For years, microchannel coil technology has been used in the automotive industry in order to increase heat transfer efficiency and improve reliability through a higher level of corrosion resistance. When 13 SEER became the new minimum efficiency for cooling systems, HVAC manufacturers saw benefits in using microchannel coils in residential equipment because their smaller size reduced the footprint of condensing units. In addition, microchannel coils improved heat transfer characteristics, enhanced durability and serviceability, and reduced refrigerant charge by up to 50 percent.

Given their success in the residential market, it seems only natural that microchannel coils would eventually be incorporated into commercial cooling equipment. Within the last year, several major manufacturers have rolled out rooftop units and chillers that utilize microchannel technology, claiming that the new equipment reduces energy usage, simplifies installation, and helps earn points for LEED® certification.

EFFICIENT BY DESIGN

Standard coils used commercially to this point have consisted of a round tube (typically copper) and plate fin (typically aluminum) design. Over the years, these coils have been made with a variety of fin designs and fin spacing to improve heat transfer, but with today’s increasing demands for higher energy efficiency, the industry has pushed the standard coil design about as far as possible using current technology.

While increasing the number of fins on a coil typically increases its efficiency, pressure drop or coil-cleaning issues can arise, noted Ted Cherubin, marketing manager, commercial unitary systems, Carrier.

“With Novation™ heat exchanger technology (Carrier’s name for its microchannel technology), we tell our customers to think of the radiator that is in front of their cars. This design utilizes small, flat, plate multi-port feeders encased with ridged aluminum waves in between and was first used in the 1980s in the automotive racing industry. The technology has since been introduced in many other industrial applications including now the commercial HVAC industry,” said Cherubin.

The microchannel coil, which is made of aluminum, has many design advantages over the copper-aluminum coil. First, its construction eliminates many of the manual or auto braze joints of a copper-aluminum coil, which helps to minimize braze joint leaks. Second, microchannel coils reduce the amount of refrigerant required. Third, its design results in better thermal performance, so the coil face area of the microchannel coil can be reduced by as much as 40 percent and weight reduced by as much as 30 percent compared to a copper-aluminum coil. This results in smaller overall product size and weight and greater flexibility in product application and handling.

Finally, manufacturers state that the quality and integrity of microchannel coils are better due to improved design and are not as susceptible to fin damage as conventional fin plate coils. All these characteristics make the microchannel a perfect addition to the line-up of commercial cooling products. So why did it take so long to be incorporated into chillers and rooftop units? Ian M. Casper, senior program manager - small tonnage chillers, Johnson Controls Inc., has the answer.

“This technology originated in the automotive industry, where a few small, standard sizes and high-volume production made the microchannel commercially viable. That formula - a few, small standard sizes and high-volume production - applies more readily to residential than commercial HVAC systems,” he said.

The commercial market is fundamentally different in that it demands far fewer units per year compared to the residential market, which affects the volume equation - and subsequent cost per unit - in a significant way. And the commercial market requires coils with larger surface areas, as well as multiple configurations, both of which just recently became available for commercial equipment, added Casper.

NEW MICROCHANNEL OFFERINGS

The first commercial microchannel product for Johnson Controls is its York® Tempo™ air-cooled scroll chiller, which is available in sizes ranging from 70 to 150 TR. The Tempo chiller uses R-410A and employs an advanced refrigerant-circuit design that requires 30-50 percent less refrigerant than comparable models.

Casper noted that microchannel coils work well in chillers, because they can deliver higher energy efficiency in a same-size package, and in some cases the designer may be able to optimize performance and achieve incremental reductions in footprint. In addition, “It requires less refrigerant volume at the same rated capacity as a predecessor copper tube-aluminum fin coil design. Minimizing the refrigerant charge (pounds per ton of cooling) is an important factor in qualifying for the LEED Advanced Refrigerant Management Credit.”

Carrier Corp. has two chillers that utilize microchannel technology: the AquaForce™® and AquaSnap® air-cooled chillers are available in sizes ranging from 60 to 500 tons and can reduce the refrigerant charge by 30 percent, while providing a significant weight reduction opportunity when compared to standard condenser coils. In addition, the company states that the microchannel condenser coil technology allows the coils to be high-pressure washed as compared to conventional coils, which are more difficult to clean.

As for rooftop units, McQuay International, Minneapolis, introduced its Maverick II™ packaged rooftop systems, which utilize microchannel technology and are available in sizes ranging from 30 to 50 tons. The company decided the time was right to incorporate microchannel coils, due to many of the reasons already discussed, including the fact that they are lighter weight, more corrosion resistant, more resistant to damage, and require less charge.

“All of these advantages improve the value of our equipment,” said Skip Ernst, marketing manager, Applied Air Products, McQuay International. “In addition, all manufacturers are in the process of redesigning their refrigerant circuits for R-410A and other HFC refrigerants in response to the phaseout of R-22. This has provided an ideal opportunity to closely examine and optimize the whole refrigeration system used in equipment.”

The Centurion® line of rooftop units from Carrier Corp. also features microchannel coils. “Using the Novation heat exchanger technology has allowed us to reduce the refrigerant in the rooftop by up to 57 percent,” said Cherubin. “The technology has also reduced the weight of our Centurion rooftops by up to 10 percent and has provided additional benefits to help achieve up to 20 percent operation savings on fan power alone.”

While microchannel technology is finding a strong reception in condenser coils of a certain size range using primarily scroll compressors and R-410A, it’s not entirely clear whether the application of this technology will be as successful with different refrigerants and in still larger sizes. The reasons for that, said Casper, have to do with the economics and viability of producing larger size coils versus using larger quantities of smaller coils joined together.

“The ability to manufacture larger size coils coupled with the rising cost of copper have finally made microchannel technology viable for commercial applications; however, the size and configuration issues need to be solved for larger-tonnage applications,” noted Casper. “But we think that microchannel is a viable technology that’s here to stay for a long time. It is simply a better, more efficient, smarter, and more environmentally responsible system, overall.”

Publication date: 05/19/2008