HEAT EXCHANGER

FIELD

[0001]The present invention relates to shell and tube heat exchangers, specifically to compact heat exchangers without the need for a tube sheet.

BACKGROUND

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

SUMMARY

[0007]Typically heat exchangers have a tube pitch of at least 1.25, such that there is sufficient volume for the first fluid to flow inbetween the tubes. In order 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 such that the first fluid remains in contact with the tubes for the maximum path length. The use of tubes that form a hypocycloidial enclosure gap, with a pitch of substantially 1, provides a path for the transfer of said first fluid, there is no further need for baffles to direct the flow along the tubes.

[0008]The hypocycloidial enclosure gap may be formed from three or more touching tubes with a pitch of substantially 1. Preferably there are three or four touching tubes, to provide a triangular or square pitch configuration, respectively. Clearly, larger numbers of tubes may be used, which will increase the flow of the first fluid through the hypocycloidial enclosure gap, however the system will be less efficient as there may be fewer tubes per unit volume. Preferably there are three touching tubes to provide triangular pitch configuration with a pitch of substantially 1.

[0009]The tubes abut along their length. The tubes may be fixedly attached along their length, however preferably there are one or more tube bundle supports, at the extremity of the bundle to provide support between the outer tube bundle and the shell. The tube bundle supports may stop flow fluid passing around the outside of the tube bundle whilst also offering structural support to the tube bundle to prevent it sagging in the middle.

[0010]Typical prior art heat exchangers may comprise fins, pleats or projections, so as to increase the surface area of the tubes. The use of only substantially smooth tubes, with no projections or relief structures, provides the maximum flow of fluid through the tubes, as the diameter of tube can be maximised.

[0011]The first and second region of reduced diameter are such that they allow the first fluid to pass from the fluid shell inlet through into the hypocycloidial enclosure gap along the length of the tubes and finally out of the fluid shell outlet.

[0012]The reduced diameter may occur for a length of up to 20% of the length of the tube, preferably up to 1%-10%, more preferably the reduced diameter may occur for a length of substantially the diameter of the fluid shell inlet/outlet nozzle.

[0013]The reduced diameter of each of said tubes may be of from 0.1% to 30% of the diameter of the tube diameter, more preferably in the range of from 1 to 10%.

[0014]The region of reduced diameter may be caused by drawing or forming a section of the tube, to the required reduced diameter, during fabrication. Tube wall reduction is a well-known technique.

[0015]Alternatively, the tube may comprise a reducer connector/coupling adaptor, to allow region of reduced diameter tube to be inserted. This may allow the retrofitting of a reduced diameter tube into an existing design, to form the region of reduced diameter, at the required point.

[0016]The first end and second end of said tubes may be secured at their open ends by a tube sheet. The tube sheet may abut the end face of the tubes and/or the hypocycloidial enclosure gap. In prior art devices, the tube sheet is a circular plate with a plurality of holes, the perforated tube sheet providing the support for the individual tubes.

[0017]In a highly preferred arrangement the first end and/or second end of said tubes comprise a cross section shape which is tessellating, such as for example triangular, square or hexagon. Whilst other complex shapes are capable of tessellating, their complex shape would not provide low cost manufacture, and are less suitable. The use of a cross section shaped, which is tessellating removes the need for a tube sheet that provides support for the tube, the tessellating shape ensures that the ends are self-supporting.

[0018]The hexagonal end to the tube allows the tubes to be stacked into a tube bundle of various shapes whilst eliminating tube plate material between each tube. Shell side pressure forces are exclusively applied to and transmitted by the tubes rather than on to a separate tube sheet. The first end and/or second end of said tubes ends may be secured by adhesives, welding, brazing, mechanical fastenings. Preferably, the transition from the tube to said tessellating tube end may comprise a back-brazed joint. Preferably, to form the mechanical joint and seal interface between the shell side and tube side the tubes are first welded together on the tube face followed by back brazing. The back brazing provides both significantly increased joint strength whilst protecting the weld surface from chemical attack from corrosive fluids.

[0019]The tubes may be manufactured from any heat conductive material, typically a metal or metalloid. The tube shape cross section may be typically circular, and remain circular along its entire length, apart from the tessellating ends when present.

[0020]The tube bundle is connected to the shell, with this design the shell may be any required shape, however where significant shell side pressure is encountered the optimum is a circular shape due to its inherent strength. To facilitate the tube to shell joint, a support ring, which adopts the outer shape of the hexagonal tube bundle end may be used. This ring may be the same diameter as the shell and attached to the shell as butt weld or can be the same as the internal diameter of the shell. Where the end support rings are designed to fit inside the shell, there are various shell to head joining methods analogous to conventional shell and tube TEMA standard available to suit each individual application.

[0021]Heat recovery from low pressure gases where differential pressure is small is complex typically due to the combined challenge of limited available pressure differential and low heat transfer performance of the fluids ie gases. These challenges are found in industry in applications such as exhaust gas recovery for power plants and industrial sites, as well as with HTGR using direct cycles with a low-pressure gas circuit.

[0022]Within these applications, to maximise performance the heat exchanger geometry on the gas side must utilise a profile which offers a high heat exchanger “goodness” factor (i.e. ratio of pressure loss to heat transfer). Typically the best profile is internal tube flow, which gives one of the lowest pressure drops per unit of heat transfer in the turbulent flow regime. Typically, internal tubular flow is optimum where you are minimising pressure drop for a given amount of heat transfer.

[0023]Considering the gas flow on the tube side the challenge presented is the frontal area. The frontal area is low for typical prior art close-pitched triangular tube layouts, (typically around 40% for 1.25 pitch×tube diameter in triangular tube layout). This low frontal flow area necessitates a large tube sheet to ensure sufficient flow area is maintained to accommodate low pressure drops (less than 0.2 bar in many cases).

[0024]The arrangement according to the invention provides a pitch equal to the tube diameter, ie pitch is substantially 1, therefore maximising the internal tubular flow cross sectional area (~60% of the frontal area). For the shell side flow, cross flow would now be restricted or prevented, such that flow becomes longitudinal through the hypocycloidial enclosure gap from the three touching tubes.

[0025]The small cross-sectional area and high heat transfer area on the shell side maintains good shell side heat transfer properties potentially comparable to cross flow designs, whilst shell side pressure drops are significantly reduced.

[0026]The tubes may be straight or U-tubes. Where the tubes are straight, there may be at least one end nozzle located in the end cover space located at each end of the tubes.

[0027]Where the heat exchanger tubes are U-tubes there is only one end cover wherein both end nozzles are located within said end cover.

Example Heat Exchanger

[0028]A compact shell and tube heat exchanger has been sized using conventional construction techniques to perform an end cycle function Table 1 provides the basic heat exchanger sizing and duty.

[0029]To illustrate the performance potential, the same heat exchanger thermal load requirements has been used to size a heat exchanger based on a conventional heat exchanger with pitch of 1.25, and the heat exchanger according to the invention. The heat exchanger according to the invention maintains the same 12.7 mm OD, 0.889 mm wall thickness tubes, the same inlet and outlet temperatures, and the same overall heat exchanger core pressure drop.

[0030]The removal of the constraint of tube sheet manufacture is an enabler for utilisation of smaller tube IDs due to removing the challenge of drilling small diameter holes at close pitch within thick tube sheets.

[0031]The reduction in tube diameter increases the available heat transfer area for a given heat exchanger size, in the example above, the tube side heat transfer area has increased from 3058 m² to 3100 m², whilst the flow frontal area remains ~constant for a given shell diameter. Further the increased performance has been achieved from a smaller “total heat exchanger volume” (internal of shell), which has decreased from 21.69 m³ down to 14.16 m³. A more compact heat exchanger unit has been provided.

[0032]Embodiments of the invention will now be described by way of example only with reference to the figures, in which:

[0033]FIGS. 1a and 1b show a shell and tube heat exchanger inlet and outlet according to the invention;

[0034]FIG. 2 shows triangular stacking of tubes; and

[0035]FIGS. 3a and 3b show triangular and square stacking of tubes

[0036]FIGS. 4a and 4b show tubes with a reduced diameter and tessellated end

[0037]FIG. 5 shows a back brazed joint of tessellated tubes of FIG. 4b

[0038]FIG. 6 shows a support ring to clamp the tubes

[0039]FIG. 7 shows a separated support ring to clamp the tubes

DETAILED DESCRIPTION

[0040]Turning to FIGS. 1a and 1b provides a shell and tube assembly 1, comprising a shell 5a, 5b with a plurality of tubes 8a, 8b running therethrough. A first fluid 6a enters via the shell nozzle inlet 2a and passes through the hypocycloidal enclosure gap 9a created by the abutting tubes 8a. The enclosure gap 9a runs the length of the pipework to ensure a large surface area of contact for the first fluid with the tube 8a. A second fluid 7a enters the tubes at the first end and traverses the length of the tubes such that the second fluid exits 7b (FIG. 1b) to the exit point. To allow the first fluid to enter the gap 9a a reduced diameter section 3a of tube, allows the shell side flow to enter and exit the tube bundle. The length and the diameter of the reduced diameter section 3a of the tubes can be optimised to minimise gas side pressure loss whilst ensuring sufficient flow area to distribute the shell side mass flow within the tube bundle. If required, the shell side header can include a larger diameter shell section to ensure the shortest path through the tube bundle to aid distributing of shell side flow. FIG. 1b, provides the exit side, wherein the first fluid 6b exits the hypocycloidal enclosure gap 9b via the reduced diameter section 3b, to exit via shell exit nozzle 2b. Finally the second fluid 7b exits the tubes 8b. The ends of the tubes 4a,4b are shown as end pieced with a tessellating shape, such that there is no gap between the abutting pipes 8a, 8b, so as to prevent the first fluid 6a, 6b from escaping, and therefore removing the need for a conventional tube web support.

[0041]The shell side pressure drop can be estimated based on analogies for flow through ducts and loss coefficient calculations for flow across tube bundles at flow entry and exit. The entrance is specifically designed to allow the shell side flow to pass around the tube bundle and enter the tube bundle from all directions. In doing so the flow losses on entry are minimised while the flow distribution through the core is optimised.

[0042]Turning to FIG. 2 there is provided three tubes 11, which each abut 15 with two other tubes to form a triangular stack. The gap 14 formed between the abutting points 15 is a hypocycloidal shape, specifically a 3 sided cusp a deltoid. The first fluid 12 is able to pass through the enclosure gap 14 along the entire length of the tubes 11. The second fluid 13 is then able to pass via the cavity within the tubes 11.

[0043]Turning to FIG. 3a, shows four tubes 21, which intersect at the abutting points 23 with three other tubes to form a square stacked arrangement. The hypocycloidal enclosure gap 22 formed is an asteroid shape.

[0044]Turning to FIG. 3b, shows three tubes 24, which intersect at the abutting points 26 with two other tubes to form a triangular stacked arrangement. The hypocycloidal enclosure gap 22 formed is a deltoid shape.

[0045]Turning to FIG. 4a there is provided a tube 30, with a first diameter 31a, such that it is capable of abutting at least two further tubes, to form a hypocycloidal enclosure gap. The tube comprising a second diameter 32a, a reduced diameter section, such that a first fluid is capable of passing into the hypocycloidal enclosure gap. The end of the tube is a regular hexagon 33a or other tessellating shapes, such that when the tube section 31a is stacked that the hexagons form a gas tight seal.

[0046]Turning to FIG. 4b, two tubes are stacked such that the first diameter pipe section 31a and 32b of the pipes abut. The second diameter regions 32a and 32b, are reduced diameter, so as to provide a large entrance or exit gap therebetween, to allow a first gas to flow in the hypocycloidal enclosure gap created between the stacked/abutting tubes.

[0047]Turning to FIG. 5 there is provided a plurality of stacked tubes 46, which have a hexagonal face. The hexagonal portions tessellate which eliminates the need for a tube plate material between each tube. However, to ensure a mechanical joint and a gas tight seal interface between the shell side and tube side the tubes are first welded 47 together on the tube face followed by back brazing 41. The two sealing means allows the flow of the second fluid to flow only though the internal cavity of the tubes 46. Similarly, the flow of the first fluid 44 is thereby confined to the hypocycloidal enclosure gap 45.

[0048]Turning to FIG. 6, the tube bundle 56 needs to be connected to a shell, with the design according to the invention, the shell can be any required shape, however where significant shell side pressure is encountered the optimum is a circular shape due to its inherent strength. To facilitate the tube 56 to shell joint, an additional ring 53 is constructed and machined to match the outer shape of the hexagonal tube bundle end. This ring may be the same diameter as the shell and attached to the shell as butt weld 54. The tube face welds 57, is shown on the face of the tube, and the back braze 55, provides the gas tight seal to the tubes.

[0049]Turning to FIG. 7 provides an alternative connection to that shown in FIG. 6, with the use of a separated end support ring 63 is that it reduces the required thickness of material to be rolled to generate the end rings. The end support ring with this design is only required to fill the gap between the outer tubes and the outer tube bundle limit plus necessary material for the butt weld 64 to maintain the ring structure. The shell wall thickness which slides over the support ring, is now only the thickness required for the pressure resistance and does not need to include the additional material to make up the gap at the tube bundle head. The tube face welds 67, is shown on the face of the tube, and the back braze 65, provides the gas tight seal to the tubes.