GB2111667A - Heat exchanger - Google Patents

Abstract

A heat exchanger has tubes 22 and a casing 10 through which two fluids pass in counterflow to set up heat exchange between the fluids. The tubes are parallel to one another and have an inlet 15 at one end and an outlet 16 at the other end. The casing is cylindrical and surrounds the tubes. The tubes have external fins 2 extending outwards of the tubes and preferably of spiral configuration to enhance the surface area of the tubes and thermal efficiency. The tubes may also have internal fins 30. <IMAGE>

Description

SPECIFICATION Heat exchanger This invention relates to heat exchangers and, in particular, to heat exchangers having tubes through which one fluid is passed in counterflowto another fluid passing over the tubes to set up heat exchange between the two fluids.

Counterflow heat exchangers generally provided enhanced heat exchange characteristics compared with cross-flow heat exchangers but counterflow heat exchangers often have the disadvantage that when tubes are provided for conducting one of the fluids the tubes are not easily cleaned and any build up of deposits on the tubes causes the effectiveness of the heat transfer to be reduced.

Shell and tube heat exchangers have also been proposed in which the tubes have been removable from the shell but such heat exchangers are often a combination of cross- and counterflowarrange- ments and include a bundle of tubes which are not easily cleaned.

Other forms of heat exchanger have been proposed, for example for use in cooling foodstuffs such as milk, which are more readily cleaned but these are not usually capable of withstanding relatively high pressure in the fluid passing therethrough.

An object of the invention is to provide a heat exchanger which is capable of effecting heat exchange between two fluids in counterflow relationship which can be easily cleaned and in which at least one of the fluids is under relatively high pressure.

According to the invention a heat exchanger comprises a plurality of tubes extending generally parallel to and spaced from one another, a casing surrounding and spaced from the tubes to define a passage extending in the same direction as the tubes, a fluid inlet at one end of the tubes for introducing a first fluid along the tubes so that the first fluid passes from the inlet towards the other end of the tubes, and a fluid inlet at the end of the casing opposite to the tube fluid inlet for introducing a second fluid along said passage to pass along the passage in counterflow to said first fluid, whereby heat exchange between the two fluids takes place, at least some of the tubes each being formed with an external, outwardly-directed fin extending in the axial direction of the tubes.

Preferably said fin extends spirally of the tube and the tube is formed of extruded metal, the fin being formed integrally with the tube.

Conveniently immediately adjacent tubes are arranged with spirally directed fins of opposite hand such that the second fluid swirls in the same direction between the tubes under the action of the fins on the tubes as the second fluid passes along the passage.

In one embodiment of the invention at least some of the tubes are formed with internal, inwardly directed fins extending in the axial direction of the tubes.

The heat exchanger may include support means for the tubes between the ends thereof in the form of a grid lying transverse to the axes of the tubes and defining a plurality of openings through which the tubes extend.

The fins may be absent from the ends of the tubes to define spaces at the ends of the passages over which the second fluid is distributed prior to passing along the exterior of the tubes.

Further features of the invention will appear from the following description of an embodiment of the invention given by way of example only and with reference to the drawings, in which: Fig. 1 is a longitudinal cross-section through a heat exchanger, Fig. 2 is an enlarged view of part of the heat exchanger of Fig. 1, Fig. 3 is a cross-section across a tube incorporated into the heat exchanger of Figs. 1 and 2, and Fig. 4 is a schematic cross-section on the line 4-4 in Fig. 1.

Referring to the drawings Figs. 1 shows a heat exchanger of the kind known as a shell and tube heat exchanger.

The heat exchanger includes a cylindrical shell 10 having a central axis 11, a fluid inlet 12 and a fluid outlet 13, the inlet being at the opposite end of the shell 10 to the outlet. At the ends of the shell 10 are manifolds 15 and 16 each having a fluid opening 17 and 18, one 17 ofthe openings being an inlet opening and the other 18 being an outlet opening.

The manifolds 15 and 16 are sealed from the shell 10 by tube plates 19 and 20 between which extends a tube bundle formed of a plurality of tubes 22 the ends of which extend through the tube plates 19 and 20. Thus the tubes 22 provide communication between the manifolds 15 and 16 whereby a first fluid passes from the manifold 15 along the tubes to the manifold 16. Similarly a second fluid is passed over the tubes 22 in travelling between the shell inlet 12 and the shell outlet 13. Heat exchange is set up between the first and second fluids during passage of the fluids and the heat exchanger operates in a counterflow system.

To enhance the heat exchange characteristics of the heat exchanger the tubes 22 are each formed with an external series of fins 24 which each extend outwardly from the tube and which, in the illustrated embodiment, extend spirally around the tubes in the axial direction of the tubes as seen in Fig. 2.

The dimensions of the fins 24 are selected to give an enhanced heat exchange coefficient both through increasing the surface area of the tube and by inducing spiral movement of the second fluid within the shell. Conveniently the fins have, for a two and one half centimetre diameter tube, a depth of 2 mm to 6 mm, a width at the base of the fin is from 2 mm to 4 mm, with a spacing between fins in a direction at right angles to the fin of between 2 mm and 6 mm.

The fins 24 each taper inwards in the radially outward direction.

The finned tube is conveniently formed by extrusion so that the fins are integral with the tube and the material of the tube is carbon steei, a copper alloy or other suitable metal.

As can be seen from Fig. 4 the tube bundle is arranged so that the tubes lie along vertical and horizontal rows, the spacing between the tubes in the rows being the same so that the tubes have a generally square configuration with each tube being spaced the same distance from immediately adjacent tubes. In addition the spirals on the tubes are handed so that adjacent tubes are of the opposite hand, as seen in Fig. 4. By this arrangement of tubers the spiral fins cause the fluid passing over the tubes to swirl in the manner shown by the arrows A in Fig.

4 eliminating dead spaces and enhancing the heat transfer coefficient.

The tubes may be spaced from one another, as shown in Fig. 4, or the tubes may be close together and in some cases with the outer edges of the fins in contact. However it is preferred to maintain a spacing between the tubes sufficient that the cleaning off of deposits on the tubes may be readily achieved. For this purpose it is also convenient to make the heat exchanger so that the tube bundle is removable.

From Fig. 2 it will be seen that at the ends of the tube the fins 24 are absent. This enables the tubes to be readily assembled with the tube plates 19 and 20 by locating the tubes through corresponding openings in the tube plates. In addition the absence of fins on the tubes in the region of their ends enables the second fluid to be readily dispersed and collected over the spaces 25 and 26 respectively at the ends of the shell 10 without the action of the fins in causing the swirling motion of the fluid adversely affecting the passage of the fluid in these regions.

Support for the tubes between the tube plates 19 and 20 is provided by a grid 28 which lies transverse to the axis 11 and is secured to the shell 10. The grid 28 includes rod or strip elements 29 at right angles to one another, the tubes 24 passing through openings in the grid 28. The tubes 24 contact the rods or strips 29 and, to improve the support against vibration given by the grid, the elements 29 may be formed of soft material.

The tubes 24 may be secured to the tube plates and sealed by welding or by expanding the tubes into the tube plate openings.

As shown in Fig. 3 the tubes may each have internally arranged fins 30 which are directed inwardly from the walls of the tubes. Such fins may extend spirally along the tubes or axially parallel to the tube axes. In the case where an internal spiral fin is employed the spirals are conveniently of the same hand as those of the external fins 24.

The use of a counterflow arrangement and the provision of at least external fins on the tubes results in a relatively small heat exchanger for a given duty, particularly when the thermal efficiency of the exchanger needs to be large, that is, when the outlet temperature of one fluid approaches the inlet temperature of the other fluid.

When internal fins are employed, as in the illustrated embodiment, the interiors of the tubes are readily cleaned by extending a suitable cleaning device along the tubes, the continuous fins not providing an obstruction to the movement of the device.

While there is described a heat exchanger through which two fluids pass in counterflow, it will be appreciated that the heat exchanger may be used as a parallel flow heat exchanger, that is, one in which the two fluids flow in the same direction. Although counterflow heat exchangers give better heat exchange characteristics there may be reasons, such as the need to reduce the temperature differential across the fluids, why a parallel flow arrangement needs to be used.

Claims (15)

1. A heat exchanger comprising a plurality of tubes extending generally parallel to and spaced from one another, a casing surrounding and spaced from the tubes to define a passage extending in the same direction as the tubes, a fluid inlet at one end of the tubes for introducing a first fluid along the tubes so that the first fluid passes from the inlet towards the other end of the tubes, and a fluid inlet at the end of the casing opposite to the tube fluid inlet for introducing a second fluid along said passage to pass along the passage in counterflowto said first fluid, whereby heat exchange between the two fluids takes place, at least some of the tubes each being formed with an external, outwardlydirected fin extending in the axial direction of the tubes.

2. A heat exchanger according to claim 1 wherein said fin extends spirally around the tube.

3. A heat exchanger according to claim 1 or 2 wherein the tubes are formed of extruded metal, the fin being formed integrally with the tube.

4. A heat exchanger according to any one of the preceding claims wherein the fin is arranged to cause said second fluid to swirl in the spaces between the tubes as said second fluid passes along the exterior of the tubes.

5. A heat exchanger according to claim 2 or 6 wherein immediately adjacent tubes are arranged with spirally directed fins of opposite hand such that the second fluid swirls in the same direction between the tubes under the action of the fins on the tubes as the second fluid passes along the passage.

6. A heat exchanger according to any one of the preceding claims wherein at least some of the tube are formed with internal, inwardly directed fins extending in the axial direction of the tubes.

7. A heat exchanger according to claim 6 wherein the internal fins extend spirally of the tubes.

8. A heat exchanger according to any one of the preceding claims wherein the tubes are arranged in rows in one direction and a direction at right angles to said one direction.

9. A heat exchanger according to claim 8 wherein the spacing between the rows in said one direction is substantially the same as the spacing between the rows at right angles thereto.

10. A heat exchanger according to any one of the preceding claims wherein said casing is circular in cross section.

11. A heat exchanger according to any one of the preceding claims comprising support means for the tubes between the ends thereof in the form of a grid lying transverse to the axes of the tubes and defining a plurality of openings through which the tubes extend.

12. A heat exchanger according to claim 11 wherein the grid is formed of a plurality of elements each in the form of a rod or a strip.

13. A heat exchanger according to any one of the preceding claims wherein at the ends of the tubes adjacent the ends of the passage the fins are absent

14. A heat exchanger according to claim 13 wherein the ends of the tubes are secured in a tube plate.

15. A heat exchanger substantially as described with reference to the drawings.