Indirect heat exchanger pressure vessel with controlled corrugation bends

Abstract

In one aspect of the disclosure, an indirect heat exchanger pressure vessel is provided that includes an inlet header for receiving a pressurized working fluid, such as water, glycol, ammonia, and / or CO2. The indirect heat exchanger pressure vessel includes an outlet header for collecting the pressurized working fluid and a serpentine tube connecting the inlet header and the outlet header. The serpentine tube allows the pressurized working fluid to flow from the inlet header to the outlet header. The serpentine tube includes a piping section and a return bend connecting the piping sections. The return bend has a controlled corrugation with alternating ridges and grooves. The alternating ridges and grooves strengthen the return bend and allow the indirect heat exchanger pressure vessel to facilitate heat transfer of the working fluid at high internal operating pressures.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 138,655, filed on January 18, 2021, and U.S. Provisional Patent Application No. 63 / 270,953, filed on October 22, 2021, both of which are incorporated by reference in their entireties into this specification.

[0002]

[0002] The present disclosure relates to an indirect heat exchanger, and more particularly to an indirect heat exchanger having a serpentine tube with multiple formed bends, conveying a pressurized working fluid through the serpentine tube and enabling heat transfer between the working fluid inside the serpentine tube and a fluid outside the serpentine tube, wherein the working fluid and the outside fluid can each be a gas, a liquid, or a mixture of gas and liquid. [Background technology]

[0003]

[0003] Heat exchangers are known, including direct and indirect heat exchangers. A direct heat exchanger transfers heat between a working fluid and another fluid through contact between the fluids. An indirect heat exchanger transfers heat between a working fluid and another fluid indirectly through a medium that separates the fluids.

[0004]

[0004] Various types of heat exchange devices are known, including direct heat exchangers, indirect heat exchangers, or both. Known heat exchange devices include open circuit heat exchange devices, such as open circuit cooling towers, and closed circuit heat exchange devices, such as closed circuit cooling towers. Open circuit cooling towers may exchange heat between a working fluid, such as water, and an external fluid, such as ambient air, by distributing the working fluid over a fill material. The working fluid is cooled directly by the ambient air as it moves along the fill material. In contrast, closed circuit cooling towers keep the working fluid separated from the external fluid.

[0005]

[0005] Closed-circuit heat exchanger devices include closed-circuit cooling towers for fluids, evaporative condensers for refrigerants, dry chillers, air-cooled condensers, and ice storage systems. These heat exchange devices utilize one or more heat exchangers to transfer heat between a pressurized working fluid and an external fluid, such as ambient air, an evaporative liquid, or a combination thereof.

[0006] For example, a heat exchanger apparatus may include a closed-circuit cooling tower having an indirect heat exchanger pressure vessel including an inlet header receiving a pressurized working fluid, an outlet header, and an indirect heat exchange coil connecting the inlet header and the outlet header. The indirect heat exchange coil may include one or more serpentine circuit tubes configured to transfer heat between the pressurized working fluid inside the indirect heat exchange coil and a fluid, such as an evaporative liquid, outside the indirect heat exchange coil. The inlet header receives the internal working fluid from an upstream component of the heat exchange apparatus, and the outlet header collects the pressurized working fluid before the working fluid is sent to a downstream component of the heat exchange apparatus.

[0007]

[0007] An indirect heat exchanger pressure vessel, including an inlet header, an outlet header, and one or more serpentine circuit tubes, must withstand high pressures appropriate for a particular application and meet national and international engineering standards, such as ASME standard B31.5. For example, an indirect heat exchanger pressure vessel of a closed-circuit cooling tower may be rated to withstand an internal pressure of 150 psig for fluids such as water, glycol, and brine. As another example, an indirect heat exchanger pressure vessel of an evaporative condenser may be able to withstand an internal pressure of up to 410 psig or more for typical refrigerants such as ammonia or R-407C. As yet another example, some evaporative condensers have an indirect heat exchanger pressure vessel with an internal pressure rating of 1200 psig or more for refrigerants such as CO2.

[0008]

[0008] The serpentine tubes of an indirect heat exchanger pressure vessel typically include straight lengths and bends connecting the straight lengths. The straight lengths of the serpentine tubes are typically joined by approximately 180 degree bends or compound bends having multiple bends, such as two 90 degree bends joined by a length of pipe. The serpentine tubes may be stacked on top of each other during assembly of the heat exchanger apparatus, typically with the serpentine tubes touching each other in the region of the return bends and with the serpentine tubes vertically staggered.

[0009]

[0009] Serpentine tubes are often made by first forming an elongated tube from a long flat piece of metal, such as mild steel or stainless steel. The flat piece of metal is rolled into a generally circular cross section, and the longitudinal edges are welded together by a continuous longitudinal weld to form a straight tube. In another approach, a seamless tube forming process is used to form the straight tube. The resulting straight tube can then be bent at spaced locations along the tube to form the tube into a serpentine shape with straight piping sections connected by bends. Tube bending is a complex process and often utilizes hydraulic, electric, or manual tube bending machines with bending dies, clamp dies, pressure dies, and optionally mandrel and wiper dies. The tube bending machine can be set to form bends with any desired angle of 180 degrees or less, such as 80 degrees, 90 degrees, 100 degrees, or 180 degrees. As mentioned above, the return bends in the serpentine tubing may include compound bends having two or more bends each, such as an 80 degree bend and a 100 degree bend, connected by lengths of straight tubing.

[0010]

[0010] To form a bend in the tube, the tube is fed into the tube bender and a portion of the tube is received in a recess in the bending die. A pressure die and a clamping die, which include a recess for the tube, are moved relative to the other side of the tube, so that the pressure die is positioned to support the tube and the clamping die clamps a portion of the tube between the clamping die and the bending die. The tube bender then rotates or pivots the bending die and the clamping die through the desired bend angle. The pressure die moves forward as the bending die and the clamping die pivot to support the tube and ensure that the tube follows the contour of the bending die. Once the bend is formed in the tube, the clamping die and the pressure die are retracted from their clamping positions and the tube is fed forward until the next bend in the tube is positioned in the tube bender, and the bending die, the clamping die, and the pressure die all return to their initial positions. The bending process is repeated for each bend formed in the serpentine tube. Some tubes are bent only once to form a single bent tube, commonly referred to as a hairpin tube or a candy cane tube, which can later be butt welded together.

[0011]

[0011] Bending of tubes that receive pressurized working fluid is a process that balances various considerations, including performance, safety, and packaging criteria for a particular application. Also, unintended deformation of the tube wall during bending can result in failure of the tube due to the pressure of the working fluid in the tube, corrosion of the tube, and / or higher pressure drop of the working fluid through the tube. In some tube bending processes, an inner mandrel is advanced inside the tube to support the tube wall during bending, and a wiper die may be used to stiffen the tube wall at the rear end inside the bend to prevent unintended deformation of the tube. The inner mandrel may be a plug mandrel or may have one or more balls or rings, in which case the inner mandrel is referred to as a ball mandrel.

[0012]

[0012] Tube bending generally includes the following parameters:

[0013]

[0013] OD = outer diameter of the pipe

[0014]

[0014] WT = pipe wall thickness

[0015] CLR = Centerline radius of bend

[0016]

[0016] Dimensions are measured using common measurement scales such as inches or millimeters. These parameters are used to calculate the ratio of two properties:

[0017]

[0017]

[0018]

number

[0018]

[0019] Two other parameters of interest in bending are the outside radius of the bend (OSR), usually called the extrados, and the inside radius of the bend (ISR), usually called the intrados.

[0019]

[0020] The W-ratio and D-ratio are further aggregated into a single factor that indicates the complexity of the bend, which is calculated as follows:

[0020]

[0021]

number

[0021]

[0022] W, D, and / or C BThe values ​​of can be used to determine whether it is possible to form a bend without an internal mandrel, called an empty bend, or whether an internal mandrel is required. In the latter case, the process is called mandrel bending. For mandrel bending, these ratios help determine whether the required internal mandrel should be a multi-ball mandrel, a single ball mandrel, or a simpler plug mandrel. Finally, these ratios help determine whether a wiper die is required in combination with the internal mandrel. As an example, process recommendations for various bend complexities are shown in the table below.

[0022] [Table 1]

[0023] To determine the type of bend required, use the W, D, and / or C on an industry standard tube bending chart. B For example, to determine the process parameters for bending a tube with an outside diameter of 1'', a wall thickness of 0.05'', and a centerline radius of 2'', the ratios W and D are:

[0024]

[0024]

[0025]

number

[0025]

[0026] An industry standard tube bending chart would recommend that with a W ratio of 20 and a D ratio of 2, a regular pitch inner mandrel with one ball should be used, with the addition of a wiper die.

[0026]

[0027] Substituting, C of the example bend above B becomes:

[0027]

[0028]

number

[0028]

[0029] Referring to the table above, this C B The values ​​also indicate that an internal mandrel is recommended, but a wiper die may be optional. The small differences in the recommendations for the mandrel and wiper indicate some flexibility in the configuration of the bend, and that tool design and tubing selection may be able to compensate for the absence of an internal mandrel and / or wiper die.

[0029]

[0030] The conventional bend chart used in the industry and the bend complexity value (C B ) range is based on the assumption that the contours of the tooling grooves formed by the bend die and clamping die in which the tube seats during bending are circular and complement the shape of the round tube. However, bending tool design has made some advances in recent years, designing bend tooling with compound radii within the tooling grooves to compress and support the tube during bending, thereby extending the range of open bends from about 5 to about 12 C. B It is possible to extend the range to a value.

[0030]

[0031] In addition, especially C B As B approaches and exceeds 20, it becomes increasingly necessary to use an internal mandrel and wiper die to successfully bend the tube. Internal mandrel bending has several disadvantages, including the use of a mandrel, which requires additional tooling, increasing costs, can increase scrap if the mandrel is not used correctly, can increase cycle time, and requires the use of lubricants, increasing the time and cost for lubrication and subsequent environmental mitigation.

[0031]

[0032] C BOne problem as φ approaches and exceeds 20 is that the associated mandrel bends impose limitations on the continuous length of the tube. Serpentine tubes can be very long, up to 400 feet long in some applications. Physical limitations on the length of the mandrel rod and settings mean that an internal mandrel cannot be used to bend long continuous serpentine tubes with several bends. This forces manufacturers to make one or two bends in short segments of tube, sometimes called candy canes, and then butt weld the tube segments together to create the larger circuit. Not only does this involve additional labor and cost, but the additional butt welds increase the chance of leaks and may not be acceptable in many applications due to the high operating pressures that the serpentine tubes will be subjected to.

[0032]

[0033] C B Another problem that can arise when approaches and exceeds 20 is that the associated internal mandrel bends can move the neutral axis of the bend closer to the inside of the bend, causing excessive thinning of the outer wall of the bend. Thinning of the outer wall of the bend can weaken the serpentine tube so that it cannot withstand the pressure of the working fluid for a particular application. Excessive thinning of the outer bend wall can also result in processing variability when forming the bend, reducing the quality of the bend area.

[0033]

[0034] The above problems make it desirable for manufacturers to avoid the use of internal mandrels for tube bending. One approach to avoid using internal mandrels for tubes with a given OD is to increase the WT or increase the CLR to an appropriate value to bring the bend into the range of the free bend. Increasing the wall thickness (WT) may not be an option for manufacturers of products that do not require such relatively thick walls from an operational standpoint. In certain cases, thicker walls may increase the fluid side pressure drop, make the product less thermally efficient, increase the weight of the assembly, and increase the material cost of the serpentine tube. Also, increasing the CLR may not be an option when the serpentine tube needs to fit into a given space due to other operational considerations. Increasing the CLR may also have a negative impact on the thermal and hydraulic efficiency of the overall coil in some cases. Summary of the Invention

[0034]

[0035] In one aspect of the disclosure, an indirect heat exchanger pressure vessel is provided that includes an inlet header for receiving a pressurized working fluid, an outlet header for collecting the pressurized working fluid, and a serpentine tube connecting the inlet header and the outlet header to allow the pressurized working fluid to flow from the inlet header to the outlet header. The pressurized fluid may be, for example, water, glycol, a glycol mixture, ammonia, or CO2, to name a few examples. The pressurized fluid may be a liquid, such as water, or a liquid / gas combination, such as a refrigerant liquid and a refrigerant vapor. The serpentine tube includes a piping section and a return bend that connects the piping sections. The return bend includes a controlled corrugation that includes alternating ridges and grooves. The controlled corrugation of the return bend provides a rigid structure that resists internal pressure during operation of the indirect heat exchanger pressure vessel. Additionally, the controlled corrugation provides a structural bend centerline radius that is greater than the actual bend centerline radius of the return bend. The larger structural bend centerline radius reduces the bend complexity factor of the return bend compared to a conventional serpentine tubing return bend having the same outer diameter and wall thickness. With the reduced bend complexity factor, the return bend with controlled corrugations can be bent without the use of an internal mandrel, which simplifies the manufacturing process of the serpentine tubing.

[0035]

[0036] The present disclosure also provides an indirect heat exchanger pressure vessel including an inlet header for receiving a pressurized working fluid, an outlet header for collecting the pressurized working fluid, and a serpentine tube connecting the inlet header and the outlet header to enable flow of the pressurized working fluid from the inlet header to the outlet header. The serpentine tube includes a piping section, a return bend connecting the piping sections, and a junction at a junction between the return bend and the piping section. The return bend includes a bend angle and a controlled crimp. The controlled crimp is spaced from the junction along the serpentine tube and has an angular extent around an inside of the return bend that is less than the bend angle. In this manner, the controlled crimp may be formed using a bending die having corresponding controlled crimping features that are less than the entire inner ring of the return bend, allowing the serpentine tube to slide lengthwise out of the bending die and increasing the speed at which the return bend may be formed in the serpentine tube. In one embodiment, the controlled corrugations include ridges having an amplitude that is smaller near the contact point and increases as the corrugations extend away from the contact point to reduce resistance to fluid flow through the return bend and reduce the internal fluid pressure drop in the return bend relative to a non-tapered or non-relaxed configuration of the corrugation ridges.

[0036]

[0037] In another aspect, an indirect heat exchanger pressure vessel is provided that includes an inlet header for receiving a pressurized working fluid, an outlet header, and a serpentine tube connecting the inlet header and the outlet header to facilitate flow of the pressurized working fluid from the inlet header to the outlet header. The serpentine tube includes a pair of piping sections and a return bend connecting the piping sections. The return bend includes an inner portion having a sinusoidal pattern on an inner ring of the return bend, the sinusoidal pattern including peaks and valleys. The inner portion of the bend includes an arc pattern that intersects the sinusoidal pattern, the arc pattern including arcs of peaks that intersect the peaks and arcs of valleys that intersect the valleys. The intersecting sinusoidal and arc patterns provide a smooth, continuously curved sidewall of the serpentine tube that strengthens the return bend against internal pressure. In one embodiment, the sinusoidal pattern has one or more end portions with shallower peaks and valleys and an intermediate portion with deeper peaks and valleys to reduce the internal fluid pressure drop across the return bend compared to a sinusoidal pattern with constant peak and valley sizes.

[0037]

[0038] The present disclosure also provides a closed-circuit cooling tower including an indirect heat exchanger comprising a plurality of serpentine tubes having piping sections and return bends connecting the piping sections. The return bends include wrinkled bends having controlled wrinkles. The closed-circuit cooling tower comprises a blower operable to generate airflow against the serpentine tubes and an evaporative liquid distribution assembly configured to distribute evaporative liquid onto the serpentine tubes. The closed-circuit cooling tower further comprises a sump to receive evaporative liquid dropping from the serpentine tubes and a pump operable to pump evaporative liquid from the sump back to the evaporative liquid distribution assembly. The controlled wrinkled bends strengthen the serpentine tubes to withstand internal pressure from a working fluid in the serpentine tubes during operation of the cooling tower. The controlled wrinkle bend also provides a structural centerline radius of the wrinkle bend that is greater than the actual centerline radius of the controlled wrinkle bend, providing a reduced bend complexity factor compared to a return bend of a conventional serpentine tube having the same outer diameter and wall thickness. The reduced bend complexity factor allows the controlled wrinkle bend to be bent without the use of an internal mandrel, which simplifies the manufacturing process of the serpentine tube. [Brief description of the drawings]

[0038] [Figure 1]

[0039] FIG. 2 is a perspective view of an indirect heat exchange device having a serpentine circuit tube, in which piping sections are connected by bends in the serpentine circuit tube. [Diagram 2]

[0040] FIG. 1 is a schematic diagram of a heat exchange device including a serpentine tube. [Diagram 3]

[0041] FIG. 2 is a side view of a serpentine pipe having piping sections connected by 180 degree bends. [Figure 4]

[0042] FIG. 4 is a close-up view of the bend shown within the dashed circle in FIG. 3, showing the controlled wrinkles on the inside of the bend. [Diagram 5]

[0043] 5 is a cross-sectional view taken along line 5-5 of FIG. 4, showing a cross-section of a bend in a groove of a corrugated portion. [Figure 6]

[0044] 6 is a cross-sectional view taken along line 6-6 of FIG. 4, showing a cross-section of a bend in a ridge of a corrugated portion. [Figure 7]

[0045] 7 is a cross-sectional view taken along line 7-7 in FIG. 4, showing a cross-section of one of the piping sections of the circuit tube. [Figure 8]

[0046] 5 is a perspective view of the bend of FIG. 4, showing a corrugated portion on the inside of the bend and a smooth outer wall portion on the outside of the bend. [Figure 9A]

[0047] 9A is a cross-sectional view taken along line 9A-9A of FIG. 8, showing a sinusoidal pattern of wrinkles spaced from the tangent point of the bend and the piping section such that the wrinkles have an angular extent less than a 180 degree bend angle of the bend. [Figure 9B]

[0048] FIG. 9B is a cross-sectional view of another embodiment of a flexure, similar to FIG. 9A, having wrinkled portions with varying amplitudes of ridges and valleys in a sinusoidal pattern. [Figure 9C]

[0049] 9B is a cross-sectional view of another embodiment of a flexure, similar to FIG. 9A, having wrinkled portions with varying period and amplitude of sinusoidal patterned ridges and valleys. [Figure 10]

[0050] 4 illustrates a process for determining a sinusoidal pattern of bends. [Figure 11]

[0050] A process for determining the sinusoidal pattern of the bend is shown. [Figure 12]

[0050] A process for determining the sinusoidal pattern of the bend is shown. [Figure 13A]

[0050] A process for determining the sinusoidal pattern of the bend is shown. [Figure 13B]

[0050] A process for determining the sinusoidal pattern of the bend is shown. [Figure 14]

[0051] 1 is a diagrammatic representation of a portion of a sinusoidal pattern of a wrinkled portion of a return bend, showing the peaks and valleys of the sinusoidal pattern. [Figure 15]

[0052] A graphical representation of a portion of a sine pattern of return bends intersecting with a circular arc pattern of return bends, the circular arc pattern including crest arcs that intersect with crests of the sine pattern and valley arcs that intersect with valleys of the sine pattern. [Figure 16A]

[0053] 16 is a diagrammatic representation of the crest arc of FIG. 15 showing the crest arc having a radius of curvature, an angular extent, and a center, the center being radially inward of the centerline of the serpentine tube. [Figure 16B]

[0054] 16B is a diagrammatic representation similar to FIG. 16A of a peak arc having a compound radius of curvature. [Figure 16C]

[0055] 16B is a diagrammatic representation similar to FIG. 16A of a mountain arc having a shape defined by a portion of an ellipse. [Figure 17A]

[0056] 16 is a diagrammatic representation of the valley arc of FIG. 15 showing a valley arc having the same radius of curvature as the peak arc, a shorter angular extent than the peak arc, and a center, the center being radially outward from the centerline of the pipe. [Figure 17B]

[0057] 17B is a diagrammatic representation similar to FIG. 17A of a valley arc having a compound radius of curvature. [Figure 17C]

[0058] 17C is a diagrammatic representation similar to FIG. 17B of a valley arc having a shape defined by a portion of an ellipse. [Figure 18]

[0059] FIG. 16 is a perspective view of a portion of the sinusoidal pattern of FIG. 15, including the peak arcs and valley arcs, and a continuous curved wrinkled surface portion connecting the peak arcs and valley arcs. [Figure 19]

[0060] 1 is a perspective view of a tube bender showing a bending die, a pressure die, and a clamp die of the tube bender; FIG. [Figure 20]

[0061] FIG. 20 is a side view of the bending die of FIG. 19 showing the ridges and grooves that form corresponding ridges and grooves in the wrinkled portion of the tube. [Figure 21]

[0062] 20 shows a process for forming a bend in a serpentine tube using the tube bending machine of FIG. 19. [Figure 22]

[0062] FIG. 19 shows a process for forming a bend in a serpentine tube using the tube bending machine. [Figure 23]

[0062] FIG. 19 shows a process for forming a bend in a serpentine tube using the tube bending machine. [Figure 24]

[0062] FIG. 19 shows a process for forming a bend in a serpentine tube using the tube bending machine. [Diagram 25]

[0062] FIG. 19 shows a process for forming a bend in a serpentine tube using the tube bending machine. [Figure 26]

[0062] FIG. 19 shows a process for forming a bend in a serpentine tube using the tube bending machine. [Figure 27]

[0063] 20 is a plan view of a tube bent using the tube bender of FIG. 19 and the lower portion of the bending die, showing the interlocking engagement between the ridges of the bend crimp portion and the ridges of the bending die. [Figure 28]

[0064] FIG. 1 is an elevation view of a bend having a 90 degree bend angle. [Figure 29]

[0064] An elevation view of a bend having an 80 degree bend angle. [Diagram 30]

[0064] An elevation view of a bend having a bend angle of 100 degrees. [Diagram 31]

[0065] FIG. 13 is a cross-sectional view of a serpentine circuit coil having tubing with a gradually flattening cross section. [Diagram 32]

[0066] FIG. 1 is an elevation view of a pair of compound bends in a serpentine tube having three contact points therebetween, each compound bend including an 80 degree bend and a 100 degree bend. [Diagram 33]

[0067] FIG. 2 is an elevational view of a bend having an asymmetric wrinkle pattern. [Diagram 34]

[0068] FIG. 34 is a perspective view of the bottom of a bending die used to form the bend of FIG. 33. [Diagram 35]

[0069] FIG. 35 is a perspective view of the lower bending die and the corresponding upper bending die of FIG. 34. [Diagram 36]

[0070] 1 is a plan view of a tube having a flattened cross section, the tube including a straight section and a return bend with a corrugated portion. [Figure 37A]

[0071] 37A is a cross-sectional view taken along line 37A-37A of FIG. 36, showing the elliptical cross-section of the tube at the valley of the crimped portion. [Figure 37B]

[0072] 37B is a cross-sectional view taken along line 37B-37B of FIG. 36, showing the elliptical cross-section of the tube at the crest of the crimp. [Figure 37C]

[0073] 37C is a cross-sectional view taken along line 37C-37C of FIG. 36, showing the elliptical cross-section of the tube at one of the straight sections of the tube. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039]

[0074] With reference to Figure 1, an indirect heat exchanger pressure vessel such as a coil assembly 10 is provided that may be used in a heat exchange device such as an evaporative condenser, closed circuit fluid cooler, or ice storage system. The coil assembly 10 includes an inlet header 12, an outlet header 14, and a serpentine tube 16. The serpentine tubes 16 each include a piping section 18 connected with a compound bend 21 that includes a 180 degree bend 20 or two 90 degree bends 23, 25 separated by a straight length 27. The serpentine tubes 16 allow working fluid to flow from the inlet header 12, through the serpentine tube 16, to the outlet header 14.

[0040]

[0075] 2, a heat exchange device such as a cooling tower 24 is provided that includes an external structure 26, one or more blowers 28 including blower blades 30 and one or more motors 32, a direct heat exchanger such as a packing material 34, and an indirect heat exchanger pressure vessel 36. The cooling tower 24 may be an evaporative condenser, a closed circuit cooling tower, or a dry cooler heat exchanger, as some examples. The indirect heat exchanger pressure vessel 36 includes an inlet header 38, one or more serpentine circuit tubes 37 with circuit piping sections 39 and bends 40, and an outlet header 42. The inlet header 38 and the outlet header 42 may be reversed depending on the application. In some embodiments, the packing material 34 is above the indirect heat exchanger pressure vessel 36 and / or the packing material 34 is located between the piping sections of the serpentine circuit tubes 37.

[0041]

[0076] 2, the cooling tower 24 includes an evaporative liquid distribution system 43 including a spray assembly 44 having spray nozzles or orifices 46 that distribute an evaporative fluid, such as water, onto the serpentine ducts 37 and the fill material 34. The evaporative liquid distribution system 43 includes a sump 50 for collecting evaporative fluid from the fill material 34 and coils 36, and a pump 52 for pumping the collected evaporative fluid through pipes 54 to the spray assembly 44. The cooling tower 24 further includes one or more air inlets 35, an inlet louver 58 for preventing evaporative liquid from escaping the cooling tower 24, an air outlet 59, and an eliminator 56 for collecting water mist from the air before the air exits through the air outlet 59. The blower 28 is operable to generate or induce an upward airflow over the serpentine ducts 37 and the fill material 34. In other embodiments, the cooling tower 24 may include one or more blowers configured to induce airflow in an upflow direction, a downflow direction, or a cross-flow direction relative to the indirect and / or direct heat exchangers of the cooling tower 24.

[0042]

[0077] With reference to FIG. 3, a serpentine tube 70 is provided that may be utilized with a heat exchange device, such as the coil assembly 10 of FIG. 1 or the cooling tower 24 described above with respect to FIG. 2. The serpentine tube 70 includes an internal passageway 72 and a tubular sidewall 74 extending therearound. The serpentine tube includes an end portion 76 that may be connected to an inlet header and an end portion 78 that may be connected to an outlet header. Depending on the application, the end portion 76 may alternatively be connected to an outlet header and the end portion 78 may be connected to an inlet header. The serpentine tube 70 includes piping sections 79, such as piping sections 80, 82, and a bend 84. In one embodiment, the piping sections 79 may be parallel. In other embodiments, one or more of the piping sections 80 extend short-side, e.g., at an angle relative to one another, to allow for drainage of the internal fluid. The serpentine tube 70 may be self-draining, whereby any liquid within the internal passageway 72 moves downward toward the end portion 78 under the influence of gravity. The material of the serpentine tube 70, the outer diameter of the serpentine tube 70, the wall thickness of the sidewall 74, the number of pipe sections 79, the length of the pipe sections 79, the number of bends 84, the angle range of the bends 84, the centerline radius of the bends 84, and the inner / outer rings of the bends 84 may be selected for a particular heat exchanger. As another example in this regard, instead of a single angle bend 84 connecting a pair of pipe sections 79, the serpentine tube may have one or more bends 84, each including a pair of 90 degree or other bends connected by a straight segment, similar to the compound bend 21 shown in FIG. 1. The pipe section 80 may have a circular cross section throughout the pipe section 80. In other embodiments, the serpentine tube 70 includes one or more pipe sections 80 with a non-circular cross section, such as an elliptical or obround cross section.

[0043]

[0078] The serpentine tube 70 may be formed from a single straight tube that is bent at spaced locations along the tube to form the bends 84. The serpentine tube 70 may be formed by gradually rolling a strip of material into a tubular shape and welding the longitudinal edges of the strip together to form a single weld that runs along the length of the serpentine tube 70. In another approach, the serpentine tube 70 may be made from multiple separately formed components. For example, the tubing section 79 may be a separate component that is welded to the bends 84. Alternatively, the serpentine tube 70 may be formed by welding separate lengths of tube together and then bending the welded longer tube. The serpentine tube 70 may be made from a metallic material, such as carbon steel or stainless steel.

[0044]

[0079] With reference to FIG. 4, each flexure 84 includes an inner ring 90, an outer ring 92, a controlled corrugated portion 94 on the inside 96 of the flexure 84, and a smooth outer surface 98 on the outside 100 of the flexure 84. The controlled corrugated portion 94 includes a continuously curved controlled corrugated surface 134 comprised of ridges 114 and grooves 116. The continuously curved controlled corrugated surface 134 is not interrupted by edges, corners, or flats to avoid areas of localized stress. The continuously curved controlled corrugated surface 134 is shaped by the ridges 114 and grooves 116 of the flexure 84, which in turn are defined, at least in part, by intersecting sinusoidal patterns 110 and arc patterns 150, as described in more detail below with reference to FIG. 15. The flexure 84 shown in FIG. 4 has a bend angle of 180 degrees. When this disclosure refers to a particular bend angle, it is intended that the bend angle be an approximation, such as + / - 5 degrees. In some embodiments, all of the bends 84 of the serpentine tube 70 have controlled corrugations 94. In other embodiments, fewer than all of the bends 84 have controlled corrugations 94.

[0045]

[0080] The serpentine tube 70 has a tube centerline 102 that extends through the piping sections 80, 82 and into the bend 84. The controlled corrugation section 94 is radially inward from the tube centerline 102 and is separated from the tube centerline by side sections 104. The smooth exterior surface section 98 and side sections 104 allow the bend 84 to be stacked with other serpentine tube bends in conventional configurations, similar to prior art tubes having smooth internal bends.

[0046]

[0081] 4, at the inner ring 90 of the bend 84, the controlled corrugation 94 has a sinusoidal pattern 110 at the inner ring 90 of the bend 84, as described below with respect to FIGS. 8 and 9A. The corrugation 94 includes a series of alternating ridges 114 and grooves 116. In one embodiment, the bend 84 has relief portions 222, 224 intermediate the sinusoidal pattern 110 and the contact points 122, 124 between the tubing sections 80, 82 and the bend 84. The relief portions 222, 224 facilitate providing a controlled corrugation angle 240 that is less than the bend angle 220, as described in more detail below. The relief portions 222, 224 extend from the contact points 122, 124 to points 216, 218. The wrinkled portion 94 further includes tapered lead-in portions 140, 142 that extend between points 216, 218 and a point 400 (see FIG. 4) where the sinusoidal pattern 110 begins and ends. In one embodiment, the relief portions 222, 224 each have a first radius and the tapered lead-in portions 140, 142 each have a smaller second radius. The sinusoidal pattern 110 begins at one point 400 and extends through the peaks 130 of the end ridge 118, undulates through the ridges 114 and grooves 116, and extends through the peaks 132 of the end ridge 120 until it reaches the other point 400.

[0047]

[0082] The ridge 114 includes end ridges 118, 120 that optionally have tapered lead-ins 140, 142. The tapered lead-ins 140, 142 provide a smooth transition between the relief portions 222, 224 and the sinusoidal pattern 110. The tapered lead-ins 140, 142 smooth the flow of working fluid through the bend 84 and aid in the flow of material in the bend 84 as it bends. The tapered lead-ins 140, 142, the ridges 114, and the grooves 116 reduce the internal fluid pressure drop caused by the working fluid flowing through the bend 84. The tapered tip portion 140 also facilitates better drainage of the serpentine tube 70. The bend 84 can have both tapered lead-ins 140, 142 if the working fluid can flow through the bend 84 in either direction 143, 145. If working fluid flows through the bend 84 in only one direction 143,145, the bend 84 may have only one tapered lead-in portion 140,142.

[0048]

[0083] With reference to FIG. 9B, a cross-sectional view of bend 84' is provided, which is similar to bend 84 and has a sinusoidal pattern 110' at the midline of bend 84'. Bend 84' has ridges 114' and grooves 116' that vary in amplitude around bend 84'. Specifically, the ridges 114' and grooves 116' closer to pipe sections 80', 82' have smaller amplitudes, while the ridges 114' and grooves 116' near the center of bend 84' have larger amplitudes. For example, ridges 114A', 114B' have larger amplitudes than ridges 114C', 114D'. The more gradual increase in amplitude of ridges 114' and grooves 116' reduces the resistance to fluid flow through bend 84', thereby reducing the pressure drop across bend 84' as compared to bend 84 in some applications. The more gradual increase in amplitude of ridges 114' and grooves 116' may also reduce stress in the material of flexure 84' during bending operations as compared to flexure 84 in some applications. In other embodiments, the amplitude of the sinusoidal pattern of flexure 84' may increase from one adjacent piping section connected to flexure 84' to the other adjacent piping section connected to flexure 84'.

[0049]

[0084] With reference to FIG. 9C, a cross-sectional view of flexure 84'' is provided, which is similar to flexure 84, but has a controlled corrugated portion 94'' with a sinusoidal pattern 110'' at an inner ring of flexure 84''. Controlled corrugated portion 94'' includes ridges 114'' and grooves 116''. Controlled corrugated portion 94'' includes a first portion 115'' having ridges 114''A,B and grooves 116''A,B of a first amplitude and a first periodicity 117''. Controlled corrugated portion 94'' includes a second portion 119'' having ridges 114''C,D and grooves 116''C,D of a second amplitude greater than the first amplitude. Ridges 114''C,D and grooves 116''C,D have a second periodicity 121'' less than the first periodicity 117''. Controlled corrugated portion 94'' further includes a third portion 123'' having ridges 114''E,F and grooves 116''E,F with a third amplitude substantially the same as the second amplitude of second portion 119'' and a third period 125'' less than the second period 121''. Bend 84'' receives fluid in direction 127'', and ridges 114''A include a tapered lead-in portion 129'' to smooth fluid flow through bend 84''. Tapered lead-in portion 129'' reduces the pressure drop across bend 84'' and improves drainage of fluid within bend 84''.

[0050]

[0085] The characteristics of the sinusoidal pattern 110 utilized for a given return bend may be selected for a particular application. For example, the number of ridges / grooves, the amplitude, the period, and / or one or more tapered introductions may be selected for a particular application. The characteristics of the return bend may vary throughout the return bend, such as varying amplitude and period throughout the return bend. The shape of the controlled wrinkled portion 94 is formed, at least in part, by two different intersecting cross-sectional profiles. With reference to Figures 4 and 15, the controlled wrinkled portion 94 includes a sinusoidal portion 110 at the inner ring 90 of the bend 84. Another pattern is an arc pattern 150 including alternating peak arcs 152 and valley arcs 154. With reference to Figures 16A and 17A, the peak arc 152 has a peak arc radius 152' and a center 182, and the valley arc 154 has a valley arc radius 158 and a center 172. In this embodiment, peak arc 152 and valley arc 154 are substantially the same. As used herein, the term substantially the same refers to dimensions that are virtually the same when considering manufacturing variations, such as within + / - 10% of each other. Peak arc 152 extends through an angle 160 that is greater than an angle 162 that valley arc 154 extends through.

[0051]

[0086] 5 and 15, the valley arc 154 forms a semicircular valley inner wall portion 170 having a valley arc radius 158 and a center 172. Opposite the semicircular valley inner wall portion 170, the bend 84 includes an outer wall portion 174, which may be semicircular. In some embodiments, the outer wall portion 174 may be curved with a flattened portion due to the outer ring 92 (see FIG. 4) of the bend 84 being placed in tension during bending. The bend 84 includes connecting wall portions 176, 178 that connect the semicircular valley inner wall portion 170 to the outer wall portion 174. The connecting wall portions 176, 178 have curvatures that may be dissimilar to the inner and outer wall portions 170, 174. The connecting wall portions 176, 178 provide a smooth transition between the geometries of the inner wall portion 170 and the outer wall portion 174 to minimize stress concentrations at the junction between the geometries of the inner wall portion 170 and the outer wall portion 174. By reducing stress concentrations at the junction between the geometries of the inner wall portion 170 and the outer wall portion 174, the connecting wall portions 176, 178 assist the flexure 84 in withstanding high internal operating pressures.

[0052]

[0087] 6 and 15, the crest arc 152 defines a semicircular inner crest wall portion 180 having a crest arc radius 156 with a center 182. The bend 84 has an outer wall portion 184 opposite the semicircular inner crest wall portion 180. Similar to the outer wall portion 174 (see FIG. 5), the outer wall portion may be semicircular. In some embodiments, the outer wall portion 184 may be curved with a flattened portion due to the outer ring 92 (see FIG. 4) of the bend 84 being placed in tension during bending. The bend 84 further includes connecting wall portions 186, 188 connecting the semicircular inner crest wall portion 180 and the outer wall portion 184. Similar to the outer wall portion 174, the outer wall portion 184 may have a semicircular or generally curved shape in some embodiments. Additionally, the connecting wall portions 186, 188 provide a smooth transition between the geometries of the inner wall portion 180 and the outer wall portion 184 to minimize stress concentrations at the junctions between the geometries of the inner wall portion 180 and the outer wall portion 184. The connecting wall portions 186, 188 contribute to the ability of the flexure 84 to withstand high internal operating pressures. The peak arc 152 and the valley arc 154 may each have a single radius, as shown in Figures 16A and 17A. In another embodiment, the peak arc 152 and / or the valley arc 154 have a compound or composite radius. For example, referring to Figure 16B, the peak arc 152' has different radii 156A', 156B'. Each radius of the peak arc 152' meets at the point where the radius joins with the adjacent radius. Similarly, in Figure 17B, the valley arc 154' has different radii 158A', 158B'.

[0053]

[0088] In another embodiment, the peak arc 152 and / or the valley arc 154 have a shape that is a portion of an ellipse. For example, the peak arc 152'' of FIG. 16C is an arc defined by an angle 160'', such as 160 degrees, between points 426'', 430'' of an ellipse 439 having a major dimension 441 and a minor dimension 443. Similarly, the valley arc 154'' of FIG. 17C has a shape defined by an angle 162'', such as 142 degrees, between points 445, 447 of an ellipse 449 having a major axis 451 and a minor axis 453.

[0054]

[0089] 7, pipe section 82 is shown with sidewall 74 having a circular cross-section centered on pipe centerline 102. Sidewall 74 may also have a non-circular cross-section, such as an elliptical or oblong cross-section. Sidewall 74 of serpentine pipe 70 has a wall thickness 190 that extends around interior passageway 72.

[0055]

[0090] 8, a perspective view of portions of tubing sections 80, 82 and bend section 84 is shown. As discussed above, controlled wrinkled portion 94 has a continuously curved controlled wrinkled surface 134 including curved ridge surface portions 200 on either side of each ridge 114 and curved groove surface portions 202 on either side of each groove 116 that connect the curved ridge surface portions of adjacent ridges 114. Ridge surface portions 200 and groove surface portions 202 form the continuous wavy appearance of controlled wrinkled portion 94.

[0056]

[0091] 9A, the serpentine tube 70 has an outer diameter 210 and a wall thickness 190. The tube centerline 102 extends through the pipe sections 80, 82 and the bend 84. The serpentine tube has a junction 214, 215 between the pipe sections 80, 82 and the bend 84. At the junction 214, 215, the tube 70 includes a junction 122, 124 between the pipe sections 80, 82 and the bend 84. The bend 84 includes a relief 222, 224 extending away from the junction 122, 124 and a tapered lead-in portion 140, 142 that slopes radially inward toward the crests 130, 132 of the end ridges 118, 120. The bend 84 has a center 230 and a centerline radius 232 extending from the center 230 to the tube centerline 102. In the illustrated embodiment, the bend 84 has a bend angle 220 of 180 degrees, and the controlled crimp 94 extends about a center 230 over a controlled crimp angle 240 that is less than the bend angle 220. For example, the controlled crimp angle 240 may be 5° or less, 10° or less, or 15° or less than the bend angle 220. In one embodiment, the bend angle is 180 degrees and the crimp angle 240 is approximately 166 degrees.

[0057]

[0092] 9A , the controlled corrugations 94 position the peaks 250 of the ridges 114 on the inner ring 90 of the bend 84 (see FIG. 4 ) and position the valleys 252 of the grooves 116 radially outward from the peaks 250. By positioning the valleys 252 outside the inner ring 90 of the bend 84, the corrugations 94 create a structural bend centerline 254. The structural bend centerline 254 has a structural bend centerline radius 256 that is greater than the centerline radius 232 of the pipe centerline 102. Because the structural bend centerline radius 256 is greater than the bend centerline radius 232, the bend complexity ratio of the bend 84 for a given inner and outer bend ring is less than the bend complexity ratio of a conventional bend having the same inner ring, outer ring, outer diameter, and wall thickness. Bend 84 has a lower bend complexity ratio due to the larger structural bend center radius 256 .

[0058]

[0093] For example, a pipe bend for a particular application may have the following ratios of properties:

[0059]

[0094]

[0095]

[0096]

number

[0060]

[0097] where OD refers to the outside pipe diameter, WT refers to the wall thickness, and CLR refers to the centerline radius of the bend. Assume the values ​​of these ratios for the pipe bend are as follows:

[0061]

[0098] W1=20 and D1=2, so C B1 =10

[0062]

[0099] Referring to Table 1 above, these values ​​indicate that if a conventional tube bender is used, an internal mandrel bend may be necessary.

[0063]

[0100] Here, certain parameters of the bends are modified to exhibit improved serpentine tube properties such as tighter bend radius for the same wall thickness, reduced coil weight, reduced internal fluid side pressure drop, reduced bend wall stress, increased tube strength, increased tube stiffness, and / or increased heat transfer efficiency. These modifications affect the ratios of properties. For example, the new property ratios may be selected as follows:

[0064]

[0101] W2=30 and D1=2, so C B2 =15

[0065]

[0102] The ratio of the bend complexity characteristics is then in a range that conventional tube bending machines can no longer compensate for, and an internal mandrel is conventionally used to perform this bend.

[0066]

[0103] Internal mandrel bending is often undesirable for a variety of reasons, as discussed above, making it impractical for manufacturers who utilize long continuous lengths of tubing to produce heat exchanger coils.

[0067]

[0104] 9A, one approach to eliminating the need for an internal mandrel is to decrease the bend complexity by increasing the bend CLR. In this example, if the bend CLR can be increased while keeping the outer diameter and wall thickness of the tube the same, the bend D can be increased from 2 to 3, decreasing the bend complexity (C B ) ratio can be obtained.

[0068]

[0105] W2=30 and D2=3, so C B2 =10

[0069]

[0106] C B2Since the ratio is in the range of 5-10, the bend can be formed without an internal mandrel. However, simply increasing the bend CLR for a given application may not be acceptable since the new bend would be larger and take up more space than the original bend. For example, the center-to-center distance between the pipe sections would be greater, which means fewer pipe sections can fit into a particular envelope or coil height. Also, since each bend in the serpentine tube would be bulkier, the serpentine tube would have fewer pipe sections for a given coil envelope or height, which would reduce the heat exchange capacity of the serpentine tube. Reducing the number of pipe sections in a serpentine coil to increase the bend CLR is not an acceptable solution for many applications.

[0070]

[0107] 9A, the controlled corrugations 94 of the bend 84 provide a structural bend centerline radius 256 that is larger than the actual bend centerline radius 232 without increasing the distance between the tubing sections 80, 82. The larger structural bend centerline radius 256 increases the CLR of the bend 84, which increases the D of the bend for a given OD, as well as the C B but within a range such that mandrel bending is not required.

[0071]

[0108] More specifically, the controlled corrugations 94 provide a structural bend centerline 254 within the available space of the bend 84, thereby allowing sufficient length along the inside of the bend 84 for the material to form the ridges 114 and grooves 116 in a controlled manner without buckling. The corrugations 94 also maintain or improve other coil properties such as internal fluid pressure drop and heat transfer efficiency. Other properties of the bend 84 are also improved, such as reduced wall thinning of the outer ring and the overall stiffness of the bend 84.

[0072]

[0109] 4, the alternating ridges 114 and grooves 116 of the controlled corrugations 94 provide space for the material of the tube 70 to fold into a smaller effective arc length as the tube 70 is bent. The material of the tube 70 folds into a sinusoidal pattern 110 along the inner ring of the bend 84. The specific variables of the sinusoidal pattern 110, such as the number of peaks / valleys, the depth of the valleys (amplitude of the sine wave), the total arc length, etc., are calculated for a particular application as described below. This method can be used to calculate variables for various combinations of materials, OD, WT, and CLR, and to optimize various properties such as pressure drop and thermal efficiency.

[0073]

[0110] The controlled corrugations 94 provide advantages over conventional tube bends. For example, compared to other bends having corrugations, the sinusoidal pattern 110 minimizes stresses generated in the material of the tube 70, allowing for much higher internal fluid pressures. The ridges 114 and grooves 116, including the tapered leads 140, 142, can be sized to limit impediments to fluid flow within the bend 84 and to minimize internal fluid pressure drop through the bend 84. The sinusoidal pattern 110 increases the length of material along the inner ring 90, increasing the total surface area of ​​the bend 84 and improving heat transfer efficiency by increasing fluid turbulence within the bend area, compared to a conventional bend with the same bend centerline radius. The ridges 114 and grooves 116 also act as corrugations that stiffen the bend 84, compared to a smooth, uncorrugated bend. Additionally, the controlled wrinkles 94 push the neutral axis of the flexure 84 outward toward the outer ring 92 of the flexure 84, thereby reducing material thinning of the flexure 84 along the outer ring as compared to a smooth, unwrinkled flexure.

[0074]

[0111] With reference to Figures 10 through 13B, a process is provided for determining the geometry of bend 84 in serpentine tube 70 to replace bend 306 in conventional serpentine tube 300, while at the same time fitting within the coil envelope of conventional serpentine tube 300 and utilizing a tighter bend radius for a given wall thickness.

[0075]

[0112] 10, conventional serpentine pipe 300 has tubing sections 302, 304, bend 306, outer diameter 308, and wall thickness 310. Bend 306 is a 180° bend, with bend 306 having an inner ring 312 and an outer ring 315 with an arc length 314. Initially, with reference to FIG. 11, serpentine pipe 70 includes outer diameter 210 that is the same as outer diameter 308, and wall thickness 190 that is less than wall thickness 310. For example, outer diameter 308 and outer diameter 210 may both be 1.05 inches, wall thickness 310 may range from approximately 0.04 inches to approximately 0.07 inches, such as 0.048 inches, and wall thickness 190 may range from approximately 0.02 inches to approximately 0.05 inches, such as approximately 0.03 inches to approximately 0.04 inches. Because outer diameter 210 is selected to be the same as outer diameter 308, bends 84 stack with adjacent bends 84 in the same manner as bends 306 stack with adjacent bends 306. A narrower bend radius for a given thickness 190 may improve the efficiency of heat transfer between the working fluid inside the serpentine tube 70 and the fluid outside the serpentine tube 70. Also, a narrower bend radius for a given wall thickness 190 may reduce the internal fluid pressure drop within the serpentine tube 70 as the inside diameter of the tubing section increases.

[0076]

[0113] 11, the process of determining the geometry of bend 84 includes first setting up serpentine pipe 70 with an initial bend 316 connecting pipe sections 80, 82. Initial bend 316 has a bend angle of 180° and a centerline radius 317 that is greater than centerline radius 313 of bend 306 shown in FIG 10. With reference to FIGs 10 and 11, initial bend 316 has an inner ring 320 with an arc length 318 that is greater than arc length 314 due to centerline radius 317 being greater than centerline radius 313.

[0077]

[0114] 12, in order for bend 84 to fit within the same coil envelope as conventional bend 306 of FIG. 10, i.e., so that the center-to-center distance between the pipe sections is equal, bend 84 has an outer ring 92 that matches the outer ring 315 of bend 306, and pipe 70 has an outer diameter 210 that matches the outer diameter 308. To provide the matching outer rings 92, 315, the step of determining the geometry of bend 84 includes moving the tangent points 122, 124 of pipe sections 70, 82 toward each other in directions 330, 332 (FIG. 11) until 1) bend 84 has an actual centerline radius 232 equal to centerline radius 313 of bend 306, and 2) the arc length of inner ring 90 of bend 84 is equal to inner ring 312 of bend 306.

[0078]

[0115] To compensate for the reduction in the vertical distance between the contacts 122, 124, the material of the serpentine tubing 70 inside the bend 84 is shaped to have a sinusoidal pattern 110. The sinusoidal pattern 110 has variables that define the shape of the sinusoidal pattern 110, such as the length of the sinusoidal pattern 110, the number of peaks / valleys, the period, and / or the amplitude.

[0079]

[0116] 13A, the step of determining the geometry of bend 84 next includes providing a line 339 having an inner ring arc length 340 that matches arc length 336 of inner ring 90 of FIG 12. Arc length 336 of inner ring 90 extends between transition points 122, 124 of FIG 12.

[0080]

[0117] The sinusoidal pattern 110 is offset from the tangent points 122, 124 of the bend 84 by two portions of the serpentine tube 70. The first portion is a relief portion 222, 224 corresponding to an offset angle, such as 7°, on either side of the sinusoidal pattern 110, measured between the angles 220, 240 (see FIG. 4). The second portion is a tapered lead-in portion 140, 142. The sinusoidal pattern 110 starts and ends at a point 400 (see FIG. 4). To create the offset of the sinusoidal pattern 110 from the tangent points 122, 124, the step of determining the geometry of the bend 84 includes removing lengths 342, 344 from the length 340 to provide a sinusoidal pattern length 346 that is shorter than the inner ring arc length 340, as shown in FIG. 13A. Thus, the lengths 342, 344 each include two length portions: 1) a length corresponding to one of the relief portions 222, 224, and 2) a length corresponding to one of the tapered lead-in portions 140, 142. The lengths 342, 344 are determined, for example, by determining the length using the inner radius and the angular offset.

[0081]

[0118] The difference between the length 340 of the line 339 (see FIG. 13A) and the arc length 318 (see FIG. 11) is captured in the total arc length 346 of the sinusoidal pattern 110. With reference to FIG. 13A, the total arc length 346 of the sinusoidal pattern 110 can be expressed as:

[0082]

[0119] Total arc length of the sine wave pattern 346 = Inner ring arc length 340 -length 342,344 [Equation 1.1]

[0083]

[0120] Once the total arc length 346 of the sinusoidal pattern 110 is known, the total arc length 346 is divided by the number of peaks 250A and valleys 252A, such as between 8 and 12 peaks and valleys, such as between 6 and 18 peaks and valleys, to determine the arc length 350 of each peak 250A and valley 252A. Each peak 250A and valley 252A has a radius 349 and arc length 350 given by:

[0084]

[0121] arc length350 =radius 349 ×θ[Equation 1.2]

[0085]

[0122] where θ is the angular extent of peak 250A and valley 252A. The radius of each peak 250A and valley 252A may be determined using the following calculation:

[0086]

[0123] 13B, a geometric shape 351 is provided having an arc line AD and a triangle formed by ABCD. Since triangle ABC is a right-angled triangle, the following equation can be seen:

[0087]

[0124]

number

[0088]

[0125] This equation can be rearranged as follows:

[0089]

[0126]

number

[0090]

[0127] The relationship a=r×θ can be substituted into equation 1.4 to give the following:

[0091]

[0128]

number

[0092]

[0129] At this point, the "a" value is known, i.e., the total arc length 346 of the sine wave pattern 110 divided by the number of peaks 250 and valleys 252 (FIG. 13A). The "c" value is known (see c / 2 in FIG. 13B), i.e., the length 346 divided by the number of peaks 250 and valleys 252 selected.

[0093]

[0130] The above equation can then be solved for theta using a numerical method such as Newton-Raphson iteration. Once theta is determined, the radii of peaks 250A and valleys 252A can be determined by solving for radius 349 in equation 1.2.

[0094]

[0131] The radius 349 and theta allow the amplitude of the sine wave pattern 110 to be determined using the following equation:

[0095]

[0132] amplitude 352 =radius 349 -(radius 349 ×cosθ)

[0096]

[0133] It will be appreciated that adjustments to the sine wave pattern 110 may be utilized to tailor the sine wave pattern 110 for a particular application.

[0097]

[0134] 12, the tapered leads 140,142 smooth out the bends in the material of the serpentine tube 70 and reduce stress risers at the transitions between the reliefs 222,224 (see FIG. 4) and the sinusoidal pattern 110. With reference to FIG.

[0098]

[0135] 14-18, the intersecting sine wave pattern 110 and the arc pattern 150 of the controlled corrugations 94 will be described in more detail. The intersecting sine wave pattern 110 and the arc pattern 150 provide a three-dimensional profile of the internal bends. The three-dimensional profile of the internal bends provides a corrugated structure with high strength to resist internal fluid pressure within the serpentine tube 70. The intersecting sine wave pattern 110 and the arc pattern 150 cause the bends 84 to experience low stress even when the bends 84 are under high internal pressure.

[0099]

[0136] With reference to FIG. 14, one half of the sine wave pattern 110 will be described. The other half of the sine wave pattern 110 is identical to the embodiment of FIG. 9A. The sine wave pattern 110 starts at point 400 and is separated from the tangent point 122 by the relief 222 and the tapered lead-in portion 140. The tapered lead-in portion 140 slopes gradually upward toward point 400 adjacent to the peak 250 of the end ridge 118. The sine wave pattern 110 oscillates about a centerline 406 that intersects the sine wave pattern 110 at the transition 410 between the recessed portion 412 and the protruding portion 414 (as viewed from the center 230). In the embodiment of FIG. 14, the centerline 406 of the sine wave pattern 110 is located on the inner ring 90 of the bend 84 (see FIG. 12). In another embodiment, the valleys 252 of the sinusoidal pattern 110 lie on the inner ring 90 of the bend 84 such that the inner ring 90 abuts the groove 116. In yet another embodiment, the peaks 250 of the sinusoidal pattern 110 lie on the inner ring 90 of the bend 84 such that the inner ring 90 abuts the ridge 114.

[0100]

[0137] 14, the centerline 406 of the sine wave pattern 110 has a radius 416. In one embodiment, the bend 84 has a centerline radius 232 (see FIG. 12) in the range of about 1.5 inches to about 2 inches, such as in the range of 1.7 inches to about 2 inches, such as 1.875 inches. The centerline 406 may have a radius in the range of about 1 inch to about 1.5 inches, such as in the range of about 1.3 inches to about 1.4 inches, such as 1.35 inches.

[0101]

[0138] 15, arc pattern 150 includes peak arcs 152 that intersect sinusoidal pattern 110 at each peak 250 and valley arcs 154 that intersect sinusoidal pattern 110 at each valley 252. Peak arcs 152 and valley arcs 154 are separated by an angle 420 around bend 84, which may range, for example, from approximately 4° to approximately 14°.

[0102]

[0139] 16A , the crest arc 152 has a center 182 of the crest arc 152 radially inward from the pipe centerline 102 of the bend 84. The center 182 is positioned along a median plane 424 of the serpentine pipe 70. The crest arc 152 extends over an angle 160, which may be in the range of 150° to approximately 170°, such as, for example, 160°. The crest arc 152 has an arc length 427 that extends from an end point 426 to an end point 430 of the crest arc 152.

[0103]

[0140] 17A , the valley arc 154 has its center 172 radially outward from the centerline 102 of the serpentine tube 70. The valley arc 154 extends over an angle 162 that is less than the angle 160 of FIG. 16A . In one embodiment, the angle 162 is within a range of approximately 100° to approximately 150°, such as 140°. The valley arc 154 has an arc length 432 between endpoints 434, 436 of the valley arc 154 that is less than the arc length 427 of the peak arc 152.

[0104]

[0141] 18, the continuously curved controlled wrinkled surface 134 (as shown in FIG. 8) of controlled wrinkled portion 94 may be formed, at least in part, by connecting peak arcs 152 and valley arcs 154 with a surface portion 440 having a convex portion 442, a concave portion 444, and a transition 446 transitioning between convex portion 442 and concave portion 444. Surface portion 440 may be mirrored on the opposite side of ridge 114 across a vertical plane that includes peak arc 152.

[0105]

[0142] In one embodiment, the continuously curved wrinkled surface 134 is perpendicular to a vertical plane including the peak arcs 152 and a vertical plane including the valley arcs 154. With reference to FIG. 15, the vertical plane including the peak arcs 152 is defined as being perpendicular to the horizontal plane 424 (see FIG. 8) and includes the origin or center 230 and the apex 250. The vertical plane including the valley arcs 154 is defined as being perpendicular to the horizontal plane 424 and includes the center 230 and the valleys 252. The vertical plane including the peak arcs 152 and the valley arcs 154 are separated by an angle 420. With reference to FIG. 18, a concave portion 442 and a convex portion 444 connect the peak and valley arcs 152, 154 to provide the wavy three-dimensional profile of the continuously curved controlled wrinkled surface 134 (FIG. 8). Each concave and convex portion 442, 444 terminates in two 4-pole splines, one of which starts at the peak arc endpoint 426 (FIG. 16A) and ends at the valley arc endpoint 434 (FIG. 17A), while the other 4-pole spline starts at the peak arc endpoint 430 (FIG. 16A) and ends at the valley arc endpoint 436 (FIG. 17A).

[0106]

[0143] 19 and 20, a tube bender 500 is provided for bending a segment of serpentine tube 70 into the bend 84 described above. The tube bender 500 includes a bending die 502 and a clamping die 504 that pivots about an axis 506. The tube bender 500 includes a pressure die 508 for supporting the outside of the bend 84 and the rear of the serpentine tube 70. The bending die 502 and the clamping die 504 include recesses 512, 514 having surfaces 516, 518 extending thereabout that clamp the tube as it is advanced in a direction 520 over a gap 522 between the bending die 502 and the clamping die 504. The clamping die 504 and the pressure die 508 may be actuated in a direction 524 to secure a portion of the tube between the clamping die 504 and the bending die 502. The pressure die 508 includes a recess that receives a portion of the tube and can be shifted in direction 526 as the tube moves when the bending die 502 and clamping die 504 pivot about axis 506 in direction 528 to support the outside of the tube during the bending operation.

[0107]

[0144] 19 and 20, bending die 502 includes an upper portion 530, a lower portion 532, and a recess 534 that receives a portion of the tube therein as bending die 502 and clamping die 504 pivot in direction 528. Bending die 502 has a crimped portion 536 that is a mirror image of crimped portion 94 of the tube, such that bending die 502 imparts crimp pattern 94 to the tube. For example, crimped portion 536 includes ridges 540 that form grooves 116 (FIG. 8) and grooves 542 that form ridges 114 (FIG. 8).

[0108]

[0145] 20, each of the ridges 540 has a middle portion 544 and an opposing end portion 546. The middle portion 544 may have a first width around the bending die 502, with the end portions 546, 548 having a width greater than the width of the middle portion 544 around the bending die 502 such that the ridges 540 widen outward as they extend away from a midline 550 of the bending die 502. The grooves 542 may correspondingly have a middle portion 552 and opposing end portions 554, 556, with the end portions being narrower than the middle portion 552 around the bending die 502 due to the increasing width of the ridges 540 as they extend away from the midline 550. The ridges 540 and grooves 542 have a wavy, continuous curvature 560 such that the wrinkled portions 536 form the continuous wrinkled surface 134 of the tube.

[0109]

[0146] With reference to Figures 21-25, a method of forming a bend 84 using a tube bender 500 is provided. The tube bender 500 shown in Figures 21-25 has similar components to the tube bender 500 shown in Figure 19, but the orientation of the components is different. For ease of discussion, like reference numbers will be used to describe the tube bender of Figures 20 and 21-25.

[0110]

[0147] 21 and 22, a tube 564 is advanced into the tube bender 500 and a pressure die 508 supports the outer surface of the tube 564. In FIG. 22, the bending die 502 and clamping die 504 engage a portion 505 of the tube 564 and begin to pivot in a direction 565 into the plane of the page of FIG.

[0111]

[0148] 23 and 24, the bending die 502 and clamping die 504 are pivoted in direction 565 to begin forming a bend 570 in the tube 564. The pressure die 508 continues to support the outside of the tube 506 and is shifted in direction 526 to move with the tube 564 during the bending operation.

[0112]

[0149] With reference to FIG. 25, tube bender 500 has just formed bend 570 by bending tube 564 180 degrees.

[0113]

[0150] 26 shows that the upper portion 530 of the bending die 502 is shifted upward in direction 569 from the lower portion 532, the clamping die 504 is shifted (into the page) away from the tube 564, and the pressure die 508 is retracted from the tube 564. The tube 564 is then shifted in direction 571 to position the next bend location along the tube 564 within the tube bender 500.

[0114]

[0151] With reference to FIG. 27, bend 570 is shown having a corrugated portion 572 including ridges 574 and grooves 576 formed on the inside of bend 570. FIG. 27 also shows how lower portion 532 has a sinusoidal pattern 578 at the midline 550 (see FIG. 20) of bending die 502 that imparts a sinusoidal pattern 580 on the inside of bend 570. More specifically, lower portion 532 has lower portions of ridges 540 that form grooves 576 in bend 570, and lower portion 532 has lower portions of grooves 542 that receive ridges 574 of bend 570. In this manner, ridges 574 of tube 564 and ridges 540 of bending die 502 form a tightly interlocking configuration. Additionally, ridges 540 and grooves 542 with their wavy, continuous surfaces support the inside of the tube. An upper portion 530 (FIG. 26) of the bending die 502 forms a corresponding mating engagement with the top of the bend 570 .

[0115]

[0152] 20, the corrugated portion 536 of the bending die 502, and with reference to FIG. 27, includes a tapered transition 590 and an end ridge 592 that cooperate to form an end ridge 594 of the bend 570. The tapered transition 590 provides a smooth lead-in to the peaks of the end ridge 594, as described above with respect to FIG. 9A.

[0116]

[0153] Various types of bends may be provided in accordance with the present disclosure. For example, Figure 28 shows a 90 degree bend 600, Figure 29 shows an 80 degree bend 620, and Figure 30 shows a 100 degree bend 640.

[0117]

[0154] 31, a cross-sectional view of serpentine tube 700 taken perpendicular to the length of serpentine tube 700 is provided. Serpentine tube 700 is similar to serpentine tube 70 and includes pipe section 701. Pipe section 701 includes pipe section 702 having a circular cross-section and pipe section 704 having a non-circular cross-section, such as an ellipse or oval. Pipe section 701 has a gradually flattening cross-section, with pipe section 706 having a width 707 that is wider than width 709 of pipe section 708.

[0118]

[0155] 32, a coil 800 is provided that includes assembled serpentine tubes 802, 804. Each serpentine tube 802, 804 includes a tubing section 803, 805 and a compound bend 806 including a first bend 808 having a first bend angle 810 of 80 degrees, a second bend 812 having a second bend angle 814 of 100 degrees, and a connecting portion 816 connecting the first and second bends 808, 812. The first and second bends 808, 812 have inner controlled corrugations similar to the controlled corrugations of the bends described above. The serpentine tubes 802, 804 have three contact points 820, 822, 824. Each serpentine tube 802, 804 has a height or distance 830 between the tubing sections 803, 805. The serpentines 802 of the coil 800 touch each other. In other embodiments, the coil may include serpentines that do not touch each other.

[0119]

[0156] 33, a portion of a tube 896 is shown that includes a straight portion 898 and a bend 900. A bend 900 is provided that is similar in many respects to the bends described above. The bend 900 includes a corrugated portion 902 having ridges 904 and grooves 906. The corrugated portion 902 includes a sinusoidal pattern 903 along the inner circumference of the bend 900, which begins and ends at points 903A and 903B. The tube 896 has tangent points 911 and 913 at the transition between the straight portion 898 and the bend 900.

[0120]

[0157] Wrinkled portion 902 is asymmetric about a plane 908 that bisects bend 900. Axes 915, 912 extend perpendicular to plane 908 and intersect tangent points 913, 911, respectively. Tangent points 911, 913 are offset a distance 910 along plane 908 such that wrinkled portion 902 extends farther along tube 896 on one side of plane 908 than the other. The portion of wrinkled portion 902 on one side of plane 908 (top of FIG. 33 ) has an offset portion 910A that includes at least one more ridge 904 and / or at least one more groove 906 than the portion of wrinkled portion 902 on the other side of plane 908.

[0121]

[0158] The wrinkled portion 910 has an end groove 906A and an end ridge 904A. In one implementation, the end ridge 904A does not have a tapered lead-in. The offset portion 910A may provide a transition for the flow in the tube 896 between the nearby straight portion 898 and the bend 900. The end ridge 904B also has a tapered lead-in 914, similar to the various end ridges described above.

[0122]

[0159] 34 and 35, a bending die 1000 is provided that is similar to bending die 502 described above, with the differences being highlighted. The bending die 1000 is used to form the bend 900 and includes an upper portion 1002 and a lower portion 1004. The upper portion 1002 and the lower portion 1004 have ridges 1006 and grooves 1008 that cooperate to form the ridges 904 and grooves 906 in the bend 900. The upper portion 1002 and the lower portion 1004 each have a pair of channels 1010, 1012. The channels 1010 in the upper portion 1002 and the lower portion 1004 form an opening 1013 on one side 1014 of the bending die 1000, and the channels 1012 in the upper portion 1002 and the lower portion 1004 form another opening 1015 on a second side 1016.

[0123]

[0160] The openings 1013, 1015 allow the bend die 1000 to have the tube fed into either of the openings 1013, 1015 of the bend die 1000 and to be rotated in a corresponding direction to form a bend 900 in the tube. For example, with reference to FIG. 35 , a first portion of the tube may be advanced in direction 1030 into the channel 1012 of the bend die lower portion 1004. The upper portion 1002 is shifted downward in direction 1032 to engage the bend die lower portion 1004 and form the opening 1015 around the tube.

[0124]

[0161] The bending die 1000 is then rotated in a direction 1034 about an axis 1036 while the rear portion of the tube is supported by the pressure die. The bending die 1000 is rotated in a direction 1034 to impart the desired angular extent to the bend 900. Once the bend 900 is formed, the bending die top 1002 is shifted upward in a direction 1033 and the tube is shifted relative to the bending die 1000 to position another portion of the tube within the bending die 1000 for bending. Continuing with the example, the tube is repositioned to advance a second portion of the tube into the opening 1013, the bending die 1000 is closed, and the bending die 1000 is rotated in a direction opposite to the direction 1034. The process of advancing and bending the tube is repeated until the desired number of bends have been imparted to the tube.

[0125]

[0162] 36, a tube 1100 is provided having a return bend 1102 and a straight section 1103. The return bend 1102 has a wrinkled section 1104 similar to the wrinkled section described above. The wrinkled section 1104 has valleys 1106 and peaks 1108. The tube 1100 has a flattened cross-section at the valleys 1106, peaks 1108, and / or straight section 1103. The flattened cross-section of the tube 1100 may allow the tube 1100 to be packed closely with adjacent tubes, such as in a cooling tower coil assembly. The flattened cross-section of the tube 1100 may also improve the thermal performance of the tube 1100.

[0126]

[0163] The flattened cross-section of tube 1100 can be, for example, an elliptical cross-section. With reference to Figure 37A, return bend 1102 includes a valley elliptical wall portion 1110 at valley 1106. Valley elliptical wall portion 1110 has a major dimension 1112 and a minor dimension 1114.

[0127]

[0164] 37B , the return bend 1102 has a peak elliptical wall portion 1116 at the peak 1108, the peak elliptical wall portion 1116 having a major dimension 1120 and a minor dimension 1122. The major dimension 1120 of the peak 1108 is greater than the major dimension 1112 of the valley 1106. In one embodiment, the minor dimension 1122 of the peak 1108 is less than the minor dimension 1114 of the valley 1106.

[0128]

[0165] 37C, the return bend 1102 has a straight section elliptical wall portion 1126 at the straight section 1103, the straight section elliptical wall portion 1126 having a major dimension 1128 and a minor dimension 1130. In one embodiment, the major dimension 1128 of the straight section 1103 is less than the major dimensions 1112, 1120, and the minor dimension 1130 is greater than the minor dimensions 1114, 1122.

[0129]

[0166] The flattened cross-section of each portion of the tube 1100 can be provided in a number of different approaches. For example, a tube bender used to bend the tube to impart the crumpled portions 1104 may flatten the bends 1102 during the bending procedure. In another approach, the tube originally has an oval cross-section, and the bending procedure imparts the crumpled portions 1104 to the bends 1102 without further flattening the tube. In yet another approach, a tube bender is used to form one or more bends in the tube, and a press is used to flatten the tube after the bending procedure.

[0130]

[0167] Use of singular terms such as "a," "an," and the like is intended to encompass both the singular and the plural unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms. As used herein, the phrase "at least one of" is intended to be construed in a disjunctive sense. For example, the phrase "at least one of A and B" is intended to encompass A, B, or both A and B.

[0131]

[0168] While specific embodiments of the present invention have been shown and described, it will be understood that numerous changes and modifications will occur to those skilled in the art, and that the present invention is intended to encompass all such changes and modifications that fall within the scope of the appended claims. For example, the flexures disclosed herein may be utilized in a variety of heat exchange devices, such as evaporative condensers, air-cooled condensers, closed circuit fluid coolers, closed circuit cooling towers, open circuit cooling towers, dry coolers, ice storage systems, thermal storage coils, and / or ice water cooling coils, to name a few.

Claims

1. an inlet header for receiving pressurized working fluid; an outlet header for collecting the pressurized working fluid; a serpentine tube connecting the inlet header and the outlet header to allow the pressurized working fluid to flow from the inlet header to the outlet header; An indirect heat exchanger pressure vessel comprising: the serpentine circuit pipe includes a piping portion and a return bend portion connecting the piping portions, the return bend has a controlled corrugation; The controlled corrugations include alternating ridges and grooves.

2. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said inlet header, said outlet header, and said serpentine tubes are configured to operate at an internal pressure of at least 150 psig.

3. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said inlet header, said outlet header, and said serpentine tubes are configured to operate at an internal pressure of at least 410 psig.

4. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said inlet header, said outlet header, and said serpentine tubes are configured to operate at an internal pressure of at least 1200 psig.

5. the serpentine pipe includes a pair of contacts at a junction between the return bend of the serpentine pipe and the piping section; The return bend has a bend angle; the controlled corrugation of the return bend is spaced along the serpentine from the contact point; 2. The indirect heat exchanger pressure vessel of claim 1, wherein said controlled corrugation of said return bend has an angular extent about the inside of said return bend that is less than said bend angle.

6. the controlled corrugation of the return bend includes a sinusoidal pattern on an inner circumference of the return bend; 2. The indirect heat exchanger pressure vessel of claim 1, wherein said sinusoidal pattern includes peaks in said ridges and valleys in said grooves of said bends.

7. the controlled corrugation of the return bend includes an arc pattern that intersects with the sinusoidal pattern of the return bend; 7. The indirect heat exchanger pressure vessel of claim 6, wherein said arc pattern comprises peak arcs intersecting said peaks and valley arcs intersecting said valleys.

8. the peak arc has a first radius of curvature; the valley arc has a second radius of curvature; 8. The indirect heat exchanger pressure vessel of claim 7, wherein said first radius of curvature and said second radius of curvature are substantially the same.

9. the ridge includes an end ridge adjacent the piping portion of the serpentine tube; 2. The indirect heat exchanger pressure vessel of claim 1, wherein at least one of said end ridges includes a tapered lead-in portion for smoothing the flow of said pressurized working fluid around said ridge and said groove.

10. the return bend has a bend radius and includes a tubular sidewall extending around an interior of the return bend; the tubular side wall includes a first semicircular inner wall portion at each ridge of the return bend, a first outer wall portion, and a pair of first connecting wall portions on opposite sides within the return bend connecting the first semicircular inner wall portion and the outer wall portion; the first semicircular inner wall portion, the outer wall portion, and the first connecting wall portion are radially aligned; the tubular side wall includes a second semicircular inner wall portion in each groove of the return bend, a second outer wall portion, and a pair of connecting wall portions on both sides inside the return bend that connect the second semicircular inner wall portion and the second outer wall portion; 2. The indirect heat exchanger pressure vessel of claim 1, wherein said second semicircular inner wall portion, said outer wall portion, and said second connecting wall portion are radially aligned.

11. the first semicircular inner wall portion has a first radius of curvature; 11. The indirect heat exchanger pressure vessel of claim 10, wherein said second semi-circular wall portion has a second radius of curvature substantially the same as said first radius of curvature.

12. the first semicircular inner wall portion has a first angular extent; the second semicircular inner wall portion has a second angular extent; 11. The indirect heat exchanger pressure vessel of claim 10, wherein said first angular range and said second angular range are each greater than 90 degrees.

13. The indirect heat exchanger pressure vessel of claim 12 , wherein the first angular range is greater than the second angular range.

14. The piping portion of the serpentine circuit pipe includes a plurality of pairs of piping portions, 2. The indirect heat exchanger pressure vessel of claim 1, wherein said return bends comprise a plurality of return bends connecting pairs of said piping sections.

15. The return bend is a first bend including a first controlled corrugation portion of the controlled corrugation portion; a second bend including a second controlled corrugation portion of the controlled corrugation portion; a straight portion of the meandering circuit tube connecting the first bent portion and the second bent portion; 2. The indirect heat exchanger pressure vessel of claim 1 , comprising:

16. the first bend has a first bend angle of 90 degrees or greater; 16. The indirect heat exchanger pressure vessel of claim 15, wherein the second bend has a second bend angle less than or equal to 90 degrees.

17. the return bend comprises a plurality of return bends; 2. The indirect heat exchanger pressure vessel of claim 1, wherein said return bends of said serpentine tubes all have coplanar centerlines.

18. The return bend has a bend angle of 180 degrees; 2. The indirect heat exchanger pressure vessel of claim 1, wherein said controlled corrugation of said bend has an arc length of 180 degrees or less.

19. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said serpentine pipe sections include pipe sections having non-circular cross-sectional shapes.

20. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said controlled corrugation section includes at least one tapered lead-in section.

21. the serpentine tube has an outer diameter (OD); The serpentine tube has a wall thickness (WT), the return bend has a centerline radius (CLR); The return bend is given by a bend complexity factor (C Ｂ 2. The indirect heat exchanger pressure vessel of claim 1, wherein said bend complexity factor is 10 or greater. [0010]

22. 22. The indirect heat exchanger pressure vessel of claim 21, wherein the bend complexity factor is 20 or less.

23. the serpentine tube includes a plurality of serpentine tubes; 2. The indirect heat exchanger pressure vessel of claim 1 wherein said serpentine tubes contact each other.

24. the serpentine tube includes a plurality of serpentine tubes; 2. The indirect heat exchanger pressure vessel of claim 1, wherein said serpentine tube return turns do not contact each other.

25. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said return bends of said serpentine tubes have a non-circular cross-sectional shape.

26. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said return bends of said serpentine tubes have an elliptical cross-sectional shape.

27. 2. The indirect heat exchanger pressure vessel of claim 1, wherein said controlled corrugations are asymmetric about a plane that bisects said return bend.

28. The return bend has a bend angle of 180 degrees; 2. The indirect heat exchanger pressure vessel of claim 1, wherein said controlled corrugations are asymmetric about a plane that bisects said return bend.

29. an inlet header for receiving pressurized working fluid; an outlet header for collecting the pressurized working fluid; a serpentine circuit pipe connecting the inlet header and the outlet header to allow flow of the pressurized working fluid from the inlet header to the outlet header, the serpentine circuit pipe including a piping section and a return bend connecting the piping sections; An indirect heat exchanger pressure vessel comprising: the return bend comprises an inner portion having a sinusoidal pattern on an inner circumference of the return bend; the sinusoidal pattern includes peaks and valleys; the inner portion of the bend includes an arc pattern that intersects with the sinusoidal pattern; the arc pattern comprises peak arcs intersecting the peaks and valley arcs intersecting the valleys.

30. the peak arc has a first radius of curvature; the valley arc has a second radius of curvature; 30. The indirect heat exchanger pressure vessel of claim 29, wherein the first radius of curvature of the peak arc and the second radius of curvature of the valley arc are substantially the same.

31. 30. The indirect heat exchanger pressure vessel of claim 29, wherein the arc of the peak has an angular extent greater than the angular extent of the arc of the valley.

32. the serpentine tube has a centerline; each of the peak arcs has a center radially inward of the center line; 30. The indirect heat exchanger pressure vessel of claim 29, wherein each of said valley arcs has a center radially outwardly of said centerline.

33. the return bend has a midline plane; the sinusoidal pattern lies in the midline plane; the peak arc is perpendicular to the midline plane; 30. The indirect heat exchanger pressure vessel of claim 29, wherein said valley arc is perpendicular to said midline plane.

34. the sinusoidal pattern includes an end crest adjacent the pipe section; 30. The indirect heat exchanger pressure vessel of claim 29, wherein at least one of said end crests includes a tapered lead-in segment.

35. The sinusoidal pattern has a period and an amplitude; 30. The indirect heat exchanger pressure vessel of claim 29, wherein at least one of said period and said amplitude varies about said return bend.

36. 36. The indirect heat exchanger pressure vessel of claim 35, wherein the sinusoidal pattern includes a first minimum amplitude adjacent one of the pipe sections, a second minimum amplitude adjacent another of the pipe sections, and a maximum amplitude intermediate the first and second minimum amplitudes along the inner ring of the bend.

37. 30. The indirect heat exchanger pressure vessel of claim 29, wherein said peak and valley arcs each have an angular subtense of at least 100 degrees.

38. each of the peak arcs includes a first radius of curvature and a second radius of curvature; the valley arcs each include a third radius of curvature and a fourth radius of curvature; the first radius of curvature and the third radius of curvature are substantially the same; 30. The indirect heat exchanger pressure vessel of claim 29, wherein said second radius of curvature and said fourth radius of curvature are substantially the same.

39. the peak arc has a shape defined by a portion of a first ellipse; 30. The indirect heat exchanger pressure vessel of claim 29, wherein said valley arc has a shape defined by a portion of a second ellipse.

40. the first ellipse has a first major dimension and a first minor dimension; the second ellipse has a second major dimension and a second minor dimension; the first major dimension is substantially the same as the second major dimension; 40. The indirect heat exchanger pressure vessel of claim 39, wherein said first minor dimension is substantially the same as said second minor dimension.

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