The present invention relates to heat exchangers e.g. such as used in air conditioning systems, and especially flat oval tube heat exchangers which are mechanically assembled by stacking of tubes and finstock and expanding the oval tubes to engage the tubes and the finstock permanently by mechanical bonding.

Numerous heat exchangers are employed in vehicles as condensers and evaporators for use in air conditioning systems, radiators to cool the engine coolant, oil coolers etc. In order to improve the heat transfer between the environment and a fluid flowing through the heat exchanger, the heat exchangers are typically designed as a tube-and-fin type in which numerous tubes thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa.

A variety of automotive air conditioning condensers is presently known, such as mechanically assembled round tube, brazed serpentine or brazed parallel flow condensers. This last type of brazed condenser with serpentine fins and flat tubes with inserts has particular high heat exchange performance capability associated with reduced physical volume. Consequently, they are widely used on high performance cars and where installation of auxiliary equipment requires enhanced cooling performance. Because of the brazing operation involved under production, this

type of condenser is however stressing the environment and is also more expensive to manufacture. On the other hand, especially the first type of mechanically assembled, round tube condenser is cheaper to manufacture and also more environmentally benign under the manufacturing process, because of the absence of the brazing operation. Unfortunately, this type of condenser has a lower performance compared to the parallel flow condenser.

Lately, new mechanically assembled and expanded heat exchangers, such as aluminium automotive radiators have been developed featuring extruded and drawn or welded flat oval tubes. These radiators achieve performance levels comparable with those of brazed radiators. Thus, a mechanically bonded flat oval tube heat exchanger radiator will give performance approaching to that of brazed units at reduced costs. Also the potential dangers to the environment will be drastically reduced. Such heat exchangers are known from EP 0 387 678 Al and EP 0 389 970 A2. These patent applications are specifically describing how to obtain firm mechanical connection with a strong constructive mounting of the header plate and connection to the individual fins in the finstock without damaging the material of the fin in the collar area.

A similar approach for making flat oval heat exchangers operating with the high internal pressure prevailing in automotive air conditioning systems would be desirable in order to combine the environmental aspect, cost effectiveness and also performance requirements in an optimal way. A new flat oval tube condenser will give a higher performance for the same front area and will be substantially smaller in size than a mechanically assembled round tube condenser so that the space needed is reduced.

Since the production sequence is absolutely free of sources of pollution and therefore will meet future environmental requirements, a mechanically assembled and expanded condenser will have considerable advantages in relation to a brazed type.

The known constructions using flat oval tubes without inserts will work satisfactorily as radiators and in other heat exchanger designs with a relatively low pressure inside the tubes. When it comes to making high internal pressure heat exchangers, e.g. condensers used in air conditioning units, especially automotive air conditioning condensers typically operating under pressures from 30 to 40 bar, there are so far no designs which are available having small size and dimensions and which can withstand the high internal pressure without deformation. While the outside tube dimensions of an automotive heat exchanger radiator are typically 28 x 5 mm, the base tube for an automotive air conditioning condenser will for example have the outside dimension of appr. 8 x 2.1 mm. Consequently, the tube dimension of such a condenser will be less than 1/3 of the tube dimensions of the known mechanically expanded radiators. This has so far resulted in difficulties in manufacturing such heat exchangers. The tendency of the flat oval tubes without inserts to get back to their original round shape under internal pressure has so far prevented the use of such flat oval tube designs for high internal pressure heat exchangers.

Consequently, conventional brazed heat exchangers are presently used for these applications as illustrated in EP Patent Application 0 379 701, and PCT Patent Application W092/15833. Such condensers are made from a pair of spaced headers with a plurality of tubes extending in hydraulic parallel between them, defining the flow path between the headers and with serpentine fins extending between facing flat side walls of adjacent tubes. To withstand the internal pressures the tubes are also provided with internal web means and the whole unit of construction is brazed together, internally and externally.

Extensive tests with mechanically expanded and assembled heat exchangers of similar design as in the above EP Patent Applications 0 387 678 Al and 0 389 970 A2 have been conducted.

With plain finstock of rolled aluminium plate and a plate thickness of 0.15 mm testing under conditions simulating the conditions prevailing for automotive applications in air conditioning condensers, a maximum pressure of 35 bar could be achieved without plastic deformation of the tubes. This is not satisfactory for such applications, as the pressure relief valve is normally set to appr. 40 bars in an air conditioning system using for expample R 134a as refrigerant. Also the high pressure during operation of such condenser makes it difficult to obtain gas tight sealing and there is a risk that the connections between the flat oval tube and the header tubes will be broken during uncontrolled expansion of the flat oval tubes. For future use of such condenser constructions, there is also to be expected a radical increase in the internal pressures required as there is a development away from using common commercial refrigerants which are environmentally unsound. New refrigerants may require even higher pressures to be workable. In a gas cooler designed for working with C0 ₂ f.e., which is now emerging as a possible new refrigerant in such automotive air conditioning systems, the internal pressures will be in excess of 100 bar.

It is therefore a main object of the present invention to provide a heat exchanger comprising oval tubes which are mechanically assembled and expanded and which can work at pressures exceeding 40 bar.

Another object of the present invention is to provide an automotive air conditioning condenser of smaller dimensions than prior known mechanically expanded round tube condensers based solely on flat oval tubes being mechanically expanded into a firm thermal contact with the finstock.

Still another object of the present invention is to provide heat exchangers having the possibility to withstand the high internal pressures in excess of 40 bar without plastic deformation of the

fin plates or tubes and without distortion of the mechanical connections between individual tubes and the spaced header tubes, thus avoiding leakages and mechanical failure.

These and other objects and advantages of the present novel heat exchangers are met by provision of a heat exchanger as defined in the accompanying claims 1-7.

Due to the small physical dimensions and the use of extremely flat oval tubes a flat oval tube condenser produced by stacking of tubes and finstock and expanding the tubes results in:

- more efficient utilization of fin surface

- lower weight with same condenser performance

- reduction of condenser depth.

Based on extending tests conducted on heat exchange core(s), it has been concluded with that the support characteristics of the finstock plate material were of decisive importance to obtain a high internal pressure heat exchanger construction having properties to withstand the high temperatures and the high pressures prevailing. Different configurations of the fin plates were tested provided with a special wavy outline forming pattern of a zig-zag or semicircular form or a combination of these forms. Preferably the waves are running in parallel in relation to the longest axis of the flat oval tube cross-sections in a direction vertical (normal) to the tubes* longitudinal axis.

Other characteristic structural features of the heat exchanger according to the present invention are apparent from the accompanying drawings, Figs. 1-4, and the invention will be readily understood from the following description.

Fig. 1 shows in a perspective view a conventional mechanically assembled heat exchanger employing flat oval tubes with straight fins.

Fig. 2 shows in a perspective view a heat exchanger made according to the invention with wavy fins.

Fig. 3 shows in a detailed perspective view of two flat oval tubes connected by means of supporting fins having the conventional straight fin design (Fig. 3a) and also in a cross-sectional view the new zig-zag shaped and semicircular formed finstock made according to the invention (Fig. 3b) .

Fig. 4 is a detailed view of a header tube suitable for connecting the flat oval tubes.

Fig. 5 shows in a partially broken view the assembled high internal pressure heat exchanger comprising the new wavy finstock and the header tube design.

The prior known mechanical assembled heat exchanger is schematically illustrated on Fig. 1. It is basically made from a number of flat oval tubes 1, which are mechanically assembled by means of a finstock 2. It consists of individual straight fins 3 made from rolled metal plate mounted with a defined distance between them.

The finstock has two purposes, primarily to conduct heat away from the flat oval tubes and secondarily to support the tubes under internal pressure so they do not become subject to plastic deformation during operation. According to the conventional and accepted state of the art, this is achieved by using plane and straight fins with louvres or the like to enhance the airside turbulences. It is also known to use corrugated fin plates or ondulations running arbitrarily in relation to circular tubes to stiffen the fin itself, but not to obtain improved support of flat oval tubes used in a high pressure heat exchanger.

Other constructional details such as header tubes, inlet and outlet of cooling medium and the connection between the header tube and the individual, flat oval tubes are not shown on the drawing.

Similarly, the heat exchanger according to the invention is illustrated on Fig. 2 and consisting of flat oval tubes 4, finstock 5 and individual fins 6. Each fin is made from metal sheet provided with a number of parallel, longitudinal waves 7 running parallel in relation to the longest axis of each flat oval tube cross-section seen in a direction vertical to the tubes' central longitudinal axis and running perpendicular to the tubes* longitudinal axis. As shown more detailed on Fig. 3a each tube 4 which has outside dimensions of e.g. 8 x 2.1 mm and a wall thickness of e.g. 0.5 mm is mounted with a distance of e.g. 10 mm from the next, neighbouring tube while the distance between each fin 6 in the finstock 5 is e.g. 1.15 mm.

The configuration of the waves or ondulations of the fins can be varied from pleated and wavy to sinuous or even meander form.

Fig. 3b illustrates in a cross-sectional view the differences between the conventional plane fins 8, pleated fins 9 , wavy semicircular fins 10 and sinous formed fins 11. The thickness of the straight fin is 0.15 mm while the thickness of the wavy fins are e.g. only 0.1 mm.

Due to the high internal pressure which is prevailing within the heat exchanger, it is important that the mechanical connections between the header tubes 12 and the open ends of the flat oval tubes 13 are strong and fluid proof. An example of such mechanical connection is shown in Fig. 5.

A preferred connection means is shown on Fig. 4. It consists of a so-called female manifold tube, which in principle is known from the applicant's U.S. Patent 4,749,033 having a protruding.

longitudinal part 14 with integrally formed, flat oval flanges 15 of a form and size which corresponds to the form of the connected individual flat oval tubes. The ends of the tubes 13 are inserted into the flanges and are brazed or otherwise firmly bonded to the female manifold.

Thus, according to the present invention, it was concluded that changing the geometrical form of the smooth finstock plate material to one with waves or ondulations before mounting and stacking the individual components of the mechanical construction, gave a surprisingly high increase of the deformation pressure of the flat oval tubes and a very high resistance to mechanical deformation due to a mutual reinforcing effect between the tube walls and the adjacent finstock.

This is also confirmed by calculations based on the newly developed finstock of pleated, semicircular, meander or sinous forms showed that it is possible to achieve considerable improvements in the support from the fins.

Example;

To prove that the concept of a flat oval tube high internal pressure heat exchanger is feasible, an extended test program was initiated. Different tubes and tube diameters were made and finstock foil of different thickness and foil materials was provided. The holes in the finstock were punched to different diameters. It had to be proved that flat oval tubes of for example outside dimensions 8 x 2.1 mm having a wall thickness of 0.5 mm or less could be mechanically expanded in a finstock of rolled plate of a thickness of 0.15 mm or less; that the finstock would support the flat oval tubes in such a way that they would hold their shape to 40 bar or more internal pressure; and that the flat oval tube bursting pressure would lie in excess of 120 bar, which is required from some automotive companies for their systems used today.

After mechanical expansion, the flat oval tube shall have an outside diameter (in the unloaded condition) larger than the diameter of the holes in the finstock. Therefore, the pressure between flat oval tubes and finstock will result in good thermal contact.

The strength of the individual finstock was calculated from the Euler equation:

where E is the elasticity quotient, I = moment of inertia of free area and 1 = distance between neighbouring fins. The results are quoted below:

Fins I FACTOR (buckling strength) straight 0.00225 1 zig-zag 0.104 59 semicircular 0.260 108

Thus, the buckling strength can be increased with a factor of 59 for zig-zag formed fins and with a factor of 108 for semicircular fins using the finstock plate thickness. With a factor of 108 the finstock would theoretically hold to 4320 bar, which is far in excess of what is required.

Operating within the limits of Euler's buckling theory and taking the material properties of the specific tubes and forms into account, it is found that a sinous formed finstock, where the individual fins have a thickness of 0.05 to 0.1 mm and the wave length of the sinus is between 3 and 6 full periods over the long axis of the tube and the amplitude of the sinus is about 0.5 to

1.0 mm is the optimal shape from a production point of view, from a tube support point of view and from an air flow resistance point of view. Thus, this is a preferred embodiment of the present invention.