Rotor clearance design and evaluation for an oil injected twin screw compressor

Designing twin screw compressors to safely operate at higher than normal temperatures poses a challenge as the compressor must accommodate larger peak thermal distortions while ideally maintaining efficiency at nominal operating conditions. This paper will present a case study of an oil injected compressor tested at elevated discharge temperatures with original and revised clearances. The local thermal distortions occurring within the compressor during operation were estimated using a procedure developed by the authors - thermodynamic results from a chamber model were used to approximate component temperature distributions that are then used to predict possible thermal distortions and the resulting affect on clearance gaps. The original and revised clearance designs are evaluated and performance penalties incurred due to the modifications are discussed

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

Calculation of clearances in twin screw compressors

Clearances between rotating and stationary parts in a screw compressor are set to ensure the efficient operation and allow for thermal deformation without unwanted contacts. The change in clearances is caused by both pressure and temperature changes within the machine. If clearances are too large, the increased leakage flows will reduce efficiency. However, if the nominal clearances are too small, contacts between the rotating and stationary parts can occur as a consequence of rotor and casing deformations. In order to determine the operational clearances, a numerical analysis of deformation of screw compressor rotors and casing has to be performed. This paper discusses how the temperature of rotor and casing surfaces calculated from the one-dimensional chamber model in the SCORG could be used as a boundary conditions for a steady state thermal and structural analysis of a screw compressor solid parts. Deformations of rotors and casing under temperature load were calculated using a commercial Finite Element Analysis code ANSYS. Operational clearance are estimated from these deformations and some recommendations for further work are proposed

Investigation of Screw Compressors for Low Pressure Ratio Applications

Screw compressors have been widely investigated for many applications including air and process gas compression, and refrigeration systems. There is however a surprising lack of literature for low pressure ratio application of these machines, defined here as application requiring volume ratios in the range of 1-1.5. The aim of this paper is to characterise the loss mechanisms for oil injected screw compressors with low volume index (defined as the ratio of maximum to minimum volumes during the internal compression process), Vi. This knowledge will be applied in the future to identify appropriate optimisation of rotor profiles and porting geometry for a range of low-pressure ratio applications. The current study involves the use of chamber models to investigate the influence of wrap angles and porting. This initial analysis will be developed in future work to allow a detailed parametric study of the factors that limit the performance of low Vi oil-injected screw compressors