An open steam generator system is disclosed which comprises a substantially enclosed vessel having an opening for exhausting steam generated and an opening for draining residual water from the vessel. Within the vessel also is a heat exchange coil having a heat transfer fluid circulating through its runs. The vessel also includes an injector for spraying water onto the heating coil to form steam. The disclosed steam generator achieves tolerance for low water quality by minimizing surface areas in contact with water. In addition, the system promotes self-cleaning of heat exchange surfaces using thermal shock to dislodge scale deposits.