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One of my favorite jobs as a mechanical engineer was as a heat exchanger designer for commercial aerospace. While it may appear straightforward, there are a lot of considerations in good heat exchanger design, including:
Now, I could rant for days about optimal fin design, methods for joining plates and fins, and even flow-stream mechanical energy losses, but this article is focused on material selection for heat exchanger design. Choosing the right material is critical to the success of your heat exchanger design because making a good choice enhances your design, while the wrong choice can cause it to fail.
Heat exchanger material should be chosen based on its physical material properties like thermal conductivity, temperature resistance, corrosion resistance, and strength &#; plus non-stable supply chain considerations such as cost and availability. The wrong material can lead to higher cost and longer production times if it&#;s more difficult to machine and weld.
Material selection has a direct impact on the ability of the heat exchanger to exchange heat from one fluid to another. If you select a poor material, you&#;ll suffer negative impacts on the rate of heat transfer, drops in pressure, and you may even require excess mechanical power. On the other hand, correct material selection for heat exchanger design allows you to optimize performance, efficiency, cost, and potentially find savings in weight or achieve a smaller footprint.
As mentioned above, these are the properties you should look to optimize when designing a heat exchanger:
Now, of course, not all materials provide an optimized combination of mechanical properties, cost, and availability &#; you&#;ll have to sacrifice one factor over the others to complete your design.
Pro-tip: To see how material properties impact CNC machining and how to design for manufacturability, check out our on-demand webinar, DFM for CNC Machining: Minimize the Challenges with Design for CNC Machining.
The Best Materials for Heat Exchanger Design
The best materials for heat exchanger design are materials with a combination of high thermal conductivity, high temperature resistance, excellent corrosion resistance, good mechanical strength, low cost, and wide availability.
Copper is the most well-known metal with high electrical conductivity. Copper also has a high thermal conductivity due to its lattice-like atomic structure, which allows efficient energy conduction throughout the material. The number of free electrons in each atom of copper also allows for fast and efficient movement of energy via thermal or electrical conduction. This high thermal conductivity means that heat transfer from the hot to cold fluid will occur as efficiently as possible and with as much speed as possible.
Copper also provides excellent corrosion resistance, fluid compatibility, and good mechanical strength. Copper comes in many alloys that are generally readily available and relatively inexpensive. In addition, it&#;s relatively easy to machine and weld, making it suitable for multiple manufacturing methods. Copper is especially suitable for tube and shell heat exchangers thanks to its high ductility &#; which allows it to be drawn into thin-walled tubes or extruded into small wires without fracturing.
Stainless steel is known for its high mechanical strength and excellent corrosion resistance, which comes from its naturally occurring, protective oxide layer. Stainless steel also has good thermal conductivity, which is why the material is an optimal choice for heat exchangers that must withstand ultra-high temperatures, high burst pressure, and extremely harsh environments.
Stainless steel is stronger than aluminum, carbon steel, and copper, and its mechanical strength is preserved at higher temperatures than other common heat exchanger metals. This reduces the potential for distortion or warping over extended exposure to extreme temperatures. Stainless steel is also simple to machine and weld, which makes it easier to fabricate and assemble heat exchangers.
For most commercial aircraft, various grades of aluminum are the predominant choice for heat exchanger material because it&#;s strong, yet lightweight. It&#;s the most common metal on earth, so it&#;s readily available and cost-effective. Aluminum also has a naturally occurring, protective oxide layer that can be modified to be stronger. Aluminum is ductile enough to be drawn into thin tubes or rolled into thin sheets and formed into fins with complex geometries. Aluminum also has excellent thermal conductivity, though it isn&#;t able to withstand temperatures above a few hundred degrees Fahrenheit.
Aluminum can be readily brazed with methods like aluminum fluxless vacuum brazing, which creates a strong joint that can withstand the pressure of a heat exchanger. During brazing, the aluminum oxide layer cracks because it&#;s more brittle than the underlying metal. This cracking occurs at high temperatures right before the braze metal melts and fills the gaps between the oxide layer, and a gettering agent is used to prevent the oxides from reforming. While aluminum isn&#;t suitable for all heat exchanger applications, it&#;s generally the first material I would recommend.
Titanium has only been prominently utilized in manufacturing since the early s. (Though it was discovered in by an English pastor!) Titanium has many excellent material properties that make it suitable for heat exchanger construction &#; it&#;s lightweight, corrosion-resistant, and has good heat transfer properties. I think of titanium as a supercharged cousin of aluminum that provides many of the same valuable material properties, but on a higher scale &#; except for its thermal conductivity, which is lower than aluminum. Still, titanium boasts the highest operating temperature on this list, and that tradeoff may be necessary in some applications.
Titanium is about 1.5x more dense than aluminum, but its strength is 4-5x higher than aluminum, so using titanium can result in significant weight savings. However, a common misconception is that titanium is lighter weight than aluminum &#; the truth is that you get the same load capacity from less titanium than you get with the same amount of aluminum. This is why many aerospace companies, car manufacturers, and even medical device companies are utilizing titanium more in order to meet requirements to minimize weight and increase strength.
Now that you know the pros and cons of the materials, you can utilize the tables below &#; which have the key data points for each &#; to simplify material selection for your next heat exchanger design:
Thermal Conductivity (BTU/hr*ft*F)Copper6.95Stainless Steel0.285Aluminum4.14Titanium0.15 Highest Operating Temperature (F)CopperStainless SteelAluminum300Titanium Density (lb/in^3) @ Room Temp (68F)Copper0.323Stainless Steel0.285Aluminum0.098Titanium0.163 Corrosion ResistanceCopperHighStainless SteelHighAluminumModerateTitaniumHigh Tensile Strength (lb/in^2)CopperStainless SteelAluminumTitaniumStainless steel is a good choice for handling condensing steam heat exchanger applications because it&#;s corrosion-resistant, resistant to high temperatures, and thermally conductive.
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Many HVAC appliances have a component that is known as a &#;heat exchanger.&#; They come in many forms, but it is the metal that composes your heat exchanger that often makes one of the biggest impacts.
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What this piece of metal does is transfer heat from one fluid (e.g., hot water in your water heater) to another (e.g., domestic water running to your tap). There is a good amount of choice in the metal you can use for that heat exchanger, ranging from bronze and titanium to brass and carbon steel.
However, copper and stainless steel heat exchangers are the most commonly used because they are less expensive and still highly effective. One of the most common questions we are asked by our customers is some variation of: which is better a copper or stainless steel hot water cylinder, water heater, boiler, or other HVAC appliance?
The main concerns of a homeowner when choosing between copper and stainless steel should be thermal conductivity, durability, and price.
In this guide, we look at the pros and cons of copper and stainless steel heat exchangers.
The thermal conductivity of a heat exchanger determines how quickly it transfers the heat from the heating source to the distribution fluid. In this regard, a heat exchanger with copper is much faster at transferring heat than stainless steel.
Here are the basic thermal conductivity levels, measured in watts per meter pre-Kelvin, of the two different metals[1]:
On average, the thermal conductivity of copper is 20 times that of stainless steel. In practical terms, this means that copper can transfer heat 20 times faster. So, if you need quick heating, copper will work to your advantage.
Why would you need to heat something quickly? That&#;s an important question to ask if you are choosing between, say, a copper vs. stainless steel tankless water heater.
For example, if you own a swimming pool and plan on going swimming on an autumn day, a water heater with a copper heat exchanger can get your pool ready for you much faster. With a stainless steel heat exchanger, you could find yourself waiting up to 72 hours before your pool is heated to 10 degrees Celsius.
Even if you don&#;t need to heat things quickly, the higher thermal conductivity offered by copper also leads to higher efficiency. As a result, using a heat exchanger with copper will lead to lower energy costs. After all, a heater or boiler that has to run for longer to heat your home, pool, or tap water is going to cost you more.
Durability is a big concern for heat exchangers when it comes to appliances like a boiler. This is because condensing boilers (the most popular type right now), release a corrosive condensate that can eat away at the metal in the heat exchanger.
A heat exchanger that cannot stand up to the condensate will quickly corrode, requiring a time-consuming and costly replacement. As a result, you will likely want to choose a heat exchanger that can resist corrosion over the long term.
In this case, the clear winner is stainless steel. Unlike standard steel, stainless steel has a property known as &#;passivation.&#; This refers to its ability to form a layer of oxide on itself in response to contacting air.[2]
This layer of oxide protects stainless steel from corrosion and rust, allowing for a longer lifespan than regular steel. It is essentially perfect to use in any heat exchanger that will be in contact with corrosive elements.
On the other hand, copper is much more vulnerable to corrosion. The condensate turns copper atoms into copper ions, effectively dissolving the metal over time. This is a big problem for two reasons. First, because of the lower lifespan; then, because a corroded copper heat exchanger loses efficiency.
Considering that higher efficiency and thermal conductivity was the advantage for copper, having it reduced balances out the other way.
Copper tends to be cheaper than stainless steel when purchased in the same quantity, and that holds true when used in heat exchangers. While that may tempt you into getting copper for your heat exchanger, remember that it is much less durable. You will have to buy more copper replacements to maintain its efficiency levels. As a result, copper can actually end up being more expensive in the long term.
Generally, you will find that heat exchanger manufacturers will offer copper as the default choice because they&#;re cheaper. These companies are aware of the trade-off between the cost and the lifespan, where the cost is a &#;pay now or pay later&#; issue. You either pay more upfront for a stainless steel heat exchanger that will last longer or pay later to replace the copper one sooner.
The ultimate choice comes down to whether you are thinking long-term or short-term. If you plan on adding value to your home by installing high-quality HVAC equipment, go with the long-term option. (e.g., gas boilers and stainless steel heat exchangers). The long-term option will save you money and reduce the need for HVAC services and replacements.
So it should be obvious that stainless steel, the more costly of the two metals, is better for long-term thinkers. However, should you really need a heat exchanger with the highest conductivity to quickly heat large bodies of water (e.g., a pool) or larger homes, then copper might be the better choice.
Of course, stainless steel can do everything copper can, just at a slower pace and slightly higher price.
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