Continuous direct steam injection systems are used in industry to rapidly raise the temperature of process streams either for heating or for sterilization purposes. High heat transfer rates can be achieved using this method, as compared with other methods e.g. shell and tube heat exchangers. Currently, there are no rational procedures available for designing steam injectors for stable operation. Flow visualization studies and pressure measurements have shown that three flow regimes; bubbling, jetting and intermittent steam/water flow exist, in direct steam injection into continuously flowing water. These flow regimes are a function of process conditions and orifice diameter. A semi-quantitative flow regime map for a range of process conditions has been drawn up. Bubbly flows give rise to the highest levels of noise due to bubble oscillations. This type of flow should therefore be avoided. The most stable flow regime, in terms of noise levels generated and hydrodynamic considerations is the jetting regime. Models have been derived for each flow regime; good agreement is found between experimental and theoretical data. A dimensionless number (Bubble/Jet number) has been defined, which can be used to predict transitions between bubbling and jetting flows. This dimensionless number, together with the flow regime map can be used to design stable, relatively quiet steam injection systems.