separator to separate water vapour and oil gas from a compressed air stream has a casing (1) into which the air passes through an inlet pipe (2). The air flow is blocked by a conical cover (4) in a sleeve (3) and this blocking of the flow causes condensation of the water vapour and oil. The air then passes through a zig-zag tube (5) and impinges on an L-shaped plate (6). This causes blocking of the air flow and additional condensation occurs. Finally, the air passes into a tube (10) and from there to the convoluted path (91) of a screw (9) in a part (72) of the outlet tube. This causes more condensation. Thus there are three condensation steps so that virtually all the water vapour and oil is removed from the compressed air. The condensed liquid collects at the bottom of the casing and can be drained through drain (11).