourth and fifth axial positions (and thus the third axial position) from the first and second axial positions, and the specific tube pitch and tube wrap angle in the above-described embodiments, represent only one set of exemplary values, and that other values may be employed as appropriate, as discussed above with respect to the embodiments of the present invention described with reference to fig. 6-9.

It should also be noted that in further embodiments of the present invention, a CWHE with a tube layer having variations in tube pitch and tube wrap angle that are a mix of the variations in tube pitch and tube wrap angle depicted in fig. 6 and 7 and fig. 10 and 11 may be used. For example, in one embodiment, the tube pitch and wrap angle may each initially remain constant from a first axial position to a fourth axial position (similar to fig. 10 and 11), then each gradually increase to a maximum value at a third axial position, then decrease back to a lower value at a fifth axial position (in a manner similar to fig. 6 and 7), and then remain at the lower value from the fifth axial position to the second axial position (similar to fig. 10 and 11). In another embodiment, the tube pitch and wrap angle may each be progressively increased (in a manner similar to the progressively increased manner in fig. 6 and 7) from the first axial position to a fourth axial position at which a maximum value is reached, then each remain constant (similar to fig. 10 and 11) at said maximum value from the fourth axial position to a fifth axial position, and then progressively decreased (in a manner similar to the progressively decreased manner in fig. 6 and 7) from the fifth axial position to the second axial position. Various other combinations and permutations are also possible as will be apparent to those of ordinary skill in the art.

Fig. 12 and 13 show variations in tube pitch and tube wrap angle in a tube layer of a CWHE according to another embodiment of the invention. In this embodiment, the tube pitch and tube winding angle are varied in a similar manner to the embodiment described with reference to fig. 10 and 11. However, in this embodiment, the tube layer is formed of fourteen spirally wound tubes as an example.

Fig. 12 is a graph schematically depicting the helically wound tube of the tube layer as being unrolled and laid flat but otherwise holding the tube pitch and tube winding angle, and fig. 13 is a graph schematically depicting the helically wound tube of the tube layer as being cut along a line parallel to the tube bundle axis and being unrolled and laid flat but otherwise holding the tube pitch and tube winding angle. In each of these figures, each tube is depicted by a different hatching. In each figure, the position along the axis of the tube bundle is plotted on the Y-axis (vertical axis), wherein 0 represents a first axial position 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 represents a second axial position 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 wherein numerals between 0 and 1 represent fractions of the total axial distance from the first axial position to the second axial position, respectively. 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 position may be, in particular, an axial position at one end of the tube layer and the second axial position may be, in particular, an axial position at the other end of the tube layer, such that fig. 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, with the tail portions of the tubes bunched together for insertion and fixation into the tube sheet.

As can be seen from fig. 12 and 13, at the first axial position, all tubes at this position have the same tube winding angle and the same tube pitch (the winding angle thus representing the first tube winding angle of the layer and the pitch thus representing the first tube pitch of the layer), and both the winding angle and the pitch of all tubes remain constant, from the first axial position to the fourth axial position of the tube layer, i.e. in this particular example one third (33%) of the total axial distance from the first axial position to the second axial position. At this fourth axial position, the winding angle and pitch of the tubes undergo a step change to a higher maximum value that is the same for all tubes, and all 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 position is reached, in this particular example, two-thirds (66%) of the total axial distance from the first axial position to the second axial position (thus, in this embodiment, the third axial position of the tube layer where each of the tube winding angle and tube pitch of the layer is maximum can be considered as any axial position between the fourth axial position and the fifth axial position). Finally, at the fifth axial position, both the winding angle and the pitch of the tube undergo a step change to the same lower value for all tubes, and then the tubes remain at this lower winding angle and lower pitch from the fifth axial position to the second axial position of the tube layer (said winding angle thus representing the second tube winding angle of the layer and said pitch thus representing the second tube pitch of the layer).

It should be noted that while the tube layers in this exemplary embodiment are formed of fourteen tubes, a greater or lesser number of tubes may be used in other embodiments as well. Likewise, the specific distances of the fourth and fifth axial positions (and thus the third axial position) from the first and second axial positions, and the specific tube pitch and tube wrap angle used in the above-described embodiments, represent only one set of exemplary values, and other values may be employed as appropriate, as discussed above with respect to the embodiments of the present invention described with reference to fig. 6-9.

Fig. 14 shows the variation of tube pitch and tube winding angle in a tube layer of a CWHE according to a further embodiment of the invention, in this embodiment, the tube layer is also formed of fourteen helically wound tubes as an example.

Fig. 14 is a graph schematically depicting the helically wound tube of the tube layers as if it were unrolled and laid flat but with the tube pitch and tube winding angle of the tube maintained, each tube depicted by a different hatching. The position along the axis of the tube bundle is plotted on the Y-axis (vertical axis), wherein 0 represents a first axial position 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 represents a second axial position 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 wherein numerals between 0 and 1 represent fractions of the total axial distance from the first axial position to the second axial position, respectively. The tube layer perimeter is plotted on the X-axis (horizontal axis). The first axial position may be, in particular, an axial position at one end of the tube layer and the second axial position may be, in particular, an axial position 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, with the tail portions of the tubes bunched together for insertion and fixation into the tube sheet.

As can be seen from fig. 14, at the first axial position, all tubes have the same tube winding angle and the same tube pitch (the winding angle thus representing the first tube winding angle of the layer and the pitch thus representing the first tube pitch of the layer), and both the winding angle and the pitch of all tubes remain constant, from the first axial position to the fourth axial position of the tube layer, i.e. in this particular example one third (33%) of the total axial distance from the first axial position to the second axial position. Starting from this fourth axial position, the winding angle and pitch of each tube gradually increase, moving in the axial direction of the bundle from the fourth axial position to a third axial position, where the tube winding angle and the tube pitch of each tube reach a maximum value, said maximum winding angle of each tube being the same (and thus representing the third tube winding angle of the layer) and said maximum pitch of each tube being the same (and thus representing the third tube pitch of the layer). In this particular example, the third axial position falls at a point that is half (50%) of the total axial distance from the first axial position to the second axial position. Starting from the third axial position, the tube wrap angle and tube pitch of each tube then gradually decrease, moving in the axial direction of the bundle from the third axial position to the fifth axial position, i.e. in this particular example, two-thirds (66%) of the total axial distance from the first axial position to the second axial position. At this fifth axial position, all tubes have the same tube winding angle and the same tube pitch, and subsequently all tubes remain at this winding angle and this pitch from the fifth axial position of the tube layer to the second axial position of the tube layer (said winding angle thus representing the second tube winding angle of the layer and said pitch thus representing the second tube pitch of the layer).

Also, while the tube layers in this exemplary embodiment are formed of fourteen tubes, a greater or lesser number of tubes may be used in other embodiments as well. Likewise, the specific distances of the third, fourth, and fifth axial positions from the first and second axial positions, as well as the specific tube pitch and tube wrap angle used in the above-described embodiments, represent only one set of exemplary values, and other values may be employed as appropriate, as discussed above with respect to the embodiments of the present invention described with reference to fig. 6-9.

Fig. 15 shows the variation of tube pitch and tube winding angle in a tube layer of a CWHE according to a further embodiment of the invention, in which the tube layer is likewise formed by fourteen helically wound tubes as an example.

Fig. 15 is a graph schematically depicting a helically wound tube of tube layers as if it were unrolled and laid flat but with the tube pitch and tube winding angle of the tube maintained, each tube depicted by a different hatching. The position along the axis of the tube bundle is plotted on the Y-axis (vertical axis), wherein 0 represents a first axial position 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 represents a second axial position 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 wherein numerals between 0 and 1 represent fractions of the total axial distance from the first axial position to the second axial position, respectively. The tube layer perimeter is plotted on the X-axis (horizontal axis). The first axial position may be, in particular, an axial position at one end of the tube layer and the second axial position may be, in particular, an axial position 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, with the tail portions of the tubes bunched together for insertion and fixation into the tube sheet.

As can be seen from fig. 15, at the first axial position, all tubes have the same tube winding angle and the same tube pitch (said winding angle thus representing the first tube winding angle of the layer and said pitch thus representing the first tube pitch of the layer), and both the tube winding angle and the pitch remain constant from the first axial position to the fourth axial position of the tube layer, i.e. in this particular example one third (33%) of the total axial distance from the first axial position to the second axial position. At this fourth axial position, the winding angle of the first half of the tubes of the layer increases, and at a fifth axial position, further a very small distance along the tube bundle axis, the winding angle of the other half of the tubes increases to match the winding angle of the first half of the tubes. This results in fourteen tubes of the layer being grouped into seven pairs of tubes at a fifth axial location, whereby the center-to-center distance between the tubes of each pair of tubes is less than the center-to-center distance between the tubes of one pair and the adjacent tubes of an adjacent pair, and therefore the tube pitch of each tube at the fifth axial location is the average of the center-to-center distances between that tube and each of its adjacent tubes. However, in this fifth axial position, both the tube pitch of the tube layer (for this purpose the average of the tube pitches of all tubes at this axial position) and the tube winding angle of the tube layer are at their maximum values (the pitch thus represents the third tube pitch of the layer and the winding angle thus represents the third tube winding angle of the layer). The tube pitch and tube winding angle of the layer remain at these maximum values until the sixth axial position is reached, i.e., in this particular example, two-thirds (66%) of the total axial distance from the first axial position to the second axial position (and accordingly in this embodiment, the third axial position at which both the tube winding angle and tube pitch of the layer are at maximum values may be considered to be any axial position between the fifth axial position and the sixth axial position). At this sixth axial position, the winding angle of the first half of the tubes of the layer is reduced, and at a seventh axial position, further a very small distance along the tube bundle axis, the winding angle of the other half of the tubes is reduced to match the winding angle of the first half of the tubes. This results in fourteen tubes of the layer being "ungrouped" such that all tubes have the same tube wrap angle and the same tube pitch at a seventh axial position where the tube wrap angle and tube pitch of the tube layer is less than the tube wrap angle and tube pitch of the tube layer at an axial position between the fifth axial position and the sixth axial position. Then from the seventh axial position of the layer to the second axial position of the tube layer, both the tube winding angle and the pitch of the layer remain constant (the tube winding angle and the tube pitch thus represent the second tube winding angle of the layer and the second tube pitch of the layer).

It should be noted that while the tube layers in this exemplary embodiment are formed of fourteen tubes, a greater or lesser number of tubes may be used in other embodiments as well. Likewise, the specific distances of the fourth, fifth, sixth and seventh axial positions (and thus the third axial position) from the first and second axial positions, and the specific tube pitch and tube wrap angle used in the above-described embodiments, represent only one set of exemplary values, and other values may be employed as appropriate, as discussed above with respect to the embodiments of the present invention described with reference to fig. 6-9. Further, while in this exemplary embodiment the tubes are grouped in pairs in the middle portion of the bundle (i.e., between the fifth axial position and the sixth axial position), in other embodiments the tubes may be grouped in 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 is discussed. However, as mentioned above, preferably all or substantially all tube layers of the tube bundle have an inconsistent tube pitch, whereby the CWHE has a larger tube pitch at a (axial) intermediate position of the tube bundle than at a position towards the axial end of the tube bundle, and thus preferably all or substantially all tube layers of the tube bundle (e.g. at least 90%, and more preferably at least 95%) have a first tube winding angle and a first tube pitch at a first axial position of the layers towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial position of the layers towards the opposite axial end of the bundle, and a third tube winding angle and a third tube pitch at a third axial position of the layers between the first axial position and the second axial position, 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 also preferred that all or substantially all tube layers (e.g., at least 90%, and more preferably at least 95%) of the tube bundles according to the embodiments of the invention in question have the same or substantially the same configuration as the tube layers already described. In particular, it is preferred that the third axial position of each of the tube layers is at the same or substantially the same axial position of the bundle (e.g., each third axial position is at the same axial position or all falls within an axial region representing 5% or less of the total axial length of the bundle). Also, preferably the fourth, fifth, sixth and/or seventh axial position (if present) of each of said tube layers is at the same or substantially the same axial position of the bundle. Also, preferably the first, second and third winding angles of each of the layers are the same or substantially the same.

It is to be understood that the invention is not limited to the details described above with reference to the preferred embodiments, but that many modifications and variations are possible without departing from the spirit or scope of the invention as defined in the appended claims.

Claims

1. A coil 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 fluid streams for heat exchange with fluid flowing through the pressure vessel outside the tubes, the tubes being helically wound around a mandrel extending in an axial direction of the bundle so as to form a plurality of tube layers around the mandrel, each of the tube layers comprising one or more of the 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 position of the layer towards one axial end of the bundle, a second tube winding angle and a second tube pitch at a second axial position of the layer towards an opposite axial end of the bundle, and a third tube winding angle and a third tube pitch at a third axial position of the layer between the first axial position and the second axial position, 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.

2. The CWHE of claim 1 wherein all or substantially all of the tube layers have a first tube wrap angle and a first tube pitch at a first axial position of the layers toward one axial end of the bundle, a second tube wrap angle and a second tube pitch at a second axial position of the layers toward an opposite axial end of the bundle, and a third tube wrap angle and a third tube pitch at a third axial position of the layers between the first axial position and the second axial position, the third tube wrap angle being greater than the first tube wrap angle and greater than the second tube wrap angle, and the third tube pitch being greater than the first tube pitch and greater than the second tube pitch for each of the layers.

3. The CWHE of claim 2 wherein the third axial position of each of the tube layers is at the same or substantially the same axial position of the bundle.

4. The CWHE of claim 1 wherein in at least one or all or substantially all of the tube layers, the third tube wrap angle at the third axial position is equal to or greater than any tube wrap angle at any other axial position of the layer between the first axial position and the second axial position of the layer, and the third tube pitch at the third axial position is equal to or greater than any tube pitch at any other axial position of the layer between the first axial position and the second axial position of the layer.

5. The CWHE of claim 1 wherein in at least one or all or substantially all of the tube layers, the third axial position of the layers is at a point between 10% and 90% of the distance in the axial direction of the bundle that falls between the first axial position and the second axial position.

6. The CWHE of claim 1 wherein the third tube wrap angle is at least 5% greater than the first tube wrap angle and at least 5% greater than the second tube wrap angle in at least one or all or substantially all of the tube layers.

7. The CWHE of claim 1 wherein in at least one or all or substantially all of the tube layers, the third tube wrap angle is at least 0.5 degrees greater than the first tube wrap angle and at least 0.5 degrees greater than the second tube wrap angle.

8. The CWHE of claim 1 wherein in at least one or all or substantially all of the tube layers, the third tube pitch is at least 5% greater than the first tube pitch and at least 5% greater than the second tube pitch.

9. The CWHE of claim 1 wherein in at least one or all or substantially all of the tube layers, the third tube pitch is at least 10% greater than the first tube pitch and at least 10% greater than the second tube pitch.

10. The CWHE of claim 1 wherein the CWHE is a vertically oriented CWHE wherein the axial direction of the bundle is vertical or substantially vertical, the CWHE further comprising a refrigerant distributor above the tube bundle for distributing refrigerant into the pressure vessel outside the tubes such that the refrigerant flows down through the pressure vessel on and through the tube bundle outside the tubes.

11. The CWHE of claim 10 wherein the CWHE has an inlet configured to introduce the one or more fluid streams into the plurality of tubes of the tube bundle at a bottom of the tube bundle and an outlet configured to withdraw the one or more fluid streams from the tubes at a top of the tube bundle such that one or more streams flow upward through the tubes of the tube bundle.

12. The CWHE of claim 10 wherein the CWHE is configured to use a liquid or two-phase refrigerant that vaporizes as it flows down the pressure vessel over and through the tube bundle outside the tubes.

13. A method of cooling and/or liquefying one or more fluid streams via heat exchange with a refrigerant using the CWHE of claim 1, wherein the method comprises passing the one or more fluid streams through the plurality of tubes of the tube bundle and passing the refrigerant outside the tubes through the pressure vessel.

14. The method of claim 13, wherein the one or more fluid streams to be cooled and/or liquefied comprise a natural gas stream.

15. The method of claim 13, wherein the refrigerant comprises a mixed refrigerant.