 The vertical pipe or shaft goes down from higher point to a separation chamber; a pipe, that is typically narrower than previous one, coming away from that chamber, allows the water to exit at a lower level, and another pipe (air-pipe) coming from the chamber allows the compressed air to exit as needed.Water rushing down the vertical pipe falls through a constriction.The constriction produces a lower pressure because of the venturi effect, and an external port allows air to be sucked in thus creating a constant air supply.