Experimental Performance of Carbon Dioxide Compressor with Parallel Compression

Carbon dioxide (CO₂) was proposed in the recent years as a natural fluid to replace the HFCs in refrigeration applications. Its implementation in refrigeration, first in subcritical and recently in trans- critical systems is becoming a technology of increasing importance.
 

Trans-critical CO₂ system presents lower COP compared to HFCs systems when the ambient temperature is high. Reciprocating compressors with two compression stages having a vapor injection port (VI) have been proposed for parallel compression to improve the efficiency of the system. Many publications have been presented to explain the application advantage but experimental data are lacking to support the theoretical analysis.
 

The work presented here highlight the advantage of the parallel compresion and explains the experimental tests carried-out on a reciprocating prototype with carbon dioxide working with two compression stages (parallel compression) and finally discusses the experimental results. The compressor is a semi-hermetic four cylinders compressor with one compression chamber (cylinder) dedicated to the parallel compression. The tests was performed on hot gas by-pass cycle at -10°C evaporating temperature varying the intermediate and discharge pressures.
 

The experimental result shows the performance at different operating conditions varying the intermediate pressure. It highlights the influence of the intermediate pressure on the efficiency of the compressor and the system. The intermediate pressure influences the volumetric efficiency of the compressor and consequently the COP of the system.
 

Finally, a system efficiency comparison between this configuration and dedicated compressor for parallel compression have been evaluated.