GB2530794A

GB2530794A - Heat exchanger

Info

Publication number: GB2530794A
Application number: GB1417478.3A
Authority: GB
Inventors: Jong-Rae Cho; San Hu Park; Man Yeong Ha; Changmin Son; Ho Seung Jeong
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2014-10-03
Filing date: 2014-10-03
Publication date: 2016-04-06
Also published as: GB201417478D0

Abstract

A heat exchanger, in particular a bent tube heat exchanger (20), wherein one or more components (18) at a hotter side of the heat exchanger (20) are thermally isolated from one or more adjacent components (19) at a colder side of the heat exchanger (20) such that the components (18,19) can expand independently, thereby reducing thermal stress experienced within the heat exchanger (20). In particular, a separate tube-sheet (12) is provided for the inlet and outlet side. A method of forming a heat exchanger in two halves and fixing together with a layer of thermal insulation between the halves is also described.

Description

Heat exchanger The present invention relates to heat exchangers, and particularly, but not exclusively, to U-tube heat exchangers.

BACKGROUND

Heat exchangers induce heat transfer between higher and lower temperature fluids, typically without direct contact or mixing. As such, most heat exchangers have at least one inlet and outlet for a high temperature fluid passing through the heat exchanger. If the heat exchanger is working effectively, there will be a significant temperature difference between the high temperature' inlet(s) and outlet(s) due to the transfer of thermal energy to the colder fluid within the heat exchanger.

These high temperature differences, while indicative of an efficient and effective heat exchanger, can be problematic for the heat exchanger itself. The temperature differences seen in the fluids can be transferred to parts of the body of the heat exchanger. These temperature differences, and the resulting thermal deformation, lead to thermal stress along the connections of the parts, which can in turn lead to deformation or damage. The issue is particularly problematic in the case, for example, of a bent tube or U-tube heat exchanger where the high temperature inlets and outlets are located close to one another. However, all heat exchangers are affected by similar processes.

It would be beneficial, therefore, if local thermal stress in heat exchangers could be minimised.

US patent 4,114,684 discloses a floating frame for supporting the curved ends of a bundle of U-shaped tubes within a heat exchanger. The floating fame disclosed in US 4,114,684 is separate from the main supporting frame, and provides support for the end of the tube bundle while still allowing for differing degrees of thermal expansion of individual tubes. Thermal stresses within the U-shaped tubes, which could arise if a single fixed frame were used, are thereby minimised.

However, US 4,114,684 provides no solution to thermal stress problems that can arise in other parts of the heat exchanger, such as in the main tube support plates or tube-sheets.

SUMMARY OF THE INVENTION

It is an object of the present disclosure to reduce or minimise locally occurring thermal stress in heat exchangers, particularly in U-tube heat exchangers.

According to a first aspect of the invention there is provided a heat exchanger as defined in the appended claim 1. Further optional features are recited in the associated dependent claims.

According to the disclosure one tube-sheet is provided for sealing the hot side inlet from the secondary flow, and another, separate, tube-sheet is provided for sealing the hot side outlet from the secondary flow. Put another way, the tube-sheet of the present disclosure is provided in two parts. Each side/part is therefore able to undergo thermal deformation separately, such that when thermal expansion does occur, the deformations do not couple. The separation of the two parts allows for independent expansion, and also minimises heat transfer from one side to the other. Both of these factors reduce the thermal stress experienced within the heat exchanger, thereby reducing or eliminating cracking and leakage at the high stress locations, and resulting in a longer life for the heat exchanger unit.

The heat exchanger may be a U-tube heat exchanger, wherein the plurality of tubes comprises a bundle of U-shaped tubes.

The heat exchanger may also comprise a first manifold provided at the hot side inlet and a second, separate, manifold provided at the hot side outlet. Additionally, or alternatively, the heat exchanger may further comprise an outer case provided in two parts, with each part being connected to either the first or second tube-sheet. The separation of the manifold and/or outer case into two parts similarly allows for independent expansion of each part, and also minimises heat transfer from one side of the heat exchanger to the other.

One or more support plates may be provided, connected to the outer case, for supporting the plurality of tubes. At least one support plate may be provided in two parts, each being joined to one part of the outer case. This arrangement provides support for the tubes while minimising the potential stresses in the tubes that may arise as a result of different expansion rates across the heat exchanger.

Thermal insulation may be provided between the first and second tube-sheets and/or the first and second manifolds and/or the two parts of the outer case. The thermal insulation helps to minimise heat transfer between the separated parts, while still ensuring a seal therebetween.

The thermal insulation may be provided as a single frame-shaped piece for ease of assembly/manufacture, and/or may be flexible so as allowing relative movement of the parts separated by the insulation layer to allow each part of the exchanger to undergo thermal deformation separately.

A further aspect of the invention provides a heat exchanger, comprising a first part having a hot side inlet and a second part having a hot side outlet, the first part being positioned adjacent the second part and the hot side outlet being positioned adjacent the hot side inlet. One or more portions of the first part and second part may be provided as separate components connected together. The first part and the second part may include a plurality of tubes arranged to connect the hot side inlet and the hot side outlet such that, in use, a primary flow can pass from the hot side inlet to the hot side outlet. The heat exchanger may be configured to permit relative movement of the first part to the second part so as to account for a difference in thermal deformation between the first part and the second part. The heat exchanger may comprise an insulator, e.g. a flexible insulator provided between the first part and the second part of the heat exchanger.

The further aspect may have one or more features of the first aspect.

The disclosure also provides a method of forming a heat exchanger according to the appended claim 12. Further optional features of the method are recited in the associated dependent claims.

The method of the disclosure allows the heat exchanger to be manufactured in a relatively simple manner. Due to an inherent flexibility in the plurality of tubes, for example a bundle of U-tubes, the two halves of the heat exchanger can be almost fully assembled before a layer of insulation is sandwiched between the two sides.

Assembly is further simplified if the layer of thermal insulation is provided as a single frame-shaped piece Each respective half of the manifold and outer case may be welded or brazed together, and the two halves of the heat exchanger may be bolted or clamped together, for example at a seam formed by flanges on each half of the case.

Alternative methods of joining the various components could also be used.

In the present application the fluid is described as being a hot fluid and a cold fluid and the heat exchanger is said to have hot features and cold features. It will be understood by the person skilled in the art that the reference to hot and cold is referring to the relative temperatures of the hot and cold fluid or the hot and cold features of the heat exchanger. The hot fluid or features may be referred to as the primary fluid or features and the cold fluid or features may be referred to as the secondary fluid or features.

Wherever practicable, any of the features defined in relation to any one aspect of the invention may be applied to any further aspect. Accordingly the disclosure may comprise various alternative configurations of the features defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Practicable embodiments of the invention are described in further detail below by way of example only with reference to the accompanying drawings, of which: Fig. I shows a general prior art U-tube heat exchanger; Fig. 1A shows a half model view of the prior art heat exchanger, taken at the line A-A in Fig. 1; Fig. 2 shows a separated U-tube heat exchanger; Fig. 3 shows a half model view of the separated U-tube heat exchanger of Fig. 2; Fig. 4 shows a cross-sectional view of the separated U-tube heat exchanger of Fig. 2; Fig. 5 shows the deformed shape at tube-sheet of the prior art U-tube heat exchanger of Fig. 1; Fig. 6 shows the deformed shape at tube-sheet of the separated U-tube heat exchanger of Fig. 2; Fig. 7 is a temperature graph in cold flow direction at tube-sheet for the

prior art U-tube heat exchanger of Fig. 1;

Fig. S is a temperature graph in cold flow direction at tube-sheet for the separated U-tube heat exchanger of Fig. 2; and Fig. 9 shows a comparison of stress between the prior art heat exchanger model of Fig. 1 and the separated model of Fig. 2.

DETAILED DESCRIPTION

Figure 1 shows the overall form of a standard U-tube heat exchanger 10. Heat exchangers of this type are conventionally made from metals such as steel, stainless steel, nickel alloys, titanium alloys, aluminium alloys, copper alloys and ceramic etc. but could be made from nickel alloys.

In brief, the heat exchanger 10 comprises a manifold 1, which provides the inlet and outlet for a primary fluid, an outer case 3 and a bundle of tubes 5 through which the primary fluid flows in use. A secondary fluid passes through openings in the sides of the heat exchanger 10 in the direction indicated by arrows 9, past the tube bundle 5. The openings in either side of the heat exchanger 10 are surrounded by flanges 4 allowing connection to a secondary fluid source.

In the illustrated example, the primary fluid is a high temperature liquid and the secondary fluid is a low temperature gas. As the secondary fluid passes the tube bundle 5, heat from the primary fluid flowing in the tube bundle 5 is transferred to the secondary fluid, raising the temperature of the secondary fluid and reducing the temperature of the primary fluid.

Figure 1 A is a cross-sectional view of the heat exchanger 10 of Figure 1, taken at the line A-A. The cross-sectional view of Figure 1A shows more detail of the constituent parts of the heat exchanger 10. The manifold 1 provides a hot side inlet 7 and a hot side outlet S for the primary fluid. The hot side inlet 7 and hot side outlet S are separated within the manifold, but are connected by the tube bundle 5. It can be seen from Figure 1A that ends of the tubes within the tube bundle 5 extend through a tube-sheet 2 at either side of the heat exchanger 10 into either the hot side inlet 7 or the hot side outlet 8.

The manifold 1 is welded directly onto the tube-sheet 2, which is a single piece of material that extends across the heat exchanger 10 sealing both primary (high temperature) inlet and outlet flows 7,8 from the secondary flow across the tube bundle 5. The flange 4 is also welded to both the tube-sheet 2 and the outer case 3, thereby firmly connecting the tube-sheet 2 to the outer case 3.

The U shape of the tubes making up the tube bundle 5 is clearly shown in Figure 1A. Also shown are support plates 6, towards the middle and upper end of the tube bundle 5. The support plates 6 together with the tube-sheet 2, reduce vibration of the tubes during use.

Under normal operating conditions, the heat exchanger 10 will experience different temperature distributions across the tube-sheet 5, manifold 1, flange 4, and case 3 as a result of heat transfer from the primary and secondary fluids.

To observe the temperature distribution and thermal stress of the U-tube heat exchanger 10 of Figure 1, a thermo-mechanical finite-element method was used.

Unsurprisingly, the contrast in temperatures was most pronounced on the tube-sheet 2. The part of the tube-sheet bordering the hot side inlet 7, where the primary fluid is at its hottest, experienced a relatively high temperature, whereas the part of the tube-sheet at the hot side outlet 8, where the primary fluid is at its coolest, showed a much lower temperature.

Since a large temperature difference exists between the two sides of the single unitary tube-sheet 2, the two sides want to expand to different degrees. This causes the tube-sheet 2 to deform by bending as the side that borders the hot side inlet 7 expands more than the side that borders the hot side outletS. This creates an undesirable compressive stress on one side of the tube-sheet 2 and an undesirable tensile force on the other.

Figure 2 shows a heat exchanger indicated generally at 20. As in the prior art heat exchanger 10 of Figure 1, the heat exchanger 20 of Figure 2 comprises a manifold 11, an outer case 13, a bundle 15 of U-shaped tubes and a flange 14 for attachment to a secondary fluid source. The key difference between the heat exchanger 20 and the prior ad heat exchanger 10 relates to a seam 22 visible in the centre of the outer case 13.

Figure 3 shows the cross sectional structural model (including internal structure) of the attachable U-tube heat exchanger, similar to the view of the prior art exchanger in Figure 1A. It can be seen from Figure 3 that the seam 22 divides each of the outer case 13, the tube-sheet 12 and the manifold 11 into two halves.

The hot side inlet 18 and hot side outlet 19 of the heat exchanger of Figure 3 no longer border a single unitary tube-sheet 2 as in the prior art. This helps to avoid the conflicting degrees of expansion and the associated bending stresses discussed above.

In addition, a layer of insulation 17 is installed between the hot side inlet 18 and hot side outlet 19. This layer further reduces the heat transfer between the two parts of the tube-sheet 12. The split line within the seam 22 and the inherent flexibility of the insulation layer 17 allows each part to move independently of the other, even under thermal deformation, which reduces stress by either sliding or deformation.

Figure 4 is a side view of the cross section shown in Figure 3, showing the seam 22 and insulation layer 17 more clearly. The insulation 17 fills the gap between the two halves of the casing 13 and tube-sheet 12. The tube support plates 16 within the heat exchanger 20 may also be split, along the line of the seam 22, but the gap left between their halves is not typically filled with insulation.

The described embodiment therefore effectively provides a heat exchanger 20 comprising two halves, each with a manifold 11, tube-sheet 12, outer case 13, flange 14, and one or more tube support plates 16, joined together by a tube bundle 15.

In assembly, each respective half of the manifold 11 and outer case 13 are welded or brazed together to form a single piece, and the two halves of the heat exchanger are connected by the bundle 15 of U-tubes. The insulation layer 17, which may be provided as a single frame-shaped piece, is then assembled between the two halves of the heat exchanger 20, and the halves are bolted or clamped together at the seam 22 to provide a seal. The tubes in the bundle 15 have a degree of inherent flexibility which is sufficient to allow this assembly operation without causing damage to the tube bundle 15.

The purpose of the described heat exchanger is to reduce thermal stress. Unlike the traditional U-tube heat exchanger, the heat exchanger 20 described above is divided in half so that the hot side outlet and inlets are separated. The result of this is that, while the temperature difference between the two sides of the tube-sheet 12 remains relatively high, the separation of this component into two halves means that the thermal stresses generated by this temperature difference in the

prior art devices are largely avoided.

Figure 5 shows the deformed shape at the tube-sheet 2 of a general prior art U-tube heat exchanger. The bending B discussed in relation to Figures 1 and 1A is clearly illustrated, as each side of the tube-sheet 2 is influenced by the other.

By way of comparison, Figure 6 shows the results of the same analysis of the described embodiment. The separation of the two halves of the model by insulation allows sliding X between the parts, allowing thermal deformation of each part of the tube-sheet 12 to occur without affecting the other.

In Figures 5 and 6 shading is used to illustrate stress in the heat exchangers. The same scale is used in Figures 5 and 6, and the arrow S indicates increasing stress.

The influence of the each half of a unitary tube-sheet 2 on the other is well illustrated by the graph of Figure 7. Figure 7 is a graph that represents the temperature (the arrow T indicates a scale of increasing temperature, and the scale in Figure 7 is the same as the scale in Figure 8) difference of the direction of the low temperature fluid flow in the single unit tube-sheet 2 of the prior art. The temperature difference experienced by the tube-sheet 2 across the hot side outlet 8 (shown at "0" in Figure 7) is marked as dT-1, while dT-2 is the difference at the hot side inlet 7 (shown at "I" in Figure 7). Both temperature differences can be seen to be quite large Figure 8 is a graph showing the temperature difference caused by the direction of the low temperature fluid flow through the separable tube-sheet 12 of the heat exchanger 20. The first point to note is that, because the two halves of the tube-sheet 12 have been separated, there is a discontinuity at the interface between the hot side inlet and hot side outlet. The lack of heat transfer between the two halves of the heat exchanger 20, as a result of the insulation layer 17, means that the minimum temperature for the hot side (the hot side inlet) is far less influenced by the cooler side of the heat exchanger 20 (the hot side outlet). As a result, the temperature differential for the hot side inlet tube-sheet (dT-4) can be seen to be far lower than that of the equivalent standard model's differential (dT-2), even though the maximum temperature is similar in each case. Similarly, the maximum temperature of the cooler side of the heat exchanger 20 (the hot side outlet) is less influenced by the hotter side (the hot side inlet), so a smaller temperature differential is also seen across the hot side outlet tube-sheet.

As can be seen above, the separation of the two halves of the tube-sheets 12 not only allows relative expansion of the two halve to avoid the bending stresses of the prior art, but also minimises the temperature differences experienced within each half of the tube-sheet 12 to further reduce the thermal stresses within each part Figure 9 further illustrates that the separable' heat exchanger provides a suitable means of reduce thermal stress. The graph of Figure 9 shows a comparison of the maximum von-Mises stress across each part of the standard and separable heat exchangers (arrow S indicates increasing von-Mises stress), including the tube-sheet 2,12 (indicated with a Tin Figure 9); the case 3,13 (indicated with a C in Figure 9); and the flange 4,14 (indicated with a F in Figure 9). From the analysis results, it can be seen the stress levels on the separable model (A) are comparatively lower than those of the standard model of the prior art (P).

The separable U-tube type heat exchanger 20 demonstrates around 20% reduced thermal stress in the flange parts compared to existing single unit type heat exchangers, representing a significant potential reduction in damage due to thermal stresses. The reduction of the thermal stress is ascribed to the separation of the high temperature and low temperature regions such that when thermal expansion does occur, the deformations do not couple. This results in a longer life for the heat exchanger unit through reduction or elimination of cracking and leakage at the high stress locations.

Claims

CLAIMS: 1. A heat exchanger, comprising a hot side inlet and a hot side outlet adjacent the hot side inlet, a plurality of tubes connecting the hot side inlet and the hot side outlet such that, in use, a primary flow can pass from the hot side inlet to the hot side outlet, wherein said plurality of tubes passes through a secondary flow, and wherein a first tube-sheet is provided for sealing the hot side inlet from the secondary flow, and a second, separate, tube-sheet is provided for sealing the hot side outlet from the secondary flow.
2. A heat exchanger according to claim 1, wherein the plurality of tubes comprises a bundle of U-shaped tubes.
3. A heat exchanger according to claim 1 or 2, wherein thermal insulation is provided between the first and second tube-sheets.
4. A heat exchanger according to any of the preceding claims, further comprising a first manifold provided at the hot side inlet and a second, separate, manifold provided at the hot side outlet.
5. A heat exchanger according to claim 4, wherein thermal insulation is provided between the first and second manifolds.
6. A heat exchanger according to any of the preceding claims, wherein the heat exchanger further comprises an outer case, and wherein the outer case is provided in two parts, each part being connected to either the first or second tube-sheet.
7. A heat exchanger according to claim 6, further comprising one or more support plates, connected to the outer case, for supporting the plurality of tubes.
8. A heat exchanger according to claim 7, wherein at least one support plate is provided in two parts, each being joined to one part of the outer case.
9. A heat exchanger according to any of claims 6 to 8, wherein thermal insulation is provided between the two pads of the outer case.
10. A heat exchanger according to any of claims 3, 5, or 9, wherein the thermal insulation is provided as a single frame-shaped piece.
11. A heat exchanger according to any of claims 3, 5, 9 or 10, wherein the insulation is flexible.
12. A method of forming a heat exchanger comprising the steps of a. joining a first half of a manifold to a first half of an outer case to form a first half of the heat exchanger; b. joining a second half of a manifold to a second half of an outer case to form a second half of the heat exchanger; c. connecting the first and second halves of the heat exchanger via a plurality of tubes; d. fixing the first and second halves of the heat exchanger together; and e. inserling a layer of thermal insulation between the first and second halves of the heat exchanger after step c and before step d.
13. A method according to claim 12, wherein the layer of thermal insulation is provided as a single frame-shaped piece
14. A method according to claim 12 or 13, wherein each respective half of the manifold and outer case 13 are welded together.
15. A method according to claim 12 or 13, wherein each respective half of the manifold and outer case 13 are brazed together.
16. A method according to any of claims 12 to 15, wherein the two halves of the heat exchanger are connected by a bundle of U-tubes.
17. A method according to any of claims 12 to 16, wherein the two halves of the heat exchanger are bolted together.
18. A method according to any of claims 12 to 16, wherein the two halves of the heat exchanger are clamped together.
19. A heat exchanger substantially as hereinbefore described with reference to the accompanying drawings.