A Twin Screw Combined Compressor And Expander For CO2 Refrigeration Systems

Recent interest in natural refrigerants has created a new impetus for studies of CO2 as a working fluid in vapour compression systems for refrigeration and air conditioning. Two major drawbacks to its use are the very high pressure differences required across the compressor and the large efficiency losses associated with the throttling process. To overcome the throttle losses, a number of proposals have been made for various types of positive displacement machine, mainly of the vane type, which combine compression with some recovery of work from the expansion process. However, how well they operate with high pressure differences across the vanes has not been confirmed. For many years, the authors have been investigating the use of twin screw machines to fulfil both the expansion and compression processes when using more conventional halocarbon refrigerants. These have many potential advantages over other types of positive displacement machine. Unfortunately, when applied to CO2 the huge bearing forces associated with the pressure distribution within them have hitherto made them appear to be unsuitable. In this paper, it is shown how the rotor forces created by the compression and expansion processes can be partially balanced in order to eliminate the axial forces and reduce the radial bearing forces. The disadvantages of twin screw compressors for such high pressure applications are thereby reduced. The balanced rotor concept is also applicable to vapour compression systems using more conventional refrigerants and even for high pressure gas compression.