Conventional high-performance, parallel-flow heat exchangers are fabricated from aluminum alloy components. At present, these heat exchangers are used primarily for automotive climate control systems. These heat exchangers use a flat, multi-channel tube known as a micro-channel tube due to its relatively small size. The micro-channel tubes are currently fabricated from aluminum alloys, primarily using direct hot extrusion through hollow dies. During the extrusion process, the aluminum must divide into two or more metal streams, and flow around a bridge (not shown) that supports a mandrel 100 (see FIG. 1). As shown in FIG. 1, the mandrel 100 incorporates weld chambers 102 in which the metal streams must rejoin to develop a solid-state weld and form continuous internal walls, thus creating the internal passages or channels.
As shown in FIG. 2, a typical condenser 200 for a vehicle climate control system (e.g., a vehicle-loaded condenser) includes an array of alternately stacked parallel aluminum micro-channel tubes 202 (e.g., from 20-50 tubes per condenser) and louvered fins 204. The aluminum micro-channel tubes 202 extend between and are connected to a pair of header tanks 206. Referring to FIG. 3, some aluminum micro-channel tubes 300, 302, 304 and 306 having varying cross-sections are shown. The header tanks 206 are often formed from cylindrical pipe. In the condenser 200, parallel flows of a fluid (e.g., a refrigerant) are established through the channels in the aluminum micro-channel tubes 202 between the header tanks 206. Heat transfer occurs between the refrigerant in the aluminum micro-channel tubes 202 and air flowing through the louvered fins and past the aluminum micro-channel tubes 202. Essentially all passenger vehicles produced with air-conditioning in North America, Europe and Japan use these heat exchangers in their vehicle climate control systems (i.e., the current R134a-refrigerant based systems).
The performance benefits of parallel-flow heat exchanger technology, as successfully implemented by the automotive industry, have begun to be recognized by the commercial and residential HVAC industry. These industries have historically been dominated by heat exchangers using round copper tubing. Nevertheless, interest currently exists in using parallel-flow heat exchangers in HVAC applications, wherein the heat exchangers are fabricated using the only currently suitable material, namely an aluminum alloy, to form aluminum micro-channel tubing in brazed assemblies. Moreover, R744 (CO2) refrigerant based systems, currently under development in the automotive and refrigeration industries, impose more severe operating conditions on the “high-pressure side” components, such as the gas cooler and associated micro-channel tube, the compressor, an internal heat exchanger/accumulator and all associated connections. Specifically, typical maximum operating pressures and temperatures are 16 MPa and 180° C., respectively (48 MPa static pressure with a factor of safety of 3). Micro-channel tube, such as those shown in FIG. 3, is ideally suited to heat exchangers using this “environmentally-friendly” refrigerant.
It is generally held that an extrusion process (e.g., the direct hot extrusion process described above) is only suitable for materials “that can be easily deformed at normal extrusion temperatures” such as 1000, 3000 and 6000 series aluminum alloys. Extrusion loads are also higher for “hollow-die” extrusion as a result of the metal separation as it enters the die. As a result, the high flow stress and high hot-working temperature of copper and other metals and alloys have precluded them from being extruded with a hollow-die extrusion process.
Hot work tool steels (with or without a surface treatment such as nitriding) rapidly wear and, thus, are not practical as a suitable wear surface for the die components (i.e., a mandrel or plate). Therefore, these die components have been fabricated from Tungsten carbide/cobalt (WC/Co) metal matrix composites (MMCs). WC/Co MMCs can provide suitable wear resistance, however their low fracture toughness imposed limits on the design of the die components and breakage was not uncommon. Currently, some extruders use die components made from tool steel coated with hard thin-film coatings. The tool steel provides the necessary die strength and fracture toughness, while the hard thin-film coatings provide the necessary wear resistance at elevated temperatures, for the extrusion of aluminum micro-channel tubes.
Copper-based heat exchangers, and specifically copper micro-channel tube, would offer several advantages over aluminum micro-channel tube for the aforementioned applications, including better strength (i.e., resistance to deformation) and elevated-temperature strength, better corrosion performance, higher thermal conductivity, better joining characteristics, and the ability for easier field service repair. Thus, there is an unmet need for a viable process for manufacturing micro-channel tube using a non-aluminum metal or alloy, such as copper or a copper alloy.