General Method for Screw Compressor Profile Generation

The paper presents a general algorithm for the specification of a screw machine geometry. The conjugancy condition was solved numerically which thereby enables a variety of primary arc curves to be introduced. In addition, an optional numerical derivation of primary arcs enables any analytical function and even discrete point functions to be used for the primary arc. The approach also simplifies the design procedure since only primary arcs need be given. Secondary arcs are then generated automatically. This procedure is not sensitive to the location of the primary arc, a primary arc can be located either on the main or gate rotor. Any other rotor and also a rack (a rotor of an infinite radius) can be used as a location of the primary arc. All combinations are allowed and the most efficient pr0files were obtained by the combined rotorrack generation procedure. INTRODUCTION A twin screw engine is a positive displacement rotary machine consisting of a meshing pair of helical lobed rotors, contained in a casing, which together form a working chamber which volume depends only on the angle of rotation. Rotation leads to a decrease in volume up to a minimum value when compression is complete. The volume ratio of compression is determined by how far the discharge port, through which the fluid is expelled, extends around the circumference of the high pressure end of the casing. At, or near, the angle of rotation corresponding to maximum compression, the discharge port in the casing is exposed. Further rotation causes the trapped volume of fluid to be expelled at approximately constant pressure. Its adiabatic and volumetric efficiencies are highly dependent on both the profile and number of the lobes in each rotor. The rotor profiles most commonly used are those developed by the Swedish company, Svenska Rotor Maskiner (SRM), the licensors of the original Lysholm patents. The first rotor combination to be developed was based on the circular symmetric profile (Fig 1 a) which all the lobe segments are