Sub-Zero, Inc., Madison, Wis., a manufacturer of residential refrigerators and freezers, is engaged in a project to explore the advantages of MicroGroove copper tubes with outer diameters of 5 mm in appliances using up to 57 g of flammable R600a (isobutane) refrigerant.

The project includes Optimized Thermal Systems, Inc. (OTS), a software developer based in Beltsville, Md., and the International Copper Association (ICA), New York.

According to a January press release from ICA, engineers from Sub-Zero approached OTS for assistance in using CoilDesigner, OTS’ heat exchanger design and simulation software tool. “The primary objective was to design a condenser coil that would equal the performance of the existing (baseline) coil while lowering the refrigerant charge,” said the ICA release. “Secondary objectives were to reduce the total footprint of the coil and the total tube-and-fin material mass.”

The baseline condenser coil in the OTS study uses quarter-inch copper tubing, flat plate fins and a low fin density. The condenser has two refrigerant circuits, with each circuit serving an independent vapor compression cycle for the refrigerator and freezer compartments.

“The idea was to explore [5 mm] diameter to address the [hydrocarbon] charge limit of 57 grams,” said Anderson Bortoletto, principal design engineer for Sub-Zero.

According to ICA, OTS has developed a CoilDesigner model of the condenser and validated it against experimental data.

Equipped with that model, Sub-Zero created a prototype coil, and has begun a “component-level and system-level assessment of its heat transfer performance,” said Bortoletto, adding that Sub-Zero will release the results of its experimental work.

Sub-Zero plans to share the results of the project with the ICA to “promote the visibility of this technology,” he said. “We’re a small company and we can’t drive 5 mm ourselves. We need others to see the benefits and jump in so everybody benefits.”

The Sub-Zero research is taking place with the policy on federal hydrocarbon charge limits in flux. The U.S. Environmental Protection Agency (EPA) recently withdrew a “direct final rule” that would have raised the charge limit to 150 g from 57 g for hydrocarbon refrigerants in domestic refrigerators and freezers because of adverse feedback from stakeholders.

However, an identical proposed rule raising the charge limit remains in play, and the EPA said it would address adverse feedback in any subsequent final action based on the proposed rule. There is a lot of push [in the U.S.] to go to 150 grams,” said Bortoletto.

The European Union and other regions have long used a 150 g limit for hydrocarbons in domestic refrigerators.

If the hydrocarbon charge limit were raised to 150 g, the need for the ICA project would be less, though there would still be an interest in the “overall heat transfer enhancement and the material savings,” resulting from the 5 mm tubing, he said.

Sub-Zero makes hydrocarbon-based refrigerators for the international market where the limit is 150 g – using 5/16-in. (7.9 mm) and 1/4-in (6.35 mm) tubes – but not so far in the U.S. with its 57 g limit. But with 5 mm tubes, the 57 g limit would be viable for domestic fridges, Bortoletto said.

“We’re a small company and we can’t drive 5 mm ourselves. We need others to see the benefits and jump in so everybody benefits.”

Anderson Bortoletto, Sub-Zero 

The OTS optimization study was conducted to identify condenser designs that could reduce the internal volume and lower the refrigerant charge.

The 5 mm tube designs were evaluated and compared to the baseline design and significant reductions were found in internal tube volume, said the ICA release, adding, “The best 5-mm design reduced the internal tube volume by as much as 41% as compared to the baseline, along with a 57% reduction in coil footprint.”

The new designs used “wavy-herringbone” fins with a reduced fin thickness as compared to the baseline. Other variables included the horizontal and vertical spacing of the tubes; number of tubes per bank; fin density; wavy fin pattern depth; tube circuitry; and tube length.

The design criteria included, compared to the baseline design: heat rejection and subcooling greater than or equal; and saturation temperature drop kept within one degree.

Multiple factors contributed to an increase in airside pressure drop with the smaller diameter tube coils, including fin type, face area of the coils, and fin density, said ICA. “Nonetheless, for this application, reduced internal volume was considered to be more important than the airside pressure drop. The fan motors used in this system can overcome the increased resistance and compensate for the increase in airside pressure drop.”

ICA called the study “a starting point for the development of high performance condenser coils for this application.” With a high-performance condenser, it added, “condenser fan speed can be reduced, resulting in a quieter refrigerator unit. Also, less fouling could be useful in the condenser, requiring less maintenance.”

“R600a will play a key role in refrigeration systems for many years to come,” said Nigel Cotton, MicroGroove Team Leader for the International Copper Association. “As MicroGroove tubes and coils are uniquely suited for use with propane, so they are also well suited for isobutane. The Sub-Zero study has set a benchmark for the use of MicroGroove with R600a.”

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