IE930881A1 - Heat exchanger with fins - Google Patents

Abstract

A panel heat exchanger 2 has a finned panel 1 with a fin array 4 joined to the panel 2 in a thermally conductive manner. A number of mechanically formed deformations are provided along lines where the bases 7 of the individual fin sections 5 make contact with the heat exchanger surface 8. The fin sections 5 are joined to the heat exchanger surface 8 by resistance welding focused on the deformations.

Description

The invention relates to heat exchangers with convective fins made from sheet metal and more particularly to central heating panel radiators with convective fins made from sheet metal.

It is well known that fins attached to a surface can in many cases increase heat transfer by convection between the surface and the fluid surrounding it. The extended surface of the fin in effect provides a greater area through which energy can flow. When heat transfer takes place by convection from both interior and exterior surfaces of a heat exchanger body, generally fins are used on the surface where the heat transfer coefficient is low. For example, in the case of a central heating panel radiator, part of the outer surface of the panel may be finned because the heat transfer coefficient for air at the outer surface is much smaller than that for water flow at the inner surface.

A large variety of geometries of heat exchanger sheet metal fins are manufactured, both for heating and cooling purposes. A very common arrangement is for the fins to comprise an array made by bending a relatively long strip of sheet metal into a repeating corrugated shape with the bases of the corrugations making mechanical and thermal contact with the heating or cooling surface. The sheet metal material thickness, the shape and size of the corrugation profile and the overall dimensions of the fin 3q array are carried out in a wide variety of arrangements.

The method for joining the fin array to the heating or cooling surface is also carried out in several different ways. The join is frequently of a soldered type or a JNL. No prestressed mechanical contact type for industrial or automotive heat exchangers. With central heating panel radiators the join is most frequently achieved by spot welding whereby most of the base makes nominal or approximate mechanical surface contact with the heating surface and is held in position by intermittent spot welds between the two materials. In an alternative arrangement, some of the base joins are spot welded and the remainder are of a prestressed mechanical contact type held in position by the spot welded base joins.

European Patent Specification No. 0 092 033 B1 (Klostermann) and British Patent Specification No. 781,928 (Dutrieux) disclose central heating panel radiators with various sheet metal fin geometries with mixed joins of the spot welded and prestressed mechanical contact types.

German Patent Specification No. 2 539 856 (Schafer) discloses a central heating panel radiator with various sheet metal fin geometries with unmixed joins made by spot welding, soldering or adhesive.

The present invention provides a heat exchanger having metal convective fins joined in a thermally conductive manner with a metal surface of the heat exchanger, characterised in that a plurality of mechanically formed deformations are provided in the region of the join between at least one fin and the surface of the heat exchanger and the fins are joined to the heat exchanger surface by resistance welding focused on said deformations.

Additionally, the fins are joined to the heat exchanger surface by arrays of resistance welds focused on deformations disposed along lines which are substantially parallel to elongate radiator contact bases of the fins.

Additionally, the deformations used to effect the resistance welds are spaced a small distance apart so that when welded the resulting welds cooperate to provide substantially continuous thermally conductive joins substantially parallel to the elongate radiator contact bases of the fins.

Additionally, the unfused portions of the joins are altered by the heat and pressure of the deformation focused resistance welding process such that a more intimate and more thermally conductive contact is made in the unfused portion of the join and is retained by the adjacent fused portions of the joins.

Additionally, each elongate fin contact base is narrow in width, being between two to five millimetres in width.

Additionally, the portions of the fin wall adjacent the region of the join are angled sharply away from the heat exchanger surface.

Additionally, the deformations used in the joins are formed on the elongate fin contact bases.

Additionally, the deformations used in the joins are formed on the elongate radiator contact bases and on the heat exchanger surface, whereby the welds are made at the intersections of the deformations on the two parts.

Additionally, the deformations used in the joins are formed as embossments on the heat exchanger surface.

Additionally, each deformation comprises an annular collar with a central hole which is formed and pierced without any removal of material from the elongate fin contact base.

Alternatively, each deformation comprises additional material from the elongate fin contact base and is formed in a compound or multiple stage operation including forming a ridge in the material parallel to the length of the elongate fin contact base and forming all or part of the deformation from the material of the ridge.

Additionally, each fin is provided with weaknesses whereby the fin may flex as portions of it are resistance welded to the heat exchanger surface.

Additionally, the materials of the fins and heat exchanger surface comprise different metals.

Additionally, one or more of the surfaces of the fins or the surface of the heat exchanger is provided with a coating of a type which is applied in advance of the joining of the fins and heat exchanger surface.

The present invention also provides apparatus for use in making heat exchangers, characterised in that the apparatus comprises one or more electrodes with narrow flat contact faces and means for applying pressure and electrical current through the electrodes and is operable to receive a heat exchanger with a metal surface and fins made from sheet metal, with a plurality of mechanically formed deformations provided in the region of the join between each fin and the surface of the heat exchanger with the apparatus being operable to join the fin contact bases of the fins to the surface of the heat exchanger by resistance welding focused on said deformations.

Additionally, the apparatus comprises two parallel electrodes or sets of electrodes with narrow flat contact faces and means for applying pressure and electrical current through the electrodes and is operable to join two fin contact bases or portions of two fin contact bases to the surface of the heat exchanger by series resistance welding.

Additionally, the apparatus is operable simultaneously to execute all of the resistance welds associated with the fin contact base of each fin.

Additionally, the electrode or set of electrodes which execute all of the resistance welds associated with a fin contact base of a fin comprises a set of electrodes with aligned flat faces which are capable of being separately moved or pressurized.

Additionally, the electrode or electrodes are provided with additional flat faces adjacent and sloping away from the main flat contact face whereby additional electrical contact is made between the electrode or electrodes and sloping side walls provided on the fins adjacent the fin contact bases.

The present invention further provides a method for making heat exchangers having metal convective fins utilizing any of the above features.

An embodiment of the invention, comprising a finned central heating panel radiator hereafter referred to as a panel or finned panel, will now be described by way of example.

Reference is made to the accompanying drawings in which: Figure 1 shows a sectional plan view of a central portion of a finned panel.

Figure 2 shows a sectional plan view, on Y-Y of Figure 3, of a base of the fin prior to welding and on a larger scale than Figure 1.

Figure 3 shows a sectional side view, on X-X of Figure 2, of the same fin base shown in Figure 2.

Referring now to Figure 1, a finned panel 1 is shown with a fin array 4 attached to a panel 2 by arrays of welds focused on deformations along lines where the bases 7 of the individual fin sections 5 make contact with the heat exchanger surface 8 which is the surface 8 of the panel 2.

Referring now to Figure 1, a section of a portion of a central heating panel radiator 1 is shown with a panel 2 comprising channels 3 through which a heating liquid such as water flows, and an array 4 of convecting fins 5 joined to the panel 2. The joins 6 comprise arrays of welds focused on deformations along a line where the bases 7 of the fins 5 make contact with the heating surface 8 of the channels 3 of the panel 2.

In the arrangement shown, the spacing of the fins 5 in the fin array 4 corresponds to the spacing of the channels 3 in the panel 2, and the fin bases 7 are joined to the channels 3. However, many other arrangements using the methods of the invention are also possible. For example, the spacing between fins 5 and the spacing between channels 3 may be equal but the fin bases 7 may be joined to the non-water bearing surfaces or flutes 9 of the panels 2. Further examples are afforded by arranging the spacing between the fins 5 such that two fins are provided for each spacing between the channels 3. Two fin bases 7 may then be joined to each channel surface 8 by arranging the channel surface wider than the flute 9, or alternatively the fin bases 7 may be alternately joined to the channel surfaces 8 and flutes 9 by arranging the walls 10 of the fins 5 to be of unequal lengths. Yet further examples are afforded by other known types of central heating panel radiator 1 which present a substantially flat surface without flutes to which the fins bases 7 may be joined without spacing constraints other than the constraints provided by the fins 5 themselves.

The welding used in the invention is a specialised type of projection or resistance welding wherein fusion is produced by the heat obtained from resistance to the flow of electric current through the work parts held together under pressure by electrodes. The resulting welds are localized at predetermined points by the design of the parts to be welded. The localization is accomplished by intersections between the parts. In this it differs from spot-welding where weld localization is determined by the electrodes.

The arrays of deformations on the workpiece are spaced a small distance apart along a line prior to welding, whereby when welding is complete they effectively form a continuous thermal join 6 between the fin base 7 and the panel surface 8. To increase the efficiency of the join 6, the welds are arranged to lie in a straight line and care is taken to prevent or minimize any reduction in the thickness of the fin base 7 material caused by the joining process.

The join will comprise a fused and unfused region between the fin and panel materials. The proportion of fused to unfused join area is maximized within the constraints of the welding process because the fused region has a much lower thermal contact resistance. Care is also taken to ensure that an intimate physical contact exists between the fin and panel surfaces to minimize the thermal contact resistance. This resistance can be made much lower than that found in the typical spot-welded join because the unfused portions of the resistance welded join 6 are altered by the heat and pressure of the resistance welding process and are retained in this more intimate and more thermally conductive contact by the adjacent fused portions. These features contrast with those of the conventional spot-welded join which consequently has a much higher average thermal contact resistance.

If the join 6 were fused over its entire area, then its optimum width would be twice the fin thickness prior to consideration of the joining process. However, because the join 6 is not fused over its entire area, its optimum width is somewhat larger but nevertheless is likely to be less than can be achieved with any practical resistance welding processes of this type. The join 6 is therefore minimized within the constraints of the welding process and will usually range from two to five millimetres in width.

The reasons for minimizing the width of the join 6 include the following.

Firstly, the base of the fin 7 does not provide an extended surface which might assist convective heat transfer. Consequently, a reduction in its size results in a saving in fin material costs and weight reduction in the finished product.

Secondly, the thermal efficiency of the convective portion of the fin 5 is maximized by positioning it as close as possible to the hottest portion of the join 6, which will be at the central fused region of the radiator. Any additional width of fin base will increase the length and consequent resistance of the conductive path from the fused to the convective portion of the fin 5.

Thirdly, the base of the fin 7 masks an equal area of the underlying panel 2. In this respect, heat emission is increased by reducing the area of the fin base because the fin base 7 is at a lower temperature than the underlying panel 2 and will therefore radiate and convect less heat than an equal area of panel surface. Furthermore, where heat can be emitted from the panel or the fin, all other things being equal it is better emitted from the panel because any additional emission from the fin must pass through the fin join 6 and thus contribute to the overall temperature drop across the join 6 with a consequent, albeit small, reduction in the other sources of heat emission from the fin 5.

Fourthly, the join 6 is kept as narrow as possible to avoid relatively large welds requiring high welding currents and high welding electrode forces. High welding currents and forces would increase the cost of apparatus and could cause unacceptable distortion in the resulting welded panel.

Finally, the relatively narrow fin base will in some instances provide greater flexibility in the design of the underlying panel 2 and may allow more than one fin base 7 to be joined to each channel surface 8 or flute 9 or may allow all of the fin bases 7 to be joined to the channel surfaces 8 instead of being alternately joined to the flutes 9 and channel surfaces 8.

The portions of the fin wall 10 adjacent to the joins 6 are angled sharply away from the panel surface 8 as close as possible to the edges of the joins 6 to facilitate heat convection in the regions 12 enclosed between the panel surface 8 and the fin walls 11. The angle and shape of the fin wall 10 profile and panel surface 8 are arranged such as to maximize the combined convection from both sides of the fin wall 10 and from the panel surface 8.

The linear array of deformations on the parts to be welded may be positioned on the panel surface or on the fin base or on both.

In resistance welding applications of this type, it would be usual to position the deformations on the thicker of two materials of different thickness, which is the panel surface in the present case. The deformations may take the form of annular or oblong embossments pressed out from the panel material simultaneous with the forming of the panel water channels and flutes.

Alternatively, the deformations may be made in both parts, a linear or intermittently linear embossment being made along the length of one part and an array of oblong embossments being made in the other part with the length of the oblongs orientated transversely to the embossment in the first part. The resistance welds are then made at the intersections of the embossments on the two parts. This method has the advantage that the embossments can be made shallower than would be required if they were only made in one of the parts.

A further alternative is to position the deformations in the fin base and this is the preferred method in the present embodiment for several reasons including the following.

Firstly, it recognizes that the required welds of the fin join are quite different to conventional resistance welds focused on deformations or projections in that their most important characteristic is to provide low thermal contact resistance rather than high strength, and that it is more important to produce a narrow although possibly weak join of low thermal resistance than a wide strong one. It is easier to produce small well defined deformations in the relatively thin fin material.

Secondly, the deformations are only made in one part and it is usually more convenient and less costly to form the lighter fin material.

Thirdly, it avoids the possibility of the forming or welding process deforming or thinning the panel material which forms part of a water bearing vessel which is liable to potential leaking or corrosion.

Fourthly, it provides wider tolerance in the relative positioning of the fin and the panel in the welding process where the relatively narrow fin base and deformations are presented to the relatively wide flat portion of the panel surface.

Finally, it allows the method of the invention to be used - 12 in circumstances where it is not feasible to form deformations on the panel surface, which is sometimes the case.

Referring now to Figure 2 and Figure 3, there is shown an array of deformations each of which comprises an annular collar with a central hole and are formed and pierced without any removal of material from the fin base. A large number of such deformations can be made with relatively simple tools and relatively low forming forces. For low carbon steel parts with fin thickness of 0.5 mm and panel thickness of 1.2 mm, the following approximate dimensions, with reference to the figures, have been found suitable a = 3.0 mm b = 1.8 mm c = 1.0 mm d = 4.0 mm e = 3.5 mm It has also been found that the welding characteristics of this type of array of deformations are of a similar order, in terms of overall electrical power and electrode forces, to those required for conventional fin base spot welds spaced at the usual approximate 35 mm distance between spot centres .

With somewhat more complicated tooling, it is possible to produce similar arrays of annular deformations in the fin base, but with the terminations of the deformations flared more outwards in contrast to the inward tapering displayed in the example shown in Figure 2 and Figure 3. The more outward flaring will have the effect of strengthening the annular collar of the deformation during the weld cycle collapse and can thereby produce a greater degree of fusion between the parts being welded.

An alternative method which can result in a yet greater degree of fusion involves a compound or multiple stage forming which draws additional material from the fin base to form a ridge parallel to the length of the fin base 7 and forming all or part of the deformations from the material of the ridge. The method may, for example, be carried out by allowing additional material in the region of the fin base and bending this material into a vee-shaped ridge running along the centre of the material which is to form the fin base. The vee-shape is then formed between a set of punches and dies which deform the ridge into an array of strong deformations, the tips of the deformations being formed from the apex of the vee-shaped ridge. These strong deformations may take the form of an aligned array of hollow shapes approximating to pyramids or cones.

Heat emission tests have indicated that the aforementioned arrangements, in accordance with the invention, can significantly increase the heat emission of a finned central heating panel radiator.

The apparatus of the invention may conveniently be arranged as an in-line closed-throat type welding machine with the panel and fin array automatically fed lengthwise through the machine with the panel orientated horizontally and with one or more fin bases being welded to the panel surface on each welding cycle of the machine.

Welding may be carried out by the series or by the direct process. In the series process current is passed from electrode to electrode through two resistance welds in series, the material of the workpieces providing a path for the current between the two welds. In the direct process current is passed from electrode to electrode through one resistance weld.

Series welding is the preferred method for panels with some or all of the fin joins on the panel waterways. Where series welding is used, advantageously the resistance weld arrays of two adjacent fin base joins are welded simultaneously. This provides equal length of current paths for all of the resistance welds.

During the resistance welding process, the deformations ideally collapse in a controlled manner and cause the two workpiece surfaces to come together. To enable all of the array of deformations associated with each fin base to collapse in the same way, it is therefore desirable simultaneously to weld the complete array of deformations associated with each fin base in view of the rigidity of the workpieces.

In some instances, for example when inadequate welding power is available or where the panel is unable to support the electrode force associated with a complete simultaneous fin base weld, it may be necessary to stagger the timing of the welds. In these instances some weaknesses may advantageously be introduced into the fin to allow it to flex as portions in the fin base are welded to the panel surface at different times. Such weaknesses may comprise, for example, slits in the fin material across the width of the fin base and part of the way into each adjacent fin wall.

Problems with regular collapse of the deformations across the fin base may also arise due to possible curvature in the panel or the fin base prior to welding. Ideally, these problems are overcome by ensuring an adequate degree of flatness in the production of the workpieces.

The problem may also be alleviated by arranging the electrode or set of electrodes to be sectioned over its length to provide a set of electrodes with aligned narrow flat faces and with each section separately moved or pressurized. This arrangement will tolerate a greater degree of curvature in the components than would a single rigid electrode. Where the welding machine is used to weld a range of panels with fins of different height, where the fin height is measured along the length of the fin base, it will be found convenient to arrange the sections of the electrode to correspond to the steps in the fin height of the panel range, as this will allow the possibility of only activating the proportion of the electrode required for the size of panel being welded.

It is important to arrange the contact area between the electrode and the fin base to be significantly greater than the contact area between the deformations and the surface to which they all are to be welded. Advantageously, the electrode should cover the full flat area of the fin base. Additional contact can be made between the electrode and a portion of the sloping side walls of the fin adjacent the fin base.

The invention may also provide further potential capabilities which are not possible with the spot-welding method of the prior art. One of these capabilities relates to the ability of the deformation welding process to join certain different metals or metals where one or both are coated or clad with a different material. The reasons for this capability over spot-welding include the ability of the tip of the deformations to break through a coating on the join contact surface early in the welding cycle and also to the much reduced likelihood of the resistance weld electrode sticking to or being contaminated by coatings on the electrode side of the material. The capability may be advantageously used by joining a material of higher conductivity or one coated with a material of higher conductivity to the base panel. It may also be used to allow a coating to be used on one of the join contact surfaces which has the ability to improve the thermal contact conductivity in the region of the join where direction fusion has not taken place. The invention may also be used to allow a protective coating to be used on one or more of the surfaces of the fin or panel.

It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention as defined in the appended claims.

Claims (23)

CLAIMS :

1. A heat exchanger having metal convective fins joined in a thermally conductive manner with a metal surface of the heat exchanger, characterised in that a plurality of mechanically formed deformations are provided in the region of the join between at least one fin and the surface of the heat exchanger and the fins are joined to the heat exchanger surface by resistance welding focused on said deformations .

2. A heat exchanger according to Claim 1, in which the fins are joined to the heat exchanger surface by arrays of resistance welds focused on deformations disposed along lines which are substantially parallel to elongate radiator contact bases of the fins.

3. A heat exchanger according to Claims 1 or 2, in which the deformations used to effect the resistance welds are spaced a small distance apart so that when welded the resulting welds cooperate to provide substantially continuous thermally conductive joins substantially parallel to the elongate radiator contact bases of the fins .

4. A heat exchanger according to any one of the preceding claims, in which the unfused portions of the joins are altered by the heat and pressure of the deformation focused resistance welding process such that a more intimate and more thermally conductive contact is made in the unfused portion of the join and is retained by the adjacent fused portions of the joins.

5. A heat exchanger according to any of the preceding claims, in which each elongate fin contact base is narrow in width, being between two to five millimetres in width.

6. A heat exchanger according to any of the preceding claims, in which the portions of the fin wall adjacent the region of the join are angled sharply away from the heat exchanger surface.

7. A heat exchanger as claimed in any one of Claims 1 to 6, in which the deformations used in the joins are formed on the elongate fin contact bases.

8. A heat exchanger as claimed in any one of Claims 1 to 6, in which the deformations used in the joins are formed on the elongate radiator contact bases and on the heat exchanger surface, whereby the welds are made at the intersections of the deformations on the two parts.

9. A heat exchanger as claimed in any one of Claims 1 to 6, in which the deformations used in the joins are formed as embossments on the heat exchanger surface.

10. A heat exchanger as claimed in Claim 7, in which each deformation comprises an annular collar with a central hole which is formed and pierced without any removal of material from the elongate fin contact base.

11. A heat exchanger as claimed in Claim 7, in which each deformation comprises additional material from the elongate fin contact base and is formed in a compound or multiple stage operation including forming a ridge in the material parallel to the length of the elongate fin contact base and forming all or part of the deformation from the material of the ridge.

12. A heat exchanger as claimed in any one of the preceding claims, in which each fin is provided with weaknesses whereby the fin may flex as portions of it are resistance welded to the heat exchanger surface.

13. A heat exchanger as claimed in any one of the preceding claims, in which the materials of the fins and heat exchanger surface comprise different metals.

14. A heat exchanger as claimed in any one of the preceding claims, in which one or more of the surfaces of the fins or the surface of the heat exchanger is provided with a coating of a type which is applied in advance of the joining of the fins and heat exchanger surface. 15. An apparatus for use in making heat exchangers, characterised in that the apparatus comprises one or more electrodes with narrow flat contact faces and means for applying pressure and electrical current through the electrodes and is operable to receive a heat exchanger with a metal surface and fins made from sheet metal, with a plurality of mechanically formed deformations provided in the region of the join between each fin and the surface of the heat exchanger with the apparatus being operable to join the fin contact bases of the fins to the surface of the heat exchanger by resistance welding focused on said deformations .

15. An apparatus as claimed in Claim 15, which comprises two parallel electrodes or sets of electrodes with narrow flat contact faces and means for applying pressure and electrical current through the electrodes and is operable to join two fin contact bases or portions of two fin contact bases to the surface of the heat exchanger by series resistance welding.

16. 17. An apparatus as claimed in Claims 15 or 16, in which the apparatus is operable simultaneously to execute all of the resistance welds associated with the fin contact base of each fin.

17. 18. An apparatus as claimed in any one of Claims 15 to 17, in which the electrode or set of electrodes which execute all of the resistance welds associated with a fin contact base of a fin comprises a set of electrodes with aligned flat faces which are capable of being separately moved or pressurized.

18. 19. An apparatus as claimed in any one of Claims 15 to 18, in which the electrode or electrodes are provided with additional flat faces adjacent and sloping away from the main flat contact face whereby additional electrical contact is made between the electrode or electrodes and sloping side walls provided on the fins adjacent the fin contact bases.

19. 20. A method of making heat exchangers having metal convective fins utilizing the steps as outlined in any one of the preceding claims.

20. 21. A heat exchanger substantially as herein described with reference to and as shown in the accompanying drawings .

21. 22. Apparatus for use in making heat exchangers, substantially as herein described with reference to and as shown in the accompanying drawings.

22.

23. A method of making heat exchangers substantially as herein described with reference to the accompanying drawings.