US2693636A - Process for fabrication of heat exchangers - Google Patents

Description

c. s. SIMPELAAR PROCESS FOR FABRICATION OR HEAT ExOHANGERs FiledApril 1, 1949 A O N/ f/// /1 /1 l/1/ /l /1 ,l ,l ,l /1 1/ I l 1/ l Nov. 9, 1954 United States Patent() PROCESS FOR FABRICATION F HEAT EXCHANGERS Clyde S. Simpelaar, Racine, Wis., assignor to Modine Manufacturing Company, Racine, Wis., a corporation of Wisconsin Application April 1, 1949, Serial No. 84,885

9 Claims. (Cl. 29-470) The invention relates generally to heat exchangers, and the like, and more particularly to a novel process for fabricating the same. The invention is particularly adapted for use in fabricating heat exchangers s uch as those illustrated in my co-pending application Serial No. 780,251, iiled on October 16, 1947, now U. S. Patent No. 2,606,007, granted August 5, 1952, in which the var1- ous elements comprising the heat exchanger are bonded into a single integral unit.

The invention has among its objects the utilization of a novel process whereby a perfect bond between the various elements of the structure, having high corrosive resistance, is achieved, with the elimination of all internal stresses in the various bonded joints.

Another object of the invention is the utilization of such a process by means of which such as exchanger may be eiiciently bonded in a minimum of time and utilizing a minimum amount of heat to perform the bonding operation, thereby also reducing the cost of manufacture.

A further object of the invention is the utilization of a novel process which may be practiced with the use of relatively simple and inexpensive machinery, at the same time permitting the eicient fabrication of relatively large, heavy heat exchangers, with the elimination of destructive annealing of the respective elements and resultant weakening of the fabricated exchanger.

Many other objects and advantages of the construction herein shown and described will be obvious to those skilled in the art from the disclosure herein given.

To this end my invention consists in the novel combination of steps herein shown and described, and more particularly pointed out in the claims.

In the drawings, wherein like reference characters represent like or corresponding parts:

Fig. l is a sectional view of one form of a machine which could be employed in conjunction with the process herein described;

Fig. 2 is a sectional 2-2 of Fig. l; and

Fig. 3 is an enlarged sectional view of a portion of a heat exchanger, illustrated in Figs. 1 and 2, of the type to which the present process is particularly applicable.

As clearly illustrated in my copending application, hereinbefore referred to, such exchangers comprise a plurality of slabs of finned passes, each adapted to conduct a respective fluid, the slabs being stacked one upon the other with separator sheets therebetween, each respective pass being sealed adjacent its edges by a suitable border member. Assuming the exchanger is adapted to handle two fluids, alternate passes would be operatively connected to one another by suitable inlets and outlets, with the intermediate passes therebetween likewise operatively connected to one another by suitable inlets and outlets, examples of which are illustrated in my co-pending application. Fig. 3 of the drawing illustrates, in section, a small portion of such type of exchanger having alternate slabs, indicated generally by the numeral 1, of interlocked fins 2, and an intermediate slab 3 of interlocked fins 4 positioned between the slabs 1, the respective slabs being separated by solid sheets 5. A cover sheet 6 extends across the outermost slab, and the peripheral edges of the slabs are sealed by border members 7 for the slabs 1, and member 8 for the slab 3. In the construction illustrated, each of the iins 2 and 4 are provided with side walls 9 terminating in offset anges 11, the latter being adapted to overlie a portion of the View taken approximately on line 2,693,636 Patented Nov. 9, 1954 side wall 9 of the next adjacent iin, as clearly illustrated.

The tins, border members, separator sheets, as well as the particular inlet and outlet ttings are preferably bonded into a single integral structure in a single honding operation, thereby eliminating numerous separate pieces, as well as separate attaching elements, such as screws, rivets, etc.

Heat exchangers of the type described nd particular application in commercial gas reduction plants as, for example, in connection with the production of commercial oxygen and nitrogen. Consequently, such type of units may be of considerable size and weight, having over two square feet of cross-sectional area, and in length of over ten feet, whereby the weight of the finished structure may be a ton or more.

lt will be appreciated that in working with exchangers of this size, and which must be constructed for high pressure and very low temperature operation, that eiiiciency of extreme importance.

of the bonding operation is Similarly, numerous complications arise as the size of the unit is increased, as a result of which attempts to fabricate large size units by conventional methods, such as, for example, the employment of baking ovens or furnaces, or the use of hot gases have proved unsuccessful due to the non-uniformity of heating whereby distortion is produced as a result of the temperature diiferentials thus created in the structure, and excessive annealing and partial exhaustion of flux activity as a result of the necessary long heating period required. Likewise, as the application of pressure on the exchanger during the bonding operation is particularly desirable, the use of an oven or the passage of hot gases through the unit is impractical from a commercial standpoint, apart from the higher cost resulting from the low production rate of the equipment employed.

In fabricating heat exchangers of this type, a number of features have been found to be of importance for the successful fabrication of relatively large units, and the present invention is directed to a process by means of which these features may be achieved. One of such features to which the invention is directed is the achieving of maximum speed in heating the exchanger to bonding temperatures, thereby preventing excessive annealing of the heat exchanger parts, particularly the extremely light thin members, as well as reducing flux deterioration prior to the melting of the bonding material and reducing cost from the standpoint of time required in the bonding operation.

A second desirable feature is the ability to cool the unit ouicklv after the bonding material has melted, without disturbing the assembly, also assisting in preventing annealing and givingy a quick solidication of the bonding material, resulting in a finer grained` better bond structure, as well as resulting in a cost reduction from a time standpoint. It is believed apparent that the use of a process reuuiring an oven structure. or the like, would be highly impractical to accomplish either rapid heating or rapid cooling, particularly cooling without removal of the unit from such an oven.

Another important factor in the fabrication of large units of the type described is the elimination of temperature differentials within planes parallel to the separator sheets of the exchanger. It will be apparent that. in large size units, considerable expansion and contraction of the elements is involved, particularly in the separator sheets, and if the heating of the assembled unit is such that the temperature adjacent the periphery of any of the separator sheets is greater than that of the center portions. stresses are introduced in the particular sheets so that during the heating of the unit the expansion would be greatest adjacent the periphery of the sheets, while in cooling the process would be reversed and the periphery of the sheets would initially contract, resulting in bulging or waviness in the center of the sheets. Obviously this distortion taking place during the solidication of the bonding material would also result in a disturbance of the bond during the critical instant of solidication and may even result in actual separation of the bond. Likewise, if not actually destroyed, the bond would be weakened due to its solidication under conditions of changing stress, whereby internal stresses between various portions of the exchanger would represent initialstress loading of the joints with a resultant' reduction 1n the ultimate strength of the finished structure. h

Another important feature in the successful fabrication of such units is the use of controlled pressure to maintain parts of the structure in intimate contact during the compacting operation as the bondingv material'melts, and as the' amount of campaetion in a large un1t may be 3/8 inch or more, it is particularly desirable to successively melt the bonding material in portions of the structure rather than. for the bonrding material to simultaneously melt throughout the structure, whereby the compacting movement is in the form of numerous slight movements rather than one large movement. It will also be` apparent that inl a heat exchanger unit such as that illustrated, failure to have uniform bonding temperatures in planes extending parallel to the separator sheets w1ll result in failure of the sheets to uniformly compact into position as non-melted center sections` would result D1n temporary high spots. Such non-uniformity likewise 1ncreases the poor temperature distribution and tends to give greater access of air to the peripheral bonding surfaces, destroying the capillary space, increasing the exhaustion of ux activity, and tending to cause segregation of ux and bonding material.

In view of the above, it is believed apparent that commonly used methods of bonding are wholly inadequat'e in the fabrication of exchangers of this type. The structure illustrated in Figs. l and 2 of the drawing is merely illustrative of one type of structure which could be employed in connection with the practice of the process herein described, and is disclosed primarily to facilitate the explanation of such process.

Referring to the drawing, 11 indicates generally a heat exchanger structure constructed, for example, as illustrated in Fig. 3, in which case the separator and outer sheets would extend horizontally. The exchanger structure 1'1 is supported between a lower metallic plate 12 and an upper metallic plate 13, the size ofthe two plates being at leasty co-extensive with the longitudinal and transverse dimensions of the exchanger 11. The lower plate 12 is carried by a base member, indicated generally by the numeral 14, a layer of suitable heat insulating material I being interposed therebetween, and, in like manner, the upper plate 13 is carried by an upper supporting structure I6, a layer of heat insulating material 17 being interposed therebetween with the plate 13 secured to the member 16 by any suitable means. Positioned adjacent the longitudinal side walls of the heat exchanger 11 are respective layers of heat insulating material 18 which are maintained in position by vertical walls 19, the latter, in the device illustrated, being suitably secured to the piece 14 by bolts 21, or the like. The ends of the exchanger 11 likewise are covered by respective layers 22 of insulating material suitably mounted on plate members 23, the latter, in the device illustrated, being hinged to the upper supporting member 16, as indicated at 24. The lower member 14 may be supported by any suitable means, and the upper member 16 is supported by suitable means operative to apply pressure through the plate 13 to the heat exchanger 11, whereby such pressure is uniformly distributed over the entire surface of the exchanger. Both plates 12 and 13 are adapted to be heated by suitable units as, for example, electrical heating elements 25 which, in the device illustrated, areA suitably embedded in the respective plates. Suitable cooling means is alsoprovided which, in the construction illustrated, comprises aplurality of transversely extending passages 26 adapted to be operatively connected to a supply line of suitable coolant as, for example, wet steam. Thus pressure may be exerted on the exchanger 11 by the plates 12 and 13 and, at the same time, heat or relative cold may be applied to the outermost sheets of the exchanger by conduction from the plates 12 and 13. It will be apparent that the plates 12 and 13, together with their respective supporting structures, may be constructed whereby the upper plate 13 isl movable relative to the lower plate 1n a vertical direction, at the same time maintaining the two plates parallel to one another, with the side walls 19 and end walls 23 constructed to permit such movement. Regardless of the particular type of structure employed to achieve the desired results, it will be ynoted such a structure would require a relatively small mass to b e heated or cooled, whereby the same could be done rapidly with a minimum amount of heat or coolant. Likewise, substantially complete inrmobility` of the exchanger structure may be achieved during both the heating and cooling operation, apart from inherent dimensional changes in the exchanger structure.

In fabricating heat exchangers in accordance with the present invention, the bonding material may be incorporated in the exchanger in one of severalways at the time of, or prior to, assembly, for example, either by coating desired surfaces of elements to be bonded with bonding material, or by laying sheets of bonding material between surfaces of elements tol be bonded, or a combination in the assembly of both, whereby the assembled exchanger includes the kdesired amount of bonding material. Where a coatingv of bonding material is employed, only one of any pair of surfaces tobe. bonded together would normally be coated with bonding material. Bonding ux may be applied by any suitable means to any exposed or uncoated surfaces to be bonded, either prior to assembly orv subsequent thereto, depending upon the nature. of the structure involved, the flux employed preferablyv having a highly volatile vehicle of low latent heat, and

including a metallic salt. The utiliaztion of such a typev flux in the process reduces, the amount of heat` required to supply the latent heat of evaporation of they flux, as well as. eliminating the tendency of gasV pocketing in the, joints, resulting with ux vehicles; of less volatility. The; use of a metallic salt, such as a. tin salt, serves a dual function in the process in that it not only insures complete wetting of all exposed or uncoated bondingA surfaces with bonding material, but results in the addition of tin to the bonding material, with improved corrosion resistance to moisture in the final bond. The amount of tin; so added to the bonding material would appear to be in the neighborhood of about 1A of one percent, and results in a bond of improved qualities and of particular advantage' in exchangers of the type involved.

Following the application of flux, the exchanger is then positioned in a bonding device, such as that herein described, pressurey is applied tothe outer sheets, and the heat conduction. plates are rapidly brought up to bonding temperatures. I have found that very satisfactory results may be obtained by the use of pressures of from 5 to 8 pounds per square inch of exterior surface. The bond-ing material employed is preferably one having a relatively very narrow liquidus-solidus range, whereby rapid solidilication of the joints is obtained, permitting rapid coolingy of the assembly without the introduction of stresses in the bond during the soliditication period, which may take place prior to the application of artificial cooling by suitable control of temperatures, etc.

Upon the application of heat to the outer sheets, such heat will be transmitted, by conduction, toward the center layers of the assembly, such action being accelerated as the bonding material of the outer layers successively melts, whereby the conduction between the adjacent elements of the structure is increased. Consequently, the bonding material will melt successively from the outer sheets inwardly toward the center, such melting action taking place uniformly throughout any plane parallel to the respective layers, whereby the joints in a single plane will reach bonding temperature simultaneously, and as such action takes place, thc pressure applied will be uniformly maintained to prevent any distortion of the structure, and, co-acting with the improved conductivity of the fused. bonds, speeds up the subsequent melting of the next layer. Following complete melting of the bonding material, heating is. discontinued and rapid cooling of the assembly is initiated, as for example, in the structure illustrated, by the circulation of a cooling medium through the passages in the upper and lower plates. Such cooling will take place in a manner similar to the heating of the assembly, hereby the cooling action will be inwardly from the surfaces adjacent the plates, by conduction, and takes place uniformly throughout any horizontal plane. Thus, as in the case of heating, distortion and internal stresses are completely eliminated.

Use of the present invention has resulted in the production of heat exchangers, of the type referred to, not only having qualities much superior to those constructed by previous methods, but has enabled the production of such exchangers in large sizes and of' great weight, heretofore considered impractical to manufacture. Likewise,

such method has reduced the fabrication time and the amount of heat required, both of which highly contribute to the considerable reduction in manufacturing costs achieved it will be noted from the above disclosure that the use of the present invention enables the production of larger and stronger units, with the maintenance of uniform high quality, at a reduction in the cost of manufacture, and the utilization of production line techniques and procedures.

Having thus described my invention, it is obvious that various immaterial modifications may be made in the same without departing from the spirit of my invention, hence l do not wish to be understood as limiting myself to the exact form, order, and number of operations herein shown and described, or uses mentioned.

What I claim as new and desire to secure by Letters Patent is:

l. The method of fabricating a metal heat exchanger comprising an assembly of a series of stacked slabs of finned passes, each adapted to conduct a iiuid in heat exchange relationship, which comprises the steps: positioning said assembly of the series of stacked slabs of nned passes together with metallic bonding material between the faces to be bonded within an insulated and enclosed chamber which has walls and ends which substantially contact the walls and ends of the assembly and which is adapted to be heated within a predetermined temperature range, applying pressure within the range of pressure from substantially five to eight pounds per square inch on the top surface of the heat exchanger to uniformly compress a pair of opposite sides of the heat exchanger assembly in a direction substantially parallel to the transverse dimension of the exchanger subject to the greatest dimensional change during the bonding operation, applying heat to the surface to which pressure is applied to successively melt, by self-conduction through the metal of the exchanger, the metallic bonding material associated with successive portions of the exchanger lying in planes extending substantially perpendicular to the direction of the pressure applied to the exchanger assembly to bonding temperature without excessive annealing of the heat exchanger parts, and maintaining the assembly immobile within the insulated chamber, other than the compacting of the structure resulting from melting of the bonding material, throughout the bonding operation.

2. The method of fabricating a metal heat exchanger comprising an assembly of a series of stacked slabs of finned passes, each adapted to conduct a iiuid in heat exchange relationship, which comprises the steps: positioning said assembly of the series of stacked slabs of finned passes together with metallic bonding material between the faces to be bonded within an insulated and enclosed chamber which has walls and ends which substantially contact the walls and ends of the assembly and which is adapted to be heated and cooled within a predetermined temperature range, applying uniform pressure to a pair of opposite sides of the exchanger assembly in a direction substantially parallel to the transverse dimension of the exehanffer subject to the greatest dimensional change during the bonding operation to prevent distortion of the heat exchanger, applying heat to a surface to which pressure is applied to successively melt, by self-conduction through the metal of the exchanger, said metallic bonding material associated with successive portions of the exchanger lying in planes extending substantially perpendicular to the direction of the pressure applied to the assembly and following complete melting of the bonding material to bonding temperature without xcessive annealing of the heat exchanger parts, cooling said exchanger by conduction within the insulated chamber, whereby portions of the bonding material will successively solidify in planes extending substantially perpendicular to the direction of the pressure applied to the exchanger assembly.

3. The method of fabricating a metal heat exchanger comprising an assembly of a series of stacked slabs of finned passes, each adapted to conduct a fiuid in heat exchange relationship, which comprises the steps: positioning said assembly of the series of stacked slabs of finned passes together with metallic bonding material between the faces to be bonded within an insulated and enclosed chamber which has walls and ends which substantially contact the walls and ends of the assembly and which is adapted to be heated and cooled within a predetermined temperature range, applying uniform pressure to a pair of opposite sides of the exchanger assembly in a direction substantially parallel to the transverse dimension of the exchanger subject to the greatest dimensional change during the bonding operation to prevent distortion of the heat exchanger, applying heat to a surface to which pressure is applied to successively melt, by selfconduction through the metal of the exchanger, the metallic bonding material associated with successive portions of the exchanger to bonding temperature without excessive annealing of the heat exchanger parts lying in planes extending substantially perpendicular to the direction of the pressure applied to the assembly, cooling said exchanger by conduction following complete melting of the bonding material, whereby portions of the bonding material will successively solidify in planes extending substantially perpendicular to the direction of the pressure applied to the exchanger assembly, and maintaining the assembly immobile within the insulated chamber, other than the compacting of the structure resulting from melting of the bonding material, throughout the bonding operation. l

4. The method of fabricating a metal heat exchanger comprising an assembly of a series of stacked slabs of finned passes, each adapted to conduct a uid in heat exchange relationship, which comprises the steps: positioning said assembly of the series of stacked slabs of finned passes together with metallic bonding material between the faces to be bonded within an insulated and enclosed chamber which has walls and ends which substantially contact the walls and ends of the assembly and which is adapted to be heated and cooled within a predetermined temperature range, applying uniform pressure to a pair of opposite sides of the exchanger assembly in a direction substantially parallel to the transverse dimension of the exchanger subject to the greatest dimensional change during the bonding operation to prevent distortion of the heat exchanger, applying heat to the surfaces to which pressure is applied to successively melt, by selfconduction through the metal of the exchanger, the metallic bonding material associated with successive portions of the exchanger to bonding temperature without excessive annealing of the heat exchanger parts lying in planes extending substantially perpendicular to the direction of the pressure applied to the assembly and following complete melting of the bonding material, cooling the surface of said exchanger to which the pressure is applied by conduction within the insulated chamber, whereby portions of the bonding material will successively solidify in a direction toward the center of the assembly in planes extending substantially perpendicular to the direction of the pressure applied to the assembly.

5. The method of fabricating a metal heat exchanger comprising an assembly of a series of stacked slabs of finned passes, each adapted to conduct a uid in heat exchange relationship, which comprises the steps: positioning said assembly of the series of stacked slabs of nned passes together with metallic bonding material between the faces to be bonded within an insulated and enclosed chamber which has walls and ends which substantially contact the walls and ends of the assembly and which is adapted to be heated and cooled within a predetermined temperature range, applying uniform pressure to a pair of opposite sides of the exchanger assembly in a direction substantially parallel to the transverse dimension of the exchanger subject to the greatest unit dimensional change during the bonding operation to prevent distortion of the heat exchanger, applying heat to the surfaces to which pressure is applied to successively melt, by self-conduction through the metal of the exchanger, said metallic bonding material associated with successive portions of the exchanger to bonding temperature without excessive annealing of the heat exchanger parts lying in planes extending substantially perpendicular to the direction of the pressure applied to the assembly, cooling the surface of said exchanger to which the pressure is applied by conduction following complete melting of the bonding material within the insulated chamber, whereby portions of the bonding material will successively solidify n a direction toward the center of the assembly in planes extending substantially perpendicular to the direction of the pressure applied to the assembly, and maintaining the assembly immobile, other than the compacting of the structure re-

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