Abstract: In a method of and a compression tube (10) for increasing pressure of a flowing gaseous medium the gaseous medium is pressed by an accelerating element (8) to flow with supersonic velocity. Heat is abstracted from the gaseous medium having supersonic velocity and by shock waves the flow is decelerated to a subsonic velocity range in an impact tube section (13) wherein by decelerating and, if necessary, further abstracting heat the pressure is increased. The power machine comprises in any pipeline section and/or instead of compressor a compression tube (10) including the accelerating element (8), a transient tube section (14) receiving supersonic flow of the gaseous medium, an impact tube section (13) comprising a shock wave tube section (12) and advantageously a passage tube section (16) for decelerating the supersonic flow to subsonic velocity and increasing the pressure to a value exceeding the inlet pressure of the accelerating element (8).

Abstract: The invention relates to plant operations for increasing the cycle times of heat exchangers by the digestion of the aluminium oxide content of a slurry according to the Bayer technology, said slurry consisting of bauxite and sodium aluminate liquor, using an equipment in which the material is heated stepwise and in more parallel material streams, then the material streams are combined and further heated, or combined and kept at the same temperature, or after combination further heated and kept at the same temperature, characterized in that a part of the parallel material streams is slurry, another part of same is aqueous sodium hydroxide solution, preferably sodium aluminate liquor containing sodium hydroxide, or water, and the parallel material streams are exchanged in the heat exchangers in a cyclical manner.