Analysis of Clearances in Combined Screw Machines

Changes caused by differential expansion between the rotors and the casing occur in the high pressure clearances in both the compressor and expander sections when expansion and compression are performed together in a single oil free machine of the twin-screw type. These are more difficult to control than when the two functions are carried out in separate machines. The clearance changes affect both the performance and reliability of the machine but can be controlled by using different materials of construction for each section. Clearances, predicted by the assumption of linear expansion of the components, were included in a well-proven software package for performance estimation of both screw compressors and expanders and the results compared with experimental data. It was found that the clearance in the machine, being dependent on the temperature, could be estimated fairly accurately by matching measured discharge temperature and the temperature obtained by the estimation model. Therefore, a simple expansion analysis of the main machine clearances appeared to be an adequate tool for use in the design of these machines in order to optimise performance and prevent the machine seizing as a result of differential thermal expansion during operation.