Thermal Analysis of a Twin Screw Vacuum Pump using CFD Model

Twin screw machines that are used in vacuum pump application have large wrap angle and length-to-diameter ratio in comparison to classical twin screw machines used in compressor applications. However, the compressor type asymmetric rotor profile with lower wrap angle can be re-designed such that the machine can switch its operation from medium pressure ratio above ambient to a vacuum operation from very low pressure to ambient i.e., high pressure ratio. This switching results into a high heat generation rate within the rotors and demands for additional cooling measures in the rotor design. In this study, CFD model of a 3-5 Twin screw vacuum pump was used to evaluate the thermal performance of the machine. SCORG grid generator and ANSYS CFX solver were used for setting the model. Experimental observations related to inefficient heat dissipation on the intake of the rotors were analysed for its causes and potential design solutions. Air injection for cooling in the current configuration was evaluated and impact of design modifications was estimated using this CFD model. The developed model will be further used to redesign the high-pressure ports and air injection system to improve the thermal cooling effectiveness and specific power of the machine.