It might be something of a paradox that smaller HVAC components can be much more effective than their larger counterparts, but microchannel coils are here to stay. Their reduced size and improved design provide manufacturing and delivery benefits for original equipment manufacturers (OEMs), and installation, efficiency, and maintenance benefits for HVAC contractors.
'Perfect Storm' Brings Opportunity
Improved coil designs have been nudged along in recent years in the interest of efficiency, and as a way of dealing with the escalating cost of raw materials.
"Units have been getting steadily larger, especially with heat pump units, as efficiency ratings have increased," says Nathan Walker, product manager for split systems, Goodman Manufacturing Company, Houston, TX. "This trend started specifically in 2006, when the industry went from a 10- to a 13-SEER minimum efficiency level. Some units got up to 15% larger in volume," Walker recalls.
Dave Anderson, vice president of sales and marketing for coil manufacturer Luvata, says higher SEER requirements, combined with rising copper prices, and the introduction of R-410A refrigerant, was a "perfect storm" of opportunity for improved coils to make their move.
"It forced the industry to look at redesigning equipment. Everyone was ready for a new technology," Anderson says.
Designed for Improved Efficiency
Microchannel coils are comprised of multiple, smaller ports — or channels — which eliminate much of the empty space inside each of those individual channels.
"Microchannel coils are very efficient heat exchangers, in part, due to the very high ratio of primary surface (direct cooling via refrigerant and tubes) versus secondary surface (fins) when compared to round tube plate fin (RTPF) coils," explains Dustan Atkinson, Product Manager for Heatcraft Refrigeration Products, Stone Mountain, GA.
"In traditional RPTF coils, the refrigerant typically flows through a 3/8-in. tube, with direct cooling really only taking place around the surface of the copper tube (and extending to aluminum fins). As a result, you essentially have a lot of wasted space —and refrigerant—in the center of the tube. This substantially adds to the refrigerant charge in the coil," Atkinson says. "An increase in the primary surface allows for a more compact coil design and substantial reduction in refrigerant charge, because there’s less total tube/coil volume required."
"The trend toward smaller diameter copper tubes is inevitable," says Nigel Cotton, Global Leader of OEM Initiative for the International Copper Association (ICA), New York, NY.
"Although various amounts of minor retooling may be required before tube suppliers can make small diameter tubes, the manufacturing methods for making coils from fins and tubes are simple, practical, and economical," Cotton says.
Goodman Manufacturing Co. was among the HVAC industry's leading OEMs to hit the drawing board to develop smaller, more efficient coils using smaller diameter copper tubing. Its SmartCoil™ — a copper tube/aluminum fin condensor coil — was developed after Goodman engineers determined that the properties of R410A refrigerant, and the heat transfer characteristics offered by 5mm diameter copper tubing were an excellent match.
"A larger percentage of the refrigerant remains in contact with the heat transfer surface areas of the 5mm copper tubing," Walker says.
Goodman has added an extra feature: grooves inside the copper tubing, to further increase the thermal transfer opportunities of R410A refrigerant, in conjunction with the physical size of the 5mm copper tubing. As the refrigerant swirls inside the grooved copper tubing, the time that it comes into contact with the outside of the tubing is increased, along with an increase in the heat transfer rate. Slots between the grooves help increase the total surface area, and increase the amount of space available for the refrigerant to touch the inside of the tubing, for optimal heat transfer.
"With the smaller tubing, refrigerant flow velocity is increased. This allows a higher degree of refrigerant flow turbulence. Increased turbulence of the refrigerant inside the copper tubing can lead to better heat transfer efficiency," Walker explains.
Condensor Size Kept in Check
Microchannel coils are helping manufacturers hold the size of higher SEER condensor units in check, which benefits both manufacturers and HVAC contractors.
"I've met with OEM equipment manufacturers who commented on the ease of assembly resulting from the weight reduction," Anderson says. "Now, they no longer need a lift to position the coil during assembly; two people can easily pick up the coil and move it into position. Additionally, contractors have commented that they only require one technician with a hand truck to deliver and install units rather than a team with special handling equipment."
According to Walker, the footprint of Goodman's 5-ton, 13-SEER residential air conditioner was reduced by six inches, from 35-in. square to 29-in.
"The SmartCoil version is up to 15% lower in weight, which is about 40 pounds," he says. "That's a significant weight reduction, which is good for everyone."
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York's new LX series condensing unit was the product of choice for HVAC contractor Mark Ensor, Ensor & Sowers, York Springs, PA, when he recently replaced a 62-year-old York unit that was still going strong, for a homeowner in Gettysburg, PA.
"The outside unit is definitely smaller than the unit we replaced indoors, and it has a really nice appearance," Ensor says.”
The LX series unit is 30% ligher than conventional coil models, and uses up to 50% less R410A refrigerant, York reports.
William Fox, owner, William C. Fox Heating & Air Conditioning, Edgewater Park, NJ, is a Trane Comfort Specialist who's installed many Trane units that use SpineFin™ coils, including the new XL20i (see sidebar).
"They're very durable, with better heat transfer compared to copper fin units," he says. I've had no problems with them in my 30 years in the HVAC industry."
Hedge Against Refrigerant Prices
Microchannel technology is a system-based solution to skyrocketing refrigerant prices, and phaseouts of popular formulations, such as R22. Smaller coils use less refrigerant.
"Refrigerant R-410A's performance characteristics allow you to have smaller internal refrigerant volume while improving system performance," adds Mike Heidenreich, Luvata's director of engineering. "These coils are 'greener.'"
Heidenreich says reduced coil size enables OEMs to user higher fin densities than with a plate fin coil for the same application, which improves their durability over RTPF coils.
"On a traditional tube and fin coil, you have exposed cantilevered fin edges that are susceptible to bending. In a microchannel coil, the tube comes out to the edge of the fin. That construction is much more resistant to pressure washing than a conventional round tube, plate fin construction," Heidenreich explains.
Combating corrosion has long been a challenge for coil developers. However, mono-metal aluminum microchannel coils, combined with a protective zinc coating, have helped to greatly reduce corrosion.
"We've had test units stationed on seaside locations for years, and they're still looking basically as good as the day we installed them," says Heatcraft's Dustan Atkinson.
In addition to corrosion caused by salty air, galvanic corrosion — which occurs when dissimilar metals (steel and copper) are electrically connected in the presence of an electrolyte — is being reduced by using single-metal coils.
Research into coil design continues. The manufacturers mentioned in this article, and others, are leveraging their knowledge of metalurgy, thermodynamics, and product design for continued product improvement. They're finding additional ways to roll with the changes and challenges, and are producing better, more efficient cooling products.
Adding up the Benefits of Improved Coils
by Will Lange
It seems as if the discussion about how to increase heat-transfer efficiency is almost as old as the industry itself. For years, manufacturers have designed and redesigned their offerings to provide homeowners with the latest innovations in coil technology and efficiency. Trane has its patented Spine Fin™ coil. Some manufacturers are focused on microchannel technology, microtube, or other ideas.
While solutions may differ, the one thing everyone can agree on is that heat-transfer is one of the most important factors in achieving cooling efficiencies.
When it comes to selecting the most reliable, durable, efficient and effective coil technology, there are five core benefits to keep in mind. While these benefits may be important primarily to HVAC contractors, some may also be important to their residential customers.
Highest long-term operating efficiencies. Look for a surface that doesn’t get loaded rapidly with dirt and debris, and is less likely to be attacked by corrosion. The result is longer-term heat transfer capability.
Lowest leak potential. A dramatic reduction in braze joints provides fewer opportunities for potential coil leaks. Fewer leaks mean fewer service calls and longer product life.
Superior heat transfer. Heat transfer is all about the interaction between airflow and the leading edge of the heat exchanger. More leading edge means more heat exchange and greater efficiency. Think of it this way: the worst heat exchanger would be a big lump of metal with refrigerant running through it. A little better method would be a tube of refrigerant attached to the same metal in a big flat sheet. That would be improved by chopping up the sheet into strips and running the refrigerant tubes through that. Trane believes the best solution is attaching millions of tiny fins to the aluminum refrigerant tubing, thereby dramatically increasing the heat exchanger capability. This also provides unbeatable retained efficiency over time.
Greatest corrosion resistance. Look for the coil that is resistant to outdoor corrosion, especially when it comes to seacoast environments, galvanic conditions and acid rain environments. This is where it makes sense to use only one type of metal, not mix two dissimilar metals that eat away at each other.
Earth-friendly technology. A coil manufactured from a single metal is simple to recycle. Reusing this material saves 95% of the energy required to make new aluminum and drastically reduces environmental impact.
Will Lange is product manager for split systems, Trane Residential Solutions.