Thermal management plays a crucial role in various industrial applications, ranging from automotive engineering to electronics cooling. In the realm of heat transfer, copper alloys have long been recognized for their exceptional performance. Although the past decades have witnessed a shift towards aluminum heat exchangers, it is important to acknowledge the historical significance and enduring benefits of copper-brass exchangers in off-road and industrial applications. The use of copper-brass radiators in cars and other vehicular applications can be traced back to the early 20th century. Copper, with its high thermal conductivity (360 W/m-K), high tensile and yield strength and excellent corrosion resistance, emerged as the material of choice for heat exchangers. Brass, an alloy of copper and zinc, provided additional strength and formability, making it suitable for the fabrication of radiator tubes, tanks, and side frames.
The benefits of copper alloys in heat transfer applications offer several advantages: Firstly, copper exhibits remarkable thermal conductivity, outperforming most other metals. Today there are lots of applications with a fin thickness of 35 microns and above which results in low air-pressure drop, requiring less forced airflow and power from the fan. Lower pressure drop means lower noise emissions and reduced power consumption. Secondly, copper alloys enable high resistance against thermal shock and fatigue resistance. Fatigue occurs when repeated application of stresses below the yield strength has a deleterious effect on a metal where each cycle of applied stress introduces defects in the metal lattice. Fatigue-resistant alloys prevent defects from accumulating and forming microcracks. Higher thickness for aluminum gives higher stresses for the same deflection and lower yield stress for Al-alloys can make much smaller deflections permanent. Thirdly, copper alloys possess excellent corrosion resistance, enabling them to withstand the harsh operating conditions encountered in thermal management systems. This resistance minimizes the risk of leaks, extends the lifespan of heat exchangers, and reduces maintenance requirements. Last but not least, cu-brass heat exchangers are safe to use in remote locations. This is due to repairability in field conditions by using soft soldering or torch brazing.
As early as 1950, aluminum heat exchangers made moderate inroads into the automotive industry. With the introduction of the vacuum brazing technique (VAB, 1970), production of aluminum-based heat exchangers began to expand. The oil crisis of the 1970s prompted automotive companies to prioritize fuel efficiency and explore lightweight materials as a means to reduce vehicle weight. Significant growth in the use of aluminum heat exchangers resulted from advantages of the controlled atmosphere brazing process (Nocolok process, 1978). Instead of brazing in vacuum, this process involves potassium alumina fluorite flux which is wetting the surface and chemically counteracting the surface oxidation. Aluminum, with its low density and favorable properties such as favorable strength-to-weight ratio, emerged as a popular choice for thermal management applications. This transition was primarily driven by the desire to reduce vehicle weight and improve fuel efficiency. Aluminum radiators offer a clear weight advantage over their copper-brass counterparts (driven by 3x lower density), contributing to lighter components and reduced energy consumption. Additionally, aluminum is less expensive than copper, making it an attractive alternative in terms of cost-effectiveness. The advancement of both VAB and CAB processes and the thermomechanical and chemical optimization of Al-alloys have enabled to push the boundaries towards larger and more demanding applications. However, brazed aluminum heat exchangers are being pushed to their limits in many heavy-duty applications in terms of durability and high operating temperatures. Cu-brass heat exchangers’ advantages are especially evident in locomotive, power generation and mining applications.
The first production process was soft soldering and that technology is still in use, after decades of continuous development. The tubes are made of brass, i.e. copper alloyed with zinc. The amount of zinc is usually in the range of 10% to 37%. The higher copper content is more resistant against corrosion forms like dezincification and stress corrosion cracking. In severe environments there is a risk of dealloying, i.e. selective dissolution of electrochemically less noble element Zn out of the brass lattice structure. This type of corrosion results in porous copper structure, which is weaker and can cause leakages. The phenomenon can be noticed by the naked eye as the yellow brass surface changes to reddish copper color. Higher zinc content saves material cost and weight. Besides increasing the copper content, the resistance against general corrosion and dezinfication can be enhanced by using alloying elements such as phosphorus (stabilizing the microstructure). Today the most popular is CuZn35P (SM2965). Tubes are produced in either lock-seam tube-mills, or HF induction welded tube-mills. The lock-seam machines are usually running slow whereas HF welded tube-mills run fast up to 150-200 m/min. Such a production speed requires high consistency in terms of strip geometry and edges (i.e., minimum width variation, camber and burr). The HF welded tubes are much stronger than the lock-seam version and eliminate the electrochemical potential difference between solder and brass alloy. The effects of internal corrosion within the cooling system are also of importance when selecting materials for radiator fabrication. In locomotive radiators, the use of CuZn15 (C2300,SM1085) the lower zinc content in brass means higher nobility and smaller tendency to dezincification and stress corrosion cracking (SCC) which are zinc content related forms of corrosion. Also, copper alloys are less sensitive to a bad coolant than aluminum, which is an important material selection factor when the quality of the coolants cannot be ensured. Brass tubes are not as sensitive to bad coolant and pitting corrosion as aluminium tubes.
The fins are required to have high thermal conductivity and good formability to complex structures. Copper has the second-best conductivity of the elements (after silver) and the fins are, therefore, made of almost pure copper. Minor alloying (often with Sn and/or Te) ( SM0702,C14415,SM 0300, 1453), is used to ascertain the required mechanical strength. The joining of the parts is made by soft soldering with tin-lead alloys in the ambient atmosphere. The soldering temperature is typically 250–350°C. A lead-free soldering process has already been developed due to environmental reasons. The solder alloy can be for instance SnCu3. Several fin designs exist, i.e., serpentine, flat, square and bumper fins. The fin surface can be either plain or louvered. Louvered fins are commonly used in many compact heat exchangers to increase the surface area and initiate new boundary layer growth. The louvers provide a means for vortex shedding from the trailing edge, which can impact the boundary layers on the neighbouring louvers and be used as a mechanism to increase the local heat transfer rates. The louvers then break up the growth of this boundary layer resulting in high heat transfer rates in the initial re-growth regions. In the case of bumper fins, the production process requires deep-drawing properties from the fin material. In all cases the fin material optimization is ensuring optimized louver bank and angles (fin forming processes are sensitive to thickness and hardness variations).
It is important to note that the choice between aluminum and copper alloys is still influenced by various factors. Each material has its own set of advantages and limitations, and OEMs and their system suppliers must carefully consider the specific requirements of their applications, including performance, cost, durability, and environmental impact. Different industries and applications have unique requirements, and a balanced approach must be taken to select the most suitable material technology. Also, both aluminum and copper are important inputs to a number of technologies critical to the energy and digital transition. Aluminum production requires substantially more energy than copper and copper generally has higher energy saving potential than aluminium. Copper is 100% recyclable and about 80 % of all copper ever used is estimated to still be in use and the reuse of copper is the highest known.
About Mayur Gupta:
Mayur Gupta is the Managing Director at Agrawal Metal Works. He has been in this position since 2006. Agrawal Metals Works is a supplier of copper and copper alloy materials to the heat exchanger industry around the world.