As the impeller vanes pass the suction port air is drawn in and trapped between the water ring and the pimp shaft. This 'slug' of air is carried around and deliverd to the discharge port, hence this pump is a positive displacement type. Due to the casing shape the water is made to flow from and towards the rotor center during each revolution. The water motion is utilised to act as suction and discharge for the air through appropriate sets of ports.