Heat exchanger for vehicles and method for the assembly of a heat exchange matrix

The present invention concerns a heat exchanger for vehicles, in particular a condenser for air-conditioning units.

Heat exchangers for motor car use can be divided into two fundamental categories based on the technology used in their production. These categories are: <ul id="ul0001" list-style="dash" compact="compact"><li>brazed heat exchangers, and</li><li>expanded car mechanically eassembled heat exchangers.</li></ul>

The aforesaid categories refer to the way in which the connection is made between the tubes through which the heat transfer liquid passes and the metal fins which give up heat to the cooling air flow.

Brazed condensers are generally formed from flat tubes folded in a serpentine configuration with corrugated or undulating fins fitted between each pair of adjacent branches of the tube. The fins are fixed to the branches of the tube by a brazing process.

In expanded or mechanically assembled condensers the heat exchange matrix is formed from a plurality of tubes and a stack of substantially flat fins. The tubes are inserted in corresponding aligned holes in the fins and the tube-fin assembly is achieved by expansion of the tubes after they have been inserted with clearance into the holes in the fins.

The present invention specifically concerns heat exchangers of the mechanically assembled type and in particular heat exchangers intended to be used as condensers in vehicle air-conditioning units.

Condensers for use in motor vehicles are subject to conditions of use which are much more onerous than for example engine-cooling radiators or heat exchangers for heating the inside of the passenger compartment.

In particular, the condensers are subjected to pressures from the heat exchange liquid which can reach values in excess of 25 bar and temperatures in excess of 110°c. Supply specifications generally require that the condenser should not suffer permanent distortion in any part from a pressure within the tubes of the order of 40 bar (test pressure), and the condenser should not split even at pressures equal to twice the test pressure (approximately 80 bar).

Such onerous conditions of use impose many limits on the choice of the tube section, and in the general dimensions of the heat exchange matrix. In particular, it can be seen that it is very difficult to make mechanically assembled condensers using tubes with oblong section, which would enable a more efficient heat exchange to be obtained than tubes with circular section.

The object of the present invention is to make a heat exchanger intended to be used in particular as a condenser in a motor vehicle air-conditioning unit, using the mechanical assembly technique with tubes of oblong section.

According to the present invention, this is achieved by making a heat exchanger having the characteristics which form the subject of Claim 1.

The present invention also concerns a method for assembling a heat exchange matrix, the characteristics of which are the subject of Claims 2, 3 and 4.

Further characteristics and advantages of the present invention will become evident from the following detailed description, given by way of non-limitative example, with reference to the accompanying drawings, in which: <ul id="ul0002" list-style="none" compact="compact"><li>Figure 1 is a schematic perspective view of a heat exchanger according to the present invention;</li><li>Figure 2 is a schematic perspective view illustrating an expansion tool used in the method according to the invention, and;</li><li>Figure 3 is a schematic plan view in the direction of the arrow III of Figure 2.</li></ul>

Referring initially to Figure 1, the reference numeral 10 indicates in general a heat exchanger comprising a stack of substantially flat metal fins 12 with holes through which extend a plurality of tubes 14 of oblong, in this case oval, transverse section. In the following description and in the Claims, "oblong" transverse section means any non-circular transverse section with a major axis and a minor axis, including oval, ovoid and elliptic transverse sections and also flat sections with two parallel walls and two semi-circular connecting zones.

Referring to Figures 2 and 3, each tube 14 is made from a wall 16 and a stiffening longitudinal septum 18 which extends along the minor axis of the transverse section of the tube 14. The tube 14 is formed by extrusion and is made from easily extrudible metal material, such as for example aluminium or copper. The internal stiffening septum 18 is formed integrally with the wall 16 during the extrusion of the tube.

Each tube 14 is inserted with slight clearance into a series of aligned holes 20 in the fins 12. Preferably, the fins 12 each have a collar 22 corresponding to each hole 20 which adheres, upon completion of the assembly, to the external surface of the wall of the tube.

The fixing of the tubes 14 to the fins 12 is achieved by causing the permanent deformation of the tube by expansion, so as to close up the clearances existing between the external surface of the tube and the internal surface of the collars 22 and causing an elastic deformation of the fins in the region of the holes 20. The deformation of the tube is achieved by using a mandrel 24 which includes two symmetrical heads 26 carried on a rod 28 which are movable together in the direction indicated by the arrows 30 through the action of an actuator of conventional type (not shown). Each head is inserted in a respective channel of the tube 14 and exerts a deforming force F on the tube along the major axis of the transverse section of the tube 14 (see Figure 3).

The deforming force F is applied to the zones 32 of the tube 14 having the least radius of curvature and causes an expansion of the tube from the original shape illustrated by the broken line in Figure 3 to the final shape illustrated in the same Figure by the continuous line. The deforming force F causes a reaction which is transferred to the stiffening septum 18. Given that the reaction forces produced by the two heads 26 are of equal magnitude and are opposing, the septum 18 and the zone of the tube with the greatest radius of curvature are substantially undeformed at the end of the expansion process.

The heads 26 have internal reaction surfaces 34 adapted for transferring the reaction forces so as to concentrate them on the connecting zones between the stiffening septum 18 and the walls 16 with the aim of reducing the sliding friction during the expansion phase.

A tube made according to the present invention is particularly suitable for the manufacture of condensers, in that it has a strong resistance to high internal pressures. This allows a free choice in the dimensions of the oval section tube in determining the dimensions of the heat exchanger. Due to the stiffening effect of the septum 18, it is possible, all other conditions being equal, to reduce the wall thickness of the tube and therefore reduce the weight. Consequently, due to the thinner tube walls, the expansion process is easier.

Also, while in the conventional condensers the fins had a structural function in the sense that they had to contribute to controlling the tendency of the tube to expand under the pressure of the fluid, with the structure according to the present invention it is possible to reduce the costs of the fins, as it is sufficient to use thinner materials with inferior mechanical properties.