The Selection Of Screw Rotor Geometry With Compressor Speed As A Design Variable

Variable frequency drives (VFD) are becoming increasingly popular in screw compressor applications. The popularity is being driven by the demand for higher part load performance. Reducing speed to modulate capacity has proven to be more efficient than traditional methods of mechanical unloading. Compressors that are designed with the intent of applying VFD’s now have speed as a variable at the design point as well. Guidance is required to choose the correct speed and other geometric parameters associated with screw compressors. An analytical study using a one-dimensional (chamber) model has been done to explore the interactions between rotor speed and diameter, length, wrap angle, number of lobes, and discharge port. Correlations of compressor efficiency revealed by the study are explained based on fundamental principles. The correlations will help guide the design engineer in selecting the proper combination of speed and compressor geometry to achieve the best compressor efficiency for an application. The results of the study are then expressed using the non-dimensional parameters of specific speed and diameter. Compressor efficiency is known to be optimized when the value of these parameters fall within a specific range. The optimum values determined in this study are compared to historical values and differences are explained