heat exchanger

Abstract

The present invention relates to a shell-and-tube heat exchanger, and more particularly to a compact heat exchanger that does not require a tubesheet. The present invention provides a shell-and-tube heat exchanger having an outer shell and a series of heat exchange tubes positioned therein, the shell having a shell-side fluid inlet and a shell-side fluid outlet for the transfer of a first fluid, the tubes capable of passing a second fluid therethrough in use, the tubes having first and second ends, the tubes arranged to meet and form a hypocycloidal enclosed gap, the hypocycloidal enclosed gap providing a path for the transfer of the first fluid, the tubes having a first region of reduced diameter adjacent to the shell-side fluid inlet nozzle and a second region of reduced diameter adjacent to the shell-side fluid outlet nozzle to allow the first fluid to form a flow path through the hypocycloidal enclosed gap.

Description

[Technical Field]

[0001] The present invention relates to a shell-and-tube heat exchanger, and more particularly to a compact heat exchanger that does not require a tubesheet. [Background technology]

[0002] Shell-and-tube heat exchangers are a common form of heat exchanger and typically comprise an arrangement of tubes supported by tube ends, baffles, grids, and tie rods to provide a controlled flow path through a shell body. Summary of the Invention

[0003] According to an aspect of the present invention, there is provided a shell and tube heat exchanger having an outer shell and a series of heat exchange tubes positioned therein, the shell having a shell-side fluid inlet and a shell-side fluid outlet for transport of a first fluid; the tube, in use, is capable of passing a second fluid therethrough, the tube having a first end and a second end, the tubes being arranged in contact to form a hypocycloidal enclosed gap therebetween, the hypocycloidal enclosed gap providing a path for transfer of the first fluid; Here, the tube has a first region of small diameter located adjacent to the shell-side fluid inlet and a second region of small diameter located adjacent to the shell-side fluid outlet to enable the first fluid to form a flow path through the hypocycloidal surrounding gap.

[0004] Typically, heat exchangers have a tube pitch of at least 1.25 to ensure sufficient volume for the first fluid to flow between the tubes. To ensure that the first fluid flows along the entire length of the tubes, baffles and tie rods are used to direct and control the flow of the first fluid so that it remains in contact with the tubes over its maximum path length. The use of tubes forming a hypocycloidal enclosure with a pitch of substantially 1 provides a path for the first fluid to travel, eliminating the need for additional baffles to direct the flow along the tubes.

[0005] The hypocycloidal enclosure gap may be formed from three or more contact tubes having a pitch of substantially 1. Preferably, there are three or four contact tubes, providing a triangular or square pitch configuration, respectively. Obviously, a greater number of tubes can be used, which will increase the flow of the first fluid through the hypocycloidal enclosure gap, but the system will be less efficient as there may be fewer tubes per unit volume. Preferably, there are three contact tubes to provide a triangular pitch configuration having a pitch of substantially unity.

[0006] The tubes abut along their length. The tubes may be fixedly attached along their length, but preferably there are one or more tube bundle supports at the ends of the bundle to provide support between the outer tube bundle and the shell. The tube bundle supports can stop fluid flow passing around the outside of the tube bundle, while also providing structural support to the tube bundle to prevent it from sagging in the middle.

[0007] A typical prior art heat exchanger may include fins, pleats, or protrusions to increase the surface area of ​​the tubes. By using only substantially smooth tubes without protrusions or relief structures, the diameter of the tubes can be maximized, thereby providing maximum fluid flow through the tubes.

[0008] The first and second regions of smaller diameter are such as to allow a first fluid to pass from the fluid shell inlet along the length of the tube into the hypocycloidal enclosure gap and ultimately out the fluid shell outlet.

[0009] The narrow diameter may occur over a length of up to 20% of the length of the tube, preferably 1% to 10%, and more preferably the narrow diameter may occur over a length substantially equal to the diameter of the fluid shell within the inlet / outlet nozzle.

[0010] The minor diameter of each of said tubes may range from 0.1% to 30%, more preferably from 1 to 10%, of the tube diameter.

[0011] The reduced diameter region can be created during manufacturing by stretching or forming a section of the tube to the required smaller diameter. Tube wall reduction is a well known technique.

[0012] Alternatively, the tubing may be provided with a reducer connector / coupling adapter to allow for the insertion of a region of smaller diameter tubing, allowing existing designs to be retrofitted with smaller diameter tubing to create a region of smaller diameter where needed.

[0013] The first and second ends of the tubes may be secured at their open ends via a tubesheet, which may abut the end faces of the tubes and / or the hypocycloidal enclosure gap. In prior art devices, the tubesheet is a circular plate with a plurality of holes, and the perforated tubesheet provides support for the individual tubes.

[0014] In a highly preferred configuration, the first and / or second ends of the tubes comprise a tessellated cross-sectional shape, such as a triangle, a square, or a hexagon. While other complex shapes can be tessellated, they do not offer low-cost manufacturing and are less suitable. The use of a tessellated cross-sectional shape eliminates the need for a tubesheet to provide support for the tubes, and the tessellated shape ensures that the ends are self-supporting.

[0015] The hexagonal tube ends allow the tubes to be stacked in various tube bundle shapes while eliminating the need for tubeplate material between each tube. Shell-side pressure is applied only to the tubes and transmitted by the tubes, not to a separate tubesheet. The first and / or second tube ends may be secured by adhesive, welding, brazing, or mechanical fastening. Preferably, the transition from the tube to the tessellated tube end may include a back-brazed joint. Preferably, the tubes are first welded together at the tube faces and then back-brazed to form a mechanical joint and sealing interface between the shell side and the tube side. Back-brazing significantly increases joint strength while protecting the weld surface from chemical attack by corrosive fluids.

[0016] The tube may be made from any thermally conductive material, typically a metal or semi-metal. The cross section of the tube shape is typically circular and may remain circular along its entire length except for tessellated ends, if present.

[0017] The tube bundle is connected to a shell. In this design, the shell can be any desired shape, but circular shells are best suited for their inherent strength when shell-side pressures are high. To facilitate the tube-to-shell connection, a support ring can be used, adopting a hexagonal tube bundle end profile. This ring can be the same diameter as the shell, butt-welded to the shell, or the same diameter as the shell's inner diameter. When the end support ring is designed to fit inside the shell, various shell-to-head connection methods are available to suit individual applications, similar to the traditional shell-and-tube TEMA standard.

[0018] Heat recovery from low-pressure gases with small differential pressures is typically complicated by the combined challenges of limited available pressure differentials and the poor heat transfer capabilities of fluids, i.e., gases. These challenges are found in applications such as exhaust gas recovery for power plants and industrial sites, as well as in industry with HTGRs that use direct cycles with low-pressure gas circuits.

[0019] In these applications, to maximize performance, the gas-side heat exchanger geometry must utilize a profile that provides a high heat exchanger "goodness" factor (i.e., the ratio of pressure drop to heat transfer). Typically, the best profile is inner-tube flow, which provides one of the lowest pressure drops per unit of heat transfer in a turbulent flow regime. Typically, inner-tube flow is optimal for minimizing pressure drop for a given amount of heat transfer.

[0020] Considering tube-side gas flow, a challenge presented is frontal area, which is small in typical prior art tight-pitch triangular tube layouts (typically about 40% for a 1.25 pitch x tube diameter in a triangular tube layout). This low frontal flow area requires a large tubesheet to ensure that sufficient flow area is maintained to accommodate the low pressure drop (often less than 0.2 bar).

[0021] The configuration according to the invention provides a pitch equal to the tube diameter, i.e. the pitch is substantially unity, thus maximizing the internal tubular flow cross-sectional area (~60% of the frontal area). For shell-side flow, cross-flow is restricted or prevented, with flow being longitudinal from the three contacting tubes through the hypocycloidal enclosure gap.

[0022] The small cross-sectional area and high heat transfer area on the shell side maintain good shell-side heat transfer characteristics potentially comparable to cross-flow designs, while the shell-side pressure drop is significantly reduced.

[0023] The tube may be straight or a U-tube. If the tube is straight, at least one end nozzle may be located in an end cover space located at each end of the tube.

[0024] If the heat exchanger tube is a U-tube, there is only one end cover and both end nozzles are located within said end cover.

[0025] Exemplary Heat Exchanger Compact shell-and-tube heat exchangers are sized using conventional construction techniques to perform end-cycle functions. Table 1 provides basic heat exchanger sizing and duties. [Table 1]

[0026] To illustrate performance potential, heat exchanger sizing was performed based on a conventional heat exchanger with a 1.25 pitch and a heat exchanger according to the present invention, using the same heat exchanger heat load requirements, maintaining the same 12.7 mm OD, 0.889 mm wall thickness tubes, the same inlet and outlet temperatures, and the same pressure drop across the heat exchanger core. [Table 2]

[0027] Removal of tubesheet manufacturing constraints allows for the utilization of smaller tube IDs due to the elimination of the challenge of drilling small diameter holes on tight pitch in thick tubesheets.

[0028] Reducing the tube diameter increases the available heat transfer area for a given heat exchanger size; in the example above, the tube side heat transfer area is 3058 m 2 3100m from 2 while the flow frontal area remains nearly constant for a given shell diameter. Furthermore, the performance improvement is 21.69 m 3 From 14.16m 3 This is achieved from a smaller "total heat exchanger volume" (inside the shell) which has been reduced to 1000 kW, providing a more compact heat exchanger unit.

[0029] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0030] [Figure 1a] FIG. 1a shows the inlet of a shell-and-tube heat exchanger according to the present invention. [Figure 1b] FIG. 1b shows the outlet of a shell-and-tube heat exchanger according to the invention. [Figure 2] Figure 2 shows a triangular stack of tubes. [Figure 3a] Figure 3a shows a square stack of tubes. [Figure 3b] Figure 3b shows a triangular stack of tubes. [Figure 4a] Figure 4a shows a tube with a small diameter and tessellated edges. [Figure 4b] Figure 4b shows a tube with a small diameter and tessellated edges. [Figure 5] FIG. 5 shows the back-brazed joint of the mosaic tube of FIG. 4b. [Figure 6]FIG. 6 shows a support ring for clamping the tube. [Figure 7] FIG. 7 shows a separate support ring for clamping the tube. DETAILED DESCRIPTION OF THE INVENTION

[0031] Referring to Figures 1a and 1b, a shell-and-tube assembly 1 is provided, comprising shells 5a, 5b with multiple tubes 8a, 8b penetrating them. A first fluid 6a enters through a shell nozzle inlet 2a and passes through a hypocycloidal gap 9a created by the abutting tubes 8a. The gap 9a extends the length of the piping, ensuring a large surface area of ​​contact between the first fluid and the tubes 8a. A second fluid 7a enters the tubes at a first end and traverses the length of the tubes, where the second fluid exits at an exit point 7b (Figure 1b). To allow the first fluid to enter the gap 9a, a small-diameter section 3a of the tube allows shell-side flow to enter and exit the tube bundle. The length and diameter of the small-diameter section 3a of the tube can be optimized to minimize gas-side pressure loss while ensuring sufficient flow area for distribution of shell-side mass flow within the tube bundle. Optionally, the shell-side header can include a larger-diameter shell section to ensure the shortest path through the tube bundle and aid in shell-side flow distribution. Figure 1b provides an outlet side where the first fluid 6b exits the hypocycloidal enclosure gap 9b via the smaller-diameter section 3b and exits through the shell outlet nozzle 2b. Finally, the second fluid 7b exits tube 8b. The tube ends 4a, 4b are shown as tessellated ends with no gaps between abutting pipes 8a, 8b to prevent the first fluids 6a, 6b from escaping, thus eliminating the need for a conventional tube web support.

[0032] The shell-side pressure drop can be estimated based on the analogy of flow through a duct and calculation of the loss coefficients for flow across the tube bundle at the inlet and outlet of the flow. The inlet is specially designed to allow the shell-side flow to pass around the tube bundle and enter the tube bundle from all directions. In doing so, flow losses at the inlet are minimized while optimizing the flow distribution through the core.

[0033] Referring to Figure 2, three tubes 11 are provided, each abutting two other tubes 15 to form a triangular stack.

[0034] The gap 14 defined between the abutment points 15 is hypocycloidal, specifically a three-sided apex of a triangle. A first fluid 12 can pass through the enclosed gap 14 along the entire length of the tube 11. A second fluid 13 can then pass through the cavity within the tube 11.

[0035] Referring to Figure 3a, four tubes 21 are shown intersecting three other tubes at abutment points 23 to form a square stacked arrangement. The resulting hypocycloidal enclosure gap 22 is star-shaped.

[0036] Referring to Figure 3b, three tubes 24 are shown intersecting two other tubes at abutment points 26 to form a triangular stacked arrangement. The resulting hypocycloidal enclosure gap 22 is triangular in shape.

[0037] Referring to Figure 4a, a tube 30 having a first diameter 31a is provided that can abut at least two additional tubes to form a hypocycloidal gap. The tube has a second diameter 32a, i.e., a smaller diameter section, to allow a first fluid to enter the hypocycloidal gap. The ends of the tube are regular hexagons 33a or other tessellated shapes such that the hexagons form an airtight seal when the tube sections 31a are stacked.

[0038] Referring to Figure 4b, two tubes are stacked such that first diameter pipe sections 31a and 32b of the pipe abut. Second diameter regions 32a and 32b are smaller to provide a large inlet or outlet gap between them, allowing the first gas to flow within the hypocycloidal enclosure gap created between the stacked / abutting tubes.

[0039] Referring to FIG. 5, a plurality of stacked tubes 46 having hexagonal faces are provided. The hexagonal sections are tessellated to eliminate the need for tube plate material between each tube. However, to ensure a mechanical bond and an airtight seal interface between the shell side and the tube side, the tubes are first welded together at the tube faces 47 and then back-brazed 41. The two sealing means allow the flow of the second fluid to flow only through the internal cavity of the tube 46. Similarly, the flow of the first fluid 44 is thereby confined to the hypocycloidal enclosure gap 45.

[0040] Referring to Figure 6, the tube bundle 56 must be connected to the shell in this design; the shell can be any desired shape, but is optimally circular due to its inherent strength when encountering significant shell-side pressures. To facilitate joining of the tubes 56 to the shell, an additional ring 53 is configured and machined to match the outer shape of the hexagonal tube bundle end. This ring may be the same diameter as the shell and may be attached to the shell as a butt weld 54. Tube face welds 57 are shown on the face of the tubes, and back brazes 55 provide an airtight seal to the tubes.

[0041] Referring to Figure 7, an alternative connection to that shown in Figure 6 is provided, in which the use of separate end support rings 63 reduces the required thickness of material rolled to create the end rings. End support rings with this design only require the material necessary to fill the gap between the outer tube and the outer tube bundle end, plus the butt weld 64 to maintain the ring structure. The thickness of the shell wall that slides over the support ring is now only that required for pressure resistance, and no additional material is required to form the gap at the tube bundle head. Tube face welds 67 are shown on the face of the tube, and back brazes 65 provide an airtight seal to the tube.

Claims

1. 1. A shell-and-tube heat exchanger having an outer shell and a series of heat exchange tubes positioned therein, the shell having a shell-side fluid inlet and a shell-side fluid outlet for transport of a first fluid; the tube, in use, is capable of passing a second fluid therethrough, the tube having a first end and a second end, the tubes being arranged in contact to form a hypocycloidal enclosed gap therebetween, the hypocycloidal enclosed gap providing a path for transfer of the first fluid; wherein the tube has a first region of reduced diameter located adjacent a shell-side fluid inlet and a second region of reduced diameter located adjacent a shell-side fluid outlet to enable the first fluid to form a flow path through the hypocycloidal surrounding gap.

2. The heat exchanger of claim 1 , wherein the first and second ends of the tubes are secured at their open ends via a tubesheet.

3. The heat exchanger of claim 1 , wherein the first end and / or the second end of the tube comprises a tessellated cross-sectional shape.

4. 4. The heat exchanger according to claim 3, wherein the cross-sectional shape is hexagonal.

5. 5. A heat exchanger according to claim 3 or 4, wherein the transitions from the tubes to the mosaic tube ends comprise back-brazed joints.

6. 6. The heat exchanger of claim 1, wherein the hypocycloidal enclosure gap is formed from three contiguous tubes to form a triangular pitch configuration.

7. 6. The heat exchanger of claim 1, wherein the hypocycloidal enclosure gap is formed from four abutting tubes to form a square pitch configuration.

8. 8. A heat exchanger according to any one of claims 1 to 7, wherein the tubes are straight and there is at least one end cap in an end cover space located at each end of the tube.

9. 8. The heat exchanger of claim 1, wherein the tubes are U-tubes and there is only one end cover, and wherein both end caps are positioned within the end cover.

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