GB2531518A

GB2531518A - Heat exchanger

Info

Publication number: GB2531518A
Application number: GB1418552.4A
Authority: GB
Inventors: Hattrell Timothy; Robertson Daniel; Pilatis Nickolaos; Lopez Ramirez Susana; Narayanan Pradeep
Original assignee: Rolls Royce Power Engineering PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 2014-10-20
Filing date: 2014-10-20
Publication date: 2016-04-27
Also published as: GB201418552D0

Abstract

A method of manufacturing a layered structure of a plate-fin heat exchanger comprises stacking (figure 8) a first sheet 150, second sheet 154 and third sheet 152 such that the third sheet 152 is positioned between the first 150 and second sheets 154. Regions of the third sheet 152 are bonded to the first sheet 150 and the second sheet 154. Recesses 162 are provided in an outer surface of the first sheet 150 and/or the second sheet 154 at a position corresponding to the respective regions bonded to the third sheet 152. The first sheet 150, second sheet 154 and third sheet 152 are placed in a die and a hollow structure is formed (figure 9) so that the third sheet 152 spans a gap between the first 150 and second sheets 154 to define channels of the heat exchanger. The first 150 and/or second sheet 154 are pressed against a surface of the die so as to remove the recesses therefrom (see figure 10).

Description

Heat Exchanger

FIELD OF INVENTION

The present invention relates to a heat exchanger, in particular but not exclusively a heat exchanger for a nuclear reactor plant.

BACKGROUND

Typically, a nuclear power plant includes a nuclear core, a heat exchanger, a turbine and a condenser. A primary fluid is circulated to the core where the primary fluid cools the core and in the process is heated. The heated primary fluid is then directed to a hot side of the heat exchanger. A secondary fluid is directed to a cold side of the heat exchanger and heat is transferred from the primary fluid to the secondary fluid. Upon exiting the heat exchanger the secondary fluid is directed to the turbine. The turbine is generally connected to a generator to generate electricity.

Generally, to deal with the heat transfer requirements of a nuclear power plant, the heat exchanger is usually of the shell and tube type. The primary or the secondary fluid may be directed through the tubes and the secondary or primary fluid may be directed around the tubes within the shell. The walls of the tubes provide a barrier to reduce the risk of the primary fluid contaminating the secondary fluid. The tube and shell type heat exchangers are generally extremely large; for example in some applications the height of the heat exchanger can be around 20m.

In non-nuclear applications, other types of heat exchangers are used. For example a plate-fin type heat exchanger. An example of such a heat exchanger used in the oil and gas industry is described in W092/15830. The heat exchanger of W092/15830 includes a plurality of layers each layer includes three sheets stacked and bonded together with the central sheet being corrugated so as to form a plurality of channels within the layer. Generally, a hot fluid will flow along the channels formed in one layer and a cold fluid will flow along the channels formed in an adjacent layer.

A method of manufacturing a heat exchanger similar to that described in W092/15830 includes providing three sheets of a metallic material amenable to super plastic forming; exemplary materials are known in the art. Selected regions of the sheets are coated in a material, such as a ceramic, that inhibits diffusion bonding in metals. The sheets of material are then stacked and external pressure is applied to the three layers and the temperature is elevated so as to diffusion bond the central layer to the two outer layers. The layered structure is then placed in a forming die and superplastically formed. The forming process elongates the central sheet so as to form the corrugated centre of a layer. The inflated structures are stacked and bonded together to form the layers of the heat exchanger.

SUMMARY OF INVENTION

A first aspect of the disclosure provides a method of manufacturing a layered structure of a plate-fin heat exchanger. The method comprises providing a first sheet, a second sheet, and a third sheet and stacking the first sheet, second sheet and third sheet such that the third sheet is positioned between the first and second sheets. Regions of the third sheet are bonded to the first sheet and the second sheet. Recesses are provided in an outer surface of the first sheet and/or the second sheet at a position corresponding to the respective regions bonded to the third sheet. The first sheet, second sheet and third sheet are placed in a die and a hollow structure is formed so that the third sheet spans a gap between the first and second sheets to define channels of the heat exchanger. The first and/or second sheet are pressed against a surface of the die so as to remove the recesses therefrom.

The recesses may be provided in the first sheet and/or the second sheet after regions of the third sheet have been bonded to the first sheet and/or the second sheet.

In alternative embodiments, the recesses may be provided in the first sheet and/or the second sheet before regions of the third sheet have been bonded to the first sheet and/or the second sheet.

The recesses provided on the first sheet may be offset from the recesses provided on the second sheet.

A second aspect of the disclosure provides a method of manufacturing a layered structure of a plate-fin heat exchanger. The method comprises providing a first sheet, a second sheet, and a third sheet and stacking the first sheet, second sheet and third sheet such that the third sheet is positioned between the first and second sheets. A plurality of recesses are provided on one or both of the inner surfaces of the third sheet (e.g. the surfaces of the third sheet that oppose the surfaces of the first and second sheet). Regions of the third sheet that correspond to the position of the recesses are bonded to the first and second sheets. The first sheet, second sheet and third sheet are placed in a die and a hollow structure is formed with the third sheet spanning a gap between the first and second sheets to define channels of the heat exchanger.

It will be understood that the layered structure may be considered as being substantially free from surface recesses.

The surfaces of the first, second and first sheets before superplastic forming and before recesses are provided therein may be substantially planar. The first and second sheets may have an inner surface to which the third sheet is bonded and an outer surface opposite (e.g. directly opposed to) the inner surface. The sheets may have two end surfaces transverse to the inner and outer surfaces.

Forming the hollow structure may be considered as expanding the hollow structure. To form the hollow structure the sheets may be subjected to elevated temperature and/or elevated internal pressure.

The hollow structure may be formed by superplastic forming.

The third sheet may be bonded to the first and/or second sheet by diffusion bonding.

The die may have a substantially planar surface that contacts the outer surface of first and/or second sheet during the expansion process.

A third aspect of the disclosure provides a method of manufacturing a plate-fin heat exchanger. The method comprising forming a plurality of layered structures and stacking and bonding said layered structures such that the outer surface of the first sheet of one layered structure is positioned against the outer surface of the second sheet of another layered structure. A portion of material is removed from an outer surface of the first and/or second sheets of one or more layered structures before stacking and bonding said layered structures together.

In the present application the thickness of the sheet is measured in a direction extending between the inner surface and the outer surface of the sheet in the case of the first and second sheet, and between the two inner surfaces in the case of the third sheet.

The layered structure may be manufactured using the method according to the first or second aspects.

A fourth aspect of the disclosure provides a plate-fin heat exchanger produced by the method according to the third aspect and/or having a layered structure produced by the method of the first or second aspect.

A fifth aspect of the disclosure provides a plate-fin heat exchanger comprising a plurality of layered structures stacked together to form a series of channels for fluid flow through the heat exchanger, wherein each layered structure comprises two planar walls and a central structure connecting between the two walls and defining a plurality of channels. The channels have a cross section that is quadrilateral in shape and has two parallel sides (for example the cross section may be considered to be a trapezoid), one of the parallel sides is shorter than the other. The thickness of the wall and the central structure in the region of the shorter side is substantially equal to the thickness of the wall and the central structure in the region of the longer side.

A sixth aspect of the disclosure provides a method of manufacturing a layered structure for a plate-fin heat exchanger. The method includes providing a sheet assembly having a first sheet, a second sheet and a third sheet, the first, second, and third sheets being stacked and the third sheet being positioned between the first and second sheets (e.g. the third sheet is sandwiched between the first and second sheets); wherein regions of the third sheet are bonded to regions of the first sheet or second sheet; and wherein recesses are provided in an outer surface of the first sheet and/or the second sheet at locations corresponding to positions where the third sheet is bonded to respective first or second sheet. The sheet assembly is expanded against a die to form a hollow structure with the third sheet spanning a gap provided between the first and second sheets and the outer surface of the first sheet and/or second sheet is substantially planar and free from recesses.

DESCRIPTION OF DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates a schematic of a sodium-cooled fast reactor (SFR) plant; Figure 2 illustrates a perspective view of a heat exchanger used in the SFR plant of Figure 1; Figures 3 to 7 illustrate steps of an exemplary method of manufacturing the heat exchanger for Figure 2; Figures 8 to 10 illustrate steps of an alternative exemplary method of manufacturing the heat exchanger of Figure 2; and Figure 11 illustrates a portion of a cross section of a heat exchanger of the related art which is used in the oil and gas industry.

DETAILED DESCRIPTION

There are many different types of nuclear reactor plants, for example pressurised water reactors (PWRs), boiling water reactors (BWRs), gas cooled reactors, molten salt reactors and liquid metal cooled reactors (e.g. a sodium fast reactor). An example of a sodium fast reactor plant is indicated generally at 10. The reactor plant includes a first sodium circuit 13, a second sodium circuit 15 and a nitrogen gas cycle 17. The first sodium circuit 13 includes a reactor core where heat energy is generated. A first flow of liquid sodium is circulated to the core for cooling. In the cooling process the liquid sodium is heated. The first flow of liquid sodium is then circulated from the core to a first heat exchanger 14 before being circulated back to the core. The liquid sodium is circulated around the first sodium circuit by a pump 16.

The second sodium circuit 15 includes a second flow of liquid sodium. The second flow of liquid sodium is circulated to the first heat exchanger 14 where heat is transferred from the first flow of liquid sodium to the second flow of liquid sodium. The second flow of liquid sodium then flows to a second or intermediate heat exchanger 20 before being circulated back to the first heat exchanger. The second flow of liquid sodium is circulated around the second sodium circuit by a pump 18.

The nitrogen gas cycle 17 includes a flow of nitrogen gas. The nitrogen gas is circulated to the intermediate heat exchanger 20 where heat is transferred from the second flow of liquid sodium to the nitrogen. Nitrogen then flows from the heat exchanger to a turbine 22. The turbine 22 is connected to a compressor 24 and an electrical generator 26. Nitrogen gas flows from the turbine 22 to a recuperator 28, then to a heat sink 30, before being directed to the compressor 24. The nitrogen gas flows from the compressor to the recuperator where the nitrogen gas is pre-heated before being directed to the intermediate heat exchanger 20. In this way electricity can be generated from a nuclear core.

The heat exchanger 20 will now be described in more detail with reference to Figure 2.

The heat exchanger 20 is a plate-fin heat exchanger. The heat exchanger includes a plurality of heat exchange modules 32. The heat exchange modules are linearly arranged adjacent each other. Each heat exchanger module is axially aligned with a neighbouring heat exchange module.

Each heat exchange module defines a plurality of hot fluid flow paths and a plurality of cold fluid flow paths. The liquid sodium (which may be referred to as the primary fluid) is circulated along the hot fluid flow paths and the nitrogen gas (which may be referred to as the secondary fluid) is circulated along the cold fluid flow paths. The fluid flow paths will be described in more detail later.

Each module 32 has a corner recess that accommodates a portion of a manifold 34, 35, 36 or 37. In the present embodiment the cross section of each manifold is circular so the recesses are a circular segment shape.

Manifolds 34, 35, 36, 37 are provided at each corner of the heat exchanger. Two entry manifolds 34, 36 and two exit manifolds 35, 37 are provided. The entry manifolds 34, 36 are, in use, positioned at an upper end of the heat exchanger and the exit manifolds 35, 37 are, in use, positioned at a lower end of the heat exchanger. The entry manifold for supplying the cold fluid to the cold fluid flow paths (referred to from here-on-in as the cold entry manifold 34) and the exit manifold for receiving the cold fluid from the cold fluid flow paths (referred to from here-on-in as the cold exit manifold 37) are at diagonally opposing corners. The entry manifold for supplying the hot fluid to the hot fluid flow paths (referred to from here-on-in as the hot entry manifold 36) and the exit manifold for receiving the hot fluid from the hot fluid flow paths (referred to from hereon-in as the hot exit manifold 35) are at diagonally opposing corners.

The manifolds 34, 35, 36 and 37 are elongate and extend from one axial end of the heat exchanger 20 to the opposing axial end of the heat exchanger 20. In the present embodiment the manifolds have a circular cross-section, e.g. they can be considered to be substantially cylindrical tubes. However, in alternative embodiments the manifolds may have any other suitable cross section, e.g. oval. The cold entry manifold 34 and the cold exit manifold 37 have a similar or equal diameter, and the hot entry manifold 36 and the hot exit manifold 35 have a similar or equal diameter. The cold entry and exit manifolds have a larger cross section (e.g. a greater diameter) than the hot entry and exit manifolds.

The entry and exit manifolds 34, 35, 36 and 37 include a plurality of openings along the length thereof that are arranged to align with the corresponding fluid flow path so that the entry and exit manifolds are in fluid communication with the corresponding cold or hot fluid flow paths of the modules 32.

In use, the inlets 44 of the hot entry manifold are connected to receive a flow of liquid sodium from the first heat exchanger 14 of the second sodium circuit 15, and the outlets 46 of the hot exit manifold are connected to deliver a flow of liquid sodium to the pump 18 to be circulated to the first heat exchanger of the secondary circuit. The inlet 38 of the cold entry manifold is connected to a flow of gaseous nitrogen from the recouperator 28 of the nitrogen gas cycle, and the outlet 40 of the cold exit manifold is connected to deliver a flow of gaseous nitrogen to the turbine 22 of the nitrogen gas cycle. By way of example only, the liquid sodium may enter the heat exchanger at a temperature exceeding 520°C and at approximately atmospheric pressure. The liquid sodium may exit the heat exchanger at a temperature below 400°C and at approximately atmospheric pressure. The gaseous nitrogen may enter the heat exchanger at a temperature below 370°C and a pressure above 150 bar. The gaseous nitrogen may exit the heat exchanger at a temperature above 500°C and at a pressure above 150 bar.

The construction of each module of the heat exchanger 20 will now be described in more detail. Each module includes a plurality of layered structures bonded together. The layered structures are optimised to improve heat transfer within the heat exchanger. Each layered structure is formed using three sheets (or plates), and the heat transfer is improved by minimising the thickness of the three sheets. The selection of the thickness of the sheets is a balance between optimising heat transfer and providing a structure that can withstand the working pressures of the heat exchanger.

The construction of the layered structure will be further understood in the following description of how the layered structures are manufactured.

An exemplary manufacturing method will now be described with reference to Figures 3 to 7. Firstly, three metallic sheets 50, 52, 54 are provided, in this embodiment the metallic sheets are capable of being superplastically formed. Regions of the metallic sheets are coated in a material that prevents diffusion bonding in metals, e.g. the sheets may be coated with a ceramic such as Yittria. One of the coated regions is indicated at 56 in Figure 3. The coated regions correspond to the position of a plurality of fluid flow channels of the heat exchanger.

The metallic sheets 50, 52, 54 are then stacked and the central metallic sheet is diffusion bonded to the outer metallic sheets. Referring in particular to Figure 3, the diffusion bonded sheets are placed in a die 58. Internal elevated pressure and elevated temperature is applied to the bonded sheets causing the bonded structure to expand to fill the die and to form the layered structure shown in Figure 4. The layered structure is then removed from the die 58.

To form a module of the heat exchanger 20, a plurality of layered structures are manufactured, stacked and bonded together. With reference in particular to Figure 5, in the present exemplary method, before stacking and bonding the layered structure, the outer sheets 50, 54 are thinned using a materials removal process such as chemical machining, adaptive machining or adaptive grinding. The outer sheets 50, 54 are thinned to an extent such that it is not possible for the layered structure to be stable under the operating conditions of the heat exchanger. However, as will now be explained in more detail, when the layered structures are stacked and bonded the effective increase in thickness of the outer sheets as a result of being positioned adjacent an outer sheet of a neighbouring layered structure is sufficient for the structure to remain stable under the operating conditions of the heat exchanger 20.

Referring now in particular to Figure 6, once material has been removed from an outer sheet of two layered structures, the layered structures are stacked and bonded together. In the present embodiment, the edges of adjacent outer sheets of neighbouring layered structures are sealed using a process such as TIG, MIG or arc welding.

The surfaces of the outer sheets of the neighbouring layered structures are then bonded together to produce the final component, as illustrated in Figure 7.

During the superplastic forming process the pressure applied to the layered structure causes the central sheet to deform rather than the outer sheets. Therefore, when the sheets are made from the same material, the outer sheets need to be thicker than or equal in thickness to the central sheet. The described method meets this process requirement by removing material from an outer surface of a layered structure after the superplastic forming process.

Figure 11 illustrates two layered structures 1060 of a heat exchanger known to be used in the oil and gas industry. Similar features of the layered structures of Figure 11 are given similar reference numerals as the layered structures previously described but with a prefix "10" to distinguish between embodiments.

Comparing Figure 7 to Figure 11, it can be seen that the thickness t1 of the sheet material between the passageways 64, 1064 formed in the layered structure is reduced compared to a process where material is not removed from the outer surface of the layered structure. This results in increased heat transfer, which is particularly important for a heat exchanger used in a nuclear power plant.

Additionally or alternatively, the heat transfer efficiency of the heat exchanger can be increased by manufacturing the layered structure using the following described method. As will be appreciated by the person skilled in the art, the following method is described in isolation from the above described method, but both methods may be used together to further optimise heat transfer.

The method of forming the layered structure 160 is similar to the previously described method, but before the sheets 150, 152, 154 are stacked and bonded material is removed from the outer sheets or from the inner sheets in a region corresponding to the region that is bonded to an adjacent sheet. Referring to Figure 8, in the presently described embodiment, a recess 162 is provided on the outer surface of the outer sheets 150, 154. The recess may be provided before the sheets are bonded together or the recess may be provided after the sheets are bonded together but before the sheets are superplastically formed. Figure 9 illustrates the position of the recesses with respect to the bonded regions, although it is to be understood that Figure 9 does not represent a step in the present method. However, in alternative methods Figure 9 may represent an intermediate step of the inflation process.

To form the layered structure 160 of Figure 10, the bonded sheets 150, 152, 154 of Figure 8 are placed in a die and superplastically formed, similar to the previously described method. During the process of superplastic forming, the recess 162 provided in the outer surface is "ironed our due to the internal pressure applied during the superplastic forming process, which presses the outer surfaces of the layered structure against a flat (or planar) face of the die.

Comparing Figure 10 to Figure 11, it can be seen that the step of providing a recess, and using the superplastic forming process to fill the recess provides outer sheets with a substantially constant thickness t2, that is the thickness doesn't increase in the region of the bond as it does in the structure of Figure 11.

Tests have found that a principal factor in determining the power density of a heat exchanger for a fixed pressure drop and fixed fluid inlet and outlet temperatures is the thermal conductivity of the wall between the two fluids (e.g. between the primary and secondary fluid or the hot and cold fluid). The thermal conductivity of the wall between the two fluids is determined by the thickness of the wall and the material used to form the wall.

The thickness of the wall between the primary and secondary fluid is determined by the geometric requirements for the fluid channels (e.g. taking into account the minimum area for plugging and corrosion allowances), the allowable stresses for the material chosen at the operating temperature, and the geometric limitations imposed by the manufacturing process.

The thickness of the central sheet is dependent upon the internal pressure within the fluid channel during operation of the heat exchanger, the pitch of the channel, the angle of the central sheet makes with the outer sheets once the central sheet has been superplastically deformed, the allowable stress for the material of the central sheet and allowances for corrosion or manufacturing tolerances.

The cell pitch (indicated at "P" in Figure 10) is set by the height of the cell, the angle of central sheet to the two outer sheets when the structure is expanded and the length of the bond between the central sheet and the outer sheets.

The bond length is dependent upon manufacturing limitations and/or allowable stress. The thickness of the central sheet is determined by how far the central sheet is stretched during the superplastic forming process.

It will be appreciated by one skilled in the art that, where technical features have been described in association with one or more embodiments, this does not preclude the combination or replacement with features from other embodiments where this is appropriate. Furthermore, equivalent modifications and variations will be apparent to those skilled in the art from this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting.

Claims

CLAIMS1. A method of manufacturing a layered structure of a plate-fin heat exchanger, the method comprising: providing a first sheet, a second sheet, and a third sheet; stacking the first sheet, second sheet and third sheet such that the third sheet is positioned between the first and second sheets; bonding regions of the third sheet to the first sheet and the second sheet; providing recesses in an outer surface of the first sheet and/or the second sheet at a position corresponding to the respective regions bonded to the third sheet; 10 and placing the first sheet, second sheet and third sheet in a die and forming a hollow structure with the third sheet spanning a gap between the first and second sheets to define channels of the heat exchanger, wherein the first and/or second sheet are pressed against a surface of the die so as to remove the recesses therefrom.
2. The method according to claim 1, wherein the recesses are provided in the first sheet and/or the second sheet after regions of the third sheet have been bonded to the first sheet and/or the second sheet.
3. The method according to any one of the previous claims, wherein the recesses provided on the first sheet are offset from the recesses provided on the second sheet.
4. A method of manufacturing a layered structure of a plate-fin heat exchanger, the method comprising: providing a first sheet, a second sheet, and a third sheet; stacking the first sheet, second sheet and third sheet such that the third sheet is positioned between the first and second sheets; providing a plurality of recesses on one or both of the inner surfaces of the third sheet: bonding regions of the third sheet that correspond to the position of the recesses to the first and second sheets; placing the first sheet, second sheet and third sheet in a die and forming a hollow structure with the third sheet spanning a gap between the first and second sheets to define channels of the heat exchanger.
5. The method according to any one of the previous claims, wherein the hollow structure is formed by superplastic forming.
6. The method according to any one of the previous claims, wherein the third sheet is bonded to the first and/or second sheet by diffusion bonding.
7. The method according to any one of the previous claims, wherein the die has a substantially planar surface that contacts the outer surface of first and/or second sheet during the expansion process.
8. A method of manufacturing a plate-fin heat exchanger comprising: forming a plurality of layered structures and stacking and bonding said layered structures such that the outer surface of the first sheet of one layered structure is positioned against the outer surface of the second sheet of another layered structure, and removing a portion of material from an outer surface of the first and/or second sheets of one or more layered structures before stacking and bonding said layered structures together.
9. The method according to claim 8, wherein the layered structure is manufactured using the method according to any one of claims 1 to 7.
10. A plate-fin heat exchanger produced by the method according to claim 8 or 9 and/or having a layered structure produced by the method of any one of claims 1 to 7.
11. A plate-fin heat exchanger comprising: a plurality of layered structures stacked together to form a series of channels for fluid flow through the heat exchanger, wherein each layered structure comprises two planar walls and a central structure connecting between the two walls and defining a plurality of channels, and wherein the channels have a cross section that is quadrilateral in shape and has two parallel sides, one the of the parallel sides being shorter than the other, and wherein the thickness of the wall and the central structure in the region of the shorter side is substantially equal to the thickness of the wall and the central structure in the region of the longer side.
12. A plate-fin heat exchanger and/or a method substantially as hereinbefore described with reference to and as shown in the accompanying drawings.