GB2131152A - A heat exchanger - Google Patents

Abstract

A shell and tube heat exchanger 10 is provided with corrugated surfaces 18 between tubes 14 extending through the heat exchanger 10 The corrugations are arranged so that a fluid flowing between the corrugated surfaces 18 is constrained to flow in continuous cross-flow about the tubes 14. <IMAGE>

Description

SPECIFICATION A heat exchanger This invention relates to a heat exchanger, and in particular to a shell and tube heat exchanger.

Shell and tube heat exchangers have been used for many years in the process industry for exchanging heat between fluid streams, and essentially comprise a relatively large cylindrical shell enclosing a bundle of tubes. One fluid stream flows through the tubes, and another fluid stream flows within the cylindrical shell and permeates the bundle of tubes.

It is common practice for the shell-side fluid to be forced to flow across the tube bundle in several passes by the use of baffles. While there are many baffle arrangements possible, the essential function of the baffles is to enhance the local heat transfer coefficient on the outside of the tubes by creating local turbulence during the flow of fluid across the tubes, using the minimum of pumping energy. In this way the heat transfer coefficient on the outside of each tube can be made to match the coefficient on the inside of the tube. Hence neither side of the tube acts as a controlling thermal resistance.

While the shell and tube heat exchangers can be regarded as the mainstay of the chemical process industry, are cheap, reliable and relatively easy to manufacture, they do have certain drawbacks. Since the energy utilised in pumping or comprssing fluids in process plant has come to be regarded as important, more and more attention is being paid to matching suitable pairs of streams of fluids for heat exchange. In some cases there will be no choice, since the chemical process may require a particular stream to be matched with another stream at a certain point in the process. However, it is becoming clear that the shell and tube heat exchanger has limitations in this direction in that there is a minimum temperature difference between streams that is essential for heat exchange.Although, in principle in an ideal heat exchanger, it is always possible to exchange heat between streams with small temperature differences by extending the length of the heat exchanger indefinitely, there is a limitation in that the cross flow of fluid, particularly on the shell side of a shell and tube heat exchanger demands higher temperature differences than are ideally necessary.

Thus, the shell and tube heat exchanger does not allow, by its very nature, fine control of heat transfer, and this is particularly obvious when temperature differences between streams become smaller and smaller in energy-saving schemes, and when true counterflow is necessary.

According to the present invention, in a heat exchanger comprising a shell in which are disposed a plurality of tubes for the flow therethrough of a first fluid stream, the tubes extending between opposite ends of the shell, and ports defined in the opposite ends of the shell to provide for passage of a second fluid stream through the shell, opposing corrugated surfaces are defined within the shell along at least part of at least some of the tubes so as to cause the second fluid stream to undergo to-and-fro cross-flow between the surfaces over the said tubes.

The tubes may be arranged in parallel arrays of coplanar tubes, with the corrugated surfaces defined by corrugated sheets or adjacent deflectors secured to the tubes. Alternatively, the tubes may be dis posed in concentric circular arrays thereof, with the corrugated surfaces defined by shaped tubes having circumferential corrugations therearound. The cor rugated sheets or corrugated tubes may span one or several said arrays of tubes, and if desired they may be shaped relative to each other so as to arrange multi-pass flow of the second fluid stream.

The corrugations may be selected from a wide variety of forms from long wave to angular or rectangular, and may change in form along the length thereof. The height of the corrugations may be selected to span one or several tubes.

Conveniently, the corrugated sheets or corrugated tubes may support the tubes of the heat exchanger and may be resiliently biased so as to spring into contact with the tubes, thereby to provide support for the tubes and to reduce vibration thereof.

The invention will now be further described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a perspective axial representation of part of a shell and tube heat exchanger; Figures la, 2a, 2b and 3 show modifications of the heat exchanger of Figure 1; Figure 1b shows a view in the direction of arrow 'A' of Figure 1a; Figure 4 shows a radial sectional representation of one form of shell and tube heat exchanger; Figure 5 shows a radial sectional representation of an alternative form of shell and tube heat exchanger to that of Figure 4; Figure 5a shows a fragmentary view on the line Va-Va of FigureS; Figure 5b shows a fragmentary partly broken away view in the direction of arrow 'B' of Figure 5;; Figure 6 shows a fragmentary side view of another modified form of the shell and tube heat exchanger of Figure 1, and Figure 6a shows a sectional view on the line Vla-Vla of Figure 6.

Referring now to Figure 1, a part of a shell and tube heat exchanger 10 is shown and comprises, a cylindrical shell 12, and a pluralityoftubes 14 extending in a plurality of horizontal rows 16 in a longitudinal direction inside the shell 12. A respective corrugated sheet 18 is disposed between pairs of adjacent rows 16 of tubes 14, and adjacent sheets 18 define between them a cross-flow channel 20.

In use of the heat exchanger 10, a first fluid flows in the tubes 14from right to left, and a second fluid flows in the channels 20 from left to right in countercurrent flow to the first fluid. Because of the effect of the corrugations of the sheets 18, the second fluid is constrained to flow in the channels 20 in continuous, to-and-fro, cross-flow about a row 16 ofthetubes 14.

The corrugated sheets 18 may be resilient and arranged so that they bear against the tubes 14, and thus provide support for the tubes 14 and reduce vibration thereof. In order to improve the firmness of the connection between the corrugated sheets 18 and the tubes 14, the sheets 18 as shown in Figures la and 1b may be deformed in the vicinity of each tube 14 to form a saddle 19 therefor. Although this does have the disadvantage of providing an extra thickness of material on the tubes 14, it enables heat to flow from the corrugated sheets 18 more easily into the tubes 14, thereby augmenting the heat transfer between the second fluid and the first fluid, (i.e. a secondary surface).

If desired as shown in Figures 2a and 2b, a corrugated sheet 18a may extend between alternate rows 16 or tubes 14, and be displaced apart by the spacing between adjacent rows 16 or between alternate rows 16 to define channels 20a, 20b respectively.

For some applications, a multi-pass flow of the second fluid stream may be required as shown in Figure 3. In Figure 3 one end of a shell and tube heat exchanger 30 is shown having a plurality of rows 32 of tubes 34 extending from a tube plate 36 having apertures 37 aligned with the respective tubes 34. A respective corrugated sheet 38 is disposed between adjacent tubes 34, but alternate sheets 38 are shortened and a deflector 40 is fitted midway between the end of the alternate sheets 38 and the tube plate 36 to reverse the flow of a fluid between the sheets 38 at that end of the heat exchanger 30. By connecting respective apertures 37 together (not shown), multi-pass flow can also be arranged for a fluid flowing through the tubes 34.

As shown in Figure 4, rows of tubers 44 in a cylindrical shell 46 may be arranged in a stacked horizontal configuration with corrugated sheets 48 therebetween, local segmental filler pieces 50 at the top and at the bottom of the shell 46 providing supportforthe tubes 44 and inhibiting vibration of the tubes 44. Alternatively, as shown in Figure 5, concentric circular arrays 52 of tubes 54 are disposed so as to extend longitudinally in a cylindrical shell 56, and have tubes 58 concentric with the arrays 52 and with circumferential corrugations 59 (see Figures 5a and 5b) to define channels 60 between the corrugated tubes 58.

It will be understood that the fluid flowing between the corrugated sheets 18, 18a, 38,48, or corrugated tubes 58 needs to be introduced therein in a uniform and controlled manner, for example from a manifold (not shown) or a plenum system (not shown) supplied from outside the shell. In this respect, the use of the well known "hair pin" arrangement of tubes has advantages since the manifold or plenum system can be located at that end of the shell free of end connections for the tubes.

Leakage from the sides of the channels between the corrugated sheets 18, 18a, 38,48 needs to be inhibited, for example by shaping the sides of the corrugated sheets 18, 1 8a, 38,48 so that they fit snugly inside the shell, or by the provision of sealing strips or plates fitted at the sides of the corrugated sheets 18, 18a, 38,48.

As an alternative to the corrugated surfaces being defined by corrugated sheets inserted between the tubes of the heat exchanger, the corrugated surfaces may be defined by curved deflectors secured to the tubes so as to simulate the effect of a corrugated sheet, as shown in Figures 6 and 6a to which reference is now made. In Figure 6 portions of horizontal arrays 70 of tubes 72 for a shell and tube heat exchanger are shown, in which a plurality of arcuate deflectors 74 secured transversely to respec tire tubes 72 are positioned with respect to each other such as to define to a substantial extent corrugated sheets extending between the tubes 72.

Narrow gaps 76 are defined between the ends of adjacent deflectors 74 of adjacent arrays 70, and narrow gaps 78 are defined between the sides of adjacent deflectors 74 in the same array 70. If desired, the ends and/or sides of adjacent deflectors 74 may overlap, touch, or otherwise engage, to close the gaps 76 and/or 78 and to provide mechanical support. It will be understood that deflectors 74 of alternative shape may be used so as to define corrugated sheets having selected corrugations, for example angular or rectangular.

Although the invention has been described in relation to counter-current flow of the fluids in the heat exchanger, the invention may also have applications where the flow of the fluids is co-directional.

It will be appreciated that the aforedescribed shell and tube heat exchangers maintain many of the characteristics of shell and tube heat exchangers having conventional baffles, but do allow closer control over heat transfer between the shell-side and the tube-side fluids. The shell-side fluid is constrained to flow in to-and-fro cross-flow close to one row or a few rows of adjacent tubes, and hence compared with known shell and tube heat exchangers, smaller temperature differences between the shell-side fluid and the tube-side fluid, and true counter-flow instead of cross-counter flow, are possible.

Although the concomitant pressure drop from the use of the corrugated sheets or tubes may be relatively high, selection of an appropriate corrugation may reduce the pressure drop to acceptable levels. Instead ofthe corrugated sheets extending in a horizontal direction, they may be disposed or defined at some alternative orientation, for example vertical. The shell may be non-cylindrical in form, and the corrugated sheets or tubes, may be formed from any suitable material depending on the environment inside the heat exchanger. Hence, for some applications plastics materials might be suitable, whilst for other applications metal may have to be used.

Claims (11)

1. A heat exchanger comprising, a shell in which are disposed a plurality of tubes for the flow therethrough of a first fluid stream, the tubes extending between opposite ends of the shell, ports defined in the opposite ends of the shell to provide for passage of a second fluid stream through the shell, and opposing corrugated surfaces defined within the shell along at least part of at least some of the tubes so as to cause the second fluid stream to undergo to-and-fro cross-flow between the surfaces over the said tubes.

2. A heat exchanger as claimed in Claim 1 wherein the tubes are arranged in parallel arrays of coplanar tubes, and the corrugated surfaces are defined by corrugated sheets.

3. A heat exchanger as claimed in Claim 1 wherein the tubes are arranged in concentric circular arrays of tubes, and the corrugated surfaces are defined by shaped tubes coaxial with the arrays and having circumferential corrugations therearound.

4. A heat exchanger as claimed in Claim 2 or Claim 3 wherein the corrugated sheets, or the corrugated tubes respectively, support the tubes of the heat exchanger.

5. A heat exchanger as claimed in Claim 4 wherein the corrugated sheets, or the corrugated tubes, are resiliently biased so ds to spring into contact with the tubes, thereby to provide support for the tubes.

6. A heat exchanger as claimed in Claim 1 wherein the corrugated surfaces are defined by adjacent deflectors secured to the tubes.

7. A heat exchanger as claimed in any one of Claims 2,3,4, Sot 6, wherein each corrugated surface spans a plurality of said arrays.

8. A heat exchanger as claimed in any one of the preceding Claims wherein the corrugated surfaces are shaped relative to each other so as to constrain the second fluid stream to undergo multi-pass flow through the heat exchanger.

9. A heat exchanger as claimed in any one of the preceding Claims wherein the corrugated surfaces are of substantially sinusoidal shape.

10. A heat exchanger as claimed in any one of the preceding Claims wherein the corrugated surfaces change in form along the length thereof.

11. A heat exchanger substantially as hereinbefore described and with reference to Figure 1, Figures 1a and 1b, Figure 2a, Figure 2b, Figure 3, Figure 4, Figures 5, 5a and Sb, or Figures 6 and 6a, of the accompanying drawings.