HAIRPIN CONFIGURED TUBES AND METHOD OF MANUFACTURE

Abstract: An integrally formed hairpin configured metallic tube, comprising two parallel leg portions (2, 2') of oblong cross-section connected together by an U-shaped bend portion (3) of substantially circular cross-section. A method of manufacturing hairpin tubes includes the steps of providing a tubular member with a substantially circular cross-section, bending said tubular member to an U-shaped hairpin tube with two parallel leg portions, and subsequent radial deformation of the leg portions to an oblong cross-section.

Description: Hairpin configured tubes and method of manufacture Field of the invention The present invention relates to tubular members, par¬ ticularly tubes forming a continuous path for the circulation of a heat exchanging fluid in heat exchangers, and methods for manufacture of such tubes. This invention relates more particularly to metallic hairpin tubes provided with secondary heat exchanger elements or fins fixed to the tubes normally by brazing/soldering, crimping, expansion or gluing. Background of the invention The large number of connections between the tubes of a heat exchanger core and the terminating end plates of fluid tanks (headers) and/or U-shaped bends represent the main manufac¬ turing and operating problems for these heat exchangers. The mechanical strength of the soldered or brazed connections between the tubes and the end plates is poor. These con¬ nections are vulnerable to damages causing leaks as a result of mechanical vibra ' tions and due to changes in pressure and temperature to which these parts are subjected. The tube end plate connections, realized by means of re¬ silient joints where the tubes are expanded into a direct, tight engagement with the end plates, are known from prior art. This expansion operation is not an easy task in mass production manufacturing and therefore tubes commonly used in heat exchangers with this type of connection have a circular cross-section which facilitates the assembly of the tube ends, e.g. with U-shaped bends, as disclosed in DE 2952736. Use of tubes with oblong or flat oval cross-section in order to improve the heat exchanging efficiency (performance) by increasing their surface area without increasing the aero¬ dynamic resistance and/or size of the heat exchangers, is also known. The drawback of the heat exchangers provided with oval tubes is the necessity to conform the ends of the oval tubes to a circular cross-section of the connecting U-shaped bends or openings in the end plates. The conformation of the .tube ends is an additional " operation in assembling of the heat exchangers, which requires narrow dimensional tolerances in order to achieve satisfactorily tight connection between the tubes and the apertures in the perforated end plates. UK patent applications Nos. GB 2.055.064 and GB 2.056.330 disclose one of the known methods of conforming the end " of an oblong metallic tube to a circular cross-section. Attempts have also been made to provide one-piece hairpin tubes of oval cross-section. Nobody has hitherto succeeded to overcome the problems related to bending flat oval, especially thin-walled metallic tubes, namely a reduction of the cross-section area of the bends and formation of wrinkles on the inner tube walls. Both phenomena have an adverse effect on the flow pattern of the circulating fluidum - in¬ crease of pressure drop in the heat exchanging systems and erosion of the bent tube parts. Object of the invention It is therefore an object of the present invention to provide one-piece hairpin .tube having a body of substantially parallel leg portions with oblong cross-section avoiding the above stated drawbacks connected to the bending operation, and methods for manufacture of such tubes. Another object of the invention is to provide tubes for heat exchanger units resulting in better heat rejection perform¬ ance, reduced number of tube joints and to avoid or facili¬ tate connections between tubes of different cross-sectional shape in heat exchangers. Still another object of the invention is to provide a simple and low cost method of manufacturing oblong hairpin tubes. These and other objects of the pre.sent invention will become apparent upon reading the following specification with accompanying drawings and patent claims. Summary of the invention According to the present invention there is provided a one-piece hairpin configured tube comprising two sub¬ stantially parallel leg portions of oblong cross-section connected by an U-shaped bend portion having a cross-section area larger than or equal to the cross-section of the leg portions. According to the present invention there is also provided a method of manufacture of thermally conductive tubes including hairpin configured U-shaped bends, comprising the steps of providing a (straight) tubular member of substantially circular cross-section, bending the said tubular member providing an U-shaped hairpin tube with two parallel leg por¬ tions which are subsequently exposed to a radial deformation operation resulting in an oblong cross-section of the entire length or parts of the leg portions. According to the present invention there is still another method of manufacture of hairpin configured tubes comprising the steps of providing a tubular member of oblong cross- section, local conformation of a predetermined part of the tubular member to a substantially circular cross-section and the consequent bending of the said conformed part providing an U-shaped tube with two parallel leg portions of oblong cross-section. Description of the drawings Embodiments of the invention are shown by way of non- restrictive examples in the accompanying drawings, Fig. 1 - 10, where Fig. 1 is a plan view of a hairpin configured tube. Fig. 2 is a sectional view of the hairpin tube taken along line I-I of Fig. 1. Fig. 3 is another sectional view taken on line " II-II of Fig. 1. Fig. 4 is a sectional view of a tool for accomplishing the step of radial deformation according to the present invention. Fig. 5 is a sectional view of the tool from Fig. 4 after performance of the deformation step. Fig. 6 is a planar view of the hairpin tube in a die after accomplished deformation step. Fig. 7 illustrates the step of conforming of a pre- deter inated part of an oblong tube to a circular cross-section in a modified method of manufacturing the hairpin tubes. Fig. 8 is a sectional view of a tool for accomplishing the conforming operation taken on line I-I of Fig. 7. Figs. 9 and 10 represent modifications in the manner of configuration of the leg portions of the hairpin tubes with regard to the U-bend portion. Referring to Figs. 1-3, it has been .established that it is possible to obtain a hairpin configured tube (1) retaining the body of the oblong cross-section and to accomplish a bending operation without encountering the above mentioned difficulties/drawbacks known from the prior art. The new one-piece hairpin tube (1) provided according to the present invention comprises two parallel leg portions (2,2') of the oblong cross-section connected together by an U-shaped bend portion (3) having a substantially circular cross-section - see Fig. 2. The new and unique configuration of the hairpin tubes is thus provided based upon a new inventive approach to solve the problem connected to bending thin-walled oblong tubes. It has been empirically established that a certain relation between the wall thickness, the central distance of the leg portions (bend-radius) and the largest diameter of the oblong tubes must be respected in order to avoid surface defects on the tube walls during the bending operation. An empirically derived factor f = , where Dm = largest diameter of the tube, c = central distance between the leg portions and d = wall-thickness should be < 10 to ensure smooth quality tubes, at f in a range from 11-14 problems can arise and at f > 14 the problems with formation of wrinkles are unavoidable. Example A typical Al-tube to be used in a motor vehicle radiator or heater core will have a wall thickness of 0.4 mm, central tube distance 22.0 mm and largest diameter of the oblong cross—section 13.0 mm (short diameter 6.0 mm) The empiric factor in this case is: f = = 19.2 so that the bending operation of such tubes in order to achieve the above central distance between the leg portions of the hairpin tube cannot be conducted without adverse effects on the U-bend surface quality. According to the present, invention the tubes are subjected to a bending operation while retaining a substantially circular cross-section over the deformed part of the tubes. In a pre¬ ferred embodiment of the invention a circular tube with a diameter of 10 mm is provided and then by means of com¬ mercially available apparatuses, e.g. hydraulically powered tube benders, a bend of 180° is formed as the first step in the manufacture of oblong hairpin tubes. Figs. 4 and 5 show schematically the subsequent step of radial deformation of the just provided hairpin tube from the circular cross-section to an oblong/elliptical cross-section along the parallel leg portions.of the tube. The hairpin tube of circular cross-section (1) is inserted between the open jaws (51,52) of a pressure die (5) so that a predetermined part or the whole leg portions (2,2") of the hairpin tube being conformed to an oblong cross-section having diameters of 13.0 and 5.7 mm respectively, see Fig. 5, showing a sec¬ tional view of the pressure die from Fig. 4 after performance of the deformation step. The hairpin tube provided in the above example exhibits no surface irregularities in the U-bend portion (3) and transition curves of intermediate portions of the tube between the oblong parts and the circular U-bend being smooth without sharp edges. Furthermore, the circular cross-section of the U-bend portion is larger than the cross-section of the leg portions so that the result is a hairpin tube of sub¬ stantially oblong cross-section body ensuring improved heat exchanging efficiency at a minimal pressure drop in the heat exchanger. Fig. 6 illustrates in planar view the hairpin tube in the pressure die (5) comprising an U-bend portion (3) of the substantially circular cross-section outside the pressure die and the leg portion ' s (2,2') of the oblong cross-section having free ends (21,21') retaining the original, circular cross-section. This configuration of the hairpin tube offers a special advantage allowing direct connection to a terminat¬ ing plate of headers provided with circular apertures. Other types of conforming apparatus than the pressure die shown and described in connection with the above example can be used for the conforming operations, e.g. drawing dies. The apparent advantage of the described pressure form ing opera¬ tion is that it ensures by simple means a. controlled fixation of the desired angle between the leg portions (2,2') and the U-bend (3) of the hairpin tubes. Figs. 3, 9 and 10 represent the most usual patterns of the -hairpin tube configuration with regard to the assembling of the heat exchanger cores - so-called parallel, staggered parallel or series connection, respectively. An .alternative manufacturing method to the one described with reference to the above example is illustrated in Figs. 7 and 8. A tube (11) of the required oblong cross-section and dimensions is provided by extrusion, extrusion followed by a drawing operation or by welding. The thus provided tube (11) is then subjected to a local radial deformation over a pre- determinated part (3 1 ) of the tube. The cross-section of this part (3') being conformed to a circular shape in a pressure die (5) - see Fig. 8 showing the die (5) in a vertical sec¬ tional view taken along line I-I of Fig. 7, facilitates the subsequent bending operation conducted by means of adapted commercial benders. The bending operation in this- case is a final step in the manufacture of oblong hairpin configurated tubes. The cross—section of the U-bend parts (3) can also exhibit other shapes deviating from the circular one shown in the drawings without departing from the scope of the present in¬ vention. Even if the circular cross-section offers advantages such as direct use of commercially available benders without any need for adaptation of the apparatuses or special acces¬ sories (mandrels), tubes with other cross-sectional shapes in the bending area can be used. It is, however, important that such cross-sectional shapes comply with the requirements to a cross-section of the U-bend being larger than or equal to a cross-section of.the leg portions, and furthermore that the relation between the wall thickness, the bend radius and the largest diameter of the tube according to the above empiric formula ensures avoidance of surface defects on the tube wall.

Claims: Cla ims 1. A hairpin configured metallic tube, particularly for thermally conductive tubing in heat exchangers, com¬ prising two parallel leg portions (2,2' ) of oblong cross-section connected together by an U-shaped bend portion (3) having a cross-section area larger than or equal to the cross-section of the leg portions, and integrally formed of a tube of non-ferrous metal. 2. Tube according to claim 1, where the bend portion (3) has a substantially circular cross-section 3. Tube according to claim 2, where free ends (22,2 * 2') of the parallel leg portions (2,2') have a circular cross-section. 4. A method of manufacturing metallic tubes according to claim 1, comprising the steps of providing a tubular member of substantially circular cross-section, bending said tubular member to an U-shaped hairpin tube including two parallel leg portions, exposing the leg portions to a radial deformation resulting in an oblong cross-section of the entire length or parts of the leg portions. 5. The method according to claim 4, where the radial deformation of said leg portions is carried out as a pressure forming operation in a die. 6. A method of manufacturing metallic tubes according to claim 1, comprising the steps of providing a tubular member of oblong cross-section, local conformation of a predetermined part of the tubular member to a substantially circular cross- section, bending said tubular member over the conformed part providing an U-shaped hairpin tube with two parallel leg portions of oblong cross-section.

Inventors: CLAUSEN EDVIN LIST, KNOLL RICHARD, CLAUSEN, EDVIN, LIST, KNOLL, RICHARD
Applicants: NORSK HYDRO AS, NORSK HYDRO A.S.
Application year: 1985
Main classification: F28D7
IPC codes: B21C37, B21D7, F28D7, F28F1