A steam generator apparatus for generating steam from a feedwater inlet stream including impurities is disclosed. The apparatus includes a tubing circuit in communication with an inlet for receiving the feedwater stream, the tubing circuit having a substantially unrifled bore defined by a metal wall, and a heat source operable to deliver a heat flux to the feedwater stream through the metal wall of the tubing circuit, the heat flux being operable to cause evaporation of feedwater within the tubing circuit and to produce an outlet stream at an outlet of the tubing circuit, the outlet stream includes a steam portion and liquid phase portion, the steam portion being greater than about 80% of the outlet stream by mass, the steam portion providing sufficient cooling of the metal wall to maintain a wall temperature at less than a threshold temperature associated with safe operation of the steam generator apparatus.