Scenarios simulated included Compressor speed as a function of molecular weight, while maintaining the same suction and discharge pressure and mass flow rate. Polytropic head and efficiencies as a function of gas molecular weight for a given compression ratio, and compressor speed. Aspen Hysys Download Free Software' title='Aspen Hysys Download Free Software' />Effects of thermodynamic properties. Compressors. Compressors generally fit into two categories Positive displacement compressors that include reciprocating, rotary screw, sliding vane, liquid ring, and rotary lobe. The compression principle is volumetric displacement, in which reducing.