The invention relates to a method and system for producing steam in a heating unit, and a method of designing a heating unit for producing steam. The method relates to receiving a liquid including impurities at a predetermine flow rate within an entry port of a monotube in the heating unit, where the monotube has a lumen of a predetermined size. The received liquid is converted into a two phase flow including turbulent and laminar flow. At least the turbulent flow is passed through the lumen at a velocity sufficient to carry the impurities through an exit port of the monotube and produce steam. The velocity of the turbulent is responsive to at least the predetermined flow rate and predetermined size of the lumen.