GB2137332A - Improvements in or relating to heat exchangers - Google Patents

Abstract

The effective external heat transfer area of a primary fluid flow tube 1 located in a secondary fluid flow enclosure 2 is extended by two interlinked coil elements 3 located by and on the primary tube 1 to project into the secondary fluid flow path, The coil elements 3 also act as turbulator elements within the enclosure 2. Each coil element 3 is similar and is helically wound, high thermal conductivity wire of triangular cross-section arranged so that the internal face of the coil loops 5 is flat and contacts the external face of the primary tube 1 for heat transfer. <IMAGE>

Description

SPECIFICATION Improvements in or relating to heat exchangers This invention concerns improvements in or relating to heat exchangers. This invention is particularly concerned with heat exchangers ofthe kind in which heat is transferred between a primary fluid contained in a heattransfertube and a secondary fluid confined externally ofthe heat transfertu be by an enclosure.

It is already known to provide heat exchangers in which the heat transfertu be has internally projecting finsorribswhich increasethe internal surface area of thetube in contact with the primaryfluid so asto increase thermal transfer. It is also already known to provide these fins or ribs so thatthey project into the primary fluid path and cause the fluid to turbulate during flow th rough the heattransfertube. In some kinds of heattransfertubes, a spirally wound wire or rod element is provided within the tube just to cause turbulation during fluid flow, but such turbulator elements are not effective internal extensions ofthe heat transfer surface of the tube.

In these heat transfer tubes, the internal configurations reduce the effective cross-sectional area and to achieve the same performance as with a plain internal tube of the same diameter, either higherflow rates are required orthe numberofthe heattransfertubes has to be increased. In both cases, there is an increase in cost when using these special heat transfertubes with internal profiles or separate inserts that have to be secured within ortothetube.

Itis also already known to provide heat transfer tubes in which the internal surface ofthe tube is plain but the external surface is extended by means of outwardly projecting fins or ribs which form an integral part ofthe tube by being brazed or bonded thereto. These externally extended tubes provide an external diameter which is substantially greater than that of the plain tube, and as a consequence, the enclosure through which the secondary fluid flows has to be largerthan that required for a plain tube, or less numbers of these special heattransfertubes can be accommodated in the same sized enclosure providing the complete heat exchanger.

In principal, the known constructions of heat exchangers using such heattransfertubes rely on assembling the desired number and size of tubes to tube plates which locate the respective ends ofthe tubes and provide the connections for the primary fluid. This assembly of heattransfertubes, called a bundle or stack, is fitted into the enclosure to complete the heat exchanger unit. The enclosure, often called a shell or cylinder, incorporates the inlet and outlet portsforthesecondaryfluid.

Various complex arrangements of such heattransfertubes andturbulators have to be provided for certain heattransfer and process performance in stallations, and in many installations, the heat exchan- gers have to be customised. This adds to the overall expanse both of original equipment and to any replacement or servicing.

The manufacture of such heattransfertubes is also complex with the need to braze or bond the elements providing the heattransfer extension to the body of thetube, whilst in known manner leaving free ends of the tubes ready for connection to the tube plates.

Generally, itwould be advantageous to provide a heat exchanger having a simple construction which can be assembled from simple modular or component parts which, by selection and arrangement, can meet the desired performance characteristics for the heat transfer and flow requirements.

It is an object of this invention to provide such a simple modular system for constructing heat exchan- gersandto provide heat exchangers constructed from such systems.

According to the broadest aspect ofthis invention we provide a heat exchanger comprising a primary fluidflowtube located at each end in an enclosurefor secondary fluid flow and two coil elements of helically wound wire received within the enclosure, the two coil elements being arranged in overlapping relationship along their lengths such that portions ofthe loops of one coil element are interjacent portions ofthe loops oftheothercoil element,andthe primarytube extends lengthwise of the coil elements and is received in the region defined between the overlapping loop portions of the two coil elements, the external face ofthe primary tube being in heattransfer contact with the respective internal faces of the overlapping loop portions to extend the effective external heattransferarea ofthe primary tube, and the primary tube locates the two coil elements together against lateral disengagement of the respective loops.

According to another aspect of this invention, we provide a method of manufacturing a heat exchanger comprising the steps of selecting two coil elements of helically wound wire, arranging the coil elements in overlapping relationship along their lengths by inserting portions ofthe loops of one coil elementinterja- cent portions of the loops of the other coil element, inserting a primary fluid flowtube lengthwise of the coil elements into a region defined between the overlapping loop portions of the two coil elements so that the e,'ernal face ofthe primary tube is in heat transfer con tact with the respective internal faces of the overlapping loop portions and the two coil elements are located together against lateral disengagement of the respective loops, and locating and enclosing the primarytubewith the assembled coil elements in an enclosure for secondaryfluid flow.

As will be appreciated, the terms "primary" and "secondary" as used herein are used for convenience only, and nd the heat exchanger according to this invention may be disposed in fluid circuits for heating or cooling with either the process or coolant fluid constituting the primaryfluid. The term "fluid"as used herein is intended to include both liquid and gaseous fluids or mixtures of same including steam.

By this invention, in both its aspects, we provide a simple form of heat exchanger in which the heat transfer surface of the primary fluid flow tube is extended byte coil elements of helically wound wire which are assembled together and located by and on the primaryfluid flow tube. The coil elements extend into the secondary fluid flow path and act as extensions of the heat transfer surface of the primary fluid flow tube and extend to act as turbulator elements associated with the primaryfluid flowtubewithin the enclosure. The two coil elements therefore serve two purposes in the heat exchanger.

The use of coil elements of helically wound wire enables simple metnods of manufacture to be employed to make up heattransfertubes of any required length which can be constructed from modular parts cutto the designed length and assembled togetherfor assembly in the enclosure.

The invention provides many possibilities and typical variations include the pitch of the coil elements, the diameter of the coil elements and the effective surface areas of both the coil elements and the primary fluid flowtube to suit the required performance criteria.

Conveniently, the coil elements are similar and the loop portions of one coil element are arranged alternately with the loop portions of the other coil element.

Advantageously,thecoil elements are constructed from wire which has a high thermal conductivity, such as copper, and which is of substantially uniform section throughout it's length.

Preferably, the section of the wire is of triangular configuration. By the use of such triangularconfiguration,the angularfaces of the loops of the coil elements presentflat internal faces for engaging the external face ofthe primaryfluid flowtube to provide heat transfer contactfaces of significant area whilstsimultaneouslypresenting angularedgestothe path of the secondary fluid flow to increase the turbulation and mixing effect whilst reducing boundary layer effects, particularlywherethesecondaryfluid isa very viscous liquid, such as oil.

Although the triangular section is preferred, other sections including round, oval, rectangular or polygonal can be employed.

In the conventional manner, the enclosure may comprise an outertube or other hollow member with inlet and outletforthe secondaryfluid. The primary fluid flow tube may be located with respect to the enclosure by tube plates through which the primary fluid is conducted with suitable inlet and outlet connections for the flow of primary fluid through the primary tube.

According to another preferred feature ofthis invention, a heat exchanger may comprise a plurality of primary fluid flowtubes with each tube being received in the region between interjacent loop portions of two overlapping coil elements.

In such a heat exchanger, the coil elements may be interlinked by the tubes to form an integrated assembly. For example, a respective tube may be inserted in the region between interjacent loop portions of successive adjacent and overlapping coil elements to form an integrated linear array.

By such interlinking of the coil elements a wide range of performance criteria can be obtained from the same modular components by selecting the total number, size and disposition ofthe primary tubes and associated coil elements within the secondaryfluid flow path.

The design possibilities are numerous from the same modular components, and the integrated assembly of interlinked coil elements may be arranged in various arrays orformations within the secondaryfluid flow path defined by the enclosure.

For example, it is envisaged thatthe integrated linear array of coil elements and primaryfluidflowtubes above-mentioned may be arranged in a spiral or circular array.

By selecting the shape and configuration of the coil elements, the effects ofturbulation and extended heat transfer surfaces can be derived having regard to flow and temperature requirements and other characteristics of the fluid mediums in the heat exchanger proposed.

Furthermore, in an array, additional tubes for primary fluid flow can be employed to extend in the regions between further interjacent loop portions of those coil elements which are purposefully overlapping. This enables an increase in the number of primarytubesto beemployedwhereitmaybe required to accommodate certain flow rates ofthe primary fluid. Alternatively, in an array, certain primary fluid flowtubes may be omitted and plain rods or blanktube members may be employed just to locate the coil elements and to further extend the heat transfersurfaceofthe heat exchanger.

As will now be appreciated, in this further aspect of this invention, a plurality of primary fluid flow tubes are provided and these are integrated with a plurality of coil elements and all ofthese components are in heat transfer contact with each other the integrated assembly ofthe coil elements and the plurality of primary tubes provides the stack or bundle of heat transfertubes. This is significantly different to the conventional approach of providing separate and independent heattransfertu bes, each with integral extension of heat transfer surfaces that are subsequently assembled in the required numberto form the bundle or stack of heat transfer tubes.

This invention will now be described with reference to exemplary embodiments depicted in the accompanying drawings wherein:- FIGURE 1 is a diagrammatic cross-sectional view of afirstembodimentofa heat exchanger according to this invention; FIGURE2 is a side view, partly sectioned for clarity, showing a part ofthe heat exchanger of thefirst embodiment; FIGURES 3 and4 are respective diagrammatic end and side views illustrating the initial stage of assembly ofthe coil elements ofthe heat exchanger of the first embodiment.

FIGURES 5 is a diagrammatic cross-sectional view of a second embodiment of a heat exchanger according to this invention; FIGURE 6is a diagrammatic end view illustrating the assembly of the heat exchanger ofthe second embodiment; FIGURE 7 is a diagrammatic end view illustrating the assembly of a third embodiment of a heat exchanger according to this invention; FIGURE 8 is a diagrammatic cross-sectional view of a fourth embodiment of a heat exchanger according to this invention; FIGURE9 is a diagrammatic cross-sectional view of a fifth embodiment of a heat exchanger according to this invention; and FIGURE 10 is a diagrammatic representation of a sixth embodiment of a heat exchanger according to this invention.

With reference to Figures 1 to 4 relating to the first embodiment of this invention,the heat exchanger comprisesa primaryfl uid flow tu be 1 and an enclosure 2 depicted as a cylindrical tubular housing concentricto the tube 1. In known manner, the enclosure 2 would be closed at each end and be provided with inlet and outlet openings (not shown) forthe flow ofsecondaryfluid through the enclosure 2. The primarytube 1 would be connected to the primary fluid path at each end of the enclosure 2 through appropriate connections which may extend through tube plates (not shown).

The heat exchangerfurther comprises two similar coil elements 3 each being helically wound from copper wire of triangular section arranged to present a flat internal face. The diameter and pitch ofthe coil loops is selected with reference to the desired geometry and dimensions ofthe primary tube 1 and the enclosure 2, and with further reference to the flow rates ofthe primaryfluid conducted through the primarytube 1 and the secondary fluid conducted through the path defined externally of the primary tube 1 bythe enclosure 2.

The two coil elements 3 are assembled together in overlapping relationship along their lengths by first aligning the elements 3 in spaced apart but parallel relationship so that clearance spaces 4 between successive loops 5 of one coil element are aligned with an adjacent portion 5' of a loop ofthe other coil element and vice versa. The two coil elements 3 are then displaced towards each othersothatthe loop portions 5' are received in the aligned clearance spaces 4. As thus assembled the loop portions 5' of one coil element 3 are interjacent and alternate with the loop portions 5' of the other coil element 3 and the overlapping loop portions 5' define a region 6 of generally elliptical shape extending lengthwise of the coil elements.

The primarytube 1 is entered and received within this region 6with the external face of the primarytube 1 engaged by the respective internal faces ofthe loop portions 5' to provide heat transfer contact faces 7 which extend regularly at spaced apart positions along that length of the primary tube 1 where surrounded by the coil elements. The contact faces 7 are arcuate having a length dependent on the relative curvatures ofthe external surface of the primary tube 1 and the internal surface ofthe loop portions 5'. As a result by appropriate selection of the curvatures and the use of wire presenting a flat internal face, heat transfer contactfaces of significant area can be obtained.

In assembling the coil elements, these may be stretched axially prior to placementofthe loop portions 5' in the aligned spaces 4to provide or increase the clearance spaces 4 between the adjacent loops 5 of each coil element 3 to assist assembly.

Furthermore, the primary tube 1 may be inserted in the region 6following the initial placement of a slave rod or tube (not shown) ofsmaller or larger diameter than the primary tube 1. The slave rod or tube merely serves to hold the assembled coil elements 3 in place temporarily pending insertion ofthe primary tube 1.

The completed assembly of the coil elements 3 and the primary tube 1 is such that the primary tube 1 locates the two coil elements 3 with respect to each other so that they cannot be laterally disengaged. The free ends ofthe primarytube are then connected to suitable tube plates which are located in the enclosure 2.

In useofthe heat exchanger, the coil elements3 extend in the flow path of the secondary fluid and the staggered array ofthe loops 5 of the two coil elements 3 provides the desired turbulator effect in the secondary fluid. Additionally, the coil elements 3, through the arcuate contact faces 7 between the respective internal faces ofthe loop portions 5' and the external face of the primaryfluid flow tube 1, provide extensions of the external surface of the primaryfluidflowtube 1 to ensure the required heat transfer between the primary fluid and the secondary fluid is obtained.

As will now be appreciated, the coil elements 3 are simple components which, in association with the primary tube 1, provide modular parts from which the heattransfertube assembly and the heat exchanger can be constructed in a simple and effective manner.

From the same modular components, a range of different sizes of heat exchanger can be constructed, and special manufacturing techniques as are required in other prior forms of heat exchanger with internal or external ribs or splints are entirely avoided.

Itis envisaged that the assembly ofthe single primary tube 1 and the associated two coil elements 3 can constitute a heattransfertubeofwhich a plurality can be separately mounted by means oftube plates within the enclosure 2 forthe secondaryfluid flow. In such an arrangement the separate assemblies would be located in the same or similar manner as the integral heattransfertubes as presently used with their complex configurations and constructions, but the cost of each heat transfertube with it's simple method of manufacture would be less.

Following the above explanation, reference is now directed to the further embodiments ofthis invention as shown in the drawings wherein the same principle and modular construction is developed to provide morecomplexforms of heat exchanger. Like reference numerals are used to indicate parts corresponding to those of the first embodiment of heat exchan ger abovedescribed.

In the second embodiment of Figure 5, an integrated assembly of six primary fluid flow tubes 1 and sixcoilelements3arranged inacirculararraytoform a circular bundle of primary tubes is shown. The primarytubes 1 are regularly spaced apart and each is received in a respective region 6 between certain interjacent loop portions 5' of adjacent overlapping coil elements 3 thereby interlinking the coil elements.

As depicted in Figure 6, such an assembly is based on a simple method of construction wherein successive adjacent ones of the coil elements 3 are relatively located on insertion of a slave rod 8 ortube in the region 6 of generally elliptical shape as described with reference to the first embodimentto form an integrated assembly comprising a linear array of coil elements 3. When the desired number of coil elements 3 have been so assembled, then the outer end coil elements are drawn together as shown bythe arrows in Figure 6 and, when the loop portions are interjacent and overlapping, a slave rod 8 ortube is entered into that last region to locate these two outer end coil elements togetherto form the circular array.

Afterthe interlinking of the several coil elements with slave rods 8 ortubes is completed, then the respective primary fluid flow tubes 1 are substituted forthe slave rods 8 ortubesto form the integrated assembly of Figure 5. If desired, and depending upon the geometry ofthe coil elements, optional extra tubes can be inserted in other zones 9 extending between the coil elements, and a typical optional extra tube loins shown in Figure 5.

In known manner as previously described, the circular bundle of primary tubes would then be connected to tube plates (not shown) and located in a suitable enclosure (not shown).

In this embodiment, it should be noted thatthe configuration and geometry ofthe coil elements 3 and primary tubes 1 is such that, in addition to the arcuate contact faces 7 between the external face of each tube 1 and the respective internal faces of the loop portions 5' of the adjacent coil elements 3 interlinked by the tube, there are two additional point contact faces 7' between the external face of each tube 1 and the respective external faces of the loop portions 5' oftwo further coil elements 3 making four contactfaces 7, 7' in all. These additional point contactfaces 7' increase the efficiency of the heat transfer between the primary fluid and the secondary fluid passing around and through the coil elements 3.

Additionally, by the provision of several coil elements 3,thetubulatorandmixing effect is increased compared with the simpler arrangement ofthefirst embodiment.

Following the method of construction ofthe integrated assembly of a plurality of coil elements as depicted in Figure 6, alternatively the linear array can be wound to form a spiral array as depicted in Figure 7. The slave rods 8 or tubes can then be replaced by respective primaryfluid flow tubesto form an integrated assembly of heat exchanger in which the primary tubes are disposed spirally. The coil ele mentsinthespiral array provide the extensions of the heat transfer surfaces of the primary tubes and the coil loops provide the turbulator effect in the flow path ofthesecondaryfluid.

With reference to the fourth embodimentofthis invention as shown in Figure 8, each coil element3 has an internal loop diameter corresponding to three times the external diameter of the primary fluid flow tubes 1 and twice the diameter or maximum sectional th ickness of the wi re. In consequence each coil element3 accomodatesthree primarytubes 1 of whichthetwo outerprimarytubesare received in respective regions 6' interjacent and overlapping loop portions of the coil element and adjacent coil elements on eitherside and the centre primarytube is received in a region 6" between interjacentand overlapping 'oop portions of two further coil ele ments which also overlap the previously mentioned coil elements.

By such close packing and disposition ofthe coil elements 3, the total heattransfersurfacearea is increased within an enclosure of unit size compared with the other embodiments afore-mentioned.

In a fifth embodiment of this invention as shown in Figure 9, each coil element 3 accommodates two primarytubes 1 each ofwhich is received in a region 6 between interjacentand overlapping loop portions of the coil element3 and adjacent coil elements 3 on either side as described in the second and third embodiments. However, in this construction each coil element3 has an internal diameter correspond ing to twice the external diameter ofthe primary tubes and oncethe diameter or maximum sectional thickness ofthewire. As a result each loop portion 5' has an internal face providing an arcuate contactface 7 with the external face of onetube and an external face providing a point contactface 7' with the external surface of an adjacent tube.The additional point contactfaces 7' provided bythis construction in creasethe heattransfer performance. It will be noted that in thefourth embodiment of Figure 8 each loop portion also has an internal and an external face in contact with two adjacenttubes.

In alltheembodimentsdescribedthusfar,the primarytubes can be straight, but dueto the inherent twisting effect ofthe helicallywound wire of each coil element, it is found desirableto employ primary tubes which have an axially extending curvature corresponding to a full or partial helix depending on the length ofthetube and the designed geometry and arrangement of the coil elements.

Byemploying curved primarytubes, these enhance the heattransfer performance due to the effect of providing a swirling motion to the secondaryfluid flow as it moves relative to the primary tube(s) and the associated coil elements.

The embodiments shown in the diagrammatic drawing of Figure 10 illustrates such an arrangement in which the primarytubes 1 are curved with thecoil elements3 also extending in curved formation and inter-engaged as aforedescribed. In this embodi ment,the enclosure 2 has an inlet 11 and outlet 12 on one side and the primarytubes 1 are connected through tube plates 13 mounted at each end in the enclosure with inlet 14 and outlet 15forthe primary fluid.

In assembling the primary tube(s) and coil ele ments as aforedescribed,thetube(s) and coil ele ments are in contact with each otherthrough the heat transfer contact faces.

The use of primary tubes which extend in a helical manner increases the thermal performance by in creasing the effective Reynolds Numberforthe primaryfluid flow. Furthermore, the helical or spiral > extentofthe primarytubesalso displacesthe loops of each coil element so that in the axial direction ofthe heat exchanger and in the direction of flow ofthe secondaryfluid,the loops are staggered orsucces sively displaced laterally presenting different por tions of successive loops to the secondary fluid.

To increase the heattransferperformance, and if required, the mechanical strength ofthe assembly, the primary tubes and coil elements may be brazed, welded or bonded together after assemblyto provide I a rigid construction. This rigid construction may have some advantages in particular applications.

In the various embodiments ofthis invention, the coil elements are all shown as being wound to a circular helical form, but in some cases other configurations of helix may be used, for instance an oval or elliptical shape. Such alternativeconfigurations of helix may be used to increase advantageously the area of the heat transfer contact faces between the coil elements and the primarytube(s) and the effects may be further enhanced by the use of primary tube(s) having otherthan the circular cross-section of the embodiments above-described,forexample primary tube(s) of oval, elliptical or polygonal crosssection may be used in combination with any configuration of helix.

Additionallywhilst it is preferred to constructthe coil elements from wire oftriangularcross-section as above-described itwill be understood that other cross-sections may be used, for example round, oval, rectangular or polygonal, with a non-circular crosssection having at least one flat face providing the internal face ofthe loop portions being generally preferred.

Furthermore, although in the embodiments described two coil elements are located by and on a primarytube received in the region between interjacent loop portions of the overlapping coil elements, it will be appreciated that three or more coil elements could be located by and on a primarytube in similar fashion.

In certain applications, the selection ofthe shape of the coil elements may be dependent on the internal shape ofthe enclosure forthe secondary fluid flow, and furtherversatility in design and geometry is permitted bythe invented heat exchanger using the modular form of construction above-described.

In heatexchangers according tothis invention employing a plurality of coil elements with more than two primary fluid flow tubes, it is also envisaged that some ofthecoil elements may be located together by rods ortubes which are not connected to the primary fluid flow.

A heat exchanger according to this invention may also include baffles or diverter plates or other means within the enclosure to control the flow path ofthe secondary fluid.

Claims (35)

1. A heat exchanger comprising a primaryfluid flow tube located at each end in an enclosure for secondary fluid flow and two coil elements of helically wound wire received within the enclosure, the two coil elements being arranged in overlapping relationship along their lengths such that portions of the loops of one coil element are interjacent portions ofthe loops of the other coil element, and the the primary tube extends lengthwise ofthe coil elements and is received in the region defined between the overlapping loop portions of the two coil elements, the external face of the primarytube being in heat transfer contact with the respective internal faces of the overlapping loop portions to extend the effective external heattransfer area ofthe primary tube, and the primarytube locates the two coil elements together against lateral disengagement of the respective loops.

2. A heat exchanger according to claim 1 wherein the coil elements are similar and the loop portions of one coil elementare arranged alternatelywith the loop portions ofthe other coil element.

3. A heat exchanger according to claim 1 or claim 2 wherein the coil elements are constructed from wire of non-circular cross-section and the internal face of the loop portions is provided buy a flat surface portion ofthe wire.

4. A heat exchanger according to claim 3 wherein the wire is of triangular cross-section.

5. A heat exchanger according to any one ofthe preceding claims wherein the coil elements are constructed from wire having a high thermal conductivity.

6. A heat exchanger according to any one ofthe preceding claims wherein the coil elements are constructed from wire of uniform cross-section.

7. A heat exchanger according to any one of the preceding claims wherein the helix of each coil element is of circular, oval or elliptical configuration.

8. A heat exchanger according to any one of the preceding claims wherein the primary tube is of circular, oval, elliptical or polygonal cross-section.

9. A heat exchanger according to any one ofthe preceding claims wherein the enclosure comprises an outertubular member having an inlet and an outlet for the secondary fluid.

10. A heat exchanger according to any one ofthe preceding claimswhereinthe primarytubeis located with respectto the enclosure by tube plates through which the primary fluid is conducted with inlet and outlet connectionsto the primarytube.

11. A heat exchanger according to any one of the preceding claims comprising a plurality of primary fluid flow tubes each ofwhich is received in the region between interjacent loop portions of two overlapping coil elements.

12. A heat exchanger according to claim 11 wherein the coil elements are interlinked to form an integrated assembly.

13. Aheat exchanger according to claim 12 wherein successive overlapping coil elements are interlinked to form an integrated linear array.

14. A heat exchanger according to claim 12 wherein the coil elements are interlinked to form a circular array.

15. A heat exchanger according to claim 12 wherein the coil elements are interlinked to form a spiral array.

16. A heat exchanger according to any one of claims 13 to 15 wherein each helical coil element is of circular configuration and each tube is of circular cross-section.

17. A heat exchanger according to claim 16 wherein each coil element has a diameter corresponding to twice the tube diameter plus oncethewire diameter.

18. A heat exchanger according to claim 16 wherein each coil element has a diametercorres- ponding to threetimes the tube diameter and twice the wire diameter.

19. A heat exchanger according to any one of claims 12 to 18 wherein the external face of each tube is in heattransfer contact with the external face of at least one adjacent coil elementto extendfurtherthe effective external heat transfer area ofthe tube.

20. A heat exchanger according to any one of claims 12 to 19 wherein the tubes have an axially extending curvature.

21. A heat exchangersubstantially as hereinbefore described with reference to Figures 1 to 4 ofthe accompanying drawings.

22. A heat exchanger substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.

23. A heatexchangersu bstantially as hereinbe foredescribedwith referenceto Figures 6 and 7 of the accompanying drawings.

24. A heat exchangersubstantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.

25. A heat exchanger substantially as hereinbefore described with reference to Figure 9 of the accompanying drawings.

26. A heat exchanger substantially as hereinbefore described with reference to Figure 10 ofthe accompanying drawings.

27. A method of manufacturing a heat exchanger comprising the steps of selecting two coil elements of helically wound wire, arranging the coil elements in overlapping relationship along their lengths by inserting portions ofthe loops of one coil element interjacent portions of the loops ofthe other coil element, inserting a primaryfluidflowtube lengthwise ofthe coil elements into a region defined between the overlapping loop portions ofthetwo coil elements so thatthe external face of the primarytube is in heat transfer contact with the respective internal faces oftheoverlapping loop portions andthetwo coil elements are located together against lateral disengagement ofthe respective loops, and locating and enclosing the primarytubewith the assembled coil elements in an enclosureforsecondaryfluid flow.

28. A method according to claim 27 wherein the loop portions of one coil element are inserted alternately with the loop portions ofthe other coil element.

29. A method according to claim 27 or claim 28 wherein each coil element is stretched axially prior to insertion of the loop portions ofthe elements interjacentone another.

30. A method according to any one of claims 27 to 29 wherein a slave rod ortube is inserted in the region between the overlapping loop portions to locate the coil elements priorto insertion of the primary tube.

31. A method according to any one one of claims 27to 30 wherein a plurality of primarytubes and associated coil elements are assembled together to form an integrated assembly by interlinking overlap ping coil elements.

32. Amethodaccordingto claim 31 wherein successive coil elements are interlinked to form an integrated linear array.

33. A method according to claim 31 wherein the coil elements are interlinked to form a circulararray.

34. A method according to claim 31 wherein the coil elements are interlinked to form a spiral array.

35. A method of manufacturing a heat exchanger substantially as hereinbefore described with reference to the accompanying drawings.