GB2047399A - Improvements in the fabrication of finned-tube heat exchangers - Google Patents

Abstract

In a heat exchanger wherein closely spaced fins 14 are brazed to a fluid carrying tube 12 passing through aligned holes in the fins, the lip of each said fin hole has a squared edge 22 except for narrow portions thereof which are shaped to form fin-spacing tabs 16. The squared edge 22 enables a satisfactory braze to be achieved between the fins and the tube, e.g. by the formation during brazing of fillets 24, 26 of brazing material, Fig. 4. The brazing material may be initially applied to the fins or as shown, to the tube as a cladding 20 on the tube wall 18. <IMAGE>

Description

SPECIFICATION Improvements in the fabrication of heat exchangers The present invention relates to the fabrication of heat exchangers of the type wherein a series of very closely spaced fins are to be bonded to a fluid-carrying tube passing through aligned holes in the fins.

One environment where heat exchangers of this type may be used is that of the viscous drag type heat exchangers utilized for the condenser and evaporator of a heat pump, such as shown in U.S. Patent 3,866,668.

Two problems arise in the fabrication of such heat exchangers. The first involves provisions of means for establishing uniform longitudinal spacing of the fins along the length of the tube. Particularly in a viscous drag air pumping type of rotating heat exchanger, there is an optimum spacing to produce maximum air flow and maximum heat exchanging efficiency. One method which has been employed in the past is the use of an annular flange or collar integral with the fin and surrounding punched tube-receiving hole in the fin. Such collar abuts a portion of the adjacent fin and thereby establishes a minimum spacing therebetween.

These gap-establishing collars have not been entirely successful. It is not possible to turn a sharp right angle flange when punching a hole in the fin, and the required radius of the bend establishes an annular conical depression surrounding the tube on the side of the fin away from the collar. The free end of the collar on the adjacent fin will tend to nest into this depression to a variable degree, destroying the intended close control of the minimum fin spacing.

These collar-like spacers also interfere with the successful brazing of aluminium fins and tubes. A sound braze is also essential to provide optimum heat flow across the joint between the tube wall and fin. The collar interferes with proper cleaning of the assembly, such as by ultrasonic bath and vapour degreasing operations, because dirt particles can be trapped in the crevices between the collar and the tube. Also, if the tube is clad with a brazing material, such cladding does not "see" the magnesium which may be placed in the brazing oven (in powder or electrode form) for the purpose of driving out impurities in the clad. Furthermore, the rounded intersection between the collar and the tube prevents the formation of an optimum brazing fillet.

Accordingly, it is an aim of the present invention to provide an improved construction of the joint between the fin and tube, to achieve reliable control over the fin spacing and to provide a sound brazed joint between the fin and tube for maximum heat conductivity.

According to the present invention there is provided a heat exchanger comprising at least one fluid-carrying metal tube which passes through a plurality of longitudinally closely spaced external sheet metal fins, each fin having a tube-receiving hole so shaped and dimensioned that the radial gap between the outer circumference of the tube and the edge of the hole is minimal consistent with free insertion of the tube into said hole; the periphery of each of said holes having a plurality of circumferentially spaced narrow tabs formed integrally with the fin, each tab comprising a formation configured to locally increase the thickness of the fin as measured lengthwise along the tube and to prevent the tab from nesting into the tube-receiving hole of an adjacent fin; the thickness of the tab formation, as measured perpendicularly to the plane of the fin, being selected and controlled to establish the desired longitudinal spacing between adjacent fins when the tabs of one fin abut the adjacent fin; the number and circumferential width of the tabs associated with each hole being selected to provide a combined tab circumference which is a very small proportion of the total hole circumference, so that the major portion of the periphery of the hole presents a sharp edge perpendicular and in close proximity to the tube; and at least one of the opposed surfaces of the fin and tube outer surface being initially clad with a brazing composition having a melting point below the melting point of both the fin and tube base material.

According to a further aspect of the present invention there is provided a method of constructing a rotating viscous drag air-pumping heat exchanger having a fluid-carrying metal tube passing through a plurality of external sheet metal fins at closely spaced intervals along the length thereof, said method comprising the steps of:: (a) forming each fin with (1) a tube-receiving hole so shaped and dimensioned that the radial gap between the outer circumference of the tube and the edge of the hole is minimal and consistent with free insertion of the tube into said hole, and (2) a series of integral narrow formations circumferentially spaced around the periphery of said hole, each formation being dimensioned to locally increase the thickness of the fin as measured along the length of the tube, said thickness being selected and controlled to establish the desired longitudinal spacing between adjacent fins when the formations of one fin abut the adjacent fin, said formations being so configured to prevent them from nesting into formations on an adajacent fin, thereby to assure the desired longitudinal spacing, the number and circumferential width of the formations associated with each hole being selected to provide a combined formation circumference which is a very small proportion of the total hole circumference, the periphery of the hole between the formations being free of any formation which increases the thickness of the fin, so that the major portion of the periphery of the hole presents a sharp edge perpendicular to and in close proximity to the tube; (b) inserting a tube through the aligned holes of a stack of fins formed in accordance with step (a) above, each fin being pre-spaced from each adjacent fin by the narrow formations thereon, at least one of the opposed surfaces of the fins and tube outer surface being clad with a brazing composition having a melting point below the melting point of both the fin and tube base materials;; (c) brazing the fins and tube into an integral assembly by placing them into a furnace at a temperature above the melting point of said cladding but below the melting point of said base materials, thereby to form a bond between said sharp edge of the fin hole and the tube along substantially the full length of said major portion of the hole periphery.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a heat exchanger employing the present invention; Figure 2 is a fragmentary elevation of a portion of the heat exchanger of Fig. 1 showing a tube having several fins spaced along its length; Figure 3 is a sectional view in the direction of arrows 3-3 of Fig. 2; and Figure 4 is an enlarged sectional view in the direction of arrows 4-4 of Fig. 3.

Fig. 1 is a perspective view of a portion of a typical viscous drag air-pumping type of heat exchanger. Such a heat exchanger 10 comprises a series of parallel fluid-carrying tubes 12, each of which passes through a plurality of closely spaced annular fins 14. As best shown in Figs. 2 to 4, each tube-receiving hole in a fin is provided with four circumferentially spaced arrow tabs 1 6. The tube-receiving holes are preferably sized to permit a snug but free insertion of the tube, so that a minimal gap will remain for the subsequent brazing step.

As shown in Fig. 4, tube wall 1 8 has an outer cladding 20 comprising a material having a melting point slightly below that of both the fin and the main body of the tube. The edge 22 of the tube-receiving hole in the fin is a clean square edge except at the location of the four spaced tabs 16, so that an optimum brazing fillet 24 may be formed by the melting of the cladding material in a brazing oven. For the reasons described above, the brazing fillet 26 formed under the tab 1 6 may or may not be sound.

The provision of only a few widely spaced and narrow tabs, rather than a full collar, is sufficient to provide the fin spacing function, while at the same time leaving most of the fin hole perimeter as a clean square edge for optimum brazing. Thus, the tube cladding can "see" the heat of the oven (except a small portion which is under tabs 16), to improve melting and bonding.

Furthermore, the narrow tab permits better cleaning of any crevices beneath it, since dirt can be removed both axially and circumferentially from beneath the free edges of the tab.

Because the free tip of each tab 1 6 is curled or double back toward the main portion of the fin, there is no danger that it will tend to nest in the annular depression or recess which exists at the bend radius of the adjacent fin. In this fashion, uniform control of the fin spacing is assured.

As shown in Fig. 3, the tabs are angularly positioned so that none lies on an ''east-west'' line through the center of the tube (i.e., a line perpendicular to a radius from the central axis of the heat exchanger out through the center of each tube). Thus, one tab at a first tube does not closely and directly oppose a tab on the same fin at the adjacent tube, which arrangement would interfere with air flow between such tubes and require greater tube spacing for adequate air flow, which in turn would reduce heat exchanging capacity or require a larger unit. The same problem arose with the use of a full collar spacer.

No particular number of tabs is critical, the important criteria being sufficient spaced contact points to assure proper location of the adjacent fin, and minimum total tab circumference. While tab-like formations have been described, it is understood that other spaceestablishing formations may be employed. It is preferred that such formations be placed at the edge 22 of the holes, to minimize interference with air flow patterns.

This invention may be further developed within the scope of the following claims. Accordingly, the above specification is to be interpreted as illustrative of only a single operative embodiment of this invention, rather than in a strictly limited sense.

Claims (6)

1. A heat exchanger comprising at least one fluid-carrying metal tube which passes through a plurality of longitudinally closely spaced external sheet metal fins, each fin having a tube-receiving hole so shaped and dimensioned that the radial gap between the outer circumference of the tube and the edge of the hole is minimal consistent with free insertion of the tube into said hole; the periphery of each of said holes having a plurality of circumferentially spaced narrow tabs formed integrally with the fin, each tab comprising a formation configured to locally increase the thickness of the fin as measured lengthwise along the tube and to prevent the tab from nesting into the tube-receiving hole of an adjacent fin; the thickness of the tab formation, as measured perpendicularly to the plane of the fin, being selected and controlled to establish the desired longitudinal spacing between adjacent fins when the tabs of one fin abut the adjacent fin; the number and circumferential width of the tabs associated with each hole being selected to provide a combined tab circumference which is a very small proportion of the total hole circumference, so that the major portion of the periphery of the hole presents a sharp edge perpendicular and in close proximity to the tube; and at least one of the opposed surfaces of the fin and tube outer surface being initially clad with a brazing composition having a melting point below the melting point of both the fin and tube base material.

2. A heat exchanger as claimed in claim 1, in which the cladding is provided on the tube rather than on the fin.

3. A heat exchanger as claimed in claim 1, in which said fin tabs are double back away from the hole.

4. A method of constructing a rotating viscous drag air-pumping heat exchanger having a fluid-carrying metal tube passing through a plurality of external sheet metal fins at closely spaced intervals along the length thereof, said method comprising the steps of: (a) forming each fin with (1) a tube-receiving hole so shaped and dimensioned that the radial gap between the outer circumference of the tube and the edge of the hole is minimal and consistent with free insertion of the tube into said hole, and (2) a series of integral narrow formations circumferentially spaced around the periphery of said hole, each formation being dimensioned to locally increase the thickness of the fin as measured along the length of the tube, said thickness being selected and controlled to establish the desired longitudinal spacing between adjacent fins when the formations of one fin abut the adjacent fin, said formations being so configured to prevent them from nesting into formations on an adjacent fin, thereby to assure the desired longitudinal spacing, the number and circumferential width of the formations associated with each hole being selected to provide a combined formation circumference which is a very small proportion of the total hole circumference, the periphery of the hole between the formations being free of any formation which increases the thickness of the fin, so that the major portion of the periphery of the hole presents a sharp edge perpendicular to and in close proximity to the tube;; (b) inserting a tube through the aligned holes of a stack of fins formed in accordance with step (a) above, each fin being pre-spaced from each adjacent fin by the narrow formations thereon, at least one of the opposed surfaces of the fins and tube outer surface being clad with a brazing composition having a melting point below the melting point of both the fin and tube base materials; (c) brazing the fins and tube into an integral assembly by placing them into a furnace at a temperature above the melting point of said cladding but below the melting point of said base materials, thereby to form a bond between said sharp edge of the fin hole and the tube along substantially the full length of said major portion of the hole periphery.

5. A method of constructing a rotating viscous drag air-pumping heat exchangers, substantially as hereinbefore described with reference to the accompanying drawings.

6. A rotatable viscous drag air-pumping heat exchangers, constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.