In a paper presented at the 2018 Purdue Conferences, optimized domestic refrigerator condenser coils using hydrocarbon refrigerants were shown to deliver needed cooling capacity while using smaller-diameter (5 mm) MicroGroove copper tubes that contain a small charge of refrigerant.

The paper, “Optimization of MicroGroove Copper Tube Coil Designs for Flammable Refrigerants,” was authored by Nigel Cotton, International Copper Association; Adam Rhoads, Optimized Thermal Systems; Anderson Bortoletto, Sub-Zero, Inc.; and Yoram Shabtay, Heat Transfer Technologies. The Purdue Conferences, encompassing 24thCompressor Engineering, 17thRefrigeration and Air Conditioning, and 5thHigh Performance Buildings, took place July 9-12 at Purdue University, Lafayette, Ind.

The paper can be downloaded here.

Reducing refrigerant charge – important for an R600a residential refrigerator, which has a 57 g charge limit in the U.S. – was the primary objective of the study. Secondary objectives included the reduction of the total footprint and the total tube-and- fin material mass.

“This study demonstrated the simulation and design of new heat exchangers that can maintain the heat transfer performance of the baseline and allow for lower refrigerant charge in a smaller, lighter envelope,” the paper said.

The baseline design employs 6.35-mm outside diameter copper tubes with a minimum wall thickness of 0.41 mm (quarter-inch tubes with 0.016-inch wall thickness). By contrast, the new design uses 5-mm outer-diameter copper tubes and wavy-herringbone fins with reduced fin thicknesses as compared to the baseline design..

The proposed design was found to have 41% less internal volume than the baseline along with a 57% reduction in coil footprint. “A significant reduction in refrigerant charge is achieved when this heat exchanger is used as a condenser in the refrigerator,” the report said.

Besides equal or better performance and reduced refrigerant charge, the prototype was 21% lighter than the baseline.

The new design has a slight improvement in heat transfer performance at a given airside pressure drop – an improvement of 2.33% at 0.033 inches H2O of airside pressure drop.  An even greater increase of heat transfer performance is expected once the proposed design is production-tooled with fins that include collars.

“At 57 g, the system becomes very sensitive to the charge variations and require a tighter control of charge.”

Anderson Bortoletto, Sub-Zero

The charge limit of hydrocarbons in domestic refrigerators remains at 57 g in the U.S.  In February, the Environmental Protection Agency (EPA) 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 a few 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 of Sub-Zero, one of the authors of the study.

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

“About 80% of domestic refrigerators could be made with 57 g, but it would be easier to design with 150 g,” said Bortoletto. “At 57 g, the system becomes very sensitive to the charge variations and require a tighter control of charge. Slight variations have big impact on performance at 57 g, compared to 150 g.”

This low-charge limit motivated evaluation of optimized heat exchangers that could deliver the needed cooling capacity using smaller diameter copper tubes, the paper said.

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