From heat recovery to air coils and refrigeration to power plants, choosing the right material for heat exchangers — particularly with reference to thermal qualities, resistance to sag during brazing and corrosion resistance — is key.
Did you know that the best example of heat exchange in the natural world is as obvious as the nose on your face? Well, technically it is the nose on your face, which warms inhaled air and cools exhaled air. But heat exchanger design depends on much more than an intuitive understanding of biology.
It requires careful consideration of the operating environment, application and, crucially, the properties of the materials used.
Fortunately, choosing materials becomes easier once you have assessed the environment and the application. If the heat exchanger will be operating outdoors, or in a processing plant with corrosive media, then a high corrosion resistance will be a necessity.
Likewise, design engineers must consider what fluid will be carried through the exchanger and specify materials accordingly.
For example, it could be critical that a substance remains pure while being passed through a standard shell and tube heat exchanger in a pharmaceutical processing application. In such an environment, the tubes must be made of an inert material, perhaps even an unconventional one that is non-metallic – such as glass.
Generally, the two most commonly selected materials for heat exchangers are aluminium and copper. Both metals have the optimum thermal properties and corrosion resistance to make them ideal choices, with most of the differences being application-specific.
Copper for heat exchangers
The typical thermal conductivity of generic pure copper is 386.00 W/(m·K) at 20°C. This makes copper the most thermally conductive common metal, which, along with its relatively low specific heat — of approximately 0.385 J/(g·°C — underpins its popularity in heat exchangers.
These characteristics do bring with them a slightly elevated price. Most design engineers and product designers consider this one of the biggest deciding factors between copper and aluminium for smaller projects.
However, there are a few practical considerations to consider when using copper. The density of the material, for example, might mean that it is unsuitable for certain applications that require a lightweight heat exchanger.
Furthermore, copper has lower flexibility than aluminium, making it more difficult to form into certain shapes. Because of this, design engineers working on a plate fin exchanger, which is a type of heat exchanger that uses plates and finned chambers to transfer heat between fluids, might find that aluminium is a better fit for the fins.
In addition, it’s important that copper tubes are joined using brazing rather than soldering, as the latter has been known to create a build-up of substances at joints. This means that design engineers should also source copper with a good sagging resistance to reduce deforming during brazing.
There are some long-term corrosion considerations with copper as well. As the material ages, it can develop verdigris — a thin layer of patina, formed by oxidation over time, that gives the material a green hue.
It’s the same chemical reaction that has made the statue of liberty the iconic green colour it is today. This process typically takes 15 or more years, depending on how the material is maintained and its environment.
Of course, there’s no guarantee that the change in a heat exchanger’s external colour will be as well-received as the statue of liberty’s verdigris, so product designers may choose an alternative to copper to deliver a different aesthetic. In any case, the patina is dielectric and may lead to reduced thermal conductivity as it accumulates.
As a matter of fact, although corrosion resistance is not a natural property of copper, Lebronze Alloys, a leading French manufacturer of high-performance materials, has worked on alloy compositions that provide copper with good oxidation resistance, even when exposed to seawater.
Despite these factors, the thermal conductivity of copper arguably compensates for maintenance considerations with its efficient transference of heat. In some cases, copper’s high comparative thermal conductivity means that a copper tube can conduct heat as effectively as two aluminium pipes.
Aluminium for heat exchangers
For design engineers that require a lighter, thermally efficient material, or are working to a tighter design budget, aluminium is the prime candidate.
Boasting a thermal conductivity of 237 W/(m·K) for pure aluminium or ~160 W/(m·K) for most alloys, aluminium is the third most thermally conductive material and arguably the most cost-effective. Aluminium also offers a specific heat of 0.44 J/(g·°C), making it very nearly as efficient at diffusing heat as copper.
Aluminium is also far more lightweight and flexible than copper, addressing many of the practical issues engineers might encounter with copper. It is far more malleable, so engineers designing a plate-fin exchanger for a gas furnace will find that it is better suited to the intricacies of the fins.
However, aluminium does typically have lower sag resistance than copper, making it more prone to deformation during the brazing process and after repeated heat cycles.
Fortunately, this can be counteracted by opting to specify an aluminium alloy that has been specifically formulated to bring the metal’s properties closer to that of copper, without significantly increasing the price.
For example, metal supplier Gränges provides aluminium alloy FA6825 H14SR that is suitable for heat exchangers in energy applications. This alloy is fortified with elements such as zinc and manganese to give the alloy a higher tensile strength after brazing. The metal forms large grains during the process, which improve its sag behaviour.
The characteristics of aluminium and copper are very closely matched in terms of suitability for heat exchangers, with the key deciding factor ultimately being the application’s practical requirements.
While the decision may not be as obvious as the nose on your face, design engineers can make it easier by understanding the properties of their materials.
Did you know? You can explore hundreds of different materials suitable for use in heat exchangers on Matmatch. You can also find materials suitable specifically for heat exchangers in energy.
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