US3399720A - Plate heat exchanger - Google Patents

Description

Sept. 3, 1968 H. DOELZ ET AL 3,399,720

PLATE HEAT EXCHANGE-R Filed Sept. 50, 1966 5 Sheets-Sheet 1 mass; l

"8 I u c- E '4- 3 Fig 1 Badman w /fl'kr fheir ATTORNEY Sept. 3, 1968 H. DOELZ ET AL 3,399,720

PLATE HEAT EXCHANGER Filed Sept. 30, 1966 5 Sheets-Sheet 2 fheir ATTORNEY p 3, 1968 H. DOELZ ETAL 3,399,720

PLATE HIS AT EXCHANGER Filed Sept. 30, 1966 5 Sheets-Sheet 3 Fig. 3

INVENTORS 28m Alia-6 r Iii f h 15:06,. w J zMM mm, \helr ATTORNEY Sept. 3, 1968 DOELZ ET AL 3,399,720

PLATE HEAT EXCHANGER Filed Sept. 30, 1966 5 Sheets-Sheet 4 Fig.4

u INVENTORS Al -w 03A Sept. 3, 1968 H. DOELZ ET AL 3,399,720

PLATE HEAT EXCHANGER Filed Sept. 30, 1966 5 Sheets-Sheet 5 nited States Patent 9 3,399,720 PLATE HEAT EXCHANGER Hans Doelz, Halle, Edgar Hipperling, Reichenhach, and Joachim Seidel, Mylau, Germany, assignors to VEB Apparatebau Mylan, Mylau, Plauen, Germany Filed Sept. 30, 1966, Ser. No. 583,405 13 Claims. (Cl. 165166) ABSTRACT OF THE DISCLOSURE The present invention relates to improvements in plate heat exchangers.

It is an important object of the present invention to provide a very compact, lightweight, rigid and inexpensive plate heat exchanger.

Another object of the invention is to provide a plate heat exchanger of the just outlined characteristics whose efliciency at least equals that of presently known plate heat exchangers and which may be utilized in existing mechanical refrigerating machines, especially in vaporcompression machines.

A further object of the invention is to provide a plate heat exchanger wherein the parts which confine the fluids and thus direct their flow simultaneously perform the function of stiffeners so that no special reinforcing parts are needed to lend necessary rigidity to the body of the heat exchanger.

An additional object of our invention is to provide a heat exchanger wherein the flow of at least one fluid may be obstructed in a very simple way to insure more effective exchange of heat with the other fluid.

A concomitant object of the invention is to provide a plate heat exchanger which may be assembled of very simple components, whose components may be massproduced by resorting to known machinery, which may be assembled by resorting to automatic machines so that the number of manual operations and man hours may be reduced to a minimum, and which can be utilized to bring about exchange of heat between fluids while such fluids are subjected to elevated pressures.

Briefly stated, one feature of the present invention resides in the provision of a method of exchanging heat between a cooler fluid and a warmer fluid. The method comprises the steps of conveying the fluids along two substantially U-shaped paths one of which surrounds the other thereof, and separating the paths from each other by relatively thin layers of heat-conducting material which takes up heat from the warmer fluid and transmits it to the cooler fluid.

The cross-sectional area of the path for cooler fluid is preferably greater than the cross-sectional area of the path for warmer fluid. The cooler fluid may be in gaseous state and the warmer fluid may be in liquid state. For

ice

example, and if the method is resorted to in a refrigerating operation, the warmer fluid may be maintained at a temperature approximating room temperature.

Each of the two paths may be subdivided into a plurality of narrower paths extending in the direction of fluid flow, and such subdivision may be carried out by inserting into the paths thin layers of heat-conducting material and by placing such thin layers in heat-conducting contact with the first-mentioned layers. In many instances, we prefer to place into one of the paths suitable obstructions which produce turbulence in the respective fluid and thus enhance the exchange of heat between such fluid and the aforementioned layers.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved heat exchanger itself, however, both as to its construction and the mode of assembling the same, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a refrigerating machine comprising a plate-type heat exchanger which embodies the present invention;

FIG. 2 is a greatly enlarged perspective view of the heat exchanger with a corner portion broken away;

FIG. 3 is a longitudinal central vertical section through the heat exchanger, substantially as seen in the direction of arrows from the line IIIIII of FIG. 2;

FIG. 3a is a diagrammatic longitudinal sectional View through a partially assembled heat exchanger which is somewhat different from the heat exchanger of FIGS. 2 and 3;

FIG. 4 is a transverse section substantially as seen in the direction of arrows from the line IVIV of FIG. 3 and illustrates one mode of bonding the components of a heat exchanger to each other;

FIG. 5 is a similar transverse section and illustrates another mode of bonding the parts of a heat exchanger to each other;

FIG. 6 is another transverse section and illustrates a third mode of bonding the parts of a heat exchanger to each other;

FIG. 7 is a smaller-scale diagrammatic perspective view of a heat exchanger which is constructed in a manner as shown in FIGS. 2 and 3;

FIG. 8 is a diagrammatic perspective view of a battery of two serially connected heat exchangers;

FIG. 9 illustrates in perspective view a battery of three serially connected heat exchangers;

FIG. 10 is a perspective view of an assembly which comprises two batteries of the type shown in FIG. 8 and wherein the heat exchangers of the adjoining batteries are connected in parallel; and

FIG. 11 is a perspective view of a modified assembly comprising two batteries of three serially connected heat exchangers each.

Referring first to FIG. 1, there is shown a mechanical refrigerating machine embodying a plate type heat exchanger 31 which is constructed and assembled in accordance with our invention. The exact construction of the remaining parts of the refrigerating machine is known and is described here for the sole purpose of fully disclosing oneadvantageous utilization of the improved heat exchanger.

The refrigerating machine of FIG. 1 is a simple vaporcompression machine comprising a compressor unit 20 which includes a prime mover and a compressor proper, preferably a reciprocating compressor which is driven by an electric motor. Gaseous refrigerant which is compressed in the unit 20 passes through a discharge valve 21 and a discharge line 22 into the coils 24 of a condenser unit 23. The coils 24 may be cooled by currents of water and/or fresh air. In the present instance, the coils are cooled by currents of air generated by a fan 27 whichis drven by the output shaft 26 of an electric motor. Such currents pass along the fins 25 of the coils 24 and the thus liquefied refrigerant is conveyed through a conduit 28 and enters a collecting tank 29. A conduit 30 conveys liquid refrigerant from the tank 29 and into one duct of the heat exchanger 31. A further conduit 32 conveys liquid refrigerant (which is pressurized by the compressor of the unit 20) to a thermostatically controlled expansion valve 33. The heat necessary for evaporation in the expansion coil 35 which receives refrigerant from the outlet 34 f the expansion valve 33 is withdrawn from the area surrounding the coil 35. The gasified refrigerant then passes through a conduit 36 and its temperature is measured by a thermostat 37 which controls the expansion valve 33 through a line 38. The conduit 36 discharges into a second duct of the heat exchanger 31 (this second duct is indicated diagrammatically as being constituted by the passage defined by a meandering coil 39), and the second duct admits heated refrigerant to a suction conduit 40 connected to a suction valve 41 which in turn admits refrigerant into the compressor of the unit 20. The cycle is then repeated in the just described manner.

In the refrigerating machine of FIG. 1, the heat exchanger 31 performs the important function of bringing about exchange of heat between the highly compressed liquid phase admitted by the conduit 30 and the gaseous phase admitted by the conduit 36. The liquid phase which enters the heat exchanger 31 is warm, i.e., its temperature approximates or slightly exceeds room temperature or the temperature of ambient air in the plant where the machine is put to use. The gaseous phase which is admitted by the conduit 36 is cold and this gaseous phase withdraws heat from the liquid phase. The cooling effect is improved if the heat exchanger 31 (and more particularly the gaseous phase flowing through this heat exchanger) withdraws more heat from the liquid phase which is compelled to flow toward the expansion valve 33. On the Other hand, and particularly if the refrigerant is dichlorodifluoromethane (CCl F or simply Freon 12), heating of the gaseous phase on its way to the suction side of the compressor in the unit results in improved performance of the compressor so that the exchange of heat in the heat exchanger 31 is desirable for a plurality of reasons. Such exchange of heat between the liquid and gaseous phases of the refrigerant results in substantial savings in energy and reduces the operating cost of the refrigerating machine.

The thermostat 37 measures the temperature of the gaseous phase and regulates the expansion valve 33 in dependency on such temperature, i.e., the valve controls the rate at which the liquid phase is admitted into the expansion coil 35 to prevent flooding of this coil and eventual admission of liquefied refrigerant to the suction side of the compressor.

FIGS. 2 and 3 show the heat exchanger 31 on a greatly enlarged scale and in full detail. This heat exchanger comprises two housings including an outer housing A composed of two mirror symmetrical pan-shaped shells 54, 55 and an inner housing B composed of two mirror symmetrical shells 51, 52. The two housings define between themselves a first or outer U-shaped duct 56 which serves to convey the warmer fluid, i.e., the liquid phase of the refrigerant utilized in the machine of FIG. 1. The inner housing B accommodates a plate-like separator 57 which 4 defines therewith a second or inner U-shaped duct 53 serving to convey the cooler fluid, i.e., the gaseous phase of the refrigerant. The inner duct 53 is surrounded by the outer duct 56 and the relatively thin walls of the inner housing B serve as 'a means for conducting heat from the warmer fluid to the cooler fluid. Since the temperature of the Warmer fluid is not substantially different from the temperature of the air surrounding the outer housing A, the exterior of the heat exchanger 31 need not be insulated when the latter is used in a refrigerating machine of the type shown in FIG. 1. The separator 57 is disposed in the central symmetry plane of the heat exchanger.

The exact configuration of the ducts in the interior of the heat exchanger 31 will be more readily understood by referring shortly to FIG. 3a which shows, very diagrammatically, one mode of forming a heat exchanger similar to that illustrated in FIGS. 2 and 3. One can start with a plate-like separator 301, with a first pan-shaped shell 302 whose open side is adjacent to the upper side of the separator 301, and with a second pan-shaped shell 303. By bending the structure of FIG. 311 about an axis 304 extending transversely of the separator 301, one would obtain a heat exchanger which is similar to the heat exchanger 31 of FIGS. 2 and 3. The space 310 between the separator 301 and the inner shell 302 would form an inner duct corresponding to the duct 53 and the space 311 between the shells 302, 303 would form an outer duct corresponding to the duct 56. Pipes 306, 307 would respectively admit and evacuate one phase from the space 311 and pipes 308, 309 would respectively admit and evacuate the other phase from the space 310.

However, and since the production of the heat exchanger is simplfied if the bending step of FIG. 3a is dispensed with, we prefer to assemble the heat exchanger 31 in a manner as outlined in connection with FIGS. 2 and 3, namely, by assembling each of the housings A and B of two separate shells (54, 55 and 51, 52) and by utilizing a separator 57 which need not be bent over itself.

Referring now again to FIGS. 2 and 3, the upper shell 54 of the outer housing A comprises a bottom wall or panel 54a of rectangular outline, two longitudinally extending side walls 54b, 54c and two transversely extending side walls 54d, 54c. Where shown, the corresponding walls of the other three shells 55, 51 and 52 are respectively denoted by similar reference numerals 55a55e, Sla-Sle and 52a52e. The side walls 51b51e, 52b-52e, 54b-54e and 55b55e are respectively provided with outwardly extending marginal portions or flanges 51 52 54f and 55 which are assembled into a package whereby the flanges 51f, 52f abut against the opposite sides of the marginal portions of the separator 57. In addition, the flanges 55f of the lower shell 55 are bent over the flanges 54 of the upper shell 54, as shown at 55g, so as to fully conceal the flanges 51 52 54f and to form a continuous annular bead around the separator 57. The manner in which the flanges 51 52 54f, 55 are sealingly bonded to each other will be described in connection with FIGS. 4 to 6. It will be noted that the bentover portions 55g of the flanges 55] by themselves maintain the four shells 51, 52, 54 and 55 and the separator 57 in assembled condition.

The separator 57 has an opening or cutout 57a which is located in a central or median zone or portion 530 of the inner duct 53 and is adjacent to the side walls 510, 52s. This separator 57 is also provided with a second opening or cutout 57b which is located in the central or median portion or zone 560 of the outer channel 56 and is adjacent to the side walls 542, 552. The opening 57!) registers with openings or cuts out 51h, 5211 provided in the flanges 51f, 52) of the side walls 51c, 52a.

The opening 5212 is formed by bending from the plane of the flange 52 a flap 52g which extends through the registering openings 57b, 5111 and is bent over the adjoining portion of the flange 51 to form a bead which holds the parts 51c, 52c and 57 together.

The two elongated portions or the inner duct 53 which extend along the inner sides of the bottom walls 51a, 52a and along the adjoining sides of the separator 57 accommodate elongated fins 58 of thin heat-conducting ductile material which divide the corresponding portions of the ducts 53 into a number of narrower passages 0r ducts 53a clearly shown in FIG. 2. Such panels 58a of the fins 58 which extend between the bottom walls 51a, 52a and the separator 57 are preferably bent back and forth to form sharply defined pockets 58b. Undulate fins 59 of thin heat-conducting ductile material are inserted between such portions of the outer duct 56 which extend between the bottom walls 51a, 54a and 52a, 55a to form narrower ducts or passages 56a. The passages 53a, 56a extend in the direction of fluid flow through the ducts 53, 56.

The specific configuration of the panels of the fins 58 products in the duct 53 a certain turbulence which enhances the exchange of heat between the two fluids. The pockets 581) are preferably provided at regular intervals and the pockets in the upper arm of the duct 53 may but need not be accurately aligned with the pockets in the lower arm.

The fins 58 and 59 are shorter than the bottom walls 510, 52a and 54a, 55a so that the leftmost portions of the ducts (as viewed in FIG. 3) are free of such fins. These portions of the ducts 53, 56 are respectively connected with fluid admitting pipes 64, 68 and with fluid evacuating pipes 65, 69. The bottom walls 51a, 52a, 54a, 55a are respectively provided with outwardly extending annular sockets 60, 61, 62, 63 which receive the ends of pipes 64, 65 in a manner best shown in FIG. 3. The bottom walls 54a, 55a are provided with additional sockets 66, 67 which receive the end portions of the pipes 68, 69. The fins 59 in the outer duct 56 preferably extend between the inner end portions of the pipes 68, 69 and the side walls 51e, 52e, and the fins 58 preferably extend between the inner end portions of the pipes 64, 65 and short of the side walls 51e, 52e so that they leave at least a portion of the opening 57a in the separator 57 exposed. As shown in FIG. 3, fluid entering the outer duct 56 via pipe 68 and leaving via pipe 69 flows in the same direction as the fluid which enters the duct 53 via pipe 64 and leaves via pipe 65. Thus, all of the pipes 64, 65 and 68, 69 can be installed at one and the same longitudinal end of the heat exchanger. The end faces of the sockets 60, 62 and 61, 63 are preferably located in common planes so that the gaps between such pairs of concentric sockets can be readily sealed to prevent communication of the duct 56 with the surrounding atmosphere. The inner end portions of the pipes 64, 65, 68, 69 are bounded by end faces which are inclined in a. manner as clearly shown in FIG. 3. Thus, the inner end faces of the pipes 64, 65 respectively extend from the separator 57 to the bottom walls 51a, 52a, and the inner end faces of the pipes 68, 69 respectively extend from the bottom walls 51a, 52a to the bottom walls 54a, 55a. Such inclination of their inner end faces enables the pipes 64, 68 to automatically direct fluids toward the intake ends of the upper passages 53a, 56a whereas the intake ends of the pipes 65, 69 face the discharge ends of the lower passages 53a, 56a. The end zones of the ducts 53, 56 which are free of fins 58, 59 and which respectively accommodate the inner end portions of the pipes 64, 65 and 68, 69 are respectively denoted by reference numerals 53b and 56b. The upper zones 53b, 56b can be called distributing compartments because they distribute fluids admitted by the pipes 64, 68 into the passages 53a, 56a. The lower zones 53b, 56b can be called collecting compartments because they receive fluid streams from the lower passages 53a, 56a and their contents can be evacuated via pipes 65, 69.

The cross-sectional area of the outer duct 56 is smaller than the cross-sectional area of the inner duct 53. This is due to the fact that the heat exchanger 31 of FIGS. 2

and 3 has been shown as having a form which is especially satisfactory for use in a refrigerating machine wherein the volume of the gas flowing through the duct 53 is a multiple of the volume of the liquid passing through the duct 56. The ratio of the two volumes may be as high as 200:1. This must be borne in mind in designing and assembling a plate type heat exchanger for use in refrigerating machines, i.e., it will have a bearing upon the relationship between the size and weight on the one hand and the efliciency of the heat exchanger on the other hand.

The feature that the outer U-shaped duct 56 surrounds the inner U-shaped duct 53 contributes to the compactness of our improved heat exchanger. It is also novel to convey the liquid phase in the outer duct 56 so that the outer housing A need not be insulated at all if the temperature of the liquid phase approximates the temperature of surrounding air. In a refrigerating machine of the type shown in FIG. 1, the temperature of the liquid phase will range between 35-40 C.

Another very important advantage of the heat-exchanger 31 is that it offers unexpectedly high resistance to deformation despite the fact that its components may and preferably consist of thin sheet or plate stock. This is attributed to the utilization of pan-shaped shells 51, 52, 54, 55. Of course, the exchange of heat between the two phases is more satisfactory if the walls of the shells 51, 52 and of the separator 57 are thin, and the utilization of thin plate or sheet stock renders it possible to produce a heat exchanger of surprisingly low weight.

Various modes of connecting the shells of the heat exchanger 31 to each other and with the fins 58, 59 and separator 57 are illustrated in FIGS. 4, 5 and 6. These parts are preferably connected to each other by bonding, for example, by soldering or brazing.

FIG. 4 illustrates length of copper wire 70 which are inserted in all such areas where the material of such wire should form fluidtight joints between the fins 58, 59 and the parts 57, 51a, 52a, 54a, 55a and also between the flanges 51], 52 54f, 55 and separator 57. The distribution of wires 70 is preferably such that, during melting, their material forms uniform seams along all such portions which are to be bonded to each other. As shown, wires 70 are inserted into the passages 53a, 56a and also along the exposed end face of the outer bead 55g. During heating, the heat exchanger 31 is preferably held in a position in which one longitudinally extending portion of the bead 55g rests on a suitable support.

FIG. 5 illustrates another mode of bonding the various parts of a heat exchanger 31 to each other. In this embodiment of the invention, the wires 70 of FIG. 4 are replaced by foils 71 which coat both sides of the separator 57, both sides of each of the inner shells 51 52, and the inner sides of the outer shells 54, 55. Portions of the foils 71 extend between the adjoining flanges 51 52 54], 55 as well as between such flanges and the marginal portions of the separator 57 so that even the flanges are bonded to each other and to the separator.

It is also possible to utilize shorter or narrower sheets or foils 72 (see FIG. 6) and to utilize some wires 70 whereby the foils 72 bond the fins 58, 59 to the bottom walls 51a, 52a, 54a, 55a and to the separator 57 whereas the wires 70 form seams which provide fluidtight seals along the exposed end face of the bead 55g. The method illustrated in FIG. 6 requires more work but the savings in foil stock are considerable.

Though not shown in FIGS. 4 to 6, the sockets 60-63 and 6667 are also soldered or brazed to the pipes 64, 65 and 68, 69 to prevent leakage. Such bonding operation can be carried out by placing rings of wire 70 along these sockets. The sockets may be formed with annular ribs (not shown) which extend into complementary annular grooves of the adjoining pipes, and the wires 70 may be forced into such grooves to insure that, after heating and subsequent cooling, the material of such wires will invariably form fluidtight seals around the pipes. Alternatively, the end portions of the pipes 64, 65, 68, 69 may have smaller diameters than the remainder of such pipes so that each pipe develops an annular shoulder which is placed adjacent to a ring of copper wire abutting against the respective socket or sockets to insure the formation of circumferentially complete annular seams. Such rings of copper wire can be clamped in position up to the time when their material is fused in response to heating in a soldering furnace.

Referring finally to FIGS. 8 to 11, there are shown several possible combinations of two or more heat exchangers 31 of the type schematically illustrated in FIG. 7. In FIG. 8, two heat exchangers 31 are assembled to form a battery of serially connected components whereby the fluid evacuating pipes 65, 69 of the upper heat exchanger extend into and constitute the fluid admitting pipes for the lower heat exchanger. The lower ends of the pipes 65, 69 of the upper heat exchanger are preferably provided with inclined end faces as illustrated in FIG. 3. The common pipes 65, 69 of the two heat exchangers may be very short so that the heat exchangers can be located in two closely adjacent planes. The free ends of the pipes 64, 68 and 65, 69 are coupled with suitable flexible or rigid conduits (not shown) which connect the battery into the machine of FIG. 1 or into another machine where the heat exchangers are being used.

FIG. 9 shows a battery of three serially connected heat exchangers 31 which form a single file whereby only the uppermost heat exchanger comprises four pipes 64, 68, 65, 69.

If the heat exchangers 31 are disposed in two or more rows, for example in two rows each of which comprises two serially connected heat exchangers 31 in a manner as illustrated in FIG. 10, the adjoining heat exchangers in the two rows have their fluid admitting pipes 64, 68 connected by h aders 73, 74 which receive fluids through supply conduits 77, 78. The fluid evacuating pipes 65, 69 of the two lower heat exchangers 31 are connected with headers 75, 76 having discharge conduits 79, 80. It will be seen that the heat exchangers of one row are connected in parallel with the heat exchangers of the other row.

FIG. 11 illustrates an assembly of six heat exchangers 31 disposed in two rows of three serially connected heat exchangers each. Such an assembly is obtained by introducing into the assembly of FIG. two intermediate heat exchangers.

It is further clear that the heat exchangers 31 may be assembled into batteries which comprise three or more rows or into batteries comprising four, five or more heat exchangers disposed in a single row.

Without further analysis, the foregoing will so fully reveal the gist of the present invention, that others can, by applying current knowledge, readily adapt it for 'various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is:

1. A plate-type heat exchanger comprising an outer housing and an inner housing received in said outer housing and defining therewith a U-shaped outer duct having a pair of legs for passage of a first fluid therethrough, each of said housings comprising a pair of substantially panshaped shells having marginal portions sealingly secured to each other; separator means installed in and defining with said inner housing a second U-s'haped duct having a pair of second legs for passage of a second fluid therethrough whose temperature is different from that of the first fluid, said separator means having marginal portions received between said marginal portions of said housing and being formed in the region of one end of said heat exchanger with a pair of openings, one located in said outer housing and providing communication between said pair of first legs and the other being located in said inner housing and providing communication between said pair of second legs, said inner housing consisting of heat conductive material to provide for heat exchange between said fluids; a first pair of passages communicating respectively with said first pair of legs in the region of the other end of said heat exchanger for respectively feeding said first fluid int-o one end of one of said first pair of legs and for discharging the first fluid from the corresponding end of the other of said pair of first legs; and a second pair of passages communicating respectively with said pair of second legs in the region of said other end of said heat-exchanger for respectively feeding said second fluid into one end of one of said pair of second legs and for discharging the first fluid from the corresponding end of the other of said pair of second legs.

2. A heatexchanger as set forth in claim 1, wherein each of said pan-shaped shells has a substantially flat bottom wall extending substantially parallel to said separator means, and wherein said passages are formed by pipes extending substantially normal to said bottom walls.

3. A heat exchanger as set forth in claim 2, wherein said bottom walls are provided with sockets integrally formed therewith and wherein said pipes extend with end portions thereof through said sockets sealingly connected thereto.

4. A heat exchanger as set forth in claim 3, wherein the pipes forming said first pair of passages and the pipes forming said second pair of passages are respectively coaxially arranged.

5. A heat exchanger as set forth in claim 4, wherein said bottom walls of said shells forming said outer housing are each formed with a pair of said sockets projecting outwardly from the respective wall and wherein said sockets formed in said bottom walls of said shell forming said inner housing respectively project through one of said pair of sockets in the corresponding outer shell and being sealingly connected thereto.

6. A heat-exchanger as set forth in claim 1, wherein said marginal portions of said shells are constituted by outwardy extending rim portions, and wherein one of said pair of shells forming said outer housing has a rim portion which is wider than the other rim portion of said pair and is folded over said other rim portion to secure said rim portions to each other.

7. A heat-exchanger as set forth in claim 6, wherein said rim portions of the shells forming said inner housing are sandwiched between said rim portions of the shells forming said outer housing, and wherein said one opening extends also through said rim portions of said inner shells.

8. A heat-exchanger as set forth in claim 7, wherein part of the trim portion of one shell forming said inner housing extends through said one opening and is folded over a corresponding part of the rim portion of the other shell forming said inner housing.

9. A heat exchanger as set forth in claim 1, further comprising fins provided in each of said ducts and subdividing the respective ducts into plural passages extending in the direction of fluid flow through said ducts.

10. A heat exchanger as set forth in claim 9, wherein the fins in one of said ducts are provided with flow obstructing portions to generate turbulence in the respective fluid.

11. A heat exchanger as set forth in claim 1, wherein said separator means comprises a panel disposed in a predetermined plane and wherein each of said housings comprises two halves which are substantially mirror symmetrical with reference to such plane.

3,399,720 9 1O 12. A heat exchanger as set forth in claim 3, wherein References Cited the end portions of said pipes are bounded by end faces UNITED STATES PATENTS located in planes which make acute angles with the axes of the respective pipes 2,571,631 10/1951 Trumpler 165-166 :13, A heat exchanger as set forth in claim 1, further 5 2940736 6/1960 O1man 165 166 comprising fins provided in each of said ducts and sub- 2979 ,310 4/1961 Nlcholson 165141 dividing the respective ducts into plural passages extend- FOREIGN PATENTS ing in the direction of fluid flow through such ducts, each 25 687 12/1899 Great Britain of said ducts comprising a pair of elongated portions which accommodate the respective fins and a median 10 ROBERT OLEARY Primwy Examiner. portion free of fins and connecting the respective longer portions CHARLES SUKALO, Asszstant Exammer.

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