| The oil-free screw vacuum pump has been widely used in industrial areas of semiconduc-tor, medicine, grain drying, chemical and pharmaceutical, spatial simulation, and many other industries, which was due to its lists of advantages, such as wide pumping speed range, simple and compact structure, no friction between the pumping component (screw rotors of the fixed lead or gradational pitch), and no oil pollution, and so on. So, the dry screw vacuum pump is also a environmental one that suits the development of world economy and energy saving.However, with alter in pumping target and object, the screw vacuum pump is inevitably faced with unprecedented challenges and opportunities. Since the problem of phase change, sublimation, deposition and so on, many researchers have paid more and more attention to the thermodynamic process, consumption of friction and flowing of internal gas. Thus, it is essen-tial to make theoretical deduction and analysis calculation for thermodynamic processes of internal gas in dry screw vacuum pump of which the lead is fixed or gradational.Firstly, the paper expounds and sums up the research background of oil-free screw vac-uum pump. Then, some products of well-known enterprises at home and abroad are described in detail. Finally, main contents and general structure scheme of this dissertation is also given.Aimed at oil-free screw vacuum pump with fixed or gradational pitch screw rotor used in many applications, some detailed research about thermodynamic calculation and analysis is carried out in this paper. According to sophisticated thermodynamics theories, some reason-able hypothesis and structural characteristics of oil-free screw vacuum pump, the working process of screw vacuum pump was divided into the suction process, the transporting process, the compressing process, the discharge process, and its theoretical derivation was also given. The effects of different compression type were discussed, which are the isothermal compres-sion, the adiabatic compression, and the polytropic compression, and the discharge tempera-ture and power consumption is calculated and compared. And leakage, backflow and thermal exchange is inevitably occur in screw vacuum pump, which also affects the thermodynamics process. The thermodynamic calculation of it was carried out on the assumption that the re- flux and heat are linearity to the change of time in transporting process. Based on the theo-retical derivation and analytical calculation, the P-v diagram, the T-s diagram and the changing rule of various thermodynamic parameters with the time, such as gas pressure, gas temperature, mass, entropy, were given and analyzed in this paper.Compared with the screw vacuum pump that having fixed lead screw rotors, the pump having gradational pitch screw rotors have the outstanding functions of energy saving and noise reduction because of its exhausting way of internal pre-compression, as a result of which, it has attracted the attention of designers, manufactures and users. Thus, thermody-namical calculation of gradational pitch screw vacuum pump is given in this paper. The vari-able pitch screw vacuum pump includes the conventional one, the combined type, the tri-section type.In view of cost and the difficulty of processing and manufacturing, thermodynamic calcu-lation and analysis of quadratic relationship between axial position and screw angle was just put forward. The combined type screw rotor is made of two different lead screw rotor, and its vacuum pump avoids the machining problem, and reduces significantly the exhaust tempera-ture. And the volume of gas is linearity to the change of time. The advantages of tri-section variable pitch screw rotor are the corresponding screw vacuum pump has a large pumping volume, and its pumping speed is high. With the similar method of the fixed pitch screw vac-uum pump, its working process was divided into the suction process, the transporting process, the pre-compressing process (the mutating compressing process, the variable lead compress-ing process.) So, theoretical derivation and correlation analysis of thermodynamic of grada-tional pitch screw vacuum pump are carried out in this paper.Based on the theoretical derivation and analytical calculation, the P-v diagram, the T-s diagram and the changing rule of various thermodynamic parameters with the time, were given and analyzed in this paper. And the influence of screw speed on discharge tem-perature and power consumption was discussed. Finally, the discharge temperature and power consumption of three kinds of variable pitch is compared, and the optimal thermal perform-ance of variable pitch screw vacuum pump was got. In the deducing process of the formulas, the form of end face profile isn’t involved, thus, the methods provided in this paper are also fit for the screw vacuum pump that screw rotor has any type of end face profile. |
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