June 3, 2013
Copper is extremely useful in the HVAC industry, but the high and volatile price can wreak havoc on profit margins. Some fabricators have found that they can substitute a couple of aluminum alloys with good results. However, aluminum isn’t as easy to work with as copper. This article tells how Harco Metal Products developed forming and brazing processes for this not-so-forgiving material.
Among the commonly used metals, few have had a bigger impact than copper. Combining it with tin creates bronze, an alloy so useful that it swept away the Stone Age and ushered in the Bronze Age nearly 6,000 years ago.
It’s just as useful today as it was in the preindustrial era. Soft and ductile, it’s easy to work, and it tarnishes but doesn’t rust, so it is a durable metal. It also transfers heat well, so all of these characteristics make it a favorite in heating, ventilation, and air-conditioning (HVAC) applications. It’s ideal for manufacturing heat exchanger components, and has ruled the HVAC industry for decades.
However, the metal does have a few drawbacks, not the least of which is its price. Throughout the 1990s and early 2000s, it rarely exceeded $1.25 per pound, but this changed as worldwide demand grew. Since 2007 the spot price rarely has been less than $3.00 per lb., and it reached $4.50 per lb. in 2012.1
Price volatility is another. The price has been known to increase by a factor of 3.5 in 24 months.2 For a fabricator or manufacturer of copper-based products, such severe price swings can bite into the company’s profit margin in a hurry.
Finally, although most suppliers have it readily available in inventory, some fabricators have trouble keeping it in stock. Because of its value, it’s a high-theft item.
Fabricators who work in the HVAC market can try a number of strategies to minimize the impact of copper’s drawbacks. Buying additional supplies when the price is low is a hedge against future price increases, and a security system can deter theft. A third strategy can sharply curtail the price problem altogether: substituting a different material. Many HVAC system manufacturers have used this tactic over the years, phasing out copper in favor of aluminum.
The use of aluminum for heat transfer components is nothing new. Brazeway, an aluminum extruder and coil supplier, was founded in 1946 for this application. Aluminum has been used almost exclusively for the coils in residential refrigerators and freezers since the early 1970s, said Scot Reagen, director of product development for Brazeway, Adrian, Mich. The transition was driven by compatibility issues, Reagen said.
“Copper is susceptible to formicary corrosion,” Reagen said, referring to corrosion caused by formic and acetic acids, which are volatile organic compounds released by many building materials, paints, and household cleaners. Aluminum isn’t vulnerable to formicary corrosion, which made it an attractive alternative back then.
Aluminum supplier Sapa took a different path into the HVAC marketplace. Three alloys in its NEXCOR™ series—AA3026, AA3110, and C47D—were developed for another industry altogether.
“The alloys were developed for corrosion resistance in the automotive sector,” said Don Brown, HVAC&R account manager for Sapa. “Automakers started with standard aluminum alloys [AA1100, AA3003, and AA3102], which are still used today, but some were looking for something with a longer life, something that would stand up to harsh road debris and the salt used to melt snow. So we developed a family of long-life aluminum alloys and now the HVAC industry is noticing some of the same benefits of these long-life alloys. Some fabricators use standard aluminum alloys while others have decided to use long-life alloys.”
These alloys are used for two heat exchanger styles.
“One uses round tube, which is expanded and press-fit to the fin,” Brown said. “The other type, which is widely used in the automotive industry and is being migrated into the HVAC industry, is a microchannel tube with several ports that run through it, which is pressed against the fin and run through a brazing furnace,” he said. Fabricators don’t typically deal with the flat channel tubing itself; they usually use round tubing to fabricate connector and jumper tubes for both design types.
Automotive and refrigeration have a couple of things in common with HVAC. All of these applications require tubing that can stand up to the stresses imparted during the forming process and withstand vibration and corrosion after they are put into use. However, these applications have many differences, and making the switch requires some research and caution.
Compatibility. First off, the material has to be compatible with the environment in which it is to be used.
“Indoor HVAC applications are relatively straightforward,” Reagen said. “The heat exchangers aren’t exposed to anything harsh and they are relatively short, so expanding the tube in the fin is fairly easy. Aluminum’s use continues to grow, and 2012 was a milestone in this conversion—it was the first year aluminum surpassed copper in the HVAC industry,” he said.
Reagen mentioned that expanding an aluminum tube isn’t quite as simple as expanding a copper tube.
“One of the big differences between copper and aluminum is the lubricity,” he said. “Copper is pretty forgiving when you slide a steel or stainless steel bullet through it to expand it. Aluminum, on the other hand, tends to gall, so you need a lubricant to help the tool slide through it.”
Brazeway produces a proprietary version of AA3020 for outdoor applications, for two reasons. First, the coils used outdoors can be more than four times longer than coils used for indoor applications.
“You need to expand the tube over a much longer distance, so column strength is very important. The strength of the alloy and the wall thickness come into play,” he said.
A bigger factor is corrosion.
“Aluminum is very stable when it’s exposed to liquids that have neutral pH,” Reagen said. “Anything very acidic or very caustic is corrosive to aluminum. If you’re near an ocean, saltwater is very corrosive, and so is rainwater if you’re in an industrial environment. Chemicals like sulfur dioxide are in the air, mix with the rain, and end up on the coils. As the rain dries, it leaves behind a very concentrated sulfuric acid.”
Even a sprinkler system spraying hard water, which has iron in it, leads to trouble.
“Iron deposits set up galvanic corrosion cells,” Reagen said.
The alloy also has to be a good match for the fluids that flow through the tube.
“These aluminum alloys have to be compatible with all the refrigerants used these days, and also with the lubricating oils used in the compressors,” Brown said. “This was one of the big questions when aluminum tubing was introduced to the HVAC market, but the testing has been done, and it has been approved for these applications,” Brown said.
Strength. The material can withstand the pressure developed by air-conditioning systems, but aluminum isn’t a direct replacement for copper. Fabricators planning on making the switch should be aware that, for a specified burst pressure, the wall thickness of an aluminum tube must be heavier than that of copper tube, Brown said. Some fabricators question the tradeoff, but it’s not a big difference, just a few thousandths of an inch for small-diameter tubes. Even with the thicker wall, the aluminum tube is still less expensive and lighter in weight than comparable copper tube, Brown said.
Ductility. Although it is suitable for heat transfer applications, aluminum doesn’t have as much ductility as copper. Fabricators need to understand aluminum’s limitations and how to deal with them.
Copper alloys typically allow quite a bit of latitude in forming. The most versatile is C65500, which stretches 70 percent before failure. Among the most ductile of aluminum alloys is 5154-0, which elongates 27 percent.
Brown acknowledged that it takes some time to research the appropriate processes and either modify existing equipment or invest in new equipment.
“Once they have the right equipment and developed the expertise, aluminum works well for these applications,” he said. “It does take some extra work to learn how to do it, but it can be done.”
When Sapa developed the refined microstructure it uses to improve its specialty alloys’ strength and corrosion resistance, it found an additional benefit: improved ductility.
“The formability of our long-life alloys is better than it is for standard 3000-series aluminum alloys,” Brown said.
The end use—pressurized lines subjected to thermal cycling and vibration—is a demanding application. It requires close attention to detail in how the material is formed and brazed to prevent weak areas that could leak or break.
The fabricator doesn’t merely do the forming and brazing processes; if it’s not too familiar with aluminum, it has to develop the processes. A project like this requires enough time, money, and expertise to figure out how to get the most out of the alloy. Jay Hall, president of Harco Metal Products Inc., Scottsdale, Ariz., took the plunge some time ago.
A Two-step Process. “The alloys comply with either ASTM B483-0 or ASTM B491,” Hall said, referring to aluminum tubes that are drawn and extruded, respectively. “We take these tubes and form a bell end, a simple expansion.”
The second phase of the fabrication process involves brazing a fitting to the formed end. While the bulk of Harco’s experience is with steel, it has learned enough about the relatively finicky aluminum to braze it successfully.
The Expansion. Hall explained that this project didn’t spawn a single end form, but actually a series of end forms; for example, the coils Harco supplies to its HVAC customers are made from 3⁄16-in.-dia. tube in several wall thicknesses from 0.035 in. and 0.050 in. It’s not a single degree of expansion, but several. Some of the parts require a 65 percent expansion, others up to 100 percent.
At a glance, the process isn’t complicated. Harco uses a series of hardened steel pins to form the end. “The first hit expands the material about 10 percent; the second takes it to about 25 percent of the expansion; and the last hit does the rest,” Hall said.
To make this work in aluminum, Harco’s process uses more steps than a similar end form in copper (three hits rather than two). It also has to run the process at an appropriate speed for aluminum. In Hall’s estimation, it runs the end forming machine about 20 to 25 percent slower than it would run it for copper.
Despite making these accommodations for aluminum, the work-hardening is severe.
“In some cases, the work-hardened area extends about an inch into the unformed area, making it very difficult to bend,” Hall said. Although Harco has been able to bend all of the components so far, Hall already has a plan to incorporate intermediate annealing if a future component needs its ductility restored.
The Braze. Brazing a fitting to the formed end is the next fabrication step, but the parts don’t go straight from forming to brazing. The lubricant used in the forming step must be removed. Although lubricant manufacturers have been formulating weld-through lubricants for some time, Hall has found that cleanness is critical in making a suitable weld, and lubricant removal is a must.
Harco also has spent quite a bit of time dialing in the heat delivered to the material.
“There’s a fine line between too little and too much heat when you’re welding aluminum,” Hall said.
“When working with copper, the temperature differential between the melting point of the braze alloy and the melting point of the tube is about 1,000 degrees F,” Reagen said. “So if you overheat the braze material a couple hundred degrees, it’s not going to matter. For the most commonly used aluminum brazing alloy, which is silicon-based, the difference is only 100 to 150 degrees F. The more user-friendly type, zinc-based, has a 400- to 500-degree-F window, but even that is quite a bit smaller than the window for copper. For a company just starting out brazing aluminum, this is the biggest issue.”
For units that are in service and require repair, some service technicians might need training to braze aluminum, but a bigger challenge is in manufacturing the units in the first place. Switching a manufacturing line from copper to aluminum can be costly, depending on whether the line produces brazed or expanded heat exchangers, Brown said.
The Manufacturing Environment. Fabricators making the switch from copper to aluminum can’t do it gradually. It takes a substantial capital investment to set up a separate processing line to prevent cross-contamination between copper and aluminum components.
It’s no different from keeping aluminum separate from steel. Every fabricator who works with both of these metals knows that using a wire brush or grinder on both steel and aluminum is a sure way to contaminate the second, a corrosion-resistant metal, with bits of the first, and the result is unsightly rust spots. The risk of contaminating aluminum with copper is just as likely and the results are similar.
“My customers were adamant that I wouldn’t run copper parts on the same equipment, or even process copper in the same environment,” Hall said.
Copper gets embedded into the tooling on benders and end formers, so running aluminum on the same machines is a recipe for trouble. This has to do with the galvanic compatibility of the two metals, measured by their anodic ratings, a number between 0 and 1.85. For harsh environments, the difference between the two metals’ anodic ratings should be less than 0.15. Copper is rated at 0.35, and 3000-series aluminum alloys rate at 0.90, so contact between these metals readily leads to corrosion and field failures. 3
Fabricators need to be aware that it doesn’t take the high pressures developed by benders and end formers to transfer bits of copper to aluminum workpieces. Any contact can lead to cross-contamination.
“In some operations, they run copper on one side of the shop floor and aluminum on the other side, and they separate the two sides by a wall or a curtain, but even this isn’t enough,” Brown said. “Often they use big hoppers to move parts from the bending station to the end forming station, and if they are short of hoppers on the aluminum side, they might borrow a few hoppers from the copper side. The problem is that some copper fines rub off onto the side of the hopper, and these get transferred over to the aluminum parts.”
If it sounds complicated, it is.
“We have a very detailed, rigid set of instructions for every one of these components,” Hall said. “In many cases, part #1 for customer A looks a lot like part #3 for customer B, for example, but the components have different applications and require different bend radii, so the procedures for making them are not similar at all,” he said.
On the other hand, if it sounds like the risk was worth the reward, in Harco’s case, it was. The HVAC industry has been rebounding from the recent downturn—in January 2013 OEMs shipped 179,492 central air-conditioning units, a 16 percent improvement compared with January 2012.
It’s too early to tell how this summer will shape up, but if it’s anything like the record-breaking temperatures in 2012, Harco is well-positioned to help sweltering homeowners beat the heat.
1. Kitco Metals Inc., www.kitco.com
2. Air-Conditioning, Heating, and Refrigeration Institute (AHRI), www.ahrinet.org
The HVAC industry’s adoption of aluminum was assisted by a federal government mandate—an increase in the seasonal energy efficiency ratio (SEER) for air conditioning units, said Don Brown, HVAC&R account manager for Sapa.
The energy efficiency ratio (EER) of a cooling device is the ratio of energy output to input; it compares cooling ability, measured in British thermal units per hour (BTU/h), with the electrical power, measured in watts (W). EER usually is based on an outside temperature of 95 degrees F and a return air temperature of 80 degrees F at 50 percent relative humidity.
The SEER is similar to the EER, but it is a measure of the expected efficiency over a typical year’s weather in a given location. The SEER is calculated with the same indoor temperature over a range of outside temperatures from 65 to 104 degrees F, with the time divided equally among eight increments of 5 degrees F.
“When the SEER rating went from 10 to 13, one way to comply was to increase the heat exchanger’s surface area to improve heat transfer,” Brown said. “Air-conditioner condensing units really grew in size when this approach was taken. It takes more material to accomplish this, and copper prices had been rising when the mandate came through, so it made aluminum look even more attractive.”
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