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One recent development involves aluminum microchannel coils. They debuted three years ago at the AHR Expo in Chicago and most recently got a boost when they were used to help the development of 1-3-hp split systems operating at 13 SEER.
In its original introduction of the technology, Heatcraft North America Heat Transfer described the product as a way "to reduce size and weight over current copper tube-in-fin-style exchanger coils, thus saving money in installation and component sizing costs."
"The exchangers are constructed completely from aluminum alloys and use high-efficiency fin designs to increase air-side heat transfer while maintaining acceptable air pressure drops," said David Davis, who is in sales and engineering for Heatcraft Aluminum Products.
He pointed out that tubes are extruded to provide the microchannels for refrigerant flow and increased tube-side heat transfer. The manufacturer brazes coils in a controlled-atmosphere furnace. The brazing technology emphasizes quality in construction and corrosion resistance, he said.
ONE APPLICATIONOne application came in the recent rollout of 1-3-hp split systems products from UPG York, A Johnson Controls company. The models have a designation of [mc]2 MicroChannel MiniCube split systems and come in 22- by 22-inch footprints.
The manufacturer described the products, which use R-22, as using the microchannel coil technology to provide higher efficiency and greater capability with a corresponding increase in coil surface area. "This technology reduces the size of the unit compared to traditional 13 SEER products, and uses less refrigerant," the company said.
Aluminum coils came to the market because of their ability to avoid galvanic corrosion. There is no copper pitting corrosion. Manufacturers consider them appropriate for indoor and outdoor applications.
The microchannels are said to have 20 percent less air-side pressure drop and 30 Btu/cu in. more fin surface than round-tube plate-fin coils with louvered fins. The reduced internal volumes are also said to reduce the refrigerant charge.
MAKING THE MATCHAs with the case of any split condensing system, the final stage in manufacturing is actually in the hands of installing contractors, who have to make sure the two components of the split are matched up.
In the textbook Refrigeration and Air Conditioning (published by Prentice-Hall), author Larry Jeffus said, "This final stage in manufacturing has a greater influence on the performance of a system than anything that the manufacturers can do. If the system is not installed with the correct size air duct system, electrical wiring, and many other factors, the result can be in the system providing poor performance or reduction in its operational life.
"It is therefore important that all manufacturer's specifications and guidelines be followed."
One contractor who has been working with the product is Thomas & Betts Corp. - HVAC Division, Memphis, Tenn. Tim Roberts, vice president of engineering, said the technology is being used in some of the air conditioning units and packaged units in the 5- to 40-ton range that the contractor is installing for commercial office buildings and fast food chains.
"With the new refrigerants, this gave us the best footprint alternative," he said. "That plus the lower weight had an impact on shipping costs. The fact that it was a corrosion-resistant product was a big part of the decision process."
He said he also appreciates the flexibility of bending the coils into L or U shapes, and the fact that repairs can be made by isolating certain coils rather than replacing the entire coil unit. Service technicians need to be aware that brazing is a bit different than with copper connections, he pointed out.
When all factors are taken into consideration, Roberts said the approach could be cost competitive with more traditional copper approaches.
For more information, visit www.heatcraft.com
Publication date: 06/05/2006