Performance analysis of air-oil separator tank in oil injected rotary screw compressors with CFD methods

Compressors are used in nearly all industrial sectors. Researches have shown that air compressors are consuming nearly 10% of industrial energy in the world [2-4]. Most of the countries are applying regulations for the energy efficiency of the compressors. Oil injected rotary screw compressors are the most common type that is being used in most of the industries. One of the most important system in oil injected rotary screw compressors is separation system. There are several techniques to perform compressed air and oil separation. In this thesis study, vertical cyclonic air oil separator tank CFD analysis has performed. Nine different vortex finder diameter and length dimension combinations have analysed with three different inlet velocity. In total, twenty seven different cases has taken into account in order to optimize the design air oil separator tank. Many studies revealed that RSM method is the most reliable method in order to predict the complex swirling flow inside air oil separator tank. Moreover, DPM method has shown best results of tracking oil droplets. Same approaches have been taken into account in this study. Velocity inlet has implemented as inlet boundary condition as 8-10-12 m/s. Pressure outlet has given as outlet boundary condition as 700000 Pa since the operating pressure of the compressor system. In addition, inlet and outlet ports have stated as escape condtion. Walls and bottom surface of the air oil separator tank are stated as reflect and trap condition respectively. Finally, Rosin-Rammler particle size distribution has used for oil particles ranging from 1 µm to 100 µm with 1.45 spread parameter. ISO-V46 synthetic rotary compressor oil is used with 868 kg/m3 density, 0.0085 kg/m-s dynamic viscosity and 60 L/min flowrate. Air density and dynamic viscosity is stated as 7.986 kg/m3 and 2.11*10-5 kg/m-s respectively. Separation efficiency and pressure drop is the most critical parameters for the performance of air oil separator tank. Inlet velocity has a big impact on pressure loss inside the air oil separator tank. Pressure loss increased rapidly with the increase in inlet velocity. Furthermore, tangential velocity increases with the increase in inlet velocity which is one of the main sources of centrifugal force acting on oil droplets so on to the separation efficiency. However, results have shown that in some cases increase in inlet velocity resulted in high separation efficiency and for some cases high inlet velocity decreased the separation efficiency. Moreover, dimensional changes of vortex finder diameter and length have more significant affect on pressure loss and separation efficiency. Increase in vortex finder diameter and length has resulted in decrease in separation efficiency and pressure loss. It is also found out that smaller oil particles have more tendancy to escape from the outlet port. Nearly all oil particle droplets larger than 10µm trapped at the bottom surface of the tank. After comparing the results of all twenty seven cases, it is found out that the tank design with 200mm vortex finder diameter and 300 mm vortex finder length at 8 m/s inlet velocity has shown the best performance by considering separation efficiency and pressure loss.