On Twin Screw Compressors Design

Twin screw compressors are widely used in many applications, such as compressing air, refrigerating, industrial gas processing, engines supercharging, etc. In the last quarter of the twentieth century the number of units manufactured registered an extraordinary increase: in fact the development of advanced manufacturing techniques permitted to exploit the basic advantages of these machines in respect to the other types of compressors. In particular twin screw compressors are characterized by the ability to satisfy varying load demands, reliability in service, high efficiency, simple maintenance. Despite the diffusion of this type of machines in many industrial fields, the available literature about them is still limited, in particular regarding the rotor profile geometry. In fact, almost every industrial profile of the rotors is protected by patents, and the companies that hold them seldom diffuse information about the generating methods. In this paper a general description of this type of machine is presented, focusing on the mechanical aspects and in particular on the shape of the rotors. The first part of the paper introduces the main advantages and disadvantages in respect to the other compressor types and the state of the art of this type of machine. Then a deep mathematical analysis of a method for the profiling of screw compressor rotors, based on the theory of gearing, and typical aspects related to the machine design, such as the rotor flow area, the sealing line on the cross-sectional plane and on the rotors and the blow-hole area, are discussed. In the second part of the work the theory above presented is applied to three different geometries of the rotors, and other possible more complex profiles are introduced.