WO2008095248A1

WO2008095248A1 - Heat exchanger

Info

Publication number: WO2008095248A1
Application number: PCT/AU2008/000148
Authority: WO
Inventors: Dirk Jacob De Boer
Original assignee: Hatch & Associates
Priority date: 2007-02-06
Filing date: 2008-02-06
Publication date: 2008-08-14

Abstract

A heat exchanger (11) in the form of a barrel heater comprising a cylindrical shell (13) containing at least one tube (19) located therein and extending therealong. The cylindrical shell (13) and the at least one tube (19) are secured to flanged end plates (31) located at each end of said heat exchanger (11). The flanged end plates (31) are each arranged to connect with a flanged connector (33) for fluid communication with said at least one tube (19). The shell (13) is provided with at least two external restraints (37) spaced apart from each other, for restraining the heat exchanger (11) on a support frame (39). The heat exchanger (11) is also provided with a plurality of internal restraints (21), spaced apart from each other, to locate and restrain the at least one tube 19 relative to the shell (13).

Description

"Heat Exchanger"

Field of the Invention

This invention relates to a heat exchanger, and in particular to a type of heat exchanger known in the chemical process technology industry as a shell and tube heater.

Background Art

Various types of heat exchanger have been described in the literature, and practiced in industry. A common problem is one caused by differential thermal expansion due to differing temperatures of the fluids in contact with different parts of the heat exchanger. These problems can be exacerbated in instances of mal-operation, for example when hot vapour or steam is inadvertently applied to one fluid pathway where the other fluid pathway is cold, resulting in greater than normal expansion of the heat exchanger componentry in contact with the hot vapour or steam, than that of the heat exchanger componentry in contact solely with cold fluid. Similarly these problems can be exacerbated in the converse situation where hot slurry is introduced into the tubes of a cold heat exchanger. In such situations the differential expansion and the corresponding reactive stresses, will be many times those that occur during normal operating conditions.

In practice such mal-operation is avoided by strict start-up and shut-down procedures. However, such procedures can be violated and should such unusual operating conditions inadvertently occur, the equipment can literally self destruct, with possible serious consequences to personnel and certain extended interruption to plant production.

While attempts may be made to protect from these extreme conditions with instrumentation, for example, by sensing differential temperatures and on the sensing of design parameters being exceeded, initiating automatic shut-down of the source of heat. Such instrumentation can only be reactive and the time interval involved in the chain of events (hysteresis) is simply too long to safeguard the equipment. There is also the issue of maintenance of such control instrumentation. Given that such control instrumentation is intended for extreme events that may not ordinarily occur, the normal operation of plant will not be impacted by the failure of such control instrumentation. It is entirely possible that such control instrumentation has not been properly maintained and thus may not be serviceable when finally called upon, years into the operational life of a plant.

Practical solutions to the internal expansion problem are corrugated shell expansion joints, and 'U' tube construction, both of which have serious disadvantages. The use of expansion joints in the mineral processing industry is problematic. The thin plate alloys employed in the construction of the corrugations are often subject to chemical attack. Also, there is a high probability of scale build-up inside the corrugations, which then render the expansion joint ineffective.

In 'U' tube construction, both the shell and the tubes contained therein are formed in a U shape. In 'U' tube construction the tubes are fixed at only one end of the heater and move independently of the shell. Such 'U' tube construction heat exchanger equipment is very cumbersome, difficult to construct and transport, difficult to repair in situ, and almost impossible to replace once installed in multiple heater arrays.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia or elsewhere as at the priority date of the application.

It is an object of this invention to provide a heat exchanger which overcomes some of the aforementioned difficulties, or at least provide an alternative. It is a preferred object of the present invention to provide a barrel heater design that could be used over a wider temperature range while preserving simplicity and cost-effectiveness.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

In accordance with the invention there is provided a heat exchanger comprising a cylindrical shell containing at least one tube located therein and extending therealong, said cylindrical shell and said at least one tube being secured to end plates located at each end of said heat exchanger, said end plates enclosing a first volume within said shell and external to said at least one tube, said end plates each being arranged to connect with a flanged connector for fluid communication with said at least one tube, said shell being provided with at least two external restraints, spaced apart, for restraining said heat exchanger on a support frame, said heat exchanger being provided with a plurality of internal restraints, spaced apart, to locate and restrain said at least one tube relative to said shell.

It should be noted that the end plates can be flanged, but it is not necessary that the end plate extend beyond the peripheral circumference of the cylindrical shell, since the flanged connector need only have flanges which extend around tube(s) for fluid communication with said at least one tube, and the flange of the flanged connector can receive bolts that secure into the end plate. However, for practical purposes, it is convenient to have the end plates arranged with flanges extending beyond the peripheral circumference of the cylindrical shell.

Preferably the spacing of internal restraints and external restraints is determined to increase buckling strength in shell and tube materials to minimise or prevent lateral deflection. Particularly, said internal and external restraints are spaced sufficiently closely to increase the buckling strength of the shell and tubes against the compressive forces induced by temperature changes and thereby minimise or prevent lateral deflection of said shell and said at least one tube.

Preferably the spatial arrangement of said shell and said at least one tube exhibits rotational symmetry. In this manner, the heat exchanger may be rotated relative to the flanged connector, so that the effect of potential wear through erosion by slurry within the tubes can be mitigated. Rotation of the heat exchanger extends the life of the heat exchanger in such circumstances, where otherwise the slurry would cause disproportionate wear on the bottom of said at least one tube.

Preferably said shell contains a plurality of tubes.

Preferably said plurality of tubes comprises from 2 to 10 tubes. Arrangements may comprise three tubes in a triangular configuration, five six or eight tubes around a central tube, and other geometrical configurations.

Preferably said plurality of tubes comprises 4 tubes.

Preferably the combined circumference of said at least one tube is at least 0.3x the circumference of the shell.

Preferably the combined circumference of said at least one tube is at least 0.5x the circumference of the shell.

Preferably the combined circumference of said at least one tube is at least 0.66x the circumference of the shell.

Preferably the combined circumference of said plurality of tubes is up to 3x the circumference of the shell.

Preferably the combined circumference of said plurality of tubes is about 2x the circumference of the shell. The diameter of the tubes may ideally be from 80mm to 200mm, although 150mm is preferred for four tubes contained within a 500mm diameter shell.

Preferably the spacing between said external restraints is from 4.5m.

Preferably the maximum spacing between said external restraints is 10m.

The most preferred spacing between said external restraints is about 9m.

Preferably the spacing between said internal restraints is from 1.5m.

Preferably the maximum spacing between said internal restraints is 5m.

The most preferred spacing between said internal restraints is about 3m.

The internal restraints are preferably constructed so that they do not inhibit longitudinal movement of said at least one tube. Preferably the internal restraints restrain lateral movements, horizontally and vertically, in said at least one tube.

The tubes are preferably a sliding fit in said internal restraint. The securing of said at least one tube to said internal restraint may act as pin joints. The material of the internal restraint is preferably selected to be compatible with the fluid contained in the first volume (ie volume within the shell).

Preferably the internal restraints surround each said at least one tube.

Preferably the internal restraints include apertures for the conducting of fluid within said first volume.

Preferably the internal restraints contact said shell in regions proximal to some of said at least one tube.

Preferably the internal restraints are secured to said shell at said regions.

The external restraints are preferably constructed so that they do not inhibit longitudinal movement of said shell. Most preferably the external restraints each are full circumferential structures that positively restrain the shell in all directions, except longitudinally. In addition, preferably the external restraints allow angular rotation and preferably do not resist bending moments.

Preferably the external restraints restrain lateral movements, horizontally and vertically (assuming the heat exchanger is deployed in a horizontal position), in said shell. The material of the external restraint may be standard structural steel.

Preferably the external restraints contact the shell at spaced radial positions around the shell. These positions may be conveniently located 90° apart, although other angles such as 45°, 60° or, 120° may also prove suitable

The end plates are preferably formed of material that is compatible with the fluid contained in the first volume. Where this is standard structural steel, preferably the end plates are approximately about 1.5 times the thickness of a standard flange.

Preferably the thickness of the end plates is sufficient to prevent plate deformation and maintain the required profile for gasket sealing.

Preferably the welds onto the plate securing said at least one tube and said shell have sufficient strength to withstand approximately about 1.33 times the allowable tensile stresses of all tube and shell materials.

The shell and tube materials are preferably selected so that the elongation or compression through differential expansion results in tensile or compressive forces in said shell or said at least one tube remaining within the allowable tensile stress of all tube and shell materials.

Preferably under conditions of varying expected temperature differentials, the change of length through compression or tension in the shell is substantially balanced by change of length through tension or compression respectively in said at least one tube, to the extent that tensile and compressive forces in said shell or said at least one tube remain within the allowable tensile stress of all shell and tube materials.

The use of the heat exchanger according to the invention significantly increases the temperature differential that can be tolerated during emergency or mal- operation, without damage to the heat exchanger.

Brief Description of the Drawings

A preferred embodiment of the invention will now be described in the following description made with reference to the drawings in which:

Figure 1 is a plan view of two heat exchangers according to the embodiment;

Figure 2 is a side elevation of the heat exchangers of figurei ;

Figure 3 is a cross section view through Section A-A of figure 2;

Figure 4 is a view of an internal restraint of the heat exchanger; and

Figure 5 is a view of an external restraint of the heat exchanger.

Best Mode(s) for Carrying Out the Invention

Referring to figure 1 two heat exchangers in the form of barrel heaters 11 are illustrated. The barrel heaters have an external cylindrical shell 13 formed of 500 mm diameter (OD), 3/8 inch (9.53mm) wall thickness mild steel, and having an expanded diameter portion 15 to which is fitted a flanged vapour inlet 17. The external shell houses at least one tube in the form of four 150 mm diameter (OD) pipes 19 which are formed of 5/16 inch (8mm) wall thickness mild steel pipe. The pipes 19 extend for the length of the shell 13. Note that shell 13 is shown cutaway for clarity of the view, in the region where the pipes 19 are shown in figures 1 and 2.

Spaced every 3 m along the length of the pipes 19, commencing 3 m from the end are internal restraints 21 to which the pipes 19 are held through apertures 23 by sliding fit, acting as pin joints therewith. The internal restraints 21 are configured so that they resist imposed lateral forces that might be imposed by differential expansion in the shell 13 and the pipes 19.

The internal restraints 21 , seen better in figures 4 and 5, are provided with arcuate regions 25 proximal to the pipes 19. The arcuate regions 25 may be welded to the internal wall of the shell 13. The remainder of the internal restraint 21 is cut away at peripheral portions 27 and centrally 29, to allow flow of heating (or cooling) fluid within the shell 13 and around the pipes 19, along the length of the shell 13 from the vapour inlet 17 to flanged outlets 30 to act as a condensate outlet 30a at the bottom and a non-condensable vent 30b at the top. The internal restraints 21 are made of 19mm thickness steel plate, and the cutting away is performed in a manner so as not to interfere with the mechanical integrity of the internal supports 21.

Referring back to figures 1 and 2, the shell 13 and the pipes 19 terminate at an end plate 31 which is flanged, to which they are welded. All welding is performed to a standard so that there is sufficient strength to withstand 1.33x the allowable tensile strength of all tube and shell materials.

The flanged end plate 31 is shown in figures 1 and 2 connected to a flanged connector 33 carrying four U-tubes 35 connected to a further like flanged connector 33 for fluidly connecting the pipes 19 to the pipes 19 of the next barrel heater 11 in the sequence.

The barrel heaters 11 have their shell 13 supported on external restraints 37, which are spaced nine metres apart, and secured to a support frame 39. Referring to figure 5, the external restraints 37 are formed of right angle section 38 mild steel, butt welded together and reinforced with reinforcing gussets 41 welded thereto. The external restraints 37 form a closed frame structure through which the shell 13 passes, and are constructed so that they contact the shell 13 in four positions, at the top, bottom, and both sides. At ambient temperature the external restrains merely contact the shell at these positions, but as the shell expands due to heat, it will be subject to lateral forces. The remaining detail shown in figure 3 is the provision of an impact plate 43, over the pipes 19 proximal to the flanged vapour inlet 17. The impact plate 43 protects the pipes 19 from external erosion by the heating medium introduced to the barrel heater 11 via the flanged vapour inlet 17.

The specific advantage of the barrel heater according to the invention is its capability of absorbing a large difference in potential thermal expansion between heater shell and heater tubes, without special provisions, such as expansion joints or complex ¹U¹ tube construction.

In the barrel heater according to the invention, these potentially large thermal movements are constrained by balancing the potential elongation of one element (e.g. the shell) against compression in the opposite element (e.g. the tubes). Either element can be in either state, depending on whether the tubes are cold and the shell hot, or vice-versa.

This capability of the barrel heater according to the invention is particularly important in cases of mal-operation. While the utility of the barrel heater according to the invention is not limited to such instances, its development was prompted to cater for two extremes of heater operation; namely, when hot vapour or steam is inadvertently applied to a heater shell, while the tubes are cold, and the obverse condition when hot fluid is driven through the tubes, while the shell is cold. In both these cases the potential differential expansion and the corresponding reactive stresses, will be many times those that occur during normal operating conditions.

The following table illustrates maximum temperature differentials that can be tolerated for differing constructions of a typical barrel heater according to the invention, and contrasts the barrel heater according to the invention with a shell and tube heater having no internal supports and external restraints. In this table the following heat exchanger constructions are referred to: Case A shows the very low temperature differentials that can be tolerated when there are no internal or external restraints, and standard grade B pipe materials are used, with the selection based on pressure requirements only.

Cases B, C, D, E₁ F, G₁ H₁ and I show how greater temperature differentials can be tolerated when internal and external restraints are used, and by variations in wall thickness and the selection of different materials. These cases also illustrate that it is important that both corroded and uncorroded conditions are considered.

Case B represents standard grade B pipe materials, in corroded condition, with the selection based on pressure requirements only.

Case C represents standard grade B pipe materials, in uncorroded condition, with the selection based on pressure requirements only.

Case D represents better grade C tube materials, in corroded condition, with the selection of materials intended to improve temperature differential.

Case E represents better grade C tube materials, in uncorroded condition, with the selection of materials intended to improve temperature differential.

Case F represents heavier, but still standard, grade C tube materials, in corroded condition, with the selection of materials intended to improve temperature differential.

Case G represents heavier, but still standard grade C tube materials, in uncorroded condition, with the selection of materials intended to improve temperature differential.

Case H represents heavier, but still standard grade C tube materials, combined with medium grade alloy shell, with corroded tubes (shell does not corrode).

Case I represents heavier, but still standard grade C tube materials, combined with medium grade alloy shell, with uncorroded tubes (shell does not corrode). CO

C DD CO

m

CO

I m m

73

C m

IO

Note: "*" tubes only

The above table also provides a useful comparison of corroded tubes with full mill tolerance on the one hand with uncorroded tubes without mill tolerance on the other.

The barrel heater according to the invention represents a departure from hitherto known heater design. The structural criteria applied in this invention are not normally a consideration in heat exchanger design. In the design of the barrel heater of the invention, the shell and tubes form columnar structures, and are alternatively treated as columns in compression.

The slenderness ratio (l/r) of both the tubes and the shell and how these relate to one another is the most critical factor in barrel heater design. The design philosophy in the invention is to space internal and external restraints at designated intervals (I) in a manner that increases the buckling strength of the shell and tubes against the compressive forces induced by differential temperature changes. Any lateral deflection in the shell and tubes at these intervals will be totally constrained.

Slenderness ratios may change over the life of the heater, resulting from changes to the radius of gyration (r), which will occur when either or both the shell or tubes corrode or erode. It should be noted that corrosion of the shell and tubes does not progress at the same pace. In addition, the mill tolerance is a code allowance, permitting the material manufacturer to deviate up to 12.5% from the standard (nominal) material thickness. The material may therefore be supplied with a thickness anywhere within that tolerance

A number of permutations with different mill tolerances and rates of shell and tube corrosion and their resulting slenderness ratios must be considered. This is complicated by the fact that in this design the shell and tubes are totally interactive and must be considered as a single entity. A change in either shell or the tubes will affect both. In addition the two possible modes of malfunction (overheated shell with cool tubes, versus a cool shell with overheated tubes) decrees that the elements that are under compression in the first mode of malfunction, will be in tension in the second malfunction. The interactive nature of the heater elements (shell and tubes) is a principle feature of the design. It requires a fine balance between the wall thickness of the shell and tubes respectively. Any over-design in one element (say the shell) in one direction of force may undermine the strength of the other element (say the tubes) when the forces act in reverse, during a different malfunction.

While pressure design criteria are met, they are a subsidiary consideration in the design approach. Safety factors are a unique combination of structural columnar criteria and pressure design allowances.

In simple terms, the barrel heater of the invention subdivides a long heater which would otherwise have insufficient buckling strength, into a series of short segments that all have sufficient buckling strength. It does this by utilisation of internal and external restraints as described, so that its continuity as a single heater is maintained.

The engineering design approach is to consider the shell and tubes as a single interactive entity. This requires special attention to the manner the two elements (shell and tubes) are connected through the tube plates.

In order to maximize a barrel heater's capacity to withstand high differential temperatures, materials of construction and material thicknesses are selected to achieve a balance of forces between shell and tubes that is not normally required in heater design.

The barrel heater according to the invention avoids the need for use of complex duplex U-tube heater head construction or thermal expansion fittings to deal with forces brought about by differential expansion.

The barrel heater according to the invention also allows for rotation of the heater body about its longitudinal axis. In slurry service the bottom of a heater tube tends to wear preferentially. The ability to rotate a heater around its axis, so that the bottom of the tubes is exchanged with the top of the tubes, can add considerably to heater service life. When a heater is rotated the vapour inlet 17 will be rotated to the other side of the heater. This will obviously entail a change in the connecting vapour piping (not shown in these drawings). While pressure pipe work is never cheap, this would only be a very minor cost compared with the cost of replacing a heater. The inlet vapour pipe work would have been arranged and flanged to accommodate a heater rotation with a simple change-over spool. When the time for rotation arrives, the change-over components can be fabricated in advance. The flanged outlets 30 forming the condensate outlet 30a and the non- condensable vent 30b, being located opposite each other allow for connection to existing piping, without any pipe work changes being required.

It should be appreciated that changes may be made to various features in the above described embodiment without departing from the spirit and scope of the invention, and that the invention is not limited to the specific embodiment described herein.

Claims

The Claims Defining the Invention are as Follows:

1. A heat exchanger comprising a cylindrical shell containing at least one tube located therein and extending therealong, said cylindrical shell and said at least one tube being secured to end plates located at each end of said heat exchanger, said end plates enclosing a first volume within said shell and external to said at least one tube, said end plates each being arranged to connect with a flanged connector for fluid communication with said at least one tube, said shell being provided with at least two external restraints, spaced apart, for restraining said heat exchanger on a support frame, said heat exchanger being provided with a plurality of internal restraints, spaced apart, to locate and restrain said at least one tube relative to said shell.

2. A heat exchanger as claimed in claim 1 wherein the spatial arrangement of said shell and said at least one tube exhibits rotational symmetry.

3. A heat exchanger as claimed in claim 1 or 2 wherein said at least one tube comprises from 2 to 10 tubes.

4. A heat exchanger as claimed in claim 3 wherein said at least one tube comprises 4 tubes.

5. A heat exchanger as claimed in claim 1 or 2 wherein the combined circumference of said at least one tube is at least 0.3x the circumference of the shell.

6. A heat exchanger as claimed in claim 5 wherein the combined circumference of said at least one tube is at least 0.66x the circumference of the shell.

7. A heat exchanger as claimed in claim 3 or 4 wherein the combined circumference of said at least one tube is up to 3x the circumference of the shell.

8. A heat exchanger as claimed in claim 7 wherein the combined circumference of said at least one tube is about 2x the circumference of the shell.

9. A heat exchanger as claimed in any one of the preceding claims wherein the spacing between said external restraints is from 4.5m.

10. A heat exchanger as claimed in any one of the preceding claims wherein the maximum spacing between said external restraints is 10m.

11. A heat exchanger as claimed in any one of claims 1 to 8 wherein the spacing between said external restraints is about 9m.

12. A heat exchanger as claimed in any one of the preceding claims wherein the spacing between said internal restraints is from 1.5m.

13. A heat exchanger as claimed in any one of the preceding claims wherein the maximum spacing between said internal restraints is 5m.

14. A heat exchanger as claimed in any one of claims 1 to 11 wherein the spacing between said internal restraints is about 3m.

15. A heat exchanger as claimed in any one of the preceding claims wherein the internal restraints allow longitudinal sliding movement of said at least one tube, and restrain lateral movement, horizontally and vertically, in said at least one tube.

16. A heat exchanger as claimed in any one of the preceding claims wherein the internal restraints surround each said at least one tube, and include apertures for the conducting of fluid within said first volume.

17. A heat exchanger as claimed in any one of the preceding claims wherein the internal restraints contact said shell in regions proximal to some of said at least one tube.

18. A heat exchanger as claimed in claim 17 wherein the internal restraints are secured to said shell at said regions.

19. A heat exchanger as claimed in any one of the preceding claims wherein the external restraints are constructed so that they do not inhibit longitudinal movement of said shell.

20. A heat exchanger as claimed in claim 19 wherein said external restraints each are full circumferential structures that positively restrain the shell in all directions, except longitudinally.

21. A heat exchanger as claimed in claim 19 or 20 wherein the external restraints are constructed so that they do not inhibit angular rotation of said shell.

22. A heat exchanger as claimed in any one of claims 19 to 21 wherein the external restraints are constructed so that they do not resist bending moments.

23. A heat exchanger as claimed in any one of the preceding claims wherein the external restraints are constructed so that they restrain lateral movement in said shell.

24. A heat exchanger as claimed in any one of claims 19 to 23 wherein the external restraints contact the shell at spaced radial positions around the shell.

25. A heat exchanger as claimed in any one of the preceding claims wherein the shell and tube materials are selected so that the elongation or compression through differential expansion results in tensile or compressive forces in said shell or said at least one tube remaining within the allowable tensile stress of all tube and shell materials.

26. A heat exchanger as claimed in any one of claims 1 to 24, wherein under conditions of varying expected temperature differentials, the change of length through compression or tension in the shell is substantially balanced by change of length through tension or compression respectively in said at least one tube, to thθ extent that tensile and compressive forces in said shell or said at least one tube remain within the allowable tensile stress of all shell and tube materials.

27. A heat exchanger as claimed in any one of the preceding claims wherein said internal and external restraints are spaced sufficiently closely to increase the buckling strength of said shell and said at least one tube against the compressive forces induced by temperature changes to minimise or prevent lateral deflection in said shell and said at least one tube.

28. A heat exchanger substantially as herein described, with reference to the drawings.