Steam-water separators are comprised of a single vertical shell mounted over a hole in a horizontal deck within a steam drum. Vanes within the cylinder imparts centrifugal force to the upwardly flowing mixture of saturated steam and water. The saturated water is thrown to the internal wall of the shell and skimmed over the upper edge of the shell to descend in flowing over the surface of a series of downwardly spiraling trays while evolved steam disengages from the saturated water.