For pure steam generation the feed water enters a tubular heat exchanger with an expansion chamber above. In the tubular heat exchanger the pre-treated water is heated and steam is generated. The tubes of the heat exchanger are connected with the expansion chamber that serves as a separating column. For production of pyrogen-free steam the chamber contains a demister. After entering the expansion chamber, the steam moves further upwards in the column. By gravity and due to the specific column design, un-evaporated water droplets fall back into the lower section of the chamber, thus separating from the steam. Since the droplets may contain contaminants like endotoxins, it is vital that they are not carried forward with the steam. As an important quality characteristics, the conductivity of the pure steam condensate can optionally be monitored. If required, the pure steam generator can be enlarged by additional columns to build up a multi effect distillation system. This allows combined generation of pure water and pure steam at the same time.