Clearance management in twin screw compressors
Although the performance of twin screw compressors is heavily dependent on the rotor clearances within them, chamber models, used as design aids, allow for the specification of their magnitude and distribution but do not account for how these may vary during operation, as a result of internal temperature changes caused by the compression process. A validated procedure has therefore been developed to enable a chamber model to predict compressor performance, while including the effects of rotor and casing distortion resulting from dependant thermal effects. This has been achieved by the use of surface boundary mapping to calculate the rotor and casing temperature exposure within the compression chamber resulting from initial performance estimates. These detailed temperature distributions are processed analytically using appropriate assumptions that allow calculation of component temperatures and thermal growth. A program for calculation of leakage area curves has been adapted to support locally calculated variations in clearances. These updated area curves can then be fed back into the chamber model in an iterative procedure to simulate performance with thermally distorted clearances. The inclusion of thermal clearance corrections generally improved the accuracy of the chamber model when predictions from it were compared with test results over a wide range of operating pressures and temperatures. Furthermore, this work was found to be applicable in the evaluation of the interlobe clearance distribution between the rotors. Predicting clearance distortions and likely areas of rotor to rotor contact at a particular operating duty allows clearances to be optimised for the correct balance between performance and reliability; the results thus obtained were supported by findings from available test and tear down results.
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