The function of the compressor in a transcritical application is the same as in a subcritical one. The compressor creates refrigerant flow, increasing discharge pressure and therefore raising refrigerant temperature to a level high enough that heat absorbed in the evaporator will be rejected in the condenser or gas cooler.
Due principally to the higher CO2 working pressures, several challenges were presented during design of the TN compressor. Some of the major challenges were the shell construction, internal leakage across the piston, piston and bearing loading, and bearing lubrication, all due to the higher pressures. At the same time, to keep costs down, a simple design was chosen.
Outer shell construction had to be of increased robustness to handle the higher pressures. Piston rings had to be added to prevent internal cylinder leakage. The piston connecting rod bearing design was modified due to a smaller piston diameter and higher specific forces on the bearing.
The volumetric refrigeration capacity of CO2 is much higher than that of traditional refrigerants, allowing system designs with smaller volumes. This also holds true for other components. Smaller cylinder displacement still provides adequate system capacity.
Compression ratio is a major factor contributing to CO2 compressor performance. Current HFC compressors for MBP-operation with R-134a are designed for compression ratios of approximately 8 to 1, and become inefficient at higher ratios.
CO2 systems operate at higher pressures, but the compression ratio is only 3 or 4 to 1. The lower compression ratio will give the compressor the capability to operate with less of a refrigerant re-expansion effect in the cylinder during operation.
Now that there are functional designs for compressors, heat exchangers, and high pressure expansion devices, the transcritical cycle with CO2 becomes a viable alternative for users seeking a solution that is acceptable worldwide. It is a natural refrigerant that will allow users to be identified as environmentally proactive. These facts will continue to fuel development of CO2 components and system designs.
Jeff Staub is manager of application engineering at Danfoss Inc., Baltimore. Bjarne Dindler Rasmussen is research engineer, Danfoss A/S, Nordborg, Denmark. Max Robinson is in Technical Communications at Danfoss Inc.
Publication date: 01/26/2004