Recently, the rotational particle separator (RPS) was introduced as a new technique for separating solid and/or liquid particles of 0.1 m and larger from gases. In this patented technique the principles of centrifugation are exploited to enhance separation of small-sized phases and particulate matter of density different from the carrier fluid. Practical designs of the RPS available in the market include equipment for purifying gases of industrial processes and portable air cleaners for domestic appliances. New developments are made in the area of the offshore industry. It concerns the separation of oil droplets from water and the separation of condensate, oil, and sand from natural gas. A particular feature of both designs is that the filter element is freely mounted in bearings and rotates, without the need of a motor, by introducing a swirl in the fluid flowing towards the filter element. The design is particularly suited for operation under high pressures as the rotating filter element is fully contained within a cylindrical pipe. The shaft does not pin through the external wall, so no sealing is required. Based on known RPS design principles and fluid flow relations an oil-water separator is designed and tested.