This paper provides a detailed design guide, optimization, and performance assessment for air-water separation of an axial flow cyclone. Axial flow cyclones (also known as swirl tube demisters, mist eliminators, or Austin-Write cyclones) have a range of applications in several different industries. This method of gas-liquid separation offers many benefits. Among these are high separation efficiency in high pressure applications (over 90% at 1 MPa) and an inline design that allows them to be more easily fitted into existing piping structures. Despite these benefits, there is a lack of recent literature on their design criteria and performance optimization. This research fills the gap in the literature by quantifying the effect of design parameters on water collection efficiency, ?_(water collection), and the air bypass efficiency, ?_(air bypass), defined as the ratio of the air mass flowrate exiting through the desired air outlet over the inlet air mass flowrate. A set of wide-ranging experiments were conducted to study the effects of gas-liquid flow rates, tube geometry, and relative injection angles to optimize water collection and air bypass efficiencies. The water collection efficiency exceeded 99.8% when the liquid streamline came in direct contact with the water drainage exit. An empirical correlation was developed to predict the swirl pitch as a function of the above design parameters. Predictions from the correlation were within 10% of the experimental results. The correlation can be used to design highly efficient in-line gas-liquid separators.