US2807074A

US2807074A - Manufacture of brazed finned tubing and the like

Info

Publication number: US2807074A
Application number: US536831A
Authority: US
Inventors: Robert W Schroeder
Original assignee: Griscom Russell Co
Current assignee: Griscom Russell Co
Priority date: 1955-09-27
Filing date: 1955-09-27
Publication date: 1957-09-24
Anticipated expiration: 1974-09-24

Description

Sept. 24, 1957 R. w. scHRoEDER MANUFACTURE oF BRAZED FINNED TUBING AND THE LIKE BY I ATTORNEYS United States Patent O 'MANUFACTURE or BRAZED FINNED TUBING AND THE Luni Robert W. Schroeder, Massillon, Ohio, assignor to The Griscom-Russell Company, Massillon, Ohio, a corporation of Delaware Application September 27, 1955, Serial No. 536,831

7 claims. (Cl. zes-157.3)

The invention relates to the manufacture of finned tubing suitable for use in heat exchangers and other purposes, and more particularly to a method for expeditiously and economically manufacturing brazed nned tubingwhereby the fins are bonded vto the, tubing along the entire interface.

In use, such finned tubing is subjected to extremely high temperatures and high pressures Vand is frequently exposed to highly corrosive iiuids. The tube must therefore necessarily be formed'of a high temperature resistant, high strength and corrosion-resisting material. such as stainless steel. For the purposes of obtaining the maximum heat exchange, the fins should be formed of a high heat conducting material such as copper.

It is important that the exterior surfaces of such a copper fin be protected against the corrosive action of iiuids to which they are exposed. For this purpose stainless steel may be clad'pupon the copper fin. The outer edge of the lin may be protected by using a copper fin clad on only one side with stainless steel and folding' the n material upon itself and attaching the edges of the folded fin to the tube.

However, in many instances the finned tube may comprise an unfolded fin of single thickness, in which the copper fin is clad on both sides with stainless steel. Thus, the outer edge of the copper is exposed. The invention contemplates the protection of this outer edge of the fin by building up a deposit of brazing alloy thereon to entirely seal and protect the copper.

Two types of fins are commonly used in the construction of such finned tubing. One type of fin comprises straight, flat ribbons extending longitudinally of the tube and attached thereto in radially disposed position. The other type of fin comprises a ribbon spirally wound around the tube, with the inner edge thereof seated in a spiral groove in the tube or contacting the periphery thereof. The spirally wound n may be. Very thin, say .008" to .010" thick, and there may be as many as twenty to thirty winds per inch of tube length.

While the present invention is applicable to either of these types of finned tubes, it is illustrated and described herein as applied to the spiral fin type to which it is especially adapted. These spiral ns are stressed and stretched as they are wound around the tube, which tends to hold the n upon the tube.

If the fin stress is released, the bond between the fin and tube becomes less secure. A differential in expansion of the two materials may aggravate these conditions,

causing the bond between the fin and tube to be destroyed.

In the manufacture of such finned tubing for use in heat exchangers it is desirable that this bond be obtained and maintained during use at high temperature, as other- Wise the conductivity is reduced, proportionately decreas-l ,of brazing alloy upon the tube at and around the base of the iin so as, after subsequent brazing, to obtain and "ice maintain a tight secure bond between the tube and the lin under all conditions to which the nned tube may be subjected in normal use. y

Likewise, where the fin `is formed of a copper ribbon clad on both sides with stainless steel, with the inner edge thereof bonded to the tube, the outer edge of the copper fin may be sealed and protected against corrosive fluids by building up a deposit of brazing alloy thereon. In order to facilitate adherence of the brazing alloy, the fin tip may be roughened by shot or grit blasting or by means of a knurled roll. The exterior surface of the tube may, in some instances, require similar roughening in'order to facilitate adherence of the brazing alloy to the tube and the base of the fin tip. Y

It is therefore a primary object of the invention to provide a method `of expeditiously and economically manufacturing brazed finned tubing whereby a tight andrsecure bond is obtained'and maintained between the fin and tubing along the entire interface. Y f

Another object is to provide such a method by which the brazing alloy is caused `to flow into all small crevices between the tube and the fin.

A further object is to provide` a method of manufacturing brazed finnedtubing which includes heating of the tube and then, spraying powdered brazing alloy onto desired portions of the heated parts'so that it will adhere thereto as a sintered deposit, and then furnace brazing the finned tube to cause the brazing alloy to ow over desired areas and also into all fine crevices between the tube and fin.

A still further object is to provide for the manufacture lof brazed finned tubing by placing a finned tube in a lathe and locating an oxy-acetylenetorch in the tool post of the lathe so that the flame thereof will impinge upon the tube and the fin, rotating the finned tube and simultaneously moving the torch from one end of the n to a point spaced therefrom to preheat the adjacent end portions of the tube and fin, then backing up the torch to the starting point, and then spraying powdered brazing alloy from the torch'upon the tube and fin as the torch is continuously moved toward'the other end of the n.

Another important object `of the invention is to provide a method of protecting the outer edge of the fin by depositing brazing alloy thereon at the same time that the brazing alloy is deposited on the tube at and around the base of the iin. l

A further object is to provide for protecting the outer edge of the fin by first rougheningsaid outer edge, then depositing brazing alloy upon the roughened outer edge of the fin, and then melting and flowing the deposited alloy.

A still further object is to provide such a method of manufacturing brazed vfinned tubing, which consists in applying a sintered deposit of powdered brazing alloyto a nned tube in the manner referred to, and then furnace brazing the finned tube to cause the brazing alloy to fiow outward on the iin and into fine crevices between the tube and fin. v v

These and other objects which will be apparentrto those skilled in the art, or which lmay be later pointed out, may be yaccomplished by the methods, constructions, arrangements, partsgcombinations `and sub-combinations comprising the present invention, the nature of which is set forth in the following general statement, a preferred embodiment of whichillustrative of the best mode in which applicant has contemplated applying the principles -is set forth in the following description. and illustrated in the accompanying drawing, and which is particularly and distinctly pointed out and set forth in the appende claims forming part hereof. Y

The method and apparatus comprising the invention provided either with fins wound spirally around the tubes or fins extending longitudinally of the tube. For the purpose of illustration, the invention is shown and described as applied to finned tubing of the spiral fin type, and the following general statement brieliy describes the manner in which the invention may be carried out.

In general terms, the invention comprises a method of and apparatus for applying brazing alloy to a tube having a fin spirally wound around the outer surface thereof, so as to securely bond the base of the fin to the tube and also to seal and protect the outer edges of the fin. The invention is carried out by rotating the finned tube and heating one end portion thereof, and then spraying powdered brazing alloy upon the rotating tube to apply a sintered deposit of brazing alloy thereto at the base of the fin and upon the outer edge of the fin, by a torch moved from the preheated end of the tube to the other end thereof, the tube and fin being heated by the torch ahead of the application of the brazing alloy.

This may be accomplished upon a lathe, the finned tube being mounted for rotation between the head stock and tail stock of the lathe, and the torch being mounted upon theV tool post. A torch is used from which a desired powdered metallic brazing alloy is blown by compressed air onto the work, with and through an oxyacetylene iiame which heats the metallic powder and deposits the same on the work on which it adheres in sintered condition.

The method is performed by first using the torch without the metallic brazing powder, to preheat one end portion of the rotating finned tube to the desired temperature. The tool post, upon which the torch is mounted, is then backed up to the starting point and powdered brazing alloy is fed to the torch, which is then advanced along the length of the tube by the usual lead screw which operates the tool post of the lathe. A deposit of sintered brazing alloy is thus sprayed upon and adheres to the rotating tube at and around the base of the lin, and upon the outer edges of the fin so that a sintered deposit of brazing alloy is applied to these portions throughout the entire finned length of the tube.

This manner of application of the brazing powder avoids -any plugging of the brazing alloy between fins which might occur by dipping or painting. In cases where the fin is copper clad material, it is especially important that any free or exposed copper edge be sealed. For this purpose the metallic brazing alloy powder is deposited by the torch on the outer edge of the clad fin, and, if necessary to promote adherence, the outer edge of the clad fin may be ronghened, as by shot or grit blasting, or by a knurled roller or the like, so that the sintered deposit of brazing alloy will adhere thereto.

The finned tube, with brazing alloy deposited thereon in this manner, is then heated to brazing temperature in a special atmosphere brazing furnace to cause the brazing alloy to flow into all fine crevices between the tube and fin, and to flow upward around-the base portions of the fin, and over the outer edge of the fin, forming a tight', unbroken bond between the tube and the fin, and also a protective coating sealing and protecting the `outer edge of the iin with a securely bonded deposit of brazing alloy.

Having thus described the invention in general terms, by way of example an embodiment of `the invention is illustrated in the accompanying drawing forming a part hereof, wherein like numerals indicate similar parts throughout the several Views, and in which;

Fig. 1 is adiagrammatic front elevation of a llathe upon which the invention may be performed;

Fig. 2 is a top plan view of the lathe shown in Fig. 1;

Fig. 3 is an enlarged, fragmentary, sectional view of a portion of a finned tube showing the sintered deposit of brazing alloy applied to the tube at the base portions of the fin and also upon the outer edges of the fin;

Fig. 4 is a fragmentary, sectional view of a further en- Cit 4., larged portion of the finned tube shown in Fig. 3, showing-the brazing alloy flowed outward upon the fin and into crevices between the fin and tube, after the furnace brazing operation;

Fig. 5 is a view similar to Fig. 4, of a slightly modified form of the finned tube after the furnace brazing; and,

Fig. 6 is a greatly enlarged fragmentary, sectional view through the tip or outer edge portion of a clad fin showing the manner in which the same is sealed and protected by la coating of brazing alloy.

Referring first more particularly to Figs. l and 2, the lathe upon which the invention may be carried out comprises generally the bed 10 having head stock 11 at one end provided with a rotatable chuck 12 of conventional design, adapted to be rotated in a usual and well known manner by conventional gearing and the like (not shown) driven by any suitable power means.

The tail stock 13 is mounted at the other end of the lathe, and a tool post 14 is mounted for longitudinal movement upon the bed of the lathe by the usual lead screw (not shown) coordinated with the mechanism which rotates the chuck in the head stock, as in usual practice.

The lathe differs from conventional lathe construction only in that a torch indicated at 15 is substituted upon the tool post 14 instead of the usual tool. The torch is of the type into which powdered metal may be blown by compressed air through an oxyacetylene flame causing the metal to be heated and deposited on the work in a sintered state.

The finned tube to be brazed comprises the tube, indicated generally at 16, which may be formed of stainless steel, ferrous metal, nickel, Inconel or the like and upon which is spirally wound a fin, indicated at 17, which may be formed of copper clad with stainless steel, or the fin may be formed of ferrous metal, nickel or Inconel.

As shown in Figs. 3V and 4, the base portions of the fin may be located in a spiral groove 18 formed in the peripheral surface of the tube 16, into which the fin is wound under stress, or, as shown in Fig. 5, the fin 17 may be spirally wound, under tension, around the peripheral surface of the tube.

These spiral fins are stressed and stretched as they are spirally wound yunder tension around the tube, the stress holding the lin upon the tube. It is evident that if the fin stress is released, the blond between the fin and tube will be at least partially, Vif not entirely destroyed. These conditions may be aggravated in use by a differential in expansion of the two materials.

In either case, there will be fine crevices between the base portions of the iin and the tube. It is highly desirable that the brazing alloy when heated sho-uld flow into these fine crevices in order to insure a secure tight bond at these points so as to pro-vide proper heat conductivity between the fin and tube when used in a heat exchanger, as well as to resist the corrosive action of uids to which the finned tube may be subjected in use.

Heretofore, it has not been possible tosatisfactorily Viiow brazing alloy into such lfine erevices, and as a result a tight bond could not be obtained and maintained between the tube and the base portions of the fin.

I have discovered that by heating the tube and fin and immediately applying a sintered deposit .of brazing alloy thereto, that the powdered metallic brazing alloy would adhere to these heated surfaces, and when the finned tube was subsequently heated in a brazing furnace, the deposited alloy would melt and flow into fine crevices, producing'a tight bond between the tube and the base portions of the fin.

After the fin has 'been attached to the tube, the finned tube is mounted `in the lathe, between the head stock and tail stock as shown 'in Figs. l and 2, with the tool post located in the starting position, adjacent to the left hand end of the finned portion of the tube, as viewed in Figs. l and 2.

The torch 15 -is operated, without the application of powdered metal, so as to direct the ame upon the finned tube, and the lathe mechanism is started to rotate the finned tube and simultaneously move the tool post carrying the torch toward the right as viewed in Figs. 1 and 2, heating the tube and iin to desired temperature.

When the torch reaches a position spaced from the starting end, approximately such as shown in full lines in Figs. 1 and 2, the tool post is backed up to the starting point and powdered brazing alloy is fed to the torch.

The lathe is then operated to rotate the finned tube and continuously move the tool post, carrying the torch, toward the iight, depositing sintered brazing alloy Aupon the tube 16 adjacent to the base of the fin as indicated at 19 in Fig. 3 and also upon the outer edgeof the iin as indicated at 20, the sintered alloy adhering to the heated surfaces of the tu-be and fin.

The sintered brazing alloy is thus deposited upon the hot surfaces of the tube and iin, and, .as ,the torch continues to move toward theright, each successive portion of the tube and fin will be heated by the torch flame ahead of the brazing alloy applied thereto, so that the sintered alloy will adhere to the tube at and around the base of the fin, and to the outer edge of the n, throughout the length thereof.

Thus, all portions of the tube and lin, from one end thereof to the other, will be heated to the necessarytemperature before the brazing alloy is deposited thereon, thus causing the sintered alloy to adhere to the tube and fin material, even though the iinmaterial is very thin, say .008 in thickness and spaced say 20 to 30 convolutions per inch of tube length. Moreover, the deposit f the brazing powder in this manner avoids any plugging of the extremely narrow spaces between iin convolutions.

In Fig. 6 is shown a greatly enlarged section of the outer edge or tip portion of a stainless steel clad copper fin, showing the manner in which the brazing alloy deposited upon the outer edge of the iin seals and protects the copper.

The copper iin is indicated at 21 and the stainless steel clad Vupon each side thereof is indicated at 22. As shown in Fig. 3, the brazing alloy 20 deposited upon the outer edge of the fin entirely covers and seals the edge of the copper fin 20 and extends over the edges of the stainless steel clad layers 22.

The outer edges of these stainless steel layers may be slightly rounded, as shown at 23, causing the brazing alloy deposit to flow downward thereover at eac'h side of the iin, during the furnace brazing operation, thus comipletely sealing and protecting the copper.

ln order to assure adherence of the deposit of brazing alloy to the outer edge of the iin, the outer edge may be roughened by shot or grit blasting, knurling or the like. If desired, the tube surfaces may be similarly roughened so as to assure adherence of the brazing alloy thereto adjacent the base of the iin.

The brazing alloy may be a commercial alloy of suitable composition including elements such as nickel, lchromium, iron, silicon and boron. After the brazing, alloy has been deposited upon the tube at the base of the fin, and upon the outer edge of the iin, as shown in Fig. 3, in the manner above described, the brazed iinned tube is then furnace brazed, with brazing temperatures and atmospheres appropriate to the type of alloy being used.

This furnace brazing is carried out at a temperature above the flow point of the brazing alloy but below the melting point of the copper in the iin so as to cause the brazing alloy to ow upward or outward upon the base portion of the fin, as indicated at 24 in Figs. 4 and 5, and into all iin crevices between the iin and the tube such as the grooves 18 in Fig. 4 and the crevices 25 between the base of the fin and the tube in Fig. 5. The iin will thus be tightly bonded to the tube at all points throughout its length, Y

As above poi-nted out, in nned tubes of this character the iin is held onto the tube becausethe iin mateesoneri.

rial is stressed as it is wound onto the tube. If the tin stress is released the bond between the iin and the tube becomes less sure. Differential in expansion of the materials may aggravate this condition. By applying the brazing alloy in the manner above described, a sure` bond between the fin and the tube is obtained and maintained under working conditions.

Although the invention has been illustrated and described in detail as applied to the manufacture of finned tubes with fins spirally wound thereon, it should be understood that tubes having fins extending longitudinally thereof may be brazed in the same manner.

In the foregoing description, certain terms have been used for brevity, clearness and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such wonds are used for descriptive purposes herein and are intended to be broadly construed. n

Moreover, the embodiments of the improved construction illustrated and described herein are by way of example, and the scope of the present invention is not limited to the exact details of construction.

Having now described the invention or discovery, the construction, the operation, and use of preferred embodiments thereof, and the advantageous new and useful results obtained thereby; the new and useful construction, and reasonable mechanical equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.

I claim:

v1.' The'methodof making finned tubes for heat exchangers of a type including a corrosion-resistant tube having high-temperature strength and iin ribbon means bonded thereto formed of high-heat-conductivity material having corrosion-resistant surfaces, which includes the steps of providing a stainless steel tube; `Securing copper ribbon iin means having stainless steel clad to both ribbon surfaces of the iin, to the exterior of the tube with the base edge of the iin ribbon in contact with the tube and with copper exposed' at the outer edge of the n'ribbon; rotating the tube with the lin means secured thereon on its axis; initially preheating one end of the rotating tube and n means; directing a flame and a spray of heated powdered metallic brazing alloy simultaneously against the heated end of the rotating tube and iin means; moving the Haine and spray axially of the rotating tube and lin means from said heated end to the other end of the tube; such directing and` moving of the flame and spray with respect to the rotating tube and iin means preheating successive unheated tube and fin means portions beyond the initially preheated rotating one end of the tube and fin means and ahead of the spray; progressively applying, by said directed and moving spray, an adherent deposit of heated powdered alloy metal on the heated tube at and around the base edge of the iin ribbon and on the copper exposed outer edge of the iin'ribbon throughout the length of the rotating tube and fin means; furnace heating the tube and 1in means with the adherent deposit thereon to brazing temperature; melting the adherent deposited alloy, by said furnace heating, and flowing the melted alloy outward on the iin surfaces from the base edge of the iin and also along the contacting interface surfaces of the base edge of the fin and tube and also over the outer edge of the tin; and the melted and flowed alloy metal forming a tight unbroken bond between the tube and fin means and protecting and sealing the outer exposed copper edge of the fin means.

2. The lmethod of making finned tubes for heat exchangers of a type including a corrosion-resistant tube having high-temperature strength and iin ribbon means bonded thereto formed of high-heat-conductivity material having corrosion-resistant surfaces, which includes the stepsr of providing a stainless steel tube; spiralling winding a `copper ribbon having stainless steel clad on both ribbon surfaces around the tube under ltension to hold the tin upon the tube with the base edge of the tin ribbon in contact with the tube and with copper exposed at the outer edge of the tin ribbon; rotating the tube with thespiral iin held thereon on its axis; initially preheating one end of the rotating tube and fin; directing a flame and a spray of heated powdered metallic brazing alloy simultaneously against the heated end of the rotating tube and fin; moving the llame and spray axially of ythe rotating tube and iin from said heated end to the other end of the tube; such directing and moving of the flame and spray with respect to the rotating tube and n preheating successive unheated tube and fin portions beyond the initially preheated rotating one end of the tube and n and ahead of the spray; progressively applying, by said directed and moving spray, an adherent deposit of heated powdered alloy metal on the heated tube at and around the base edge of the iin ribbon and on the copper exposed outer edge of the fin ribbon throughout the length of the rotating tube and iin; furnace heating the tube and iin with the adherent deposit thereon to brazing temperature; melting the adherent deposited alloy, by said furnace heating, and flowing the melted alloy outward on the iin surfaces from the base edge of the fin and also along the contacting interface `surfaces of the base edge of the tin and tube and also over the outer edge of the fin; and the melted and flowed alloy metal forming a tight unbroken bond between the tube and n and protecting and sealing the outer exposed copper edge of the n.

l3. The method of making finned tubes for heat exchangers of a type including a corrosion-resistant tube having high-temperature strength and n ribbon means bonded thereto formed of highheatconductivity material having corrosion-resistant surfaces, which includes the steps of providing a corrosion-resistant tube; securing fin ribbon means formed of high-heat-conductivity material having,y corrosion-resistant `surfaces to the exterior of the tube with the base edge of the iin ribbon in contact with the tube and with the high-heat-conductivity fin material exposed at the outer edge of the n ribbon; rotating the tube with the tin means secured thereon on its axis; initially preheating one end of the Arotating tube and iin means; directing a flame and a spray of heated powdered metallic brazing alloy simultaneously against the heated end of the rotating tube and n means; movingthe flame and spray axially of the rotating tube and n means from said heated end to the other end of the tube; such directing and moving or" the flame and spray with respect to the rotating tube and n means preheating successive unheated tube and tin means portions beyond the initially preheated rotating one end of the tube and lin means and ahead of the spray; progressively applying, by said directed and moving spray, anradherent deposit of heated powdered alloy metal on the heated tube at and around the base edge of the tin ribbon and on the exposed outer edge of the tin ribbon throughout the length of thc rotating tube and iin means; furnace heatingthe tube and tin means with the adherent deposit thereon to brazing temperature; melting the adherent deposited alloy, by said furnace heating, and owing the melted alloy outward on the tin surfaces from the base edge of the fin and along the contacting interface surfaces of the base edge ofthe lin and tube and also over the outer edge of the tin; and the melted and owed alloy metal forming a tight unbroken bond between the tube and iin means and protecting and sealing the outer exposed edge of the fin means.

4. The method of making nned tubes for heat exchangers of a type including a corrosion-resistant tube having high-temperature strength and fin ribbon means bonded thereto formed of high-heat-conductivity material having corrosion-resistant surfaces, which includes the steps of providing a heat-resistant metal tube, securing iin ribbon means formed of highheat=conductivity material to the exterior of the tube with the base edge of the tin ribbon in contact with the tube; rotating the tube with the n means secured thereon on its axis; initially preheating one end of the rotating tube and tin means; directing a Harrie and a spray of heatedpowdered metallic bra'zing alloy simultaneously against the heated end of the rotating tube and n means; 'moving the flame and spray axially of the rotating tube and n means from said heated end to the other end of the tube; such directing and moving of the flame and spray with respect to the rotating tube and tin means preheating successive uriheated tube and fin means portions beyond the initially preheated rotating one end of the tube and tin means and ahead of the spray; progressively applying, by said directed and moving spray, an adherent deposit of heated powdered alloy metal on the heated tube at and around the base edge of the fin ribbon throughout the length of *the rotating tube and lin means; furnace heating the tube and fin means with the adherent deposit thereon to brazing temperature; melting the adherent deposited alloy, by said furnace heating, and owing the melted alloy outward on the fin surfaces from the base edge of the tin and also along the contacting interface surfaces of the base edge of the iin and tube; and the melted and owed alloy metal forming a tight unbroken bond between the tube and fin means.

5. The method of making finned tubes for heat exchangers of a type including a corrosion-resistant tube having high-temperature strength and n ribbon means bonded thereto formed of l'iigh-heat-conductivity material having corrosion-resistant surfaces, which includes the steps of providing a stainless steel tube; securing copper ribbon n means having stainless steel clad to both ribbon surfaces of the fin, to the exterior of the tube with the base edge of the iin ribbon in contact with the tube and with copper exposed at the outer edge of the fin ribbon; rotating the tube with the iin means secured thereon on its axis; directing a ame and a spray of heated powdered metallic brazing alloy simultaneously against one end of the rotating tube and n means; moving the llame and spray axially of the rotating tube and iin means from said one end to the other end of the tube; such directing and moving of the flame and spray with respect to the rotating tube and fin means preheating successive unheated tube and tin means portions ahead of the spray; progressively applying, by said directed and moving spray, an adherent deposit of heated powdered alloy metal on the tube heated by said flame at and around the base edge of the tin ribbon and on the copper exposed outer edge of the fin ribbon throughout the length of the rotating tube and tin means; furnace heating the tube and tin means with the adherent deposit thereon to brazing temperature; melting the adherent deposited alloy, by said furnace heating, and flowing the melted alloy outward on the n surfaces from the base edge of the fin and also along the contacting interface surfaces of the base edge of the n and tube and also over the outer edge of the tin; and the melted and flowed allo-y metal forming a tight unbroken bond between vthe tube and fin means and protecting and sealing the outer exposed copperedge of the tin means.

6,v The method of brazing a finned tube with a brazing alloy which consists in rotating the finned tube on its axis, directing a flame upon one end of the rotating finned tube, moving the ame axiallyof the rotating tube to a point spaced from said one end thereby preheating the tube from said one end to said point, returning the fiamc to saidy one end, directing said ame and a spray of heated metallic brazing alloy powder simultaneously against the heated tube at said one end, moving the flame and heated powder spray axially of the rotating tube to the other end of the tube thereby preheating successive portions of the finned tube ahead of the spray and depositing heated powder on the heated tube as an adherent metal coating adjacent the base of the fin progressively throughout the length of the heated finned tube, and then furnace heating the finned tube to brazing temperature and thereby melting and owing the adherent deposited metal along the iin 9 surfaces at the base of the iin and into interface crevices between the tube and n.

7. The method of brazing a finned tube with a brazing alloy which consists in rotating the finned tube on its axis, directing a ame upon one end of the rotating nned tube, moving the flame axially of the rotating tube to a point spaced from said one end thereby preheating the tube from said one end to said point, returning the ame to said one end, directing said ame and a spray of heated metallic brazing alloy powder simultaneously against the heated tube at said one end, moving the ame and heated powder spray axially of the rotating tube to the other end of the tube thereby preheating successive portions of the finned tube ahead of the spray and depositing heated powder on the heated tube as an adherent metal coating adjacent the base of the iin and upon the outer edge of the iin progressively throughout the length 10 of the heated vfinned tube, and then furnace heating the nned tube to brazing temperature and thereby melting and flowing the adherent coating metal along the n surfaces at the base of the n and into interface crevices between the tube and iin and over and around the outer edge of the fin.

References Cited in the tile of this patent UNITED STATES PATENTS 1,256,599 Schoop Feb. 19, 1918 1,412,656 Jenkins Apr. 11, 1922 2,092,018 Quarnstrom Sept. 7, 1937 2,164,737 Ford July 4, 1939 2,314,902 Shepard Mar. 30, 1943 2,440,698 Patterson May 4, 1948