US3368547A - Finned-tube heat exchanger - Google Patents

Description

Feb. 13, 1968 w. A. HALE 3,368,547

FINNEDTUBE HEAT EXCHANGER Filed om. 25, 1965 5 sheets-Sheet 1" MVML ` l lINVENTOR. n//gggzmz HQ@ uy /QMToR-NEYS Feb.13,196s W,A,HALE I 3,368,541v

-FINNED-TUBE HEAT EXCHANGER Filed oct. 25, 1965 5 sheets-sheet 2 t l INVENTOR. //zawZ/ fa/Ze f v y l) ATTORNEYS Feb. 13, 1968 w. A. HALE 3,368,547

' FINNED-TUBE HEAT EXCHANGER Filed oct. 25, 1965 5 sheets-sheet z INVENTOR Feb. 13, 1968 w. A. HALE 3,368,547

FINNED-TUBE HEAT EXCHANGER Filed oct. 25, 1965 v 5 sheets-sheet 4 INVENTOR. v

Feb. 13, 1968 W. A.; HALE 3,368,547

FINNED-TUBE HEAT EXCHANGER Filed oct. 25, 1965 5 sheets-sheet s) INVENTOR.

/f/zawZ/ /Y/*Ze UY /MATTORNEYS United States Patent() 3,368,547 FINNED-TUBE HEAT EXCHANGER William A. Hale, 4503 N. Knoxville Ave.,

Peoria, Ill. 61614 Y Filed Oct. 23, 1965, Ser. No. 503,323 8 Claims. (Cl. 126-350) ABSTRACT F THE DISCLSURE A water tube type heat exchanger is provided with sheet metal fins mechanically fixed in place thereon in appropriate position to improve transfer efficiency. The fins may be employed as spacing means, in addition to their heat transfer function, for positioning the adjacent coils of a multiple coil tube and, further, the fins may be provided with a configuration substantially controlling the flow of heat transfer medium, such as combustion gas, along and around the water tube coil. A number of specific embodiments of fin construction are disclosed.

The present invention relates to heat transfer devices and is, more particularly, concerned with the provision of a novel ,and substantially improved hot water boiler. As those skilled in the art of boiler construction are aware, increased efficiency in high speed water circulation sys'- tems, as well as improved materials have led to very substantial simplifications in modern heating systems. Increasing attention has, accordingly, been given to the continued improvement of such systems along the lines of efficiency. However, as new, more compact, and efficient hot water boilers have developed, improvements in the form of cost reduction have lagged behind.

By employing several novel techniques of manufacture and assembly, I have provided an extremely inexpensive and yet unusually highly efficient hot water boiler. As a result of the novel configuration of the boiler components of my invention, increased boiler capacity is achieved in an extremely simple manner. This simplicity has very substantially reduced the cost a manufacture of the boiler and has, at the same time, provided an improved over-all boiler efficiency throughout the range of normal sizes for home heating purposes.

In accordance with the principles of my invention, a copper coil is preformed into its final configuration in a bare, unfinned state. Utilizing this initial forming step permits the simple manipulation of the copper tubing. Following the forming step, a plurality of individual fins are mechanically clipped to the tubing along the length thereof. The mechanical attachment of the fins to the tubing is designed to provide close contact between the fin l and the tube such that dipping of the tube with assembled fins clipped thereon will provide a very satisfactory metallic bond. This is particularly true when the tubing is copper and bonding agents such as tin or zinc are employed to provide a permanently tinned or galvanized, bonded connection between each of the fins and the tubing. By this arrangement, manipulation of the fins onto the tubing is readily accomplished while the final positioning of the fins is rendered permanent as a result of the bonding operation. Following the bonding operation, portions of the method of manufacture of the assembled unit is novel and has minimized cost of the boiler assembly.

It is, accordingly, an object of the present invention to provide a simplified hot water boiler of improved eiciency.

Another object of the present invention is to provide an improved finned coil for hot water boilers or the like.

Still a further object of the present invention is to provide an improved fin element adaptable for use in heat transfer equipment.

Still a further object of the present invention is to provide a novel and improved method of assembly and disassembly of a hot water boiler or the like.

A feature of the invention is the provision of a snap-on mechanically retained heat conducting fin.

Still .another feature of the invention is the provision of a preformed coil with permanently bonded sheet metal heat conducting fins projecting therefrom.

Still other and further objects and features of the present invention will at once be understood from a consideration of the attached specification and drawings in which various embodiments of my invention are shown by way of illustration only, and wherein:

FIGURE 1 is a side-elevational view, in partial crosssection, of a hot-water boiler incorporating the principles of the present invention;

FIGURE 2 is a plan view, in partial cross-section, of the boiler illustrated in FIGURE l;

FIGURE 3 is an end-elevational view of the boiler constructed in accordance with the principles of the present invention, in the same manner as FIGURE l, but incorporating a pair of heat transfer coils and related burner equipment.

FIGURE 4 is a side-elevational view in partial crosssection, of a round boiler modification constructed in accordance with the principles of the present invention;

FIGURE 5 is a plan view of a first form of fin ernployed in accordance with the principles of the present invention;

FIGURE 6 is an end-elevational View of the clip shown in FIGURE 5;

FIGURE 7 is a plan view of a fin constructed similarly to the fin shown in FIGURE 5 but with a positioning tang lstruck up therefrom;

FIGURE 8 is an end-elevational view of the fin shown in FIGURE 7 in position in a boiler;

FIGURE 9 is a further embodiment of a fin constructed in accordance with the principles of the present invention shown in plan view;

FIGURE 10 is an end-elevational view of the fin shown in FIGURE 9;

FIGURE 11 is a plan view of a still `further modified form of the fin of the present invention;

FIGURE 12 is an end-elevational view of the fin illustrated in FIGURE 11;

FIGURE 13 is an end-elevational view of a still further v `modified form of fin;

FIGURE 14 is a plan view of astill further form of FIGURE 15 is a partial side-elevational view of the fin shown in FIGURE 14;

FIGURE 16 is an end-elevational view of the fin shown in FIGURE 14 shown in position relative to a boiler coil; I

FIGURE 17 is a plan view of a still further embodiment of the invention;

FIGURE 18 is a cross-sectional view, in part, taken along the line XVIII-XVIII in FIGURE 17;

FIGURE 19 is an end-elevational view of the fin shown in FIGURE 17 in its assembled relation;

FIGURE 20 is a further embodiment of a fin constructed in accordance with the principles of the present invention;

FIGURE 21 is an end-elevational view of the fin shown in FIGURE 20 in assembled relation with boiler components;

FIGURE 22 is a plan view of a still further modified form of fin design for use with a circular or round boiler;

FIGURE 23 is an end-elevational View of the fin illustrated in FIGURE 22;

FIGURE 24 is a still further modified form of fin; and

FIGURE 25 is an end-elevational view of the fin illustrated in FIGURE 24.

As shown on the drawings:

As has been above noted, boilers of the water tube type commonly employed in domestic heating systems have received close scrutiny from the point of view of cost and etiiciency in recent years. Increasingly improved insulation, water circulating equipment, and automatic burner and circulation control have materially reduced the over-all size of individual boiler units. My own studies in the area of improving the efficiency and lowering the cost of gas-to-Water heat transfer systems has proved that although the recent developments in these areas have been significant, extremely important improvements in both of these areas are possible through the method and apparatus of the present invention. In accordance with the principles of the present invention the gas flow outside of, and around, the water tube of the boiler has been directed into a more efficient heat transfer contact, and the technique of positioning and applying heat transfer fins directly to the water tubes has been improved. The improved over-all system is clearly shown in FIG- URES 1 through 4, in several embodiments, and specific for-ms of heat transfer fins configuration are more fully illustrated in the remaining figures.

As may be seen from a consideration of FIGURES 1 and 2, a first form of improved heat exchanger is comprised of a housing containing an oil or gas burner element 11 positioned at the base of a firebox or combustion chamber 12 above which a generally spiral shaped coil 15 is positioned. The coil 15 comprises a conduit 16 having an inlet connection 17, an outlet connection 18, and a plurality of heat-conducting fins 19. Combustion gases leaving the chamber 12 circulate generally longitudinally of the conduit 16 and, at the same time, generally upwardly toward the fiue 20. As shown in FIG- URES 1 and 2, the coil 15 is wound in a flattened spiral when viewed from above. This spiral is of a dimension permitting the fins 19 to project immediately adjacent one another in the central area, a fact which can readily be seen from FIGURE 2 where the edges 19a of the fins 19 approach each other along the central isle. Similarly, the exterior edges 19b of the individual fins extend substantially .against the insulation 13. In the embodiment illustrated the insulation 13 comprises a conventional fibrous insulation such as for example the material marketed under the trademark Kaowool comprising 2200 insulation material or similar product such as Carborundum Fiber Frax insulation material. This `insulation material is positioned inside of an additional insulating layer 14 of conventional type, such as for example 2 thick 1200 U.S. Gypsum oven insulation positioned between the outermost housing wall 10a and the inside housing wall 10b.

As a result of the close `proximity of the outermost edges 19b with the inside layer of insulation, and the close proximity between the centermost edges 19a with corresponding edges 19a of other fins, the circulation of the combustion gases is forced into a generally helical path with some longitudinal movement along the longitudinal axis 15a, permitted between adjacent fins. A tortuous path is thus provided which requires that the combustion gases pass in intimate contact with the fins in a wiping or scrubbing manner on their way to the flue 20.

It will be clear that the lconduit 16, upon which the fins 19 are positioned, is extremely rugged. As shown, it has no internal seams or connections in the combustion chamber area. In addition, as will now be more fully discussed, the individual fins 19 are bonded in an efficient heat transfer relationship in a simple, inexpensive manner permitting simple manufacture or on-site repair of the boiler without in any way endangering the boiler from the introduction of seams, fittings, or the like.

I have found that as a result of the tortuous path provided in this manner, a single copper conduit 16, formed in an output heat exchanger coil as shown in FIGURES 1 and 2, employing 7A" O.D. copper tubing coiled as illustrated in a space having a width x, shown in FIGURE 2, of 5" and a length y of 13" with six coils will produce a gross output on the order of 50,000 B.t.u.s per hour per unit, when firing the unit with gas. The unit may readily be fired with oil by lowering the burner unit, possibly necessitating a lengthening of the combustion chamber 12 in the vertical dimension, and in such installations I have found that the same tubing arrangement will provide a gross output on the order of 56,000 B.t.u.s per hour. As will be recognized by those skilled in the art, such an output is extremely high for a coil configuration of such small size and compactness.

A major advantage of the system and technique employed in the present invention is that multiples of the system may readily be achieved merely by the addition of further coils and burners. This is readily observable from a consideration of FIGURE 3 where the combustion chamber 12 is shown to have a pair of identical burners 11 positioned beneath a pair of identical coils 15. The ue 20 is, as in the case of a single coil embodiment of FIGURES 1 and 2, positioned centrally in the top of the housing 100. Essentially all of the basic components are identical, except twice in number, in the embodiment shown in FIGURE 3. One difference in construction is, of course, that a header is desirable for connecting the coil inputs 17 to a single circulation input header 17a, and connecting the two outputs 18 to a joint output header. Such input and output headers may, of course, take conventional form and as shown, are preferably positioned externally of the housing 10 in the general position shown by the header connections 17a and 18a of FIGURE 1. It has been found that the utilization of two burners with two coils in the manner illustrated provides a ygross output rating on the order of 95,000 B.t.u.s per hour that similar utilization of three coils with three burners provides a rating of 145,000 B.t.u.s per hour, four similar units a rating of 192,000 B.t.u.s per hour and the combination lof five similar units will provide a gross output rating of approximately 240,000 B.t.u.s per hour, all when fired with gas. A single thermal switch 11a senses the water temperature in the coil and shuts off the burner 11 upon a tube temperature of 300 F. indicating lack of Water therein. With this system, it becomes extremely simple to construct an entire range of boiler units with an absolute minimum of different parts.

In the embodiment of the boiler shown in FIGURES 1 and 2, baffles are provided at the front and rear of the coil 15 in the form of ceramic separators 21. Such a ceramic lining may encircle the coil 15 in the form of ceramic sides 22, as well. A second embodiment of peripheral coil construction is found in the cylindrical boiler embodiment of FIGURE 4. As may readily be observed from FIGURE 4, the peripheral edges of the individual fins substantially contact the Kaowool insulation directly. In this form of the invention, which will be more fully described below, the individual fins completely encircle the coil providing a substantially continuous flow path without baflies. When such substantial contact between the individual fins and the Kaowool insulation is employed in the embodiments of the invention illustrated in FIGURES 1, 2 and 3, ceramic elements 21 and 22 are eliminated and iins 19 are snapped into position continuously around the ends of the coil. Under these circumstances, such individual tins 19 positioned 'at the `ends of the coil may take the form of a one-piece continuous iin covering substantially the end of the coil as a generally segmental circular 1in larger than that of the same circular conduit-gripping shape as illustrated in connection with the burner of FIGURE 4.

Contrary to conventional boiler construction, the fins of the present invention run generally longitudinally of the tube and transversely to the direction of intended gas flow relative to the axis a of the coil 15. In conventional practice, -ins are generally applied in some manner to the water tube such that the `fins lie transverse to the longitudinal axis of the water tube and in a plane running substantially parallel to the axis of the coil. I have found that the provision of the fin generally longitudinally of the conduit and transversely positioned relative to the coil axis provides a much more tortuous path and greatly superior wiping action of the flue gases relative to the water coil. This in turn has provided a substantially improved over-all boiler efiiciency without an increase in space requirements. Still further, .as will now become apparent, the utilization of the longitudinally extending iin has permitted a greatly improved iin-to-conduit connection capable of simple manufacture and inexpensive assembly.

Preferably, the individual fins 19 are formed of stamped sheet metal snapped over a tubular form conduit into a stable, spring-retained condition on the conduit. Preferably, the retaining force for maintaining the individual 1in rmly on the conduit is supplied by the fin itself. A primary form of finis illustrated in FIGURES 5 and 6. As may there be seen, the iin 19 comprises a pair of laterally extending ears 19e joined by a segmental cylindrical portion 19d struck on a curve about a center 19e. As shown, the segmental cylindrical portion 19d extends over-center on opposite sides of a diametral line 19j so that the dimension Z forming a restricted neck portion is of smaller length than the diameter of curvature of the portion 19d. Accordingly, the fin 19 may be snapped over the conduit into a friction lit position.

In assembling a heat transfer coil, the coil is initially formed in a generally helical form with a continuous length of conduit. After taking the iinal generally helical form, the individual ns are snapped into place and pivoted into the position generally illustrated in FIGURES 1 and 2. After this assembly procedure the coil, comprising the conduit with the assembled fins, is preferably dipped into a tin or zinc bath or like metallurgical treatment to provide a bond between the individual fins and the conduit. This bond provides an extremely efficient heat transfer medium between the tins and the conduit. Following the metallurgical treatment, the assembled coil is inserted from the right-hand wall of the boiler as Viewed in FIGURE 1, by removal of the front wall 10c of the housing 10. In such assembly, the inlet conduit portion 17 may have a dowel, not shown, inserted therein and projecting substantially therebeyond for insertion into the hole 10d in the side wall of the boiler. This acts as a guide and permits extremely rapid insertion of the coil into the position shown in FIGURE 2. During such assembly the header 18a is not assembled with the outlet portion 18 of the conduit and following insertion of the coil into positions, the side wall portion 10c is slipped over the extension 18 into the position sh'own and rigidly retained in such position by assembly screws, not shown, and slip-on foot connections 10e and 10f. As a result of such a slipon connection, the wall 10c may readily be removed by removal of the header 18a, for cleaning of the coil in position in the boiler. Alternatively, of course, the entire coil may readily be removed from the boiler for repair or cleaning by removal of the header 17a and movement of the coil and the wall 10c as a unit toward the right.

-6 If found desirable, a positioning guide pin 25 may be rigidly imbedded in the lwall 10b for co-operation with ceramic locator 21 or, alternatively, with a bracket (not shown) welded or otherwise secured to conduit 16.

Due to the fact that the coil 15 of the boiler of the present invention comprises a plurality of turns, the fins may be employed as a device for accurately spacing the coils. Such an arrangement is shown in the embodiment of FIGURE 7 where lin 119 is shown as having an upstruck tang 119a. The tang 119a co-operates with the next adjacent turn of the coil conduit 16 to space adjacent coils as shown in FIGURE 8. In constructions in which the turns of the coil 15 are accurately and xedly spaced by external means, the fin need not comprise an arrangement having an over-center retaining action. Such an arrangement is shown, for example, in FIGURES 14, l5 and 16. There, the iin 219 is provided with an upstruck adjustably bendable tang 219a having an arcuate portion 219b which co-operates with the next adjacent turn of the coil conduit 16 in the manner illustrated in FIGURE 16. In view of the curved shape of the tang 219b, the turns of the coil will accurately locate and position the fins 219. During the metallurgical bonding operation, the ins become integrated with the tubing and subsequent variation and separation between the turns of the coil 15 will not cause separation of the ns 219 from the conduit 16.

A further modification of the fin 19 is illustrated in FIGURES 9 and 10 where 1in 319 is illustrated. There, a very substantial pierced opening 320 is provided. The opening operates to substantially relieve the amount of Vmaterial in the arcuate portion 319d thereby rendering the assembly of the device to the conduit simpler by increasing the flexibility of the fin. Under such circumstances the thickness of the tin material may be increased, if desired, without increasing the diliiculty of manual assembly. Another technique of rendering the lateral portions of the iin more rigid is illustrated in FIG- URES 11 and l2 where the laterally extending portions 419C of the iin 419 are bent backwardly upon themselves to provide a double thickness. This arrangement may be employed with a relatively thin arcuate portion 419d to, again, provide a relatively flexible arcuate portion with a substantial, relatively stili, laterally extending iin portion.

In some installations it is desirable to increase the restriction of gas flow beyond that available with the arrangements above described. One technique of accomplishing this is to provide for a downward deflection of the lateral iin portions in the manner illustrated in FIG- URE 13. As there shown, portions 519e deflect downwardly from the arcuate portions 519d. This downward deflection is in the upstream direction of gas flow and requires gas to pass backwardly upon itself and outwardly before it can proceed upstream, as illustrated by the arrow 520. It will be clear that this downward positioning or deection of the fins may be employed with any of the fin forms of the present invention.

A further embodiment of fin construction provides a particularly simple assembly procedure. For example, the arrangement shown in FIGURES 17 through 21 incorporate a separate spring element for maintaining the fins in position on the tube. This separate spring element permits utilization of a 1in not having a snap-over feature and at the same time permits a very stable, assembled, relationship. In the embodiment shown in FIGURES 17 through 19, a wire spring 621 is employed with fin 619. The wire `spring 621 has an end projection 622 that cooperates with an upstruck tunnel 619:1 and locating guide tab 619b. The wire spring 621 is provided with an arcuate form extension 623 co-operating with the next adjacent conduit 16 and co-operates therewith to maintain the iin l 619 pressed downwardly in its position against conduit 16.

Similarly, in the embodiment shown in FIGURES 20 and 21, the fin 719 is provided with an upstruck portion 719a that acts in co-operation as a spacer with the next adjacent conduit 16. The lin 719 is maintained resiliently in contact with the lower coil conduit portion 16 by means of a resilient spring 721 that is provided with hook portions 722 that project through apertures 719d for cooperation wit-h fin 719. In assembly, the spring 721 is passed beneath the lowermost conduit portion 16, upwardly through the aperture 719b on one side of the fin to a point at which it projects substantially therebeyond. With the fin and spring thus positioned, the other end of the spring is pressed upwardly into axial alignment with the other aperture 719b and is slid relative to the conduit portion 16 through the other aperture 719b into the assembled position shown in FIGURE 21.

The relatively flexible form of spring employed in the embodiment of FIGURES l7 through 21 may be achieved by the use of a relatively flexible upstruck tangv portion of the fin itself as shown in the embodiment of FIGURES 24 and 25. As there shown, the fin 819 is provided with an upstruck tang spring portion 819a which projects in cantilever fashion slightly beyond the center condition. As a result the fin may be snapped over the conduit 16 by movement .in the direction of the arrow 820. The fin construction permits simple assembly and an absolute minimum of parts. It will, of course, be apparent that if desired a still further tang may be upstruck for co-operation as a spacer with the next adjacent turn of the coil.

In the fin configurations above discussed, the fins have been provided with a generally straight segmental cylindrical portion for co-operation with a generally straight portion of the coil conduit. In constructions .in which the boiler is generally circular as shown in the embodiment of FIGURE 4, or in which the fin is constructed for attachment to the ends, or arcuate portions of coils such as illustrated in FIGURES l and 2, it is desired that the n be curved to co-operate with the curved portion of the coil conduit.

An example of such a fin is found in FIGURES 22 and 23. As there shown, the fin 919 is provided with an arcuately configurated segmental cylindrical portion 919d. It will be apparent that fins of the curved type as shown in FIGURES 22 and 23 may readily take any of the forms in the remaining figures.

In the specific embodiment of the boiler incorporating a circular form, and .illustrated in FIGURE 4, it is necessary to employ the form of fin described in FIGURES 22 and 23. In view of the fact that a relatively large diameter coil turn is employed, it is desirable that the central area of the coil be filled with a gas flow controller. This controller takes the form of a generally cylindrical plug 30. Preferably the plug 30 has a helical flange 31 in the form of a thread projecting around its peripheral surface. The thread 31 cooperates with the inwardly facing edges 919a to provide a restriction to gas flow. The plug is preferably constructed of ceramic and is provided with ears 32 which act as stops against the uppermost turn of the coil 116 to position the plug accurately with respect to the individual coil turns. The burner 111 may, of course, be of any conventional form and forms no part of the present invention.

The improved boiler and fin configuration, as well as technique of boiler construction and assembly, described above are of great importance in providing an inexpensive highly eflicient hot water boiler system. It will be apparent to those skilled in the art that the fins may be constructed of a wide variety of materials, for example, stainless or spring steel, which provide very strong snap-action retention. Alternatively, the fins may be constructed of such materials as copper, if desired. Copper is particularly satisfactory where separate springs are employed, such as for example in the embodiments shown in FIGURES 17 through 2l. Similarly, although the fins have been described as metallurgically bonded to the coil turns by means of a molten dip of tin or zinc, other fastening means may be employed. For example, the fins may be spotwelded, brazed, soldered, or otherwise secured to the conduit if desired. Since many variations will be readily apparent to those skilled in the art from the numerous embodiments above described, it is my intent that the scope of the present invention be limited solely by that of the hereinafter appended claims.

I claim as my invention:

1. In combination in a heat exchanger, means providing a source of high temperature combustion gas, a gas outlet and means Iproviding a gas flow current from said source to said outlet, a fluid conducting tube formed in a coil about an axis generally parallel to the axis of combustion gas flow, a plurality of fins in heat transfer contact with said tubing and extending laterally outwardly therefrom transverse to the direction of gas flow and generally parallel to the axis of the tubing at their contact junction therewith, said fins extending outwardly and inwardly of the coil in both directions substantially restricting the open flow area outside of the coil and inside of the coll whereby the gas flow will follow generally the contour of the fluid conducting tube.

2. The structure set forth in claim 1 wherein said fins are mechanically maintained in contact with said fluid conducting tube and are metallurgically lbonded thereto.

3. A heat transfer element comprising a conduit formed in a coil having a plurality of turns along its longitudinal axis for distribution of a heat transfer uid, fins secured to said conduit and projecting laterally thereof and generally parallel to the axis of the conduit, and mechanical means for retaining said fins on said conduit, the innermost and outermost edges of the fins being substantially constant along said coil and the innermost edges of the fins substantially blocking the interior of said coil.

4. In combination in a heat exchanger, means providing a source of high combustion gas, a gas outlet, and means providing a gas flow current from said source to said outlet a fluid conducting tube formed in a coil about an axis generally parallel to the axis of combustion gas flow, a plurality of fins in heat transfer contact with said tubing and extending laterally outwardly therefrom transverse to the direction of gas flow and generally parallel to the axis of the tubing at their contact junction therewith, and means on said fins co-operable with spaced turns of said coil to retain said fins between adjacent coils in mechanical contact with and positioned by said adjacent coils.

5. In combination in a heat exchanger, means providing a source of high combustion gas, a gas outlet, and means providing a gas flow current from said source to said outlet a fluid conducting tube formed in a coil about an axis generally parallel to the axis of combustion gas flow, a plurality of fins in heat transfer contact with said tubing and extending laterally outwardly therefrom transverse to the direction of gas flow and generally parallel to the axis of the tubing at their contact junction therewith, means on said fins co-operable with spaced turns of said coil to retain said fins between adjacent coils in mechanical contact with and positioned by said adjacent coils, and metallurgical bonding means for bonding said fin to at least one of the coils with which it has contact.

6. A heat transfer iin for cooperation with a tubular heat transfer conduit, which comprises a sheet of heatconducting material having a channel stamped therein to co-operate in surface-to-surface contact with said conduit, and means integral with said sheet and co-operating with said conduit whereby said fin snaps mechanically into position on said conduit in a mechanically secure position thereon, said last-named means comprising a tang struck from the body of said sheet and snapping over a projecting portion of said conduit for applying pressure to opposite sides of said conduit to retain said fin thereon.

7. In combination in a heat exchanger, a fluid conducting conduit formed in a coil about an axis generally parallel `to the axis of combustion of gas flow in said heat exchanger, a plurality of fins in heat transfer contact with said conduit and extending laterally outwardly and inwardly therefrom transverse to the direction of gas flow and generally parallel to the axis of the conduit at their contact junction therewith, an insulated housing surrounding said coil and in immediate proximity to the outer edges of said tins whereby gas ow generallyv parallel to the said axis and outside said tins, said ns projecting inwardly of said coil to substantially block free flow of said gas longitudinally of said coil within the contines thereof whereby combustion gases are restricted to tortuous paths through said coil. Y

8. In combination in a heat exchanger, a fluid conducting conduit formed in a coil about an axis generally parallel to the axis of combustion of gas flow in said heat exchanger, a plurality of fins in heat transfer contact With said conduit and extending laterally outwardly and inwardly therefrom transverse to the direction of gas flow and ygenerally parallel to the axis of the conduit at their Contact junction therewith, said fins extending substantially Icontinuously along said coil conduit to provide said conduit with at least one completev spiral turn,

' an insulated housing surrounding said coil j and in immediate proximity to the outer edges of said ns whereby combustion gas ow generally parallel to the said axis and outside said tins is restricted, said ns projecting in- 10 wardly of said coil to substantially block free flow of said gas longitudinally of said coil within the confines thereof whereby combustion gases are restricted to tortuous paths through said coil.

References Cited UNITED STATES PATENTS 473,176 4/1892 Holden 165-183 672,412 4/ 1901 Birdsall 165-181 1,942,211 1/1934 Hartwig 122-367 2,036,961 4/ 1936 Collier 165-181 X 2,086,647 7/ 1937 Sterrick 122-250 X 2,203,718 6/1940 Burch 165-168 X 2,205,984 6/1940 Kromas 165-148 3,219,017 11/1965 Thybault 122-250 FOREIGN PATENTS 604,025 8/ 1960 Canada. 403,899 7/ 1934 Great Britain.

ROBERT A. O'LEARY, Primary Examiner. A. W. DAVIS, Assistant Examiner.

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