US3795125A

US3795125A - High-fin integral finned tube of heat-resisting alloys, and multi-pass process for making the same

Info

Publication number: US3795125A
Application number: US00221358A
Authority: US
Inventors: D Laing; J Issott
Original assignee: Universal Oil Products Co
Current assignee: Bank of Nova Scotia; Wolverine Tube Inc; Universal Oil Products Co
Priority date: 1972-01-27
Filing date: 1972-01-27
Publication date: 1974-03-05
Anticipated expiration: 1991-03-05
Also published as: FR2169178B1; AR199096A1; ATA53473A; DE2303192A1; AU5098173A; IL41228A0; AT345234B; ZA7386B; JPS529435B2; ES410739A1; GB1409016A; CA980099A; AU470554B2; IL41228A; BR7300626D0; JPS4886770A; FR2169178A1; EG10818A; IT976955B; CH557015A

Abstract

Rolling of high-fin integral finned tube from stainless steel or other difficult to form material, in two operations with an annealing step between rolling operations. Each rolling operation quickly forms narrow helical grooves to displace metal into wide helical ribs or rib portions, followed by more gradual widening and deepening of the grooves with additional metal displacement, and lateral application of pressure to the ribs to form it to required fin shape.

Description

United States Patent 1 Laing et al.

.1111 3,795,125 1 Mar.5, 1974 1 HIGH-FIN INTEGRAL FINNED TUBE OF HEAT-RESISTING ALLOYS, AND MULTI-PASS PROCESS FOR MAKING THE SAME [75] Inventors: David D. Laing, Plymouth; John W.

issott, Dearborn Heights, both of Mich.

[731 Assignee: Universal Oil Products Company, Des Plaines, Ill.

22 Filed: Jan. 27, 1972 211 Appl. No.: 221,358

3,383,893 5/1968 Counts 72/98 3,174,319 3/1965 Sayukoyama et al... 72/96 3,379,042 4/1968 Brown 1 l 72/78 3,648,502 3/1972 Klug et al. 72/78 Primary ExaminerLowell A. Larson Attorney, Agent, or Firm-Whittemore, Hulbert &

Belknap [57] ABSTRACT Rolling of high-fin integral finned tube from stainless steel or other difficult to form material, in two operations with an annealing step between rolling operations. Each rolling operation quickly forms narrow helical grooves to displace metal into wide helical ribs or rib portions, followed by more gradual widening and deepening of the grooves with additional metal displacement, and lateral application of pressure to the ribs to form it to required lin shape. 3 I

11 Claims, 6 Drawing Figures l/VS/DE T086 SURFACE PATENTED 51974 FIGI l/vsnna Tuag mF/vce FIG.5

HIGH-FIN INTEGRAL FINNED TUBE OF HEAT-RESISTING ALLOYS, AND MULTI-PASS PROCESS FOR MAKING THE SAME BRIEF SUMMARY OF THE INVENTION The rolling up of material from the outer wall of a tube by effecting relative revolution of a plurality of sets of axially aligned finning discs, while rotating the discs, is well known. When working with malleable materials such as copper, particularly where relative high fins are not required, present techniques are satisfactory.

However, when working with metals or alloys which are difficult to work, particularly when work hardening occurs as in stainless steel, it has heretofore been impossible to attain the required results. The problem is made more difficult where the fin is to be of substantial thickness throughout and thus to have a relatively blunt outer edge as compared to fins the outer edge of which are substantially knife edged.

An example of finned tubing of this type is designed for use in cooling liquid sodium by air, as is done in nuclear reactor applications, more specifically fast breeder reactors. finned tube is produced by two or more rolling operations with annealing of the tube intermediate successive rolling operations. Successive rolling operations are substantially similar, except that in subsequent operations, the leading discsoperate in the helical grooves left by the preceding operation.

In each operation a set comprising a multiplicity of discs are assembled in engagement in axial alignment and contact the tube with the axes crossed in space to conform to helical fin formation to the assembly of circular discs. The first few discs of each set have the edges thereof transversely curved on a relatively short radius to facilitate penetration into the material of the tube, and successive discs increase in diameter by relatively large amounts. This quickly forms one or mor relatively deep, relatively narrow helical grooves, and displaces a substantial quantity of metal into a relatively wide, relatively low helical rib in the first operation and into the base or root portions of previously formed and shaped ribs in subsequent operations. Thereafter the successive discs are shaped to hav gradually wider edges, and have relatively less increase in diameter from disc to disc, so as to produce a gradual widening and deepening ofthe groove, with consequent displacement of more metal into the eventual fin. In addition, the angularity of the sides of the discs from radial surfaces decreases gradually to a very low value, so that fins are formed having a thickness at their tips amounting to a very substantial percentage of the fin thickness adjacent the base.

The successive rolling operation or operations ar carried out with sets of discs of increased thickness or pitch, as compared to the pitch of the discs of the immediately preceding set, thus producing helical formations on the tube of longer lead.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary section through a finished finned tube produced in accordance with the present invention.

FIG. 2 is a generalized outline of a single fin produced in accordance with the present invention.

FIG. 3 is a sectional view similar to FIG. I of the same tube following its initial rolling' operation with partially formed fins thereon.

FIG. 4 is an enlarged diagrammatic view with wall sections of FIGS. 1 and 2 superimposed.

FIG. 5 is a fragmentary outline view of successive disc edge portions of a set of discs.

FIG. 6 is a fragmentary outline of a finning disc to show dimensions defined in the specification.

DETAILED DESCRIPTION Referring first to FIG. 1, there is shown an enlarged fragmentary cross-section of a finished tube 10 produced in accordance-with the present invention. Very approximately, the critical dimensions of this tube are as follows:

DIMENSIONS INCHES Fin height l2 (H30 Fin thickness at tip l4 0027 Fin thickness at root I6 004] Fin thickness, average l8 0.034 Fin space, average 20 0.058

Wall thickness 22 0.107

From the foregoing tabulation it will be noted that with an exceptional fin height of 0.130 inches, the fin thickness at the tip is over 20 percent of fin height, and is over 65.percent of the fin thickness near the root.

A more generalized fin cross section outline of a single fin is seen in FIG. 2, where dimensions are applied to show fin thickness at tip and base, fin height and wall thickness.

' From the foregoing it is apparent that the fin of the present invention differs from conventional fins in its exceptional thickness adjacent its tip, and its height as compared to its average thickness. In general terms the present invention may be considered as producing fins having a minimum height of 0.100 inches, a thickness adjacent its tip which is not less than 20 percent of the fin height, a thickness adjacent is base or root which is not less than 25 percent of the fin height, and a thickness at the tip which is not less than 60 percent of its thickness adjacent its base. This represents a fin crosssection which is characterized by substantial tip thickness and which is advantageous in rendering the entire fin surface useful in heat transfer, as well as in providing a substantial increase in area.

In the production of this new finned tubing, mention has been made of a plurality of successive rolling operations with an annealing step intermediate successive rolling operations. In FIG. 3 there is shown the crosssectional shape of the tube wall with ribs or partially formed fins following the first rolling operation, where this operation is designed to be the first of two, the second of which produces the tube configuration as seen in FIG. I. In FIG. 3, the partially rolled tube 26 has the approximate dimensions tabulated below:

DIMENSION INCHES Rib height 28 0.085 Rib thickness at tip 30 0.032 Rib thickness at root 32 0.039 Rib thickness, average 34 0.035 Rib space, average 36 0.054 Wall thickness (H29 Here it will be observed that the average rib thickness is about 40 percent of the rib height, and the thickness at the tip is over percent of the thickness near the fin root.

In FIG. 4 theoutline of the cross-sectional shapes of the finished tube is superimposed over the outline of the cross-section of the semi-finished tube. The inside tube diameter at 40 is used as the common reference line, and the rib or partially formed fin 42 seen at the left of the Figure is superimposed so far as lateral position is concerned over the finished fin configuration 44. By comparing these Figures, it will be noted that the groove or channel between fins has been deepened by the amount shown at 46, while the crest or tip of the fin has been displaced radially outward by the amount shown at 48. At the same time it will be noted that the outline of the entire upper portion of the partially formed fin'or rib conforms very closely with the mid portion of the finished fin, as indicated by the matching side surfaces seen generally at 50.

FIG. 4 also graphically illustrates a very important feature of the present invention and that is the increase in pitch or axial spacing of adjacent fin or rib convolutions as produced by the second rolling operation as compared to the first. In the specific example illustrated in FIGS. 1, 3 and 4 the pitch or axial spacing of the roll discs which produced the semirolled tube of FIG. 3 was 0.082 inches, while that of the discs which produced the finished finned tube of FIG. 1 was 0.086 inches. This represents an increase in pitch or lead of nearly 5 percent. This of course requires that in multiple pass rolling, the pitch produced by the first pass must be sufficiently smaller than the final desired pitch as to produce the required number of fins per inch, in this case, eleven. Referring now to FIGS. 5 and 6, there is an enlarged diagrammatic showing of the edge of an assembly or set of discs which are separately designated 56a to 56 An important feature of the present invention is that the profiles, or cross-sectional shape of the side surfaces of the edge portions of the corresponding discs is substantially the same in the two or more setsor assemblies. The only significant difference in correspoinding discs is the thickness thereof as indicated by the dimension 58, which as previously described, is increased in successively acting sets of discs.

In FIG. 6 there is shown in cross-sectional outline the generalized shape of the periphery of a fin-forming disc. In this Figure the sides of the edge portion of the disc a re shown at 60 and are substantially straight in radial cross-section of the disc. In other words, the straight line side elements together define a frustoconical surface, the included cone of which is determined by the angle A between straight line elements 60 and the line 62, which is parallel to the central radial plane 64 of the disc. Where the element 60 is inclined at 50 to the plane 64 (or intersects the line 62 at this angle) the included cone angle is of course 170.

The disc has a periphery 65 which is rounded in cross-section, and its radius of curvature is designated R in the Figure. Portions 66 of the edge surface smoothly interconnect the rounded edge with the conical sides. It will be apparent that the smaller the radius R, the more readily the disc will penetrate into the material of the tube.

The thickness of the disc adjacent the

rounded edge portion

65,66 is designated 68. This dimension is determined by the distance between the intersections of the line 67, tangent to the tip 65, with the extension of straight line side elements 60. The thickness of the disc at an annular zone spaced inward of the edge at a constant dimension 69 is designated 70.

The progressive change in shapes and dimensions of the discs 56a56q of course determines the specific manner in which the metal of the tube is displaced and shaped into final form.

Initially, it isrequired to obtain a quick penetration into the tube, to establish a relatively narrow, relatively deep groove or channel, adjacent convolutions of which define therebetween a relatively wide and short rib, themetal of which will eventually be shaped into the outer edge of the fin. Thereafter, by the progression in shapes and dimensions of successive discs, the

groove or channel is gradually deepened and widened by a gradual increase in the diameter and tip width and radius of transverse curvature, to move more metal into ribs establishing the eventual fin portion, and to apply lateral pressure to shape the material. In general terms, the first few discs, as for example the first six discs 56a-56f have a tip thickness dimension 68 which is constant and desirably are about percent of the tip thickness dimension 68 of the final few discs, as for example the final five discs 56m-56q, with a gradual increase in tip thickness in the intermediate discs.

At the same time, as the tip thickness dimension 68 increases, the radius of the rounded periphery 65 in creases, and the radius R of the first few discs are desirably about one-half the radius R of the last few discs, with a gradual increase from disc to disc in the centrally located discs.

The root or gage thickness 70, as measured at a constant gage distance 69 is constant for the first few discs, as for example the six discs 56a-56f, and in successive discs is decreased gradually to the last few discs, as for example the last five discs 56m-56q, which have a uniform gage dimension desirably about percent of that of the leading discs 56a-56f.

It will of course be apparent that the widening of the tip width 68 and the reduction in gage width 70 results in a reduction in the angle A, which represents the inclination of the conical side surfaces 60 to the radial plane 64. In production of the fin structure specifically disclosed herein, the angle A is reduced from about 5 throughout the first few discs to about l throughout the last few discs, and this reduction is gradual throughout intermediate discs 56f56m.

The first few discs are arranged to provide a quick penetration into the tube wall, and to accomplish this the disc diameter in the first few discs increase very substantially as compared to the increase in diameter of successive discs thereafter. In addition, the first sets of discs are positioned relative to the tube such that the first disc 56a penetrates to a substantial depth, as for example 0.030-0040 inches, into the metal of the tube.

The next few discs, for example the two discs 56!? and 561' desirably have a diameter exceeding the diameter of the preceding disc by from 0010-0020 inches.

The following discs show a more gradual increase in diameter, as for example about one-third of that of the first few discs, and in addition, in the last discs of the series some discs may be identical with an adjacent disc, thus further decreasing the average change in dimensions to a more gradual condition.

The penetration of the edges of the discs into the material of the tube and the pressure applied to the tube by the discs has the effect of forming fins, reducing the inside diameter of the tube, reducing effective wall thickness as measured between fin convolutions, and elongating the tube. In. order to accommodate the tube elongation that accompanies each rolling operation, the axial spacing of median planes of discs, which is uniform for each set of rolls, is increased in succeeding sets.

It is customary to employ three sets of discs at uniform circumstantial spacing around the tube. Thus the radial components of force acting between the rolls and the tube are balanced. However, a pin or mandrel is also provided to control the inside diameter of thefinished tube. The pin is cylindrical and is smaller by a few thousandths of an inch than the inside diameter of the tube as it approaches the sets of finning discs. The initial discs, which have narrower edges which are rounded to smaller diameters than subsequent discs, penetrate the material of the tube without forcing its inner surface into engagement with the pin. Somewhere adjacent the center of each set, radial pressure is sufficient to force the inside surface of the tube into engagement with the pin, and thereafter increase in effective diameter of discs and increase in tip width is effective to displace more metal than when the tube inside diameter is being reduced.

It has been mentioned that the profiles of the edge portions of corresponding discs in subsequently operat ing sets of discs are substantially the same as in preceding sets, differing only in thickness. It will be-understood that alldiscs have flat sides and are assembled in tight side-by-side abutment with each other.

In carryingout the procedure, a tube has its end slightly pointed to provide smooth engagement with the leading discs of the sets which perform the initial rolling operation, and the sets are positioned at radial spacings such that when fully engaged beyond the pointed end section, the leading disc penetrates the tube material to a greater depth, for example double the depth to which the next few discs increase the penetration. In subsequent rolling operations, after annealing, sets of discs are positioned so that the leading disc bottoms in the groove or channel in the partly rolled tube without substantial penetration.

The annealing step of course depends on the particular material, but where the tube is Type 304 stainless steel, the tube is advanced through an annealing furnace having a muffle length of about feet at a speed of 6 inch/min., the furnace being at l,950F. The tube initially had a hardness of R 77 after the initial rolling operation, and after annealingits hardness was reduced to R ,72.

In general terms, the included cone angle at each side of the discs in each set increases from a value of about l65l70 to about 178.

An inspection of FIG. 4 shows that in increasing fin height in the second operation, the increase in fin outside diameter 48 is approximately equal to the increase in fin space depth or reduction in wall thickness shown at 46.

In the foregoing it will be noted that the term fin. is reserved for the final shape produced by the final rolling operation, and that until thus formed, the material that ultimately becomes part of the fin is referred to as a rib.

In the sets of discs, the disc which initially engages the tube is referred to as the first or leading disc of the set, and the disc at the opposite end is referred to as the last, final or trailing disc.

What we claim as our invention is:

1. The method of forming fins of at least 0.100 inches fin height and of predetermined pitch on tubes of difficult-to-roll metal such as stainless steel which comprises forming partial fins of substantially less than final height and of less than said predetermined pitch in a first operation by supporting the tubes on mandrels and applying pressure of metal deforming intensity to limited zones radially inwardly and progressively around the tubes in helical paths of less than said predetermined pitch to produce partiallyformed fins, annealing the tubes, and providing a second forming operation by supporting the tubes on a mandrel and applying pressure of metal deforming intensity to limited zones both at the bottom of the spaces beween partially formed fins and to the sides of the partially formed fins along helical paths of said predeermined pitch to displace metal from the bottom of the spaces between adjacent fins, and to displace metal laterally of the fins to increase the outside diameter of the tin crests, both metal displacements serving to increase the height of the individual fins.

2. The method of claim 1 which comprises in each forming operation both before and after the annealing step, initially applying pressure to relatively narrow zones to form relatively deep narrow grooves leaving relatively wide ribs formed partially of material of the tubes displaced by formation of the grooves, and thereafter gradually widening the zones of pressure application and deepening the grooves to displace additional material into the ribs and to apply pressure to the sides of the ribs to form ribs in the first operation and fins in the second operation of exceptional height and width adjacent the outer edge portion thereof.

3. The method of claim 2 in which the increase in the pitch of fin convolutions formed by the second operation over the pitch of rib convolutions formed by the first operation is about 3-5 percent.

4. The method of claim 1, which comprises using a set of axially aligned discs in each operation, the discs of each set having the first few discs provided with rounded peripheral portions of a predetermined small radius in radial cross-section and of a predetermined substantial increase in diameter from disc to disc whereby to penetrate into the material of the tube quickly to substantial depths, the remainder of the discs of each set having peripheral portions of progressively greater radius of curvature in cross-section and of sub: stantially less increase in diameter from disc to disc, whereby in each rolling operation a relatively thick rib portion of material is displaced from the tube material and thereafter shaped into a narrower higher crosssectional shape.

5. The method of claim 4 in which the side surfaces of each disc adjacent its periphery is conical and in which the included cone angle of each side increases from about 165-l at the first few discs to approach at a reduced rate of penetration into the tube material while applying lateral pressure to shape the rib into a tin of the required dimension.

7. The method of claim 6 which comprises at the start of each rib forming operation, applying pressure to obtain a metal displacing penetration into the tube material several times as great as the penetration obtained 7 upon subsequent applications of presusre.

Ti. The method of claim 7 in which the tin height pro duced in the first operation is at least 0.060 inches and in the second operation is at least 0.100 inches.

9. The method of claim 7 in which the fin height produced in the first operation is at least 0.075 inches and in the second operation is at least 0.1 15 inches.

10. The method of claim 6, which comprises the step ofincreasing the pitch of fin convolutions between successive rolling operations.

11. The method as defined in claim 10 in which the increase of pitch between successive rolling operations is about 3-5 percent.

Claims

1. The method of forming fins of at least 0.100 inches fin height and of predetermined pitch on tubes of difficult-to-roll metal such as stainless steel which comprises forming partial fins of substantially less than final height and of less than said predetermined pitch in a first operation by supporting the tubes on mandrels and applying pressure of metal deforming intensity to limited zones radially inwardly and progressively around the tubes in helical paths of less than said predetermined pitch to produce partially formed fins, annealing the tubes, and providing a second forming operation by supporting the tubes on a mandrel and applying pressure of metal deforming intensity lo limited zones both at the bottom of the spaces beween partially formed fins and to the sides of the partially formed fins along helical paths of said predeermined pitch to displace metal from the bottom of the spaces between adjacent fins, and to displace metal laterally of the fins to increase the outside diameter of the fin crests, both metal displacements serving to increase the height of the individual fins.

4. The method of claim 1, which comprises using a set of axially aligned discs in each operation, the discs of each set having the first few discs provided with rounded peripheral portions of a predetermined small radius in radial cross-section and of a predetermined substantial increase in diameter from disc to disc whereby to penetrate into the material of the tube quickly to substantial depths, the remainder of the discs of each set having peripheral portions of progressively greater radius of curvature in cross-section and of substantially less increase in diameter from disc to disc, whereby in each rolling operation a relatively thick rib portion of material is displaced from the tube material and thereafter shaped into a narrower higher cross-sectional shape.

5. The method of claim 4 in which the side surfaces of each disc adjacent its periphery is conical and in which the included cone angle of each side increases from about 165*-170* at the first few discs to approach 180* at the last few discs.

6. The method of forming fins on the exterior of metal tubes in which the fins are characterized by exceptional height and a thickness at the crest which is at least half the thickness adjacent the root which comprises repeatedly applying pressure in a first rolling operation to adjacent narrow zones progressively around the tube in a helical path to penetrate into the tube material and to initially displace a substantial quantity of metal into a helical, short, thick rib and thereafter progressively displacing material from wider zones with reduced penetration while applying lateral pressure to the rib, terminating the application of pressure and annealing the tube, and thereafter in a second rolling operation repeatedly applying pressure in the helical spaces between rib convolutions in narrow zones with increased penetration into the tube material to displace a further substantial quantity of material into the rib, and finally displacing material by penetrating into the tube material over zones of increased width at a reduced rate of penetration into the tube material while applying lateral pressure to shape the rib into a fin of the required dimension.

7. The method of claim 6 which comprises at the start of each rib forming operation, applying pressure to obtain a metal displacing penetration into the tube material several times as great as the penetration obtained upon subsequent applications of presusre.

8. The method of claim 7 in which the finheight produced in the first operation is at least 0.060 inches and in the second operation is at least 0.100 inches.

9. The method of claim 7 in which the fin height produced in the first operation is at least 0.075 inches and in the second operation is at least 0.115 inches.

10. The method of claim 6, which comprises the step of increasing the pitch of fin convolutions between successive rolling operations.