Patent Application
20070227690
The present disclosure relates generally to gas to air heat exchangers, and more specifically to cooling potentially corrosive gases, such as engine exhaust, in an envelope with relatively tight spatial constraints.
In recent times, when an engine included a turbocharger and exhaust gas recirculation, it might be only the incoming air that is compressed via the turbocharger before being combined with recirculated exhaust gas that is supplied to the engine. Such an engine, for example, is shown in co-owned U.S. Pat. No. 6,526,753. More recently, there have arisen reasons for adding the exhaust gas to the incoming air before passing the combined mixture through the turbocharger for compression. The compressed exhaust gas/air mixture often needs to be cooled before being supplied to the engine intake. Because the exhaust gases can contain corrosive constituents, such as sulfuric and/or nitric acid, the wetted surfaces of the cooler can, and often will, corrode over time. After a prolonged period, the fluid isolation between the cooling tubes and the air fins can be undermined, and in more extreme situations, the inlet or outlet tank can become corroded leading to holes allowing the hot exhaust gases to vent to atmosphere.
Some heat exchanger applications have additional problematic constraints. For instance, the spatial envelope available in an over the road truck can severely limit the space available for inclusion of a necessary gas to air heat exchanger. When relying on construction techniques according to the conventional wisdom these spatial constraints can become even more acute. Typically, a heat exchanger will include tubes, air fins and heads all constructed from a similar material that are joined together in a conventional well known brazing process. However, when corrosion resistance is a substantial issue, and that problem is combined with a severe spatial constraint, the conventional wisdom in some instances will suggest that the cooling demands of a given engine system in a specific application, such as an over the road truck, simply can not be met in the space available. Completely, redesigning the remaining portion of the engine to gain additional volume for a gas to air heat exchanger is too expensive an option for realistic consideration.
The present disclosure is directed to overcoming one or more of the problems set forth above.
In one aspect, a gas to air heat exchanger comprises a core with a plurality of tubes that are fluidly isolated from, but in heat transfer contact with, a plurality of air fins. The tubes comprise a tube material, and the air fins are comprised of an air fin material. The tube material has a high corrosive resistance relative to the air fin material. The air fin material has a high thermal conductivity relative to the tube material.
In another aspect, an engine system includes a gas to air heat exchanger fluidly positioned in between a compressor outlet and an engine intake. An exhaust gas recirculation system is fluidly connected between an engine exhaust and a compressor inlet. The heat exchanger includes an inlet tank with a first minimum wetted wall thickness, a plurality of tubes with a second minimum wetted wall thickness, and a plurality of air fins with a third minimum wetted wall thickness. The first minimum wetted wall thickness is greater than the second minimum wetted wall thickness, which is greater than the third minimum wetted wall thickness.
In still another aspect, a method of making a gas to air heat exchanger includes a step of assembling a core out of at least two different materials in a two step braising process at high and low temperatures, respectively. A tank is mechanically attached to the core with a seal positioned between the two.
Referring now to
Referring now in addition to
In order to meet tight spatial constraints while having superior heat transfer capability in the face of potentially corrosive gases, gas to air heat exchanger 28 includes a number of unique features. By putting an appropriate amount of the appropriate material in the right locations, gas to air heat exchanger 28 can provide adequate heat exchange while avoiding many of the problems associated with corrosive gases, and do so in a tight spatial envelope. From one perspective, this is accomplished by making the minimum wetted wall thickness of tanks 61 and 63 thicker than the minimum wetted wall thickness of tubes 67, which have a greater thickness than the minimum wetted wall thickness of air fins 66. Using this strategy, and realizing that the air fins need not be substantially corrosive resistant, they can be made of a relatively thin highly thermally conductive material, such as thin sheeting made predominantly of copper. Although not preferred, air fin material could also be cupro brazed copper, and less preferably a suitable stainless steel alloy, such as 409 stainless steel. Those skilled in the art will appreciate that the air fin material can include any of a variety of a materials exhibiting thermal conductive properties typical of the materials just identified. In any instance, the air fin material should be more thermally conductive than a tube material for tubes 67.
Like air fins 66, tubes 67 must have substantial thermal conductivity, but resistance to corrosion is also an important consideration. Those skilled in the art will recognize that the more thermally conductive a material is, generally the lower its ability to resist corrosion, and vice versa. With this in mind, tubes 67 might be made of stainless steel, with that being chosen in order of preference from 409 stainless steel, 304 and possibly even 316 stainless steel. Apart from stainless steel, tubes 67 might also be constructed from a suitable corrosive resistant material such as titanium, nickel plated aluminum, or possibly even nickel plated steel. Given these examples, those with ordinary skill in the art will recognize a family of materials that could be used for tubes 67 that have significant corrosive resistance, yet retain sufficient thermal conductivity for use in a heat exchanger application. As shown in
As in a conventional heat exchanger, the gas to be cooled is isolated from air fins 66 by attaching heads 69 and 70 at opposite ends of tubes 67. Like turbulators 78, heads 69 and 70 are preferably made from a material similar to that of tubes 67, to ease the attachment between the two. Thus, in one specific example, the heads 69 and 70, as well as tubes 67 and turbulators 78, if any, would all be made from a common stainless steel material and then brazed to one another with a high temperature brazing material 71 in a conventional manner. Some suitable high temperature brazing alloys include 613 nickel based alloys, nickel plating alloys, and possibly even Bnix alloys. After brazing together the tubes 67, heads 69, 70 and any turbulator 78, the air fins are fitted between tubes 66 and attached to the tubes in a relatively low temperature brazing process that facilitates good heat transfer between the tubes 67 and air fins 66. Some suitable low temperature alloys might include OKC 600, nickel plating alloys, and copper based alloys. Those skilled in the art will appreciate that based upon these example brazing alloys, a number of different alternatives would be available without departing from the scope of the disclosure.
Inlet tank 61 must take into account other considerations, including but not limited to, corrosive resistance and cost considerations as well as high temperatures. With these considerations in mind, tank 61 could be constructed from aluminum, such as a cast aluminum alloy with relatively thick walls that can tolerate expected corrosive concentrations and durations without allowing corrosive holes to develop. Outlet tank 63 could be made of a like material, or further weight and cost savings might be achieved by employing some other material, such as a non-metallic composite since the gases arriving at outlet area 75 are much lower in temperature. Referring to
The gas to air heat exchanger 28 according to the present disclosure finds potential application where corrosive gases need to be cooled with, but isolated from, air, and this cooling must be done in a relatively tight spatial constraint. For instance, in some work machines, such as over the road trucks, engines have evolved to include turbocharging and exhaust gas recirculation upstream from the compressor. When this occurs, the mixture of incoming air and exhaust gases must often need to be cooled prior to entry into the engine so that the engine can better function to achieve good efficiency and low emissions. Prior to such an engine evolution, the same over the road truck might have a simple air to air after cooler that did not need substantial corrosive resistance since there may not have been exhaust gas recirculation, but if there was, the exhaust gases were added to the intake downstream from the air cooler. But the spatial envelope available for gas cooling remained about the same, leading to the need for a high density corrosive resistant gas to air heat exchanger of a type described in this disclosure.
The gas to air heat exchanger seeks to address the needs of specific portions of the heat exchanger with materials having specific properties and in specific quantities (wall thicknesses) necessary to perform there needed function. For instance, the air fins need not necessarily be corrosive resistant but should be made from a highly thermally conductive material, such as one predominantly made of copper so that good heat transfer can occur from the air fins 66 to air passing through the heat exchanger 28. The tubes 67, on the other hand also need substantial heat exchanging capabilities, but this must be tempered with the need for corrosive resistance. Although the air fins 66 can be made extremely thin, the tubes 66 generally have a wetted wall thickness thicker than that of the air fins 67. The respective heads 69 and 70 attached to opposite ends of the tubes 67 should have a thickness on the order of that of the tubes and are preferably made of the same materials, for ease in attaching the two during a high temperature brazing process. The core 60 can be completed with a low temperature brazing process when attaching the relatively thin predominantly copper air fins 66 to the outer surfaces of the tubes 67. Although the tanks 61 and 63 could also be made from stainless steel, substantial costs savings can be achieved by making them from a less expensive material, such as cast aluminum, and possibly a composite for the cooler outlet tank 63. However, because aluminum has less corrosive resistance than stainless steel, the walls would generally have to be thicker than that of the tubes 67 and head 69 and 70 so that the inevitable corrosion to the wetted inner surface of the tanks could be tolerated over the expected life of the heat exchanger 28 without holes developing. Any problems associated with attaching aluminum and/or composite tanks to the stainless steel heads of core 60 are remedied via a mechanical attachment process, such as by crimping an extension of the heads about a flange on the respective tank 61 and 63. Before doing so, a suitable o-ring seal is positioned between the tank and head to inhibit leakage of corrosive gases from the heat exchange 28. Thus, the present disclosure brings a unique combination of heat exchanger features together, and assembles them in a unique way to arrive at a cost effective heat exchanger that can tolerate corrosive gases, and cool the same in a relatively small spatial volume.
It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Thus, those skilled in the art will appreciate that other aspects, objects, and advantages of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims.
