Copper tube/aluminum fin (CTAF) coils are “the standard” to some, just as aluminum tube/aluminum fin (ATAF) coils are to others. Forty was reported to have seen many trends in his storied career, but he and the hvacr industry are about to experience a paradigm shift.
Many of Forty’s issues with maintenance difficulty and repair challenges with all-aluminum coils are non-issues with the other aluminum coil: the brazed aluminum microchannel (BAM) heat exchanger. This is very relevant today, because the Air-Conditioning and Refrigeration Institute (ARI) rates and lists several nationally distributed BAM condensers, and 21-CR research by the Air-Conditioning and Refrigeration Technology Institute (ARTI) into flat-tube BAM heat exchangers and applications with CO2 as a refrigerant are under way.
WHAT IS A BAM COIL?
Introduced in the automotive industry, the BAM coil is composed of three key components: the flat multi-void microchannel tube, the finstock sandwiched between alternating layers of microchannel tube, and two manifolds. The slotted manifolds seat each tube in a parallel flow orientation.A one-pass coil has gas entering through one manifold and liquid exiting out of the other. A two-pass coil employs an entering manifold baffle to direct flow back to the first manifold to exit the condenser. Three- and four-pass coil designs are also applied.
Clad-coated manifolds and finstock join the three components into a unitized coil in a nitrogen-charged brazing furnace. Repeatability and process control of this technology are exceptional. Many Tier-1 automotive suppliers produce thousands of coils per day, striving for zero defects.
PARALLEL FLOW MAINSTREAMS
BAM coil applications in the hvacr industry have accelerated quickly over the last three years. But to understand why, let’s understand the technology’s late 1980s introduction in the automotive industry.When the Montreal Protocol mandated R-12’s phaseout, its replacement, R-134a, presented 20% capacity losses for automotive copper-aluminum condensers. To maintain vehicle air conditioning capacity, larger core designs would be needed.
Smaller, but higher performance, all-aluminum BAM condensers were then designed and applied with R-134a. The automotive industry vertically integrated this technology within three years and, today, over 75% of vehicles apply parallel flow condensers based on the BAM design’s exceptional performance.
Even the quality-demanding Forty would be impressed with the stellar track records attained by the six-sigma requirements of Tier-1 suppliers to today’s automotive industry. Repairability is not an issue when the BAM condensers outlast the life of the automobiles. And imagine leak rates measured in ppm (parts per million) instead of fractional percentage.
GRUELING ROAD TOUGHNESS
Consider this comparison. The typical hvac condenser sits still on a level pad and performs its comfort cooling duties. By marked contrast, the automotive BAM condenser, located right behind the grill of a motor vehicle, endures regular high-speed collisions with stones, insects, tire fragments, and debris, while hurtling down bumpy roads at high speed. Oh, to be so lucky as to have your own pad.Pad-based condensers do not experience the high-velocity attacks of oil, salt, material spills, sand, ash, and other chemical road treatments seen by a car’s condenser in hot, humid coastal locations, or Snow Belt extremes. The all-aluminum BAM condenser has endured the tough duty, demonstrating its robust design application in vehicles as well as in homes and businesses.
TO REPAIR OR NOT TO REPAIR
ATAF field repair was a significant concern for Forty. Often cited is the repair difficulty involved in sealing a condenser’s aluminum hole vs. a copper hole. But this argument assumes that field repair must be a part of our industry’s existence.It doesn’t have to be that way, and it’s not for the automotive industry, where outstanding first-time braze rates are the standard in the supply chain. Remember dumping Barr’s into your radiator to seal leaks from the inside? When was the last time you did that? New automotive technologies, like BAM coils, did away with that vestige of yesteryear.
Automotive heat exchangers are extremely reliable, because the six-sigma quality demanded by the supply chain requires them to be more robust, reliable, and hassle-free.
When a field repair is needed, low-temperature brazing solutions exist today which were not available years ago. For example, Omni Technologies (www.omni-technologies-corp.com) offers easy-to-use, low-temperature aluminum brazing kits. And the nature of brazed BAM coils makes repairs simpler and easier.
BAM COIL TRIAGE
BAM coils will offer service technicians a field repair service technique that is not possible with traditional CTAF heat exchangers. Let’s consider an air-cooled BAM condenser coil that does indeed have a microchannel tube leak.The technician finds the leak, but the evacuation, repair, and re-charge may require downtime that the business just cannot endure. The technician simply crimps the microchannel tube on either side of the leak, to close that entire tube off from the circuit, balances the charge, and puts the unit back into service. Refrigerant flows to all of the other tubes in the BAM coil, with minimal performance loss.
The technician can then return at a better time to complete the job while avoiding costly downtime. The critical comfort of a homeowner, a senior center, or process refrigeration requirements can be met with this field triage. BAM coil technology provides the service technician flexibility, unavailable with traditional copper-aluminum coils.
THERMAL BENEFITS
In a given surface area, copper is a better conductor than aluminum. However, when considering weight, aluminum overwhelms copper’s thermal transfer. And extruded aluminum microchannel can exploit this surface area to weight ratio, not simply with wall-thinning, as done with round copper tube, but with flat aluminum tube multi-voids to transport refrigerant.BAM coil design applications typically permit equivalent system performance in a smaller condensing unit chassis, i.e., 20% to 25% smaller BAM coils. Other design approaches can target more than 10% to 15% improved performance with the same coil face area.
And with higher efficiency BAM coils, simpler high-efficiency condensing units are possible, without the added requirement of complex electronics, advanced compressors, and variable speed fans to meet these efficiencies.
An exciting application involves space-constrained units, like through-the-wall products, where a fixed unit size cannot accommodate a larger coil to meet higher efficiency.
Application models have shown that BAM air conditioner condensers typically require between 25% to 45% less refrigerant charge per system than with CTAF systems. And these smaller and lighter condensers enable more units shipped per truck and more units stored in less area.
LOOK MA, NO MASH
Our industry began painting all or part of its condenser coils in an effort to cosmetically conceal unsightly fin-mash, even after combing and straightening. The BAM coil requires no such primping.As the microchannel tubes and finstock are brazed between the manifolds to form a unitized coil, no damage occurs when the coil surface is slapped with your hand or bumped by one’s knee. Toss a baseball at it — there’s no mash.
CORROSION
A constant concern of CTAF designs was clearly explained by Forty. Galvanic action between the dissimilar metals of copper and aluminum can induce leaks from corrosion. And one only has to take a walk along a seacoast to see condensers rapidly sacrificing their fins, which dangle off oxidized copper tubes. Besides heading toward failure, system heat transfer capacity and efficiency have fallen well below the original, nominal ratings.Significantly improved corrosion resistance results from the same aluminum alloy brazing in BAM coils vs. CTAF dissimilar metal expansion. In fact, many BAM coil components are entirely the same alloy of aluminum, i.e., AA3003, and nitrogen-charged brazing facilitates high-quality, repeatable braze rates.
Testing done with two different hvacr manufacturers has independently demonstrated that corrosion resistance test results were highest first with BAM coils, followed by ATAF coils, while CTAF coils were the last of the three.
This is the engineering reason why the all-aluminum BAM coil in the front of your car, or the one on its seacoast pad, can handle all the rigors both man and Mother Nature can throw at it.
THE FUTURE
The inherent design for BAM coils will require advancements in design, orientation, and application for either an evaporator or heat pump condenser. Present challenges involve condensate management and equalized refrigerant distribution. Accordingly, BAM coils are in study by many companies for their condenser applications as well as their evaporators at this time. Innovative designs are in consideration.Research utilizing CO2 as a refrigerant is also underway in “gas coolers” which employ round-port extruded aluminum microchannel tubes, capable of handling the very high operating pressures of this natural gas.
But the greatest promise for the hvacr industry is forecast in the words of a former president of a major equipment manufacturer. His prediction is that microchannel technology will represent nearly 50% of the air conditioning condensers of our industry within the next five years. The paradigm shift is underway.
Panopoulos is HVACR sales manager for Thermalex, Inc., Baltimore, MD, and he coordinates the company’s HVACR BAM tubing applications. He is a member of ARI and ASHRAE. Contact him at MPanopTMX@msn.com (e-mail).
Publication date: 04/15/2002
