A study on the use of liquid separators in variable-speed small-scale carbon dioxide refrigerating systems.

Concerns about the environmental impact and safety of refrigerants have become an important aspect in the design of new refrigerating systems. Carbon dioxide (CO2) is seen as a promising non-flammable, non-toxic and low global warming potential (GWP) refrigerant alternative. Several theoretical studies have shown that CO2 systems can benefit significantly from modifications at the cycle level. This investigation focuses on the thermodynamic performance of a CO2 refrigerating system comprised of a liquid separator, two-stage rolling piston variable-capacity compressor, intercooler and internal heat exchanger. The experimental apparatus is essentially a refrigeration loop, where the vapor outlet of the liquid separator is directly connected to the suction port of the second stage compressor. An energy optimization exercise was carried out at different compressor speeds and the results compared to those obtained in previous studies carried out without an internal heat exchanger and liquid separator. It was found that the liquid separator provides a slight gain in the coefficient of performance (COP) but adds considerable complexity and cost to the system. It was also found that the COP is affected by the intermediate pressure, with the best value being that resulting from a natural balance between the stages.