CA1303605C - Heat exchanger tube having embossed ring bell and brazing ring - Google Patents

Abstract

Inventions HEAT EXCHANGER TUBE HAVING EMBOSSED RING BELL
WITH BRAZING RING
Inventors: STEPHEN B. UNRUH and DANIEL P. GAFFANEY JR.
Abstract of the Disclosure A heat exchanger tube has a bell formed on one end. At the base of the bell there is embossed a ring large enough to provide the necessary holding force required for tension expansion of the tubing. A brazing ring can be emplaced in the inside of the embossed ring prior to inserting a male tube member into the bell. When the bell is brazed, capillary action between the end of the male member and the inner surface of the bell will draw the braze up into the bell.

Description

130~'3605 HEAT EXCHANGER TUBE HAVING
EMBOS8ED RING BELL AND ~RAZING ~ING
Background of the Invention This invention relates to l~provement~ in heat e~changer fabrication, especially for plate-fin heat exchangers which employ copper tubing. The inventlon i9 more particularly directed to improvements in the technique of belling the end of a tube which facilitates the process of expansion, under tension, of the tubing outward against the heat exchanger fin and thereby achieve good thermal contact.
The invention is also directed to an improvement in the assembly of plate-fin type heat exchangers which facilitates the tension expanding of hairpin tubes into the fin pack, and also facilitates the connection of return bends to the open end of the hairpin tubes. In connection with this, the invention is further directed to improvements of the brazing of heat exchanger tubing to mating tubing members, to wit, for the mating of hairpin tubes of a condenser or evaporator coil to associated return bends.
A~ discussed in U.S. Pats. Nos. 4,195,5iO; 4,285,256 and 4,286,486, techniques have been developed for fabrication of plate-fin heat exchangers for refrigeration systems or automotive cooling systems. The plate-like fins are stamped from good thermal conducting materials (such as aluminum). The fins are stacked with the fin plates aligned one over the other in a column 80 that their through-apertures are in registry. The ~ assembled fin pack units are then fitted with end baffles or tube - 25 sheets, and are laced with hairpin tubes. The term "hairpin" or "hairpin tube" refers to a U-shaped tubular member having two ' ' 6~

equally elongated legs that are in generally parallel alignment.
The legs of the hairpin tubes form flow channels that are perpendicular to the fln plates. The legs of ad~acent hairpin~
are later interconnected by means of short U-~haped tubing bends referred to as return bends. These members complete the flow of the circuit through the fin pack unit. To receive the return bends, the open ends of the hairpin are generally belled, or expanded to a larger diameter. The return bends are inserted into the belled openings and are brazed in place to provide a fluid-tight joint. Also, in order to ensure a good thermal contact between the tubing and the fins, the legs of the hairpin tubes are expanded into locking contact against the fin plates by driving an expanding rod through the tube leg~.
The expansion process is described generally in U.S. Pat.
No. 4,228,573 and also in U.S. Pat. No. 4,584,765. The tube expansion process is generally carried out by passing an expansion "bullet", or rounded member of larger diameter than the tube inside diameter, through the tube from the open ends of the hairpin tubes. The "bullets" are affixed atop elongated rods which are driven mechanically or hydraulically into the hairpin legs. In so-called "compression" expansion, a backing plate is placed at the tube bends during an expansion to prevent the tubes from being driven out of the unit as the expansion bullets are driven through it. As a consequence of this, the tubes are compressed rearwardly as they are being expanded outwardly by the expansion tools. This results in tube shrinkage, on the order of about three percent, 80 that the axial length of each tube can 1;~.36~S

vary dramatically in regard to its neighbor in final assembly.
Because of differences in tube lengths, belling of the tubes is dificult and generally results ln uneven or mlsaligned bells being formed in the tube ends. The return bend therefore cannot always be properly seated within the bells leading to formation of relatively weak or incomplete brazing in this critical region.
Another problem with compression expansion is that the coupled force~ of the expansion device and backing plate on the hairpins can cause the coil unit to bow or to bend. An elaborate, rigid "iron maiden" coil clamp is required to hold the coil assembly to prevent this from happening. Even so, a significant amount of scrap does occur as a result of compression expansion.
In order to avoid the problems of "compression"
expansion, various proposals have been advanced for "tension"
expansion, that is, supporting the hairpin tubes by the open or bell end when the expansion tools are driven through. Here, it has been the practice to bring the open ends of the hairpins a considerable distance out from the adjacent tube sheet. Then, a clamping fixture can hold the belled ends of the tube against axial displacement and expansion tools can be arranged to move through the open end~ of the tubes to expand the tube walls into contact against the fin collars and tube sheets of the unit, while the bells are clamped in place. It has been found that expansion under tension conditions results in a lesser shrinXage in tube length, on the order of about one percent.
In addition, tension expansion results in greater 130:3605 evenness in bell height or stand off from the bell-end tube sheet. This permits the return bends to be properly seated within all of the bell~ of a partlcular coil unlt. Moreover the tension ~xpansion operation has much less tendency to bend or bow out the unit, as the tubes are always under a tension force, and there is never a force component tending to bow or warp the tube~
Unfortunately, the belled end of a copper tube typically has only a tiny increase in diameter over the unexpanded tube.
Consequently it is difficult for a gripping device to achieve a secure grip without damaging the belled end. Further, while it may be advantageous to support the belled ends of the hair pins directly from the upper tube plate, the lack of a substantial shoulder on the bell end makes this proposal difficult.
Another problem of the current techniques for assembling these fin pack units is that the brazing or soldering tends to place the metal primarily at the outer end of the bell, and not at the inner end, where weakness of tube integrity is most iikely. In hydrostatic tests of these coil units, the weakest point~ tend to be at the tip of the male end of the return bends, that is, at the inner end of the bell of the hairpins. This occurs because if the space between the male end of the return bend and the bell i8 incompletely filled with braze alloy, or the braze alloy is incompletely bonded to the joint members, the hydraulic fluid under pressure tends to peel the inner layer of the outer member i.e., the bell, away from the outer layer of the return bend tip. Ideally, therefore, the brazing should occur at the tip of the return ben-d, i.e., at the base of the bell, but _ 4 _ , ~

' `' , , ' ~ ' .

1:~0~'36~5 this cannot be achieved using current techniques.
It is currently the practlce to as~emble the return bend~
into the hairpin bells by first placlng a brazing ring over each leg of the return bend, and then inserting the return bends into the hairpin bells. After this, the tubing i9 heated sufficiently to melt the brazing ring, and the brazing material is then drawn between the return bend and the bell to form a seal. It is clear that with this technique the brazing operation can be carried OUt only in a "vertical" or upright mode, that i8, with the fin pack coils arranged with the bells upwards, 80 that the braze is drawn by gravity downward into the gap between the hairpin bell and the return bend. However, the vertical mode i8 not practical for very large coils or for coil~ in ~hich there may be obstructing flanges on the tube sheet. In such cases, it may be better to dispose the entire fin pack coil horizontally or with the return bends oriented vertically downwards. However, there is no prior method which will ensure that the brazing material would flow into the gap between the return bend and the hairpin bells if a non-upright mode of brazing is used.
Objects and Summary of the Invention Accordingly, it is an object of this invention to improve fin pack heat exchanger coils and to improve the methods of manufacturing them.
It is another object of this invention to eliminate the problems associated with the gripping of heat exchanger tubes during the process of expanding them.
¦ rt 8 a rther obj-ct of thi~ tnvontion to reduce the ~ ~ ,.................... I
'~

`~
, 130;~61~5 scrap rate in the manufacture of heat exchanger colls.
It is still a further object of this invention to facilitate the brazing of the tubular return bend onto the bell3 of their associated hairpin tubes ln a heat exchanger coil.
It is yet another object of this invention to make it possible to braze the tubular return bends onto the hairpin bells in orientations other than the vertical or upright orientation, and to strengthen the resulting brazed joint.
According to one of many possible aspects of this invention, a heat transfer tube of the type that comprises a hairpin tube or the like, has an elongated main tube portion and an end portion which is belled out to a suitable inside diameter to receive a male tube member, such as a return bend, which has substantially the same outside diameter as the main portion of the hairpin tube. The belled end portion of the heat exchanger tube has a ring or torus formed in it at the junction of the mai~
tube portion with the belled-out end portion. The ring or torus has a greater diameter than the outer diameter of the end portion. This forms a shoulder which can be supported by gripping means, or even by the tube sheet itself, which can sustain the forces that are encountered during a tension expansion operation. A ring of brazing material can be situated within the embossed ring or torus. When the return bend is inserted into the hairpin bells, the tip of the return bend will lodge against the ring of brazing material. With this configuration, when the braze melts, it will flow into the gap between the bell and the return bend tip at the lower end of the - 6 - ~

13036r~s bell. This yields a substantially stronger braze joint than that resulting from the technique of the prior art descrlbed above.
Moreover, a brazing operation can be carried out automattcally in any arbitrary orientation of the tubing.
The ends of the hairpin tube can be belled or expanded by means of a collet device which has a number of radially movable fingers, and in which an embossing device i8 carried at the ends of these finger~. The collet i9 expanded outward radially to enlarge the tube end to form the bell, and, at the same time, to form the embos~ed ring or torus at the base of the bell. The embossing device can comprise a wire ring di~posed over the collet fingers, and preferably can be a ring of brazing wire, which simply remains in the embossed ring or torus after the bell expansion operation.
The above and many other objects, features, and advantages of this invention will be more fully understood from the ensuing description of a preferred embodiment, which should be read in connection with the accompanying drawing.
Briof De~cription of the Drawing Fig. 1 is a partial perspective elevation of a heat exchanger tube bell, shown with a portion of the associated tube sheet .
Fig. 2 is an elevational cross-section of the heat exchanger tube bell of Fig. 1.
Fig. 3 is another elevational cross-section of the heat exchanger twbe bell of Fig. 1, shown with a brazing ring and an associated male tubular member.
7- ~

1~036~)5 Figs. 4 and 5 are cross-sectional elevatione of the tube and a device for forming the bell end in the heat exchanger tubing.
Fig. 6 i9 a sectional elevation for illustrating and explaining the tension tube expansion operation according to the prior art.
Figs 7 and 8 are sectional elevations for explaining tension tube expan~ion operations according to this invention.
Fig. 9 is a cross-sectional view of another embodiment of this invention.
Detailed De-cription of th- Preferred Eibodiment With reference initially to Figs. 1 and 2, a heat exchanger tube 10, which can here represent the upper end of one leg of a hairpin tube, has a bell 12 or an expanded end. The bell 12 i~ generally cylindrical and is usually enlarged just enough to accommodate the male end of another tube with which it i8 to be connected. This means that its inside diameter is just slightly larger than the original outside diameter. At the junction of the bell 12 with the remainder of the tube 10, there is a torus or ring 14 embossed into the tube, which provides a definite shoulder which can seat against a tubing collar of a tube sheet 16.
As shown in Fig. 3, a ring 18 of brazing wire, for instance, an alloy of copper and phosphorus, is lodged within the embossed ring 14. A male end 20 of an as~ociated connecting tube member is inserted within the bell 16. The male end 20 here has about the same outside diameter as the unenlarged outside :::
.

' ` ,~ .' ' ' ' ' ' . ' " ' l~O~G~5 diameter of the tube 10. There i~ a very small clearance between the inner wall of the bell 12 and the outer wall of the male end 20, 80 that when the bell 12 and ring 14 are heated aufflciently to melt the brazing wire 18, the brazing materlal i9 drawn up into thi~ clearance by capillary action. The resulting braze joint i9 then strongest at the tip~of the male end 20, which formerly had been the weakest point in other brazing operations.
A~ shown in Fig. 4, the unexpanded heat exchanger tube 10 has its bell 12 formed by means of a suitable tube enlarging tool. In this example, this tool includes a collet 22 formed of a number of finger~ 24 which radially surround an axially tapered expansion pin 26. When the collet 22 i8 inserted into the open end of the tube 10, the pin 26 is driven axially, thus wedging the fingers 24 outward and expanding the tube material to form the bell 12, as shown in Fig. 5. This technique, at least to the extent described thus far, is generally described in U.S. Pat.
No. 4,584,765. According to this invention, a boss-forming ring 28 is disposed at the distal end of the collet fingers 24. This ring 28 can either be comprised of rounded members formed on the end of the fingers 24, or can be a ring of steel wire or a ring of brazing metal carried on the fingers 24. When the fingers 24 are expanded, as shown in Fig. 5, the boss-forming ring 28 urges the tube metal outward to emboss the torus or ring 14.
After this, the pin 26 is withdrawn and the collet 22 contracts, as in Fig. 4. The collet is withdrawn to leave the bell generally as shown in Fig. 1.
There are, of course, other means available for belling ., .... ., ; .. , . ~..... .. . .

~ 605 tubing ends which could provide equivalent result~.
As aforementioned, with the bell-end ring or torus structure of this invent~on, the process of expanding heat exchanger tubes under ten~ion 18 described inltially with reference to Fig. 6, which illustrate~ an existlng technique. In this technique a heat exchanger tube 10' is provided with a bell 12', which is simply a gradually expanded end portion of the tube 10'. The tube 10' passes through a tube sheet 16, and through a plurality of fin plates 30. A claw device 32 grips the bell 12' of the tube. However, because of the notable lack of any shoulder on the bell 12', it is difficult for the device 32 to obtain a good purchase on the outside of the bell 12'.
Accordingly, extreme force must sometimes be used, which can damage the tube 10'. The expansion proces~ is carried out by lS ramming a "bullet" type expansion device 34 through the unexpanded tube 10' 80 that its outer surface is expanded into good thermal contact with the openings in the associated fin plate~ 30. The bullet device 34 is driven or rammed through the tube 10' by means of an expander rod 36. While the rod 36 is driven through the tube 10', the claw 32 maintains tension on the tube 10'. However, if the claw 32 cannot maintain its grip on the tube bell 12', the bell can be pushed through the tube sheet 16, necessitating scrapping of the entire coil unit.
As illustrated in Fig. 7, in a tube expansion process according to the present invention, the claw 32 can grab the tube behind the embossment or torus 14. This provides a sure grip on the tube 10' without the need for a great deal of force, and . :

1~0~ 5 without requiring substantial standoff between the bell 12 and the tube sheet 16.
As shown in Fiq. 8, lt is posslble ~or the tube expanslon process to be carried out wlthout a claw device 32, that is, wlth the tube sheet 16 being directly supported or held, and transmitting tension directly to, the distal surface of the embo6sment or torus 14. That is, in the process as illustrated in Fig. 8, the shoulder of the ring 14 rests directly on the collar or opening in the tube sheet 16. This mechanical support of the embossment 14 prevents the tube 10 from being pulled through the tube sheet opening when the expansion device 34 and the rod 36 are being driven through the tube 10. With this configuration, the tube 10 can be provided with a zero standoff between the tube sheet 10 and the embossment 14. Thi~ permits lS the use of shorter hairpin tubes, and achieves a greater burst strength, and a more compact structure.
As shown in Fig. 9, the principles of this invention can be achieved by an assembly 110 comprising a tube 111 of copper or other suitable metal in combination with a union 112. The latter is a cylindrical sleeve with an inner cylindrical surface to receive tubes of a given outside diameter. A second tube 113 has an end also received in the union 112. Here there is an annular embossment 114 midway along the union to define an annular void serving as means to hold a ring 118 of a wire of fusible metal, such a~ a braze or a solder. The ring 118 also serves as means to seat the ends of the two tubes 111 and 113 within the union `: - 11 -, i30~ i05 In all of the above example~, the ring 14 can be of a braze, a solder, or other fusible material as would be suitable for the intended end use. Also instead of the embo~ed annulus 14 or 114, the copper tube could be staked or otherwise worXed to provide means for mechanical retention of the braze or ~older ring 18.
Thase joints which employ the principles of this invention are not limited to heat exchanger tubes or joiQts formed in heat exchanger tubes or joints formed in heat exchanger fabrication. The invention can be applied to advantage in joining other metal tubing where increased joint reliability and ease of brazing or soldering would be desirable. This for example could be in joining the compressor to the evaporator coil or the condenser coil. The invention could al80 find lS advantageous application outside of mechanical refrigeration or air conditioning apparatus.
While the invention has been described with reference to selected preferred embodiments, it should be understood that the invention is not limited to the preferred embodiments, but that many modifications and variations woul~ become apparent to those of skill in the art without departing from the scope and spirit of this invention, as defined in the appended claims.

Claims (18)

1. In a tube and plate type heat exchanger of the type in which a metal tube has an elongated main tube portion passing through a tube sheet and a plurality of heat exchanger fin plates and an end portion which is belled out to a suitable inside diameter to receive a mail tube member of substantially the same outside diameter as the main portion of the tube, the improvement comprising an embossed ring joining the main, unbelled portion and the belled-out end portion and having a greater diameter than the outer diameter of the belled-out end portion.

2. A heat exchanger as in claim 1 in which said belled-out end portion is substantially cylindrical.

3. A heat exchanger as in claim 1 in which said embossed ring is substantially a torus.

4. A heat exchanger as in claim 1 further comprising a ring of fusible metallic material situated in said embossed ring.

5. A method of belling an end portion of a metal tube of a given outside diameter comprising expanding the end portion to a suitable inside diameter for receiving a male tube member also substantially of said given outside diameter, and embossing a ring at the junction of the expanded end portion with the remainder of the tube so that the ring has a greater outside diameter than that of the expanded end portion.

6. The method of claim 5 further comprising inserting a ring of fusible metal inside the embossed ring.

7. The method of claim 5 wherein the step of expanding the end portion includes inserting a collet which has a plurality of radially movable fingers into the end portion of the tube, and expanding said fingers radially outward; and wherein the step of embossing said ring is carried out by an embossing device carried on said fingers.

8. The method of claim 7 wherein the embossing device comprises a wire ring disposed over the fingers of said collet.

9. A method of expanding a heat exchanger tube under tension in which an expansion device carried on the end of a drive rod is forced through a heat exchanger tube while the tube is being gripped at a belled end portion of the heat exchanger tube, and held by a tensive force in which said tube has an embossed ring, of greater outside diameter than the outside diameter of the belled end portion, that joins the end portion with the remainder of the tube, and said gripping is carried out by holding said tube by the embossed ring.

10. A method of expanding a heat exchanger tube as in claim 9, wherein said gripping is carried out by gripping jaws placed over said embossed ring.

11. A method of expanding a heat exchanger tube as in claim 9, in which said tube is disposed in a tube sheet that is seated against the embossed ring on the side thereof opposite the belled end portion, and said gripping is carried out by supporting the tube sheet while said bullet is driven through the tube.

12. A method of expanding a heat exchanger tube as in claim 9, and additionally comprising joining another tubular member to the belled end portion, by inserting a ring of brazing material inside said embossed ring, inserting a male portion of said other tubular member into the belled end portion and against said brazing material ring, and heating the belled end portion sufficiently to braze the male portion of the tubular member to the belled-end portion of the heat exchanger tube.

13. In combination, a tube sheet, a plurality of heat exchanger plates, a metal tube of a given outside diameter projecting through said tube sheet and said plates, a cylindri-cal sleeve at one end of said metal tube beyond said tube sheet and having an inner surface with a diameter sufficient to receive a male tube member having the same outside diameter as said metal tube, and an embossed ring formed at a junction of the metal tube and the sleeve, and forming an annular void on an inner surface of the junction.

14. The combination of claim 13 further comprising a ring of a fusible material disposed in said annular void.

15. A union for joining a metal tube of a given outside diameter to another tube of substantially the same outside diameter, comprising a cylindrical metal sleeve having an inner surface with a diameter to receive the ends of said metal tubes, a ring of a fusible metal material disposed in said sleeve at a position to seat the ends of said tubes; and means integral in said sleeve retaining said ring of fusible metal.

16. The union of claim 15 in which said means includes an annular embossment in said sleeve defining an annular void in which said ring is reposed.

17. A heat exchanger as in claim 1 wherein said embossed ring is formed substantially against said tube sheet.

18. The combination of claim 13 wherein said embossed ring is formed substantially against said tube sheet.