Computer aided design of twin screw compressors
The twin screw refrigeration compressor is required to run over a large range of working conditions. In order to design an advanced and high-efficiency compressor economically, computer aided design techniques are required. This thesis presents such techniques, which include profile generation, geometrical characteristic calculation, working process simulation, rotor cutter blade calculation and optimisation techniques. All the basic theory and equations and the derived equations are presented. Four important computer programs, ie the profile generation program, the geometrical characteristics calculation program, the working process simulation program and the cutter blade calculation program, are developed and presented in the thesis. A few other support programs are also developed by the author to display the calculated results. All the programs developed form a program library for the CAD of twin screw compressors. All the programs except the profile generation program are universal, which means that they can be used for any shape of rotor profile. For the working process simulation program, only refrigeration twin screw compressors are considered, but it is easy to extend the use of the programs to other kinds of compressor. The thermodynamic effects of the following are discussed and taken into account: internal leakage of gas through all paths; oil, injected and drained from bearings; refrigerant injection, both gaseous and liquid; the flashing of injected refrigerant and that dissolved in oil; friction effects, in both end and main casings; the use of measured performance data in the determination of essential empirical coefficients in the mathematical model. The application of the programs and the design optimisation technique are presented, which include leakage analysis, compressor geometrical parameter optimisation, rotor-to-rotor clearance distribution optimisation and cutter blade shape optimisation etc. The author believes that the research work presented in this thesis is of practical value. Further, it presents new knowledge: of the compression start blow hole and its influence; of leakage quantitative analysis; of compressor design optimisation; of the quantitative analysis of the influence of different determination procedures of inter-rotor clearances.