US2586653A - Method of producing heat exchange elements - Google Patents

Description

W. P. HILL 7 METHOD OF PRODUCING HEAT EXCHANGE ELEMENTS Feb. 19, 1952 2 SHEETSSHEET 1 Filed June 22, 1951 FIG.2.

INVENTOR.

WALTER P. HILL Feb. 19, 1952 w. P. HILL METHOD OF PRODUCING HEAT EXCHANGE ELEMENTS Filed June 22, 19511 2 SHEETS-SHEET 2 FIG.7.

INVENTOR.

WALT ER P. HILL wyw ATTORNEYS Patented Feb. 19, 1952 METHOD PRODUCING HEAT EXCHANGE ELEMENTS Walter P. Hill, Pontiac, Mich., assignor to Calumet and Hecla Consolidated Copper Company, Calumet, Mich., a corporation of Michigan Application June 22, 1951, serial No. 238,043

2 Claims. 1

This invention relates to an improved method of producing a heat exchange element and is a. continuation-in-part of my co-pending application, Serial No. 42,048 on Heat Exchange Device and Method of Forming the Same."

One of the objects of this invention is to provide a heat exchange unit comprising an outer finned tube and an inner tube secured in intimate contacting relation with the outer tube to form a liner for the latter.

Another object of this invention is to provide a heat exchange unit wherein a helical fin is integrally formed on the outer surface of the outer tube and wherein the inner surface of the outer tube is pressed into intimate engagement with the outer surface of the inner tube by the fin forming operation.

Still another object of this invention is to mechanically interlock the two tubes together by the fin forming operation in a manner such that any heat loss resulting from the joint between the tubes is negligible even where extremely high temperature differentials are encountered.

A further object of this invention is to provide a composite tubular structure of the type noted above wherein the outer tube may be formed of one metal and the inner tube may be formed of a different metal. For example, the outer tube may be formed of a white metal such as aluminum and the inner tube may be formed of copper or steel. Thus, the part of the composite tubing from which the fins are extruded may be formed of an inexpensive, ductile material capable of being easily worked during the fin forming operation which, of course, is desirabl in that it reduces tool breakage as well as cost to a minimum. At the same time the above arrangement enables employing a copper or copper alloy liner characterized in that it resists corrosion by fluids ordinarily used, for example, in refrigeration and renders it possible to obtain highly effective brazed joints with such materials as silver solder for example. Moreover, since the two tubes are pressed into intimate interlocking relationship during the fin forming operation, the above advantages are obtained without an appreciaable loss in heat transfer efliciency. In actual use of such a composite finned tube, the loss may be five per cent or less which, in most installations, is insignificant especially when the saving in cost is considered.

The foregoing as well as other objects will be made more apparent as this description proceeds, especially when considered in connection with the accompanying drawings, wherein:

Figure 1 is a longitudinal sectional view through a. composite finned tube or heat ex= change device enlarged approximately three times its actual size;

Figure 2 is a. cross sectional view taken substantially on the plane indicated by the line 2-2 of Figure 1;

Figure 3 is a semi-diagrammatic cross sectional view through one type of apparatus that may be employed for forming an integral helical fin on the outer tube of the heat exchange device;

Figure 4 is a sectional view taken substantially on the plane indicated by the line 4-4 of Figure Figure 5 is a greatly enlarged fragmentary cross sectional view showing one of the roll sections in fin forming relationship with the outer tube;

Figure 6 is a sectional view taken substantially on the plane indicated by the line 6-6 of Figure Figure 7 is an enlarged fragmentary sectional view illustrating a part of one of the rolls in operative relation to the outer tube; and

Figure 8 is a fragmentary sectional view of the plain tubes prior to being subjected to the fin forming operation.

The reference numeral 10 in Figure 1 of the drawings designates generally a heat exchange device which may take any shape required for the particular use to which the device is to be put. For example, in the event the device is to be used as either a condenser or an evaporator in a refrigerating system, the device may be bent into a flat or helical coil or may be fashioned to a serpentine contour.

In any case, the device comprises an outer tube H and a liner or inner tube If. The outer tube has a helically extending fin l3 integral with the outer surface thereof and is also provided with a helically extending groove H in the inner surface. The groove 14 extends in substantially the same helicoidal path as the fin l3 and results from the fin forming operation which will be presently described.

The tube II has walls which are thin in comparison to the tube II and is sleeved within the tube l I with the outer surface in contact with the inner surface of the tube H. As shown particularly in Figure 1 of the drawings, the outer surface of the tube "is formed with a helically extending rib l5 and this rib fitswithin the helical groove I to provide an interlocking connection between the two tubes. As will be presently set forth, the rib Ii is rolled up from the inner 3 tube l2 at the same time the material of the outer tube is extruded to form the helical fin Hi.

In the present instance ,the outer tube H is formed of one material and the inner tube 12 is formed of still another material. Both materials are preferably of non-ferrous metals, although in some cases the inner tube or liner l2 may be formed of steel. In most applications the outer tube H is formed of a white metal such, for example, as aluminum, and the inner tube 12 is preferably formed of copper. These two materials, although having diflerent characteristics, are nevertheless sufiiciently similar to avoid appreciable relative movement by substantial changes in temperature.

The fabrication of the tube H of aluminum is advantageous not only because this material has .a high thermal conducting characteristic but also because it may be more easily worked or extruded than many other types of materials such as copper or steel. As a consequence, tool breakage is reduced to a minimum and the cost of manufacture is correspondingly reduced. The tendency for the aluminum to corrode in the presence of certain fluids ordinarily employed in refrigerating systems, for example, is overcome by the copper liner or inner tube l2. Also, the copper inner tube lends itself more readily to brazing operations with silver solder and thereby assures obtaining highly efiective joints between the tubing and suitable fittings. As will be apparent from the following description, the two tubes are mechanically bonded together during the fin forming operation and the joint therebetween is so tight that the loss of efiiciency in the transfer of heat is practically negligible. In actual practice, it has been found that the efficiency loss through the joint between the two tubes does not exceed five per cent.

Although the helical fin may be. formed on the 4 formed on the outer tube H. The construction is such that the discs of the respective rolls actually track with one another and cooperate to form a continuous helical fin on the peripheral surface of the outer tube H in a manner to be presently described. Referring again to Figure 3 of the drawings, it will be noted that the arbors are respectively supported on the free ends of swinging arms 2| enabling the rolls to be moved in directions toward and away from the axis of the mandrel [8. In the present instance, the

. arbors 20 are driven and suitable power means (not shown) is universally connected to the arbors for this purpose.

The number of discs which cooperate to form each roll may vary considerably but for the purouter tube H by various methods and different types of apparatus, nevertheless, I prefer to employ the method and apparatus shown in Patent No. 2,508,518, dated May 23, 1950. This method and apparatus is preferred because it renders it possible to develop a helical fin from the outer surface of the tube H without appreciably rubbing or bending the fin and without hardening or reducing the ductility of the metal forming the fin.

With the above in view, reference is made more in detail to Figures 3 to 6 inclusive wherein one form of apparatus is diagrammatically illustrated. This apparatus comprises a mandrel it having an outside diameter approximately the same as the inside diameter of the tube l2 and adapted to be sleeved into the latter. In addition, the apparatus embodies three rolls indicated generally in Figure 3 of the drawings by the numerals ll, i8 and I9. These rolls are spaced equal distances from each other around the axis of the mandrel l8 and are adapted to engage the peripheral surface of the outer tube II when the latter, together with the inner tube l2, are supported on the mandrel. Each roll comprises a series of individual circular portions or discs having hubs centrally apertured to receive an arbor 20 and having the hub portions keyed or otherwise secured to the arbor 20. The axes of the arbors 20, or in other words the axes of the rolls, are crossed with respect to the axis of the mandrel and are also arranged at such an angle to the axis of the mandrel as to travel a helical path about the outer tube H which corresponds generally to the helix angle of the fin l3 to be pose of illustration, each roll is shown in Figure 4 of the drawings as comprising seven discs designated by the numerals 22 to 28 inclusive. The opposite sides of the peripheral portions of the discs are relieved to provide spaces or grooves 28 between adjacent discs with converging fin forming surfaces 30. It will further be noted from Figure 4 of the drawings that the peripheral portions of the discs progressively increase in width from the first disc 22 to the final disc 28 with the result that the widths of the grooves 29 between the discs are correspondingly reduced.

Figure 8 of the drawings shows the relationship of the inner and outer plain tubes prior to subjecting the same to the fin forming operation. The outer plain tube is indicated by the numeral HA and the inner plain tube is designated by the numeral I2A. It will be noted that the outer tube HA has a wall thickness exceeding twice the wall thickness of the inner tube HA, and has a length substantially less than the inner tube HA. The purpose of such an arrangement will become more apparent as this description proceeds. Briefly the outer plain tube HA elongates relative to the inner plain tube IZA as a result of the fin rolling operation, and the wall thickness of the outer plain tube HA is sufficient to not only permit the required degree of elongation of this tube, but to also provide sufilcient material for the fins l3 while at the same time assuring the provision of a specified wall thickness for the finished tube II. The length differential between the plain tubes HA and HA is important only if it is desired to provide the tubes II and I2 with approximately the same length after the rolling operation. In such case the length of the inner plain tube 12A should exceed the length of the outer plain tube HA by an amount approximating the degree of elongation of the outer tube during the fin forming operation.

It is important to note that the external diameter of the inner plain tube |2A is sufiiciently less than the internal diameter of the outer plain tube HA to provide a loose fit between the plain tubes when arranged in the telescoping relationship shown in Figure 8 of the drawings. The

two plain tubes HA and l2A are installed upon the mandrel H5 in the manner shown in Figure 8 at the start of the forming operation. In this connection it is pointed out that the use of the mandrel I6 may be dispensed with providing the inner tube possesses the required rigidity to adequately support the outer tube during the fin forming operation.

After the two telescopically arranged plain tubes are properly positioned between the fin forming rolls. the arbors 20 together with the associated fin forming rolls are swung inwardly toward the plain tubes, and are locked in their innermost positions shown in Figure 3 of the drawings by any suitable means not shown herein.

discs on each roll progressively exert a reducing pressure on the outer plain tube A and contract or size the latter so that the inner surface thereof frictionally engages the outer surface of the inner plain tube HA. The fin forming rolls l1, l8 and iii are rotating during the above operation so that rotation is imparted to both the inner and outer plain tubes.

As stated above, the discs of each fin forming roll are spaced axially from one another so that the peripheral ,portions exert a circumferential rolling pressure in a generally radially inward direction on axially spaced portions of the outer plain tube HA and this pressure is so determined with respect to the malleable characteristic of the outer tube A that material from the tube HA is extruded outwardly into the spaces between adjacent roll discs. Attention'is now called to the fact that the axes of the rolls l1, l8 and [9 also cross the axes of the Plain tubes and that the discs on the several rolls operate in tracking relationship. Hence the telescopically engaging tubes are advanced by the rolls in the direction of their axes, and the peripheral edge surfaces of the roll discs apply the required circumferential rolling pressure along ahelical path depending upon the angle at which the axes of the forming rolls cross the axes of the tubes. Thus the material progressively rolled up or extruded from the outer tube A into the spaces between adjacent discs forms a helical fin'on the outer tube HA.

During the rolling operation the material of the outer tube HA is extruded throughout the wall thickness of this tube so that the inner surface of the tube HA is formed with a helically extending groove l4 directly opposite the root of the helical fin l3. Referring now to Figure 4 of the drawings it will be understood that as material from the relatively thick walled outer tube A is extruded outwardly into the spaces provided between adjacent roll discs, the tube i IA is reduced against the inner tube I2A and is elongated. Elongation of the outer tube HA takes place relative to the irmer tube I2A and causes a migration of the inner surface of the outer tube along the outer surface of the inner tube. Since the inner surface of the outer tube HA is brought into intimate frictional contact with the outer surface of the inner tube while this migration or elongation takes place a rubbing or scufilng action results between the outer and 0 formed of materials having widely varying thermal expansion characteristics. This feature is further assured by reason of the fact that the relatively malleable outer tube ll of the finished product not only has integral fins which provide a large surface area, but, in addition,-has a wall thickness greater than the inner tube l2. Hence even in instances where the relatively thin walled inner tube 12 is in the form of a steel tube and the outer tube II is made of aluminum or some equivalent material, a tight interlocking joint As shown in the drawings. the side surfaces 20 v of the grooves 29 or spaces between the peripheral portions of adjacent roll discs serve to guide the material extruded during the rolling operation to form the fin Il and do not squeeze the material therebetween 'or apply appreciable axial compression to the material. Thus the material is not excessively work-hardened during the extruding operation and a fin ll of substantial height may be raised from the outer tube without danger of fracturing or cracking the fin. It will be noted in Figure 7 of the drawings that a slight clearance space 2| is shown at the trailing sides of the fin l2. This clearance results from displacement of the material axially of the tube IIA during the fin rolling operation.

inner tubes. Also at the same time the roll discs apply a radially inwardly directed rolling pressure on the inner tube II A along a helical path and as a result the helically extending rib I5 is raised from the outer surface of the inner tube I2A. This rib l5 projects into the groove I4 and the migration of the grooved surface of the outer tube HA along the ribbed outer surface of the inner tube has a peening action on the rib which forces the same into exceptionally intimate contact with the walls of the grooves It. In actual use this. interlocking connection retains its tight or intimate contacting relationship throughout extremely wide ranges of temperature differentials and very little if any noticeable heat loss is apparent at the joint beween the. tubes even in cases where the tubes are During the fin rolling operation there is a tendency for the discs to bend the fin convolutions back and forth due to interference between the sides of the discs and adjacent convolutions of the fin l3. In order to overcome this objection which has a tendency to work harden the fin, the

, surfaces 30 are ground or otherwise formed to a spheroidal contour or, in other words,'are provided with a non-rectilinear curved radial contour determined so that the peripheral portions of the discs will pass through the helicoidal path between adjacent convolutions of the fin without laterally displacing the fin or working the material to any practical extent. There is, of course, contact between the respective forming discs and the material of the fin ii. In Figures 5 and 6 of the drawings, a typical forming disc is shown in relation to the'fin and it will be noted that the spheroidal surface 30 at one side of the disc contacts the fin at a point 32 above the axis of the tube II, and the spheroidal surface at the opposite side of the disc contacts the fin at a point 33 located below the axis of'the tube Ii. However, the curvature of the spheroidal surfaces on opposite sides of the discs is such that no part of either surface will either interfere with or displace the formed fin l3. Thus, regardless of the diameter of the fins being formed, the rolls do not excessively work harden the extruded material during the fin forming operation, but on the other hand. merely serve to in effect guide the material as it is extruded outwardly into the grooves 29.

Attention is further called to the fact that while the spaces or grooves 29 progressively decrease in width from the first groove between the adjacent discs 22 and 23 to the last groove between the discs 21 and 22, nevertheless, no appreciable axial compression of the displaced material takes place because the shape of the radial surfaces 30 of the grooves 29 and the depth of the latter are such that the material flows relatively freely into the grooves during the extruding operation. Thus, the density of the material forming the fins is very little, if any, greater than the normal density of the material prior to the fin forming operation.

What I claim as my invention is: 1. The method 01' forming a composite tub having an integral helically extending fin projecting outwardly from the outer tube and having an integral helically extending rib projecting outwardly from the inner tube into a helically extending groove formed in the inner surface of the outer tube opposite the root of the -fln, which comprises telescopically engaging inner and outer plain tubes, the inner plain tube having an external diameter sufllciently less than the internal diameter of the outer plain tube to permit elonation of the outer tube relative to the inner tube and the outer plain tube having a wall thickness substantially greater than the wall thickness of the inner plain tube, exerting a circumferential rolling pressure in an inward direction on axially spaced helically aligned portions of the outer plain tube to extrude material from the outer tube throughout its wall thickness to form an external helical fin on the outer tube and at the same time form a helical groove in the inner surface of the outer tube directly opposite the root of the helical fin, reducing the outer tube against the inner tube by the circumferential rolling pressure and at the sametime elongating the outer tube relative to the inner tube to cause migration of the inner surface of the outer tube along the inner tube, filling the grooves formed in the inner surface of the outer tube with ribs rolled up from the outer surface of the inner tube by continued application of the rolling pressure and pressing the ribs into intimate contact with the adjacent walls of the grooves by the migration of the outer tube along the inner tube.

2. The method defined in claim 1 in which the plain outer tube has a malleable characteristic greater than the inner tube and has an initial length less than the length of the inner tube and in which the application of the circumferential rolling pressure against the outer tube along a helically extending path elongates the outer tube relative to the inner tube an amount approximating the difference in the initial length between the two plain tubes.

WALTER P. HILL.

No references cited.

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