BACKGROUND OF THE INVENTION Oil coolers found in vehicles are typically stacked, multi-plate constructions, which are mounted separately inside the radiator tank and plumbed with an oil inlet and outlet that open through the tank wall to the outside. Coolant is fed to the radiator tank, and washes over the outside of the oil cooler within. Oil is fed independently into the oil cooler, and conducts oil heat to the radiator coolant. Obviously, the oil cooler plate stack must be carefully, internally sealed to prevent a cross exchange of oil and radiator coolant inside the tank. Just as important, the oil inlet and outlet must be carefully sealed where they pass through the radiator tank wall so as to prevent leakage of radiator coolant to the outside. A stacked plate oil cooler is disclosed in U. S. Pat. 5,636, 685.

Patented designs propose to integrate the oil cooler into the radiator coolant tank in such a way as to reduce or eliminate the possibility of oil-coolant cross exchange, while having oil inlets and outlets that do not create a potential leakage of radiator coolant to the outside. A design like this is known from US-5823250. In this design the oil cooler takes up a part of the radiator tank volume. The cooling efficiency is hereby limited by the fact that only two sides are available for heat transfer between the oil and the radiator coolant. The design is also not optimised for the most efficient transfer of heat through the oil cooler/radiator interface.

The cooler components are brazed together, which limits the oil pressure that may be used. Brazing of some aluminium, such as those of the AA6000 series, is also not very feasible, which makes the choice of alloy composition limited.

From US-4821797 it is known to manufacture a fluid cooler by extruding tubes with different diameters having heat exchange fins extending inwardly and/or outwardly from the tube surface and inserting in this tube a second tube, having heat exchange fins extending outwardly and welding together the tube ends so as to form a closed chamber. The height of the fins and the diameters of the tubes are adjusted so as to make the fins reach the surface of the other tube. This limits the flexibility in design to optimise the construction for different requirements of cooling efficiency or different cooling media. Also the ability to withstand high fluid pressures are severely limited due to the fact that the fins of this construction are not integral with the surrounding tube. At high fluid pressures the tubes will be prone to separation and leak-proofness can not be assured.

The same problem will occur due to difficulties of getting an exact correspondence of the shape of the tubes when extruding these, which will cause some of the fins not being in contact with the outer tube inner surface.

Another problem with this cooler is that to seal the ends of the cooler the end parts have to be widened/compressed so as to make the ends join. This operation is very sensitive to precision in the bending operation as to make the surfaces of the inner and outer tube meet around the whole length of the tube circumference. The fins at the end parts have to be removed to make joining possible. These extra manufacturing steps are unpractical and considerably increases the cost for manufacturing of the cooler. The fact that the cooler has a tubular inner space does also not provide for the most compact solution possible.

Another prior art document, GB2231142, discloses a tube oil cooler where an inner and outer tube is connected together by end walls. Nothing is said about the method of joining the end walls to the tubes. The construction does also not provide the most compact cooler possible, which may be a requirement for some applications, as has been discussed above in connection with US-4821797.

SUMMARY OF THE INVENTION The present invention provides an extruded fluid cooler, preferably an oil cooler, made from extruded aluminium profiles, a elongated tank body (1) comprising at least one extruded element having one exterior and one interior surface, each of said exterior and/or interior surface having a web of extending heat dissipating or emanating fins (2), said extruded element (s) forming a chamber with two open ends, a fluid inlet opening into said chamber a fluid outlet opening into the other end of said chamber two lids sealing off said open ends forming a leak-proof/sealed chamber with fins extending inwardly and outwardly of said chamber. in a preferred embodiment of the invention the fluid flow path within the fluid cooler section of the tank is serpentine, with a total path length that is a multiple of the end to end length of tank. The cooler has a high cooling capacity, a small size, and is resistant to high fluid pressures.

In the preferred embodiment disclosed, a tank body comprises an elongated tank body, initially open at each end. The tank body preferably comprises four extruded elements, each having one exterior and one interior surface, each of said exterior and/or interior surface having a web of extending heat dissipating or emanating fins, said extruded elements forming a chamber with two open ends when put together.

The tank may be manufactured from one or two parts, but the extrusion process becomes more complicated when fewer parts than four are used.

The invention also relates to a method for manufacturing a fluid cooler comprising the step of extruding an elongated tank cooler, comprising at least one element, having one exterior and one interior surface, each of said exterior or interior surface having a web of extending heat dissipating or emanating fins. If the tank consists of several elements, these elements are put together as to form an elongated tank body having first and second open ends. Said lids are sealed to said tank body open ends to form a leak-proof/sealed chamber with fins extending inwardly and outwardly of said chamber.

To obtain the most efficient heat transfer the AA6000 aluminium alloy series are preferably used, although other metals or aluminium alloys may also be used.

The sealing is preferably done by friction stir welding as this method affects the material to a lesser degree than other known joining methods and thereby the construction is more resistant to leakage, even at high pressures. Other welding methods may also be used when the requirements on the joints are less severe.

Friction stir welding is described in EP-A-615480. Typically, a friction welding tool (a probe) is inserted into a joint region to be welded, undergoes a cyclic motion to generate plasticised material and is typically traversed along the joint region, although in some cases it could simply be withdrawn without traversing.

To further increase the strength of the construction at least one integral web (3) defining an interior wall, may be inserted into slots formed in the interior side of the tank body, thereby defining interior parallel sub-chambers.

This inserted web, which joins the inner and outer walls, terminates short of the other end of the tank body. In this way a narrow clearance space is left, interconnecting the two sub-chambers. When more than one web divides the interior chamber into more than two adjacent sub-chambers, the webs terminate short of opposite ends of the tank body, in an axially alternating pattern.

Another outer wall of the tank body is provided with a conventional coolant inlet and/or outlet, so that coolant is thermally exposed to the sub-chambers across the inner wall, but physically sealed therefrom. A fluid inlet opens to the first of the sub- chambers at a point remote from the clearance space in the nearest web, and an fluid outlet opens to the last of the sub-chambers, also at a point remote from the clearance space in the nearest web. The fluid enters the inlet to flow axially in one direction, along the length of the sub-chamber that it enters, then through the remote clearance space and in the opposite axial direction along the length of next sub- chamber, and so on, in a serpentine pattern, until it exits the fluid outlet in the last sub-chamber. The fluid thus flows over at least twice the basic length of the tank. As it flows back and forth through the sub-chambers, it is continually thermally exposed to coolant across the inner wall.

The fluid cooler may be put inside a tank containing another cooling media, e g water if the fluid inside the cooler is oil. If the area of the total surface of the fins extending inwardly, towards the oil side, are bigger than the total surface of the fins extending outwardly the most efficient heat transfer is obtained due to the higher heat transfer coefficient of the water.

The shape of the cooler may be rectangular or circular or any other shape.

The exact height and separation distance of the fins depends on the other dimensions of the cooler, the properties or the fluids, such as the oil/water temperature, the velocity of the fluids and the required cooling capacity etc. The construction used in the present invention makes the design very flexible and adaptable to different demands.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features of the invention will appear from the following written description, and from the drawings, in which: FIG. 1 is a cross section of an oil cooler of the prior art; and FIG. 2 is a cross section of an oil cooler according to the invention.

FIG. 3 is a cross section of a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, an oil cooler made according to the prior art comprises an extruded tank body A, which comprises an extruded inner and an extruded outer tube, the length of the fins being adjusted so that the fins pointing inwardly as well as the fins pointing outwardly radially bridge a circumferencial passageway formed between the assembled tubes.

Referring next to FIG. 2, tank body 1 is an axially elongated, hollow metal extrusion, preferably of aluminium alloy, with a generally rectangular cross section, cut to length to provide square top and bottom end edges. All components, the tank body and the lid, can be welded in one operation. The use of friction stir welding for sealing the tank makes the construction more resistant too high oil pressures, such as up to 60 bar, and the joints also receive a more attractive appearance.

Example Four aluminium extrusions (one U-shaped extrusion with fins, one flat extrusion with fins and two flat extrusions, according to figure 3), having fins extending from one side are put together in a fixture to form a cooler with a rectangular cross section, having fins extending outwardly and inwardly from said cooler. The fins extending from the outside of the cooler may have the same separation distance but different height of the fins extending from the inside of the cooler. The height of the fins extending inwardly and the height of fins outwardly are determined according to fluids properties, temperature gradient and speed of fluids. The spacing of the fins SUBSTITUTE SHEET (RULE 26) are determined according to the required cooling capacity and maximum allowed pressure drop in the system.

Holes for the inlet and outlet of oil is drilled through the construction and a filler pipe with a hole overlapping the drilled hole is onto the construction. The assembled cooler is cut to length to provide square top and bottom end edges. All components of the cooler body and the lid are welded together by friction stir welding in one operation.

The cooler is assembled inside a tank with cooling liquids flowing through the outer surfaces. Tubes for the oil inlet provides a circulation of the oil through the oil cooler.

The cooler of the invention has a high cooling capacity and a small size and the pressure fall of the fluid is low compared to previously used constructions.