CuproBraze

The ability to withstand elevated temperatures is essential in high-heat applications. Aluminium alloys are challenged at higher temperatures due to their lower melting points. The yield strength of aluminium is compromised above 200 °C. Problems with fatigue cracking are exacerbated at elevated temperatures.[5][7][8] CuproBraze heat exchangers are capable of operating at temperatures of 290 °C and above. Special anneal-resistant copper and brass strip ensure that radiator cores maintain their strength without softening, despite exposures to high brazing temperatures.[3]

Cooling efficiency is a measure of heat rejection from a given space by a heat exchanger.[9] The overall thermal efficiency of a heat exchanger core depends on many factors, such as thermal conductivity of fins and tubes; strength and weight of the fins and tubes; spacing, size, thickness and shape of fins and tubes; velocity of the air passing through the core; and other factors.[5][9]

The main performance criterion for heat exchangers is cooling efficiency. Heat exchanger cores made from copper and brass can reject more heat per unit volume than any other material. This is why copper-brass heat exchangers generally have a greater cooling efficiency than alternate materials. Brazed copper-brass heat exchangers are also more rugged than soldered copper-brass and alternate materials, including brazed aluminium serpentine.[2]

Air pressure drop is a good evaluator of heat exchanger design. A heat exchanger core with a smaller air pressure drop from the front to the back of the core (i.e. from the windward to the leeward side in a wind tunnel test) is more efficient. Air pressure drops typically are 24% less for CuproBraze versus aluminium heat exchangers. This advantage, responsible for a 6% increase in heat rejection, contributes to CuproBraze’s overall greater efficiency.[10][11]

Since copper’s thermal conductivity is higher than aluminium’s, copper has a higher capacity to dissipate heat. By using thinner material gauges in combination with higher fin density, heat dissipation capacity with CuproBraze can be increased with air pressure drops still at reasonable levels.[4]

Due to its high heat transfer efficiency, CuproBraze offers a significant amount of cooling capacity in a small size. This is because the same heat rejection level can be achieved with a smaller-sized core. Hence, a significant reduction in frontal area and volume is achievable with CuproBraze versus other materials.[3][5]

Three new alloys were developed to enhance the strength and durability of CuproBraze heat exchangers: 1) an anneal-resistant fin material that maintains its strength after brazing; 2) an anneal-resistant tube alloy that retains its fine grain structure after brazing and provides ductility and fatigue strength in the brazed heat exchanger core; and 3) the brazing alloy.[12] Brazing at 650 °C creates a joint that is stronger than a soldered joint and comparable in strength to a welded joint.[13] Unlike welding, brazing does not melt the base metals. Therefore, brazing is more amenable to joining dissimilar alloys.[3]

CuproBraze has more strength at elevated temperatures than soldered copper-brass or aluminium. Due to the lower thermal expansion of copper versus aluminium, there is less thermal stress during the manufacturing of CuproBraze and in its use as a heat exchanger. CuproBraze heat exchangers have stronger tube-to-header joints than other materials. These braze joints are the most critical in heat exchangers and must be leak-free. CuproBraze also has higher tolerances to internal pressures because its thin-gauge high strength materials provide stronger support for the tubes. The material is also less sensitive to bad coolants than aluminium heat exchangers.[14][12]

Test results demonstrate a much longer fatigue life for CuproBraze joints compared to similar soldered copper-brass or brazed aluminium joints.[15] Stronger joints allow for the use of thinner fins and new radiator and cooler designs.[16][5]

The copper fins are not easily bent when dirty radiators are washed with high pressure water. Anticorrosive coatings further improve strength and resistance against humidity, sand erosion, and stone impingement on copper fins.

For further information, see: CuproBraze: Durability and reliability (Technology Series):[12] and CupropBraze durability (design criteria series).[15]

New legislation in Europe, Japan and the U.S. call for strong reductions in NO_X and particulate emissions from diesel engines used in trucks, buses, power plants, and other heavy equipment.[17] These goals can in part be accomplished by using cleaner-performing turbocharged diesel engines and charge air coolers. Turbocharging enables better power outputs. Charge air coolers allow power to be produced with more efficiency by reducing the temperature of the air charge entering the engine, thereby increasing its density.[18]

The charge air cooler, located between the turbocharger and the engine air inlet manifold, is an air-to-air heat exchanger.[3] It reduces the inlet air temperatures of turbocharged diesel engines from 200 °C to 45 °C while increasing inlet air densities to increase engine efficiencies. Even higher inlet temperatures (246 °C or higher) and boost pressures may be necessary to comply with the emissions standards in the future.[17][11]

Present-day charge air cooler systems, based on aluminium alloys, experience durability problems at temperatures and pressures necessary to meet the U.S. Tier 4i standards for stationary and mobile engines.[19][2][4][20] Published reports estimate that the average life of an aluminium charge air cooler is currently only about 3,500 hours.[21] Aluminium is near its upper technological limit to accommodate higher temperatures and thermal stress levels[17] because the tensile strength of the metal declines rapidly at 150 °C and repetitive thermal cycling between 150 °C and 200 °C substantially weakens it. Thermal cycling creates weak spots in aluminium tubes, which in turn causes charge air coolers to fail prematurely. A potential option is to install stainless steel precoolers in aluminium charge air coolers, but limited space and the complexity of this solution is a hampering factor for this option.[11]

A CuproBraze charge air cooler can operate at temperatures as high as 290 °C without creep, fatigue, or other metallurgical problems.[11][22]

Exterior corrosion resistance in a heat exchanger is especially important in coastal areas, humid areas, polluted areas, and in mining operations. Corrosion mechanisms of copper and aluminium alloys are different. CuproBraze tube contains 85% copper which provides high resistance against dezincification and stress corrosion cracking. The copper alloys tend to corrode uniformly over entire surfaces at known rates. This predictability of copper corrosion is important for proper maintenance management.[2][23] Aluminium, on the other hand, is more likely to corrode locally by pitting, resulting eventually in holes.[23]

In accelerated corrosion tests, such as SWAAT for salt spray and marine conditions, CuproBraze performed better than aluminium.[2]

The corrosion resistance of CuproBraze is generally better than soft soldered heat exchangers. This is because the materials in CuproBraze heat exchangers are of equal nobility, so galvanic differences are minimized. On soft soldered heat exchangers, the solder is less noble than fin and tube materials and can suffer from galvanic attack in corrosive environments.[23]

CuproBraze can be easily repaired. This advantage of the technology is especially important in remote areas where spare parts may be limited. CuproBraze can be repaired with lead-free soft solder (for example 97% tin, 3% copper) or with common silver-containing brazing alloys.

Biofouling is often a problem in HVAC systems that operate in warm, dark, and humid environments. The antimicrobial properties of CuproBraze alloys eliminate foul odors, thereby improving indoor air quality. CuproBraze is being investigated in mobile air conditioner units as a solution to bad odors from fungus and bacteria in aluminium-based heat exchange systems.[24][25]