EP0055711A4

EP0055711A4 - Low profile heat exchanger and method of making the same.

Info

Publication number: EP0055711A4
Application number: EP19810900865
Authority: EP
Inventors: Alexander Goloff
Original assignee: Caterpillar Tractor Co
Current assignee: Caterpillar Inc
Priority date: 1980-07-07
Filing date: 1980-07-07
Publication date: 1983-06-09
Also published as: CA1140531A; JPS57500945A; EP0055711B1; DE3071178D1; WO1982000194A1; EP0055711A1

Abstract

A low profile heat exchanger module (46) and method for forming the same wherein the heat exchanger module (46) is formed from one or more compact, single sheet primary surface heat exchanger core units (38a and 38b). Each single sheet primary surface core unit (38a and 38b) is made from a rectangular sheet of a suitable heat exchange material (10) which has been serrated along the longitudinal edges (18) to provide entrance ramps (20) and to minimize flow blockage. Fluid flow is controlled by closures (42, 44) in the ends of alternate flow passages (58, 64) which serve to isolate the fluid in one passage from the fluid in an adjacent passage.

Description

Low Profile Heat Exchanger and Method of Making the Same

Technical Field This invention relates to a low profile heat exchanger module for use in heat furnaces, steel melting furnaces, gas turbines which use recuperators and the like, and a method for forming the same.

Background Art United States Patent 3,759,323 issued to Harry J. Dawson et al describes an important prior art primary surface heat exchanger for use as a recupera¬ tor core of a gas turbine. Dawson et al discloses a heat exchanger core made from a multiplicity of thin metal sheets which have been individually cor¬ rugated, or folded in a wavy pattern according to the method described in United States Patent 3,892,119 issued to Miller et al. A large number of these metal plates are stacked on top of each other and the edges of the sheets are crushed to form the flat sections necessary to encase the assembly and to allow the attachment of suitable manifolding for conveying hot and cold fluids. The necessity for crushing the edges results in blockage, and hence the restriction of fluid flow. In addition, the depth of the corrugations in the unit described by Dawson et al is limited by the need to crush the edges. Deep corrugations or pleats cannot be crushed in an organized or predictable manner without causing severe blockage.

The pattern of the corrugations and the relatively thick crushed edges of the individual sheets described by Dawson et al result in rigidity in all directions in the heat exchanger unit. This may lead to the development of high thermal strains, especially when transient loads are characterized by steep gradients. A major drawback of the prior art heat exchanger construction has been the presence of high stress concentration factors which have resulted from the need to crush the edges. In some applications the effect has been that of producing a multiplicity of cracks. Another problem has been the failure of the weld to penetrate at certain junctions, which results in a preformed crack. While such stress concentration factors may not be significant when the assembly is preloaded in compression, as intended, and when the transients are not steep, high stresses which lead to premature failures may appear under severe operating conditions and after prolonged periods of operation during which the preload is likely to be relaxed.

Disclosure of the Invention

The present invention is directed to a low pro¬ file heat exchanger module made from one or more compact, single sheet primary surface heat exchanger core units. Each unit is formed from a single sheet of thin material which has been pleated to any depth desired. Before pleating, the longitudinal edges of the thin sheet are serrated to provide fluid en¬ trance ramps that minimize blockage, and the surface of the sheet is embossed to define flow passages and provide a means for directing flow and controlling turbulence.

Other objects, advantages and features of the invention will be more readily apparent from the following detailed description of a preferred embodi¬ ment of the invention when taken together with the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a plan view of a portion of a single sheet of heat exchange material employed to form the heat exchange core of the present invention;

Figure 2 is a plan view of a portion of the single sheet of Figure 1 after embossing to show a "U" flow concept.

Figure 2* is a plan view of a portion of the single sheet of Figure 1 after embossing to show a "Z" flow concept. Figure 3 is a view in side elevation of the single sheet of Figure 2 during the pleating thereof;

Figure 4 is a diagrammatic illustration of a side view of a pleated assembly of the present inven¬ tion; Figure 5 is a diagrammatic illustration of an end view of a pleated assembly of the present inven¬ tion;

Figure 6 is a cross sectional view of the heat exchanger module of the present invention; Figure 7 is a diagrammatic illustration of an end view of the heat exchanger of the present invention;

Figure 8 is a plan view of a portion of a single sheet of heat exchange material of a second embodi¬ ment of the present invention; Figure 9 is a view in side elevation of the single sheet of the embodiment of Figure 8;

Figure 10 is a view in side elevation of the single sheet of the embodiment of Figure 8 during

O PI pleating thereof;

Figure 11 is a diagrammatic illustration of a third embodiment of the present invention; and

Figure 12 is a diagrammatic illustration of the method of producing the embodiment of Figure 11.

Best Mode For Carrying Out the Invention

Figure 1 illustrates a single sheet used to form one of the pleated assemblies of the heat exchanger module of the present invention. The single sheet, indicated generally at 10, is a long rectangular strip of heat exchange material, for example a suit¬ able thin metal, such as heat resistant steel. The width of the sheet 10 indicated at 12 determines the length of the resulting pleated assembly, and may be varied by the designer to fit the desired heat exchanger application. The longitudinal edges of the sheet are serrated' or cut in a sawtooth pattern so that the distance 14 between notches 16 is about equal to the desired height of the pleated assembly. The apexes or points of the serrated edge are shown at 18, and the distance of an edge 20 extending be¬ tween notch 16 and point 18 should preferably be equal to the height of the pleated assembly as indi- cated at 14 to eliminate fluid flow blockage. For example, the assembly may be about 1-2 inches in height at 14 and the width 12 may be about 6-7 inches.

Obviously, the configuration of the serrated edges of the sheet 10 can be altered to meet varying structural requirements. For example, the apexes 18 may be rounded instead of pointed, and the edge 20 can be curved rather than straight.

The pleated assembly is formed in a manner de- picted in Figures 2 and 3 or 2' and 3. The serrated sheet of heat exchange material 10 is divided into sections or walls 22 and 24 whcih are embossed by means of conventional dies, shaped rollers or any other embossing techniques;. The embossing serves to separate the subsequently formed pleats of the heat exchanger accurately and to guide the fluid flow through the completed heat exchanger. The em¬ bossing may space the pleats for a distance of about .030 inches.

As illustrated in Figures 2 and 2*, each section 22 and 24 may be embossed in a U-shaped or Z-shaped configuration respectively as shown by the rows of bosses 26 on section 22 and the rows of bosses 28 on section 24. While only four rows of bosses 26 and four rows of bosses 28 are shown, the number of rows could be much higher and is limited only by the size of the sections 22 and 24. Fluid flow channels 27 are defined on^' the face of each section 22 and 24 between the rows of bosses 26 and 28. The interruptions between bosses in a row induce fluid turbulence in the flow path defined thereby and thus enhance the heat transfer characteristics of the pleated assembly formed from the sheet 10. Since the sheet 10 will be pleated into a pleated assembly which forms part of the core of the heat exchanger module depicted in Figure 6, it is necessary to emboss the sheet so that the pleats are spaced far enough apart to allow fluid flow between them. This is accomplished by embossing sections 22 and 24 in opposite directions. Thus in Figure 2, the bosses 26 on section 22 project upwardly, while the bosses 28 on section 24 project downwardly. This alternate arrangement is maintained throughout the

_ O.MPI length of the sheet 10.

Once the sheet 10 is embossed, the sheet is pleated by folding it along lines or crest portions 34 between the sections or walls 22 and 24. Pleat- ing may be accomplished mechanically in any conven¬ tional manner, such as on machines utilizing dull- edged knife blades like those used for pleating filter paper in the manufacture of air cleaners and oil fil¬ ters, but which have been modified to pleat thin metal or heat exchange material rather than paper. Figure 3 shows the embossed sheet 10 being pleated and compressed at the lower end 36 to form the pleated assembly shown in Figure 4. The sections 22 and 24 of the sheet are compressed together until the raised bosses 26 and 28 contact the next adjacent section of the sheet. Thus the height of such bosses accurately controls the spacing between adjacent sections when the sheet 10 is pleated and compressed. To enable the bosses to perform this spacing function, it is imperative that the bosses on adjacent sections 22 and 24 be precluded from nesting when the sec¬ tions are compressed together. This may be accom¬ plished, as illustrated in Figure 2, by placing the bosses on section 24 so that they fall in the spaces between the bosses on section 22 when the two sections are pleated. In addition, the bosses may be of different depths, and it is possible to have both deep and shallow bosses on the same sheet. When sheet 10 is pleated and compressed as at 36 in Figure 3, a side view of the pleated assembly would appear as illustrated in Figure 4, but without items 40 in which the pleated assembly is indicated generally as 38. The embossing is omitted for clarity, but in actuality the fluid flow paths are defined by the bosses previously described. Inclined fluid entrance ramps are formed by edges 20, between notches 16 and points 18, and the pleated assembly is in part held together by means of corrugated strips 40 which are welded to points 18 across each end of the assembly. This is shown in Figure 5, which depicts an end view of the pleated assembly, not showing the corrugated strip 40. The pleated as¬ sembly is also held together by welds 42 and 44 which plug one side of the open ends of the fluid passages between sections 22 and 24, leaving the remaining sides 43 and 45 open. The passages are plugged at both ends, although only one end is shown in Figure 5. Additional external clamping may be provided to preload the heat exchanger pleats.

The junction of points 18 and welds 42 and 44 creates a relativley inelastic, rigid assembly. However, if such an inelastic area is attached to a relatively elastic area, high thermal stresses cannot be generated. Since the corrugated strip permits limited flexibility along the overall width of the edge 38* which is distant from the weld zone and provides flexibility, and therefore high thermal stresses are substantially elimi- nated.

It will be noted with reference to Figure 5, that the welds 42 above the corrugated strip 40 are staggered with relation to the welds 44 below the strip. Thus fluid entry to every other fluid passage occurs above the strip while fluid entry to the inter¬ mediate fluid passages occurs below the strip. How¬ ever, since the edges 20 bordering the entry to a

.VI fluid passage form an inclined ramp having a length equal to the total height of the pleated assembly, unrestricted fluid flow into each passage is assured. The heat exchanger module 46 of the present invention may be formed by stacking at least two pleated assemblies 38 within a housing 48 as shown in Figure 6. The upper pleated assembly 38a is placed over the lower pleated assembly 38b with a spacer 50 between them. The spacer 50 is essentially either a solid sheet or a mesh or perforated strip and is shown extending along the entire length of line 34 between sections of the pleated assemblies from point 52 to point 54. However, the spacer 50 may be placed so that it stops short of points 52 and 54. It is possible, by varying the thickness and the length of the spacer 50, to reduce fluid flow blockage be¬ yond the reduction achieved by means of the fluid entry ramps defined by edges 20. As previously men¬ tioned in discussing Figure 1, the length of edges 20 should be equal to pleat depth 14 to minimize fluid flow blockage. Although the ramps are shown to be straight, longer ramps may be achieved within the same dimensions by curving edges 20, thus length¬ ening the ramps while maintaining the compactness of the unit.

Hot and cold fluid manifolds are attached to the ends of the two stacked pleated assemblies as shown in Figure 6, and result in a low profile heat exchanger. Inlet manifolds 56 provide hot fluid, for example hot exhaust gas from a gas turbine, through fluid passages 58. This hot fluid follows the path shown by the white arrows 58 to outlet manifolds 60 which collect the previously hot fluid after heat has been transferred therefrom in the heat exchanger σore. Since the heat exchanger of the present inven¬ tion is a counterflow type heat exchanger, cool fluid, as, for example air from the compressor of a gas turbine engine, is supplied to the path shown by arrows 64 by a cool fluid inlet manifold 62. This cool fluid flows through the pleated assemblies 38a and 38b along the paths indicated by the dark arrows, is heated, and then is collected by an outlet mani¬ fold 66. The corrugated strips 40 are preferably welded to the housing 48 to form the manifolding. The width of corrugated strip 40 introduces a desired flexi¬ bility into the heat exchanger unit, as it allows the reduction of stresses in the presence of thermal gradients. It is also possible, but less desirable, to weld rigid manifolding directly to the pleated assemblies, which is not shown in Figure 6. However, this means of attaching the manifolding to the pleated assembly does not provide the flexibility and conse- quent dissipation of thermal stresses possible with the arrangement shown in Figure 6.

Figure 7 illustrates an end view of the heat exchanger module 46 used as a recuperator for re¬ ceiving hot exhaust gas and compressed air from a gas turbine engine. Hot gas input manifolds 56 are at the top and bottom and the hot combustion air output manifold 66 is in the center. Hot gas flows in alternate passages 58 while air to be heated flows in the opposite direction through the intermediate passages 64. The housing 48 closes the open ends of the passages 58.

A second embodiment of the present invention is diagrammatically shown in Figures 8, 9, and 10. Heat exchanger sheet material 72, which has been serrated along the longitudinal edges in the same manner as sheet 10 in Figure 1 and subsequently em¬ bossed, is bent between points 68 and 70 by passing it over shaped rollers, which are not shown in Figure 8, prior to pleating to form a sheet which is rippled in cross section (Figure 9). This structure provides flexibility to the pleats in the direction 68-70. Figure 10, which corresponds to Figure 3, shows the rippled sheet 72 being pleated and compressed into a bent pleated assembly 74. The method depicted in Figures 8, 9, and 10 results in increased flexibil¬ ity, not only in the pleats themselves, but also in the welded plugs 42 and 44 used to seal alternate flow passages as shown in Figures 5 and 7. Additional flexibility in the direction of sheet width 12 shown in Figure 1 can be introduced in still another embodiment of the present invention. The compressed pleated assembly shown at 36 in Figure 3 can be made to assume an arcuate shape or even an *S" shape, (not shown) when viewed in the direction of the crest portions or edges 34, as depicted in Figure 11.

Finally, the sheet 10 can be treated in an al¬ ternate method after pleating to enhance fluid flow and flexibility_* This is accomplished as shown in Figure 12. A pleated and compressed sheet, indi¬ cated generally at 76, is passed over a roller 78, which causes the pleats to separate as at 80 while remaining compressed at 82, thus allowing cams, rollers, pawls, or other suitable mechanisms to be introduced into the wide gaps at 80 to spread the pleats at edge 34 to any desired distance. After passing over roller 78, the pleated and compressed sheet passes over roller 84 where the wide gaps 80 become compressed

- K E

O PI at 86 and the pleats which were compressed at 82 become separated at 88, thus permitting the same type of cams, rollers or pawls to bow the pleats as at 34, resulting in a compact and bowed, pleated assembly after the core passes over the roller 84 as shown in Figure 11. This introduces flexibility in the direction of the width of the strip thus minimizing thermal stresses.

Industrial Applicability The heat exchanger module 46 may be effectively employed as a recuperator for a gas turbine engine or for other heat exchange applications, as, for example, in steel heat treating or melting furnaces. The inlet manifold 62 is connected to a source of cool fluid to be heated while the inlet manifolds 56 are connected to a source of heated fluid. In a gas turbine engine, the inlet manifolds 56 would be connected to receive hot exhaust gases from the engine while the inlet manifold 62 would be connected to receive compressor discharge air from the engine. As the cooler discharge air passes through the pleated assemblies 38a and 38b with the counter flowing hot exhaust gas, the air is heated by the heat transfer provided by heat exchange sections 22 and 24. The exhaust gas then passes out of the outlet manifolds 60 and is normally vented to the atmosphere while the heated air passes to the outlet manifold 66. This outlet manifold is connected to the σombustor of the gas turbine engine and proceeds on through the engine in the conventional manner.

Other aspects, objects and advantages of this invention can be obtained from a study of the draw¬ ings, the disclosure, and the appended claims.

Claims

1. A primary surface heat exchanger (46) com¬ prising:

(a) at least one heat exchanger core (38a) including a plurality of sequentially arranged, sub¬ stantially parallel, open ended fluid passages (58, 64) extending therethrough, one side of each open end of every other one of said fluid passages being closed (44) to form a first group of fluid passages (58) having adjacent aligned closed portions (44) and open portions (43) at either end thereof, the remaining fluid passages (64) intermediate said first group of fluid passages (58) having one side of each open end thereof closed (42) to provide adjacent aligned closed (42) and open portions (45) at the ends thereof, the open portions (43) of said first group of fluid passages (58) being adjacent and inter¬ mediate the closed portions (42) of said second group of fluid passages (64) , (b) first fluid inlet means (56) extending across the open portions (43) of said first group of fluid passages (58) and the closed portions (42) of said second group of fluid passages (64) at an end of said heat exchange core (38a) , (c) first fluid outlet means (60) positioned at the end of said heat exchange core (38a) opposite to said first fluid inlet means (56) and extending across the open portions (43) of said first group of fluid passages (58) and the closed portions (42) of said second group of fluid passages (64) at said end, said first fluid inlet and outlet means (56, 60) communicating with said first group of fluid passages (58) ,

O- PI (d) second fluid inlet means (62) extending across the open portions (45) of said second group of fluid passages (64) and the closed portions (44) of said first group of fluid passages (58) at an end of said heat exchanger core (38a) ,

(e) and second fluid outlet means (66) posi¬ tioned at the end of said heat exchanger core (38a) opposite to said second fluid inlet means (62) and extending across the open portions (45) of said second group of fluid passages (64) and extended across the closed portions (44) of said first group of fluid passages (58) , said second inlet and outlet means (62, 66) communicating wit said second group of fluid passages (64).

2. The primary surface heat exchanger (46) according to claim 1 wherein said first fluid inlet means (56) is positioend at a first end of said heat exchanger core (38a) and said second fluid inlet means (62) is positioned at a second end of said heat exchanger core, said second end being opposite to said first end.

3. The primary surface heat exchanger (46) according to claim 1 wherein said heat exchanger core (38a) is formed from a unitary strip of heat conducting material (10) formed to provide said plu¬ rality of interleaved fluid passages (58, 64) , each such fluid passage (58, 64) opening along one side of said heat exchange core (38a) in a direction op¬ posite to that of the next adjacent fluid passages on either side thereof.

4. The primary surface heat exchanger (46) according to claim 3 wherein said unitary strip of heat conducting material (10) forming said heat ex¬ changer core (38a) is pleated to define a plurality of sequential sections (22, 24) of substantially equal size which define said fluid passages (58, 64) and means (26, 28) extending between adjacent pleated sections (22, 24) to determine the space between such sections.

5. The primary surface heat exchanger (46) according to claim 4 wherein said means (26, 28) extending between adjacent pleated sections (22, 24) define fluid flow channels (27) in said fluid passages (58, 64), the fluid flow channels (27) in said first group of fluid passages (58) extending between the open portions (43) at the ends of said first group of fluid passages and the fluid flow channels (27) in said second group of fluid pass¬ ages (64) extending between the open portions (45) at the ends of said second group of fluid passages.

6. the primary surface heat exchanger (46) according to claim 5 wherein said means (26, 28) extending between adjacent pleated sections (22, 24) is formed to create turbulence in fluid flowing through said fluid flow channels 27.

7. The primary surface heat exchanger (46) according to claim 5 wherein said first fluid inlet means (56) is positioned at a first end of said heat exchanger core (38a) and said second fluid inlet means (62) is positioned at a second end of said heat exchanger core, said second end being opposite to said first end and said first and second fluid inlet means (56, 62.) operating to provide fluid flow through -said fluid flow channels (27) to said first and second fluid outlet means (60, 66) respectively.

8. The primary surface heat exchanger ( 46) according to claim 7 wherein said first and second fluid inlet and outlet means (56, 60, 62, 66) provide fluid flow in opposite directions through said first and second groups of fluid passages (58, 64).

9. The primary surface heat exchanger (46) according to claim 5 wherein said means (26, 28) extending between adjacent pleated sections are elong¬ ated bosses on said sequential sections (22, 24).

10. The primary surface heat exchanger (46) according to claim 9 wherein said elongated bosses (26, 28) are formed on only one side of every one of said sequential sections (22, 24), the bosses on adjacent sections (22, 24) extending from oppo¬ site sides thereof.

11. The primary surface heat exchanger (46) according to claim 10 wherein the bosses (26, 28) on adjacent sections (22, 24) are misaligned to pre¬ vent nesting of said sections.

12. The primary surface heat exchanger (46) according to claim 3 wherein said unitary strip of heat conducting material (10) forming said heat ex¬ change core (38a) is pleated to define a plurality of sequential sections (22, 24) of substantially equal size, adjacent sections (22, 24) forming two sidewalls joined at one edge (34) to define an open ended fluid passage (58, 64), and a strip (40) is attached to said sequential sections (22, 24) and extends across at least one end of said heat exchange core (38a) between the closed and open portions (42, 43, 44, 45) of said fluid passages (58, 64).

5 13. The primary surface heat exchanger (46) according to claim 12 wherein said strip (40) is corrugated to provide flexibility lengthwise thereof.

14. The primary surface heat exchanger (46) according to claim 13 wherein a strip (40) is se- Q cured to extend across opposite ends of said heat exchange core (38a) between said first and second fluid inlet and outlet means (56, 66, 60, 62).

15. The primary surface heat exchanger (46) according to claim 12 wherein the ends (20) of said 5 sections (22, 24) are formed to provide an inclined open side (43, 45) at either end of said passage

(58, 64), the length of said open sides (43, 45) being substantially equal to the height of said pass¬ ages.

0 16. The primary surface heat exchanger (46) according to claim 3 wherein siad unitary strip of heat conducting material (10) forming said heat ex¬ change core (38a) is pleated to define a plurality of sequential sections (22, 24) of substantially 5 equal size, adjacent sections (22, 24) forming two sidewalls joined at one edge (34) to define an open ended fluid passage (58, 64) .

17. The primary surface heat exchanger (46) according to claim 16 wherein said sections (22, 24) are arcuate in cross section to define curved fluid passages (58, 64) in said heat exchange core (38a).

18. The primary surface heat exchanger (46) according to claim 16 wherein said sections (22,

24) are formed to provide undulations in the side- walls formed thereby extending between the ends of said fluid passages (58, 64).

19. The primary surface heat exchanger (46) according to claim 18 wherein a strip (40) is at¬ tached to said sequential sections (22, 24) and ex¬ tends across at least one end of said heat exchange core (38a) between the closed and open sides (42, 43, 44, 45) of said fluid passages (58, 64), said strip (40) being corrugated to provide flexibility lengthwise thereof.

20. The primary surface heat exchanger (46) according to claim 19 wherein said sections (22, 24) are arcuate in cross section to define substan- tially curved fluid passages (58, 64) in said heat exchange core (38a) .

21. A primary surface heat exchanger (46) com¬ prising:

(a) at least a first heat exchange core (38a) , (b) a second heat exchange core (38b) ,

(c) each such heat exchange core including a plurality of interleaved, substantially parallel fluid passages (58, 64) formed by a unitary strip of heat conducting material (10) pleated to define a plurality of sequential sections (22, 24) of sub- stantially equal size, adjacent sections (22, 24) forming two sidewalls joined at one edge (34) to define an open ended fluid passage (58, 64), each such fluid passage opening along one side of said heat exchange core (38a, 38b) in a direction opposite to that of the next adjacent fluid passages on either side thereof, the fluid passages opening along one side of said heat exchange core in a first direction forming a first group of fluid passages (58) and the intermediate fluid passages opening along an opposite side of said heat exchange core in a second direction opposite to said first direction forming a second group of fluid passages (64) ,

(d) said first group of fluid passages (58) having a first portion of each open end thereof closed (44), said second group of said fluid passages (64) having a second portion_.of each open end thereof closed (42) , said second portion (42) being opposite to said first portion (44) whereby the remaining^' open portions (43) at the ends of said first group of fluid passages (58) are opposite to the remaining open portions (45) at the ends of said second group of fluid passages (64) ,

(e) said first and second heat exchange cores (38a, 38b) being relatively positioned so that the portions (45) thereof along which said second group of fluid passages (64) open are adjacent and in align¬ ment,

(f) closure means (48) secured to said first and second heat exchange cores (38a, 38b) to close the remaining open sides of said first group of fluid passages (58) along one side of each said first and second heat exchange cores (38a, 38b) ,

(g) first fluid inlet means (56) extending across the open portions (43) of said first group of fluid passages (64) at one end of said heat ex¬ change cores (38a, 38b) ,

(h) first fluid outlet means (60) positioned at the end of said heat exchange cores (38a, 38b) opposite to said first fluid inlet means (56) and extending across the open portions (43) of said first group of fluid passages (58) and the closed portions (42) of said second group of fluid passages (64) at said end,

(i) second fluid inlet means (62) extending across the open portions (45) of the second group of fluid passages (64) and the closed portions (44) of the first group of fluid passages (58) at an end of said first and second heat exchange cores (38a, 38b) , and

(j) second fluid outlet means (66) positioned at the end of said heat exchange cores (38a, 38b) opposite to said second fluid inlet means (62) and extending across the open portions (45) of said second group of fluid passages (64) and the closed portions (44) of said first group of fluid passages (58) .

22. The primary surface heat exchanger (46) according to claim 21 wherein a separator means (50) is positioned between said first and second heat exchange cores (38a, 38b) , said separator means being formed to permit fluid flow between the second group of fluid passages (64) in said first and second heat exchange cores (38a, 38b) .

23. The primary surface heat exchanger (46) of claim 21 wherein said fluid inlet means (56) in¬ cludes a first fluid inlet manifold (56) connected to a first end of said first heat exchange core (38a) and a second fluid inlet manifold (56) connected to a corresponding first end of said second heat exchanger core (38b) , said first fluid outlet means (60) includes a first fluid outlet manifold (60) connected to a second end of said first heat exchange core (38a) and a second fluid outlet manifold (60) connected to a corresponding second end of said second heat exchange core (38b) , said second fluid inlet means (62) includes a third single fluid inlet manifold (62) connected to the second ends of said first and second heat exchange cores (38a, 38b) between said first and second fluid outlet manifolds (60) , and said second fluid outlet means (66) includes a third single fluid outlet manifold (66) connected to the first ends of said first and second heat exchange cores (38a, 38b) between said first and second fluid inlet manifolds (56) .

24. The primary surface heat exchanger (46) according to claim 23 wherein said first and second fluid inlet and outlet means includes a housing (48) spaced from and enclosing opposite ends of said first and second heat exchange cores (38a, 38b) , said first and second heat exchange cores (38a, 38b) each includ- ing a strip (40) whcih is attached to said sequential sections (22,^' 24) and extends across the ends of such heat exchange core (38a, 38b) between the closed and open portions (42, 43, 44, 45) of said first and second groups of fluid passages (58, 64), said strips (40) extending outwardly from the ends of said first and second heat exchange cores (38a, 38b) and being attached to said housing (48) to divide the interior of said housing into said first, second. and third fluid inlet and outlet manifolds (56, 62, 60, 66).

25. The primary surface heat exchanger (46) according to claim 24 wherein said strip (40) is corrugated to provide flexibility lengthwise thereof.

26. The primary surface heat exchanger (46) of claim 21 wherein the ends 20 of the sections (22, 24) of each heat exchange core (38a, 38b) are formed to provide an inclined open side (43, 45) at either end of said fluid passages (58, 64), the length of said open sides (43, 45) being substantially equal to the height of said passages.

27. A method for forming a primary surface heat exchanger (46) which includes: (a) forming a heat exchange core (38a) having a plurality of sequentially arranged, substantially parallel open ended, fluid passages (58, 64) extend¬ ing therethrough,

(b) closing one portion (44) of each open end of every other one of said fluid passages (58) to form a first group of fluid passages (58) having adjacent aligned closed portions (44) and open por¬ tions (43) at either end thereof and

(c) closing one portion (42) of each open end of the remaining intermediate fluid passages (64) which is opposite to the closed portion (44) of said first group of fluid passages (58) to form a second group of fluid passages (64) having adjacent aligned closed portions (42) and open portions (45), the open portions (43, 45) of each group of fluid passages being adjacent the closed portions (42, 44) of the remaining group of fluid passsages.

28. The method for forming a primary surface heat exchanger (46) according to claim 27 which in¬ cludes securing a first fluid inlet manifold (56) at one end of said heat exchange core (38a) to extend across the open portions (43) of said first group of fluid passages (58) and the closed portions (42) of said second group of fluid passages (64) so as to communicate with said first group of fluid passages (58), securing a first fluid outlet manifold (60) to an end of said heat exchange core (38a) opposite to said first fluid inlet manifold (56) to extend across the open portions (43) of said first group of fluid passages (58) and the closed portions (42) of said second group of fluid passages (64) so as to communicate with said first group of fluid pass¬ ages (58) , securing a second fluid inlet manifold (62) to one end of said heat exchange core (38a) to extend across the open portions (45) of said second group of fluid passages (64) and the closed portions (44) of said first group of fluid passages (58) so as to communicate with said second group of fluid passages (64), and^* securing a second fluid outlet manifold (66) to an end of said heat exchange core (38a) opposite to said second fluid inlet manifold

(62) to extend across the open portions (45) of said second group of fluid passages (64) and the closed portions (44) of said first group of fluid passages (58) so as to communicate with said second group of fluid passages (64) .

29. The method for forming a primary surface heat exchanger (46) according to claim 28 which in- cludes securing said first fluid inlet manifold (56) and said second fluid outlet manifold (66) to one end of said heat exchange core (38a) , and said second fluid inlet manifold (62) and first fluid outlet manifold (60) to an opposite end of said heat ex¬ change core (38a) .

30. A method for forming a primary surface heat exchanger which includes:

(a) dividing an elongated unitary strip of heat conducting material (10) into a plurality of sequential sections (22, 24) of substantially equal size,

(b) pleating said elongated strip (10) along the dividing lines (34) between sections (22, 24) to form a plurality of interleaved, substantially parallel fluid passages (58, 64), the open side of each such fluid passage opening in a direction op¬ posite to that of the next adjacent fluid passages on either side thereof, (c) closing one portion (44) of each open end of every other one of said fluid passages (58) to form a first group of fluid passages (58) having adjacent, aligned closed portions (44) and open por¬ tions (43) at either end thereof, and (d) closing one portion (42) of each open end of the remaining intermediate fluid passages (64) which is opposite to the closed portion (44) of said first group of fluid passages (58) to form a second group of fluid passages (64) having adjacent aligned closed portions (42) and open portions (45) , the open portions (43, 45) of each group of fluid passages being adjacent the closed portions (42, 44) of the remaining group of fluid passages.

/^ UR 31. The method for forming a primary surface heat exchanger according to claim 30 which includes cutting said unitary strip of heat conducting material (10) along each side thereof to a sawtooth configura- tion prior to pleating to provide an apex (18) at each end of each sequential section (22, 24) at the center of said section and an edge (20) inclined away from either side of said apex (18) to a point (16) at the dividing line (34) between sections.

32. The method for forming a primary surface heat exchanger according to claim 31 which includes forming said inclined edges (20) to be at least sub¬ stantially equal in length to the distance (14) across a section (22,24) between the dividing lines (34) with the adjacent sections on either side thereof.

33. The method for forming a primary surface heat exchanger according to claim 31 which includes securing an elongate strip (40) across each end of said unitary strip (10) after the pleating thereof, said elongate strip (40) being secured to the apex (18) of each of said sequential sections (22, 24).

34. The method of claim 33 which includes closing the ends of each fluid passage in said first group of fluid passages (58) along the inclined edges (20) extending in a first direction away from said apex (18) and closing the ends of each fluid passage in said second group of fluid passages (64) along the inclined edges (20) extending in a second direction away from said apex (18) , said second direction being opposite to said first direction. 35. The method for forming a primary surface heat exchanger according to claim 30 which includes bending said unitary strip (10) into an arcuate configuration after pleating to cause said sequen- tial sections (22, 24} to be arcuate in cross section along a line extending between the dividing lines (34) between said sections.

36. The method for forming a primary surface heat exchanger according to claim 30 which includes forming undulations extending across said unitary strip (10) between the side edges thereof prior to pleating such strip (10) .

37. A heat exchanger core unit (38) comprising:

(a) a pleated sheet (10) having a plurality of walls (22, 24), each wall having an outwardly extending apex (18) at each end thereof,

(b) interconnecting, alternating first and second crest portions (34) extending between said walls (22, 24), and (c) closure means (42, 44) extending between the ends of adjacent walls (22, 24), said closure means (42, 44) being formed to close the space be¬ tween said walls (22, 24) from each of the crest portions (34) to said apexes (18) in a repetitively alternating opposite manner at each end of said core unit (38) to define alternating fluid flow passages (58, 64) between said walls 922,24).

38. The heat exchanger core unit (38) of claim 37 which includes spacing means (26, 28) extending between adjacent walls (22, 24) to determine and maintain the spacing between said walls (26, 28) .

39. The heat exchanger core unit (38) of claim 37 which includes guide means (26, 28) on said walls ( 22, 24y for guiding the flow of fluid within said passages (58, 64).

40. The heat exchanger core unit (38) of claim 37 which includes separator means (40) for separating fluid flow to and from said alternating fluid flow passages (58, 64), said separator means (40) being connected to extend across said apexes (18) .

41. A method for making a heat exchanger core unit (38) which includes:

(a) forming a plurality of serrations (16) in the edge of a strip (10) of material,

(b) pleating the strip (10) between each of said serrations (16) to form a plurality of crest portions (34) and a plurality of alternating fluid flow passages (58, 64) , and

(c) closing off a portion (42, 44) at opposite ends of each said flow passage (58, 64) in a repeti- tive alternating pattern, the closure (42, 44) of adjacent flow passages (58, 64) extending from the crest portion (34) for said flow passages toward each other.

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